1 00:00:00,090 --> 00:00:02,490 The following content is provided under a Creative 2 00:00:02,490 --> 00:00:04,030 Commons license. 3 00:00:04,030 --> 00:00:06,330 Your support will help MIT OpenCourseWare 4 00:00:06,330 --> 00:00:10,720 continue to offer high quality educational resources for free. 5 00:00:10,720 --> 00:00:13,320 To make a donation or view additional materials 6 00:00:13,320 --> 00:00:17,280 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:17,280 --> 00:00:18,450 at ocw.mit.edu. 8 00:00:20,957 --> 00:00:22,540 JOHN ESSIGMANN: Hi, I'm John Essigman. 9 00:00:22,540 --> 00:00:25,290 I'm one of the instructors in 5.07. 10 00:00:25,290 --> 00:00:28,350 I teach with JoAnne Stubbe and Bogdan Fedeles, 11 00:00:28,350 --> 00:00:31,410 you've probably seen in earlier videos of this type. 12 00:00:31,410 --> 00:00:34,590 Today we're going to talk about the pentose phosphate pathway. 13 00:00:34,590 --> 00:00:36,420 Professionally, I'm a toxicologist. 14 00:00:36,420 --> 00:00:38,310 The pentose phosphate pathway is one 15 00:00:38,310 --> 00:00:41,310 that is very central to the people who work in my field, 16 00:00:41,310 --> 00:00:45,720 specifically because the pentose phosphate pathway provides us 17 00:00:45,720 --> 00:00:48,180 with one of the reagents that we need in order 18 00:00:48,180 --> 00:00:50,520 to be able to combat or counter the effects 19 00:00:50,520 --> 00:00:52,140 of oxidative stress. 20 00:00:52,140 --> 00:00:55,380 Oxidative stress is involved in many pathologies. 21 00:00:55,380 --> 00:00:58,290 One of the reasons that the pentose phosphate pathway is 22 00:00:58,290 --> 00:01:01,860 challenging to teach is because it's 23 00:01:01,860 --> 00:01:04,560 a pathway that interacts very, very closely 24 00:01:04,560 --> 00:01:07,050 with other pathways-- for example, glycolysis, 25 00:01:07,050 --> 00:01:09,840 the energy-generating pathway in gluconeogenesis, 26 00:01:09,840 --> 00:01:11,820 a pathway by which, in some organs, 27 00:01:11,820 --> 00:01:14,700 you're able to synthesize glucose 28 00:01:14,700 --> 00:01:17,370 from non-carbohydrate precursors. 29 00:01:17,370 --> 00:01:20,640 The fact that these tend to be so closely related 30 00:01:20,640 --> 00:01:23,280 makes it difficult to see the distinct features 31 00:01:23,280 --> 00:01:25,390 of the pentose phosphate pathway. 32 00:01:25,390 --> 00:01:27,712 So let me show you the pentose phosphate pathway 33 00:01:27,712 --> 00:01:29,670 and how it interacts with these other pathways. 34 00:01:34,182 --> 00:01:35,890 First of all, let me start with the roles 35 00:01:35,890 --> 00:01:37,450 of the pentose phosphate pathway. 36 00:01:37,450 --> 00:01:39,290 As I see it, there are three. 37 00:01:39,290 --> 00:01:43,120 The first is this is the cell's principle 38 00:01:43,120 --> 00:01:48,250 source of an NADPH, which is our reductive cofactor. 39 00:01:48,250 --> 00:01:51,070 It's used for the synthesis of such things 40 00:01:51,070 --> 00:01:54,050 as lipids and other carbohydrates, and so on. 41 00:01:54,050 --> 00:01:56,350 So this is how the reducing equivalent, 42 00:01:56,350 --> 00:01:58,570 the source of electrons that we use in order 43 00:01:58,570 --> 00:02:01,750 to be able to synthesize complex molecules. 44 00:02:01,750 --> 00:02:04,690 The second major role of the pentose phosphate 45 00:02:04,690 --> 00:02:08,889 pathway is that it's our biosynthetic source of ribose. 46 00:02:08,889 --> 00:02:11,560 We have to make ribose for things like ATP. 47 00:02:11,560 --> 00:02:13,510 We have to make it for the nucleotides that 48 00:02:13,510 --> 00:02:15,130 are in our nucleic acids. 49 00:02:15,130 --> 00:02:16,750 This is where it comes from. 50 00:02:16,750 --> 00:02:21,130 What we're also going to see is that when we eat foods 51 00:02:21,130 --> 00:02:22,390 that contain ribose-- 52 00:02:22,390 --> 00:02:25,030 for example, ATP and nucleotides-- 53 00:02:25,030 --> 00:02:27,700 this is how they enter the mainstream of metabolism 54 00:02:27,700 --> 00:02:31,120 so that we can utilize them as raw material or energy 55 00:02:31,120 --> 00:02:32,350 material. 56 00:02:32,350 --> 00:02:34,060 And the third thing about the pathway 57 00:02:34,060 --> 00:02:35,800 is that some of the intermediates you'll 58 00:02:35,800 --> 00:02:39,970 see in what we call this carbon scrambling phase 59 00:02:39,970 --> 00:02:43,960 are critical for the synthesis of certain biomolecules. 60 00:02:43,960 --> 00:02:45,850 This is where the aromatic residues 61 00:02:45,850 --> 00:02:50,060 come from in our aromatic amino acids, such as tyrosine. 62 00:02:50,060 --> 00:02:52,690 So the pentose phosphate pathway is very important 63 00:02:52,690 --> 00:02:55,390 because it provides us with our reducing equivalents. 64 00:02:55,390 --> 00:02:58,600 It provides us with a biosynthetic source 65 00:02:58,600 --> 00:03:00,310 of ribose for nucleotides. 66 00:03:00,310 --> 00:03:02,230 And it provides us with building blocks 67 00:03:02,230 --> 00:03:03,910 for more complicated molecules. 68 00:03:09,910 --> 00:03:14,890 Because this pathway is associated with biosynthesis, 69 00:03:14,890 --> 00:03:17,890 in terms of its providing reducing equivalents, what 70 00:03:17,890 --> 00:03:20,650 we'll find is that the pathway is very highly 71 00:03:20,650 --> 00:03:23,860 expressed in tissues that make a lot of lipid. 