1 00:00:00,000 --> 00:00:01,440 [SQUEAKING] 2 00:00:01,440 --> 00:00:02,880 [RUSTLING] 3 00:00:02,880 --> 00:00:06,240 [CLICKING] 4 00:00:10,477 --> 00:00:11,560 MATTHEW VANDER HEIDEN: OK. 5 00:00:11,560 --> 00:00:14,680 Today we will start our discussion of-- 6 00:00:14,680 --> 00:00:18,310 with a discussion of glycolysis as a pathway 7 00:00:18,310 --> 00:00:22,600 and how we can use favorable oxidation of glucose 8 00:00:22,600 --> 00:00:25,600 to enable phosphate addition while at the same time 9 00:00:25,600 --> 00:00:27,760 producing useful intermediates that 10 00:00:27,760 --> 00:00:31,300 enable us to do ATP synthase-- 11 00:00:31,300 --> 00:00:36,760 ATP synthesis and maintain a high ATP-ADP ratio in cells. 12 00:00:36,760 --> 00:00:40,840 Now I drew up here the entire glycolytic pathway, 13 00:00:40,840 --> 00:00:43,570 just like you saw at the end of the last lecture, 14 00:00:43,570 --> 00:00:47,690 and I want to go through it here step by step. 15 00:00:47,690 --> 00:00:53,710 And so the first step in glycolysis, 16 00:00:53,710 --> 00:00:56,650 as we already talked about, is we have glucose. 17 00:00:56,650 --> 00:01:00,290 This here is, of course, alpha-D-Glucose in the-- 18 00:01:00,290 --> 00:01:01,590 I drew it in the alpha form. 19 00:01:01,590 --> 00:01:04,090 If the OH was pointing up, that would be the beta form, just 20 00:01:04,090 --> 00:01:05,239 some quick review. 21 00:01:05,239 --> 00:01:08,800 I'll drop the alpha and beta and the stereochemistry denotion 22 00:01:08,800 --> 00:01:10,480 going forward. 23 00:01:10,480 --> 00:01:13,930 This glucose molecule, as we discussed previously, 24 00:01:13,930 --> 00:01:17,380 is first trapped in cells by being phosphorylated 25 00:01:17,380 --> 00:01:21,280 by the enzyme hexokinase, sometimes abbreviated HK, 26 00:01:21,280 --> 00:01:24,370 to give glucose 6-phosphate. 27 00:01:24,370 --> 00:01:26,830 Not shown here, this glucose 6-phosphate 28 00:01:26,830 --> 00:01:29,890 then undergoes an isomerase reaction 29 00:01:29,890 --> 00:01:31,720 via the open chain form. 30 00:01:31,720 --> 00:01:33,580 Again, we discussed this reaction 31 00:01:33,580 --> 00:01:35,260 when we discussed sugars. 32 00:01:35,260 --> 00:01:39,070 And so this would be catalyzed by glucose phosphate isomerase, 33 00:01:39,070 --> 00:01:41,860 and takes it from the aldose to the ketose, 34 00:01:41,860 --> 00:01:45,340 and now you have the fructose-- the ketose fructose 35 00:01:45,340 --> 00:01:50,860 6-phosphate shown here in the furanose form. 36 00:01:50,860 --> 00:01:54,880 This fructose 6-phosphate is then phosphorylated 37 00:01:54,880 --> 00:01:58,570 a second time by an enzyme called phosphofructokinase, 38 00:01:58,570 --> 00:02:00,865 typically abbreviated PVK. 39 00:02:00,865 --> 00:02:05,050 It's phosphorylated on the 1 position to give fructose 40 00:02:05,050 --> 00:02:08,560 1,6-bisphosphate, shown here. 41 00:02:08,560 --> 00:02:13,450 That fructose 1,6-bisphosphate can then be cleaved via 42 00:02:13,450 --> 00:02:15,220 the enzyme aldolase. 43 00:02:15,220 --> 00:02:19,360 Cleavage takes place in the open chain form which I drew up here 44 00:02:19,360 --> 00:02:20,470 in the corner. 45 00:02:20,470 --> 00:02:23,890 Basically splitting the molecule in half 46 00:02:23,890 --> 00:02:28,570 to form two trioses, one of which is the ketose 47 00:02:28,570 --> 00:02:32,680 dihydroxyacetone phosphate, basically the top half 48 00:02:32,680 --> 00:02:33,950 of the molecule. 49 00:02:33,950 --> 00:02:36,760 The other is the bottom half of the molecule, 50 00:02:36,760 --> 00:02:40,720 the aldose glyceraldehyde 3-phosphate. 51 00:02:40,720 --> 00:02:45,040 These two trioses can be interconverted, the aldose 52 00:02:45,040 --> 00:02:47,572 and the ketose via on isomerase. 53 00:02:47,572 --> 00:02:49,030 The enzyme that does that is called 54 00:02:49,030 --> 00:02:51,220 trioses phosphate isomerase. 55 00:02:51,220 --> 00:02:54,370 And moving forward basically takes the dihydroxyacetone 56 00:02:54,370 --> 00:02:58,510 phosphate that comes from the top half and can turn all 57 00:02:58,510 --> 00:03:02,590 the carbon from fructose 1,6-bisphosphate into two 58 00:03:02,590 --> 00:03:05,560 glyceraldehyde 3-phosphates. 59 00:03:05,560 --> 00:03:09,100 That glyceraldehyde 3-phosphate, as we discussed last time, 60 00:03:09,100 --> 00:03:12,130 is the substrate for the GAPDH reaction, 61 00:03:12,130 --> 00:03:16,510 the glyceraldehyde 3-phosphate dehydrogenase, GAPDH. 62 00:03:16,510 --> 00:03:18,640 which, as we discussed last time, 63 00:03:18,640 --> 00:03:22,450 couples oxidation of this aldehyde 64 00:03:22,450 --> 00:03:26,350 to this acid with phosphate addition. 65 00:03:26,350 --> 00:03:29,420 That oxidation reaction, the electrons have to go somewhere, 66 00:03:29,420 --> 00:03:32,170 that's where the cofactor NAD comes in. 67 00:03:32,170 --> 00:03:35,110 Picks up two electrons, generates NADH, 68 00:03:35,110 --> 00:03:38,750 and in the process, this enzyme generates this molecule, 69 00:03:38,750 --> 00:03:43,960 1,3-bisphosphoglycerate or 1,3-BPG. 70 00:03:43,960 --> 00:03:48,160 Remember, that 1,3-BPG molecule was one of the useful 71 00:03:48,160 --> 00:03:53,440 intermediates whereby we could undergo favorable synthesis 72 00:03:53,440 --> 00:03:58,660 of ATP even at the high ATP-ADP ratio found in cells, 73 00:03:58,660 --> 00:04:01,030 and that's because we can transfer the phosphate from 74 00:04:01,030 --> 00:04:04,600 this acid group onto ADP. 75 00:04:04,600 --> 00:04:10,120 That gives us an ATP and the product 3-phosphoglycerate 3-PG 76 00:04:10,120 --> 00:04:13,060 carried out by the enzyme phosphoglycerate kinase. 77 00:04:16,750 --> 00:04:20,230 That 3-PG molecule can then undergo 78 00:04:20,230 --> 00:04:22,850 what's referred to as a mutase reaction. 79 00:04:22,850 --> 00:04:25,180 And so the phosphate is moved from the 3 80 00:04:25,180 --> 00:04:27,220 to the 2 position of glycerate. 81 00:04:27,220 --> 00:04:30,640 So 3-phosphoglycerate to 2-phosphoglycerate, 82 00:04:30,640 --> 00:04:35,380 carried out by phosphoglycerate mutase, often abbreviated PGAM. 83 00:04:35,380 --> 00:04:39,730 This 2-phosphoglycerate can then undergo a dehydration 84 00:04:39,730 --> 00:04:43,720 to-- by an enzyme enolase to give the molecule 85 00:04:43,720 --> 00:04:45,490 phosphoenolpyruvate. . 86 00:04:45,490 --> 00:04:47,860 Phosphoenolpyruvate was the second 87 00:04:47,860 --> 00:04:51,160 of those useful intermediates that 88 00:04:51,160 --> 00:04:55,600 allow ATP synthesis despite the high ATP-ADP ratio in cells, 89 00:04:55,600 --> 00:04:57,670 and that reaction, as we already discussed, 90 00:04:57,670 --> 00:04:59,770 is carried out by pyruvate kinase 91 00:04:59,770 --> 00:05:06,600 and allows you to generate another ATP. 92 00:05:06,600 --> 00:05:11,480 Now, as I drew out the glycolytic pathway here, 93 00:05:11,480 --> 00:05:16,400 I also listed the delta G0 prime for every single step 94 00:05:16,400 --> 00:05:17,780 in the reaction. 95 00:05:17,780 --> 00:05:23,120 Remember, this is informative as to the equilibrium for each 96 00:05:23,120 --> 00:05:25,940 of these individual reactions. 97 00:05:25,940 --> 00:05:28,670 You'll note that for many of the steps, 98 00:05:28,670 --> 00:05:31,820 that delta G0 prime is positive. 99 00:05:31,820 --> 00:05:32,950 Remember what that means. 100 00:05:32,950 --> 00:05:35,600 That means that equilibrium favors the left. 101 00:05:35,600 --> 00:05:39,930 That is the reverse direction of the entire pathway. 102 00:05:39,930 --> 00:05:42,350 However, if you add all of these up, 103 00:05:42,350 --> 00:05:46,130 the delta G0 prime for the entire pathway-- 104 00:05:46,130 --> 00:05:58,398 so delta G0 prime for the entire pathway, glucose to pyruvate 105 00:05:58,398 --> 00:06:07,240 adds up to negative 6.9 kcals per mole. 106 00:06:07,240 --> 00:06:11,980 And so that negative 6.9 means that for the entire pathway, 107 00:06:11,980 --> 00:06:15,250 equilibrium favors the right. 108 00:06:15,250 --> 00:06:20,230 So glucose to pyruvate, the right direction. 109 00:06:20,230 --> 00:06:25,930 And that is why this can work as an overall pathway 110 00:06:25,930 --> 00:06:29,440 even though some of the individual steps, equilibrium 111 00:06:29,440 --> 00:06:32,570 may favor the opposite direction. 112 00:06:32,570 --> 00:06:36,700 Now you'll notice that there's several favorable steps where 113 00:06:36,700 --> 00:06:40,850 delta G0 prime is quite negative. 114 00:06:40,850 --> 00:06:43,236 Pyruvate kinase, phosphoglycerate kinase, 115 00:06:43,236 --> 00:06:46,360 hexokinase, phosphofructokinase. 116 00:06:46,360 --> 00:06:50,860 As we talked about before, these are able to pull the less 117 00:06:50,860 --> 00:06:55,450 favorable steps upstream forward by keeping concentrations 118 00:06:55,450 --> 00:07:00,010 of products such as 1,3-bisphosphoglycerate low, 119 00:07:00,010 --> 00:07:04,490 thus allowing this net reaction to move forward. 120 00:07:04,490 --> 00:07:08,480 You'll notice I drew three steps here as irreversible-- 121 00:07:08,480 --> 00:07:13,430 hexokinase, phosphofructokinase, and pyruvate kinase. 122 00:07:13,430 --> 00:07:17,930 Remember, these are all reactions in our-- 123 00:07:17,930 --> 00:07:20,600 there's no such thing as an irreversible reaction. 124 00:07:20,600 --> 00:07:23,450 There are some conditions where the reverse reaction, 125 00:07:23,450 --> 00:07:25,190 of course, can happen. 126 00:07:25,190 --> 00:07:28,550 Although remember, this-- when we discussed irreversible 127 00:07:28,550 --> 00:07:30,800 in metabolism, what we're referring to 128 00:07:30,800 --> 00:07:34,190 is irreversible under physiological conditions-- that 129 00:07:34,190 --> 00:07:36,680 is, conditions found in cells. 130 00:07:36,680 --> 00:07:38,900 You'll notice that those three steps 131 00:07:38,900 --> 00:07:42,395 are three of the four steps that have very negative delta 132 00:07:42,395 --> 00:07:43,940 G0 prime. 133 00:07:43,940 --> 00:07:45,440 And so you would expect that if we 134 00:07:45,440 --> 00:07:47,030 were going to reverse those reactions, 135 00:07:47,030 --> 00:07:49,460 you would need some kind of energy input, 136 00:07:49,460 --> 00:07:52,160 and you'll see later that these are the control points or also 137 00:07:52,160 --> 00:07:54,500 regulated steps in this pathway. 138 00:07:54,500 --> 00:07:56,570 And there are also the sites, just to give you 139 00:07:56,570 --> 00:07:58,028 a preview, that we're going to have 140 00:07:58,028 --> 00:08:01,580 to deal with if we're going to try to synthesize glucose, 141 00:08:01,580 --> 00:08:05,690 gluconeogenesis, the reverse of glycolysis. 142 00:08:05,690 --> 00:08:08,690 Now of course, these delta G0 primes 143 00:08:08,690 --> 00:08:13,010 are very informative of what happens at equilibrium, 144 00:08:13,010 --> 00:08:16,370 but of course, what actually happens in a cell 145 00:08:16,370 --> 00:08:18,170 or in any condition, remember, is 146 00:08:18,170 --> 00:08:19,880 dependent on the equilibrium constant, 147 00:08:19,880 --> 00:08:21,330 but also on the conditions. 