1 00:00:00,000 --> 00:00:01,908 [SQUEAKING] 2 00:00:01,908 --> 00:00:03,339 [RUSTLING] 3 00:00:03,339 --> 00:00:05,247 [CLICKING] 4 00:00:10,048 --> 00:00:11,590 MATTHEW VANDER HEIDEN: So today we're 5 00:00:11,590 --> 00:00:17,170 going to begin by going over the Calvin cycle, which 6 00:00:17,170 --> 00:00:20,320 I know I went quite quickly on just 7 00:00:20,320 --> 00:00:22,885 to introduce it at the end of the last lecture. 8 00:00:22,885 --> 00:00:24,760 And so we'll spend a little time there first. 9 00:00:24,760 --> 00:00:26,950 And then we'll move on to discuss 10 00:00:26,950 --> 00:00:30,040 the main topic of today, which is the pentose phosphate 11 00:00:30,040 --> 00:00:31,390 pathway. 12 00:00:31,390 --> 00:00:32,890 But before we get there, I just want 13 00:00:32,890 --> 00:00:37,960 to remind you about the Calvin cycle, which you'll recall 14 00:00:37,960 --> 00:00:40,550 happened in three phases. 15 00:00:40,550 --> 00:00:42,970 So there was the fixation, reduction, 16 00:00:42,970 --> 00:00:45,520 and regeneration phases. 17 00:00:45,520 --> 00:00:50,140 And so remember, fixation was this special reaction that 18 00:00:50,140 --> 00:00:54,220 occurs only in photosynthetic organisms catalyzed by rubisco 19 00:00:54,220 --> 00:00:58,630 ribulose-1,5-bisphosphate carboxylase/oxygenase, 20 00:00:58,630 --> 00:01:01,540 where a ribulose-1,5-bisphosphate 21 00:01:01,540 --> 00:01:06,430 molecule combines with CO2, and then is split to produce 22 00:01:06,430 --> 00:01:08,200 2,3-phosphoglycerates. 23 00:01:08,200 --> 00:01:12,160 This was the reaction that Calvin described 24 00:01:12,160 --> 00:01:16,120 as the dark reaction, that now bears the name of this 25 00:01:16,120 --> 00:01:18,760 as the Calvin cycle. 26 00:01:18,760 --> 00:01:22,120 Once you get those 2,3-phosphoglycerates, 27 00:01:22,120 --> 00:01:26,170 you can then enter them into gluconeogenesis. 28 00:01:26,170 --> 00:01:27,820 This, of course, happens in the stroma 29 00:01:27,820 --> 00:01:31,690 of the chloroplast in eukaryotic photosynthetic organisms, 30 00:01:31,690 --> 00:01:35,590 and it's exactly the reactions for the reduction phase 31 00:01:35,590 --> 00:01:40,210 that we learned about for glycolysis or gluconeogenesis 32 00:01:40,210 --> 00:01:42,650 when we described it at the beginning of the course. 33 00:01:42,650 --> 00:01:48,310 And so that 3PG is turned into glyceraldehyde 3-phosphates and 34 00:01:48,310 --> 00:01:49,690 dihydroxyacetone phosphate. 35 00:01:49,690 --> 00:01:51,790 Of course, that has to go backwards 36 00:01:51,790 --> 00:01:56,110 through the phosphoglycerate kinase as well 37 00:01:56,110 --> 00:02:01,540 as the glyceraldehyde 3-phosphate dehydrogenase step 38 00:02:01,540 --> 00:02:03,850 that we learned about when we discussed 39 00:02:03,850 --> 00:02:05,860 glycolysis and gluconeogenesis. 40 00:02:05,860 --> 00:02:12,160 And so therefore, our cost ATP, as well as need a source of 41 00:02:12,160 --> 00:02:14,320 electrons to go through glyceraldehyde 3-phosphate 42 00:02:14,320 --> 00:02:18,250 dehydrogenase, and that those come from NADPH 43 00:02:18,250 --> 00:02:21,220 when this is occurring in photosynthetic organisms 44 00:02:21,220 --> 00:02:22,840 and the chloroplast. 45 00:02:22,840 --> 00:02:25,610 Once you get these trioses, of course, 46 00:02:25,610 --> 00:02:27,670 you can continue with gluconeogenesis, 47 00:02:27,670 --> 00:02:30,520 combine them at the aldolase step to make fructose 48 00:02:30,520 --> 00:02:31,780 1,6-bisphosphate. 49 00:02:31,780 --> 00:02:36,010 That can be-- take the phosphate off by FPPase 50 00:02:36,010 --> 00:02:38,230 to give you fructose 6-phosphate. 51 00:02:38,230 --> 00:02:43,060 That can then be isomerized to glucose 6-phosphate, which, 52 00:02:43,060 --> 00:02:45,040 of course, could be further-- 53 00:02:45,040 --> 00:02:47,200 remove the phosphate and go to glucose, 54 00:02:47,200 --> 00:02:51,880 or be turned into glucose 1-phosphate or whatever else 55 00:02:51,880 --> 00:02:56,530 to build starch or another storage sugar for the plant-- 56 00:02:56,530 --> 00:02:59,200 combine glucose and a fructose, say, 57 00:02:59,200 --> 00:03:03,130 to make sucrose, table sugar. 58 00:03:03,130 --> 00:03:07,060 And so dose reduction steps we didn't spend a lot of time 59 00:03:07,060 --> 00:03:10,270 on, because it's really gluconeogenesis with the use 60 00:03:10,270 --> 00:03:15,130 of NADPH rather than NADH, as we described it 61 00:03:15,130 --> 00:03:18,700 when we describe gluconeogenesis back 62 00:03:18,700 --> 00:03:20,930 at the early part of the course. 63 00:03:20,930 --> 00:03:25,400 Now, once you run this part of the cycle that, of course, 64 00:03:25,400 --> 00:03:26,440 gives you glucose. 65 00:03:26,440 --> 00:03:29,590 But of course, in order to net run this as a cycle, 66 00:03:29,590 --> 00:03:31,990 much like we saw for the TCA cycle, 67 00:03:31,990 --> 00:03:33,910 that if this is going to run as a cycle, 68 00:03:33,910 --> 00:03:37,720 you need a source of carbon acceptors for the next CO2. 69 00:03:37,720 --> 00:03:40,540 Anaplerosis, that we described in the TCA cycle-- 70 00:03:40,540 --> 00:03:43,690 remember, we needed a source of oxaloacetate 71 00:03:43,690 --> 00:03:46,240 that could net accept the next acytal-CoA 72 00:03:46,240 --> 00:03:48,010 to continue running the cycle. 73 00:03:48,010 --> 00:03:49,180 Same thing happens here. 74 00:03:49,180 --> 00:03:51,970 You need a source of ribulose-1,5-bisphosphate 75 00:03:51,970 --> 00:03:55,270 so rubisco has something to add CO2 to. 76 00:03:55,270 --> 00:03:59,770 And so anaplerosis of this cycle involves regeneration 77 00:03:59,770 --> 00:04:02,680 of ribulose-1,5-bisphosphate. 78 00:04:02,680 --> 00:04:05,260 And that's the confusing set of reactions 79 00:04:05,260 --> 00:04:07,150 that I introduced last time that we'll 80 00:04:07,150 --> 00:04:10,340 spend much more time talking about today. 81 00:04:10,340 --> 00:04:16,180 And so, effectively, this comes from both 6 and 3 carbon 82 00:04:16,180 --> 00:04:19,120 units-- so glyceraldehyde 3-phosphate and fructose 83 00:04:19,120 --> 00:04:25,360 6-phosphate with a series of 2- and 3-carbon swaps that 84 00:04:25,360 --> 00:04:29,620 ultimately generate these three pentoses, xylulose 5-phosphate, 85 00:04:29,620 --> 00:04:33,520 ribose 5-phosphate, and another xylulose 5-phosphate. 86 00:04:33,520 --> 00:04:36,430 Once you have those xylulose 5-phosphate, 87 00:04:36,430 --> 00:04:39,880 you can see differs from ribulose-1,5-bisphosphate 88 00:04:39,880 --> 00:04:42,460 by the stereochemistry at the three position. 89 00:04:42,460 --> 00:04:46,060 And so an epimerase reaction can fix that. 90 00:04:46,060 --> 00:04:47,800 We talked about those earlier. 91 00:04:47,800 --> 00:04:51,610 And then, of course, you need to phosphorylation the 1 position 92 00:04:51,610 --> 00:04:55,150 on the sugar to give you ribulose-1,5-bisphosphate. 93 00:04:55,150 --> 00:04:58,060 That's how you deal with the 2 xylulose 5-phosphates. 94 00:04:58,060 --> 00:05:00,880 And ribose 5-phosphate, of course, 95 00:05:00,880 --> 00:05:03,910 you need to change it from the aldose to the ketose. 96 00:05:03,910 --> 00:05:09,010 So that's an isomerase reaction plus a phosphorylation to give 97 00:05:09,010 --> 00:05:13,140 you ribulose-1,5-bisphosphate. 98 00:05:13,140 --> 00:05:15,450 Now, if you run this entire cycle, 99 00:05:15,450 --> 00:05:18,320 if you're going to net generate a hexose-- 100 00:05:18,320 --> 00:05:21,420 a glucose or some other hexose-- 101 00:05:21,420 --> 00:05:24,630 for the plant to store as a storage sugar-- 102 00:05:24,630 --> 00:05:26,400 of course, if it's coming from CO2, 103 00:05:26,400 --> 00:05:28,590 you need to run the cycle six times 104 00:05:28,590 --> 00:05:33,930 to take six CO2s and net generate one hexose that 105 00:05:33,930 --> 00:05:36,270 can be stored for later. 106 00:05:36,270 --> 00:05:39,000 And so I just want to start by showing you 107 00:05:39,000 --> 00:05:41,460 that this accounting can work. 108 00:05:50,060 --> 00:05:56,920 So if we start here at the top with six 109 00:05:56,920 --> 00:06:02,410 ribulose-1,5-bisphosphates and add 6 CO2 molecules, 110 00:06:02,410 --> 00:06:07,530 you're going to net generate 12 3-phosphoglycerates. 111 00:06:13,820 --> 00:06:19,850 Those 12 3-phosphoglycerates can be turned into, of course, 112 00:06:19,850 --> 00:06:22,500 12 glyceraldehyde 3-phosphate. 113 00:06:22,500 --> 00:06:27,020 So that's going to cost 12 ATPs. 114 00:06:27,020 --> 00:06:33,990 And 12 is from the light reactions and photosynthesis. 115 00:06:33,990 --> 00:06:38,060 And so now you have 12 glyceraldehyde 3-phosphates. 116 00:06:38,060 --> 00:06:42,660 I'm going to divide them up in the following way. 117 00:06:42,660 --> 00:06:46,970 If I take two of those and place them over here, 118 00:06:46,970 --> 00:06:51,320 then take the remaining 10, use five of those 119 00:06:51,320 --> 00:06:57,200 here and turn five more into dihydroxyacetone phosphate-- 120 00:06:57,200 --> 00:07:02,060 so that gives me 5 plus 5 plus 2 equals 12 total. 121 00:07:02,060 --> 00:07:06,140 Of these five glyceraldehyde 3-phosphates and five 122 00:07:06,140 --> 00:07:09,680 dihydroxyacetone phosphates, I can run them through 123 00:07:09,680 --> 00:07:13,385 the aldolase step to generate five fructose 124 00:07:13,385 --> 00:07:18,920 1,6-bisphosphates, turn those five fructose 1,6-bisphosphates 125 00:07:18,920 --> 00:07:23,360 into five fructose 6-phosphates. 126 00:07:23,360 --> 00:07:25,430 And, of course, I need one of those 127 00:07:25,430 --> 00:07:31,210 to ultimately generate a glucose 6-phosphate or whatever 128 00:07:31,210 --> 00:07:36,050 hexose that I'm going to have as my net product. 