72 00:03:23,860 --> 00:03:28,080 Lipid is our bioenergetically most expensive 73 00:03:28,080 --> 00:03:31,330 biosynthetic process. 74 00:03:31,330 --> 00:03:34,090 Requires a lot of NADPH. 75 00:03:34,090 --> 00:03:36,040 If the cell is dividing, what it's going to do 76 00:03:36,040 --> 00:03:37,480 is make membranes. 77 00:03:37,480 --> 00:03:40,180 And that's a lot of lipid biosynthesis. 78 00:03:40,180 --> 00:03:42,760 This pathway, the pentose phosphate pathway, 79 00:03:42,760 --> 00:03:45,010 not surprisingly, is upregulated when 80 00:03:45,010 --> 00:03:46,780 we have to make a lot of membranes-- 81 00:03:46,780 --> 00:03:48,640 for example, in a cell that's growing. 82 00:03:48,640 --> 00:03:51,600 And that means, for example, that this could be 83 00:03:51,600 --> 00:03:53,650 a target for anti-cancer drugs. 84 00:03:53,650 --> 00:03:55,810 In other words, if you wanted to find a pathway 85 00:03:55,810 --> 00:03:59,980 to block in order to stop the provision of the resources, 86 00:03:59,980 --> 00:04:01,960 the reductive resources for growth, 87 00:04:01,960 --> 00:04:04,060 this would be one that would be looked at-- 88 00:04:04,060 --> 00:04:06,830 in fact, is being looked at by the modern pharmaceutical 89 00:04:06,830 --> 00:04:07,330 industry. 90 00:04:11,470 --> 00:04:13,960 Over here toward the center of the top 91 00:04:13,960 --> 00:04:20,320 panel is what's often considered to be a little bit confusing, 92 00:04:20,320 --> 00:04:22,330 but it's really actually quite straightforward, 93 00:04:22,330 --> 00:04:26,350 which is the fact that this pathway can run in two modes. 94 00:04:26,350 --> 00:04:29,410 I said earlier that the pentose phosphate pathway 95 00:04:29,410 --> 00:04:32,290 is an offshoot of other pathways, which 96 00:04:32,290 --> 00:04:35,770 makes it a little bit difficult for some people to understand. 97 00:04:35,770 --> 00:04:37,780 So in this line right across here 98 00:04:37,780 --> 00:04:40,120 that I'm just moving across, that's 99 00:04:40,120 --> 00:04:43,540 going from glucose to pyruvate as we go left to right. 100 00:04:43,540 --> 00:04:45,550 And that's, of course, glycolysis. 101 00:04:45,550 --> 00:04:47,620 If we went from pyruvate to glucose-- 102 00:04:47,620 --> 00:04:49,280 that is, going from right to left-- 103 00:04:49,280 --> 00:04:51,020 that's gluconeogenesis. 104 00:04:51,020 --> 00:04:53,380 So gluconeogenesis, for example, might 105 00:04:53,380 --> 00:04:55,720 be a situation in a diabetic. 106 00:04:55,720 --> 00:04:59,570 For example, I'm a diabetic, and when I go to sleep at night, 107 00:04:59,570 --> 00:05:02,980 I'm not eating, and what my liver is doing is it's saying, 108 00:05:02,980 --> 00:05:04,660 OK, you haven't eaten for a while. 109 00:05:04,660 --> 00:05:06,740 Your blood sugar is dropping. 110 00:05:06,740 --> 00:05:09,370 And my gluconeogenesis pathways start 111 00:05:09,370 --> 00:05:12,730 to manufacturing-- in my case, unfortunately, too much. 112 00:05:12,730 --> 00:05:14,590 From non-carbohydrate precursors, 113 00:05:14,590 --> 00:05:17,110 they move the carbon from right to left 114 00:05:17,110 --> 00:05:21,280 and put out a lot of glucose into my blood. 115 00:05:21,280 --> 00:05:26,800 That's one of the uses of the gluconeogenic pathway. 116 00:05:26,800 --> 00:05:29,200 Similarly, if a Doberman Pinscher, 117 00:05:29,200 --> 00:05:33,204 which has become commonly seen in the 5.07 course so far, 118 00:05:33,204 --> 00:05:34,870 starts to chase me down the street, what 119 00:05:34,870 --> 00:05:36,430 I'm going to want to do is I'm going 120 00:05:36,430 --> 00:05:39,040 to want to take and break down my glucose as quickly 121 00:05:39,040 --> 00:05:41,890 as possible to bring it toward pyruvate 122 00:05:41,890 --> 00:05:45,100 to get fast ATP that I can use to power my muscles 123 00:05:45,100 --> 00:05:46,510 to be able to run away. 124 00:05:46,510 --> 00:05:51,700 So that's the gluconeogenesis and the glycolysis pathways. 125 00:05:51,700 --> 00:05:55,210 Now imagine that you've got a railroad track 126 00:05:55,210 --> 00:05:57,790 and that you've got a side track off of it. 127 00:05:57,790 --> 00:06:00,210 And that's what the pentose phosphate pathway is. 128 00:06:00,210 --> 00:06:05,560 It is a track that comes off of glycolysis or gluconeogenesis, 129 00:06:05,560 --> 00:06:09,940 stuff happens, and then you can re-enter the mainstream 130 00:06:09,940 --> 00:06:11,530 of the train line. 131 00:06:11,530 --> 00:06:13,960 The interesting thing is that you can basically 132 00:06:13,960 --> 00:06:19,070 go in either direction at certain parts of this pathway. 133 00:06:19,070 --> 00:06:23,870 What this really means is we have common components 134 00:06:23,870 --> 00:06:27,440 in both the pentose phosphate pathway 135 00:06:27,440 --> 00:06:31,547 and along this main line of gluconeogenesis/glycolysis. 136 00:06:31,547 --> 00:06:33,380 And the common components we're going to see 137 00:06:33,380 --> 00:06:39,020 are, first, glucose 6-phosphate; second, fructose 6-phosphate; 138 00:06:39,020 --> 00:06:42,020 and third, glyceraldehyde 3-phosphate, or GAP. 139 00:06:42,020 --> 00:06:44,930 So these are the common features of all three pathways. 140 00:06:50,540 --> 00:06:54,300 OK, so I'm going to give you a little bit of an overview first 141 00:06:54,300 --> 00:06:57,550 and show you how this side track works. 