148 00:08:21,330 --> 00:08:24,860 Remember, it's delta G, delta G equals delta G0 prime plus RT 149 00:08:24,860 --> 00:08:27,900 times the log of the products over the reactants. 150 00:08:27,900 --> 00:08:31,550 And so delta G is what really matters 151 00:08:31,550 --> 00:08:34,010 for any individual reactions, and what's 152 00:08:34,010 --> 00:08:37,280 shown here on this slide is basically 153 00:08:37,280 --> 00:08:43,159 a approximate delta G0 change across the glycolytic pathway 154 00:08:43,159 --> 00:08:47,300 based on someone's approximation of the conditions found 155 00:08:47,300 --> 00:08:48,410 in cells. 156 00:08:48,410 --> 00:08:50,000 Now we can see and looking at this 157 00:08:50,000 --> 00:08:53,180 that there's really three big drops in delta G 158 00:08:53,180 --> 00:08:54,590 across the pathway. 159 00:08:54,590 --> 00:08:56,780 Here's the hexokinase reaction, here's 160 00:08:56,780 --> 00:08:58,940 the phosphofructokinase reaction, 161 00:08:58,940 --> 00:09:01,870 here's the pyruvate kinase reaction. 162 00:09:01,870 --> 00:09:05,250 And so it makes sense that these would be, 163 00:09:05,250 --> 00:09:07,500 then, the irreversible steps because there 164 00:09:07,500 --> 00:09:13,090 is what is actually pulling the pathway forward. 165 00:09:13,090 --> 00:09:16,170 It also makes sense that one would exert control 166 00:09:16,170 --> 00:09:18,810 at each of these steps, because this 167 00:09:18,810 --> 00:09:21,750 is what drives the flux of the pathway forward, 168 00:09:21,750 --> 00:09:24,630 but it's also where it's difficult to go back. 169 00:09:24,630 --> 00:09:26,760 Once you get here on this flat part of the curve 170 00:09:26,760 --> 00:09:29,250 energetically it doesn't matter so much which 171 00:09:29,250 --> 00:09:31,200 direction you go along the curve, 172 00:09:31,200 --> 00:09:33,600 but to try to climb back up each of these hills 173 00:09:33,600 --> 00:09:37,320 becomes difficult. And so we'll revisit this later 174 00:09:37,320 --> 00:09:39,480 in the lecture, but this is basically 175 00:09:39,480 --> 00:09:41,460 where the control points will end up 176 00:09:41,460 --> 00:09:45,300 happening in this pathway. 177 00:09:45,300 --> 00:09:49,020 Now, I want to note a couple other features 178 00:09:49,020 --> 00:09:53,460 about this pathway, and that is that we had 179 00:09:53,460 --> 00:09:55,510 to invest some energy early. 180 00:09:55,510 --> 00:09:58,800 You see you cost an ATP at the hexokinase 181 00:09:58,800 --> 00:10:01,320 and the phosphofructokinase step and then 182 00:10:01,320 --> 00:10:06,030 you harvest that ATP much later at the phosphoglycerate kinase 183 00:10:06,030 --> 00:10:09,780 and the pyruvate kinase steps. 184 00:10:09,780 --> 00:10:14,340 The textbook typically will split glycolysis 185 00:10:14,340 --> 00:10:18,270 into what they refer to as two stages, an investment 186 00:10:18,270 --> 00:10:22,470 stage and a harvesting stage, and that basically 187 00:10:22,470 --> 00:10:24,780 relates to that fact. 188 00:10:24,780 --> 00:10:28,980 And that's because you have a glucose molecule that then you 189 00:10:28,980 --> 00:10:32,010 use to generate two trioses. 190 00:10:36,150 --> 00:10:39,750 And then each of those trioses can 191 00:10:39,750 --> 00:10:44,670 be used to generate a pyruvate molecule. 192 00:10:50,160 --> 00:10:59,710 And so it costs 2 ATP to generate those to trioses, 193 00:10:59,710 --> 00:11:08,660 but then each triose generated allows you to recapture 2 ATPs. 194 00:11:12,240 --> 00:11:16,530 And so what that means, then, is that the net output 195 00:11:16,530 --> 00:11:20,310 from glycolysis is 2 ATPs. 196 00:11:20,310 --> 00:11:27,720 And so glucose going to two glyceraldehyde 197 00:11:27,720 --> 00:11:44,110 3-phosphates costs you, whereas using those two glyceraldehyde 198 00:11:44,110 --> 00:11:50,890 3-phosphates to generate two pyruvates 199 00:11:50,890 --> 00:12:00,880 allows you to recover 4 ATP for a net of plus 2 ATP molecules 200 00:12:00,880 --> 00:12:05,230 produced per glucose metabolized. 201 00:12:05,230 --> 00:12:10,000 Now if we add up the entire stoichiometry of what's 202 00:12:10,000 --> 00:12:13,120 going on here, we can also draw it another way, 203 00:12:13,120 --> 00:12:16,480 and that is we can basically have glucose 204 00:12:16,480 --> 00:12:24,730 plus 2 ATP molecules plus 2 inorganic phosphates, 205 00:12:24,730 --> 00:12:34,910 goes to 2 pyruvates plus 2 ATP, and that's 206 00:12:34,910 --> 00:12:37,790 because we're able to incorporate 207 00:12:37,790 --> 00:12:47,020 2 phosphate molecules here at the GAPDH step. 208 00:12:47,020 --> 00:12:50,110 Now that's great, but if you're paying attention, 209 00:12:50,110 --> 00:12:52,390 you notice that we're missing one other thing 210 00:12:52,390 --> 00:12:54,550 in this stoichiometry, and that's 211 00:12:54,550 --> 00:13:00,790 that cofactor NAD that was also required to maintain electron 212 00:13:00,790 --> 00:13:04,870 balance across the GAPDH step. 213 00:13:04,870 --> 00:13:10,360 And so remember that for these pathways to work, 214 00:13:10,360 --> 00:13:13,150 we have to balance all things that are going in 215 00:13:13,150 --> 00:13:14,680 with all things are going out-- we 216 00:13:14,680 --> 00:13:16,870 can't create or destroy matter. 217 00:13:16,870 --> 00:13:19,370 And so those electrons have to be dealt with. 218 00:13:19,370 --> 00:13:22,060 When we discussed the GAPDH reaction last time, 219 00:13:22,060 --> 00:13:27,640 we needed that cofactor NAD in order to act as an acceptor 220 00:13:27,640 --> 00:13:31,840 for the electrons to allow that oxidative phosphorylation, 221 00:13:31,840 --> 00:13:35,260 if you will, of glyceraldehyde 3-phosphate 222 00:13:35,260 --> 00:13:38,650 to 1,3-bisphosphoglycerate. 223 00:13:38,650 --> 00:13:41,410 And so in order for this pathway to work, 224 00:13:41,410 --> 00:13:46,600 we need a way to regenerate that NADH back to an NAD 225 00:13:46,600 --> 00:13:49,900 in order to make electron balance. 226 00:13:49,900 --> 00:13:52,930 In other words, we have to deal with the fact 227 00:13:52,930 --> 00:13:54,400 that we've generated-- 228 00:13:54,400 --> 00:14:03,860 or that we required 2 NAD+ to run this pathway and generate 2 229 00:14:03,860 --> 00:14:06,290 NADH. 230 00:14:06,290 --> 00:14:16,730 And so we need a way to get rid of those electrons from NADH 231 00:14:16,730 --> 00:14:23,430 to regenerate that NAD and allow the pathway to be balanced. 232 00:14:23,430 --> 00:14:27,950 Now it should be clear that this has 233 00:14:27,950 --> 00:14:32,990 to happen stoichiometrically for every single glucose molecule 234 00:14:32,990 --> 00:14:34,880 that goes through glycolysis. 235 00:14:34,880 --> 00:14:40,160 That is, we have to regenerate 2 NAD plusses for every glucose 236 00:14:40,160 --> 00:14:44,240 that we're going to convert into 2 pyruvates. 237 00:14:44,240 --> 00:14:50,640 And so for this NADH reoxidation to occur, 238 00:14:50,640 --> 00:14:55,490 we need a place to transfer those two electron pairs. 239 00:14:55,490 --> 00:14:59,450 And so just like any oxidation and reduction reaction, 240 00:14:59,450 --> 00:15:04,940 if we're going to oxidize NADH back to NAD+, 241 00:15:04,940 --> 00:15:07,760 something else has to be reduced, 242 00:15:07,760 --> 00:15:12,290 and that thing has to be reduced stoichiometrically with every 243 00:15:12,290 --> 00:15:18,140 single pyruvate that's produced from glucose and glycolysis. 244 00:15:18,140 --> 00:15:20,630 And effectively you will see that this 245 00:15:20,630 --> 00:15:23,210 is the role of fermentation. 246 00:15:23,210 --> 00:15:27,500 It's an ability for us to solve this redox problem, 247 00:15:27,500 --> 00:15:31,790 that we have to dispose of those electrons in the oxidation 248 00:15:31,790 --> 00:15:37,520 at the GAPDH step in order to deal with this electron 249 00:15:37,520 --> 00:15:40,010 waste of the pathway. 250 00:15:40,010 --> 00:15:42,920 Now, the useful way to do this is 251 00:15:42,920 --> 00:15:47,180 that we're generating a pyruvate stoichiometrically. 252 00:15:47,180 --> 00:15:50,270 And so we can use that stoichiometrically, 253 00:15:50,270 --> 00:15:54,290 that product pyruvate by reducing it to form the waste 254 00:15:54,290 --> 00:15:57,140 product, and that's effectively what fermentation is. 255 00:15:57,140 --> 00:16:00,590 It's turning that pyruvate into ethanol or lactate, which 256 00:16:00,590 --> 00:16:04,130 is reducing that pyruvate, picking up the electron 257 00:16:04,130 --> 00:16:06,470 waste from the NADH, and allowing 258 00:16:06,470 --> 00:16:12,530 you to regenerate the NAD that's required to run the GAPDH step. 259 00:16:12,530 --> 00:16:15,550 And so I'll just illustrate this. 260 00:16:27,205 --> 00:16:31,740 The simplest fermentation reaction that we can do 261 00:16:31,740 --> 00:16:34,080 is the run that's done by animals, 262 00:16:34,080 --> 00:16:36,450 and that's to generate lactate. 263 00:16:36,450 --> 00:16:40,680 And so to just illustrate this explicitly, here is-- 264 00:16:55,300 --> 00:17:02,420 so this here would be the reduced form of NAD, NADH. 265 00:17:04,950 --> 00:17:06,738 So this here this-- 266 00:17:10,482 --> 00:17:12,450 this is the nicotinamide group. 267 00:17:12,450 --> 00:17:14,970 Remember, it's hooked up to an ADP ribose 268 00:17:14,970 --> 00:17:18,690 to give nicotinamide adenine dinucleotide in the reduced 269 00:17:18,690 --> 00:17:20,550 form. 270 00:17:20,550 --> 00:17:25,680 So this can transfer electrons to pyruvate. 271 00:17:36,750 --> 00:17:38,920 Here's pyruvate. 272 00:17:38,920 --> 00:17:46,130 So as we showed last time, we can take 2 electrons 273 00:17:46,130 --> 00:17:48,570 from NADH reduced. 274 00:17:48,570 --> 00:18:14,040 That gives us this hydride ion, which can generate lactate. 275 00:18:14,040 --> 00:18:31,440 Plus, what we're left with is the oxidized form 276 00:18:31,440 --> 00:18:33,240 of the cofactor NAD+. 277 00:18:37,890 --> 00:18:43,560 And now this NAD+ can be reduced back to NADH at the GAPDH step, 278 00:18:43,560 --> 00:18:49,200 and the electrons from NADH, as it's being reoxidized to NAD, 279 00:18:49,200 --> 00:18:53,290 can go and take pyruvate and reduce it to lactate. 280 00:18:53,290 --> 00:18:56,430 So NAD gets reoxidized to NAD. 281 00:18:56,430 --> 00:19:01,110 Pyruvate now gets-- the ketone gets reduced to the alcohol 282 00:19:01,110 --> 00:19:06,010 as you go from pyruvate to lactate. 283 00:19:06,010 --> 00:19:08,770 Opposite of what's going on in the GAPDH step. 284 00:19:08,770 --> 00:19:12,880 There, the aldehyde is oxidized to the acid, 285 00:19:12,880 --> 00:19:16,600 while NAD+ oxidized goes-- 286 00:19:16,600 --> 00:19:18,610 is reduced to NADH. 287 00:19:18,610 --> 00:19:23,260 Here, NADH is reoxidized to NAD while taking the pyruvate 288 00:19:23,260 --> 00:19:26,470 and re-reducing it to lactate. 