129 00:07:36,050 --> 00:07:39,870 If I do that, that leaves me four left over. 130 00:07:39,870 --> 00:07:41,630 And so I'm going to allocate them such 131 00:07:41,630 --> 00:07:46,190 that I have two here and two here. 132 00:07:46,190 --> 00:07:50,270 So the two I have here can be combined with the remaining two 133 00:07:50,270 --> 00:07:53,960 glyceraldehyde 3-phosphates to generate 134 00:07:53,960 --> 00:07:57,815 two xylulose 5-phosphates and two 135 00:07:57,815 --> 00:08:02,070 erythrose 4-phosphates, a 5-carbon sugar, 136 00:08:02,070 --> 00:08:04,230 and a 4-carbon sugar. 137 00:08:04,230 --> 00:08:07,920 This 5-carbon sugar-- this xylulose 5-phosphate-- 138 00:08:07,920 --> 00:08:10,800 I can then use a epimerase and ATP. 139 00:08:10,800 --> 00:08:14,970 So that's two ATPs running back through. 140 00:08:14,970 --> 00:08:19,530 And that's going to net give me two ribulose-1,5-bisphosphates, 141 00:08:19,530 --> 00:08:22,860 which I'll draw up there later. 142 00:08:22,860 --> 00:08:25,440 Coming back down here, I have this two erythrose 143 00:08:25,440 --> 00:08:30,900 4-phosphates and two fructose 6-phosphates. 144 00:08:30,900 --> 00:08:32,820 So this was a two carbon swap. 145 00:08:32,820 --> 00:08:35,640 So I took the two carbons for this step 146 00:08:35,640 --> 00:08:38,520 to generate the xylulose 5-phosphate and erythrose 147 00:08:38,520 --> 00:08:39,570 4-phosphate. 148 00:08:39,570 --> 00:08:42,809 I took two carbons from fructose 6-phosphate 149 00:08:42,809 --> 00:08:45,300 added them to glyceraldehyde 3-phosphate. 150 00:08:45,300 --> 00:08:48,810 That's how I got the 5-carbon xylulose 5-phosphate. 151 00:08:48,810 --> 00:08:52,560 And that left me with a 4-carbon erythrose 4-phosphate. 152 00:08:52,560 --> 00:08:54,210 So all the carbons add up. 153 00:08:54,210 --> 00:08:57,420 Now I take this 4-carbon erythrose 4-phosphate 154 00:08:57,420 --> 00:09:01,530 and the two remaining fructose 6-phosphates. 155 00:09:01,530 --> 00:09:04,530 And if I move three carbons from here 156 00:09:04,530 --> 00:09:10,290 to there, what I end up getting is a 7-carbon sedoheptulose 157 00:09:10,290 --> 00:09:16,990 7-phosphate and a 3-carbon glyceraldehyde 3-phosphate. 158 00:09:16,990 --> 00:09:21,060 And so two in, two out-- 159 00:09:21,060 --> 00:09:23,780 now I'm going to move two carbons again-- to carbons 160 00:09:23,780 --> 00:09:25,940 from sedoheptulose 7-phosphate to 161 00:09:25,940 --> 00:09:28,820 the glyceraldehyde 3-phosphate. 162 00:09:28,820 --> 00:09:31,340 And that's going to give me 2. 163 00:09:31,340 --> 00:09:33,200 7 minus 2 is 5. 164 00:09:33,200 --> 00:09:35,150 3 plus 2 is 5. 165 00:09:35,150 --> 00:09:39,500 So two 5-carbon sugars, two ribose 5-phosphates, 166 00:09:39,500 --> 00:09:42,650 and two xylulose 5-phosphates-- 167 00:09:42,650 --> 00:09:45,830 xylulose 5-phosphates through the epimerase, 168 00:09:45,830 --> 00:09:48,290 costing me two ATP. 169 00:09:48,290 --> 00:09:54,320 Ribose 5-phosphate through isomerase, costing me two ATP. 170 00:09:54,320 --> 00:09:59,510 And when I net generate is two ribulose 5-phosphates here, 171 00:09:59,510 --> 00:10:01,490 two ribulose 5-phosphates there, two 172 00:10:01,490 --> 00:10:03,350 ribulose 5-phosphates there. 173 00:10:03,350 --> 00:10:05,360 2 plus 2 plus 2 equals 6. 174 00:10:05,360 --> 00:10:09,260 And so by running this cycle six times, 175 00:10:09,260 --> 00:10:17,330 six ribulose-1,5-bisphosphate in, six CO2 in, six 176 00:10:17,330 --> 00:10:25,430 ribulose-1,5-bisphosphate out, and one hexose out. 177 00:10:25,430 --> 00:10:27,890 So all the accounting adds up. 178 00:10:30,690 --> 00:10:35,660 I know there's a lot of moving pieces there. 179 00:10:35,660 --> 00:10:36,770 It's confusing. 180 00:10:36,770 --> 00:10:40,680 But we'll see as we go through today why it works this way. 181 00:10:40,680 --> 00:10:42,680 However, before we get there, I just 182 00:10:42,680 --> 00:10:48,290 want to stress what we've actually accomplished here. 183 00:10:48,290 --> 00:10:56,090 So basically, we started with six CO2 molecules. 184 00:10:56,090 --> 00:11:05,430 And we added to that 12 NADPHs. 185 00:11:05,430 --> 00:11:09,210 And so those 12 NADPHs came there 186 00:11:09,210 --> 00:11:14,490 through the 3-phosphoglycerate to generate 187 00:11:14,490 --> 00:11:16,950 the glyceraldehyde 3-phosphate-- so running 188 00:11:16,950 --> 00:11:20,250 through the GAPDH reaction using NADPH. 189 00:11:20,250 --> 00:11:22,770 And so we had to do that 12 times 190 00:11:22,770 --> 00:11:28,500 plus a total of 18 ATP molecules. 191 00:11:28,500 --> 00:11:32,430 And so 12 of them up there through the phosphoglycerate 192 00:11:32,430 --> 00:11:38,880 kinase cost me 12 ATPs, plus I needed an additional six ATPs 193 00:11:38,880 --> 00:11:42,900 to regenerate the ribulose-1,5-bisphosphates 194 00:11:42,900 --> 00:11:48,810 re-phosphorylation the six pentoses that I generated 195 00:11:48,810 --> 00:11:52,000 through the regeneration phase of the Calvin cycle. 196 00:11:52,000 --> 00:11:58,850 And so that's 12 plus 2 plus 2 plus 2, or 12 plus 6 equals 18. 197 00:11:58,850 --> 00:12:07,830 And what I net generate is one glucose molecule 198 00:12:07,830 --> 00:12:10,930 from my six CO2. 199 00:12:10,930 --> 00:12:15,420 And, of course, to do this, I needed a lot of energy input. 200 00:12:15,420 --> 00:12:20,610 That comes from the 12 NADPHs and the 18 ATPs, which, 201 00:12:20,610 --> 00:12:29,780 of course, are the products of the light reactions 202 00:12:29,780 --> 00:12:31,010 of photosynthesis. 203 00:12:38,920 --> 00:12:46,450 And so you'll recall that if we go back to our chloroplast, 204 00:12:46,450 --> 00:12:48,500 the light reactions are happening here 205 00:12:48,500 --> 00:13:00,810 at the thylakoid membranes of the chloroplast. 206 00:13:00,810 --> 00:13:02,460 And that's where we're generating 207 00:13:02,460 --> 00:13:07,590 our ATP and our NADPH from the light reactions. 208 00:13:07,590 --> 00:13:11,250 And then here in the stroma of the chloroplast 209 00:13:11,250 --> 00:13:15,300 is where we're doing the dark reactions, and so perfectly 210 00:13:15,300 --> 00:13:19,110 positioned where the ATP and from the light 211 00:13:19,110 --> 00:13:24,090 reactions are there to fuel the dark reactions, 212 00:13:24,090 --> 00:13:28,860 the Calvin cycle, and net have this turn CO2 213 00:13:28,860 --> 00:13:31,830 and generate a pentose-- 214 00:13:31,830 --> 00:13:35,550 glucose, fructose, combine them to make a sucrose, 215 00:13:35,550 --> 00:13:39,480 or a bunch of glucose that we can build as alpha-1,4 polymers 216 00:13:39,480 --> 00:13:44,340 to make starch, depending on the organism and how it chooses 217 00:13:44,340 --> 00:13:50,260 to store its sugars. 218 00:13:50,260 --> 00:13:54,610 So that is the Calvin cycle. 219 00:13:54,610 --> 00:13:59,230 But what I want to do now is look a bit more closely 220 00:13:59,230 --> 00:14:02,080 on why it works this way-- that is, 221 00:14:02,080 --> 00:14:07,870 why nature chose this confusing system with two and three 222 00:14:07,870 --> 00:14:08,710 carbon swaps. 223 00:14:08,710 --> 00:14:10,840 Wouldn't it just have been easier to just, 224 00:14:10,840 --> 00:14:14,750 I don't know, take CO2s and build a glucose 225 00:14:14,750 --> 00:14:16,460 molecule together? 226 00:14:16,460 --> 00:14:18,423 Well of course, the reason it does 227 00:14:18,423 --> 00:14:20,090 this has to do with the chemistry of how 228 00:14:20,090 --> 00:14:22,410 the reactions happen. 229 00:14:22,410 --> 00:14:28,550 And if we look more closely at the various steps and these 230 00:14:28,550 --> 00:14:36,890 swaps that take place in the regeneration phase, 231 00:14:36,890 --> 00:14:41,580 basically what we see is in every single situation-- 232 00:14:41,580 --> 00:14:44,400 so it's basically this reaction, this reaction, 233 00:14:44,400 --> 00:14:50,840 and this reaction, which are the swap part of the regeneration 234 00:14:50,840 --> 00:14:51,650 phase-- 235 00:14:51,650 --> 00:14:55,910 you see that in every case, we're moving two carbons. 236 00:14:55,910 --> 00:14:57,770 And so here two carbons were removed 237 00:14:57,770 --> 00:15:03,200 from fructose 6-phosphate to glyceraldehyde 3-phosphate. 238 00:15:03,200 --> 00:15:06,560 So 6 minus 2 equals 4. 239 00:15:06,560 --> 00:15:08,720 3 plus 2 equals 5. 240 00:15:08,720 --> 00:15:10,490 That's how we get erythrose 4-phosphate 241 00:15:10,490 --> 00:15:12,830 and xylulose 5-phosphate. 242 00:15:12,830 --> 00:15:15,500 Here we started with our fructose 6-phosphate 243 00:15:15,500 --> 00:15:19,820 and our erythrose 4-- phosphate, moved three carbons from here 244 00:15:19,820 --> 00:15:20,820 to there. 245 00:15:20,820 --> 00:15:24,980 So 6 minus 3 equals 3. 246 00:15:24,980 --> 00:15:27,620 4 plus 3 equals 7. 247 00:15:27,620 --> 00:15:29,750 So that's a three carbon swap. 248 00:15:29,750 --> 00:15:31,860 And then here, again, is another two carbon swap-- 249 00:15:31,860 --> 00:15:35,480 so three carbon glyceraldehyde 3-phosphate, 7-carbon 250 00:15:35,480 --> 00:15:37,610 sedoheptulose 7-phosphate. 251 00:15:37,610 --> 00:15:39,590 Move two carbons from here to there. 252 00:15:39,590 --> 00:15:40,910 7 minus 2 is 5. 253 00:15:40,910 --> 00:15:42,470 3 plus 2 is 5. 254 00:15:42,470 --> 00:15:46,160 And that's how we get those two pentoses. 255 00:15:46,160 --> 00:15:47,720 Now, if we look more closely at this, 256 00:15:47,720 --> 00:15:50,510 what you'll see in every case, the swap 257 00:15:50,510 --> 00:15:55,460 involves a ketose and an aldose. 258 00:15:55,460 --> 00:15:58,430 So here we have fructose 6-phosphate. 259 00:15:58,430 --> 00:16:00,320 It's a ketose. 260 00:16:00,320 --> 00:16:02,570 Here we have glyceraldehyde 3-phosphate. 261 00:16:02,570 --> 00:16:03,740 It's an aldose. 262 00:16:03,740 --> 00:16:07,940 And what we end up with is the aldose 263 00:16:07,940 --> 00:16:10,610 getting longer by two carbons to become 264 00:16:10,610 --> 00:16:15,580 a ketose and the ketose getting shorter by two carbons 265 00:16:15,580 --> 00:16:17,020 to become an aldose. 