142 00:06:57,550 --> 00:07:02,514 Now, the pathway can be run in the oxidative mode exclusively. 143 00:07:02,514 --> 00:07:03,930 And what we'll see is-- and again, 144 00:07:03,930 --> 00:07:06,150 the details will come in a few minutes-- 145 00:07:06,150 --> 00:07:11,730 that this glucose 6-phosphate is going to be oxidized in order 146 00:07:11,730 --> 00:07:13,330 to generate-- 147 00:07:13,330 --> 00:07:15,990 and oxidation, of course, is when electrons 148 00:07:15,990 --> 00:07:19,350 are removed from the substrate. 149 00:07:19,350 --> 00:07:21,990 And they're going to be put into an NADP 150 00:07:21,990 --> 00:07:25,560 to form NADPH, which is our reductive cofactor. 151 00:07:25,560 --> 00:07:28,380 So this is an oxidative arm. 152 00:07:28,380 --> 00:07:30,892 Now, one of the consequences of this oxidation 153 00:07:30,892 --> 00:07:32,600 is going to be-- we're going to be losing 154 00:07:32,600 --> 00:07:35,710 a carbon from the six-carbon glucose 6-phosphate 155 00:07:35,710 --> 00:07:38,340 and we're going to be forming a pentose, specifically 156 00:07:38,340 --> 00:07:40,510 ribulose 5-phosphate. 157 00:07:40,510 --> 00:07:43,260 It's going to be easily interconverted 158 00:07:43,260 --> 00:07:47,040 with other pentoses, five-carbon sugars. 159 00:07:47,040 --> 00:07:49,320 And then I want you to notice that from here on, 160 00:07:49,320 --> 00:07:51,510 everything is reversible in the pathway. 161 00:07:51,510 --> 00:07:54,760 Loss of CO2 typically is irreversible, 162 00:07:54,760 --> 00:07:58,480 so this first arm, the first oxidative arm, is irreversible. 163 00:07:58,480 --> 00:08:01,020 So if we go from glucose 6-phosphate to pentoses, 164 00:08:01,020 --> 00:08:03,160 that's a one-way street. 165 00:08:03,160 --> 00:08:07,110 The pentoses, if we are in the process, say, of cell division, 166 00:08:07,110 --> 00:08:09,180 these can be used to make nucleotides. 167 00:08:09,180 --> 00:08:12,780 And this is how we make pentoses for nucleic acids. 168 00:08:12,780 --> 00:08:16,020 If we don't need the pentoses, then the carbon 169 00:08:16,020 --> 00:08:18,990 from the pentoses will continue along the pathway 170 00:08:18,990 --> 00:08:22,140 and will go into this complicated carbon-scrambling 171 00:08:22,140 --> 00:08:25,770 network, which we see underlined with a squiggly line over here 172 00:08:25,770 --> 00:08:27,300 in the center. 173 00:08:27,300 --> 00:08:30,300 And the scrambling will result in the pentoses being 174 00:08:30,300 --> 00:08:34,109 restructured into fructose 6-phosphate, glyceraldehyde 175 00:08:34,109 --> 00:08:36,299 3-phosphate, so the carbon can then 176 00:08:36,299 --> 00:08:38,320 continue along the pathway. 177 00:08:38,320 --> 00:08:41,120 So I can imagine the pathway going like this. 178 00:08:41,120 --> 00:08:45,300 A cell-- let's say it's in a situation where it is growing 179 00:08:45,300 --> 00:08:47,970 and it needs NADPH, and it needs pentoses 180 00:08:47,970 --> 00:08:49,590 for making nucleotides. 181 00:08:49,590 --> 00:08:53,040 So it takes and it gets up to the glucose 6-phosphate step. 182 00:08:53,040 --> 00:08:56,920 And so rather than progressing in glycolysis, 183 00:08:56,920 --> 00:08:59,670 it takes, as you'll see, a right-hand turn. 184 00:08:59,670 --> 00:09:01,290 Make the NADPH that's going to be 185 00:09:01,290 --> 00:09:04,290 needed for making those membranes of the new cell. 186 00:09:04,290 --> 00:09:05,850 Getting pentoses, which are going 187 00:09:05,850 --> 00:09:07,800 to convert into nucleotides. 188 00:09:07,800 --> 00:09:09,560 And if you don't use all of the pentoses, 189 00:09:09,560 --> 00:09:12,150 then what you want to do is you don't want any waste material, 190 00:09:12,150 --> 00:09:14,580 so you go through the carbon-scrambling network, 191 00:09:14,580 --> 00:09:16,680 and the remainder of the material 192 00:09:16,680 --> 00:09:19,650 will go back as fructose 6-phosphate and glyceraldehyde 193 00:09:19,650 --> 00:09:21,180 3-phosphate. 194 00:09:21,180 --> 00:09:24,930 So that's what we'll call the full pentose phosphate pathway, 195 00:09:24,930 --> 00:09:30,390 with the oxidative and the non-oxidative portions 196 00:09:30,390 --> 00:09:32,220 distinctly shown. 197 00:09:32,220 --> 00:09:36,090 Everything's oxidative up to the pentoses generation, 198 00:09:36,090 --> 00:09:38,640 and then it's non-oxidative from pentoses 199 00:09:38,640 --> 00:09:43,870 through the fructose 6-phosphate and glyceraldehyde 3-phosphate. 200 00:09:48,910 --> 00:09:51,570 Now in the Case B over here-- we call it 201 00:09:51,570 --> 00:09:53,970 the purely non-oxidative mode. 202 00:09:53,970 --> 00:09:56,680 Let's say you're in a situation where you're not growing, 203 00:09:56,680 --> 00:09:58,680 but you do need, for example, ATP, 204 00:09:58,680 --> 00:10:00,900 and you need pentoses for that. 205 00:10:00,900 --> 00:10:03,390 That's where the back door comes in. 206 00:10:03,390 --> 00:10:07,590 We're going to be taking, in this case out of probably 207 00:10:07,590 --> 00:10:09,840 gluconeogenesis-- remember, gluconeogenesis 208 00:10:09,840 --> 00:10:13,390 is the flow of carbon from right to left in this pathway. 209 00:10:13,390 --> 00:10:15,930 So it could be amino acids and things 210 00:10:15,930 --> 00:10:18,900 from the tricarboxylic acid, or TCA cycle, 211 00:10:18,900 --> 00:10:21,750 are flowing carbon in this direction. 