289 00:19:26,470 --> 00:19:32,860 This, by adding here, allows us now to regain balance 290 00:19:32,860 --> 00:19:34,900 across the entire pathway. 291 00:19:43,920 --> 00:19:57,230 This step is catalyzed by the enzyme lactate dehydrogenase, 292 00:19:57,230 --> 00:20:01,370 often abbreviated LDH. 293 00:20:01,370 --> 00:20:16,270 And that allows regenerating the NAD+ that's needed 294 00:20:16,270 --> 00:20:20,470 for the GAPDH step, which allows us to deal with 295 00:20:20,470 --> 00:20:25,310 the stoichiometry problem of the entire pathway. 296 00:20:25,310 --> 00:20:28,180 And so what fermentation does for cells 297 00:20:28,180 --> 00:20:31,420 is allows electron disposal by taking 298 00:20:31,420 --> 00:20:33,820 the product of the pathway pyruvate, 299 00:20:33,820 --> 00:20:38,230 reducing it stoichiometrically to form a reduced product-- 300 00:20:38,230 --> 00:20:40,930 as shown here, lactate, and that allows 301 00:20:40,930 --> 00:20:45,710 you to maintain electron balance across the entire pathway. 302 00:20:45,710 --> 00:20:50,500 And so this entire fermentation pathway, glucose to lactate, 303 00:20:50,500 --> 00:20:56,980 now allows the generation of the net generation of 2 ATP 304 00:20:56,980 --> 00:21:01,600 and run this pathway in cells so cells 305 00:21:01,600 --> 00:21:06,130 can maintain a high ATP-ADP ratio by fermenting glucose 306 00:21:06,130 --> 00:21:08,610 into lactate. 307 00:21:08,610 --> 00:21:11,730 Now, if you're paying attention, this also 308 00:21:11,730 --> 00:21:15,930 now begins to describe why it is that oxygen 309 00:21:15,930 --> 00:21:20,640 is such a useful molecule for metabolism 310 00:21:20,640 --> 00:21:24,060 and actually why it's quite key to supporting 311 00:21:24,060 --> 00:21:26,940 the bioenergetics of cells. 312 00:21:26,940 --> 00:21:32,760 And so recall, oxygen was there when we burned wood 313 00:21:32,760 --> 00:21:34,440 and we used that example. 314 00:21:34,440 --> 00:21:39,180 And why oxygen is so important for burning wood, why 315 00:21:39,180 --> 00:21:44,790 it's important for doing glucose catabolism in our cells 316 00:21:44,790 --> 00:21:48,060 is that oxygen is a really great electron acceptor. 317 00:21:48,060 --> 00:21:55,470 That is, I can draw out oxygen can take 2 electrons that 318 00:21:55,470 --> 00:21:59,280 are generated as NADH waste, if you 319 00:21:59,280 --> 00:22:07,200 want to call it that, plus protons, 320 00:22:07,200 --> 00:22:10,140 and this generates water. 321 00:22:10,140 --> 00:22:15,540 And so an alternative to fermentation in order 322 00:22:15,540 --> 00:22:19,410 to allow glucose oxidation is rather than do fermentation, 323 00:22:19,410 --> 00:22:23,280 is to transfer those electrons to oxygen 324 00:22:23,280 --> 00:22:25,920 to run the pathway another way. 325 00:22:25,920 --> 00:22:34,230 In other words, if we write glucose to pyruvate and realize 326 00:22:34,230 --> 00:22:38,470 that this is an oxidation reaction, 327 00:22:38,470 --> 00:22:41,430 and so therefore, generates electron 328 00:22:41,430 --> 00:22:48,690 waste in the form of NADH that has to be recycled to NAD, 329 00:22:48,690 --> 00:22:52,770 we can do so by putting those reoxidizing NADH, 330 00:22:52,770 --> 00:22:55,540 means something else has to be reduced. 331 00:22:55,540 --> 00:23:00,380 This something else could be oxygen as a good electron 332 00:23:00,380 --> 00:23:05,060 acceptor to be reduced to water and find an alternative place 333 00:23:05,060 --> 00:23:06,980 to put those electrons. 334 00:23:06,980 --> 00:23:10,130 Or, if oxygen is not present, now we 335 00:23:10,130 --> 00:23:14,225 can do fermentation, turning pyruvate into lactate. 336 00:23:17,100 --> 00:23:18,930 It should be clear that if we want 337 00:23:18,930 --> 00:23:24,870 to completely oxidize glucose, as in burning wood-- 338 00:23:24,870 --> 00:23:26,610 that is, turn it-- 339 00:23:26,610 --> 00:23:29,760 all the carbon and glucose into the most oxidized form 340 00:23:29,760 --> 00:23:35,790 of carbon, CO2, this also requires 341 00:23:35,790 --> 00:23:39,520 places to put electrons. 342 00:23:39,520 --> 00:23:43,570 That is, we could generate more NADH. 343 00:23:43,570 --> 00:23:46,000 And of course, each of those electrons 344 00:23:46,000 --> 00:23:48,140 have to be dealt with. 345 00:23:48,140 --> 00:23:50,320 And the final home for those electrons 346 00:23:50,320 --> 00:23:52,390 needs to be something that's a good electron 347 00:23:52,390 --> 00:23:58,240 acceptor like oxygen, allowing you to continually reoxidize 348 00:23:58,240 --> 00:24:03,550 your NADH by reducing oxygen to water. 349 00:24:03,550 --> 00:24:07,350 So, in the absence of fermentation, 350 00:24:07,350 --> 00:24:12,180 oxygen can be used to maintain electron balance, 351 00:24:12,180 --> 00:24:14,340 and of course, much more energy is 352 00:24:14,340 --> 00:24:18,270 going to be released if we completely burn glucose 353 00:24:18,270 --> 00:24:21,060 than if we only partially oxidize glucose. 354 00:24:21,060 --> 00:24:24,450 And this is why oxygen allows more energy 355 00:24:24,450 --> 00:24:26,580 to be released, or, as you probably 356 00:24:26,580 --> 00:24:29,940 learned in high school, more ATP to be produced 357 00:24:29,940 --> 00:24:32,920 from glucose metabolism. 358 00:24:32,920 --> 00:24:36,120 Now the details of how all of this works 359 00:24:36,120 --> 00:24:39,960 are much more complicated than what I have drawn. 360 00:24:39,960 --> 00:24:42,480 Obviously fermentation happens the way I drew it, 361 00:24:42,480 --> 00:24:45,120 but all the other details about the role of oxygen 362 00:24:45,120 --> 00:24:47,552 and how it fits in to these pathways 363 00:24:47,552 --> 00:24:49,260 is more complicated than what I've drawn, 364 00:24:49,260 --> 00:24:51,840 and we'll cover this in the upcoming lectures. 365 00:24:51,840 --> 00:24:54,180 But the key concept is here, and that 366 00:24:54,180 --> 00:24:58,050 is the relationship between oxygen and fermentation 367 00:24:58,050 --> 00:24:59,790 is really what's illustrated here. 368 00:24:59,790 --> 00:25:02,790 These are two different ways that cells 369 00:25:02,790 --> 00:25:06,720 can use to dispose of the electron waste that's 370 00:25:06,720 --> 00:25:11,520 produced from carbon oxidation and glucose metabolism. 371 00:25:11,520 --> 00:25:13,110 Those electrons have to go somewhere. 372 00:25:13,110 --> 00:25:16,590 They can go to the product of glycolysis, 373 00:25:16,590 --> 00:25:19,920 say pyruvate to lactate, by lactate dehydrogenase, 374 00:25:19,920 --> 00:25:23,010 reduce the pyruvate to lactate, fermentation, 375 00:25:23,010 --> 00:25:24,960 or they can go somewhere else. 376 00:25:24,960 --> 00:25:27,730 A great electron acceptor like oxygen 377 00:25:27,730 --> 00:25:31,140 which allows these pathways to happen 378 00:25:31,140 --> 00:25:34,500 while saving the pyruvate to do something else, 379 00:25:34,500 --> 00:25:39,260 like be completely oxidized to CO2. 380 00:25:39,260 --> 00:25:41,210 Now it should be clear here, then, 381 00:25:41,210 --> 00:25:44,870 that one of the things that's special about fermentation 382 00:25:44,870 --> 00:25:48,680 is that absolutely no oxygen is required to do this, 383 00:25:48,680 --> 00:25:52,250 and this is part of the reason why this pathway is so ancient. 384 00:25:52,250 --> 00:25:56,720 Fermentation evolved in the pre-oxygen atmosphere, 385 00:25:56,720 --> 00:26:00,650 and that really is why it's such an ancient pathway. 386 00:26:00,650 --> 00:26:03,590 Only when oxygen levels rose in the atmosphere 387 00:26:03,590 --> 00:26:07,940 due to photosynthesis that enough oxygen was around that 388 00:26:07,940 --> 00:26:12,890 then other oxidative pathways could evolve as an alternative 389 00:26:12,890 --> 00:26:15,600 to fermentation. 390 00:26:15,600 --> 00:26:19,100 Now, you'll note that the original description 391 00:26:19,100 --> 00:26:24,060 of fermentation by Pasteur was ethanol, not lactate. 392 00:26:24,060 --> 00:26:26,880 We'll cover the details of ethanol in a minute, 393 00:26:26,880 --> 00:26:30,330 but I just want to point out that making ethanol instead 394 00:26:30,330 --> 00:26:34,500 of lactate is simply an alternative product 395 00:26:34,500 --> 00:26:38,230 to dispose of pyruvate as a more reduced product. 396 00:26:38,230 --> 00:26:42,090 So ethanol is a more reduced product than pyruvate 397 00:26:42,090 --> 00:26:48,270 in that ethanol generation is an additional adaptation that 398 00:26:48,270 --> 00:26:50,580 allows you to dispose of the electron waste 399 00:26:50,580 --> 00:26:55,410 while also generating a molecule that's toxic, and therefore, 400 00:26:55,410 --> 00:26:58,710 allow organisms that make ethanol to better kill off 401 00:26:58,710 --> 00:27:00,330 their neighbors and their environment 402 00:27:00,330 --> 00:27:02,970 and compete for resources. 403 00:27:02,970 --> 00:27:05,580 But the overall concept is identical. 404 00:27:05,580 --> 00:27:09,090 Ethanol is really an alternative to lactate and fermentation 405 00:27:09,090 --> 00:27:13,440 but solves exactly the same problem. 406 00:27:13,440 --> 00:27:16,250 I want to talk about ethanol metabolism in more detail, 407 00:27:16,250 --> 00:27:19,020 but before we do that first I want to go through and just 408 00:27:19,020 --> 00:27:23,370 describe at a very high level some of the chemistry that 409 00:27:23,370 --> 00:27:27,570 allows all these reactions in glycolysis to work, 410 00:27:27,570 --> 00:27:31,140 because you'll see that the chemistry itself is not 411 00:27:31,140 --> 00:27:35,280 all that complicated even though we can draw it 412 00:27:35,280 --> 00:27:37,290 and it seems like somewhat overwhelming 413 00:27:37,290 --> 00:27:39,660 when drawn out as a whole pathway. 414 00:27:39,660 --> 00:27:41,700 Now I want to point out, we've already 415 00:27:41,700 --> 00:27:45,040 covered the chemistry for a lot of these steps. 416 00:27:45,040 --> 00:27:48,360 First of all, there's two dehydrogenase reactions. 417 00:27:48,360 --> 00:27:53,290 There's GAPDH and there's lactate dehydrogenase. 418 00:27:53,290 --> 00:27:55,620 Here's the mechanism for lactate dehydrogenase, 419 00:27:55,620 --> 00:27:57,240 it's a hydride transfer. 420 00:27:57,240 --> 00:28:00,450 The same thing happened at GAPDH with the additional details 421 00:28:00,450 --> 00:28:01,800 of adding the phosphate. 422 00:28:01,800 --> 00:28:05,800 We covered that previously in an earlier lecture. 423 00:28:05,800 --> 00:28:11,533 Note, we also covered before the chemistry of pyruvate kinase 424 00:28:11,533 --> 00:28:12,700 and phosphoglycerate kinase. 425 00:28:15,440 --> 00:28:19,070 In addition, there's two more kinase reactions 426 00:28:19,070 --> 00:28:23,330 that exist here, hexokinase and phosphofructokinase. 427 00:28:23,330 --> 00:28:26,030 These are really just straightforward phosphotransfer 428 00:28:26,030 --> 00:28:29,990 reactions using ATP to transfer a phosphate. 