266 00:16:17,020 --> 00:16:18,920 Same thing happens here-- 267 00:16:18,920 --> 00:16:21,940 here we have a ketose and an aldose. 268 00:16:21,940 --> 00:16:23,020 We do the swap. 269 00:16:23,020 --> 00:16:27,550 We have the aldose getting longer by 3 carbons 270 00:16:27,550 --> 00:16:31,570 to become a ketose, and the ketose becoming 271 00:16:31,570 --> 00:16:36,610 shorter three carbons, in this case, to become an aldose. 272 00:16:36,610 --> 00:16:39,610 And lastly, same swap we showed before-- 273 00:16:39,610 --> 00:16:44,410 here we have a ketose and an aldose. 274 00:16:44,410 --> 00:16:51,490 The ketose gets shorter by two carbons to become an aldose. 275 00:16:51,490 --> 00:16:57,205 And the aldose gets longer by two carbons to become a ketose. 276 00:17:05,619 --> 00:17:08,230 So I recognize that it's very hard 277 00:17:08,230 --> 00:17:12,250 to follow that when I'm saying aldose and ketose over and over 278 00:17:12,250 --> 00:17:13,490 and over again. 279 00:17:13,490 --> 00:17:17,349 But effectively in every single case, what we're doing is 280 00:17:17,349 --> 00:17:24,400 we're moving two carbons or three carbons 281 00:17:24,400 --> 00:17:35,180 and we're moving them from a ketose to an aldose. 282 00:17:35,180 --> 00:17:41,090 And so that means that our product, when we do that, 283 00:17:41,090 --> 00:17:49,250 is we end up getting a longer ketose. 284 00:17:49,250 --> 00:18:04,720 So our former aldose plus 2 or plus 3 carbons 285 00:18:04,720 --> 00:18:07,240 becomes a longer ketose. 286 00:18:07,240 --> 00:18:17,940 And then we end up getting a shorter aldose 287 00:18:17,940 --> 00:18:28,920 from the former ketose minus two or minus three carbons. 288 00:18:28,920 --> 00:18:30,930 So if you go and look at that, you 289 00:18:30,930 --> 00:18:32,640 will see in every single case where 290 00:18:32,640 --> 00:18:34,950 we're doing those two and three carbon swaps, 291 00:18:34,950 --> 00:18:39,060 we're effectively following that rule. 292 00:18:39,060 --> 00:18:44,640 So these reactions, which I've abbreviated up there, 293 00:18:44,640 --> 00:18:52,110 are catalyzed by enzymes called TK, 294 00:18:52,110 --> 00:19:01,310 which stands for transketolase or TA, 295 00:19:01,310 --> 00:19:04,640 which stands for transaldolase-- 296 00:19:08,670 --> 00:19:15,500 TK, transketolase, TA, transaldolase. 297 00:19:15,500 --> 00:19:19,730 I want to point out that these are the enzymes that 298 00:19:19,730 --> 00:19:23,945 catalyze the transketolase is going to catalyze the two 299 00:19:23,945 --> 00:19:26,900 carbon swaps. 300 00:19:26,900 --> 00:19:32,030 And the transaldolase is going to catalyze the three carbon 301 00:19:32,030 --> 00:19:33,500 swaps. 302 00:19:33,500 --> 00:19:36,200 And so twice we used transketolase, 303 00:19:36,200 --> 00:19:38,360 once we used transaldolase. 304 00:19:38,360 --> 00:19:44,450 Transketolase moved the two carbons from the ketose 305 00:19:44,450 --> 00:19:49,130 to the aldose, giving you a longer ketose and a shorter 306 00:19:49,130 --> 00:19:50,210 aldose. 307 00:19:50,210 --> 00:19:53,150 And transaldolase also catalyzed, 308 00:19:53,150 --> 00:19:56,810 in this case, the 3-carbon swap, giving you 309 00:19:56,810 --> 00:20:01,250 a longer ketose and a shorter aldose following 310 00:20:01,250 --> 00:20:03,910 the exact same rules. 311 00:20:03,910 --> 00:20:10,220 Now, these enzymes is why it works this way. 312 00:20:10,220 --> 00:20:14,810 And I want to point out that the enzyme names are absolutely not 313 00:20:14,810 --> 00:20:17,300 helpful for remembering what they do. 314 00:20:17,300 --> 00:20:22,340 Both enzymes use a ketose donor and an aldose acceptor. 315 00:20:22,340 --> 00:20:26,240 Both generate products where the former aldose 316 00:20:26,240 --> 00:20:31,040 becomes longer as a ketose and the former ketose 317 00:20:31,040 --> 00:20:34,040 becomes shorter as an aldose. 318 00:20:34,040 --> 00:20:40,730 So don't rely on the enzyme names to remember what they do. 319 00:20:40,730 --> 00:20:42,950 You'll see partially why they're named 320 00:20:42,950 --> 00:20:46,080 what they are as we go through the mechanisms. 321 00:20:46,080 --> 00:20:51,170 But for now, just remember that transketolase, two carbons, 322 00:20:51,170 --> 00:20:54,410 transaldolase, three carbons-- 323 00:20:54,410 --> 00:21:00,290 different numbers of carbons moved, but always follow 324 00:21:00,290 --> 00:21:03,870 the exact same rules. 325 00:21:03,870 --> 00:21:06,870 I know this is confusing and so it's probably better 326 00:21:06,870 --> 00:21:10,110 to go through exactly what is going 327 00:21:10,110 --> 00:21:13,815 on using a couple examples. 328 00:21:18,070 --> 00:21:27,990 So let's start off here with a ketose. 329 00:21:49,970 --> 00:21:52,100 So here is a ketose. 330 00:21:52,100 --> 00:21:58,340 This is, of course, fructose 6-phosphate. 331 00:21:58,340 --> 00:22:02,670 And so fructose 6-phosphate is a ketose. 332 00:22:02,670 --> 00:22:05,360 If we use this step here as an example-- 333 00:22:05,360 --> 00:22:12,200 fructose 6-phosphate and glyceraldehyde 3-phosphate-- 334 00:22:12,200 --> 00:22:14,165 so here we start with our ketose. 335 00:22:16,880 --> 00:22:23,915 And if we also use with it a aldose-- 336 00:22:45,660 --> 00:22:49,125 so here is glyceraldehyde 3-phosphate, 337 00:22:49,125 --> 00:22:51,180 which is, of course, an aldose. 338 00:22:51,180 --> 00:22:59,230 So if we subject this to a transketolase reaction, which, 339 00:22:59,230 --> 00:23:03,970 remember, moving two carbons, we're 340 00:23:03,970 --> 00:23:07,120 going to take the two carbons from the ketose 341 00:23:07,120 --> 00:23:11,050 and move them to the aldose. 342 00:23:11,050 --> 00:23:15,940 So if we do that, what we end up with is-- 343 00:23:15,940 --> 00:23:17,410 if we take two carbons from here, 344 00:23:17,410 --> 00:23:19,810 we end up with a shorter aldose. 345 00:23:47,460 --> 00:23:52,775 And we end up with a longer ketose. 346 00:24:27,820 --> 00:24:34,680 So transketolase reaction-- ketose donor of two carbons, 347 00:24:34,680 --> 00:24:38,970 give them to an aldose, acceptor of two carbons. 348 00:24:38,970 --> 00:24:44,190 That gives me an aldose that's shorter-- 349 00:24:44,190 --> 00:24:46,000 minus 2 carbons. 350 00:24:46,000 --> 00:24:50,670 So the bottom four carbons from fructose 6-phosphate 351 00:24:50,670 --> 00:24:54,360 now become a erythrose 4-phosphate, an aldose. 352 00:24:54,360 --> 00:24:59,140 And here we have our aldose accepts the two carbons. 353 00:24:59,140 --> 00:25:05,490 And so now we generate a ketose that is longer by two carbons. 354 00:25:05,490 --> 00:25:11,640 And that gives me this longer ketose-- 355 00:25:11,640 --> 00:25:16,080 so in this case, xylulose 5-phosphate. 356 00:25:16,080 --> 00:25:19,950 So this is the exact transketolase reaction 357 00:25:19,950 --> 00:25:23,790 that is right there-- this first part, 358 00:25:23,790 --> 00:25:26,970 fructose 6-phosphate glyceraldehyde 3-phosphate 359 00:25:26,970 --> 00:25:31,890 give me an erythrose 4-phosphate and a xylulose 5-phosphate. 360 00:25:31,890 --> 00:25:35,700 So two carbons move from a ketose to an aldose, 361 00:25:35,700 --> 00:25:38,615 gives me a shorter aldose and a longer ketose. 362 00:25:47,460 --> 00:25:53,870 Now, if we do exactly the same thing, 363 00:25:53,870 --> 00:25:57,760 but this time rather than do a transketolase reaction, 364 00:25:57,760 --> 00:26:00,220 do a transaldolase reaction-- so now 365 00:26:00,220 --> 00:26:02,800 I'm going to move three carbons. 366 00:26:02,800 --> 00:26:08,620 So same thing-- ketose donor, aldose acceptor. 367 00:26:08,620 --> 00:26:11,800 So now I'm going to end up with-- if I donate three 368 00:26:11,800 --> 00:26:15,280 carbons from that ketose, what I end up 369 00:26:15,280 --> 00:26:25,280 with is 3 carbons remaining. 370 00:26:37,320 --> 00:26:41,940 And if I start with this 3-carbon as an acceptor, 371 00:26:41,940 --> 00:26:46,320 move three carbons over, now I end up with a longer ketose. 372 00:27:31,910 --> 00:27:35,390 So instead do a transaldolase reaction-- 373 00:27:35,390 --> 00:27:38,540 take three carbons from the ketose, 374 00:27:38,540 --> 00:27:41,120 transfer them to the aldose. 375 00:27:41,120 --> 00:27:44,720 That means the ketose now gets shorter by three carbons 376 00:27:44,720 --> 00:27:46,840 and becomes an aldose. 377 00:27:46,840 --> 00:27:50,590 And the former aldose gets longer by three carbons 378 00:27:50,590 --> 00:27:53,080 and becomes a ketose. 379 00:27:53,080 --> 00:27:55,780 And so if I use these two things as substrate-- fructose 380 00:27:55,780 --> 00:27:58,490 6-phosphate and those are glyceraldehyde 3-phosphate, 381 00:27:58,490 --> 00:27:59,410 what do I generate? 382 00:27:59,410 --> 00:28:03,220 I generate another glyceraldehyde 3-phosphate and 383 00:28:03,220 --> 00:28:05,620 fructose 6-phosphate, where all I do 384 00:28:05,620 --> 00:28:08,710 is scramble which carbons are aware. 385 00:28:08,710 --> 00:28:11,500 And so this, obviously, is not a reaction 386 00:28:11,500 --> 00:28:14,350 I showed for how you do anaplerosis of the Calvin 387 00:28:14,350 --> 00:28:17,830 cycle, but clearly can happen. 388 00:28:17,830 --> 00:28:20,650 These transketolase and transaldolase enzymes 389 00:28:20,650 --> 00:28:22,910 are highly reversible. 390 00:28:22,910 --> 00:28:28,120 And so this is, of course, is happening-- scrambling carbons 391 00:28:28,120 --> 00:28:32,530 around, but there's no net generation or consumption 392 00:28:32,530 --> 00:28:34,390 of anything, because, of course, we're 393 00:28:34,390 --> 00:28:37,180 just moving carbons between a ketose and an aldose, 394 00:28:37,180 --> 00:28:41,560 generating the same ketose and the same aldose using 395 00:28:41,560 --> 00:28:43,480 these particular substrates. 