212 00:10:21,750 --> 00:10:26,450 And it goes up into the gluconeogenic pathway, 213 00:10:26,450 --> 00:10:30,140 and then the molecule repositories 214 00:10:30,140 --> 00:10:32,960 fill up for a glyceraldehyde 3-phosphate and fructose 215 00:10:32,960 --> 00:10:34,280 6-phosphate. 216 00:10:34,280 --> 00:10:38,840 If I were to block gluconeogenesis right here, 217 00:10:38,840 --> 00:10:41,620 then carbon would flow in this direction, 218 00:10:41,620 --> 00:10:44,040 would go through the backdoor of the pathway. 219 00:10:44,040 --> 00:10:46,550 The carbon-scrambling phase would result ultimately 220 00:10:46,550 --> 00:10:47,600 in pentoses. 221 00:10:47,600 --> 00:10:50,540 So this would be a biosynthetic route to pentoses. 222 00:10:50,540 --> 00:10:54,270 Again, that's a situation in which we do not need NADPH. 223 00:10:54,270 --> 00:10:55,610 Perhaps we just need pentoses. 224 00:11:00,450 --> 00:11:03,420 OK, so now let me get into the details. 225 00:11:03,420 --> 00:11:06,180 In the center of this piece of paper, what you'll see 226 00:11:06,180 --> 00:11:09,270 is the stoichiometry of the pathway. 227 00:11:09,270 --> 00:11:12,180 In most of 5.07 so far, we have seen 228 00:11:12,180 --> 00:11:15,570 that we work with a single molecule of glucose 229 00:11:15,570 --> 00:11:19,800 working its way to two molecules of pyruvate, and so on. 230 00:11:19,800 --> 00:11:22,440 Here the tricarboxylic geometry works out best, 231 00:11:22,440 --> 00:11:25,440 or it's easiest to teach and to appreciate, 232 00:11:25,440 --> 00:11:27,810 if you start with three molecules of glucose. 233 00:11:27,810 --> 00:11:29,710 And we'll see why in a few minutes. 234 00:11:29,710 --> 00:11:31,020 So what we'll do is-- 235 00:11:31,020 --> 00:11:33,390 I'm going to do is take three molecules of glucose 236 00:11:33,390 --> 00:11:34,630 6-phosphate. 237 00:11:34,630 --> 00:11:36,210 Now looking up at the top here-- so 238 00:11:36,210 --> 00:11:38,400 this is glucose to glucose 6-phosphate. 239 00:11:38,400 --> 00:11:42,510 This is the entry portion right here, with glucose 6-phosphate, 240 00:11:42,510 --> 00:11:45,310 into the oxidative arm of the pathway. 241 00:11:45,310 --> 00:11:46,860 So that's 18 carbons. 242 00:11:46,860 --> 00:11:49,260 3 times 6 is 18. 243 00:11:49,260 --> 00:11:51,900 We're going to lose three CO2s. 244 00:11:51,900 --> 00:11:54,690 That's going to leave us 15 carbons left. 245 00:11:54,690 --> 00:11:56,970 And those 15 carbons are going to be distributed 246 00:11:56,970 --> 00:12:01,920 among two fructose 6-phosphates and glyceraldehyde 3-phosphate. 247 00:12:01,920 --> 00:12:04,290 So here's the fructose 6-phosphate. 248 00:12:04,290 --> 00:12:06,810 Here's the glyceraldehyde 3-phosphate. 249 00:12:06,810 --> 00:12:08,920 That's the overall stoichiometry of the pathway. 250 00:12:13,365 --> 00:12:14,740 So now what I'm going to do is go 251 00:12:14,740 --> 00:12:17,650 through the chemical details of the pathway, 252 00:12:17,650 --> 00:12:21,760 down here at the bottom and on the next page. 253 00:12:21,760 --> 00:12:25,180 We've got glucose going to glucose 6-phosphate. 254 00:12:25,180 --> 00:12:27,610 And I draw three lines there because I'll 255 00:12:27,610 --> 00:12:30,610 say this glucose 6-phosphate is the abbreviation that I 256 00:12:30,610 --> 00:12:33,490 use for the structure that's shown right here. 257 00:12:33,490 --> 00:12:35,200 So this is glucose 6-phosphate with 258 00:12:35,200 --> 00:12:38,020 the proper stereochemistry. 259 00:12:38,020 --> 00:12:44,470 Now the hydroxyl group on the glucose-- 260 00:12:44,470 --> 00:12:47,890 we don't really know what its stereochemistry is, 261 00:12:47,890 --> 00:12:55,000 but we've got it up or in beta in this drawing. 262 00:12:55,000 --> 00:12:58,540 The first step is for the enzyme glucose 263 00:12:58,540 --> 00:13:02,800 6-phosphate dehydrogenase, G6PDH, 264 00:13:02,800 --> 00:13:07,060 to attack the one carbon. 265 00:13:07,060 --> 00:13:10,510 The electrons go down to make a keto functionality. 266 00:13:10,510 --> 00:13:14,770 We lose hydride from the one carbon, which is used then 267 00:13:14,770 --> 00:13:17,560 to reduce NADP to NADPH. 268 00:13:17,560 --> 00:13:19,990 So that's our first oxidation step. 269 00:13:19,990 --> 00:13:23,230 I want you to notice that this is a one-way arrow. 270 00:13:23,230 --> 00:13:26,800 Typically at the top of a pathway-- and in this case, 271 00:13:26,800 --> 00:13:28,420 it's at the very top-- 272 00:13:28,420 --> 00:13:32,200 these are the steps that are thermodynamically irreversible. 273 00:13:32,200 --> 00:13:35,620 These are also the steps where you principally effect 274 00:13:35,620 --> 00:13:37,420 regulation of the pathway. 275 00:13:37,420 --> 00:13:38,920 And as I said earlier, we're going 276 00:13:38,920 --> 00:13:41,890 to be running three molecules of glucose 6-phosphate 277 00:13:41,890 --> 00:13:44,380 through this step. 278 00:13:44,380 --> 00:13:47,770 The next molecule in the pathway is this one. 279 00:13:47,770 --> 00:13:48,760 This is a lactone. 280 00:13:48,760 --> 00:13:52,480 A lactone is a cyclic ester. 281 00:13:52,480 --> 00:13:55,510 So this is an ester, and you can see it's cyclic. 