1 00:00:00,000 --> 00:00:01,980 [SQUEAKING] 2 00:00:01,980 --> 00:00:03,465 [RUSTLING] 3 00:00:03,465 --> 00:00:10,400 [CLICKING] 4 00:00:10,400 --> 00:00:12,350 MATTHEW VANDER HEIDEN: Last time, we 5 00:00:12,350 --> 00:00:17,930 discussed standard reduction potential, this E0 prime value, 6 00:00:17,930 --> 00:00:20,330 that really is a way to quantitate 7 00:00:20,330 --> 00:00:25,940 the propensity of a molecule to accept or donate electrons. 8 00:00:25,940 --> 00:00:30,590 And this is useful to think about how energy is transduced 9 00:00:30,590 --> 00:00:31,820 in biological systems. 10 00:00:31,820 --> 00:00:38,390 Because remember, as I have now tried to stress many times, 11 00:00:38,390 --> 00:00:41,600 how energy is moved through biology 12 00:00:41,600 --> 00:00:46,010 is largely about oxidation and reduction reactions 13 00:00:46,010 --> 00:00:49,910 involving carbon and ultimately oxygen. 14 00:00:49,910 --> 00:00:55,910 And so remember that oxidation of carbon is favorable. 15 00:00:55,910 --> 00:00:58,460 That is removing electrons from the carbon 16 00:00:58,460 --> 00:01:01,250 and giving them to oxygen, exactly what we 17 00:01:01,250 --> 00:01:04,550 described for burning wood or burning gasoline. 18 00:01:04,550 --> 00:01:07,670 And so that's why coming from the most reduced 19 00:01:07,670 --> 00:01:10,910 carbon like fatty acids or other reduced carbon 20 00:01:10,910 --> 00:01:12,830 like carbohydrates, not as reduced 21 00:01:12,830 --> 00:01:18,960 as fatty acids, that oxidizing that carbon releases energy. 22 00:01:18,960 --> 00:01:22,040 And of course, it means that storing carbon 23 00:01:22,040 --> 00:01:26,180 as reduced carbon either as carbohydrates or fatty acids 24 00:01:26,180 --> 00:01:29,240 is a way to store energy for later. 25 00:01:29,240 --> 00:01:35,190 And so moving from left to right, equilibrium strongly 26 00:01:35,190 --> 00:01:36,540 favors moving to the right. 27 00:01:36,540 --> 00:01:40,290 That means that delta G is less than 0 28 00:01:40,290 --> 00:01:43,030 for the oxidation of carbon. 29 00:01:43,030 --> 00:01:46,260 Or we can describe this as moving from a smaller 30 00:01:46,260 --> 00:01:50,040 standard reduction potential to a larger standard reduction 31 00:01:50,040 --> 00:01:54,570 potential because, remember, the relationship between the change 32 00:01:54,570 --> 00:01:56,790 in standard reduction potential is related 33 00:01:56,790 --> 00:02:01,663 to the equilibrium constant or delta G0 prime, 34 00:02:01,663 --> 00:02:03,330 which is also related to the equilibrium 35 00:02:03,330 --> 00:02:07,360 constant by the above formula. 36 00:02:07,360 --> 00:02:13,230 And so we closed last time by saying that with this formula 37 00:02:13,230 --> 00:02:15,960 we know that, if we move from a smaller standard reduction 38 00:02:15,960 --> 00:02:18,480 potential-- that is donate electrons from a smaller 39 00:02:18,480 --> 00:02:21,870 standard reduction potential to a larger standard reduction 40 00:02:21,870 --> 00:02:25,830 potential-- that that number, that difference, 41 00:02:25,830 --> 00:02:31,140 will then be positive, which is the opposite of delta G 42 00:02:31,140 --> 00:02:32,210 because of that formula. 43 00:02:32,210 --> 00:02:35,010 And so a positive change in standard reduction potential, 44 00:02:35,010 --> 00:02:39,810 moving from lower to higher, equilibrium 45 00:02:39,810 --> 00:02:43,200 is going to favor that direction-- 46 00:02:43,200 --> 00:02:47,430 should make sense from everything we discussed before. 47 00:02:47,430 --> 00:02:49,260 And of course, that change tells you 48 00:02:49,260 --> 00:02:51,870 the propensity of electron transfer 49 00:02:51,870 --> 00:02:55,860 that ultimately is going to be favorable at equilibrium. 50 00:02:55,860 --> 00:03:00,540 However, exactly as we discussed for delta G, 51 00:03:00,540 --> 00:03:03,210 whether an exact reaction is favorable 52 00:03:03,210 --> 00:03:05,670 is still going to be dependent on delta G. 53 00:03:05,670 --> 00:03:08,100 And so the change in standard reduction potential 54 00:03:08,100 --> 00:03:10,110 tells you about the equilibrium constant, 55 00:03:10,110 --> 00:03:12,210 and so which direction is going to be 56 00:03:12,210 --> 00:03:14,340 net favorable at equilibrium. 57 00:03:14,340 --> 00:03:16,950 However, what actually happens is still 58 00:03:16,950 --> 00:03:19,830 going to depend on conditions for all the reasons 59 00:03:19,830 --> 00:03:22,310 we talked about previously. 60 00:03:22,310 --> 00:03:25,200 Still, this change in standard reduction potential 61 00:03:25,200 --> 00:03:28,800 is still a useful concept to think about 62 00:03:28,800 --> 00:03:32,250 because it tells us about how energy transfer can 63 00:03:32,250 --> 00:03:34,710 occur across redox pairs. 64 00:03:34,710 --> 00:03:37,410 And that will be useful in thinking 65 00:03:37,410 --> 00:03:40,650 about this process of oxidative phosphorylation 66 00:03:40,650 --> 00:03:45,840 and how cells really couple electron transfer to oxygen 67 00:03:45,840 --> 00:03:50,070 as a way to do lots of useful things 68 00:03:50,070 --> 00:03:55,060 in cells, including making ATP. 69 00:03:55,060 --> 00:03:57,840 Now, one point I want to come back to again, 70 00:03:57,840 --> 00:04:02,670 which we discussed at length when we talked about changes 71 00:04:02,670 --> 00:04:06,180 in free energy and how that worked for pathways, 72 00:04:06,180 --> 00:04:10,590 is that, remember, we discussed that, if we start with wood 73 00:04:10,590 --> 00:04:14,610 or oil and all in one step burn it to CO2, 74 00:04:14,610 --> 00:04:17,279 it releases a certain amount of energy. 75 00:04:17,279 --> 00:04:19,920 Or we can break that into individual steps, 76 00:04:19,920 --> 00:04:22,260 as we did in glycolysis. 77 00:04:22,260 --> 00:04:24,030 The same amount of energy is released. 78 00:04:24,030 --> 00:04:26,620 But by breaking into individual steps, 79 00:04:26,620 --> 00:04:29,750 we can now capture that energy as intermediates 80 00:04:29,750 --> 00:04:32,970 such that we can, say, favorably synthesize ATP 81 00:04:32,970 --> 00:04:35,670 at a high ATP/ADP ratio in cells. 82 00:04:35,670 --> 00:04:38,850 The exact same thing is true about any of these electron 83 00:04:38,850 --> 00:04:40,050 transfers. 84 00:04:40,050 --> 00:04:42,870 There is the change in standard reduction potential. 85 00:04:42,870 --> 00:04:47,580 Or the change in free energy as one moves from electron 86 00:04:47,580 --> 00:04:50,670 transfer from one acceptor to one donor 87 00:04:50,670 --> 00:04:53,070 is the same if you go in one step 88 00:04:53,070 --> 00:04:55,330 or in many different steps. 89 00:04:55,330 --> 00:05:00,350 And so by doing these stepwise electron transfers 90 00:05:00,350 --> 00:05:05,060 in exactly the same way we discussed about for glycolysis, 91 00:05:05,060 --> 00:05:09,050 we can capture that energy release in smaller packets. 92 00:05:09,050 --> 00:05:12,500 And this is a way to basically better harness this energy 93 00:05:12,500 --> 00:05:15,500 release to do useful things for cells. 94 00:05:15,500 --> 00:05:17,480 Ultimately, you'll see it's how you make ATP, 95 00:05:17,480 --> 00:05:19,520 but it can do other work as well, 96 00:05:19,520 --> 00:05:22,760 generate heat, move ions, et cetera. 97 00:05:22,760 --> 00:05:28,040 And so this is really what I want to discuss today. 98 00:05:28,040 --> 00:05:32,680 But it's important to remember these concepts when we think 99 00:05:32,680 --> 00:05:35,020 through how these systems work. 100 00:05:35,020 --> 00:05:38,320 And so we've now spent several lectures 101 00:05:38,320 --> 00:05:42,640 discussing the chemical details of how we can oxidize. 102 00:05:42,640 --> 00:05:44,360 First, we discussed carbohydrates. 103 00:05:44,360 --> 00:05:46,520 Then we discussed fatty acids. 104 00:05:46,520 --> 00:05:50,560 And what we saw is that many of the electrons that 105 00:05:50,560 --> 00:05:53,740 are lost from the carbon oxidation 106 00:05:53,740 --> 00:06:02,830 are used to generate molecules such as NADH or FADH2 107 00:06:02,830 --> 00:06:09,430 and effectively charge up a ratio, if you will, of 108 00:06:09,430 --> 00:06:12,970 reduce oxidized cofactors. 109 00:06:12,970 --> 00:06:17,650 And these molecules then donate those electrons, ultimately, 110 00:06:17,650 --> 00:06:23,260 to oxygen. And it's that, how you couple those electron 111 00:06:23,260 --> 00:06:27,250 transfers from these carriers to oxygen, 112 00:06:27,250 --> 00:06:30,040 ultimately is what oxidative phosphorylation is 113 00:06:30,040 --> 00:06:37,660 about as a way to allow cells to convert this energy released 114 00:06:37,660 --> 00:06:41,500 into a biologically useful form. 115 00:06:41,500 --> 00:06:43,360 Now, effectively, the way biology 116 00:06:43,360 --> 00:06:48,730 does this is by using that favorable electron transfer 117 00:06:48,730 --> 00:06:52,300 to oxygen to charge a battery. 118 00:06:52,300 --> 00:06:57,170 So really oxidative phosphorylation, if you will, 119 00:06:57,170 --> 00:06:59,690 is about a biological battery. 120 00:06:59,690 --> 00:07:01,670 And it's really the same concepts 121 00:07:01,670 --> 00:07:03,830 that you may have learned in other courses 122 00:07:03,830 --> 00:07:06,032 about how chemical batteries work. 123 00:07:06,032 --> 00:07:07,490 And so let's just think about that. 124 00:07:07,490 --> 00:07:09,200 What is a battery? 125 00:07:09,200 --> 00:07:13,790 Well, a battery is two different solutions of ions usually 126 00:07:13,790 --> 00:07:17,960 that is separated into two compartments. 127 00:07:17,960 --> 00:07:22,070 It has a unit of volts. 128 00:07:22,070 --> 00:07:24,560 That's exactly what the units are 129 00:07:24,560 --> 00:07:29,260 for this change in standard reduction potential is volts. 130 00:07:29,260 --> 00:07:33,850 By using that voltage to run a current between the two 131 00:07:33,850 --> 00:07:36,730 compartments, you can do work. 132 00:07:36,730 --> 00:07:39,760 And if you short circuit a battery, of course, 133 00:07:39,760 --> 00:07:41,920 you can get some heat. 134 00:07:41,920 --> 00:07:45,460 And so how are these really made? 135 00:07:45,460 --> 00:07:49,240 Well, you basically just need to get that voltage. 136 00:07:49,240 --> 00:07:52,570 You need to have something that insulates between those two 137 00:07:52,570 --> 00:07:53,140 compartments. 138 00:07:53,140 --> 00:07:55,960 And you can then control electron flow 139 00:07:55,960 --> 00:07:59,070 from one side of the circuit to the other. 140 00:07:59,070 --> 00:08:03,430 And it's exactly the same issue that happens in biology, 141 00:08:03,430 --> 00:08:06,850 except biology needs some insulator 142 00:08:06,850 --> 00:08:08,590 between two compartments. 143 00:08:08,590 --> 00:08:11,170 How do we create different compartments in cells? 144 00:08:11,170 --> 00:08:13,790 Well, we create them with membranes. 145 00:08:13,790 --> 00:08:18,100 And so an important function of membranes in cells, 146 00:08:18,100 --> 00:08:21,850 other than things like separating cellular contents 147 00:08:21,850 --> 00:08:24,280 from the outside world or breaking up 148 00:08:24,280 --> 00:08:27,220 different organelles, is that cells actually 149 00:08:27,220 --> 00:08:28,810 require a membrane. 150 00:08:28,810 --> 00:08:32,320 They're essential for life because they 151 00:08:32,320 --> 00:08:36,490 are what allows an insulation between two aqueous 152 00:08:36,490 --> 00:08:38,710 compartments in a way that you can build 153 00:08:38,710 --> 00:08:45,010 a biological battery that is really central to how life uses 154 00:08:45,010 --> 00:08:47,630 energy as a way to do work. 155 00:08:47,630 --> 00:08:53,290 And so life creates a voltage, which I'm going to call delta 156 00:08:53,290 --> 00:08:56,320 psi, delta pH-- we'll come back to that in a little bit-- 157 00:08:56,320 --> 00:09:06,680 to basically couple favorable electron transfer from, say, 158 00:09:06,680 --> 00:09:15,680 an electron carrier like NAD/NADH to oxygen, 159 00:09:15,680 --> 00:09:22,200 so reoxidizing NADH to reduce oxygen to water. 160 00:09:22,200 --> 00:09:27,630 That is moving from a lower to a higher standard reduction 161 00:09:27,630 --> 00:09:29,670 potential that's favorable. 162 00:09:29,670 --> 00:09:32,400 Delta G0 prime is negative. 163 00:09:32,400 --> 00:09:34,800 Net transfer is favorable. 164 00:09:34,800 --> 00:09:42,060 This can be coupled to doing some work that otherwise would 165 00:09:42,060 --> 00:09:45,210 be unfavorable, in this case pumping protons 166 00:09:45,210 --> 00:09:47,850 from one side of the membrane to another. 