396 00:28:43,480 --> 00:28:47,410 But if we look at how it worked where we showed it here-- 397 00:28:47,410 --> 00:28:50,230 fructose 6-phosphate erythrose 4-phosphate-- 398 00:28:50,230 --> 00:28:53,260 we end up moving those three carbons 399 00:28:53,260 --> 00:28:55,210 onto erythrose 4-phosphate. 400 00:28:55,210 --> 00:28:57,700 That gives us sedoheptulose 7-phosphate 401 00:28:57,700 --> 00:29:01,240 and leaves us with a shorter aldose 402 00:29:01,240 --> 00:29:04,480 glyceraldehyde 3-phosphate. 403 00:29:04,480 --> 00:29:07,930 And so I color-coded the carbons here 404 00:29:07,930 --> 00:29:11,110 to show how transketolase and transaldolase work. 405 00:29:11,110 --> 00:29:14,840 And if you go back over here through the Calvin cycle, 406 00:29:14,840 --> 00:29:19,600 you will note that every single one of the reactions where 407 00:29:19,600 --> 00:29:22,810 I drew transketolase and transaldolase 408 00:29:22,810 --> 00:29:27,540 follows those exact rules. 409 00:29:27,540 --> 00:29:29,820 So why does it work this way? 410 00:29:29,820 --> 00:29:33,120 Well, it works this way because of how transketolase 411 00:29:33,120 --> 00:29:36,630 and transaldolase work as enzymes. 412 00:29:36,630 --> 00:29:40,980 And so as I said at the beginning of the course 413 00:29:40,980 --> 00:29:44,190 and have now alluded to several times, 414 00:29:44,190 --> 00:29:48,360 is that biochemistry is just nature repurposing 415 00:29:48,360 --> 00:29:52,950 the same chemistries, the same reactions, 416 00:29:52,950 --> 00:29:55,140 with slight variations over and over 417 00:29:55,140 --> 00:29:58,420 and over again to build that complexity. 418 00:29:58,420 --> 00:30:01,800 And I think that is very evident here. 419 00:30:01,800 --> 00:30:05,490 So a lot of complexity in what we described, but it's really 420 00:30:05,490 --> 00:30:07,770 just using two enzymes-- transketolase 421 00:30:07,770 --> 00:30:09,300 and transaldolase. 422 00:30:09,300 --> 00:30:11,790 And transketolase and transaldolase basically 423 00:30:11,790 --> 00:30:14,520 use variations on chemistry that we've already 424 00:30:14,520 --> 00:30:17,010 seen to catalyze these reactions, which 425 00:30:17,010 --> 00:30:20,173 is almost certainly why it evolved this way, 426 00:30:20,173 --> 00:30:21,840 because you already have a reaction that 427 00:30:21,840 --> 00:30:23,370 catalyzes something. 428 00:30:23,370 --> 00:30:24,690 You tweak it. 429 00:30:24,690 --> 00:30:27,960 And now you can get these swaps and nature figures 430 00:30:27,960 --> 00:30:31,860 out a way to have it work. 431 00:30:31,860 --> 00:30:35,020 And so what are those reactions? 432 00:30:35,020 --> 00:30:38,100 So transketolase, which, remember, 433 00:30:38,100 --> 00:30:41,550 catalyzes the 2-carbon movement. 434 00:30:41,550 --> 00:30:43,560 That's two carbons that are moving. 435 00:30:43,560 --> 00:30:47,940 It uses TPP+. 436 00:30:47,940 --> 00:30:51,540 So that's, remember, thiamine pyrophosphate. 437 00:30:51,540 --> 00:30:52,950 We saw it before. 438 00:30:52,950 --> 00:30:57,120 It was involved in those decarbonization reactions 439 00:30:57,120 --> 00:30:59,820 of alpha-keto acids. 440 00:30:59,820 --> 00:31:03,090 We saw it in the pyruvate dehydrogenase reaction 441 00:31:03,090 --> 00:31:05,910 to turn pyruvate into acetyl-CoA. 442 00:31:05,910 --> 00:31:08,940 We saw it in ethanol metabolism to turn pyruvate 443 00:31:08,940 --> 00:31:10,380 into acetaldehyde. 444 00:31:10,380 --> 00:31:15,030 And we saw it in the TCA cycle to turn alpha-ketogluterate 445 00:31:15,030 --> 00:31:18,330 into succinate. 446 00:31:18,330 --> 00:31:21,690 And in all of those cases, we decarboxylated-- 447 00:31:21,690 --> 00:31:25,327 that is broke a carbon-carbon bond of an alpha-ketoacid. 448 00:31:27,900 --> 00:31:31,620 And that used TPP+. 449 00:31:31,620 --> 00:31:35,460 And basically what happens in the transketolase reaction is 450 00:31:35,460 --> 00:31:39,850 TPP+ plus is also used to break a carbon-carbon bond. 451 00:31:39,850 --> 00:31:42,660 But this time, rather than releasing CO2, 452 00:31:42,660 --> 00:31:47,070 it's basically moving the 2-carbon unit over 453 00:31:47,070 --> 00:31:52,280 to another molecule, as I'll show in a minute. 454 00:31:52,280 --> 00:31:58,110 Transaldolase, which, remember, moves three carbons, 455 00:31:58,110 --> 00:32:01,320 does not use TPP+. 456 00:32:01,320 --> 00:32:10,290 Instead, it uses an aldolase-like mechanism 457 00:32:10,290 --> 00:32:12,520 to move the three carbons. 458 00:32:12,520 --> 00:32:16,530 And so aldolase, you remember, if you think way back to when 459 00:32:16,530 --> 00:32:20,370 we described glycolysis, aldolase broke 460 00:32:20,370 --> 00:32:23,325 the carbon-carbon bond to take you from fructose 461 00:32:23,325 --> 00:32:28,230 1,6-bisphosphate to glyceraldehyde 3-phosphate 462 00:32:28,230 --> 00:32:31,000 and dihydroxyacetone phosphate. 463 00:32:31,000 --> 00:32:34,500 And so that was done with an active site lysine. 464 00:32:34,500 --> 00:32:37,830 And transaldolase basically repurposed 465 00:32:37,830 --> 00:32:41,130 that reaction-- also has an active site lysine, 466 00:32:41,130 --> 00:32:43,470 breaks a carbon-carbon bond, but this time 467 00:32:43,470 --> 00:32:49,290 to transfer the 3-carbon unit from ketose donor 468 00:32:49,290 --> 00:32:51,108 to an aldose acceptor. 469 00:32:51,108 --> 00:32:52,650 And when we go through the mechanism, 470 00:32:52,650 --> 00:32:55,710 you'll see it will be very reminiscent of, 471 00:32:55,710 --> 00:32:57,720 if you look back in your notes, how 472 00:32:57,720 --> 00:33:02,730 aldolase worked in glycolysis. 473 00:33:02,730 --> 00:33:13,050 So let's start with the mechanism of transketolase. 474 00:33:13,050 --> 00:33:18,240 So transit delays uses thiamine pyrophosphate. 475 00:33:22,070 --> 00:33:27,350 So remember this here is the active part 476 00:33:27,350 --> 00:33:31,310 of thiamine pyrophosphate, the stabilized carbanion. 477 00:34:01,070 --> 00:34:06,740 So let's here just draw a generic ketose donor. 478 00:34:55,000 --> 00:34:59,260 So this intermediate is very similar to what 479 00:34:59,260 --> 00:35:03,145 we saw for the-- 480 00:35:06,030 --> 00:35:10,580 when we did decarboxilation of the alpha-ketoacid. 481 00:35:10,580 --> 00:35:14,930 Basically, this same mechanism would occur. 482 00:35:14,930 --> 00:35:18,200 But rather than release CO2, which gets 483 00:35:18,200 --> 00:35:30,640 released here instead is this shorter aldose 484 00:35:30,640 --> 00:35:34,570 with two carbons that are lost and remain here 485 00:35:34,570 --> 00:35:47,419 on the active site of the enzyme on thiamine pyrophosphate. 486 00:36:02,300 --> 00:36:07,910 Now, this molecule here then needs to be transferred to a-- 487 00:36:14,680 --> 00:36:19,345 so here we have our aldose acceptor. 488 00:37:50,990 --> 00:37:56,030 So that then will regenerate TPP+. 489 00:37:56,030 --> 00:37:59,750 And, of course, what we're left with 490 00:37:59,750 --> 00:38:22,110 is our former aldose, which now becomes a ketose that's 491 00:38:22,110 --> 00:38:24,120 two carbons longer. 492 00:38:24,120 --> 00:38:29,970 And so ketose donor, TPP+ breaks off the two carbons, 493 00:38:29,970 --> 00:38:37,750 releases that ketose two carbons shorter as an aldose. 494 00:38:37,750 --> 00:38:39,730 Those two carbons are then transferred 495 00:38:39,730 --> 00:38:41,320 to an aldose acceptor. 496 00:38:41,320 --> 00:38:46,330 And it becomes two carbons longer and becomes a ketose. 497 00:38:46,330 --> 00:38:48,430 And so the net result is is that we 498 00:38:48,430 --> 00:38:51,970 take two carbons from the ketose donor 499 00:38:51,970 --> 00:38:54,520 give them to the aldose acceptor. 500 00:38:54,520 --> 00:38:57,940 The ketose gets shorter by two carbons 501 00:38:57,940 --> 00:39:00,250 to become a shorter aldose. 502 00:39:00,250 --> 00:39:04,090 And the former aldose acceptor gets longer by two carbons 503 00:39:04,090 --> 00:39:06,610 to become a ketose. 504 00:39:06,610 --> 00:39:11,050 And this is, basically, repurposing very similar 505 00:39:11,050 --> 00:39:15,700 chemistry to what we saw for the decarbonization 506 00:39:15,700 --> 00:39:17,320 of alpha-ketoacids. 507 00:39:21,120 --> 00:39:25,090 So that's how transketolase works. 508 00:39:25,090 --> 00:39:29,620 This is how transaldolase works. 509 00:39:29,620 --> 00:39:37,810 And so this works to move, of course, three carbons. 510 00:39:37,810 --> 00:39:40,410 And so this is going to work much more 511 00:39:40,410 --> 00:39:48,890 like what we saw for the aldolase reaction 512 00:39:48,890 --> 00:39:49,780 in glycolysis. 513 00:40:18,970 --> 00:40:33,600 So let's here use this here as our generic ketose donor. 514 00:40:33,600 --> 00:40:37,345 So this is going to be a donor of three carbons-- 515 00:40:45,670 --> 00:40:48,550 so active site lysine and transaldolase place. 516 00:42:35,110 --> 00:42:40,210 So basically, this should look very familiar to you 517 00:42:40,210 --> 00:42:45,760 as the mechanism for how aldolase worked to break 518 00:42:45,760 --> 00:42:49,240 the carbon-carbon bond to divide fructose 1,6-bisphosphonate 519 00:42:49,240 --> 00:42:53,170 to glyceraldehyde 3-phosphate and dihydroxyacetone phosphate, 520 00:42:53,170 --> 00:42:58,900 except, in this case, what we're left with is taking this ketose 521 00:42:58,900 --> 00:43:00,190 donor. 522 00:43:00,190 --> 00:43:05,530 And it releases an aldose that's three carbons shorter 523 00:43:05,530 --> 00:43:13,560 while retaining, down to the enzyme, the three 524 00:43:13,560 --> 00:43:25,460 carbons from the ketose donor, just like we 525 00:43:25,460 --> 00:43:28,280 saw for the aldolase reaction. 