282 00:13:55,510 --> 00:13:58,990 The next step involves the addition of water 283 00:13:58,990 --> 00:14:03,610 by lactonase in order to open up this six-membered ring 284 00:14:03,610 --> 00:14:06,970 to give you this linear structure that we'll see here. 285 00:14:06,970 --> 00:14:13,770 And this is basically 6-phosphoglucuronic acid. 286 00:14:13,770 --> 00:14:16,950 The next step will involve the oxidation 287 00:14:16,950 --> 00:14:19,020 of the 1, 2, 3 carbon. 288 00:14:19,020 --> 00:14:23,160 I always try to find places in any pathway 289 00:14:23,160 --> 00:14:25,380 where we've seen the same chemistry before 290 00:14:25,380 --> 00:14:27,250 but with different molecules. 291 00:14:27,250 --> 00:14:29,400 This step right here, specifically 292 00:14:29,400 --> 00:14:34,250 working with lactonase and the enzyme 6 phosphogluconate 293 00:14:34,250 --> 00:14:35,580 dehydrogenase-- 294 00:14:35,580 --> 00:14:40,030 I'd like you to look back at the tricarboxylic acid cycle, 295 00:14:40,030 --> 00:14:44,220 specifically at the enzyme isocitrate dehydrogenase. 296 00:14:44,220 --> 00:14:47,880 While some of the specifics are different, basically what 297 00:14:47,880 --> 00:14:50,130 you'll see is the overall concept 298 00:14:50,130 --> 00:14:53,110 is the same for what's happening in the next couple of steps. 299 00:14:53,110 --> 00:14:56,320 So let me now get back to the drawing. 300 00:14:56,320 --> 00:14:59,370 So what we have here is the 1, 2, 301 00:14:59,370 --> 00:15:03,780 3 carbon is vulnerable to what's going to be an oxidation event. 302 00:15:03,780 --> 00:15:05,940 So a base will remove the hydrogen 303 00:15:05,940 --> 00:15:09,150 from the 3-hydroxyl group. 304 00:15:09,150 --> 00:15:13,140 The electrons that were holding the hydrogen to the oxygen 305 00:15:13,140 --> 00:15:18,510 will now move down to the position between the oxygen 306 00:15:18,510 --> 00:15:20,200 and the 3 carbon. 307 00:15:20,200 --> 00:15:23,490 And then the hydride on the 3 carbon 308 00:15:23,490 --> 00:15:27,060 will be transferred to NADP to make NADPH. 309 00:15:27,060 --> 00:15:30,840 So this is the second of two oxidation steps. 310 00:15:30,840 --> 00:15:33,960 That gives us this ring open structure here. 311 00:15:33,960 --> 00:15:37,440 And just as happens in the isocitrate dehydrogenase 312 00:15:37,440 --> 00:15:41,430 step, what we've done is we've generated a beta ketoacid. 313 00:15:41,430 --> 00:15:43,740 Whenever you see a beta ketoacid, 314 00:15:43,740 --> 00:15:46,590 you really always want to think that that's 315 00:15:46,590 --> 00:15:50,520 prone to either enzymatic or spontaneous decarboxylation. 316 00:15:50,520 --> 00:15:54,000 What you can see is the electrons can go like this 317 00:15:54,000 --> 00:15:57,000 and like this and like this, and that 318 00:15:57,000 --> 00:16:00,600 will result in the cleavage of the bond connecting 319 00:16:00,600 --> 00:16:02,640 carbon 1 and carbon 2. 320 00:16:02,640 --> 00:16:06,570 So now the new carbon 1 will be the 321 00:16:06,570 --> 00:16:08,940 what was carbon 2, which is the carbon that has 322 00:16:08,940 --> 00:16:11,490 this little purple ball on it. 323 00:16:11,490 --> 00:16:15,090 And we're going to lose the terminal, or 1 carbon, as CO2. 324 00:16:18,252 --> 00:16:21,630 The CO2 will have the triangle, which up until this point 325 00:16:21,630 --> 00:16:25,710 has defined our 1 carbon. 326 00:16:25,710 --> 00:16:28,140 Now I just want to mention a couple of things. 327 00:16:28,140 --> 00:16:29,720 One is that we'll look at-- 328 00:16:29,720 --> 00:16:32,400 I put on-- like here's a dark ball. 329 00:16:32,400 --> 00:16:36,120 Here's a purple ball, and here's a black triangle. 330 00:16:36,120 --> 00:16:38,280 These could be-- for example, experimentally, 331 00:16:38,280 --> 00:16:41,730 you could use a radioactive chemical, a radioactive atom, 332 00:16:41,730 --> 00:16:45,600 in order to help you trace the reaction as you 333 00:16:45,600 --> 00:16:48,360 go from precursor to product. 334 00:16:48,360 --> 00:16:52,560 I'm using these just to help you see the relationship 335 00:16:52,560 --> 00:16:54,480 of a precursor and a product. 336 00:16:54,480 --> 00:16:57,660 And what we see here is that the 1 carbon 337 00:16:57,660 --> 00:17:01,140 is lost from the beta ketoacid as CO2 338 00:17:01,140 --> 00:17:03,390 and according to the kind of chemistry that we've seen 339 00:17:03,390 --> 00:17:06,030 many times for decarboxylation. 340 00:17:06,030 --> 00:17:08,890 Now, this is going to give us a five-carbon product. 341 00:17:08,890 --> 00:17:10,710 We've lost the CO2. 342 00:17:10,710 --> 00:17:12,867 And that is ribulose 5-phosphate. 343 00:17:12,867 --> 00:17:14,950 And for this, I'm going to go on to the next page. 344 00:17:19,720 --> 00:17:22,630 On the previous page, we saw the loss of CO2 345 00:17:22,630 --> 00:17:25,780 from the 1 carbon of the beta ketoacid. 346 00:17:25,780 --> 00:17:29,800 And this gives us this linear structure, five-carbon ribulose 347 00:17:29,800 --> 00:17:31,570 5-phosphate. 348 00:17:31,570 --> 00:17:36,100 I've drawn it this way so you can see it 349 00:17:36,100 --> 00:17:41,980 from the perspective of what was a six-carbon molecule that's 350 00:17:41,980 --> 00:17:43,420 lost one carbon. 351 00:17:43,420 --> 00:17:46,090 But the more formal chemical way to draw this 352 00:17:46,090 --> 00:17:48,880 would be as the linear chain of five carbons 353 00:17:48,880 --> 00:17:53,770 starting with the new 1 carbon, which is the alcohol, 354 00:17:53,770 --> 00:17:56,410 with a purple circle. 