167 00:09:47,850 --> 00:09:52,470 By pumping protons to one side of a membrane or the other, 168 00:09:52,470 --> 00:09:56,610 you create a voltage membrane potential, often 169 00:09:56,610 --> 00:09:58,500 abbreviated delta psi. 170 00:09:58,500 --> 00:10:04,020 It's also a pH gradient, so delta H. The protons 171 00:10:04,020 --> 00:10:08,440 have lower pH on one side of the membrane than the other. 172 00:10:08,440 --> 00:10:12,300 And it's this charging of a battery at a membrane that 173 00:10:12,300 --> 00:10:16,620 is what is captured by oxidative phosphorylation, what's 174 00:10:16,620 --> 00:10:21,480 typically referred to as that process, as a way to make ATP. 175 00:10:21,480 --> 00:10:24,190 But of course, this is a useful thing. 176 00:10:24,190 --> 00:10:24,930 It's a battery. 177 00:10:24,930 --> 00:10:31,270 And so it can also do other work for cells. 178 00:10:31,270 --> 00:10:35,580 Now, I just want to say and really 179 00:10:35,580 --> 00:10:40,230 stress that this is really critical, this process. 180 00:10:40,230 --> 00:10:45,150 And it's really central to how energy works for cells. 181 00:10:45,150 --> 00:10:48,150 You will see that, effectively, this 182 00:10:48,150 --> 00:10:51,030 is the key steps in photosynthesis, 183 00:10:51,030 --> 00:10:52,960 do exactly the same thing. 184 00:10:52,960 --> 00:10:54,720 It's how mitochondria work. 185 00:10:54,720 --> 00:10:57,930 It's also how bacteria work. 186 00:10:57,930 --> 00:11:03,360 And really prokaryotes end up doing this at their plasma 187 00:11:03,360 --> 00:11:08,220 membranes, whereas mitochondria and chloroplasts end up 188 00:11:08,220 --> 00:11:12,930 doing this at intracellular membranes in eukaryotes. 189 00:11:12,930 --> 00:11:16,170 But ultimately, it's the same idea 190 00:11:16,170 --> 00:11:21,360 of charging this biological battery that really allows 191 00:11:21,360 --> 00:11:25,680 cells to convert, do energy conversions 192 00:11:25,680 --> 00:11:33,060 in a way that's useful for them to allow otherwise 193 00:11:33,060 --> 00:11:36,720 unfavorable things to happen in cells. 194 00:11:36,720 --> 00:11:40,530 Now, this works largely because you 195 00:11:40,530 --> 00:11:44,790 have this insulating membrane system, which is, 196 00:11:44,790 --> 00:11:47,620 of course, made of lipids. 197 00:11:47,620 --> 00:11:50,160 Lipids are what separates the two compartments. 198 00:11:50,160 --> 00:11:54,090 Remember, we discussed earlier that these lipid bilayers 199 00:11:54,090 --> 00:11:59,760 are useful because they create a hydrophobic interface 200 00:11:59,760 --> 00:12:02,670 between two aqueous compartments. 201 00:12:02,670 --> 00:12:05,220 It's also useful because lipids are 202 00:12:05,220 --> 00:12:08,380 poor conductors of electrons. 203 00:12:08,380 --> 00:12:13,380 And so this really ends up being the insulating system 204 00:12:13,380 --> 00:12:15,370 that makes this work. 205 00:12:15,370 --> 00:12:20,900 Now, in the previous lecture, we discussed 206 00:12:20,900 --> 00:12:25,670 a lot about the polar and non-polar properties 207 00:12:25,670 --> 00:12:29,090 of phospholipids really allow them to assemble 208 00:12:29,090 --> 00:12:31,640 into these membrane barriers. 209 00:12:31,640 --> 00:12:34,160 And I just want to do a slight diversion 210 00:12:34,160 --> 00:12:36,950 to revisit this in the context of how 211 00:12:36,950 --> 00:12:41,670 this works for bioenergetics. 212 00:12:41,670 --> 00:12:46,050 So remember, you have this lipid bilayer where 213 00:12:46,050 --> 00:12:59,210 you separate two aqueous compartments 214 00:12:59,210 --> 00:13:01,430 with this hydrophobic membrane. 215 00:13:01,430 --> 00:13:07,850 In this membrane are these phospholipids, 216 00:13:07,850 --> 00:13:13,160 where you have these polar head groups with two lipid tails. 217 00:13:13,160 --> 00:13:16,520 You can look back to a former lecture where I drew 218 00:13:16,520 --> 00:13:19,670 and we discussed what those phospholipids 219 00:13:19,670 --> 00:13:21,380 are in great detail. 220 00:13:21,380 --> 00:13:25,310 But it's really having that polar, non-polar interface 221 00:13:25,310 --> 00:13:27,710 that allows you to form these bilayers that's 222 00:13:27,710 --> 00:13:32,270 creating this separation between the aqueous compartments, 223 00:13:32,270 --> 00:13:37,100 but also this insulator that allows a battery to be built 224 00:13:37,100 --> 00:13:40,160 across the membrane. 225 00:13:40,160 --> 00:13:44,770 Now, there are several consequences, though, 226 00:13:44,770 --> 00:13:49,000 to this system that is worth thinking about at least 227 00:13:49,000 --> 00:13:53,830 in terms of how lipids and membranes work. 228 00:13:53,830 --> 00:13:56,290 Now, a consequence of this is that, if we're 229 00:13:56,290 --> 00:13:58,480 going to create this system, now we 230 00:13:58,480 --> 00:14:05,280 create a system where we have issues with transport 231 00:14:05,280 --> 00:14:09,400 across from one compartment to another. 232 00:14:09,400 --> 00:14:11,190 Now, this, of course, is an opportunity 233 00:14:11,190 --> 00:14:15,330 for a layer of regulation because it also 234 00:14:15,330 --> 00:14:16,980 allows then these conditions where 235 00:14:16,980 --> 00:14:19,610 you could have different chemical conditions, 236 00:14:19,610 --> 00:14:23,040 say, different ATP/ADP ratios, NAD/NADH ratios 237 00:14:23,040 --> 00:14:25,540 in the different compartments, which, of course, 238 00:14:25,540 --> 00:14:27,930 for all the thermodynamic reasons we've already discussed 239 00:14:27,930 --> 00:14:31,260 can make things more or less favorable to occur 240 00:14:31,260 --> 00:14:33,930 in different compartments in cells. 241 00:14:33,930 --> 00:14:36,030 But it also means metabolism then 242 00:14:36,030 --> 00:14:40,080 has to have ways to transport stuff across these membranes. 243 00:14:40,080 --> 00:14:42,240 And we've discussed this a couple of times. 244 00:14:42,240 --> 00:14:44,160 A few examples-- I talked briefly 245 00:14:44,160 --> 00:14:45,930 about the pyruvate carrier. 246 00:14:45,930 --> 00:14:47,880 That is, if glycolysis is happening 247 00:14:47,880 --> 00:14:50,130 in this compartment in the cytosol 248 00:14:50,130 --> 00:14:53,310 and we need to get the pyruvate inside the mitochondria 249 00:14:53,310 --> 00:14:57,120 across a membrane for the pyruvate dehydrogenase 250 00:14:57,120 --> 00:15:00,300 reaction, now you needed this pyruvate carrier molecule. 251 00:15:00,300 --> 00:15:02,190 And it turns out that that transport process 252 00:15:02,190 --> 00:15:04,200 is an opportunity for regulation, 253 00:15:04,200 --> 00:15:06,630 but it still needs to occur. 254 00:15:06,630 --> 00:15:09,360 We discussed the carnitine shuttle, right? 255 00:15:09,360 --> 00:15:13,890 Fatty acid, we generate fatty acyl-CoAs 256 00:15:13,890 --> 00:15:17,130 from breakdown of lipids in one side 257 00:15:17,130 --> 00:15:18,510 in the cytosolic compartment. 258 00:15:18,510 --> 00:15:22,170 We need to get those fatty acyl-CoAs into the mitochondria 259 00:15:22,170 --> 00:15:25,080 where they can be oxidized by fatty acid oxidation. 260 00:15:25,080 --> 00:15:29,070 And that required that carnitine shuttle system. 261 00:15:29,070 --> 00:15:34,260 Now, ultimately, what mediates this transport is, of course, 262 00:15:34,260 --> 00:15:40,860 proteins that can sit within this membrane bilayer. 263 00:15:40,860 --> 00:15:42,690 And you can imagine you can have proteins 264 00:15:42,690 --> 00:15:44,880 that span the membrane bilayer, such 265 00:15:44,880 --> 00:15:47,400 that there's hydrophilic surfaces that 266 00:15:47,400 --> 00:15:51,330 are on either face of the membrane and hydrophobic 267 00:15:51,330 --> 00:15:53,940 residues in the middle with various ways 268 00:15:53,940 --> 00:15:57,480 to make channels and other transport mechanisms to get 269 00:15:57,480 --> 00:16:00,180 things from one side of the membrane to the other, 270 00:16:00,180 --> 00:16:01,110 all right? 271 00:16:01,110 --> 00:16:04,980 Or you could have complexes that float freely within the lipid 272 00:16:04,980 --> 00:16:08,670 bilayer, such that there's a hydrophilic surface that 273 00:16:08,670 --> 00:16:12,870 faces one compartment and a hydrophobic surface that faces 274 00:16:12,870 --> 00:16:15,640 the inside of the membrane. 275 00:16:15,640 --> 00:16:20,850 Now, having these complexes will become very important 276 00:16:20,850 --> 00:16:25,710 for ways in which one can regulate processes that 277 00:16:25,710 --> 00:16:28,050 occur either within the membrane or by moving things 278 00:16:28,050 --> 00:16:31,440 across membranes, like generating moving protons 279 00:16:31,440 --> 00:16:37,350 and generating a voltage, or even 280 00:16:37,350 --> 00:16:40,350 move large things like proteins across the membrane. 281 00:16:40,350 --> 00:16:44,400 But it has additional consequences because it now 282 00:16:44,400 --> 00:16:46,680 means that the properties of the membrane 283 00:16:46,680 --> 00:16:49,390 also matter somewhat as well. 284 00:16:49,390 --> 00:16:51,540 And this brings us back to the other discussion 285 00:16:51,540 --> 00:16:53,220 we had in the last lecture. 286 00:16:53,220 --> 00:16:57,360 And that is how the various properties of the fatty acids 287 00:16:57,360 --> 00:17:03,720 that exist within this hydrophobic membrane area also, 288 00:17:03,720 --> 00:17:06,569 now, can really matter because they're 289 00:17:06,569 --> 00:17:09,930 going to affect things about how leaky this 290 00:17:09,930 --> 00:17:13,410 is as an insulator between the two compartments, 291 00:17:13,410 --> 00:17:15,690 as well as how fluid the membrane is, 292 00:17:15,690 --> 00:17:18,119 such that things like these proteins 293 00:17:18,119 --> 00:17:23,190 can actually move within this hydrophobic barrier 294 00:17:23,190 --> 00:17:25,890 between the two compartments. 295 00:17:25,890 --> 00:17:39,410 In other words, the fatty acid composition, as well as 296 00:17:39,410 --> 00:17:43,700 the lipid species themselves, that 297 00:17:43,700 --> 00:17:50,930 is which phospholipid, other polar hydrophobic lipid 298 00:17:50,930 --> 00:17:54,650 species analogous to phospholipids that form up 299 00:17:54,650 --> 00:17:59,000 membranes, can change properties such as how leaky 300 00:17:59,000 --> 00:18:05,030 the membrane is or how fluid the membrane is. 301 00:18:05,030 --> 00:18:09,380 You can imagine, if you have a really rigid membrane surface, 302 00:18:09,380 --> 00:18:12,630 you're not going to get any flow of materials. 303 00:18:12,630 --> 00:18:15,168 It's not going to be very leaky, which might be great 304 00:18:15,168 --> 00:18:17,210 if you want to build an insulator between the two 305 00:18:17,210 --> 00:18:18,170 compartments. 306 00:18:18,170 --> 00:18:21,590 But it's going to be very difficult to regulate transport 307 00:18:21,590 --> 00:18:23,270 across that compartment. 308 00:18:23,270 --> 00:18:26,990 And so nature has come up with a bunch of different solutions 309 00:18:26,990 --> 00:18:32,000 for how to play with fatty acid and other lipid composition 310 00:18:32,000 --> 00:18:34,490 to change the properties of membranes. 311 00:18:34,490 --> 00:18:37,070 We don't have time to get into that in great detail 312 00:18:37,070 --> 00:18:38,960 in this course, but I want you to appreciate 313 00:18:38,960 --> 00:18:40,880 that these things exist. 314 00:18:40,880 --> 00:18:43,460 But one thing I want to discuss is that often, 315 00:18:43,460 --> 00:18:46,610 when you see phospholipids drawn like this, 316 00:18:46,610 --> 00:18:48,680 they're drawn in this way where the polar head 317 00:18:48,680 --> 00:18:50,150 group is a circle. 318 00:18:50,150 --> 00:18:53,660 And then they'll draw the two lipids tails often 319 00:18:53,660 --> 00:18:56,030 with a kink in one of the lipid tails. 320 00:18:56,030 --> 00:18:59,150 That's because many times phospholipids, of the two lipid 321 00:18:59,150 --> 00:19:01,460 tails, one is saturated. 322 00:19:01,460 --> 00:19:05,780 And one has a monounsaturation in it. 323 00:19:05,780 --> 00:19:08,630 And I just want to go back to the models here. 324 00:19:08,630 --> 00:19:11,570 Remember, the monounsaturated, the double bond, 325 00:19:11,570 --> 00:19:14,790 ends up being this cis double bond. 326 00:19:14,790 --> 00:19:18,770 And so here's two fatty acids. 327 00:19:18,770 --> 00:19:23,270 If you put these here in with the head groups on one side-- 328 00:19:23,270 --> 00:19:26,900 so this would be esterified to the glycerol, the phospholipid, 329 00:19:26,900 --> 00:19:29,750 you end up getting this kink in the membrane, which 330 00:19:29,750 --> 00:19:32,570 is much different than two straight chains. 331 00:19:32,570 --> 00:19:34,460 You can imagine that within the membrane 332 00:19:34,460 --> 00:19:39,680 itself this ends up basically not letting those lipid tails 333 00:19:39,680 --> 00:19:43,160 pack quite so tightly in the membrane, which certainly 334 00:19:43,160 --> 00:19:47,090 contributes to an ability to have space 335 00:19:47,090 --> 00:19:49,670 to put other stuff in the membrane to allow 336 00:19:49,670 --> 00:19:52,520 other processes to happen. 