429 00:28:29,990 --> 00:28:32,660 Very straightforward, exactly what 430 00:28:32,660 --> 00:28:35,600 Professor Yaffe has already talked to you about, 431 00:28:35,600 --> 00:28:39,170 about how one does phosphotransfer 432 00:28:39,170 --> 00:28:42,860 for kinase reactions on proteins. 433 00:28:42,860 --> 00:28:46,930 Now, two other steps are isomerase reactions, 434 00:28:46,930 --> 00:28:52,780 triose phosphatase isomerase and glucose phosphate isomerase. 435 00:28:52,780 --> 00:28:56,290 We discussed this chemistry back when we did the sugar lectures. 436 00:28:56,290 --> 00:28:59,920 This is just acting on the open chain form of the carbohydrate 437 00:28:59,920 --> 00:29:02,710 and allows you to interconvert between the ketose 438 00:29:02,710 --> 00:29:06,670 and the aldose as I already drew during that lecture. 439 00:29:06,670 --> 00:29:09,250 And so that just leaves a few steps 440 00:29:09,250 --> 00:29:13,190 left to discuss that we can cover briefly. 441 00:29:13,190 --> 00:29:15,610 The first one that we'll talk about 442 00:29:15,610 --> 00:29:20,740 is this enolase reaction, which is simply a dehydration. 443 00:29:20,740 --> 00:29:25,570 I'll draw it here just to be explicit as to what's going on. 444 00:29:36,810 --> 00:29:41,530 So here is 2-phosphoglycerate. 445 00:29:46,864 --> 00:29:50,490 Simply dehydration to remove water, 446 00:29:50,490 --> 00:30:00,745 and that gives us phosphoenolpyruvate, PEP. 447 00:30:03,400 --> 00:30:08,870 The next step I want to discuss, phosphoglycerate mutase, 448 00:30:08,870 --> 00:30:12,020 is an example of a mutase reaction that is, remember, 449 00:30:12,020 --> 00:30:14,900 it's moving the phosphate from the 3 450 00:30:14,900 --> 00:30:18,530 to the 2 position of glycerate. 451 00:30:18,530 --> 00:30:19,790 This is a class of reactions. 452 00:30:19,790 --> 00:30:23,330 It has somewhat of an interesting mechanism. 453 00:30:23,330 --> 00:30:26,330 And basically precedes by a phosphorylated 454 00:30:26,330 --> 00:30:28,250 enzyme intermediate. 455 00:30:28,250 --> 00:30:31,310 In the case of phosphoglycerate mutase, this is a histidine, 456 00:30:31,310 --> 00:30:36,540 and so there is a histidine with a nitrogen on it 457 00:30:36,540 --> 00:30:38,580 in the active site. 458 00:30:38,580 --> 00:30:44,160 And effectively, this nitrogen can pick up a phosphate and be 459 00:30:44,160 --> 00:30:52,290 primed via a reaction involving a molecule you've seen already 460 00:30:52,290 --> 00:30:56,860 from your discussion of hemoglobin with Professor 461 00:30:56,860 --> 00:31:00,070 Yaffe, and that's 2,3-bisphosphoglycerate. 462 00:31:00,070 --> 00:31:02,560 And so again, here's-- 463 00:31:09,290 --> 00:31:13,980 this is glycerate phosphorylated on the 2 and 3 position. 464 00:31:13,980 --> 00:31:16,310 So this is 2,3-bisphosphoglycerate. 465 00:31:18,840 --> 00:31:27,030 And effectively, this molecule can bind in the active site 466 00:31:27,030 --> 00:31:28,950 and pick up a phosphate from either 467 00:31:28,950 --> 00:31:30,810 of the 2 or the 3 position. 468 00:31:30,810 --> 00:31:34,980 I drew it here is picking it up from the 2 position. 469 00:31:34,980 --> 00:31:39,060 That would, of course, generate a 3-phosphoglycerate. 470 00:31:39,060 --> 00:31:43,980 But more importantly, it ends up with the enzyme 471 00:31:43,980 --> 00:31:49,020 having this phosphorylated intermediate 472 00:31:49,020 --> 00:31:51,420 in the active site. 473 00:31:51,420 --> 00:31:54,360 Once it has that phosphorylated intermediate 474 00:31:54,360 --> 00:32:00,660 in the active site, now it's ready to catalyze 475 00:32:00,660 --> 00:32:02,415 the mutase reaction. 476 00:32:06,640 --> 00:32:08,310 So I'll draw here first, this here 477 00:32:08,310 --> 00:32:11,586 would be 3-phosphoglycerate. 478 00:32:15,932 --> 00:32:19,470 Color the phosphate so you can see what's going on. 479 00:32:19,470 --> 00:32:23,510 This would basically, in the active site of the enzyme, 480 00:32:23,510 --> 00:32:30,410 pick up the phosphate from the active site and generate 481 00:32:30,410 --> 00:32:49,680 transiently this 2,3-bisphosphoglycerate 482 00:32:49,680 --> 00:33:01,510 intermediate, and then it can retransfer the phosphate from 483 00:33:01,510 --> 00:33:06,380 the other position back on to the active site of the enzyme 484 00:33:06,380 --> 00:33:09,820 so it's ready to carry out another catalytic cycle, 485 00:33:09,820 --> 00:33:24,110 and in the process, move the phosphate effectively from 486 00:33:24,110 --> 00:33:27,090 the 3 position to the 2 position. 487 00:33:27,090 --> 00:33:30,410 So you'll see that the phosphate is not actually 488 00:33:30,410 --> 00:33:32,090 being-- the same phosphate is not 489 00:33:32,090 --> 00:33:34,250 being moved within the same molecule, 490 00:33:34,250 --> 00:33:36,950 it's being ping-ponged back and forth 491 00:33:36,950 --> 00:33:40,040 off of this phosphate enzyme intermediate, 492 00:33:40,040 --> 00:33:45,930 and basically this is how cells catalyze various mutase 493 00:33:45,930 --> 00:33:50,750 reactions to move phosphates between hydroxyl groups. 494 00:33:50,750 --> 00:33:52,940 And so you have to prime this once with 495 00:33:52,940 --> 00:33:56,060 a separately-generated 2,3-bisphosphoglycerate, 496 00:33:56,060 --> 00:33:59,180 but once you have that phosphorylated enzyme 497 00:33:59,180 --> 00:34:02,690 in the active site, it can now continually ping-pong that 498 00:34:02,690 --> 00:34:07,190 phosphate around to effectively convert 3-phosphoglycerate 499 00:34:07,190 --> 00:34:10,530 to 2-phosphoglycerate, or an example of another mutase 500 00:34:10,530 --> 00:34:13,190 reaction, move the phosphate between two other hydroxyl 501 00:34:13,190 --> 00:34:16,010 groups on the same molecule. 502 00:34:16,010 --> 00:34:17,489 OK. 503 00:34:17,489 --> 00:34:22,370 Now the final reaction that we haven't talked about is this 504 00:34:22,370 --> 00:34:26,389 one, the aldolase reaction, which is a really key step 505 00:34:26,389 --> 00:34:30,500 in glycolysis because it splits that carbon-carbon bond to take 506 00:34:30,500 --> 00:34:32,000 the hexose-- 507 00:34:32,000 --> 00:34:36,320 fructose 1,6-bisphosphate, and split it into two trioses, 508 00:34:36,320 --> 00:34:40,290 dihydroxyacetone phosphate and glyceraldehyde 3-phosphate, 509 00:34:40,290 --> 00:34:47,420 which ultimately allows you to make this pathway work. 510 00:34:47,420 --> 00:34:53,880 And so I'll take you quickly through how aldolase works. 511 00:34:53,880 --> 00:35:02,350 So aldolase also has this amine in the active site. 512 00:35:21,900 --> 00:35:26,830 So here's FBP drawn in the open chain form. 513 00:36:35,330 --> 00:36:38,030 So you end up with this intermediate 514 00:36:38,030 --> 00:36:46,360 in the active site, which can undergo this chemistry 515 00:36:46,360 --> 00:36:49,930 to split this into the-- 516 00:36:49,930 --> 00:36:53,740 remove the lower half of the molecule. 517 00:36:59,640 --> 00:37:11,960 So you can see here, this generates 518 00:37:11,960 --> 00:37:17,590 this lower half of the molecule, glyceraldehyde 3-phosphate, 519 00:37:17,590 --> 00:38:52,210 as well as the top half, which can regenerate 520 00:38:52,210 --> 00:39:09,410 the active site, as well as the other product, 521 00:39:09,410 --> 00:39:13,170 dihydroxyacetone phosphate. 522 00:39:13,170 --> 00:39:18,970 And so very briefly, that's effectively the chemistry that 523 00:39:18,970 --> 00:39:22,600 allows aldolase to carry out this important reaction 524 00:39:22,600 --> 00:39:26,320 in glycolysis, splitting that fructose 1,6-bisphosphate 525 00:39:26,320 --> 00:39:30,190 into the aldose, glyceraldehyde 3-phosphate, 526 00:39:30,190 --> 00:39:35,510 and the ketose dihydroxyacetone phosphate. 527 00:39:35,510 --> 00:39:36,620 OK. 528 00:39:36,620 --> 00:39:40,730 So that is effectively the chemistry that 529 00:39:40,730 --> 00:39:44,830 allows you to turn glucose into pyruvate, 530 00:39:44,830 --> 00:39:47,440 as well as pyruvate into lactate, 531 00:39:47,440 --> 00:39:50,230 but what about making ethanol? 532 00:39:50,230 --> 00:39:54,190 How does that work? 533 00:39:54,190 --> 00:39:58,580 Now ethanol is an alternative fermentative product. 534 00:39:58,580 --> 00:40:03,640 And so the reactions to make ethanol is basically this. 535 00:40:09,180 --> 00:40:10,350 So here's pyruvate. 536 00:40:16,420 --> 00:40:23,410 First thing that happens is we lose this CO2. 537 00:40:38,180 --> 00:40:42,590 Without a change in oxidation state at the second carbon 538 00:40:42,590 --> 00:40:44,450 at the ketone in pyruvate. 539 00:40:44,450 --> 00:40:47,480 And so this ketone stays the same oxidation state, 540 00:40:47,480 --> 00:40:50,360 it's now an aldehyde. 541 00:40:50,360 --> 00:40:55,580 This is a molecule called acetaldehyde. 542 00:40:59,680 --> 00:41:15,600 And now we can pick up those two electrons from NADH and reduce 543 00:41:15,600 --> 00:41:28,600 this aldehyde to the alcohol, reoxidizing that NADH back 544 00:41:28,600 --> 00:41:47,330 to NAD+ and generating ethyl alcohol or ethanol. 545 00:41:47,330 --> 00:41:53,210 So, the fermentative part is turning the acetaldehyde 546 00:41:53,210 --> 00:41:57,650 into the ethanol because that allows you to dispose of those 547 00:41:57,650 --> 00:42:01,610 two electrons, reduce the aldehyde to an alcohol while 548 00:42:01,610 --> 00:42:08,360 reoxidizing the NADH to an NAD+, which is what allows glycolysis 549 00:42:08,360 --> 00:42:14,650 to continue so that NAD+ is available for the GAPDH step. 550 00:42:14,650 --> 00:42:17,235 How this reaction works is-- 551 00:42:17,235 --> 00:42:17,860 I just drew it. 552 00:42:17,860 --> 00:42:22,870 It's exactly how the lactate dehydrogenase works. 553 00:42:22,870 --> 00:42:26,050 However, what we also now need to discuss 554 00:42:26,050 --> 00:42:27,820 is how you carry out this reaction, 555 00:42:27,820 --> 00:42:33,670 this carboxylation to take this CO2 off of pyruvate 556 00:42:33,670 --> 00:42:37,030 and make acetaldehyde. 557 00:42:37,030 --> 00:42:39,820 Now decarbonization reactions are really 558 00:42:39,820 --> 00:42:43,330 common reactions in metabolism. 559 00:42:43,330 --> 00:42:45,910 They end up being important for lots of pathways. 560 00:42:45,910 --> 00:42:49,870 Obviously if we're going to completely oxidize glucose 561 00:42:49,870 --> 00:42:53,290 into CO2, we have to remove CO2 for a molecule, 562 00:42:53,290 --> 00:42:55,450 and here's the first one-- 563 00:42:55,450 --> 00:43:00,260 first one that we're going to see for how that happens. 564 00:43:00,260 --> 00:43:02,530 Now there's two general mechanisms 565 00:43:02,530 --> 00:43:06,790 that metabolism uses for decarboxylation reactions, 566 00:43:06,790 --> 00:43:11,090 and I want to introduce the concept here of both of them. 567 00:43:11,090 --> 00:43:14,560 Now, the first one here is that these 568 00:43:14,560 --> 00:43:17,080 are referred to as decarboxylation reactions that 569 00:43:17,080 --> 00:43:20,890 occur in the context of either an alpha keto 570 00:43:20,890 --> 00:43:31,890 acid or a beta keto acid. 571 00:43:31,890 --> 00:43:33,720 Now, what do I mean by that? 572 00:43:33,720 --> 00:43:40,280 Well, pyruvate is an alpha keto acid, 573 00:43:40,280 --> 00:43:44,120 and that's because here's a acid group. 