355 00:17:56,410 --> 00:17:59,540 The next carbon would have the carbonyl group, 356 00:17:59,540 --> 00:18:00,850 which I'll use a blue box. 357 00:18:00,850 --> 00:18:05,650 And all the way down to what is now 358 00:18:05,650 --> 00:18:08,390 the five-carbon-- what was originally the six-carbon-- 359 00:18:08,390 --> 00:18:11,500 which has the dark black dot. 360 00:18:11,500 --> 00:18:17,350 Now because we're processing three molecules of hexose 361 00:18:17,350 --> 00:18:23,250 and now pentose, where we've got three molecules 362 00:18:23,250 --> 00:18:26,190 of ribulose 5-phosphate, and the two enzymes that 363 00:18:26,190 --> 00:18:30,180 are going to process this are ribulose 5-phosphate isomerase 364 00:18:30,180 --> 00:18:33,280 and ribulose 5-phosphate epimerase. 365 00:18:33,280 --> 00:18:35,640 Now, just because of the relative activities 366 00:18:35,640 --> 00:18:38,790 of these enzymes, we're going to see that about one-third 367 00:18:38,790 --> 00:18:41,850 of the molecules will be processed by the isomerase 368 00:18:41,850 --> 00:18:45,330 and two-thirds will be processed by the epimerase. 369 00:18:45,330 --> 00:18:48,330 Now, let me tell you a little bit about what isomerization 370 00:18:48,330 --> 00:18:52,150 is, versus epimerization. 371 00:18:52,150 --> 00:18:55,290 Bogdan and others have told you about the importance 372 00:18:55,290 --> 00:19:01,270 of enediol or diolate intermediates in biochemistry. 373 00:19:01,270 --> 00:19:05,130 The section of the molecule in the ribulose 5-phosphate 374 00:19:05,130 --> 00:19:09,000 that I'm circling here, if you just take and form 375 00:19:09,000 --> 00:19:12,030 an enediolate here, what you can do 376 00:19:12,030 --> 00:19:16,110 is you can move the carbonyl functionality from the 2 carbon 377 00:19:16,110 --> 00:19:17,620 up to the 1 carbon. 378 00:19:17,620 --> 00:19:22,020 And that's what happens when you convert ribulose 5-phosphate 379 00:19:22,020 --> 00:19:24,060 into ribose 5-phosphate. 380 00:19:24,060 --> 00:19:28,770 So it's a simple enediol conversion. 381 00:19:28,770 --> 00:19:33,240 Very similarly, I could have made the enediol 382 00:19:33,240 --> 00:19:36,610 between the 2 and the 3 carbon. 383 00:19:36,610 --> 00:19:39,450 And if I did that, I'll form an enediol. 384 00:19:39,450 --> 00:19:43,530 And depending on how I put the hydrogen 385 00:19:43,530 --> 00:19:48,360 back on that I take off, I can change the stereochemistry 386 00:19:48,360 --> 00:19:49,560 at the 3 carbon. 387 00:19:49,560 --> 00:19:53,040 In other words, I can form the epimer at the 3 carbon. 388 00:19:53,040 --> 00:19:57,240 So I can go from this epimer on the top 389 00:19:57,240 --> 00:20:00,600 to where the hydroxyl group is pointed to the right, 390 00:20:00,600 --> 00:20:03,450 to this epimer on the bottom, where the hydroxyl group is 391 00:20:03,450 --> 00:20:04,890 pointed to the left. 392 00:20:04,890 --> 00:20:09,270 This molecule, the epimer of ribulose 5-phosphate, where 393 00:20:09,270 --> 00:20:11,700 the hydroxyl on 3 is pointed to the left, 394 00:20:11,700 --> 00:20:14,340 is called xylulose 5-phosphate. 395 00:20:14,340 --> 00:20:18,750 As I said, this enzyme, this epimerase enzyme, 396 00:20:18,750 --> 00:20:22,260 is a little more active than the isomerase enzyme. 397 00:20:22,260 --> 00:20:26,070 So we get about two molecules of the xylulose 5-phosphate 398 00:20:26,070 --> 00:20:27,810 for every molecule that we process-- 399 00:20:27,810 --> 00:20:30,330 again, by enediolate-type intermediates 400 00:20:30,330 --> 00:20:32,640 to form the ribose 5-phosphate. 401 00:20:32,640 --> 00:20:35,900 So this gives us a-- 402 00:20:35,900 --> 00:20:38,910 ribulose 5-phosphate is now converted 403 00:20:38,910 --> 00:20:42,720 into one molecule of ribose 5-phosphate and two 404 00:20:42,720 --> 00:20:45,199 molecules of xylulose 5-phosphate. 405 00:20:49,510 --> 00:20:52,210 I'm going to give you a little bit of a mechanistic interlude 406 00:20:52,210 --> 00:20:55,840 here to remind you that some of the chemistry I'm 407 00:20:55,840 --> 00:20:58,330 going to be showing you with the enzyme transketolase 408 00:20:58,330 --> 00:21:00,090 is very reminiscent of chemistry we 409 00:21:00,090 --> 00:21:04,150 have seen several times in 5.07, specifically 410 00:21:04,150 --> 00:21:07,060 the enzymes pyruvate dehydrogenase, 411 00:21:07,060 --> 00:21:11,530 pyruvate decarboxylase, and alpha-ketoglutarate 412 00:21:11,530 --> 00:21:13,060 dehydrogenase. 413 00:21:13,060 --> 00:21:15,130 Now, the molecule that I have right here 414 00:21:15,130 --> 00:21:17,560 in this little mechanistic interlude box 415 00:21:17,560 --> 00:21:19,660 is one of the ones we were just talking about. 416 00:21:19,660 --> 00:21:22,960 Specifically, this is xylulose 5-phosphate. 417 00:21:22,960 --> 00:21:27,430 You see the epimer with the 3-hydroxyl off on the left. 418 00:21:27,430 --> 00:21:31,990 This carbonyl group is exactly the type of carbonyl 419 00:21:31,990 --> 00:21:35,550 that's attackable by thiamine pyrophosphate, TPP. 420 00:21:35,550 --> 00:21:38,590 Let me remind you a little bit about why 421 00:21:38,590 --> 00:21:43,750 TPP can be a nucleophile to attack this carbonyl group. 