337 00:19:52,520 --> 00:19:55,190 Now, recall that there's other ways 338 00:19:55,190 --> 00:20:01,210 we can modulate the properties of fatty acids, 339 00:20:01,210 --> 00:20:05,660 introduce more double bonds, chain length, et cetera. 340 00:20:05,660 --> 00:20:09,550 And different organisms use different strategies 341 00:20:09,550 --> 00:20:11,030 to deal with this. 342 00:20:11,030 --> 00:20:15,820 And so plants, remember, have all kinds 343 00:20:15,820 --> 00:20:20,920 of polyunsaturated fatty acids, just like we talked about 344 00:20:20,920 --> 00:20:24,280 with plant oils being liquid at room temperature. 345 00:20:24,280 --> 00:20:29,860 Having more polyunsaturated fatty acids in their lipid 346 00:20:29,860 --> 00:20:34,030 tails is one way plants use to keep 347 00:20:34,030 --> 00:20:36,610 their membranes more fluid. 348 00:20:36,610 --> 00:20:40,690 Animals, remember, have more saturated fatty acids. 349 00:20:40,690 --> 00:20:44,290 All right, those saturated fatty acids-- 350 00:20:44,290 --> 00:20:48,190 less double bonds, going to be higher melting 351 00:20:48,190 --> 00:20:52,130 temperature, more likely to be solid at room temperature. 352 00:20:52,130 --> 00:20:54,670 So that's not going to be very good for membrane fluidity. 353 00:20:54,670 --> 00:20:58,390 And it turns out animals use a different strategy 354 00:20:58,390 --> 00:21:00,550 to keep membrane fluidity that's not 355 00:21:00,550 --> 00:21:02,680 polyunsaturated fatty acids. 356 00:21:02,680 --> 00:21:05,830 And that is that they incorporate this molecule shown 357 00:21:05,830 --> 00:21:09,530 here on the slide, which is cholesterol, 358 00:21:09,530 --> 00:21:11,030 which you're well aware of. 359 00:21:11,030 --> 00:21:15,070 And so animals use you sterols, like cholesterol, 360 00:21:15,070 --> 00:21:19,370 as a way to keep their membranes more fluid. 361 00:21:19,370 --> 00:21:24,700 And so that cholesterol that is present in animal products 362 00:21:24,700 --> 00:21:26,750 is not there to give you heart attacks. 363 00:21:26,750 --> 00:21:29,320 It's really there as a way animals use 364 00:21:29,320 --> 00:21:31,450 to keep their membranes fluid. 365 00:21:31,450 --> 00:21:33,640 And this is really, at heart, why 366 00:21:33,640 --> 00:21:37,570 plants are cholesterol free foods and animals have 367 00:21:37,570 --> 00:21:38,650 cholesterol. 368 00:21:38,650 --> 00:21:41,530 And it's because animals use cholesterol as a way 369 00:21:41,530 --> 00:21:43,870 to keep their membranes fluid, whereas plants 370 00:21:43,870 --> 00:21:48,970 use a different strategy, polyunsaturated fatty acids. 371 00:21:48,970 --> 00:21:54,090 All right, now, ultimately setting up 372 00:21:54,090 --> 00:21:57,720 the system this way is important because it allows 373 00:21:57,720 --> 00:21:59,400 for these flexible membranes that 374 00:21:59,400 --> 00:22:03,930 can enable ion and other transport between compartments, 375 00:22:03,930 --> 00:22:10,650 as well as hold a charge so that you can form this battery which 376 00:22:10,650 --> 00:22:14,820 allows oxidative phosphorylation or other processes to occur 377 00:22:14,820 --> 00:22:17,070 where you can charge this membrane 378 00:22:17,070 --> 00:22:22,740 and ultimately use that voltage to do work, 379 00:22:22,740 --> 00:22:26,130 like make ATP, all right? 380 00:22:26,130 --> 00:22:31,450 Now, how this membrane charging occurs, 381 00:22:31,450 --> 00:22:36,310 again, is really taking advantage of favorable electron 382 00:22:36,310 --> 00:22:41,110 transfer across a series of redox pairs, 383 00:22:41,110 --> 00:22:45,970 ultimately, to oxygen or another good electron acceptor. 384 00:22:45,970 --> 00:22:49,450 And it's really that favorable electron transfer 385 00:22:49,450 --> 00:22:53,380 that's being coupled to this pumping of proton ions 386 00:22:53,380 --> 00:22:55,570 to charge a battery. 387 00:22:55,570 --> 00:22:57,730 That's the oxidation part. 388 00:22:57,730 --> 00:23:04,210 And then that battery doing work such as driving favorable ADP 389 00:23:04,210 --> 00:23:08,140 to ATP conversion, the phosphorylation 390 00:23:08,140 --> 00:23:11,050 part of oxidative phosphorylation, 391 00:23:11,050 --> 00:23:14,620 is the type of work that you can then 392 00:23:14,620 --> 00:23:18,790 do once you generate this chemical battery, although not 393 00:23:18,790 --> 00:23:20,920 the only work you can do with that battery. 394 00:23:20,920 --> 00:23:22,330 And it's why this is so much more 395 00:23:22,330 --> 00:23:26,160 flexible than just making ATP. 396 00:23:26,160 --> 00:23:31,830 All right, so ultimately this process 397 00:23:31,830 --> 00:23:35,520 by coupling transfer of electrons 398 00:23:35,520 --> 00:23:37,860 from favorable oxidation of carbon 399 00:23:37,860 --> 00:23:41,370 to charge up these other electron carriers that 400 00:23:41,370 --> 00:23:46,740 ultimately transfer to oxygen is a major way 401 00:23:46,740 --> 00:23:52,350 that life uses nutrient oxidation to convert it 402 00:23:52,350 --> 00:23:54,810 to biologically useful energy. 403 00:23:54,810 --> 00:23:59,250 And this is where we get to this process of what is typically 404 00:23:59,250 --> 00:24:07,530 referred to as oxidative phosphorylation, which 405 00:24:07,530 --> 00:24:14,850 is a concept or a process that was often abbreviated oxphos. 406 00:24:14,850 --> 00:24:19,350 And it was discovered by or first described by a gentleman 407 00:24:19,350 --> 00:24:20,790 by the name of Peter Mitchell. 408 00:24:24,590 --> 00:24:29,150 And like many novel or revolutionary ideas, 409 00:24:29,150 --> 00:24:32,240 when Peter Mitchell first described or proposed 410 00:24:32,240 --> 00:24:34,340 the processes I'm about to tell you about, 411 00:24:34,340 --> 00:24:38,600 his ideas were initially dismissed. 412 00:24:38,600 --> 00:24:41,900 It was thought at the time, when people were thinking about how 413 00:24:41,900 --> 00:24:45,830 do you couple metabolism to, say, charge up your ATP/ADP 414 00:24:45,830 --> 00:24:49,280 ratio, that that process would work 415 00:24:49,280 --> 00:24:53,630 via the oxidative phosphorylation type reactions 416 00:24:53,630 --> 00:24:55,910 that we otherwise described, things 417 00:24:55,910 --> 00:24:59,660 like what happens at the GAPDH step or the succinic thiokinase 418 00:24:59,660 --> 00:25:01,760 step in the TCA cycle. 419 00:25:01,760 --> 00:25:04,340 Those are two examples where we could directly 420 00:25:04,340 --> 00:25:10,820 couple the oxidation of carbon to a phosphorylation event that 421 00:25:10,820 --> 00:25:14,540 ultimately allowed us to favorably produce 422 00:25:14,540 --> 00:25:20,060 ATP to maintain an ATP/ADP ratio that's high 423 00:25:20,060 --> 00:25:23,090 and is present in cells. 424 00:25:23,090 --> 00:25:25,760 However, what Peter Mitchell described 425 00:25:25,760 --> 00:25:29,210 was really this battery phenomena 426 00:25:29,210 --> 00:25:30,750 that we're talking about. 427 00:25:30,750 --> 00:25:33,500 And it turned out to be largely right 428 00:25:33,500 --> 00:25:36,380 and represents something that's actually 429 00:25:36,380 --> 00:25:40,610 quite unique about biological processes 430 00:25:40,610 --> 00:25:43,250 even if you think about other biological processes that 431 00:25:43,250 --> 00:25:44,090 occur. 432 00:25:44,090 --> 00:25:50,540 And that is what he proposed was the oxidation 433 00:25:50,540 --> 00:26:04,260 and phosphorylation events were actually separate processes. 434 00:26:04,260 --> 00:26:09,330 That is nature does not have to have these two things touch 435 00:26:09,330 --> 00:26:11,370 in order for them to work. 436 00:26:11,370 --> 00:26:16,470 That is that you use oxidation to create 437 00:26:16,470 --> 00:26:18,780 this battery as one event. 438 00:26:23,560 --> 00:26:36,460 So oxidation generates this potential across a membrane. 439 00:26:36,460 --> 00:26:40,390 And then that potential across a membrane 440 00:26:40,390 --> 00:27:02,630 is a totally separate event, can drive phosphorylation, ATP, 441 00:27:02,630 --> 00:27:05,090 or do other work, ultimately making 442 00:27:05,090 --> 00:27:08,780 this a more flexible system for biology. 443 00:27:08,780 --> 00:27:14,040 And so turns out, as I said before, in a prokaryotic cell-- 444 00:27:14,040 --> 00:27:20,480 so this here is some kind of prokaryote, OK? 445 00:27:20,480 --> 00:27:23,630 Prokaryotes have a membrane that separates them 446 00:27:23,630 --> 00:27:24,870 from the outside world. 447 00:27:24,870 --> 00:27:30,620 This cell membrane effectively couples that oxidation step 448 00:27:30,620 --> 00:27:32,570 to the pumping of protons. 449 00:27:32,570 --> 00:27:41,770 That creates a battery at the cell membrane 450 00:27:41,770 --> 00:27:43,420 of the prokaryote. 451 00:27:43,420 --> 00:27:47,530 And separate in space, that can then 452 00:27:47,530 --> 00:27:53,440 be used to drive ATP synthesis or do 453 00:27:53,440 --> 00:28:00,420 other work that's useful to maintain that prokaryotic cell. 454 00:28:00,420 --> 00:28:09,940 Now, in eukaryotes, this process occurs at membranes 455 00:28:09,940 --> 00:28:11,180 within the mitochondria. 456 00:28:11,180 --> 00:28:14,890 So this here is a mitochondria, OK? 457 00:28:14,890 --> 00:28:21,010 So just to remind you of some very basic cell biology-- 458 00:28:21,010 --> 00:28:24,140 and so, remember, mitochondria have two membranes. 459 00:28:24,140 --> 00:28:35,080 So there's this outer mitochondrial membrane, 460 00:28:35,080 --> 00:28:37,060 outer mitochondrial membrane. 461 00:28:37,060 --> 00:28:44,200 And then there's this inner mitochondrial membrane, 462 00:28:44,200 --> 00:28:46,930 so two membranes in the mitochondria. 463 00:28:46,930 --> 00:28:52,380 Remember, there's this matrix space. 464 00:28:52,380 --> 00:28:55,020 The cytosol would be out here. 465 00:28:55,020 --> 00:28:59,250 And then there's this space between the two membranes 466 00:28:59,250 --> 00:29:14,430 called the intermembrane space, intermembrane space 467 00:29:14,430 --> 00:29:16,690 between the two membranes. 468 00:29:16,690 --> 00:29:21,690 If you'll recall, many of these oxidation reactions, the TCA 469 00:29:21,690 --> 00:29:26,760 cycle, fatty acid oxidation, pyruvate dehydrogenase step, 470 00:29:26,760 --> 00:29:30,840 all of those were happening in the matrix of the mitochondria. 471 00:29:30,840 --> 00:29:33,390 Remember, glycolysis was in the cytosol, 472 00:29:33,390 --> 00:29:35,460 but all these other processes were 473 00:29:35,460 --> 00:29:37,830 happening in the matrix of the mitochondria. 474 00:29:37,830 --> 00:29:41,940 And really what's going on is that this battery is 475 00:29:41,940 --> 00:29:46,920 being charged across this inner mitochondrial membrane, OK? 476 00:29:46,920 --> 00:29:50,550 So protons are being pumped into the inner mitochondrial 477 00:29:50,550 --> 00:29:55,680 membrane into the intermembrane space of the mitochondria 478 00:29:55,680 --> 00:30:01,170 and then used to synthesize ATP in the matrix 479 00:30:01,170 --> 00:30:03,090 of the mitochondria. 480 00:30:03,090 --> 00:30:07,050 Now, part of this came, many people know-- remember, 481 00:30:07,050 --> 00:30:09,640 mitochondria have their own DNA, et cetera. 482 00:30:09,640 --> 00:30:12,540 And so really where people thought, 483 00:30:12,540 --> 00:30:15,900 think eukaryotic cells came from is basically 484 00:30:15,900 --> 00:30:21,060 the mitochondria is an enslaved bacteria or an obligate 485 00:30:21,060 --> 00:30:26,040 captured symbiont, if you will, where one cell engulfed 486 00:30:26,040 --> 00:30:28,480 a different prokaryotic cell. 487 00:30:28,480 --> 00:30:34,610 And if you track this, think of the inner mitochondrial 488 00:30:34,610 --> 00:30:39,290 membrane as a enslaved prokaryotic cell, 489 00:30:39,290 --> 00:30:41,090 you can see that you're generating 490 00:30:41,090 --> 00:30:51,440 this membrane potential across the inner mitochondrial 491 00:30:51,440 --> 00:30:53,660 membrane, which is really equivalent 492 00:30:53,660 --> 00:30:59,330 to the bacterial membrane of the enslaved prokaryotic cell. 493 00:30:59,330 --> 00:31:02,990 In both cases, it's creating this battery 494 00:31:02,990 --> 00:31:05,540 at a membrane, the inner mitochondrial membrane 495 00:31:05,540 --> 00:31:08,630 or the bacterial membrane of a prokaryote, that is then 496 00:31:08,630 --> 00:31:12,830 being used to separately in space 497 00:31:12,830 --> 00:31:17,360 use that gradient to do work, such as synthesize ATP. 498 00:31:21,410 --> 00:31:29,250 So if I draw this more explicitly as this here 499 00:31:29,250 --> 00:31:31,090 being the membrane-- 500 00:31:31,090 --> 00:31:35,960 so the inner mitochondrial membrane 501 00:31:35,960 --> 00:31:45,600 of a mitochondria or the bacterial membrane 502 00:31:45,600 --> 00:31:48,090 of a prokaryotic cell, such that this 503 00:31:48,090 --> 00:31:51,970 is outside the bacteria, that's inside the bacteria. 