574 00:43:44,120 --> 00:43:50,150 The ketone is alpha to the carboxylic acid. 575 00:43:50,150 --> 00:44:02,570 This would be in contrast to this molecule. 576 00:44:02,570 --> 00:44:05,410 So I'll just draw with a generic R group. 577 00:44:05,410 --> 00:44:11,160 Where here, the ketone is beta to the carboxylic acid-- 578 00:44:11,160 --> 00:44:12,720 alpha, beta. 579 00:44:12,720 --> 00:44:15,900 So pyruvate is an alpha keto acid. 580 00:44:15,900 --> 00:44:20,770 This here is a generic beta keto acid. 581 00:44:20,770 --> 00:44:25,620 Now I want to point out that decarboxylations of bait ketone 582 00:44:25,620 --> 00:44:28,500 acids are very favorable, and they're 583 00:44:28,500 --> 00:44:30,585 favorable for the following reason. 584 00:44:34,340 --> 00:44:43,360 And that's because if I remove this CO2 from a beta keto acid, 585 00:44:43,360 --> 00:44:52,900 I'm left with this molecule, which hopefully you'll 586 00:44:52,900 --> 00:44:56,620 recognize as an enol. 587 00:44:56,620 --> 00:45:00,250 And just like we talked about for the pyruvate kinase 588 00:45:00,250 --> 00:45:13,620 reaction, enols much prefer to rearrange into the keto form. 589 00:45:18,030 --> 00:45:22,140 And so decarbonization of a beta keto acid 590 00:45:22,140 --> 00:45:27,000 generates an enol which will rearrange to the ketone. 591 00:45:27,000 --> 00:45:29,100 This becomes very favorable, and so it 592 00:45:29,100 --> 00:45:34,240 becomes favorable to decarboxylate a beta keto acid. 593 00:45:34,240 --> 00:45:37,350 Now, we can't use that exact same chemistry 594 00:45:37,350 --> 00:45:40,420 to decarboxylate an alpha keto acid. 595 00:45:40,420 --> 00:45:43,290 However, we can help that process, 596 00:45:43,290 --> 00:45:45,810 effectively mimicking the same thing that 597 00:45:45,810 --> 00:45:50,330 happens with beta keto acid, by instead introducing a cofactor. 598 00:46:27,360 --> 00:46:29,840 So I introduced the concept of cofactors 599 00:46:29,840 --> 00:46:33,500 when we talked about NAD and NADH. 600 00:46:33,500 --> 00:46:36,620 And remember, cofactors are molecules 601 00:46:36,620 --> 00:46:38,510 that provide functional groups that help 602 00:46:38,510 --> 00:46:42,360 facilitate the chemistry of various reactions. 603 00:46:42,360 --> 00:46:47,960 And so there's a cofactor that facilitates the chemistry that 604 00:46:47,960 --> 00:46:51,920 allows decarboxylation of alpha keto acids, 605 00:46:51,920 --> 00:46:57,950 and that cofactor is abbreviated TPP+, 606 00:46:57,950 --> 00:47:07,680 which stands for the thiamine pyrophosphate. 607 00:47:11,830 --> 00:47:24,180 TPP+, which is derived from the vitamin thiamine, 608 00:47:24,180 --> 00:47:33,670 sometimes also referred to as the vitamin B1. 609 00:47:33,670 --> 00:47:36,850 And so this is what TPP+ looks like. 610 00:48:21,010 --> 00:48:21,640 OK. 611 00:48:21,640 --> 00:48:25,810 So like many of our vitamins or cofactors, 612 00:48:25,810 --> 00:48:27,230 it's a complex molecule. 613 00:48:27,230 --> 00:48:30,550 So this is vitamin B1 or thiamine, 614 00:48:30,550 --> 00:48:37,150 and the cofactor TPP+ is basically the same molecule 615 00:48:37,150 --> 00:48:42,850 with a pyrophosphate group added on the end, TPP+. 616 00:48:42,850 --> 00:48:44,830 Now, to see what's going on, we're 617 00:48:44,830 --> 00:48:53,860 going to focus here just on this part, this reactive part 618 00:48:53,860 --> 00:49:02,870 of the molecule, which is right here, which for simplicity, I 619 00:49:02,870 --> 00:49:04,520 will just draw like this. 620 00:49:10,260 --> 00:49:15,450 So this here is just this basically part of the molecule 621 00:49:15,450 --> 00:49:18,025 here, that's the reactive part of TPP+. 622 00:49:21,140 --> 00:49:24,470 And what makes this cofactor useful 623 00:49:24,470 --> 00:49:46,480 is that this basically can exist in this stabilized carbanion 624 00:49:46,480 --> 00:49:47,860 form. 625 00:49:47,860 --> 00:49:53,530 This stabilized carbanion can react with alpha keto 626 00:49:53,530 --> 00:49:59,620 acid like pyruvate, shown here. 627 00:50:25,580 --> 00:50:32,630 And then this intermediate now becomes 628 00:50:32,630 --> 00:50:37,070 favorable for decarboxylation of the CO2 that's 629 00:50:37,070 --> 00:50:39,200 alpha to the ketone. 630 00:51:38,700 --> 00:51:51,180 And then here, we can regenerate the active TPP+ that we started 631 00:51:51,180 --> 00:51:58,980 with plus the other product, acetaldehyde. 632 00:52:05,950 --> 00:52:14,980 And then that acetaldehyde can then be reduced, 633 00:52:14,980 --> 00:52:26,610 regenerating NAD+ and giving alcohol for fermentation. 634 00:52:26,610 --> 00:52:28,210 OK. 635 00:52:28,210 --> 00:52:34,310 Now incidentally, this is how you produce alcohol. 636 00:52:34,310 --> 00:52:37,790 How you metabolize alcohol is effectively 637 00:52:37,790 --> 00:52:42,560 reversing that last step and further oxidizing 638 00:52:42,560 --> 00:52:44,390 the acetaldehyde. 639 00:52:44,390 --> 00:52:55,970 And so this is ethanol. 640 00:52:55,970 --> 00:53:00,890 So the way ethanol is metabolized. 641 00:53:00,890 --> 00:53:09,370 And so the famous alcohol dehydrogenase enzyme 642 00:53:09,370 --> 00:53:15,030 basically does that reverse reaction, 643 00:53:15,030 --> 00:53:18,840 generates 2 electrons as a hydride ion. 644 00:53:18,840 --> 00:53:21,420 They get transferred to NAD+. 645 00:53:21,420 --> 00:53:30,690 That NAD+ gets reduced to NADH as this alcohol in ethanol is 646 00:53:30,690 --> 00:53:38,280 oxidized to the aldehyde in acetaldehyde. 647 00:53:41,640 --> 00:53:47,580 And this can then be further oxidized, that aldehyde 648 00:53:47,580 --> 00:53:48,180 to the acid. 649 00:54:03,970 --> 00:54:10,890 So here again, two electrons from the hydride iron can 650 00:54:10,890 --> 00:54:13,680 reduce NAD+ to NADH. 651 00:54:13,680 --> 00:54:17,310 And that generates, in the process, 652 00:54:17,310 --> 00:54:22,340 oxidizes the aldehyde to the acid. 653 00:54:22,340 --> 00:54:27,850 In this case, this is acetate. 654 00:54:27,850 --> 00:54:33,340 The acid form it would be acetic acid, also known as vinegar. 655 00:54:33,340 --> 00:54:38,650 And so effectively, the way you turn alcohol into vinegar 656 00:54:38,650 --> 00:54:42,310 is by the microorganisms oxidizing 657 00:54:42,310 --> 00:54:49,180 the alcohol they produce into acetic acid or vinegar. 658 00:54:49,180 --> 00:54:51,190 Now note, this is-- 659 00:54:51,190 --> 00:54:55,430 alcohol metabolism is two oxidation reactions. 660 00:54:55,430 --> 00:54:58,300 So we talked about last time, oxidation reactions 661 00:54:58,300 --> 00:55:01,940 are generally favorable, therefore energy is released. 662 00:55:01,940 --> 00:55:05,350 This is why alcohol has calories and it's 663 00:55:05,350 --> 00:55:10,420 impossible to make diet alcohol, because it will-- 664 00:55:10,420 --> 00:55:12,195 just no way around this. 665 00:55:12,195 --> 00:55:15,100 A couple other things about this, as I mentioned. 666 00:55:15,100 --> 00:55:16,840 This enzyme catalyzes. 667 00:55:16,840 --> 00:55:21,220 This is the famous alcohol dehydrogenase. 668 00:55:21,220 --> 00:55:26,020 This enzyme is an aldehyde dehydrogenase. 669 00:55:26,020 --> 00:55:29,770 The issue is is that this aldehyde hydrogenate 670 00:55:29,770 --> 00:55:34,750 is a less efficient enzyme than alcohol dehydrogenase, 671 00:55:34,750 --> 00:55:39,910 thus it can become rate-limiting for alcohol metabolism. 672 00:55:39,910 --> 00:55:42,340 And so if you overwhelm the system 673 00:55:42,340 --> 00:55:45,910 by drinking too much, effectively this enzyme can't 674 00:55:45,910 --> 00:55:48,930 keep up with this enzyme, you generate 675 00:55:48,930 --> 00:55:52,960 excess of the toxic acetaldehyde product. 676 00:55:52,960 --> 00:55:57,250 Acetaldehyde product is toxic, and this is effectively 677 00:55:57,250 --> 00:56:00,310 what generates hangovers. 678 00:56:00,310 --> 00:56:01,960 It's also the case that there are 679 00:56:01,960 --> 00:56:05,950 individuals with polymorphisms of this aldehyde dehydrogenase. 680 00:56:05,950 --> 00:56:08,800 This was, I believe, discussed in Professor Yaffe's lectures 681 00:56:08,800 --> 00:56:13,060 on enzyme kinetics that have different catalytic efficiency 682 00:56:13,060 --> 00:56:16,270 at this step, and the people who have those polymorphisms 683 00:56:16,270 --> 00:56:18,770 can't metabolize alcohol as well. 684 00:56:18,770 --> 00:56:21,340 They tend to get flushing and whatnot because they build up 685 00:56:21,340 --> 00:56:23,470 this toxic acetaldehyde, and in fact, 686 00:56:23,470 --> 00:56:26,950 can be quite dangerous if they drink too much. 687 00:56:26,950 --> 00:56:31,300 Finally, this also illustrates effectively 688 00:56:31,300 --> 00:56:35,200 why when commercially you want to generate alcohol, 689 00:56:35,200 --> 00:56:40,580 like in beer or a wine, you need to keep oxygen levels low. 690 00:56:40,580 --> 00:56:41,480 Why is that? 691 00:56:41,480 --> 00:56:44,800 Because, well, the microbes will read their ethanol. 692 00:56:44,800 --> 00:56:47,920 It's a perfectly good source of calories for them as well 693 00:56:47,920 --> 00:56:49,870 because of these reactions. 694 00:56:49,870 --> 00:56:53,770 But they generate all this electron waste in the process. 695 00:56:53,770 --> 00:56:56,080 Those electrons have to go somewhere. 696 00:56:56,080 --> 00:56:56,830 Where do they go? 697 00:56:56,830 --> 00:56:59,470 Well, oxygen is a great electron acceptor. 698 00:56:59,470 --> 00:57:03,010 And so to metabolize this ethanol, 699 00:57:03,010 --> 00:57:06,340 they need to put those electrons somewhere. 700 00:57:06,340 --> 00:57:09,190 They need to put them on oxygen. And so oxygen 701 00:57:09,190 --> 00:57:12,730 is required to carry out this ethanol metabolism 702 00:57:12,730 --> 00:57:14,570 by the microorganisms. 703 00:57:14,570 --> 00:57:19,750 And so by keeping oxygen out of your fermentation of alcohol 704 00:57:19,750 --> 00:57:22,690 in commercial beer and wine production, 705 00:57:22,690 --> 00:57:27,730 it prevents the formation of aldehyde and acetic acid 706 00:57:27,730 --> 00:57:32,920 products which would be undesirable in your finished 707 00:57:32,920 --> 00:57:35,045 drink product. 708 00:57:35,045 --> 00:57:35,545 OK. 709 00:57:42,080 --> 00:57:45,740 Now this whole discussion also begins 710 00:57:45,740 --> 00:57:51,260 to suggest one big way in which glycolysis itself is regulated, 711 00:57:51,260 --> 00:57:56,240 and that's effectively by the availability of oxygen. 712 00:57:56,240 --> 00:57:57,930 And so to be explicit about this, 713 00:57:57,930 --> 00:58:01,640 I want to draw the relationship between fermentation, 714 00:58:01,640 --> 00:58:06,980 glycolysis, and oxygen in terms of maintaining electron 715 00:58:06,980 --> 00:58:09,720 balance across these pathways. 