422 00:21:43,750 --> 00:21:46,300 TPP, when you look at the whole cofactor, 423 00:21:46,300 --> 00:21:50,210 it has attached up here an aminopyridine. 424 00:21:50,210 --> 00:21:53,130 And the amino group can be in the imino tautomer, 425 00:21:53,130 --> 00:21:57,040 and that imino tautomer can help pull off this hydrogen, 426 00:21:57,040 --> 00:21:58,540 lowering the PK. 427 00:21:58,540 --> 00:22:00,670 So the PK of this is actually something 428 00:22:00,670 --> 00:22:03,610 in the order of 18 or 19, which is 429 00:22:03,610 --> 00:22:07,660 sufficient to make this carbon a good nucleophile 430 00:22:07,660 --> 00:22:10,060 to be able to attack the carbonyl that's 431 00:22:10,060 --> 00:22:13,330 on our, in this case, xylulose 5-phosphate. 432 00:22:13,330 --> 00:22:17,320 That attack will result in the formation of an initial adduct 433 00:22:17,320 --> 00:22:21,460 in which the vitamin, the TPP, is covalently associated 434 00:22:21,460 --> 00:22:24,580 with the xylulose 5-phosphate. 435 00:22:24,580 --> 00:22:28,210 You can see that the keto group has been converted 436 00:22:28,210 --> 00:22:30,080 into a hydroxyl group. 437 00:22:30,080 --> 00:22:34,390 So we've now got a dihydroxy molecule at this point. 438 00:22:34,390 --> 00:22:37,540 And just as we've seen with the other enzymes I mentioned 439 00:22:37,540 --> 00:22:41,410 earlier-- for example, the pyruvate dehydrogenase-- 440 00:22:41,410 --> 00:22:45,160 the delocalization of electrons through this system 441 00:22:45,160 --> 00:22:47,470 makes it possible to be able to remove 442 00:22:47,470 --> 00:22:50,740 this hydrogen. These electrons move to this position. 443 00:22:50,740 --> 00:22:52,660 These electrons move up here. 444 00:22:52,660 --> 00:22:55,810 And that means that the bond connecting the bottom three 445 00:22:55,810 --> 00:23:01,420 carbons of the xylulose 5-phosphate is broken, 446 00:23:01,420 --> 00:23:03,070 and that gives you-- 447 00:23:03,070 --> 00:23:05,060 if you look at the chemistry-- 448 00:23:05,060 --> 00:23:08,290 glyceraldehyde 3-phosphate, the very common biochemical 449 00:23:08,290 --> 00:23:09,460 intermediate. 450 00:23:09,460 --> 00:23:12,790 And what's left over is the vitamin TPP 451 00:23:12,790 --> 00:23:16,120 attached to two-carbon unit. 452 00:23:16,120 --> 00:23:20,980 And it's called dihydroxyethyl TPP. 453 00:23:20,980 --> 00:23:24,420 Hydroxy, hydroxy, this is an ethyl group, 454 00:23:24,420 --> 00:23:26,860 and it's an anion, the TPP anion. 455 00:23:26,860 --> 00:23:31,450 So I abbreviate that minus, and then with a box, C2-TPP. 456 00:23:31,450 --> 00:23:35,590 It's a chemically reactive nucleophilic intermediate 457 00:23:35,590 --> 00:23:38,537 that we're going to be able to use to do powerful chemistry. 458 00:23:43,010 --> 00:23:45,200 The enzyme transketolase, which I'm 459 00:23:45,200 --> 00:23:46,970 going to talk about from the standpoint 460 00:23:46,970 --> 00:23:50,000 of medical importance somewhat later, 461 00:23:50,000 --> 00:23:54,410 enables the removal of the top two carbons, 462 00:23:54,410 --> 00:23:55,960 via the chemistry I just showed you. 463 00:23:55,960 --> 00:23:59,030 It's a kind of a chemical decapitation 464 00:23:59,030 --> 00:24:05,900 to remove the C2-TPP in order to liberate it to be able to be 465 00:24:05,900 --> 00:24:09,110 used as a chemical reagent. 466 00:24:09,110 --> 00:24:13,940 So both of our molecules of xylulose 5-phosphate are going 467 00:24:13,940 --> 00:24:17,330 to be susceptible to this chemistry, 468 00:24:17,330 --> 00:24:19,820 and that means we're going to get two molecules of this 469 00:24:19,820 --> 00:24:22,280 C2-TPP anion-- 470 00:24:22,280 --> 00:24:23,990 one here and one here-- 471 00:24:23,990 --> 00:24:25,790 and two molecules of glyceraldehyde 472 00:24:25,790 --> 00:24:28,370 3-phosphate-- here and here. 473 00:24:28,370 --> 00:24:29,360 OK? 474 00:24:29,360 --> 00:24:31,680 What are we going to do with the C2-TPP? 475 00:24:31,680 --> 00:24:34,760 Well, I want to draw your attention back up here 476 00:24:34,760 --> 00:24:37,600 to the ribulose 5-phosphate. 477 00:24:37,600 --> 00:24:40,130 Remember, I told you that it's possible, 478 00:24:40,130 --> 00:24:42,980 by way of an enediolate intermediate, 479 00:24:42,980 --> 00:24:45,440 with this isomerase enzyme, to move 480 00:24:45,440 --> 00:24:48,470 the carbonyl group from the 2 carbon up to the 1 carbon. 481 00:24:48,470 --> 00:24:51,410 And that produces ribose 5-phosphate. 482 00:24:51,410 --> 00:24:53,720 One of the things that I just want to point out here 483 00:24:53,720 --> 00:24:56,870 is ribose 5-phosphate was one of the things that's one 484 00:24:56,870 --> 00:24:58,080 of the goals of the pathway. 485 00:24:58,080 --> 00:25:02,540 So I could, for example, stop the pathway here and shunt off 486 00:25:02,540 --> 00:25:06,830 this ribose 5-phosphate to be able to make nucleotides. 487 00:25:06,830 --> 00:25:12,710 But this carbonyl is exactly the kind of functional group 488 00:25:12,710 --> 00:25:16,470 that the C2-TPP looks for, for chemical reaction. 489 00:25:16,470 --> 00:25:19,360 So I can plug in, just like a LEGO, 490 00:25:19,360 --> 00:25:23,660 the C2-TPP on the five-carbon ribose 5-phosphate. 491 00:25:23,660 --> 00:25:25,460 What that does is gives me-- 492 00:25:25,460 --> 00:25:27,330 5 plus 2 is 7-- 493 00:25:27,330 --> 00:25:29,900 sedoheptulose 7-phosphate. 