504 00:31:51,970 --> 00:31:56,100 This is the intermembrane space often referred to 505 00:31:56,100 --> 00:31:58,590 as the cytosol side. 506 00:31:58,590 --> 00:32:01,860 Of course, there's another outer mitochondrial membrane here 507 00:32:01,860 --> 00:32:04,390 that, for the sake of this class, 508 00:32:04,390 --> 00:32:07,570 you can just think of as being generally permeable. 509 00:32:07,570 --> 00:32:10,860 This would be the matrix inside of the inner mitochondrial 510 00:32:10,860 --> 00:32:11,980 membrane. 511 00:32:11,980 --> 00:32:19,320 And so all this NADH is being generated 512 00:32:19,320 --> 00:32:22,380 from nutrient oxidation TCA cycle, 513 00:32:22,380 --> 00:32:24,690 fatty acid oxidation, pyruvate dehydrogenase, 514 00:32:24,690 --> 00:32:28,020 whatever, within the matrix or within the bacteria. 515 00:32:32,430 --> 00:32:39,360 Those electrons are ultimately being transferred from NADH, 516 00:32:39,360 --> 00:32:45,730 reoxidizing it to NAD, to reduce oxygen to water. 517 00:32:45,730 --> 00:32:48,370 This is favorable. 518 00:32:48,370 --> 00:32:55,210 This is, therefore, coupled to pumping protons. 519 00:32:55,210 --> 00:33:04,860 That creates a membrane potential and gradient 520 00:33:04,860 --> 00:33:09,030 across this membrane, which can be 521 00:33:09,030 --> 00:33:22,940 used to do work such as driving the synthesis of ATP 522 00:33:22,940 --> 00:33:24,570 for other work. 523 00:33:24,570 --> 00:33:28,570 So this all has to happen at the same membrane, 524 00:33:28,570 --> 00:33:33,360 but this process does not have to touch that process. 525 00:33:33,360 --> 00:33:36,660 It's coupled because you create this battery, 526 00:33:36,660 --> 00:33:42,180 this membrane potential and pH gradient, that 527 00:33:42,180 --> 00:33:45,570 can be used elsewhere along the membrane 528 00:33:45,570 --> 00:33:48,860 to do some kind of work. 529 00:33:48,860 --> 00:33:51,340 Now, I draw this is delta psi, delta pH. 530 00:33:51,340 --> 00:33:53,750 You can imagine, if you're moving protons, 531 00:33:53,750 --> 00:33:56,420 you're going to create a voltage. 532 00:33:56,420 --> 00:33:58,972 You're also going to create a pH gradient. 533 00:33:58,972 --> 00:34:00,680 It turns out I don't have time to discuss 534 00:34:00,680 --> 00:34:01,763 this further in the class. 535 00:34:01,763 --> 00:34:04,070 A lot is written about how to think 536 00:34:04,070 --> 00:34:09,320 about this membrane potential as either a true voltage 537 00:34:09,320 --> 00:34:10,940 or as a pH gradient. 538 00:34:10,940 --> 00:34:12,889 It turns out that they're useful to think 539 00:34:12,889 --> 00:34:15,784 about in different ways for different processes. 540 00:34:15,784 --> 00:34:17,159 But if you're interested in this, 541 00:34:17,159 --> 00:34:21,290 there's certainly lots written about this on various papers 542 00:34:21,290 --> 00:34:23,389 and texts on bioenergetics. 543 00:34:23,389 --> 00:34:26,880 And I would encourage you to read more about that. 544 00:34:26,880 --> 00:34:28,135 OK. 545 00:34:28,135 --> 00:34:29,510 All right, what I want to do next 546 00:34:29,510 --> 00:34:32,540 then is discuss each of these processes 547 00:34:32,540 --> 00:34:34,850 in turn, that is how does this work 548 00:34:34,850 --> 00:34:41,020 and how does that work, to really understand 549 00:34:41,020 --> 00:34:44,739 what's going on in mitochondria and bacteria 550 00:34:44,739 --> 00:34:49,370 to carry out this process of oxidative phosphorylation. 551 00:34:49,370 --> 00:34:51,280 So first, let's talk here. 552 00:34:51,280 --> 00:35:06,190 First, how does one use favorable electron transfer 553 00:35:06,190 --> 00:35:16,590 to make this membrane potential? 554 00:35:16,590 --> 00:35:19,860 Obviously, if we're going to pump protons 555 00:35:19,860 --> 00:35:24,780 against a gradient, such that we have different concentration 556 00:35:24,780 --> 00:35:26,790 of protons on either side of the membrane, 557 00:35:26,790 --> 00:35:29,430 that's going to require energy input. 558 00:35:29,430 --> 00:35:33,340 And that energy is going to come from favorable electron 559 00:35:33,340 --> 00:35:33,840 transfer. 560 00:35:36,650 --> 00:35:49,490 And really this process is taking electrons from, say, 561 00:35:49,490 --> 00:36:00,130 a cofactor like NADH, and ultimately transferring 562 00:36:00,130 --> 00:36:05,180 those electrons to oxygen to get water. 563 00:36:05,180 --> 00:36:09,320 The standard reduction potential of NAD/NADH 564 00:36:09,320 --> 00:36:12,020 is less than the standard reduction 565 00:36:12,020 --> 00:36:14,240 potential of oxygen and water. 566 00:36:14,240 --> 00:36:18,020 That means that equilibrium is going to favor electron 567 00:36:18,020 --> 00:36:20,420 transport in this direction. 568 00:36:20,420 --> 00:36:25,170 To say it another way, relative to oxygen-water, 569 00:36:25,170 --> 00:36:28,130 NAD/NADH is a better electron donor. 570 00:36:28,130 --> 00:36:32,480 Oxygen-water is a better electron acceptor pair. 571 00:36:32,480 --> 00:36:37,460 And so by this electron transfer being favorable, 572 00:36:37,460 --> 00:36:42,770 it can be used then to release energy 573 00:36:42,770 --> 00:36:46,760 coupled to other processes, such as moving protons 574 00:36:46,760 --> 00:36:51,530 and make that otherwise unfavorable process favorable. 575 00:36:51,530 --> 00:36:55,130 Now, you'll note that, in doing this, 576 00:36:55,130 --> 00:36:59,510 we now regenerate the oxidized cofactor NAD+. 577 00:36:59,510 --> 00:37:05,000 And so that solves our electron balance problem for glycolysis, 578 00:37:05,000 --> 00:37:07,910 solves any electron balance problem 579 00:37:07,910 --> 00:37:12,380 that would exist for fatty acid oxidation or the TCA cycle 580 00:37:12,380 --> 00:37:13,850 in the mitochondria. 581 00:37:13,850 --> 00:37:15,620 Because ultimately, those electrons 582 00:37:15,620 --> 00:37:18,800 are being given to oxygen. And so oxygen, 583 00:37:18,800 --> 00:37:22,030 being this final electron acceptor, 584 00:37:22,030 --> 00:37:26,620 being stoichiometrically produced to generate water, 585 00:37:26,620 --> 00:37:30,550 is effectively water being the alternative waste product 586 00:37:30,550 --> 00:37:32,620 to, say, lactate or ethanol that we 587 00:37:32,620 --> 00:37:36,520 saw in fermentation, all right? 588 00:37:36,520 --> 00:37:39,280 Now, as we started the lecture with and discussed 589 00:37:39,280 --> 00:37:43,120 in glycolysis, if we do this in one step, it's favorable. 590 00:37:43,120 --> 00:37:48,610 But it's also favorable if we do it in lots of little steps. 591 00:37:48,610 --> 00:37:50,570 By doing it and lots of little steps, 592 00:37:50,570 --> 00:37:52,810 just like we did in glycolysis, we 593 00:37:52,810 --> 00:37:55,180 can break up that energy release in a way 594 00:37:55,180 --> 00:37:58,060 to allow us to do more work, right? 595 00:37:58,060 --> 00:38:04,680 So in glycolysis, we could turn carbohydrate into CO2-- 596 00:38:04,680 --> 00:38:07,920 one step, burning wood, release a lot of light and heat 597 00:38:07,920 --> 00:38:09,510 all in one step. 598 00:38:09,510 --> 00:38:11,100 That can be used to do work. 599 00:38:11,100 --> 00:38:14,100 But it was more efficient to break it up into little steps 600 00:38:14,100 --> 00:38:15,900 where we could then capture intermediates 601 00:38:15,900 --> 00:38:20,850 that made ATP synthase favorable despite high ATP/ADP ratio. 602 00:38:20,850 --> 00:38:24,300 Same idea here, we could transfer this all in one step. 603 00:38:24,300 --> 00:38:26,370 But instead, you break it up into a bunch 604 00:38:26,370 --> 00:38:31,050 of individual steps, which can then be coupled individually 605 00:38:31,050 --> 00:38:33,490 to pumping protons across the membrane 606 00:38:33,490 --> 00:38:37,980 to generate more efficiently this delta PSI delta pH 607 00:38:37,980 --> 00:38:40,260 to charge this battery. 608 00:38:40,260 --> 00:38:46,620 And so this occurs across a series of electron carriers, 609 00:38:46,620 --> 00:38:49,860 multi-protein complexes, electron carriers, 610 00:38:49,860 --> 00:38:52,650 called the electron transport chain. 611 00:38:55,750 --> 00:38:58,470 And you can think of this electron transport chain 612 00:38:58,470 --> 00:39:01,320 as being entirely analogous to what we already 613 00:39:01,320 --> 00:39:03,660 talked about when we talked about how 614 00:39:03,660 --> 00:39:06,090 pyruvate dehydrogenase or alpha-ketoglutarate 615 00:39:06,090 --> 00:39:09,330 dehydrogenase works when we talked about the TCA cycle. 616 00:39:09,330 --> 00:39:12,480 Remember, these had these different protein complexes, 617 00:39:12,480 --> 00:39:15,420 this E1, E2, E3, where electrons were 618 00:39:15,420 --> 00:39:17,250 transferred across this chain. 619 00:39:17,250 --> 00:39:20,010 It's exactly the same idea here, a number 620 00:39:20,010 --> 00:39:24,900 of multi-polypeptide encoded protein complexes 621 00:39:24,900 --> 00:39:29,760 that come together to hold a bunch of electron carriers 622 00:39:29,760 --> 00:39:33,400 that break up the energy release, if you will, 623 00:39:33,400 --> 00:39:38,880 as we oxidize NADH and reduce oxygen and transfer 624 00:39:38,880 --> 00:39:43,440 those electrons along the chain with the complexes themselves 625 00:39:43,440 --> 00:39:47,040 coupling the favorable electron transfer to the moving 626 00:39:47,040 --> 00:39:52,040 of protons against a gradient across the membrane to work. 627 00:39:52,040 --> 00:39:55,700 And so for this electron transport chain to work, 628 00:39:55,700 --> 00:39:59,000 we obviously need a bunch of electron carriers. 629 00:39:59,000 --> 00:40:01,220 And why you need more than one electron carrier 630 00:40:01,220 --> 00:40:02,750 should now be obvious. 631 00:40:02,750 --> 00:40:05,270 Because having different carriers that 632 00:40:05,270 --> 00:40:08,150 sit at different standard reduction 633 00:40:08,150 --> 00:40:12,380 potentials is going to be useful if we 634 00:40:12,380 --> 00:40:18,260 want to build a chain such that we can have favorable oxidation 635 00:40:18,260 --> 00:40:21,800 and reduction to move electrons in a favorable direction 636 00:40:21,800 --> 00:40:26,210 and couple that to processes that allow 637 00:40:26,210 --> 00:40:28,970 us to charge this battery. 638 00:40:28,970 --> 00:40:31,820 And so it turns out that for this electron transport 639 00:40:31,820 --> 00:40:33,200 chain to work-- 640 00:40:33,200 --> 00:40:35,670 well, we know about some of our electron carriers. 641 00:40:35,670 --> 00:40:39,890 So we've discussed NAD, NADH. 642 00:40:39,890 --> 00:40:43,270 There's FAD, FADH2. 643 00:40:43,270 --> 00:40:44,470 There was lipoic acid. 644 00:40:47,800 --> 00:40:53,460 Lipoic acid is not part of the electron transport chain. 645 00:40:53,460 --> 00:40:54,890 NAD, NADH is a donor. 646 00:40:54,890 --> 00:40:59,090 You'll see FAD can be part of the electron transport chain, 647 00:40:59,090 --> 00:41:00,740 but there's a different-- 648 00:41:00,740 --> 00:41:02,510 or there's additional electron carriers. 649 00:41:02,510 --> 00:41:05,160 I want to discuss those now. 650 00:41:05,160 --> 00:41:07,970 So another important one is something called 651 00:41:07,970 --> 00:41:13,606 FMN or Flavin Mononucleotide. 652 00:41:16,890 --> 00:41:21,360 What FMN, or Flavin Mononucleotide is, is it's 653 00:41:21,360 --> 00:41:24,270 effectively FAD. 654 00:41:24,270 --> 00:41:27,750 But it's a FAD not in a dinucleotide state, 655 00:41:27,750 --> 00:41:29,950 but a mononucleotide state. 656 00:41:29,950 --> 00:41:33,630 So if you take FAD and you remove an AMP group from it, 657 00:41:33,630 --> 00:41:36,420 what you're left with is FMN. 658 00:41:36,420 --> 00:41:40,465 So just to remind you to be explicit about this-- 659 00:41:40,465 --> 00:41:40,965 so. 660 00:42:30,760 --> 00:42:36,050 So you have this flavin group attached to this ribitol group 661 00:42:36,050 --> 00:42:37,550 with a phosphate on it. 662 00:42:37,550 --> 00:42:40,670 If I put another phosphate and an adenine nucleotide here, 663 00:42:40,670 --> 00:42:44,150 an AMP group attached here, that would be FAD. 664 00:42:44,150 --> 00:42:49,040 This would be FMN, nucleotide instead of a dinucleotide. 665 00:42:49,040 --> 00:42:55,460 This is FMN in the oxidized form. 666 00:42:55,460 --> 00:43:03,530 And just like FAD, it can accept an electron pair. 667 00:43:03,530 --> 00:43:08,675 So here's our hydride ion, two electrons 668 00:43:08,675 --> 00:43:12,180 if we just draw here the middle part of the molecule. 