716 00:58:09,720 --> 00:58:13,490 And so glucose is, of course, a major fuel for us. 717 00:58:13,490 --> 00:58:17,090 It's a major sugar in our blood. 718 00:58:17,090 --> 00:58:19,910 Yeast use a lot of glucose from the grapes 719 00:58:19,910 --> 00:58:21,940 in their environment. 720 00:58:21,940 --> 00:58:25,780 Plants, of course, make glucose as a storage carbohydrate. 721 00:58:25,780 --> 00:58:29,650 It's-- we discussed, starch is a polymer of glucose that uses 722 00:58:29,650 --> 00:58:31,420 glucose to survive the night. 723 00:58:31,420 --> 00:58:34,240 And effectively what they do is turn that glucose 724 00:58:34,240 --> 00:58:37,660 into two pyruvate molecules which 725 00:58:37,660 --> 00:58:41,980 can be used to generate net 2 ATP as we discussed. 726 00:58:41,980 --> 00:58:49,960 But creates this problem that it also generates NADH that needs 727 00:58:49,960 --> 00:58:52,180 to be recycled back to NAD+. 728 00:58:54,700 --> 00:58:59,260 In the absence of oxygen, you can ferment that pyruvate 729 00:58:59,260 --> 00:59:04,450 into lactate or alcohol or some other product that allows you 730 00:59:04,450 --> 00:59:07,780 to regenerate NAD+. 731 00:59:07,780 --> 00:59:16,220 Or-- and this is a pathway that requires no oxygen. 732 00:59:16,220 --> 00:59:23,950 However, that pyruvate can be further oxidized to CO2, 733 00:59:23,950 --> 00:59:29,170 but that's going to generate even more electron waste. 734 00:59:29,170 --> 00:59:32,590 That electron waste also has to be dealt with, 735 00:59:32,590 --> 00:59:38,190 and that requires oxygen or some other place 736 00:59:38,190 --> 00:59:40,800 to put those electrons, oxygen to water 737 00:59:40,800 --> 00:59:47,580 being a major driver of further oxidative metabolism. 738 00:59:47,580 --> 00:59:52,700 Now the beauty of fermentation as a pathway 739 00:59:52,700 --> 00:59:57,140 is that no oxygen is needed, but the trade-off 740 00:59:57,140 --> 01:00:00,140 is that it's much less efficient. 741 01:00:00,140 --> 01:00:05,900 You only get 2 ATP per mole of glucose that's-- 742 01:00:05,900 --> 01:00:09,860 2 moles of ATP per of glucose that's fermented. 743 01:00:09,860 --> 01:00:12,410 You can imagine that a lot more energy is released-- 744 01:00:12,410 --> 01:00:14,840 if you can further oxidized glucose, you can generate, 745 01:00:14,840 --> 01:00:17,360 therefore, a lot more ATP if you use 746 01:00:17,360 --> 01:00:22,940 a complete oxidation of glucose, but this requires oxygen. 747 01:00:22,940 --> 01:00:26,930 And so a major way that glycolysis and certainly 748 01:00:26,930 --> 01:00:33,350 fermentation is regulated is that low oxygen 749 01:00:33,350 --> 01:00:48,840 is what promotes fermentation, whereas high oxygen suppresses 750 01:00:48,840 --> 01:00:52,450 fermentation, and that makes sense. 751 01:00:52,450 --> 01:00:53,120 It's intuitive. 752 01:00:53,120 --> 01:00:54,880 You get a lot more energy. 753 01:00:54,880 --> 01:00:58,050 If you completely oxidize your glucose, 754 01:00:58,050 --> 01:01:00,840 you can more efficiently get energy 755 01:01:00,840 --> 01:01:06,330 from oxidation of available reduced glucose carbon 756 01:01:06,330 --> 01:01:08,770 if you do it by oxidative pathways, 757 01:01:08,770 --> 01:01:11,820 and so therefore, in the presence of plenty of oxygen, 758 01:01:11,820 --> 01:01:14,670 you don't need to do fermentation. 759 01:01:14,670 --> 01:01:19,440 Whereas if you outstrip oxygen supply, as, say, 760 01:01:19,440 --> 01:01:24,090 might happen if your muscles are exercising, burning more ATP 761 01:01:24,090 --> 01:01:28,170 than can be kept up with by oxygen delivery from the blood, 762 01:01:28,170 --> 01:01:31,320 now you switch over to a more affirmative metabolism, 763 01:01:31,320 --> 01:01:34,080 something like lactic acid builds up 764 01:01:34,080 --> 01:01:37,260 that can then be dealt with later. 765 01:01:37,260 --> 01:01:42,990 Now, this fact that high oxygen can suppress fermentation 766 01:01:42,990 --> 01:01:46,890 is experimentally true in many organisms in many contexts. 767 01:01:46,890 --> 01:01:50,640 It's likely because the high ATP-ADP that's 768 01:01:50,640 --> 01:01:56,040 produced with oxidative glucose metabolism 769 01:01:56,040 --> 01:02:00,570 can directly affect some steps or deplete the NAD that's 770 01:02:00,570 --> 01:02:02,610 required to drive glycolysis. 771 01:02:02,610 --> 01:02:04,620 Exactly the mechanism that causes 772 01:02:04,620 --> 01:02:06,840 this is somewhat debated and likely 773 01:02:06,840 --> 01:02:10,120 depends on context and conditions. 774 01:02:10,120 --> 01:02:13,450 But it also sets out at least a way 775 01:02:13,450 --> 01:02:16,210 for us to begin discussing, how is it 776 01:02:16,210 --> 01:02:20,350 at a level of individual enzymes or reactions might 777 01:02:20,350 --> 01:02:24,580 something like high ATP-ADP ratio effect flux 778 01:02:24,580 --> 01:02:26,395 or flow through a pathway? 779 01:03:09,400 --> 01:03:09,900 All right. 780 01:03:09,900 --> 01:03:11,400 Well, let's just start by talking 781 01:03:11,400 --> 01:03:13,920 about what are the two things that could affect flux 782 01:03:13,920 --> 01:03:15,090 or flow through the pathway? 783 01:03:15,090 --> 01:03:25,620 Well, remember, thermodynamics, delta G, 784 01:03:25,620 --> 01:03:28,750 is what determines whether a pathway happens at all. 785 01:03:28,750 --> 01:03:31,530 And because delta G is proportional to things 786 01:03:31,530 --> 01:03:34,740 like the ATP-ADP ratio, you can imagine 787 01:03:34,740 --> 01:03:37,590 if this ratio gets too high, some of the steps 788 01:03:37,590 --> 01:03:41,450 might no longer be favorable to occur. 789 01:03:41,450 --> 01:03:43,970 That's certainly one possibility. 790 01:03:43,970 --> 01:03:48,740 But the other way that regulation can be done 791 01:03:48,740 --> 01:03:55,280 is actually kinetic, and this is because even though enzymes 792 01:03:55,280 --> 01:03:57,890 don't determine whether a reaction can happen, 793 01:03:57,890 --> 01:04:00,680 at least thermodynamically, they oftentimes 794 01:04:00,680 --> 01:04:03,140 do determine the rate of the reaction, 795 01:04:03,140 --> 01:04:06,170 and when we're talking about rate and flow through-- 796 01:04:06,170 --> 01:04:08,540 flux through pathways, flow through pathways, 797 01:04:08,540 --> 01:04:10,890 now rate can become very important. 798 01:04:10,890 --> 01:04:13,280 And so the kinetics of enzymes, exactly what 799 01:04:13,280 --> 01:04:16,880 you talked-- heard about with Professor Yaffe, properties 800 01:04:16,880 --> 01:04:20,960 like V max or Km of an enzyme, which, of course, can 801 01:04:20,960 --> 01:04:24,470 be affected by allostery, and Professor Yaffe covered 802 01:04:24,470 --> 01:04:28,040 in great detail, these also, you can imagine, 803 01:04:28,040 --> 01:04:32,840 can affect how a pathway is regulated. 804 01:04:32,840 --> 01:04:37,550 And of course, things like ATP, ADP, and AMP as small molecules 805 01:04:37,550 --> 01:04:42,950 can be good allosteric affectors of some enzymes. 806 01:04:42,950 --> 01:04:45,380 And so let's now talk about, if we're 807 01:04:45,380 --> 01:04:49,820 going to regulate steps in a pathway, which steps should 808 01:04:49,820 --> 01:04:52,520 actually be regulated. 809 01:04:52,520 --> 01:04:58,280 And so shown here is, again, coming back to our delta G 810 01:04:58,280 --> 01:05:02,240 change map across glycolysis, as we alluded to earlier, 811 01:05:02,240 --> 01:05:03,920 the steps where it's hard to go back 812 01:05:03,920 --> 01:05:07,730 is this hexokinase step, this phosphofructokinase step, 813 01:05:07,730 --> 01:05:09,950 and this pyruvate kinase step, because these 814 01:05:09,950 --> 01:05:12,440 are where there's the big change in delta G 815 01:05:12,440 --> 01:05:15,920 that now makes it hard to reverse those steps. 816 01:05:15,920 --> 01:05:19,610 And it's exactly those steps that are regulated, 817 01:05:19,610 --> 01:05:20,900 because that makes sense. 818 01:05:20,900 --> 01:05:23,900 If this delta G is thermodynamically driving flux 819 01:05:23,900 --> 01:05:26,930 through the pathway, changing the rate 820 01:05:26,930 --> 01:05:29,420 of the enzymes that catalyze those steps 821 01:05:29,420 --> 01:05:34,490 is also going to change the rate at which the pathway as a whole 822 01:05:34,490 --> 01:05:35,750 can happen. 823 01:05:35,750 --> 01:05:38,780 Now as a result, it's these steps, hexokinase, 824 01:05:38,780 --> 01:05:41,330 phosphofructokinase, pyruvate kinase, 825 01:05:41,330 --> 01:05:45,020 that are often referred to as the rate-limiting steps 826 01:05:45,020 --> 01:05:47,450 of glycolysis. 827 01:05:47,450 --> 01:05:51,890 Personally, I don't like referring to memorizing things 828 01:05:51,890 --> 01:05:54,710 like these are the rate-limiting steps of the pathway, 829 01:05:54,710 --> 01:05:57,500 because in reality, what step of any pathway 830 01:05:57,500 --> 01:05:59,630 is going to depend on the context. 831 01:05:59,630 --> 01:06:03,350 However, often this concept of rate limitation when people 832 01:06:03,350 --> 01:06:06,110 refer to pathways really is coming 833 01:06:06,110 --> 01:06:11,600 from this energetic argument about which steps 834 01:06:11,600 --> 01:06:15,050 have the biggest drop in delta G, and it's important for you 835 01:06:15,050 --> 01:06:17,570 to understand that, not just memorize 836 01:06:17,570 --> 01:06:20,130 what steps are rate-limiting. 837 01:06:20,130 --> 01:06:22,970 Now I also want to point out that these steps actually 838 01:06:22,970 --> 01:06:26,280 are in interesting places in glycolysis. 839 01:06:26,280 --> 01:06:29,130 So this is basically entry into the pathway, 840 01:06:29,130 --> 01:06:32,190 and this is exit from the pathway. 841 01:06:32,190 --> 01:06:33,560 And this makes sense, too. 842 01:06:33,560 --> 01:06:37,460 You don't want to start committing carbon 843 01:06:37,460 --> 01:06:42,120 into a pathway unless you're actually going to need it, 844 01:06:42,120 --> 01:06:44,870 and you also need to match output to input. 845 01:06:44,870 --> 01:06:48,020 And what you'll see is that many steps in pathways 846 01:06:48,020 --> 01:06:50,210 that are regulated, in addition to being 847 01:06:50,210 --> 01:06:52,760 the ones with the biggest delta G changes, 848 01:06:52,760 --> 01:06:57,080 are also often the entrance and exit from the pathway, which, 849 01:06:57,080 --> 01:06:59,390 of course, is the case in glycolysis, 850 01:06:59,390 --> 01:07:01,880 but also in most pathways it makes 851 01:07:01,880 --> 01:07:06,320 a lot of sense just from logical considerations. 852 01:07:06,320 --> 01:07:06,920 All right. 853 01:07:06,920 --> 01:07:08,810 Now if we're going to control things 854 01:07:08,810 --> 01:07:12,230 that the level of enzymes and rate, 855 01:07:12,230 --> 01:07:16,580 how can we control rate of a reaction? 856 01:07:16,580 --> 01:07:20,030 That is, how can we control enzyme kinetics? 