526 00:43:41,150 --> 00:43:44,270 Now, when we saw the aldolase reaction-- 527 00:43:44,270 --> 00:43:49,053 glycolysis, of course-- we just took this off the molecule. 528 00:43:49,053 --> 00:43:51,470 And that's how we ended up with glyceraldehyde 3-phosphate 529 00:43:51,470 --> 00:43:54,560 from the bottom and dihydroxyacetone on the top. 530 00:43:54,560 --> 00:43:56,780 The difference here is is now we're 531 00:43:56,780 --> 00:44:01,460 going to use this as a way to combine 532 00:44:01,460 --> 00:44:06,140 it, move this 3-carbon unit rather than just releasing it, 533 00:44:06,140 --> 00:44:10,985 transferred over to an aldose acceptor. 534 00:46:44,500 --> 00:46:53,350 And then, once we re-take the 3-carbon unit and add it 535 00:46:53,350 --> 00:46:57,750 to the aldose acceptor, you'll see that then you release this, 536 00:46:57,750 --> 00:47:01,230 now, aldose three carbons longer as a ketose-- 537 00:47:01,230 --> 00:47:04,410 again, very similar mechanism, if you look back on your notes, 538 00:47:04,410 --> 00:47:08,670 to how aldolase worked in glycolysis. 539 00:47:08,670 --> 00:47:11,730 And so the net effective the transaldolase reaction 540 00:47:11,730 --> 00:47:16,410 is that we take three carbons from a ketose donor. 541 00:47:16,410 --> 00:47:23,160 We end up releasing an aldose that's three carbons shorter, 542 00:47:23,160 --> 00:47:25,410 take those three carbons, transfer them 543 00:47:25,410 --> 00:47:29,280 to an aldose acceptor, and now generate a ketose 544 00:47:29,280 --> 00:47:33,020 that is three carbons longer. 545 00:47:33,020 --> 00:47:40,190 And so this ultimately moves to carbon units 546 00:47:40,190 --> 00:47:43,610 in the case of transketolase, three carbon units 547 00:47:43,610 --> 00:47:46,190 in the case of transaldolase. 548 00:47:46,190 --> 00:47:51,720 It evolved this way because it's repurposing existing chemistry 549 00:47:51,720 --> 00:47:54,420 to allow these two and three carbon swaps. 550 00:47:54,420 --> 00:47:57,660 That ultimately gives you the flexibility 551 00:47:57,660 --> 00:48:01,620 to build a cycle like the Calvin cycle, where 552 00:48:01,620 --> 00:48:06,180 one can interconvert 3-carbon sugars and 6-carbon sugars 553 00:48:06,180 --> 00:48:11,160 with 5-carbon sugars and ultimately make that cycle 554 00:48:11,160 --> 00:48:12,330 work. 555 00:48:12,330 --> 00:48:16,590 And so on the Calvin cycle this is useful because we can start 556 00:48:16,590 --> 00:48:19,560 with the 5-carbon ribulose-1,5-bisphosphate 557 00:48:19,560 --> 00:48:23,070 and net generate a hexose as a storage sugar, 558 00:48:23,070 --> 00:48:28,530 while regenerating our 5-carbon ribulose-1,5-bisphosphate. 559 00:48:28,530 --> 00:48:32,010 But it has an additional benefit in that this is also a way 560 00:48:32,010 --> 00:48:34,050 to generate 5-carbon sugars. 561 00:48:34,050 --> 00:48:37,590 And so you can see that ribose 5-phosphate 562 00:48:37,590 --> 00:48:41,550 is one of the intermediates in the carbon cycle. 563 00:48:41,550 --> 00:48:45,420 And, of course, ribose is a useful sugar 564 00:48:45,420 --> 00:48:47,010 to generate nucleoside. 565 00:48:47,010 --> 00:48:50,890 So all the time you spent talking about RNA and DNA 566 00:48:50,890 --> 00:48:53,760 with Professor Yaffe, you saw lots 567 00:48:53,760 --> 00:48:57,840 of 5-carbon sugars, lots of pentoses, 568 00:48:57,840 --> 00:49:00,090 as the ribose in those backbones. 569 00:49:00,090 --> 00:49:04,080 And so it's not just-- you know now, not just nucleosides. 570 00:49:04,080 --> 00:49:06,750 It's also part of lots of cofactors-- 571 00:49:06,750 --> 00:49:11,310 NAD, FAD, coenzyme A. All of those cofactors 572 00:49:11,310 --> 00:49:16,620 ended up having nucleosides in them-- 573 00:49:16,620 --> 00:49:20,220 and so lots of ribose needed by the cell 574 00:49:20,220 --> 00:49:22,500 in order to make nucleosides. 575 00:49:22,500 --> 00:49:27,690 And this is a way to, for at least photosynthetic organisms, 576 00:49:27,690 --> 00:49:33,570 the Calvin cycle, a way for them to make ribose to generate 577 00:49:33,570 --> 00:49:35,430 all those nucleosides. 578 00:49:35,430 --> 00:49:39,570 Now, that's great if you're a photosynthetic organism, 579 00:49:39,570 --> 00:49:44,010 because you can run the Calvin cycle and do it in a way 580 00:49:44,010 --> 00:49:49,050 that you can store carbon for the night as some hexose 581 00:49:49,050 --> 00:49:51,090 that you can burn later when you don't 582 00:49:51,090 --> 00:49:53,730 have light to carry out the light reactions 583 00:49:53,730 --> 00:49:55,170 of photosynthesis. 584 00:49:55,170 --> 00:49:57,360 Or, if you want to grow, you can use 585 00:49:57,360 --> 00:50:00,450 it to make nucleosides and replicate your genome, 586 00:50:00,450 --> 00:50:04,230 or make RNA, or make ribosomes. 587 00:50:04,230 --> 00:50:08,370 And in fact, the Calvin cycle generating ribose 588 00:50:08,370 --> 00:50:10,230 is a way to get those. 589 00:50:10,230 --> 00:50:12,330 You'll see when we get to this in a few lectures 590 00:50:12,330 --> 00:50:15,660 that ribose 5-phosphate is the starting point 591 00:50:15,660 --> 00:50:20,880 to generate both purine and pyrimidine nucleotides. 592 00:50:20,880 --> 00:50:25,500 However, this reactions, the Calvin cycle, 593 00:50:25,500 --> 00:50:28,110 is unique to photosynthetic organisms. 594 00:50:28,110 --> 00:50:31,170 But of course animals, including us-- are all 595 00:50:31,170 --> 00:50:35,400 nonphotosynthetic organisms-- also need to make ribose. 596 00:50:35,400 --> 00:50:38,970 And we also need a way to make NADPH. 597 00:50:38,970 --> 00:50:41,640 And we need a source of NADPH for the same reason 598 00:50:41,640 --> 00:50:43,200 plants needed it. 599 00:50:43,200 --> 00:50:48,210 And that is we need something to have an NADPH/NADP ratio that's 600 00:50:48,210 --> 00:50:51,030 useful to keep our glutathione reduced. 601 00:50:51,030 --> 00:50:55,260 That allows us to keep our cells in a reducing state, 602 00:50:55,260 --> 00:50:58,020 battle reactive oxygen species, help 603 00:50:58,020 --> 00:51:01,230 with reductive biosynthesis. 604 00:51:01,230 --> 00:51:04,890 We all know that if we eat too much food, 605 00:51:04,890 --> 00:51:09,010 we can all store those excess calories as fat. 606 00:51:09,010 --> 00:51:10,890 Fat is reduced carbon. 607 00:51:10,890 --> 00:51:13,650 Making their reduced carbon in the fatty acids 608 00:51:13,650 --> 00:51:17,520 is going to require a source of electrons. 609 00:51:17,520 --> 00:51:19,770 All organisms, that source of electrons 610 00:51:19,770 --> 00:51:22,380 is going to be NADPH, which makes sense, 611 00:51:22,380 --> 00:51:26,910 because an NADPH/NADP ratio that's more reduced 612 00:51:26,910 --> 00:51:29,460 can favor reduction reactions. 613 00:51:29,460 --> 00:51:32,010 And in our next lecture, we will see 614 00:51:32,010 --> 00:51:36,270 how that's used to actually store carbon as fat when 615 00:51:36,270 --> 00:51:40,020 there's excess NADPH around. 616 00:51:40,020 --> 00:51:43,810 Now, that makes a lot of sense. 617 00:51:43,810 --> 00:51:48,150 But obviously, as nonphotosynthetic organisms, 618 00:51:48,150 --> 00:51:51,090 we, as well as all non photosynthetic organisms, 619 00:51:51,090 --> 00:51:53,950 can't rely on photosynthesis in the Calvin cycle. 620 00:51:53,950 --> 00:51:57,540 And so we need another pathway that 621 00:51:57,540 --> 00:52:03,240 allows us to do both NADPH production as well 622 00:52:03,240 --> 00:52:07,680 as ribose production. 623 00:52:07,680 --> 00:52:11,400 And that's where we get to the pentose phosphate pathway. 624 00:52:38,960 --> 00:52:43,400 And so the pentose phosphate pathway, the PPP-- 625 00:52:43,400 --> 00:52:50,110 also sometimes referred to as the pentose phosphate shunt-- 626 00:52:50,110 --> 00:52:53,950 is a major pathway that all organisms, 627 00:52:53,950 --> 00:52:56,440 photosynthetic and nonphotosynthetic, 628 00:52:56,440 --> 00:53:02,620 can use to generate NADPH, which is useful for cells 629 00:53:02,620 --> 00:53:08,910 as a source of reducing power, as well as ribose, which 630 00:53:08,910 --> 00:53:10,770 is going to be necessary if we're 631 00:53:10,770 --> 00:53:17,250 going to build nucleosides and cofactors that 632 00:53:17,250 --> 00:53:18,885 contain nucleosides sites. 633 00:53:22,060 --> 00:53:27,790 Now, how one runs the pentose phosphate pathway to get 634 00:53:27,790 --> 00:53:33,640 these two products is conceptually akin to the Calvin 635 00:53:33,640 --> 00:53:35,190 cycle. 636 00:53:35,190 --> 00:53:38,070 But it's a Calvin cycle in reverse. 637 00:53:38,070 --> 00:53:42,420 And so if we look at what really happens in the Calvin cycle, 638 00:53:42,420 --> 00:53:46,710 we start with a 5-carbon sugar, we add CO2, 639 00:53:46,710 --> 00:53:50,730 we reduce it with NADPH and ATP, and we end up 640 00:53:50,730 --> 00:53:54,010 generating a 6-carbon sugar. 641 00:53:54,010 --> 00:53:56,790 And so the pentose phosphate pathway 642 00:53:56,790 --> 00:54:01,320 is we're going to start with a 6-carbon sugar. 643 00:54:01,320 --> 00:54:07,680 We're going to oxidize to generate 644 00:54:07,680 --> 00:54:12,120 CO2 and a 5-carbon sugar. 645 00:54:12,120 --> 00:54:15,210 And if we oxidize, we're going to end up-- those electrons 646 00:54:15,210 --> 00:54:16,710 have to go somewhere. 647 00:54:16,710 --> 00:54:20,970 And so we can put those electrons and net 648 00:54:20,970 --> 00:54:23,700 generate NADPH. 649 00:54:23,700 --> 00:54:27,270 The Calvin cycle is, of course, doing reduction. 650 00:54:27,270 --> 00:54:30,720 Remember, reduced carbon is energy storage. 651 00:54:30,720 --> 00:54:33,870 Oxidizing carbon is energy release. 652 00:54:33,870 --> 00:54:36,930 And so the Calvin cycle, we had to put energy in. 653 00:54:36,930 --> 00:54:39,240 That energy, of course, came from photosynthesis. 654 00:54:39,240 --> 00:54:44,160 That's the magic of us harvesting energy from the sun 655 00:54:44,160 --> 00:54:46,050 in order to do biology. 