669 00:43:20,870 --> 00:43:37,410 And so this here would be FMNH2 or the reduced form 670 00:43:37,410 --> 00:43:42,440 of this electron carrier, OK? 671 00:43:42,440 --> 00:43:47,320 So flavin mononucleotide-- part of the electron transport 672 00:43:47,320 --> 00:43:48,260 chain. 673 00:43:48,260 --> 00:43:52,690 Now, note FMN, FAD, NAD, all of these things 674 00:43:52,690 --> 00:43:55,120 are two electron carriers, right? 675 00:43:55,120 --> 00:43:57,820 We disguise the mechanism of all of them, 676 00:43:57,820 --> 00:44:00,220 just reminded you of it, these hydride ion 677 00:44:00,220 --> 00:44:02,780 transfers, two electron carriers. 678 00:44:02,780 --> 00:44:05,920 It turns out the electron transport chain also 679 00:44:05,920 --> 00:44:09,130 uses metal ions as electron carriers. 680 00:44:09,130 --> 00:44:12,820 And so-- lots of minerals in our diet, things like iron. 681 00:44:12,820 --> 00:44:16,240 It turns out elemental iron is a great electron carrier. 682 00:44:16,240 --> 00:44:21,320 And it's very important for electron transport chain. 683 00:44:21,320 --> 00:44:26,770 Now, elemental metals are one electron carrier. 684 00:44:26,770 --> 00:44:30,490 And so there's a few ways you incorporate iron. 685 00:44:30,490 --> 00:44:33,940 One is something called iron sulfur clusters. 686 00:44:33,940 --> 00:44:38,110 And so iron sulfur clusters is, within a protein, 687 00:44:38,110 --> 00:44:40,580 you have some cysteine residues. 688 00:44:43,100 --> 00:44:49,170 And those cysteine residues can coordinate an iron atom. 689 00:44:49,170 --> 00:44:57,410 And so that iron atom can sit in the 3+ versus 2+ state, 690 00:44:57,410 --> 00:44:58,745 oxidized, reduced. 691 00:45:02,860 --> 00:45:04,780 So reduction of the iron-- 692 00:45:04,780 --> 00:45:07,780 oxidation of the iron, OK? 693 00:45:07,780 --> 00:45:09,640 So one electron carrier-- 694 00:45:09,640 --> 00:45:11,740 move one electron that way. 695 00:45:11,740 --> 00:45:19,090 Also, ion sits within molecules called cytochromes. 696 00:45:19,090 --> 00:45:21,070 So what are cytochromes? 697 00:45:21,070 --> 00:45:24,940 This is an iron atom that's basically 698 00:45:24,940 --> 00:45:28,480 chelated into a porphyrin very similar to what 699 00:45:28,480 --> 00:45:30,530 you saw for hemoglobin. 700 00:45:30,530 --> 00:45:54,870 And so you have an iron 2+ or 3+ that's chelated here into this 701 00:45:54,870 --> 00:45:57,140 porphyrin ring structure. 702 00:46:09,050 --> 00:46:10,040 OK. 703 00:46:10,040 --> 00:46:12,560 Lots of conjugated double bond systems in it, 704 00:46:12,560 --> 00:46:14,410 this should look very similar. 705 00:46:14,410 --> 00:46:16,940 Porphyrin is the thing on the outside, coordinates iron 706 00:46:16,940 --> 00:46:18,020 atom in the middle. 707 00:46:18,020 --> 00:46:20,900 That can move between the 2+ and the 3+ state just like we do 708 00:46:20,900 --> 00:46:24,320 for iron sulfur clusters, so carry electrons be oxidized 709 00:46:24,320 --> 00:46:27,110 or reduced, 3+ or 2+. 710 00:46:27,110 --> 00:46:32,750 And ultimately, this porphyrin can 711 00:46:32,750 --> 00:46:40,930 have some R groups added here, can be associated with it 712 00:46:40,930 --> 00:46:44,560 or bound to cysteine residues within proteins. 713 00:46:44,560 --> 00:46:49,480 Effectively, these molecules varying these R groups 714 00:46:49,480 --> 00:46:51,970 and how it's attached to proteins 715 00:46:51,970 --> 00:46:56,020 gives you different families of cytochrome is referred 716 00:46:56,020 --> 00:47:04,720 to as cytochrome A, the B type cytochromes, and cytochrome C. 717 00:47:04,720 --> 00:47:07,720 These are associated with enzyme complexes, 718 00:47:07,720 --> 00:47:10,090 cytochrome C. It's actually covalently bound 719 00:47:10,090 --> 00:47:13,630 to the polypeptide itself. 720 00:47:13,630 --> 00:47:15,790 Basically, these are named because you 721 00:47:15,790 --> 00:47:21,560 have these different conjugated ring structures here. 722 00:47:21,560 --> 00:47:24,770 That and how the R groups are will slightly change. 723 00:47:24,770 --> 00:47:26,300 So this will absorb visible light. 724 00:47:26,300 --> 00:47:29,640 That will slightly change the absorption spectra. 725 00:47:29,640 --> 00:47:32,120 And so classically, based on absorption spectra, 726 00:47:32,120 --> 00:47:35,810 these were classified into A, B, and C type cytochromes. 727 00:47:35,810 --> 00:47:37,850 And you can imagine, by these properties 728 00:47:37,850 --> 00:47:40,640 having slightly different-- 729 00:47:40,640 --> 00:47:42,650 affecting the iron in ways that it slightly 730 00:47:42,650 --> 00:47:45,620 affects its electron transport properties. 731 00:47:45,620 --> 00:47:48,440 Of course, hemoglobin almost certainly 732 00:47:48,440 --> 00:47:52,550 evolved from these molecules as oxygen carrying ways 733 00:47:52,550 --> 00:47:54,350 in multicellular life. 734 00:47:54,350 --> 00:47:58,195 But of course, the original use was as electron carriers 735 00:47:58,195 --> 00:47:59,570 for the processes here that we're 736 00:47:59,570 --> 00:48:01,700 going to talk about, to hold that iron as a way 737 00:48:01,700 --> 00:48:06,170 to control electron transport across these electron transport 738 00:48:06,170 --> 00:48:07,700 chains. 739 00:48:07,700 --> 00:48:08,810 All right, so that's iron. 740 00:48:08,810 --> 00:48:11,840 It turns out iron is not the only metal that 741 00:48:11,840 --> 00:48:13,530 can be used to do this. 742 00:48:13,530 --> 00:48:16,640 And so cells also use copper. 743 00:48:16,640 --> 00:48:32,400 And so histidine can coordinate a copper that can move between 744 00:48:32,400 --> 00:48:36,210 an oxidized and reduced state, so copper 2+ reduced to copper 745 00:48:36,210 --> 00:48:41,490 +, copper + to copper 2+ or copper + oxidized to copper 2+, 746 00:48:41,490 --> 00:48:44,250 another one electron carrier. 747 00:48:44,250 --> 00:48:49,137 And so iron and copper can act as one electron carriers. 748 00:48:49,137 --> 00:48:50,970 And so you have these two electron carriers. 749 00:48:50,970 --> 00:48:53,640 Here's a bunch of metals as one electron carriers. 750 00:48:53,640 --> 00:48:57,875 Obviously, you need ways to carry both one and two 751 00:48:57,875 --> 00:48:59,250 electrons if you're going to move 752 00:48:59,250 --> 00:49:02,190 electrons between these different carriers. 753 00:49:02,190 --> 00:49:05,700 And an important molecule to do that is something 754 00:49:05,700 --> 00:49:12,160 called coenzyme Q. Coenzyme Q-- 755 00:49:12,160 --> 00:49:22,030 sometimes referred to as ubiquinone ubiquinol, 756 00:49:22,030 --> 00:49:29,198 because it's ubiquitous, often abbreviated co-Q. Many of you 757 00:49:29,198 --> 00:49:30,740 have probably heard of the supplement 758 00:49:30,740 --> 00:49:34,910 CoQ10, very popular supplement out there. 759 00:49:34,910 --> 00:49:38,840 CoQ10 is basically a version of coenzyme Q, 760 00:49:38,840 --> 00:49:40,970 ubiquinone, ubiquinol. 761 00:49:40,970 --> 00:49:42,620 What this looks like is as follows. 762 00:50:03,750 --> 00:50:12,470 So this here is ubiquinone, the oxidized form. 763 00:50:12,470 --> 00:50:15,860 So this middle part is the quinone. 764 00:50:15,860 --> 00:50:18,710 And then you have these decorations on the outside, 765 00:50:18,710 --> 00:50:20,240 including this R group. 766 00:50:20,240 --> 00:50:23,390 This R group is typically a long acyl tail. 767 00:50:23,390 --> 00:50:27,530 If it's 10 carbons long, that's CoQ10 supplement. 768 00:50:27,530 --> 00:50:29,810 Having this long acyl chain makes this 769 00:50:29,810 --> 00:50:31,520 a very hydrophobic molecule. 770 00:50:31,520 --> 00:50:35,540 And it actually lives within the membrane itself 771 00:50:35,540 --> 00:50:37,310 as an electron carrier. 772 00:50:37,310 --> 00:50:45,955 And so this ubiquinone can pick up an electron as well as 773 00:50:45,955 --> 00:50:46,455 a proton. 774 00:50:49,192 --> 00:50:49,692 Whoops. 775 00:50:57,490 --> 00:51:04,280 And so by picking up one electron, 776 00:51:04,280 --> 00:51:25,110 it can then basically go here to this semiquinone stabilized 777 00:51:25,110 --> 00:51:27,570 free radical state and then pick up 778 00:51:27,570 --> 00:51:34,535 a second electron to go to the fully reduced form. 779 00:51:45,550 --> 00:51:46,180 ubiquinol. 780 00:51:51,180 --> 00:51:52,080 OK. 781 00:51:52,080 --> 00:51:55,800 So one electronic carrier and two electron carrier-- 782 00:51:55,800 --> 00:51:59,850 so basically, it can carry two electrons 783 00:51:59,850 --> 00:52:01,710 from the oxidized, the reduced form, 784 00:52:01,710 --> 00:52:05,010 but can transfer them as single electrons 785 00:52:05,010 --> 00:52:11,250 by going through this stabilized free radical ubiquinone state. 786 00:52:11,250 --> 00:52:14,400 And so, basically, these various electron carriers 787 00:52:14,400 --> 00:52:18,810 are associated with these protein complexes 788 00:52:18,810 --> 00:52:22,740 within this electron transport chain that ultimately works 789 00:52:22,740 --> 00:52:26,340 to couple favorable electron transport down 790 00:52:26,340 --> 00:52:30,660 the chain to pumping protons to generate this potential 791 00:52:30,660 --> 00:52:33,330 across the membrane. 792 00:52:33,330 --> 00:52:47,920 All right, so this process occurs, again, 793 00:52:47,920 --> 00:52:51,850 at the bacterial membrane or the inner mitochondrial membrane 794 00:52:51,850 --> 00:53:01,340 and, in animals and most organisms, 795 00:53:01,340 --> 00:53:10,490 operates with four large multi-subunit protein 796 00:53:10,490 --> 00:53:13,340 complexes, all right? 797 00:53:13,340 --> 00:53:19,940 Each of these protein complexes has multiple polypeptides, 798 00:53:19,940 --> 00:53:20,440 as I said. 799 00:53:20,440 --> 00:53:22,910 It's coming together to form these complexes just like we 800 00:53:22,910 --> 00:53:25,400 described for pH, for instance. 801 00:53:25,400 --> 00:53:27,680 Most of the components of this are 802 00:53:27,680 --> 00:53:31,160 encoded in the nuclear genome, but a subset of these 803 00:53:31,160 --> 00:53:33,500 are encoded in the mitochondrial genome. 804 00:53:33,500 --> 00:53:36,680 Now, what exactly is encoded in different mitochondrial genomes 805 00:53:36,680 --> 00:53:38,690 varies by organisms. 806 00:53:38,690 --> 00:53:41,960 But across organisms, it's basically 807 00:53:41,960 --> 00:53:47,210 mostly complex components of these protein complexes. 808 00:53:47,210 --> 00:53:49,790 And so this is why mitochondrial DNA is retained. 809 00:53:49,790 --> 00:53:53,150 It's kind of this vestige of this symbiotic relationship. 810 00:53:53,150 --> 00:53:56,030 The mitochondria have retained some of the ability 811 00:53:56,030 --> 00:53:59,525 to control their own energy transduction, sort 812 00:53:59,525 --> 00:54:02,510 of core components of these complexes. 813 00:54:02,510 --> 00:54:06,020 There is one soluble protein. 814 00:54:06,020 --> 00:54:09,410 So these complexes are embedded within the membrane. 815 00:54:09,410 --> 00:54:12,320 There's also one soluble protein that's 816 00:54:12,320 --> 00:54:14,030 associated with the membrane. 817 00:54:14,030 --> 00:54:19,190 And then there's coenzyme Q, which also 818 00:54:19,190 --> 00:54:21,460 floats within the membrane. 819 00:54:21,460 --> 00:54:25,600 So that's in the membrane. 820 00:54:25,600 --> 00:54:29,172 These are embedded in the membrane. 821 00:54:29,172 --> 00:54:30,880 And then there's a soluble protein that's 822 00:54:30,880 --> 00:54:32,410 associated with the membrane. 823 00:55:11,390 --> 00:55:15,650 OK, now I want to discuss the details of what these protein 824 00:55:15,650 --> 00:55:23,790 complexes are and also few comments about how they work. 825 00:55:23,790 --> 00:55:26,150 So if this is the inner membrane space 826 00:55:26,150 --> 00:55:28,160 and this is the matrix side, that'd 827 00:55:28,160 --> 00:55:34,460 be out or in if we're discussing a prokaryote. 828 00:55:34,460 --> 00:55:53,260 The first is we have something called complex I, also 829 00:55:53,260 --> 00:56:06,595 referred to as NADH oxidase because it oxidizes NADH back 830 00:56:06,595 --> 00:56:07,095 to NAD. 831 00:56:11,530 --> 00:56:20,020 NADH oxidase is a giant complex greater than 900 kilodaltons 832 00:56:20,020 --> 00:56:21,115 in total. 833 00:56:21,115 --> 00:56:31,230 It has more than 25 individual protein subunits, 834 00:56:31,230 --> 00:56:37,410 contains some flavin mononucleotides as well as 835 00:56:37,410 --> 00:56:42,750 some iron sulfur clusters as electron carriers. 836 00:56:42,750 --> 00:56:47,370 And it is the least well-understood 837 00:56:47,370 --> 00:56:49,050 of all of the protein complexes. 