857 01:07:20,030 --> 01:07:23,270 Well, if you remember from Professor Yaffe's lectures, 858 01:07:23,270 --> 01:07:28,160 you can increase the rate of a reaction-- that is, 859 01:07:28,160 --> 01:07:29,960 catalyzed by an enzyme-- 860 01:07:29,960 --> 01:07:34,790 by increasing the Vmax of that enzyme; or, if 861 01:07:34,790 --> 01:07:37,850 the substrate concentration is close to Km, 862 01:07:37,850 --> 01:07:43,700 by lowering the Km of that enzyme. 863 01:07:43,700 --> 01:07:45,672 How can you decrease the rate? 864 01:07:45,672 --> 01:07:47,130 Of course, you can do the opposite. 865 01:07:47,130 --> 01:07:52,640 You can decrease the Vmax; or, if the substrate concentration 866 01:07:52,640 --> 01:07:57,890 is close to Km, you can increase the Km. 867 01:07:57,890 --> 01:07:59,600 So how can I change these things? 868 01:07:59,600 --> 01:08:05,060 How can I alter Vmax or Km of an individual enzyme? 869 01:08:05,060 --> 01:08:07,160 Well, there's a number of ways I can do this. 870 01:08:07,160 --> 01:08:09,515 One is is I can make more enzyme. 871 01:08:14,445 --> 01:08:16,840 What will making more enzyme do? 872 01:08:16,840 --> 01:08:21,670 Well, effectively that will increase the Vmax of an enzyme. 873 01:08:21,670 --> 01:08:23,170 Or if I get rid of enzyme, that will 874 01:08:23,170 --> 01:08:26,229 decrease the Vmax of an enzyme. 875 01:08:26,229 --> 01:08:35,439 I can use a different version of the same enzyme. 876 01:08:35,439 --> 01:08:39,069 That is, have an enzyme like pyruvate kinase 877 01:08:39,069 --> 01:08:41,560 but have it come in a bunch of different varieties, 878 01:08:41,560 --> 01:08:44,620 have different genes encode different pyruvate kinase 879 01:08:44,620 --> 01:08:49,540 isoforms such that those isoforms have different Vmax 880 01:08:49,540 --> 01:08:53,710 and Km relationships with respect to substrate, 881 01:08:53,710 --> 01:08:55,990 and effectively, by using a different version 882 01:08:55,990 --> 01:09:00,490 of the enzyme, a so-called isoform of an enzyme, 883 01:09:00,490 --> 01:09:04,540 you can now have different properties. 884 01:09:04,540 --> 01:09:07,990 Now you'll note that the change enzyme 885 01:09:07,990 --> 01:09:14,260 kinetics this way basically is new transcription translation. 886 01:09:14,260 --> 01:09:17,800 That's fairly long on the timescale 887 01:09:17,800 --> 01:09:24,010 of needing to adapt to metabolism, but that can work. 888 01:09:24,010 --> 01:09:26,725 But sometimes a lion's attacking you, 889 01:09:26,725 --> 01:09:28,149 you need to run away quickly, you 890 01:09:28,149 --> 01:09:31,270 may not have time to make more enzyme. 891 01:09:31,270 --> 01:09:35,350 You also need direct control of the enzymes, 892 01:09:35,350 --> 01:09:39,370 and this is where allosteric regulation of enzyme function 893 01:09:39,370 --> 01:09:40,750 comes into play. 894 01:09:40,750 --> 01:09:45,890 Allosteric regulation can be a result of signaling. 895 01:09:45,890 --> 01:09:48,200 Phosphorylate, acetylate an enzyme, 896 01:09:48,200 --> 01:09:50,450 that can change its properties. 897 01:09:50,450 --> 01:09:54,590 Or you can have metabolite binding. 898 01:09:54,590 --> 01:10:00,260 Bind ATP, bind AMP into some allosteric site 899 01:10:00,260 --> 01:10:07,580 that can change Vmax or Km and operate on a short time scale 900 01:10:07,580 --> 01:10:13,655 to allow metabolism to adapt in a more acute setting. 901 01:10:16,240 --> 01:10:19,660 I want to say that time scales matter for real biology 902 01:10:19,660 --> 01:10:22,150 if you want to understand physiology 903 01:10:22,150 --> 01:10:24,880 and how it applies to how metabolism 904 01:10:24,880 --> 01:10:28,030 and physiology relate to biology that you'll 905 01:10:28,030 --> 01:10:30,220 encounter in other contexts. 906 01:10:30,220 --> 01:10:34,360 You realize that some responses need to be acute. 907 01:10:34,360 --> 01:10:38,040 Those allow you to adapt to conditions quickly, 908 01:10:38,040 --> 01:10:41,760 be able to get enough ATP to run away from the lion, et cetera. 909 01:10:41,760 --> 01:10:44,310 But you also need adaptations that 910 01:10:44,310 --> 01:10:46,560 work on longer time scales, because it's 911 01:10:46,560 --> 01:10:49,200 things like this that allow you to enact programs that allow 912 01:10:49,200 --> 01:10:52,950 you to adapt long-term to whatever new conditions a cell 913 01:10:52,950 --> 01:10:55,930 may face in its environment. 914 01:10:55,930 --> 01:10:56,680 All right. 915 01:10:56,680 --> 01:11:00,970 Now, before I get into talking exactly how these things 916 01:11:00,970 --> 01:11:03,610 actually work to regulate glycolysis, 917 01:11:03,610 --> 01:11:08,950 I want to discuss a few details about regulation 918 01:11:08,950 --> 01:11:11,740 of pathways in general. 919 01:11:11,740 --> 01:11:14,590 And while we talked about these with respect to glycolysis, 920 01:11:14,590 --> 01:11:17,860 these points will end up coming up over and over again 921 01:11:17,860 --> 01:11:20,900 and really apply to other pathways. 922 01:11:20,900 --> 01:11:23,650 Now I may not explicitly point this out, 923 01:11:23,650 --> 01:11:26,260 but the same considerations for how glycolysis 924 01:11:26,260 --> 01:11:28,540 works, how it's regulated has to be 925 01:11:28,540 --> 01:11:30,820 true for all metabolic pathways. 926 01:11:30,820 --> 01:11:32,770 And remember, any pathway is just 927 01:11:32,770 --> 01:11:36,040 variations on relatively few chemical-- 928 01:11:36,040 --> 01:11:39,340 chemistry reactions that are repurposed in a way 929 01:11:39,340 --> 01:11:44,300 to build some other thing that's useful to the cell, 930 01:11:44,300 --> 01:11:47,540 and you'll see this over and over again. 931 01:11:47,540 --> 01:11:51,970 Now, it has to be true for every pathway 932 01:11:51,970 --> 01:11:55,180 that it has to be thermodynamically favorable, 933 01:11:55,180 --> 01:11:58,120 and that means either that the pathway itself, glucose 934 01:11:58,120 --> 01:12:01,600 to pyruvate, is thermodynamically favorable, 935 01:12:01,600 --> 01:12:04,720 or it's coupled to some kind of energy input, 936 01:12:04,720 --> 01:12:11,030 like ATP hydrolysis that allows the pathway to be favorable. 937 01:12:11,030 --> 01:12:13,580 Every pathway is going to be built in a way 938 01:12:13,580 --> 01:12:16,220 that intermediates are generated along the way that 939 01:12:16,220 --> 01:12:18,410 allow that pathway to accomplish its goals. 940 01:12:18,410 --> 01:12:21,230 In the case of glycolysis it was the two steps 941 01:12:21,230 --> 01:12:24,290 that allowed us to synthesize and incorporate phosphate 942 01:12:24,290 --> 01:12:27,260 despite the high ATP-ADP ratio in cells. 943 01:12:27,260 --> 01:12:29,330 And in all cases this has to obey 944 01:12:29,330 --> 01:12:30,650 the laws of thermodynamics. 945 01:12:30,650 --> 01:12:33,950 That is, we have to obey constant conservation of mass. 946 01:12:33,950 --> 01:12:37,220 That means we have to account for all carbon, all electrons, 947 01:12:37,220 --> 01:12:42,110 delta G has to be less than 0 across the entire pathway. 948 01:12:42,110 --> 01:12:45,920 You'll see that the regulated steps will often 949 01:12:45,920 --> 01:12:49,940 be those with the largest thermodynamic considerations, 950 01:12:49,940 --> 01:12:51,640 like the big drops across hexokinase, 951 01:12:51,640 --> 01:12:54,920 phosphofructokinase, and pyruvate kinase. 952 01:12:54,920 --> 01:12:56,900 They'll often be at the entrance and exits 953 01:12:56,900 --> 01:12:58,640 to pathway because you want to regulate, 954 01:12:58,640 --> 01:13:00,350 you don't want to commit something to a pathway 955 01:13:00,350 --> 01:13:01,940 if you're not going to use it, and you 956 01:13:01,940 --> 01:13:04,408 want to match input and output. 957 01:13:04,408 --> 01:13:06,200 And all of this is going to have regulation 958 01:13:06,200 --> 01:13:08,540 that acts on both short and long time scales 959 01:13:08,540 --> 01:13:13,310 so it allows it to match physiology. 960 01:13:13,310 --> 01:13:14,240 All right. 961 01:13:14,240 --> 01:13:16,430 Now with that in mind, now let's discuss 962 01:13:16,430 --> 01:13:20,720 how we can regulate glycolysis using both short and long time 963 01:13:20,720 --> 01:13:22,190 scales as-- 964 01:13:22,190 --> 01:13:25,400 using really short time scales, allosteric regulation of 965 01:13:25,400 --> 01:13:29,790 glycolysis in a way that makes it most useful to cells. 966 01:13:29,790 --> 01:13:30,290 All right. 967 01:13:30,290 --> 01:13:37,490 So the first step is uptake of glucose 968 01:13:37,490 --> 01:13:46,030 into the cell, which in animals, like us, is passive, but can 969 01:13:46,030 --> 01:13:51,730 still be regulated by glucose transporters, often regulated 970 01:13:51,730 --> 01:13:53,290 glute proteins. 971 01:13:53,290 --> 01:13:55,420 And it's really these glucose transporters 972 01:13:55,420 --> 01:13:57,880 that can often determine whether glucose is actually 973 01:13:57,880 --> 01:13:59,680 taken up into cells. 974 01:13:59,680 --> 01:14:04,040 A famous example of this is FDG PET scanning, 975 01:14:04,040 --> 01:14:08,920 which is used to measure glucose uptake into tissues in humans, 976 01:14:08,920 --> 01:14:11,590 where basically a hydroxyl group is replaced 977 01:14:11,590 --> 01:14:13,240 with a positron-emitting fluorine 978 01:14:13,240 --> 01:14:17,320 atom, fluorodeoxyglucose, that is taken up and phosphorylated 979 01:14:17,320 --> 01:14:19,780 in cells, and it's this lighting up 980 01:14:19,780 --> 01:14:23,440 of this on a positron-emitting scan 981 01:14:23,440 --> 01:14:26,830 you can tell that which tissues in the body take up glucose 982 01:14:26,830 --> 01:14:29,020 and that's a property of cancer, and so it 983 01:14:29,020 --> 01:14:32,500 is often used as a way to detect or stage cancer. 984 01:14:32,500 --> 01:14:34,840 Another example of this is insulin. 985 01:14:34,840 --> 01:14:36,940 Basically one of the major actions 986 01:14:36,940 --> 01:14:40,360 of insulin on your tissues is that when glucose is high, 987 01:14:40,360 --> 01:14:42,460 your body makes more insulin. 988 01:14:42,460 --> 01:14:44,920 That causes glucose transporters to be 989 01:14:44,920 --> 01:14:48,490 put on the surface of your muscle and your fat, 990 01:14:48,490 --> 01:14:51,340 which allows that glucose now to get into those cells 991 01:14:51,340 --> 01:14:53,500 and effectively allows those cells 992 01:14:53,500 --> 01:14:55,420 to dispose of the glucose in the blood 993 01:14:55,420 --> 01:14:59,270 and keep glucose in the right physiological range. 994 01:14:59,270 --> 01:15:01,750 And so uptake of glucose, at least in animals, 995 01:15:01,750 --> 01:15:06,880 is one important step that is really controlled largely by 996 01:15:06,880 --> 01:15:11,030 whether or not there is a transporter on the surface. 997 01:15:11,030 --> 01:15:11,530 All right. 998 01:15:11,530 --> 01:15:14,560 Once that's trapped as-- 999 01:15:14,560 --> 01:15:17,380 sorry, once it's in the cell as glucose, 1000 01:15:17,380 --> 01:15:20,710 hexokinase then traps that molecule 1001 01:15:20,710 --> 01:15:25,390 as glucose 6-phosphate. 