656 00:54:46,050 --> 00:54:49,590 And so the light products, the NADPH 657 00:54:49,590 --> 00:54:51,030 came from photosynthesis in order 658 00:54:51,030 --> 00:54:53,100 to net run the Calvin cycle. 659 00:54:53,100 --> 00:54:56,400 But just like we saw with glycolysis and gluconeogensis, 660 00:54:56,400 --> 00:54:59,440 if one direction of a pathway requires energy input, 661 00:54:59,440 --> 00:55:02,200 well the other direction is going to be favorable. 662 00:55:02,200 --> 00:55:05,070 And so if we oxidize a 6-carbon sugar 663 00:55:05,070 --> 00:55:07,590 to make a 5-carbon sugar and CO2, 664 00:55:07,590 --> 00:55:11,590 that intuitively, hopefully by now, should be favorable. 665 00:55:11,590 --> 00:55:13,500 And so we can use that energy release 666 00:55:13,500 --> 00:55:20,160 to charge up an NADPH to NDP ratio that now gives the cell 667 00:55:20,160 --> 00:55:23,050 a source of reducing power-- 668 00:55:23,050 --> 00:55:25,440 so energetically makes sense. 669 00:55:25,440 --> 00:55:27,630 Now, this pentose phosphate pathway 670 00:55:27,630 --> 00:55:30,000 is also sometimes called a shunt, 671 00:55:30,000 --> 00:55:32,280 because really the way this works 672 00:55:32,280 --> 00:55:36,690 is as a branch of glycolysis-- a shunt of glycolysis, 673 00:55:36,690 --> 00:55:39,360 if you will, because it's an alternative way 674 00:55:39,360 --> 00:55:42,900 to run glycolysis and get NADPH. 675 00:55:42,900 --> 00:55:50,250 And so the glucose 6-phosphate-- remember, 676 00:55:50,250 --> 00:55:56,070 for glycolysis we can isomerize to give 677 00:55:56,070 --> 00:55:58,260 us fructose 6-phosphate. 678 00:55:58,260 --> 00:56:02,205 And then ultimately in glycolysis, 679 00:56:02,205 --> 00:56:08,600 we can generate glyceraldehyde 3-phosphate down to pyruvate, 680 00:56:08,600 --> 00:56:13,760 which can go to TCA cycle, et cetera, et cetera, et cetera. 681 00:56:13,760 --> 00:56:16,250 Of course, glucose is a 6-carbon sugar. 682 00:56:16,250 --> 00:56:18,770 Fructose is a 6-carbon sugar. 683 00:56:18,770 --> 00:56:23,990 glyceraldehyde 3-phosphate is a 3-carbon sugar-- 684 00:56:23,990 --> 00:56:28,010 aldose, ketose, aldose. 685 00:56:28,010 --> 00:56:30,950 So if we take our glucose 6-phosphate 686 00:56:30,950 --> 00:56:35,210 and we send it into the pentose phosphate 687 00:56:35,210 --> 00:56:38,460 pathway, the pentose phosphate shunt, 688 00:56:38,460 --> 00:56:41,060 and rather than oxidize the glucose 689 00:56:41,060 --> 00:56:44,090 as we did in glycolysis, we oxidize 690 00:56:44,090 --> 00:56:52,230 it to release CO2 and generate a 5-carbon sugar, 691 00:56:52,230 --> 00:56:56,310 like a ribose 5-phosphate. 692 00:56:56,310 --> 00:57:02,600 Now we have a way to get 5-carbon sugars pentoses 693 00:57:02,600 --> 00:57:06,660 by oxidizing glucose 6-phosphate. 694 00:57:06,660 --> 00:57:08,880 Those electrons have to go somewhere. 695 00:57:08,880 --> 00:57:19,750 And so that ends up being a way to generate NADPH. 696 00:57:24,430 --> 00:57:26,800 Once we get those 5-carbon sugars, 697 00:57:26,800 --> 00:57:29,230 that's great if we want to build nucleotides. 698 00:57:29,230 --> 00:57:33,680 But it turns out we could also run 699 00:57:33,680 --> 00:57:37,400 a bunch of transketolase and transaldolase reactions 700 00:57:37,400 --> 00:57:40,580 and interconvert 5-carbon sugars with 6-carbon sugars 701 00:57:40,580 --> 00:57:44,730 and 3-carbon sugars, just as we did in the Calvin cycle. 702 00:57:44,730 --> 00:57:46,250 And if we run it this way, you can 703 00:57:46,250 --> 00:57:49,430 see that I can now make this effectively 704 00:57:49,430 --> 00:57:51,620 an alternative version of glycolysis-- 705 00:57:51,620 --> 00:57:52,910 a shunt, if you will. 706 00:57:52,910 --> 00:57:55,010 Start with glucose 6-phosphate rather than 707 00:57:55,010 --> 00:57:56,900 going the traditional way. 708 00:57:56,900 --> 00:57:59,330 First, go to make 5-carbon sugars 709 00:57:59,330 --> 00:58:04,970 and then bring those carbons back into glycolysis 710 00:58:04,970 --> 00:58:08,390 that I can then further oxidize to pyruvate 711 00:58:08,390 --> 00:58:11,760 and oxidize in the TCA cycle, et cetera. 712 00:58:11,760 --> 00:58:15,020 And so all of this-- so glycolysis happens 713 00:58:15,020 --> 00:58:18,490 and the cytosol, the pentose phosphate 714 00:58:18,490 --> 00:58:21,580 pathway happens in the cytosol. 715 00:58:21,580 --> 00:58:26,650 And really, this can be thought of as two separate phases. 716 00:58:26,650 --> 00:58:36,370 And so this glucose 6-phosphate going to a pentose, 717 00:58:36,370 --> 00:58:40,210 like ribose 5-phosphate, plus CO2, 718 00:58:40,210 --> 00:58:47,345 that is referred to as the oxidative pentose phosphate 719 00:58:47,345 --> 00:58:47,845 pathway-- 720 00:58:58,720 --> 00:59:01,020 the oxidative pentose phosphate pathway, 721 00:59:01,020 --> 00:59:04,410 because it's using oxidation of glucose carbon 722 00:59:04,410 --> 00:59:07,950 to generate ribose 5-phosphate and NADPH. 723 00:59:14,290 --> 00:59:18,760 Or, this swaps here between 5-carbon sugars 724 00:59:18,760 --> 00:59:22,450 and 6-carbon sugars and 3-carbon sugars-- very similar to what 725 00:59:22,450 --> 00:59:29,560 we saw in the Calvin cycle, which is really interconverting 726 00:59:29,560 --> 00:59:36,260 6-carbon and 3-carbon sugars with 5-carbon sugars. 727 00:59:36,260 --> 00:59:42,970 So that is referred to as the nonoxidative pentose 728 00:59:42,970 --> 00:59:43,780 phosphate pathway. 729 00:59:53,308 --> 00:59:54,100 And it makes sense. 730 00:59:54,100 --> 00:59:57,250 All those transketolase and transaldolase reactions 731 00:59:57,250 --> 01:00:00,670 that I just discussed with you in great detail-- 732 01:00:00,670 --> 01:00:04,120 no electron movement, no oxidation reduction, 733 01:00:04,120 --> 01:00:06,340 nonoxidative PPP. 734 01:00:06,340 --> 01:00:08,650 Similar to the Calvin cycle, it's 735 01:00:08,650 --> 01:00:14,530 converting pentoses with trioses and hexoses. 736 01:00:14,530 --> 01:00:22,020 So this should be very clear now that this is actually 737 01:00:22,020 --> 01:00:25,260 a really useful things for cells, 738 01:00:25,260 --> 01:00:30,120 because having this pentose phosphate pathway 739 01:00:30,120 --> 01:00:32,920 work in these two different ways, the oxidative 740 01:00:32,920 --> 01:00:36,810 than the nonoxidative, gives cells a lot of flexibility 741 01:00:36,810 --> 01:00:41,910 to make pentoses when they need ribose to make nucleosides, as 742 01:00:41,910 --> 01:00:46,230 well as NADPH when they need NADPH to deal 743 01:00:46,230 --> 01:00:49,080 with, say, oxidative stress. 744 01:00:49,080 --> 01:00:53,580 Suppose you take a drug, causes oxidative stress in your cells, 745 01:00:53,580 --> 01:00:56,250 pentose phosphate pathway can increase 746 01:00:56,250 --> 01:01:00,270 as a way to make more NADPH. 747 01:01:00,270 --> 01:01:03,720 So if a cell needs NADPH because there's oxidative stress, 748 01:01:03,720 --> 01:01:07,470 it can run the oxidative pentose phosphate pathway 749 01:01:07,470 --> 01:01:09,900 if it also needs to make ribose great 750 01:01:09,900 --> 01:01:13,020 it has a source of pentoses to make ribose. 751 01:01:13,020 --> 01:01:16,110 However, if it just needs NADPH and not ribose, it 752 01:01:16,110 --> 01:01:18,030 and then take those ribose carbons, 753 01:01:18,030 --> 01:01:20,360 re-enter them into glycolysis. 754 01:01:20,360 --> 01:01:26,070 So running this shunt allows you to get NADPH as well as ATP 755 01:01:26,070 --> 01:01:30,150 while oxidizing glucose carbon. 756 01:01:30,150 --> 01:01:34,080 However, if you need ribose and you don't need NADPH, 757 01:01:34,080 --> 01:01:38,190 well, forget the oxidative PPP, just take sugars 758 01:01:38,190 --> 01:01:41,580 from glycolysis and shunt them over via the nonoxidative 759 01:01:41,580 --> 01:01:43,770 PPP to make ribose. 760 01:01:43,770 --> 01:01:47,730 And so you can run this direction if you need NADPH. 761 01:01:47,730 --> 01:01:53,610 And you can run this direction if you just need ribose. 762 01:01:53,610 --> 01:01:59,830 So this is great because it gives cells 763 01:01:59,830 --> 01:02:05,440 a lot of flexibility. 764 01:02:05,440 --> 01:02:11,530 And so these two pathways, really, can work together. 765 01:02:11,530 --> 01:02:13,480 Or, they can work separately depending 766 01:02:13,480 --> 01:02:15,910 on what the cell needs, and can really 767 01:02:15,910 --> 01:02:21,460 be tuned in a way to allow the cell to get both ribose 768 01:02:21,460 --> 01:02:24,550 and NADPH and match it as demands. 769 01:02:24,550 --> 01:02:27,430 And so in a lot of ways, it's probably better 770 01:02:27,430 --> 01:02:31,480 to think of them as two separate pathways-- 771 01:02:31,480 --> 01:02:37,570 a so-called oxidative PPP, which is really glucose 6-phosphate 772 01:02:37,570 --> 01:02:58,840 plus 2 NADP+, generating 2 NADPH plus pentose plus CO2. 773 01:02:58,840 --> 01:03:07,440 And a nonoxidative PPP, which is really 774 01:03:07,440 --> 01:03:21,890 interconverting 3 pentoses with 2 hexoses and 1 triose. 775 01:03:21,890 --> 01:03:26,030 So 3 times 5 is 15 carbons. 776 01:03:26,030 --> 01:03:34,620 2 times 6 is 12 carbons plus 3 carbons, 15 carbons. 777 01:03:34,620 --> 01:03:38,460 This reaction, oxidative PPP, is only 778 01:03:38,460 --> 01:03:40,595 going to be favorable in one direction. 779 01:03:40,595 --> 01:03:41,970 You would need a separate pathway 780 01:03:41,970 --> 01:03:44,012 if you're going to run it the opposite direction. 781 01:03:44,012 --> 01:03:46,870 That doesn't happen in organisms. 782 01:03:46,870 --> 01:03:50,430 So that's, under physiological conditions, a one-way pathway. 