838 00:56:49,050 --> 00:56:53,520 In fact, it is still a major field of study 839 00:56:53,520 --> 00:56:57,930 to try to understand how NAD oxidase really works. 840 00:56:57,930 --> 00:57:03,120 But it effectively is one of the complexes that can pump protons 841 00:57:03,120 --> 00:57:07,770 across the membrane as those two electrons are transferred 842 00:57:07,770 --> 00:57:17,070 through the complex from NADH ultimately to coenzyme Q that 843 00:57:17,070 --> 00:57:20,720 exists within the membrane. 844 00:57:20,720 --> 00:57:34,010 OK, the next complex discuss is complex II. 845 00:57:34,010 --> 00:57:44,440 Complex II is succinate dehydrogenase, the exact enzyme 846 00:57:44,440 --> 00:57:47,830 that we heard about from the TCA cycle. 847 00:57:47,830 --> 00:57:51,520 So what does succinate dehydrogenase do? 848 00:57:51,520 --> 00:57:54,760 Remember, it converts fumarate to succinate. 849 00:57:54,760 --> 00:58:04,200 It gave those electrons to FAD to make FADH2. 850 00:58:04,200 --> 00:58:07,420 Well, that occurs within this complex, 851 00:58:07,420 --> 00:58:14,010 which can also reoxidize the FADH2 with those two electrons 852 00:58:14,010 --> 00:58:18,540 and give them, also, to coenzyme Q 853 00:58:18,540 --> 00:58:21,790 that sits within the membrane. 854 00:58:21,790 --> 00:58:27,810 So I want to point out that this, ultimately, you will see, 855 00:58:27,810 --> 00:58:34,530 is why it is that electron transfer from NADH and FADH2 856 00:58:34,530 --> 00:58:37,800 has different energy yields. 857 00:58:37,800 --> 00:58:44,250 And so the electron transfer from NADH to ubiquinone 858 00:58:44,250 --> 00:58:48,030 is more of a change in standard reduction potential 859 00:58:48,030 --> 00:58:51,450 than from FADH2 to ubiquinone. 860 00:58:51,450 --> 00:58:53,460 Why use FADH2? 861 00:58:53,460 --> 00:58:57,030 Well, because, remember when we did electron transfers, 862 00:58:57,030 --> 00:59:05,370 we used, when we oxidized a carbon, say, 863 00:59:05,370 --> 00:59:15,360 in a carbohydrate to a ketone, we tended to use NAD+/NADH 864 00:59:15,360 --> 00:59:18,990 as the electron transport pair. 865 00:59:18,990 --> 00:59:21,240 So this change in standard reduction 866 00:59:21,240 --> 00:59:26,160 potential is such that we can make that reaction work 867 00:59:26,160 --> 00:59:28,140 and ultimately drive that. 868 00:59:28,140 --> 00:59:34,320 It turns out that putting a double bond in-- 869 00:59:34,320 --> 00:59:40,410 this going from a not oxidizing an alcohol to a ketone, 870 00:59:40,410 --> 00:59:43,380 but oxidizing the carbon to a double bond-- 871 00:59:43,380 --> 00:59:50,510 that basically you need a different change 872 00:59:50,510 --> 00:59:55,040 in standard reduction potential to carry out that reaction 873 00:59:55,040 --> 00:59:57,140 and make it work. 874 00:59:57,140 --> 01:00:01,670 FADH/FADH2 sits at a higher standard reduction 875 01:00:01,670 --> 01:00:04,580 potential than NAD/NADH. 876 01:00:04,580 --> 01:00:08,090 And so that means to further transfer them 877 01:00:08,090 --> 01:00:14,678 to something further downstream, like coenzyme Q, 878 01:00:14,678 --> 01:00:16,970 there's a bigger change in standard reduction potential 879 01:00:16,970 --> 01:00:17,470 here. 880 01:00:17,470 --> 01:00:19,130 That can be used to drive protons. 881 01:00:19,130 --> 01:00:20,840 There's not enough of a change here. 882 01:00:20,840 --> 01:00:23,120 You can't use that to drive protons. 883 01:00:23,120 --> 01:00:25,400 And it's because of these differences 884 01:00:25,400 --> 01:00:29,360 in what's actually being oxidized that ultimately leads 885 01:00:29,360 --> 01:00:35,750 to this difference in energetics when we think about FADH2 886 01:00:35,750 --> 01:00:39,800 versus NADH and how much ATP equivalence 887 01:00:39,800 --> 01:00:42,280 that they might have for cells. 888 01:00:42,280 --> 01:00:49,850 All right, once electrons are in the ubiquinone pool 889 01:00:49,850 --> 01:00:57,980 or coenzyme Q pool, they can now be transferred to complex III. 890 01:00:57,980 --> 01:01:11,840 So complex III is also referred to as cytochrome C reductase. 891 01:01:11,840 --> 01:01:14,240 You'll see why in a second. 892 01:01:14,240 --> 01:01:19,910 It's a 250 kilodalton complex. 893 01:01:19,910 --> 01:01:23,810 There are 10 subunits. 894 01:01:23,810 --> 01:01:35,720 It has a bunch of B type and C type cytochromes, 895 01:01:35,720 --> 01:01:40,890 as well as some iron sulfur clusters. 896 01:01:40,890 --> 01:01:43,590 Shown here on the slide from a textbook 897 01:01:43,590 --> 01:01:50,730 is a picture of complex III and what it actually looks like. 898 01:01:50,730 --> 01:01:54,330 Fairly well-understood how it works, 899 01:01:54,330 --> 01:01:58,440 this coupling of basically electron transfer through the Q 900 01:01:58,440 --> 01:02:00,825 pool can pump protons. 901 01:02:04,470 --> 01:02:08,940 And this ultimately allows transfer 902 01:02:08,940 --> 01:02:15,060 of electrons to this small soluble protein that 903 01:02:15,060 --> 01:02:18,600 lives in the intermembrane space or associated with the outside 904 01:02:18,600 --> 01:02:22,050 of the bacteria called cytochrome C-- 905 01:02:22,050 --> 01:02:25,650 or outside in the intermembrane space called cytochrome C. 906 01:02:25,650 --> 01:02:34,840 Cytochrome C is a 13 kilodalton protein. 907 01:02:34,840 --> 01:02:38,520 It's a very small highly conserved protein 908 01:02:38,520 --> 01:02:41,273 with a C type cytochrome commonly used 909 01:02:41,273 --> 01:02:43,440 because it's found across many, many different types 910 01:02:43,440 --> 01:02:44,170 of organisms. 911 01:02:44,170 --> 01:02:47,430 And so it's been sequenced from many, many organisms 912 01:02:47,430 --> 01:02:50,550 as a way to identify evolutionary relationships. 913 01:02:50,550 --> 01:02:52,590 And then cytochrome C, this is also 914 01:02:52,590 --> 01:02:55,410 why it's called cytochrome C reductase because it's 915 01:02:55,410 --> 01:02:59,640 reducing cytochrome C to transfer electrons 916 01:02:59,640 --> 01:03:07,200 along the chain, ultimately to the final complex, complex IV. 917 01:03:07,200 --> 01:03:14,580 Complex IV-- also referred to as cytochrome C oxidase. 918 01:03:14,580 --> 01:03:20,790 And this is ultimately the place where electrons are transferred 919 01:03:20,790 --> 01:03:24,700 to reduce oxygen to water. 920 01:03:24,700 --> 01:03:30,300 So this is 160 kilodalton protein complex 921 01:03:30,300 --> 01:03:45,970 with 13 subunits, uses A type cytochromes and copper 922 01:03:45,970 --> 01:03:50,710 as electron carriers and, ultimately, 923 01:03:50,710 --> 01:03:55,150 allows you to get electrons from cytochrome C to oxygen. 924 01:03:55,150 --> 01:03:57,340 And so here, again, from the textbook 925 01:03:57,340 --> 01:04:03,190 is a picture of complex IV and what it looks like. 926 01:04:03,190 --> 01:04:08,590 Also, fairly well-understood-- also couples electron transport 927 01:04:08,590 --> 01:04:12,140 to proton pumping. 928 01:04:12,140 --> 01:04:18,050 And so this is effectively the electron transport chain 929 01:04:18,050 --> 01:04:20,000 as it exists in mitochondria. 930 01:04:20,000 --> 01:04:25,650 And you can draw it a different way as I will do here. 931 01:04:25,650 --> 01:04:35,710 So here's complex I, allows oxidation of NADH back to NAD+. 932 01:04:40,890 --> 01:04:48,480 That oxidation can be coupled to reduction of FMN. 933 01:04:48,480 --> 01:05:03,500 Ultimately, those are passed through some iron sulfur 934 01:05:03,500 --> 01:05:12,620 clusters leading to the reduction of ubiquinone. 935 01:05:12,620 --> 01:05:17,660 And in the process, this electron transport 936 01:05:17,660 --> 01:05:20,720 can pump on the order of four protons, 937 01:05:20,720 --> 01:05:23,150 or at least that's what's estimated. 938 01:05:23,150 --> 01:05:29,510 Complex II, which is succinate dehydrogenase from the TCA 939 01:05:29,510 --> 01:05:37,900 cycle, really converts succinate to fumarate. 940 01:05:37,900 --> 01:05:44,980 That oxidation reaction within the complex 941 01:05:44,980 --> 01:05:50,600 leads to the reduction of FAD to FADH2. 942 01:05:50,600 --> 01:06:00,170 That can be reoxidized through iron, 943 01:06:00,170 --> 01:06:08,710 ultimately driving the reduction of coenzyme Q 944 01:06:08,710 --> 01:06:11,440 from ubiquinone to ubiquinol. 945 01:06:11,440 --> 01:06:15,760 That ubiquinone to you ubiquinol in something 946 01:06:15,760 --> 01:06:19,960 called the Q pool within the membrane 947 01:06:19,960 --> 01:06:32,310 can then be reoxidized in complex III. 948 01:06:32,310 --> 01:06:38,670 That reoxidized in complex III puts electrons 949 01:06:38,670 --> 01:06:42,570 through complex III that can be coupled to pumping 950 01:06:42,570 --> 01:06:45,930 on the order of two protons. 951 01:06:45,930 --> 01:06:52,020 Those electrons are transferred to cytochrome C. Ultimately, 952 01:06:52,020 --> 01:07:00,800 those electrons are transferred to copper in complex IV 953 01:07:00,800 --> 01:07:08,840 to cytochrome A and, ultimately, oxygen to water, 954 01:07:08,840 --> 01:07:12,070 reducing oxygen to water and pumping 955 01:07:12,070 --> 01:07:16,270 on the order of four protons. 956 01:07:16,270 --> 01:07:22,390 And so you can see here that, basically, this 957 01:07:22,390 --> 01:07:26,920 is a series of oxidation and reduction reactions 958 01:07:26,920 --> 01:07:32,260 that happens across these complexes, such that complex I, 959 01:07:32,260 --> 01:07:35,170 the transfer electrons from the Q pool through complex III. 960 01:07:35,170 --> 01:07:40,000 And complex IV can pump protons across the membrane 961 01:07:40,000 --> 01:07:42,400 and generate this delta psi delta 962 01:07:42,400 --> 01:07:44,980 pH that can be used to do work. 963 01:07:44,980 --> 01:07:47,680 And it's really the favorable transfer of electrons 964 01:07:47,680 --> 01:07:51,580 either from NAD/NADH or the oxidation of the succinate 965 01:07:51,580 --> 01:07:56,200 to fumarate with those electrons being given to oxygen to drive 966 01:07:56,200 --> 01:07:58,460 this process. 967 01:07:58,460 --> 01:08:08,040 And so really just to schematize this, in a way, is that, 968 01:08:08,040 --> 01:08:19,080 if this is our membrane, NAD+ to NADH gives electrons to complex 969 01:08:19,080 --> 01:08:24,660 I, which goes to coenzyme Q, which goes to complex III, 970 01:08:24,660 --> 01:08:28,830 which goes to cytochrome C, which goes to complex IV 971 01:08:28,830 --> 01:08:34,319 and reduces oxygen to water. 972 01:08:34,319 --> 01:08:38,250 That is one way to carry out this set of reactions. 973 01:08:38,250 --> 01:08:43,295 The other way is we can convert fumarate-- 974 01:08:43,295 --> 01:08:48,630 sorry, succinate to fumarate. 975 01:08:48,630 --> 01:08:50,520 That's the FAD. 976 01:08:50,520 --> 01:08:54,390 That's complex II, also gives it to the Q pool. 977 01:08:54,390 --> 01:08:56,790 And so you can see that the electron transport 978 01:08:56,790 --> 01:09:02,790 chain would be complex I, Q, III C, IV or II Q, III, C IV. 979 01:09:02,790 --> 01:09:05,160 It is not I, II, III, IV. 980 01:09:05,160 --> 01:09:11,766 It is actually I to Q or II to Q, ultimately going to III. 981 01:09:11,766 --> 01:09:15,870 Now, the way protons are pumped is a process 982 01:09:15,870 --> 01:09:19,290 that I don't have time this year to discuss in class. 983 01:09:19,290 --> 01:09:21,810 I will post something on Stellar that is just 984 01:09:21,810 --> 01:09:24,990 a short section from the textbook that describes 985 01:09:24,990 --> 01:09:28,859 at least what is known about how that process works, 986 01:09:28,859 --> 01:09:33,000 such as the Q cycle, basically how oxidation 987 01:09:33,000 --> 01:09:37,050 and reduction of coenzyme Q, which will take protons 988 01:09:37,050 --> 01:09:39,660 on and off the molecule on either side of the membrane, 989 01:09:39,660 --> 01:09:44,340 can be used to show how this can be coupled to proton 990 01:09:44,340 --> 01:09:47,200 pumping across the membrane. 991 01:09:47,200 --> 01:09:51,170 And you can read that if you're interested. 992 01:09:51,170 --> 01:09:52,760 OK. 993 01:09:52,760 --> 01:09:58,890 Now, a couple other points about this, this 994 01:09:58,890 --> 01:10:05,450 works because you're moving from a lower 995 01:10:05,450 --> 01:10:08,810 to a higher standard reduction potential across the chain. 996 01:10:08,810 --> 01:10:13,340 And it turns out oxygen is a particularly good electron 997 01:10:13,340 --> 01:10:14,540 acceptor. 998 01:10:14,540 --> 01:10:18,320 And this is really why oxygen and respiration is really 999 01:10:18,320 --> 01:10:22,280 so vital to a lot of the way energy transduction 1000 01:10:22,280 --> 01:10:24,140 works in metabolism. 1001 01:10:24,140 --> 01:10:25,760 Oxygen is now abundant. 