1002 01:15:25,390 --> 01:15:28,510 And it turns out the glucose 6-phosphate 1003 01:15:28,510 --> 01:15:34,240 is an inhibitor of hexokinase. 1004 01:15:34,240 --> 01:15:35,170 Why is that? 1005 01:15:35,170 --> 01:15:37,780 Well, we will see in the next lecture 1006 01:15:37,780 --> 01:15:41,560 that glucose 6-phosphate can be turned 1007 01:15:41,560 --> 01:15:44,770 into glycogen in the case of mammals or starch 1008 01:15:44,770 --> 01:15:46,400 in the case of plants. 1009 01:15:46,400 --> 01:15:49,270 And so glucose 6-phosphate, if you're going to store glucose, 1010 01:15:49,270 --> 01:15:51,340 you can ship it off into glycogen, 1011 01:15:51,340 --> 01:15:56,350 or you could commit it further down into glycolysis. 1012 01:15:56,350 --> 01:15:58,150 But if you don't need it for a glycolysis 1013 01:15:58,150 --> 01:16:02,260 and your stores are full, you'll build up glucose 6-phosphate, 1014 01:16:02,260 --> 01:16:05,710 you want to inhibit hexokinase to keep things from flowing 1015 01:16:05,710 --> 01:16:08,060 through the system. 1016 01:16:08,060 --> 01:16:17,030 The next step that's regulated is phosphofructokinase. 1017 01:16:17,030 --> 01:16:21,175 So phosphofructokinase, of course, makes FTP. 1018 01:16:27,590 --> 01:16:30,830 The step downstream of that that's regulated 1019 01:16:30,830 --> 01:16:35,600 is pyruvate kinase, PEP to pyruvate, 1020 01:16:35,600 --> 01:16:44,670 and it turns out that FTP will stimulate pyruvate kinase, 1021 01:16:44,670 --> 01:16:51,870 whereas PEP will inhibit phosphofructokinase. 1022 01:16:51,870 --> 01:16:52,590 Makes sense. 1023 01:16:52,590 --> 01:16:54,760 You want to match input to output. 1024 01:16:54,760 --> 01:16:57,930 If you have a lot of FTP, activate the enzyme 1025 01:16:57,930 --> 01:16:59,170 to dispose of it. 1026 01:16:59,170 --> 01:17:04,530 If you're building up PEP, tell you to stop sending molecules 1027 01:17:04,530 --> 01:17:07,200 down the pipeline. 1028 01:17:07,200 --> 01:17:12,360 It turns out, PFK is actually the major regulatory step 1029 01:17:12,360 --> 01:17:14,880 of glycolysis, because this is really 1030 01:17:14,880 --> 01:17:18,810 what commits that carbon to going through glycolysis 1031 01:17:18,810 --> 01:17:22,720 as opposed to upstream of that, it could be stored or not. 1032 01:17:22,720 --> 01:17:25,710 So it's really that step that's a commitment step. 1033 01:17:25,710 --> 01:17:28,230 And so it's also regulated by some 1034 01:17:28,230 --> 01:17:31,210 of the outputs of the pathway. 1035 01:17:31,210 --> 01:17:40,027 And so some of those outputs are ATP downstream 1036 01:17:40,027 --> 01:17:42,360 of pyruvate, which we'll talk about later in the course. 1037 01:17:42,360 --> 01:17:43,890 There's some other molecules. 1038 01:17:43,890 --> 01:17:47,820 Alanine, citrate. 1039 01:17:47,820 --> 01:17:55,440 It turns out that high levels of citrate will also inhibit PFK. 1040 01:17:55,440 --> 01:18:00,780 High levels of ATP will inhibit phosphofructokinase. 1041 01:18:00,780 --> 01:18:11,740 Whereas high levels of AMP will activate phosphofructokinase. 1042 01:18:11,740 --> 01:18:12,760 Makes sense. 1043 01:18:12,760 --> 01:18:17,590 A major output of glycolysis is to keep the ATP-ADP ratio high, 1044 01:18:17,590 --> 01:18:20,260 the energy charge high in the cell. 1045 01:18:20,260 --> 01:18:22,510 If the energy charge is very high, 1046 01:18:22,510 --> 01:18:25,930 high ATP, shut off phosphofructokinase, 1047 01:18:25,930 --> 01:18:27,820 don't need to do more glycolysis. 1048 01:18:27,820 --> 01:18:31,120 If the energy charge low, high AMP, 1049 01:18:31,120 --> 01:18:33,940 activate phosphofructokinase. 1050 01:18:33,940 --> 01:18:37,660 If production of other molecules downstream like citrate 1051 01:18:37,660 --> 01:18:43,180 are high, come back and stop sending things 1052 01:18:43,180 --> 01:18:45,100 into the pathway. 1053 01:18:45,100 --> 01:18:50,260 Turns out, alanine can inhibit pyruvate kinase, another step 1054 01:18:50,260 --> 01:18:52,910 downstream. 1055 01:18:52,910 --> 01:18:54,590 Great. 1056 01:18:54,590 --> 01:18:58,040 This is effectively general regulation, 1057 01:18:58,040 --> 01:19:03,180 allosteric regulation of how glycolysis works. 1058 01:19:03,180 --> 01:19:05,700 And what I wrote is was generally true 1059 01:19:05,700 --> 01:19:09,240 across different tissues, but of course, these regulations 1060 01:19:09,240 --> 01:19:12,090 can be tweaked to be more or less important to match 1061 01:19:12,090 --> 01:19:14,460 physiology and function. 1062 01:19:14,460 --> 01:19:17,100 Now I know many of you are going to take the MCAT. 1063 01:19:17,100 --> 01:19:21,030 And while I would never want to teach just to take a test, 1064 01:19:21,030 --> 01:19:23,070 a couple of things that you should 1065 01:19:23,070 --> 01:19:26,640 remember for that test is phosphofructokinase 1066 01:19:26,640 --> 01:19:28,020 is a major regulator. 1067 01:19:28,020 --> 01:19:32,400 Its big allosteric regulators are AMP, as well as 1068 01:19:32,400 --> 01:19:36,150 ATP and citrate. 1069 01:19:36,150 --> 01:19:38,970 Whereas the other steps, hexokinase, 1070 01:19:38,970 --> 01:19:41,280 you should know that it's regulated by-- negatively 1071 01:19:41,280 --> 01:19:43,410 regulated by glucose 6-phosphate, 1072 01:19:43,410 --> 01:19:49,740 and pyruvate kinase is positively regulated by FBP. 1073 01:19:49,740 --> 01:19:51,870 Now one other point that I want to mention, 1074 01:19:51,870 --> 01:19:55,770 just because it will come up sometimes on MCAT exams, 1075 01:19:55,770 --> 01:20:00,270 is that there's an additional detail about how PFK is 1076 01:20:00,270 --> 01:20:03,300 allosterically regulated, and that's because there's this 1077 01:20:03,300 --> 01:20:07,380 side reaction where fructose 6-phosphate can be 1078 01:20:07,380 --> 01:20:13,800 phosphorylated on the 2 rather than the 1 position to make 1079 01:20:13,800 --> 01:20:17,430 this molecule, fructose 2,6-bisphosphate. 1080 01:20:17,430 --> 01:20:22,680 This is a reaction that is catalyzed by bifunctional 1081 01:20:22,680 --> 01:20:24,570 enzyme called PFK-2/FBPase. 1082 01:20:27,240 --> 01:20:32,160 So basically phosphofructokinase on the 2 position, FBPase, 1083 01:20:32,160 --> 01:20:35,250 we're moving it from the 2 position which allows you 1084 01:20:35,250 --> 01:20:41,190 to regulate the levels of fructose 2,6-bisphosphate. 1085 01:20:41,190 --> 01:20:45,960 It turns out that PFK in glycolysis, which, of course, 1086 01:20:45,960 --> 01:20:53,970 generates FBP, is positively regulated by fructose 1087 01:20:53,970 --> 01:20:56,340 2,6-bisphosphate. 1088 01:20:56,340 --> 01:21:01,080 And the production of fructose 2,6-bisphosphate is something 1089 01:21:01,080 --> 01:21:03,900 that can be inhibited by ATP, inhibited 1090 01:21:03,900 --> 01:21:11,620 by phosphoenolpyruvate, and activated by ATP. 1091 01:21:11,620 --> 01:21:16,890 So exactly what is shown here is allosteric regulation. 1092 01:21:16,890 --> 01:21:20,970 Just some of this allosteric regulation is via this side 1093 01:21:20,970 --> 01:21:22,590 enzyme to make a separate product, 1094 01:21:22,590 --> 01:21:26,130 fructose 2,6-bisphosphate, and there's control theory 1095 01:21:26,130 --> 01:21:30,480 considerations for why that might happen. 1096 01:21:30,480 --> 01:21:31,980 All right. 1097 01:21:31,980 --> 01:21:37,410 So at this point, we've learned how we can start with glucose, 1098 01:21:37,410 --> 01:21:42,140 oxidase that glucose via a pathway that 1099 01:21:42,140 --> 01:21:48,200 now allows us to get the energy that cells need to maintain 1100 01:21:48,200 --> 01:21:52,580 ATP-ADP ratio in a good physiological range 1101 01:21:52,580 --> 01:21:58,830 despite the high ATP-ADP ratio in cells. 1102 01:21:58,830 --> 01:22:04,290 However, it should also be clear that in order for this to work, 1103 01:22:04,290 --> 01:22:07,490 we need a way to actually get that glucose from somewhere 1104 01:22:07,490 --> 01:22:09,350 to begin with. 1105 01:22:09,350 --> 01:22:11,570 You probably learned in high school 1106 01:22:11,570 --> 01:22:14,870 that plants use photosynthesis as a way 1107 01:22:14,870 --> 01:22:16,640 to get energy from the sun. 1108 01:22:16,640 --> 01:22:18,680 Well, the synthesis part of photosynthesis 1109 01:22:18,680 --> 01:22:21,530 is generating the sugar that those plants can then 1110 01:22:21,530 --> 01:22:25,370 use to burn and keep ATP-ADP high at night 1111 01:22:25,370 --> 01:22:27,800 and survive the night until the sun comes up again 1112 01:22:27,800 --> 01:22:31,460 when they can again use energy from the sun directly 1113 01:22:31,460 --> 01:22:33,710 to get energy. 1114 01:22:33,710 --> 01:22:37,070 We as animals evolve the way that we have to eat the plants. 1115 01:22:37,070 --> 01:22:40,090 We have to get the glucose that the plants make, 1116 01:22:40,090 --> 01:22:43,550 but of course, we also use-- 1117 01:22:43,550 --> 01:22:47,870 generate-- have the ability to generate some glucose, 1118 01:22:47,870 --> 01:22:53,030 and this is best illustrated from muscle physiology. 1119 01:22:53,030 --> 01:22:55,640 And that is, we alluded to earlier, 1120 01:22:55,640 --> 01:22:59,210 that if we're running away from the lion, 1121 01:22:59,210 --> 01:23:05,730 our muscles might outstrip the supply of oxygen in the blood 1122 01:23:05,730 --> 01:23:09,620 and basically generate a whole lot of lactate. 1123 01:23:09,620 --> 01:23:12,950 Well, that lactate is perfectly good fuel. 1124 01:23:12,950 --> 01:23:17,270 Just like the yeast, can oxidize or we can reoxidize ethanol. 1125 01:23:17,270 --> 01:23:20,900 We can also use lactate as a fuel, but it turns out, 1126 01:23:20,900 --> 01:23:25,760 a big job of our liver is to take that lactate 1127 01:23:25,760 --> 01:23:29,090 from the blood, as well as other molecules, 1128 01:23:29,090 --> 01:23:33,980 and regenerate glucose so that our blood glucose remains 1129 01:23:33,980 --> 01:23:37,880 in a constant level. 1130 01:23:37,880 --> 01:23:40,070 This cycling of glucose and lactate 1131 01:23:40,070 --> 01:23:45,320 across the body between the muscle and the liver 1132 01:23:45,320 --> 01:23:49,400 is sometimes referred to as the Cori cycle 1133 01:23:49,400 --> 01:23:53,000 and involves a clear set of reactions 1134 01:23:53,000 --> 01:23:56,960 to turn that lactate back into glucose, 1135 01:23:56,960 --> 01:23:59,810 a process called gluconeogenesis. 1136 01:24:03,230 --> 01:24:06,950 And unfortunately we're out of time today, 1137 01:24:06,950 --> 01:24:10,430 but next time I will start off the lecture 1138 01:24:10,430 --> 01:24:15,620 by going through how it is that gluconeogenesis works 1139 01:24:15,620 --> 01:24:20,120 to turn something like lactate back into glucose so 1140 01:24:20,120 --> 01:24:22,520 that organisms have the glucose to begin 1141 01:24:22,520 --> 01:24:26,180 with to run glycolysis. 1142 01:24:26,180 --> 01:24:28,030 Thank you.