783 01:03:50,430 --> 01:03:53,700 You can only go one direction through the oxidative PPP 784 01:03:53,700 --> 01:03:56,840 in cells, whereas a nonoxidative PPP is just 785 01:03:56,840 --> 01:04:02,340 this interconversion of trioses and hexoses with pentoses. 786 01:04:02,340 --> 01:04:06,960 This is very reversible, just like the Calvin cycle 787 01:04:06,960 --> 01:04:08,980 would be very reversible. 788 01:04:08,980 --> 01:04:12,300 And so while they can work together as I described, 789 01:04:12,300 --> 01:04:15,220 it's really useful to think of them as separate pathways. 790 01:04:15,220 --> 01:04:18,120 And so let's go through each of them 791 01:04:18,120 --> 01:04:21,360 separately so you can see how they work. 792 01:04:42,561 --> 01:04:46,940 So let's start first here with the oxidative pentose phosphate 793 01:04:46,940 --> 01:04:47,440 pathway. 794 01:05:02,740 --> 01:05:09,990 So this here, it would be alpha-glucose 6-phosphate. 795 01:06:13,800 --> 01:06:16,800 So first step of the oxidative pentose phosphate 796 01:06:16,800 --> 01:06:20,070 pathway is we're going to oxidize 797 01:06:20,070 --> 01:06:22,960 the 1-carbon of glucose. 798 01:06:22,960 --> 01:06:27,087 That's, of course, going to have two electrons. 799 01:06:27,087 --> 01:06:28,920 So if we oxidize that carbon, something else 800 01:06:28,920 --> 01:06:29,740 has to be reduced. 801 01:06:29,740 --> 01:06:33,210 That hydride ion, those two electrons can go to NADP+ 802 01:06:33,210 --> 01:06:34,560 to make NADPH-- 803 01:06:34,560 --> 01:06:37,650 a reaction that we've now seen a million times. 804 01:06:37,650 --> 01:06:43,350 That oxidizes the alcohol, that carbon 1 to the ketone. 805 01:06:43,350 --> 01:06:45,240 And that gives us this lactone-- 806 01:06:45,240 --> 01:06:47,370 very similar to what we saw as the first step 807 01:06:47,370 --> 01:06:49,920 in the epimerase reaction, when we described that, 808 01:06:49,920 --> 01:06:52,900 to give us this molecule, 6-phosphogluconolactone. 809 01:06:56,010 --> 01:06:59,850 Now, so, in essence, what this did, remember, 810 01:06:59,850 --> 01:07:03,510 is it oxidized-- remember, this 1-carbon 811 01:07:03,510 --> 01:07:07,890 was the aldehyde of glucose, this now becomes, 812 01:07:07,890 --> 01:07:10,650 on this lactose, it's actually an acid. 813 01:07:10,650 --> 01:07:15,480 And that becomes evident if we break open this ring here 814 01:07:15,480 --> 01:07:16,350 with water. 815 01:07:57,940 --> 01:08:01,596 That causes this molecule, 6-phosphogluconate. 816 01:08:05,240 --> 01:08:07,880 Oh, I forgot to mention this first reaction here 817 01:08:07,880 --> 01:08:11,450 is catalyzed by this enzyme, G6PD-- 818 01:08:11,450 --> 01:08:14,660 glucose 6-phosphate dehydrogenase. 819 01:08:14,660 --> 01:08:17,479 Makes sense-- the oxidation reduction reaction, 820 01:08:17,479 --> 01:08:20,982 dehydrogenase, glucose 6-phosphate dehydrogenase. 821 01:08:20,982 --> 01:08:23,149 Just as an aside for those of you who are interested 822 01:08:23,149 --> 01:08:26,960 in medical school, turns out glucose 6-phosphate deficiency 823 01:08:26,960 --> 01:08:31,100 in humans is actually a quite common genetic disorder-- 824 01:08:31,100 --> 01:08:33,500 very common in Mediterranean regions 825 01:08:33,500 --> 01:08:38,420 because it is thought to have evolved because it confers 826 01:08:38,420 --> 01:08:41,160 some resistance to malaria. 827 01:08:41,160 --> 01:08:45,109 And so having less G6PD means you deal less well 828 01:08:45,109 --> 01:08:46,580 with oxidative stress. 829 01:08:46,580 --> 01:08:49,069 And as a result, some foods and drugs 830 01:08:49,069 --> 01:08:53,510 can cause increased symptoms in those patients, 831 01:08:53,510 --> 01:08:59,540 but also having that means that the cells are 832 01:08:59,540 --> 01:09:01,160 more resistant to malaria infection 833 01:09:01,160 --> 01:09:04,490 because the malaria ends up killing the cell 834 01:09:04,490 --> 01:09:07,399 before it can establish an infection because it 835 01:09:07,399 --> 01:09:10,430 causes some oxidative stress. 836 01:09:10,430 --> 01:09:11,970 Back to the pathway-- 837 01:09:11,970 --> 01:09:15,050 so if we open up this lactone, what we have here 838 01:09:15,050 --> 01:09:17,240 is this molecule, 6-phosphor gluconate. 839 01:09:17,240 --> 01:09:20,600 It's basically glucose where the aldehyde has now 840 01:09:20,600 --> 01:09:23,630 been oxidized to the acid. 841 01:09:23,630 --> 01:09:26,479 That's how we generated NADPH. 842 01:09:26,479 --> 01:09:32,540 The next step is we're going to oxidize carbon 3. 843 01:09:32,540 --> 01:09:45,279 So if we oxidize the alcohol on carbon 3, 844 01:09:45,279 --> 01:09:47,319 those electrons have to go somewhere. 845 01:09:53,700 --> 01:09:55,320 Put them on the net plus. 846 01:09:55,320 --> 01:09:58,050 That gives us an edge. 847 01:09:58,050 --> 01:10:06,050 This is catalyzed by 6 phophogluconate dehydrogenase-- 848 01:10:06,050 --> 01:10:09,350 generates an NADPH. 849 01:10:09,350 --> 01:10:16,501 And what ends up being generated is this intermediate. 850 01:10:33,700 --> 01:10:36,520 If you look at this intermediate, 851 01:10:36,520 --> 01:10:42,190 this is a beta-keto acid. 852 01:10:42,190 --> 01:10:49,330 So here's our acid that is alpha beta to the ketone, so 853 01:10:49,330 --> 01:10:51,880 a beta-keto acid. 854 01:10:51,880 --> 01:10:58,240 Remember, decarboxylation of beta-keto acids is favorable-- 855 01:11:07,650 --> 01:11:12,120 same track that we saw before and the TCA 856 01:11:12,120 --> 01:11:15,310 cycle and other places. 857 01:11:15,310 --> 01:11:40,890 And so remember that generates this enol. 858 01:11:40,890 --> 01:11:46,710 And why it's so favorable is because the enol prefers 859 01:11:46,710 --> 01:11:56,125 to rearrange to the keto form. 860 01:12:25,970 --> 01:12:32,020 And when we rearrange the keto form, this, of course, 861 01:12:32,020 --> 01:12:36,810 is the pentose ribulose 5-phosphate. 862 01:12:40,130 --> 01:12:43,740 And so ribose 5-phosphate, of course, 863 01:12:43,740 --> 01:12:45,515 we saw in the Calvin cycle. 864 01:12:45,515 --> 01:12:47,390 So we started with phosphorylation at the one 865 01:12:47,390 --> 01:12:49,880 position, ribulose-1,5-bisphosphate. 866 01:12:49,880 --> 01:12:53,480 So ribulose 5-phosphate can be produced 867 01:12:53,480 --> 01:12:57,710 by the oxidative pentose phosphate pathway glucose 868 01:12:57,710 --> 01:12:58,400 6-phosphate. 869 01:12:58,400 --> 01:13:03,470 First oxidize the 1-carbon by G6PD. 870 01:13:03,470 --> 01:13:05,900 That generates 6--phosphogluconate. 871 01:13:05,900 --> 01:13:07,820 Oxidized the 3-carbon. 872 01:13:07,820 --> 01:13:10,520 That allows favorable decarboxylation 873 01:13:10,520 --> 01:13:15,560 to generate another NADPH and ribulose 5-phosphate. 874 01:13:15,560 --> 01:13:18,260 Once you add that ribulose 5-phosphate, 875 01:13:18,260 --> 01:13:24,110 we can, of course, use an isomerase. 876 01:13:24,110 --> 01:13:43,410 And with an isomerase, we can generate ribose 5-phosphate, 877 01:13:43,410 --> 01:13:47,060 which is now available to be sent 878 01:13:47,060 --> 01:13:50,690 to do a nucleic acid synthesis. 879 01:13:50,690 --> 01:13:58,130 Or, we can use an epimerase. 880 01:14:01,280 --> 01:14:08,510 And if we use an epimerase, we can change the stereochemistry 881 01:14:08,510 --> 01:14:10,610 at this three position. 882 01:14:19,000 --> 01:14:22,940 And that gives me xylulose 5-phosphate, 883 01:14:22,940 --> 01:14:30,160 which was the ketose donor and acceptor for the transketolase, 884 01:14:30,160 --> 01:14:35,110 transaldolase reactions that were so useful for entering 885 01:14:35,110 --> 01:14:41,110 the nonoxidative pentose phosphate pathway or basically 886 01:14:41,110 --> 01:14:43,280 some version of the Calvin cycle. 887 01:14:43,280 --> 01:14:45,850 And so we can get ribose 5-phosphate, 888 01:14:45,850 --> 01:14:48,940 xylulose 5-phosphate, ribulose 5-phosphate 889 01:14:48,940 --> 01:14:52,540 all as products of the oxidative pentose phosphate 890 01:14:52,540 --> 01:14:57,430 pathway, which will allow us to either generate ribose as a way 891 01:14:57,430 --> 01:15:02,890 to make nucleosides or have carbon in the right form 892 01:15:02,890 --> 01:15:06,460 that I can now run the oxidative pentose phosphate pathway 893 01:15:06,460 --> 01:15:09,130 and have it re-enter glycolysis. 894 01:15:11,480 --> 01:15:11,980 Great. 895 01:15:11,980 --> 01:15:15,960 So that's the oxidative pentose phosphate pathway. 896 01:15:15,960 --> 01:15:20,640 Now, let's discuss the nonoxidative pentose phosphate 897 01:15:20,640 --> 01:15:24,930 pathway, which just like the Calvin cycle, 898 01:15:24,930 --> 01:15:29,640 is interconversion of 3-carbon and 6-carbon sugars 899 01:15:29,640 --> 01:15:32,190 with 5-carbon sugars. 900 01:15:32,190 --> 01:15:34,845 Basically it's the Calvin cycle, same idea-- 901 01:15:34,845 --> 01:15:36,660 transketolase, transaldolase. 902 01:15:36,660 --> 01:15:39,450 Obviously, it evolved from the Calvin cycle, 903 01:15:39,450 --> 01:15:42,420 because photosynthesis had to be first. 904 01:15:42,420 --> 01:15:46,990 So let me first put it in context, how it works. 905 01:15:46,990 --> 01:15:48,615 And then we can draw it in more detail. 906 01:15:58,200 --> 01:16:00,540 So this here is glycolysis. 907 01:16:03,170 --> 01:16:12,132 If I run the oxidative pentose phosphate pathway, 908 01:16:12,132 --> 01:16:25,810 I can generate two NADPHs as well as a ribulose 5-phosphate. 909 01:16:25,810 --> 01:16:35,710 That ribulose 5-phosphate can be turned into ribose 5-phosphates 910 01:16:35,710 --> 01:16:40,090 and xylulose 5-phosphates-- 911 01:16:40,090 --> 01:16:44,650 aldose and ketose acceptors. 912 01:16:44,650 --> 01:16:49,840 I can then use these 5-carbon sugars. 913 01:16:49,840 --> 01:16:52,570 If I do a transketolase reaction, 914 01:16:52,570 --> 01:17:00,580 I'll get a 3-carbon aldose and a 7-carbon ketose-- 915 01:17:00,580 --> 01:17:05,170 remember transketolase moves two carbons 916 01:17:05,170 --> 01:17:09,580 from the ketose to an aldose, giving you a shorter 917 01:17:09,580 --> 01:17:12,640 aldose and a longer ketose. 