1002 01:10:25,760 --> 01:10:28,260 And being a great electron acceptor is why it's used. 1003 01:10:28,260 --> 01:10:30,200 But of course, oxygen was not abundant 1004 01:10:30,200 --> 01:10:32,600 2 and 1/2 billion years ago. 1005 01:10:32,600 --> 01:10:37,200 And it doesn't have to be oxygen for this to work. 1006 01:10:37,200 --> 01:10:40,220 If you have another electron acceptor that 1007 01:10:40,220 --> 01:10:42,980 basically makes this process favorable, 1008 01:10:42,980 --> 01:10:44,520 that can work just as well. 1009 01:10:44,520 --> 01:10:48,500 And in fact, that happens still in some very extreme 1010 01:10:48,500 --> 01:10:49,670 environments. 1011 01:10:49,670 --> 01:10:52,610 This so-called chemosynthesis basically 1012 01:10:52,610 --> 01:10:56,210 can use other electronic acceptors 1013 01:10:56,210 --> 01:10:59,300 to drive the energetics of cells. 1014 01:10:59,300 --> 01:11:04,060 And you really just need the right ways 1015 01:11:04,060 --> 01:11:07,720 to build an electron transport chain to make this whole system 1016 01:11:07,720 --> 01:11:11,150 work, such you can build a battery and use it to do work, 1017 01:11:11,150 --> 01:11:11,650 OK? 1018 01:11:11,650 --> 01:11:15,070 So oxygen-- commonly used because it's abundant and good 1019 01:11:15,070 --> 01:11:18,250 at it, but it's not the only way that this can work. 1020 01:11:18,250 --> 01:11:19,810 And there are examples in biology 1021 01:11:19,810 --> 01:11:23,830 where oxygen is not used, that there is other acceptors that 1022 01:11:23,830 --> 01:11:25,980 are used instead. 1023 01:11:25,980 --> 01:11:29,430 Now, I also want to note, as I've alluded to in discussing 1024 01:11:29,430 --> 01:11:33,570 complex II, this FAD/FADH2. 1025 01:11:33,570 --> 01:11:37,260 So when we talk about that in the TCA cycle 1026 01:11:37,260 --> 01:11:40,350 or in fatty acid oxidation, we discuss it 1027 01:11:40,350 --> 01:11:44,040 as if it's kind of like NAD/NADH, but it's not. 1028 01:11:44,040 --> 01:11:48,330 NAD/NADH are basically cofactors that 1029 01:11:48,330 --> 01:11:50,100 move around between enzymes. 1030 01:11:50,100 --> 01:11:54,930 Fad/fadh2 are not free. 1031 01:11:54,930 --> 01:11:59,490 They're electron carriers that are part of protein complexes, 1032 01:11:59,490 --> 01:12:03,000 like what we described up here. 1033 01:12:03,000 --> 01:12:05,970 And they're part of enzymes like succinate dehydrogenase. 1034 01:12:05,970 --> 01:12:13,170 So complex II just happens to be an FAD/FADH2 containing enzyme 1035 01:12:13,170 --> 01:12:16,980 that's part of the TCA cycle and part of the electron transport 1036 01:12:16,980 --> 01:12:20,880 chain as a way to transfer electrons in the TCA cycle 1037 01:12:20,880 --> 01:12:24,790 into the Q pool via this succinate 1038 01:12:24,790 --> 01:12:27,000 to fumarate conversion. 1039 01:12:27,000 --> 01:12:30,660 Acyl-CoA dehydrogenase, the FAD/FADH2 enzyme 1040 01:12:30,660 --> 01:12:33,030 we discussed in fatty acid oxidation, 1041 01:12:33,030 --> 01:12:37,020 could easily be drawn instead in electron transport chain that 1042 01:12:37,020 --> 01:12:41,850 goes acyl-CoA dehydrogenase, coenzyme Q, III, C, IV, 1043 01:12:41,850 --> 01:12:43,860 just like we can say succinate to fumarate 1044 01:12:43,860 --> 01:12:47,880 is the same as complex II, Q, III, C, IV, OK? 1045 01:12:47,880 --> 01:12:51,540 And so there's really lots of alternative electron transport 1046 01:12:51,540 --> 01:12:55,830 chains that involve these FAD/FADH2 containing enzymes 1047 01:12:55,830 --> 01:12:59,850 that really sit in the membrane and directly transfer electrons 1048 01:12:59,850 --> 01:13:03,780 to the Q pool. 1049 01:13:03,780 --> 01:13:06,420 Ultimately, the goal of all of this 1050 01:13:06,420 --> 01:13:10,260 is, of course, to create this delta psi delta 1051 01:13:10,260 --> 01:13:14,790 pH, which can be used to charge this battery that then can then 1052 01:13:14,790 --> 01:13:16,840 be done to do work. 1053 01:13:16,840 --> 01:13:22,110 And so this work includes the phosphorylation of ATP. 1054 01:13:22,110 --> 01:13:25,200 And so if we charge this battery, 1055 01:13:25,200 --> 01:13:36,250 we can now use this as a way to couple current in this battery 1056 01:13:36,250 --> 01:13:41,680 to phosphorylate ADP to make ATP, the phosphorylation part 1057 01:13:41,680 --> 01:13:44,830 of oxidative phosphorylation. 1058 01:13:44,830 --> 01:13:48,730 Now, this occurs also by a large protein complex sometimes 1059 01:13:48,730 --> 01:13:52,210 referred to as complex V. But remember, 1060 01:13:52,210 --> 01:13:55,690 complex V, or this ATP synthase machinery, 1061 01:13:55,690 --> 01:13:58,720 is not really part of the electron transport chain. 1062 01:13:58,720 --> 01:14:02,890 It is separate in space and is basically something, a way, 1063 01:14:02,890 --> 01:14:07,450 to utilize this delta psi delta pH across the membrane 1064 01:14:07,450 --> 01:14:11,590 to drive ATP synthesis. 1065 01:14:11,590 --> 01:14:19,190 All right, how does this complex V or ATP synthesis work? 1066 01:14:22,370 --> 01:14:29,200 So this is also a large protein complex, 1067 01:14:29,200 --> 01:14:32,300 sits within the same membrane, of course, 1068 01:14:32,300 --> 01:14:36,640 because it has to utilize the membrane potential to function. 1069 01:14:51,050 --> 01:14:58,930 And the so-called complex V is also 1070 01:14:58,930 --> 01:15:12,750 referred to as the ATP synthase or the F0 F1 ATPase. 1071 01:15:12,750 --> 01:15:19,190 And basically, if this is the intermembrane space, 1072 01:15:19,190 --> 01:15:22,650 this is the inner mitochondrial membrane. 1073 01:15:22,650 --> 01:15:25,240 This is the matrix. 1074 01:15:25,240 --> 01:15:29,360 You have a protein complex that sits 1075 01:15:29,360 --> 01:15:33,590 within the membrane called F0. 1076 01:15:33,590 --> 01:15:35,420 It's actually FO. 1077 01:15:35,420 --> 01:15:39,470 The O stands for a Oligomycin because there's 1078 01:15:39,470 --> 01:15:42,470 a drug called oligomycin that inhibits the FO, 1079 01:15:42,470 --> 01:15:46,250 or F0, part of the molecule. 1080 01:15:46,250 --> 01:15:49,700 And then there's this F1 component 1081 01:15:49,700 --> 01:15:52,760 that is associated with the F0 component that 1082 01:15:52,760 --> 01:15:54,950 sits on the matrix side of the membrane. 1083 01:15:54,950 --> 01:15:58,250 And it is effectively the machine 1084 01:15:58,250 --> 01:16:04,830 that phosphorylates or interconverts 1085 01:16:04,830 --> 01:16:11,080 ATP and ADP on the matrix side of the membrane. 1086 01:16:11,080 --> 01:16:15,100 Now, it's pretty well-understood how this system works. 1087 01:16:15,100 --> 01:16:17,790 And basically, F1 works by a process 1088 01:16:17,790 --> 01:16:21,360 called rotational catalysis. 1089 01:16:21,360 --> 01:16:24,870 How this works was awarded the Nobel Prize in 1997, 1090 01:16:24,870 --> 01:16:27,930 figured out not all that long ago. 1091 01:16:27,930 --> 01:16:32,760 And basically, this is F1 is a large protein complex where 1092 01:16:32,760 --> 01:16:36,480 you have a gamma subunit that is attached 1093 01:16:36,480 --> 01:16:41,520 to F0 the couples, basically, proton pumping to this gamma 1094 01:16:41,520 --> 01:16:46,860 subunit to literally turn the F1 component 1095 01:16:46,860 --> 01:16:53,040 and change the confirmation of attached regions, parts, 1096 01:16:53,040 --> 01:16:55,830 of this F1 machine. 1097 01:16:55,830 --> 01:16:57,270 Because it turns out you can have 1098 01:16:57,270 --> 01:17:01,050 three different conformations of the rest of F1, 1099 01:17:01,050 --> 01:17:04,380 the so-called L, or Loose, conformation, the T, 1100 01:17:04,380 --> 01:17:09,480 or Tight, conformation, or the O, or Open, conformation. 1101 01:17:09,480 --> 01:17:24,290 And so in the open conformation, ADP and phosphate or ATP 1102 01:17:24,290 --> 01:17:30,880 can exchange in and out of this complex, all right? 1103 01:17:30,880 --> 01:17:34,720 In the loose confirmation, you basically 1104 01:17:34,720 --> 01:17:38,500 have ADP and phosphate bound. 1105 01:17:38,500 --> 01:17:43,630 And in the tight conformation, this 1106 01:17:43,630 --> 01:17:49,900 is favored to synthesize ATP. 1107 01:17:49,900 --> 01:17:55,510 And so you can envision that what actually happens here 1108 01:17:55,510 --> 01:17:59,980 is that you first have ADP and ATP 1109 01:17:59,980 --> 01:18:05,840 bind in this open conformation. 1110 01:18:10,820 --> 01:18:12,650 Protons are moved. 1111 01:18:12,650 --> 01:18:16,680 You get rotation around the gamma subunit. 1112 01:18:16,680 --> 01:18:20,610 That changes the conformation of the O subunit 1113 01:18:20,610 --> 01:18:21,960 to the loose subunit. 1114 01:18:21,960 --> 01:18:26,040 So now, you have ADP plus PI bound. 1115 01:18:26,040 --> 01:18:30,870 Another proton moves, changes the conformation 1116 01:18:30,870 --> 01:18:36,030 of the loose subunit into the tight confirmation. 1117 01:18:36,030 --> 01:18:40,800 Now, ATP synthase is favored. 1118 01:18:40,800 --> 01:18:46,920 Another proton moves, comes back to the open conformation. 1119 01:18:46,920 --> 01:18:49,620 ATP is released from the molecule, 1120 01:18:49,620 --> 01:18:52,920 and you can run another rotational catalysis cycle 1121 01:18:52,920 --> 01:18:58,740 again to ultimately drive ATP to ATP synthase using 1122 01:18:58,740 --> 01:19:03,750 this transfer of protons across the F0 subunit 1123 01:19:03,750 --> 01:19:10,160 to ultimately rotate this machine to synthesize ATP. 1124 01:19:10,160 --> 01:19:14,210 Now, the way F0 works is within the membrane-- 1125 01:19:14,210 --> 01:19:18,140 and this is always hard to draw-- 1126 01:19:18,140 --> 01:19:32,120 you basically have a central complex 1127 01:19:32,120 --> 01:19:34,700 with another set of complexes around it 1128 01:19:34,700 --> 01:19:38,930 that, within that, have two half-channels that 1129 01:19:38,930 --> 01:19:42,780 are open to either side of the membrane. 1130 01:19:42,780 --> 01:19:45,780 And so if you look at this from the top view, 1131 01:19:45,780 --> 01:19:47,990 so looking at it straight down, you'd 1132 01:19:47,990 --> 01:19:52,400 have this central piece with this external piece 1133 01:19:52,400 --> 01:19:58,010 with the two half-channels that basically can rotate around 1134 01:19:58,010 --> 01:19:59,540 the central stalk. 1135 01:19:59,540 --> 01:20:02,750 This is hooked up to the gamma subunit of F1 1136 01:20:02,750 --> 01:20:05,690 in which drives rotational catalysis. 1137 01:20:05,690 --> 01:20:07,700 And how this works is that attached 1138 01:20:07,700 --> 01:20:13,220 to this inner piece that is open to the half-channels 1139 01:20:13,220 --> 01:20:17,070 is an aspartate residue. 1140 01:20:17,070 --> 01:20:21,350 So this is an aspartic acid residue that's there. 1141 01:20:21,350 --> 01:20:26,810 And recall that aspartic acid, it's an acid. 1142 01:20:26,810 --> 01:20:33,060 So it can pick up a proton or release a proton 1143 01:20:33,060 --> 01:20:37,590 from its acid group depending on the pH 1144 01:20:37,590 --> 01:20:39,520 on either side of the membrane. 1145 01:20:39,520 --> 01:20:42,030 And so, remember, there's a different pH 1146 01:20:42,030 --> 01:20:46,180 on the two sides of the membrane. 1147 01:20:46,180 --> 01:20:50,550 And it's really that difference in pH 1148 01:20:50,550 --> 01:20:56,640 that will allow favorable turning around this center 1149 01:20:56,640 --> 01:20:57,710 stalk. 1150 01:20:57,710 --> 01:21:02,130 And so the pH in the intermembrane space is lower. 1151 01:21:02,130 --> 01:21:05,160 So that means you're more likely to favor 1152 01:21:05,160 --> 01:21:07,530 protonation of the aspartate. 1153 01:21:07,530 --> 01:21:10,920 It can then rotate, such as the channel, that aspartate, is now 1154 01:21:10,920 --> 01:21:13,590 exposed to the other channel, to the matrix side. 1155 01:21:13,590 --> 01:21:17,400 There, the pH is higher, so favors deprotonation. 1156 01:21:17,400 --> 01:21:22,560 And basically, this will drive motion of this piece 1157 01:21:22,560 --> 01:21:24,270 around the central stalk. 1158 01:21:24,270 --> 01:21:28,220 That being coupled to the gamma subunit of F1 1159 01:21:28,220 --> 01:21:33,510 will cause the rotation of the F1 subunit changing 1160 01:21:33,510 --> 01:21:37,890 the attached conformation of the rest of the complex to favor 1161 01:21:37,890 --> 01:21:39,420 ATP synthesis. 1162 01:21:39,420 --> 01:21:44,670 And it's really this is how moving of protons back 1163 01:21:44,670 --> 01:21:47,070 from the intermembrane space to the matrix 1164 01:21:47,070 --> 01:21:50,580 can drive the synthesis of ATP, the phosphorylation 1165 01:21:50,580 --> 01:21:54,220 part of this process. 1166 01:21:54,220 --> 01:21:56,280 And so now you have a full picture 1167 01:21:56,280 --> 01:22:01,570 of how carbon oxidation really releases energy in a way that's 1168 01:22:01,570 --> 01:22:03,900 coupled to do work in biology. 