918 01:17:12,640 --> 01:17:17,770 Now, if I run a transaldolase reaction, 919 01:17:17,770 --> 01:17:19,780 I'll move three carbons. 920 01:17:19,780 --> 01:17:25,900 That gives me a 6-carbon ketose and a 4-carbon aldose. 921 01:17:25,900 --> 01:17:37,990 If I run another transketolase reaction 922 01:17:37,990 --> 01:17:42,520 with this 4-carbon sugar with this 5-carbon sugar, 923 01:17:42,520 --> 01:17:48,070 I will then generate another 6-carbon ketose and a three 924 01:17:48,070 --> 01:17:49,960 carbon aldose. 925 01:17:49,960 --> 01:17:57,530 These six carbon are fructose phosphate 926 01:17:57,530 --> 01:18:02,150 and this 3-carbon aldose is glyceraldehyde 3-phosphate. 927 01:18:02,150 --> 01:18:04,820 And I can re-enter glycolysis. 928 01:18:04,820 --> 01:18:09,230 The stoichiometry works up if I do this three times. 929 01:18:09,230 --> 01:18:13,940 So 3 times 5 equals 15 carbons. 930 01:18:13,940 --> 01:18:19,580 So three ribulose 5-phosphates from the oxidative PPP. 931 01:18:19,580 --> 01:18:24,290 If I turn them into one ribose 5-phosphate and two 932 01:18:24,290 --> 01:18:28,610 xylulose 5-phosphate, I can allocate one here and one here. 933 01:18:28,610 --> 01:18:30,560 These two can come together to give me 934 01:18:30,560 --> 01:18:34,760 one fructose 6-phosphate and one erythrose 4-phosphate. 935 01:18:34,760 --> 01:18:38,510 That erythrose 4-phosphate can use the other xylulose 936 01:18:38,510 --> 01:18:41,690 5-phosphate to give me another fructose 6-phosphate 937 01:18:41,690 --> 01:18:44,450 and the glyceraldehyde 3-phosphate. 938 01:18:44,450 --> 01:18:50,900 So [INAUDIBLE] I have two hexoses and one triose coming 939 01:18:50,900 --> 01:18:54,440 from my three pentoses-- 940 01:18:54,440 --> 01:18:59,790 and so total here, again, of 15 carbons. 941 01:18:59,790 --> 01:19:03,540 And so like the Calvin cycle, these 942 01:19:03,540 --> 01:19:07,450 transketolase and transaldolase reactions are all reversible. 943 01:19:07,450 --> 01:19:10,020 And so if I don't want to run the oxidative pentose 944 01:19:10,020 --> 01:19:14,220 phosphate pathway, I can start with fructose 6-phosphates and 945 01:19:14,220 --> 01:19:16,530 glyceraldehyde 3-phosphates from glycolysis 946 01:19:16,530 --> 01:19:19,890 and run it this direction to generate riboses 947 01:19:19,890 --> 01:19:25,680 in order to make nucleosides. 948 01:19:25,680 --> 01:19:31,220 Just to be explicit about this, hopefully this all 949 01:19:31,220 --> 01:19:33,110 makes sense after the long discussion 950 01:19:33,110 --> 01:19:35,720 we had of the Calvin cycle and how these reactions work. 951 01:19:35,720 --> 01:19:37,400 And you could work this out yourself. 952 01:19:37,400 --> 01:19:42,620 But I'll close today by just showing this and how it works. 953 01:19:42,620 --> 01:19:45,800 I'll actually show it in the opposite direction. 954 01:19:45,800 --> 01:19:48,710 I described it here moving from products 955 01:19:48,710 --> 01:19:52,580 of the oxidative PPP running through the nonoxidative PPP 956 01:19:52,580 --> 01:19:54,440 to re-enter glycolysis. 957 01:19:54,440 --> 01:19:57,620 But I'm going to now draw it for you, starting with products 958 01:19:57,620 --> 01:20:00,560 of glycolysis, running the nonoxidative PPP 959 01:20:00,560 --> 01:20:04,650 in the other direction to generate pentoses 960 01:20:04,650 --> 01:20:08,240 just because this is so reversible, but also 961 01:20:08,240 --> 01:20:11,720 point out to you how this whole thing can work. 962 01:20:14,800 --> 01:20:33,640 So here is a glyceraldehyde 3-phosphate from glycolysis. 963 01:20:33,640 --> 01:20:37,720 That's a aldose acceptor. 964 01:21:11,130 --> 01:21:15,810 Here's a fructose 6-phosphate from glycolysis. 965 01:21:15,810 --> 01:21:18,030 That can be a keto [INAUDIBLE]. 966 01:21:18,030 --> 01:21:29,990 And so if I run a transketolase reaction-- 967 01:21:29,990 --> 01:21:34,610 so remember, transketolase-- transfer two carbons 968 01:21:34,610 --> 01:21:38,300 from the ketose to the aldose. 969 01:21:38,300 --> 01:21:39,680 What am I left with? 970 01:21:39,680 --> 01:21:44,210 Well, I'm left with a shorter aldose. 971 01:21:54,900 --> 01:21:59,650 And this is a erythrose 4-phosphate. 972 01:21:59,650 --> 01:22:03,055 And I'm left with a longer ketose. 973 01:22:27,340 --> 01:22:33,160 There is a pentose xylulose 5-phosphate. 974 01:22:33,160 --> 01:22:43,000 Now I have my erythrose 4-phosphate 975 01:22:43,000 --> 01:22:45,220 and my fructose 6-phosphate. 976 01:22:50,510 --> 01:22:56,000 So if I now carry out a transaldolase reaction-- 977 01:22:56,000 --> 01:22:57,730 so I'm going to move three carbons now 978 01:22:57,730 --> 01:23:01,270 for my ketose donor, fructose 6-phosphate, 979 01:23:01,270 --> 01:23:03,730 to my aldose acceptor. 980 01:23:03,730 --> 01:23:05,810 And so what do I end up with? 981 01:23:05,810 --> 01:23:07,150 So I end up with a-- 982 01:23:16,320 --> 01:23:21,630 this becomes 3 carbons shorter, so glyceraldehyde 3-phosphate. 983 01:23:21,630 --> 01:23:25,890 Move the three carbons from the ketose to the aldose acceptor. 984 01:23:25,890 --> 01:23:28,920 Now I end up with a longer ketose. 985 01:24:00,210 --> 01:24:04,860 Now I have this 7-carbon sedoheptulose 7-phosphate. 986 01:24:12,280 --> 01:24:18,620 If I now, again, run a transketolase reaction-- 987 01:24:18,620 --> 01:24:24,070 so two carbons here transferred from the ketose 988 01:24:24,070 --> 01:24:27,970 to the aldose, what I end up with now 989 01:24:27,970 --> 01:24:51,030 is a 5-carbon xylulose 5-phosphate. 990 01:24:51,030 --> 01:25:01,710 And so aldose becomes a longer ketose by two carbons. 991 01:25:01,710 --> 01:25:15,110 And our ketose becomes a shorter aldose 992 01:25:15,110 --> 01:25:31,930 by two carbons, which is a ribose 5-phosphate. 993 01:25:31,930 --> 01:25:37,480 I can now have two xylulose 5-phosphates 994 01:25:37,480 --> 01:25:39,410 and a ribose 5-phosphate. 995 01:25:39,410 --> 01:25:45,510 I can take my xylulose 5-phosphates 996 01:25:45,510 --> 01:25:51,810 and carry out an epimerase reaction. 997 01:25:51,810 --> 01:25:55,050 If I carry out an epimerase reaction, I, of course, get-- 998 01:26:00,780 --> 01:26:15,250 change the stereochemistry at the three position 999 01:26:15,250 --> 01:26:20,890 to get a ribulose 5-phosphate. 1000 01:26:31,120 --> 01:26:39,670 Or, I can carry out an isomerase reaction 1001 01:26:39,670 --> 01:26:45,070 to take my ribose 5-phosphate to ribulose 5-phosphate. 1002 01:26:45,070 --> 01:26:49,170 And so if you note that I started 1003 01:26:49,170 --> 01:26:51,960 this time with products of glycolysis, 1004 01:26:51,960 --> 01:26:56,400 glyceraldehyde 3-phosphate, fructose 6-phosphate, 1005 01:26:56,400 --> 01:27:00,450 did a series of transketolase and transaldolase reactions, 1006 01:27:00,450 --> 01:27:06,370 ultimately to generate two fructose 6-phosphates, 1007 01:27:06,370 --> 01:27:08,470 one glyceraldehyde 3-phosphate-- so 1008 01:27:08,470 --> 01:27:12,160 that's 6 plus 6 plus 3 equals 15 carbons. 1009 01:27:12,160 --> 01:27:18,160 I get three pentoses out, a xylulose 5-phosphate, 1010 01:27:18,160 --> 01:27:20,350 a ribose 5-phosphate, and a xylulose 1011 01:27:20,350 --> 01:27:23,680 5-phosphate, which I can then use epimerase or isomerase 1012 01:27:23,680 --> 01:27:27,490 reactions to give me ribulose 5-phosphate, which, of course, 1013 01:27:27,490 --> 01:27:29,680 all could be shuttled into ribose 5-phosphate, 1014 01:27:29,680 --> 01:27:32,290 giving me the flexibility to start with products 1015 01:27:32,290 --> 01:27:35,170 of glycolysis and net generate ribose 1016 01:27:35,170 --> 01:27:38,700 5-phosphate for nucleoside synthesis. 1017 01:27:38,700 --> 01:27:42,840 If I run the oxidative pentose phosphate pathway, 1018 01:27:42,840 --> 01:27:46,470 I end up with ribulose 5-phosphates. 1019 01:27:46,470 --> 01:27:49,140 I can take those ribulose 5-phosphates, 1020 01:27:49,140 --> 01:27:51,840 turn them into xylulose 5-phosphates and ribose 1021 01:27:51,840 --> 01:27:54,750 5-phosphates, run the same series 1022 01:27:54,750 --> 01:27:57,570 of transketolase and transaldolase reactions 1023 01:27:57,570 --> 01:28:01,050 in reverse and get glyceraldehyde 3-phosphates and 1024 01:28:01,050 --> 01:28:02,790 fructose 6-phosphates. 1025 01:28:02,790 --> 01:28:07,260 Either direction, 15 carbons in, 15 carbons out, 1026 01:28:07,260 --> 01:28:09,720 can start with products of the oxidative PPP 1027 01:28:09,720 --> 01:28:12,690 and put them back in glycolysis or start with products 1028 01:28:12,690 --> 01:28:15,690 of glycolysis and generate pentoses, 1029 01:28:15,690 --> 01:28:19,710 such as ribose 5-phosphate for nucleoside synthesis, 1030 01:28:19,710 --> 01:28:21,660 giving cells a lot of flexibility 1031 01:28:21,660 --> 01:28:24,930 to take carbohydrates from glycolysis 1032 01:28:24,930 --> 01:28:27,600 and make pentoses for nucleosides, 1033 01:28:27,600 --> 01:28:32,820 as well as have a way to generate NADPH. 1034 01:28:32,820 --> 01:28:36,690 I know there's a lot of carbons moving around 1035 01:28:36,690 --> 01:28:38,340 and a lot of swapping, but hopefully 1036 01:28:38,340 --> 01:28:43,020 after today how it works and why it works this way is 1037 01:28:43,020 --> 01:28:45,570 a little bit clearer than if you just 1038 01:28:45,570 --> 01:28:48,300 sit down and try to memorize these really 1039 01:28:48,300 --> 01:28:49,215 confusing pathways. 1040 01:28:50,900 --> 01:28:53,870 Next time we'll talk about how one uses NADPH 1041 01:28:53,870 --> 01:28:56,720 as a source of reducing power in all organisms 1042 01:28:56,720 --> 01:28:59,360 in order to synthesize fatty acids. 1043 01:28:59,360 --> 01:29:00,910 Thanks.