1169 01:22:03,900 --> 01:22:06,120 So we can, of course, couple this directly, 1170 01:22:06,120 --> 01:22:10,050 as we did at GAPDH or succinic thiokinase. 1171 01:22:10,050 --> 01:22:15,630 But mostly, it's captured to charge up these NAD/NADH ratios 1172 01:22:15,630 --> 01:22:18,120 where favorable electron transport can 1173 01:22:18,120 --> 01:22:21,990 be used to make this delta psi delta pH in a membrane that 1174 01:22:21,990 --> 01:22:26,280 can be used also to synthesize ATP despite a high ATP/ADP 1175 01:22:26,280 --> 01:22:34,740 ratio because that battery can be used to drive this machine. 1176 01:22:34,740 --> 01:22:37,740 Now, I want to point out that the ATP synthase is just 1177 01:22:37,740 --> 01:22:40,870 like any other enzyme in any pathway. 1178 01:22:40,870 --> 01:22:42,690 And that is, if conditions are right, 1179 01:22:42,690 --> 01:22:45,090 there's no reason that this machine 1180 01:22:45,090 --> 01:22:47,160 has to drive ATP synthesis. 1181 01:22:47,160 --> 01:22:50,880 It could also consume ATP as a way 1182 01:22:50,880 --> 01:22:53,400 to create a proton gradient. 1183 01:22:53,400 --> 01:22:57,450 That is use ATP hydrolysis to pump 1184 01:22:57,450 --> 01:22:59,670 protons in the opposite direction 1185 01:22:59,670 --> 01:23:01,835 into the intermembrane space. 1186 01:23:01,835 --> 01:23:03,210 Well, why would you ever do that? 1187 01:23:03,210 --> 01:23:06,030 Well, you would do that because having this battery 1188 01:23:06,030 --> 01:23:09,960 is useful because ATP is not the only way that cells 1189 01:23:09,960 --> 01:23:11,940 can use energy to do work. 1190 01:23:11,940 --> 01:23:14,790 It turns out that having a membrane potential 1191 01:23:14,790 --> 01:23:16,070 is useful for other things. 1192 01:23:16,070 --> 01:23:17,790 So if you're in course nine, you know 1193 01:23:17,790 --> 01:23:21,510 that action potentials are carried, basically electricity 1194 01:23:21,510 --> 01:23:23,250 along cells. 1195 01:23:23,250 --> 01:23:23,880 What is that? 1196 01:23:23,880 --> 01:23:26,370 That is basically changing membrane potentials 1197 01:23:26,370 --> 01:23:27,870 across cells. 1198 01:23:27,870 --> 01:23:29,580 More generally, membrane potentials 1199 01:23:29,580 --> 01:23:33,030 can be used to do lots of other types of work. 1200 01:23:33,030 --> 01:23:40,320 And so ATP is only one way that cells can use biological energy 1201 01:23:40,320 --> 01:23:41,940 because it can be coupled to otherwise 1202 01:23:41,940 --> 01:23:43,620 unfavorable reactions. 1203 01:23:43,620 --> 01:23:45,960 But you can also couple ion gradients 1204 01:23:45,960 --> 01:23:50,260 to unfavorable reactions and use that to do work-- 1205 01:23:50,260 --> 01:23:54,300 and so lots of useful ways that delta psi delta 1206 01:23:54,300 --> 01:23:59,620 pH can be used by cells. 1207 01:23:59,620 --> 01:24:00,990 And so what are some of these? 1208 01:24:00,990 --> 01:24:06,970 Well, the first one is this can be used to generate heat. 1209 01:24:06,970 --> 01:24:09,810 So if you're checking your phone right now, 1210 01:24:09,810 --> 01:24:12,120 your phone is obviously a very useful device 1211 01:24:12,120 --> 01:24:14,280 to do all kinds of work for you. 1212 01:24:14,280 --> 01:24:16,710 You can read whatever you want on your phone, 1213 01:24:16,710 --> 01:24:18,505 use it to call people. 1214 01:24:18,505 --> 01:24:20,130 Well, what happens if you short circuit 1215 01:24:20,130 --> 01:24:21,480 the battery in your phone? 1216 01:24:21,480 --> 01:24:23,370 Well, it'll get really hot. 1217 01:24:23,370 --> 01:24:26,100 And the same thing happens here. 1218 01:24:26,100 --> 01:24:30,480 Rather than use this delta psi delta pH to generate ATP, 1219 01:24:30,480 --> 01:24:34,080 if I just put a hole in the membrane, 1220 01:24:34,080 --> 01:24:37,980 something referred to as an uncoupling protein, UCP, 1221 01:24:37,980 --> 01:24:42,510 Uncoupling Protein, this will just allow the leak of protons 1222 01:24:42,510 --> 01:24:44,880 back across the membrane. 1223 01:24:44,880 --> 01:24:47,430 Well, that's short circuiting the battery. 1224 01:24:47,430 --> 01:24:49,170 Energy has to be dissipated somewhere, 1225 01:24:49,170 --> 01:24:51,930 and it's dissipated as heat. 1226 01:24:51,930 --> 01:24:58,290 And this is effectively ways in which biology can create heat. 1227 01:24:58,290 --> 01:25:01,230 And so uncoupling proteins are expressed in a tissue called 1228 01:25:01,230 --> 01:25:03,570 brown fat in mammals. 1229 01:25:03,570 --> 01:25:06,900 Brown fat is a way to generate heat. 1230 01:25:06,900 --> 01:25:10,200 And it's one way that we maintain our body temperatures. 1231 01:25:10,200 --> 01:25:11,730 Now, the system is imperfect. 1232 01:25:11,730 --> 01:25:14,370 And lots of cells, all metabolism, 1233 01:25:14,370 --> 01:25:15,900 generates some heat. 1234 01:25:15,900 --> 01:25:17,760 If you've ever done composting, you 1235 01:25:17,760 --> 01:25:20,070 know your compost pile gets hot. 1236 01:25:20,070 --> 01:25:21,660 That's because the organisms there 1237 01:25:21,660 --> 01:25:24,930 that are metabolized in the material in the compost pile 1238 01:25:24,930 --> 01:25:26,277 are generating some heat. 1239 01:25:26,277 --> 01:25:28,110 And that's because membranes aren't perfect. 1240 01:25:28,110 --> 01:25:30,060 They're somewhat leaky. 1241 01:25:30,060 --> 01:25:33,930 Now, as humans, we work hard to maintain a constant body 1242 01:25:33,930 --> 01:25:34,950 temperature. 1243 01:25:34,950 --> 01:25:36,990 And there's actually a huge amount of energy 1244 01:25:36,990 --> 01:25:40,540 that goes into just maintaining our body temperature. 1245 01:25:40,540 --> 01:25:43,080 But you can imagine how efficiently 1246 01:25:43,080 --> 01:25:47,760 we couple these processes will determine how many calories we 1247 01:25:47,760 --> 01:25:49,800 need to maintain heat. 1248 01:25:49,800 --> 01:25:52,110 And that's actually a theory of weight control, 1249 01:25:52,110 --> 01:25:54,180 that those of us out there who have 1250 01:25:54,180 --> 01:25:56,100 less well-coupled mitochondria can 1251 01:25:56,100 --> 01:25:59,280 eat more because they waste more energy in generating 1252 01:25:59,280 --> 01:26:02,650 heat than those of us who are really efficient in doing this. 1253 01:26:02,650 --> 01:26:04,410 And in fact, there's drugs that allow 1254 01:26:04,410 --> 01:26:07,880 you to uncouple electron transport 1255 01:26:07,880 --> 01:26:11,540 generation of delta psi delta pH from utilizing 1256 01:26:11,540 --> 01:26:13,700 that membrane potential. 1257 01:26:13,700 --> 01:26:15,895 And these are great weight loss drugs. 1258 01:26:15,895 --> 01:26:21,050 Dinitrophenol was used middle part of the last century 1259 01:26:21,050 --> 01:26:22,210 as a weight loss agent. 1260 01:26:22,210 --> 01:26:23,970 It was very effective. 1261 01:26:23,970 --> 01:26:26,080 The problem is it's also incredibly dangerous 1262 01:26:26,080 --> 01:26:27,080 because people will die. 1263 01:26:27,080 --> 01:26:29,240 Because it turns out, if you uncouple this process 1264 01:26:29,240 --> 01:26:32,280 and you don't have enough ATP, you die really fast. 1265 01:26:32,280 --> 01:26:35,750 And so don't use that drug, but it illustrates 1266 01:26:35,750 --> 01:26:38,810 how you can use this battery to generate 1267 01:26:38,810 --> 01:26:42,270 heat, another form of energy. 1268 01:26:42,270 --> 01:26:46,640 Another big one is membrane transport. 1269 01:26:50,400 --> 01:26:53,300 So if I want to concentrate something-- and this 1270 01:26:53,300 --> 01:26:57,020 could be moving proteins. 1271 01:26:57,020 --> 01:26:59,930 Or it could be moving other ions, 1272 01:26:59,930 --> 01:27:03,880 like in an action potential. 1273 01:27:03,880 --> 01:27:05,740 Those are unfavorable processes, right? 1274 01:27:05,740 --> 01:27:10,420 I'm causing dissociation of ions across membranes 1275 01:27:10,420 --> 01:27:14,020 or moving protein from one side of a membrane to another. 1276 01:27:14,020 --> 01:27:18,790 One can couple those transfer processes to these delta psi 1277 01:27:18,790 --> 01:27:20,420 delta pH as well. 1278 01:27:20,420 --> 01:27:23,410 And in fact, protein import in the mitochondria 1279 01:27:23,410 --> 01:27:26,680 depends very much on a membrane potential 1280 01:27:26,680 --> 01:27:28,540 to get those proteins in. 1281 01:27:28,540 --> 01:27:30,430 The mitochondria is an important site 1282 01:27:30,430 --> 01:27:32,890 of calcium storage for cells. 1283 01:27:32,890 --> 01:27:35,200 Calcium is concentrated in the mitochondria. 1284 01:27:35,200 --> 01:27:37,570 And that concentration can be driven 1285 01:27:37,570 --> 01:27:40,070 by this membrane potential. 1286 01:27:40,070 --> 01:27:42,700 And of course, this potential can also 1287 01:27:42,700 --> 01:27:46,780 use to power lots of shuttles, like the carnitine shuttle, 1288 01:27:46,780 --> 01:27:48,850 and actually create situations where 1289 01:27:48,850 --> 01:27:52,270 you have different conditions across membranes as a way 1290 01:27:52,270 --> 01:27:55,780 to favor different chemistries. 1291 01:27:55,780 --> 01:27:57,430 And so the last thing I want to mention 1292 01:27:57,430 --> 01:28:01,300 is, in fact, these shuttles, which is just ways 1293 01:28:01,300 --> 01:28:05,260 that one can couple membrane potential to drive 1294 01:28:05,260 --> 01:28:08,590 other things, can also be really important for how 1295 01:28:08,590 --> 01:28:10,270 metabolism works. 1296 01:28:10,270 --> 01:28:12,950 And I just want to illustrate one aspect of this. 1297 01:28:12,950 --> 01:28:16,273 So if I draw here my mitochondria-- 1298 01:28:16,273 --> 01:28:19,660 let me draw it up here. 1299 01:28:19,660 --> 01:28:26,980 If I draw a mitochondria, so here's 1300 01:28:26,980 --> 01:28:30,460 all these processes are happening here 1301 01:28:30,460 --> 01:28:34,630 where we have electron transport generating 1302 01:28:34,630 --> 01:28:38,320 a potential across the intermitochondrial membrane 1303 01:28:38,320 --> 01:28:43,100 that can be used to synthesize ATP. 1304 01:28:43,100 --> 01:28:47,690 Well, this synthesis is occurring in the mitochondria. 1305 01:28:47,690 --> 01:28:52,010 But ultimately, I need to get that ATP out 1306 01:28:52,010 --> 01:28:57,260 into the cytosol for it to be useful for the cell 1307 01:28:57,260 --> 01:29:00,770 to run otherwise unfavorable processes in the cytosol. 1308 01:29:00,770 --> 01:29:02,870 And so one needs a transporter for this. 1309 01:29:02,870 --> 01:29:05,120 There's some transporter called the adenine nucleotide 1310 01:29:05,120 --> 01:29:06,440 transporter. 1311 01:29:06,440 --> 01:29:09,920 And this partially takes advantage of the fact 1312 01:29:09,920 --> 01:29:14,350 that there's a charge difference between ATP and ADP 1313 01:29:14,350 --> 01:29:18,220 to actually couple membrane potential to exchange 1314 01:29:18,220 --> 01:29:24,370 of ATP/ADP between the cytosol and the mitochondrial matrix. 1315 01:29:24,370 --> 01:29:27,500 Now, there's other issues we'll talk about next time as well. 1316 01:29:27,500 --> 01:29:30,070 And so, remember, some metabolism 1317 01:29:30,070 --> 01:29:32,620 is happening in the cytosol. 1318 01:29:32,620 --> 01:29:41,080 So glycolysis turning glucose into pyruvate 1319 01:29:41,080 --> 01:29:42,850 is occurring in the cytosol. 1320 01:29:42,850 --> 01:29:43,850 We've already discussed. 1321 01:29:43,850 --> 01:29:45,267 That means you got to get pyruvate 1322 01:29:45,267 --> 01:29:47,620 in the mitochondria for the PDH reaction, 1323 01:29:47,620 --> 01:29:51,970 but this is also generating NADH in the cytosol. 1324 01:29:51,970 --> 01:29:55,330 And if that NADH needs to transfer its electrons 1325 01:29:55,330 --> 01:29:57,010 to oxygen, we also have to get that 1326 01:29:57,010 --> 01:30:02,170 NADH into the mitochondrial matrix or, more correctly, 1327 01:30:02,170 --> 01:30:05,090 transfer those electrons into the mitochondrial matrix. 1328 01:30:05,090 --> 01:30:08,770 It turns out cells maintain distinct NAD/NADH pools 1329 01:30:08,770 --> 01:30:10,660 in the mitochondria and the cytosol. 1330 01:30:10,660 --> 01:30:12,280 That's part of the conditions that 1331 01:30:12,280 --> 01:30:17,560 make different reactions favorable in each compartment. 1332 01:30:17,560 --> 01:30:21,310 And how cells do that, transfer those electrons 1333 01:30:21,310 --> 01:30:25,060 into the mitochondria, ends up being another important thing 1334 01:30:25,060 --> 01:30:28,630 that has to be transport that's driven across these membranes 1335 01:30:28,630 --> 01:30:31,720 where one can use delta psi delta pH. 1336 01:30:31,720 --> 01:30:34,490 And we'll talk about that at the start of the next lecture. 1337 01:30:34,490 --> 01:30:36,040 Thanks.