494 00:25:29,900 --> 00:25:33,340 1, 2, 3, 4, 5, 6, 7. 495 00:25:33,340 --> 00:25:35,480 Sedoheptulose 7-phosphate. 496 00:25:35,480 --> 00:25:39,560 And just because of the way the chemistry works, 497 00:25:39,560 --> 00:25:43,100 we've basically pushed out the hydroxyl group at this position 498 00:25:43,100 --> 00:25:46,790 off on the left, on the number 3 carbon. 499 00:25:46,790 --> 00:25:51,230 OK, so now we have a seven-carbon molecule. 500 00:25:51,230 --> 00:25:54,530 And again, this is another biosynthetic opportunity. 501 00:25:54,530 --> 00:25:57,530 As I said earlier, ribose 5-phosphate 502 00:25:57,530 --> 00:26:00,180 can go off to make ribonucleotides. 503 00:26:00,180 --> 00:26:02,900 If you look up the pathway of synthesis 504 00:26:02,900 --> 00:26:05,960 of the aromatic amino acid tyrosine, what you'll see 505 00:26:05,960 --> 00:26:12,710 is that basically this is the biosynthetic intermediate that 506 00:26:12,710 --> 00:26:15,860 will go off to produce that aromatic amino acid. 507 00:26:15,860 --> 00:26:18,140 OK, so this is sedoheptulose 7-phosphate. 508 00:26:22,860 --> 00:26:27,150 Now if I look at the top three carbons here, 509 00:26:27,150 --> 00:26:32,930 you've got basically an alcohol, a carbonyl, and an alcohol. 510 00:26:32,930 --> 00:26:38,600 This looks almost identical to the carbon 1, 2, and 3 511 00:26:38,600 --> 00:26:44,030 that you see in the aldolase reaction back in glycolysis. 512 00:26:44,030 --> 00:26:46,850 In other words, the linear form of-- 513 00:26:46,850 --> 00:26:49,920 in that case, it's fructose 1,6-bisphosphate-- 514 00:26:53,970 --> 00:26:57,060 in the linear form looks very much like this. 515 00:26:57,060 --> 00:27:01,050 And what that means is that the aldolase enzyme, 516 00:27:01,050 --> 00:27:03,690 with its amino group at its active site, 517 00:27:03,690 --> 00:27:06,680 can attack this carbonyl group to form 518 00:27:06,680 --> 00:27:11,130 a protonated imine that will-- 519 00:27:11,130 --> 00:27:12,660 in the process of the chemistry, you 520 00:27:12,660 --> 00:27:14,550 can break the bond that connects carbon 521 00:27:14,550 --> 00:27:17,200 3 to carbon 4 of this molecule. 522 00:27:17,200 --> 00:27:20,790 So the enzyme transaldolase that does this chemistry 523 00:27:20,790 --> 00:27:23,820 is very similar to the aldolase enzyme 524 00:27:23,820 --> 00:27:26,890 that we have looked at in the glycolysis pathway. 525 00:27:26,890 --> 00:27:28,920 This is the splitting, in this case, 526 00:27:28,920 --> 00:27:32,250 of a seven-carbon compound into a three-carbon 527 00:27:32,250 --> 00:27:35,800 and a four-carbon, whereas in the case of glycolysis, 528 00:27:35,800 --> 00:27:38,730 it was splitting a six-carbon compound into two 529 00:27:38,730 --> 00:27:40,560 three-carbon compounds. 530 00:27:40,560 --> 00:27:45,595 This intermediate that I'm encircling here with the imine 531 00:27:45,595 --> 00:27:49,730 is very similar to the dihydroxyacetone phosphate 532 00:27:49,730 --> 00:27:50,710 precursor. 533 00:27:50,710 --> 00:27:54,880 So I'll call it a DHA-like fragment-- 534 00:27:54,880 --> 00:28:01,340 two methanol groups with a methyl imine in the middle. 535 00:28:01,340 --> 00:28:05,230 So what we've done is I've taken the top three 536 00:28:05,230 --> 00:28:07,840 carbons of the sedoheptulose 7-phosphate 537 00:28:07,840 --> 00:28:11,530 and made it into something that looks a lot like carbons 538 00:28:11,530 --> 00:28:18,320 1, 2, and 3 from the aldolase step of the glycolysis pathway. 539 00:28:18,320 --> 00:28:21,340 So it's a dihydroxyacetone-like phosphate. 540 00:28:21,340 --> 00:28:24,220 I'm just going to put that off to the side for a minute. 541 00:28:24,220 --> 00:28:28,210 The bottom four carbons, carbons 4, 5, 6, and 7 542 00:28:28,210 --> 00:28:30,130 of the sedoheptulose 7-phosphate, 543 00:28:30,130 --> 00:28:33,490 are something we haven't actually seen in 5.07 before. 544 00:28:33,490 --> 00:28:35,560 But if we look at what the structure is, 545 00:28:35,560 --> 00:28:37,240 it's going to be an aldehyde-- 546 00:28:37,240 --> 00:28:39,220 one, two, three, four-carbon aldehyde. 547 00:28:39,220 --> 00:28:43,090 And this is erythrose 4-phosphate, E4P. 548 00:28:43,090 --> 00:28:47,570 Now, that in itself is not a valuable molecule. 549 00:28:47,570 --> 00:28:49,570 But it turns out that we can make it 550 00:28:49,570 --> 00:28:52,510 into a valuable molecule very easily. 551 00:28:52,510 --> 00:28:54,340 Remember back when we were talking 552 00:28:54,340 --> 00:28:59,620 about this C2-TPP electrophilic molecule that 553 00:28:59,620 --> 00:29:02,350 can put two carbons onto anything that 554 00:29:02,350 --> 00:29:05,320 has an aldehyde at the end? 555 00:29:05,320 --> 00:29:09,430 Well, erythrose 4-phosphate has an aldehyde at the end. 556 00:29:09,430 --> 00:29:13,270 If we do the chemistry and we plug this C2 fragment 557 00:29:13,270 --> 00:29:17,500 onto the erythrose 4-phosphate fragment up at the aldehyde, 558 00:29:17,500 --> 00:29:21,580 you get a six-carbon compound that is fructose 6-phosphate. 559 00:29:21,580 --> 00:29:24,580 So this is the source of fructose 6-phosphate 560 00:29:24,580 --> 00:29:28,660 that can then become part of the gluconeogenesis or glycolysis 561 00:29:28,660 --> 00:29:29,440 pathways. 562 00:29:29,440 --> 00:29:30,790 So we haven't wasted anything. 563 00:29:30,790 --> 00:29:33,544 We've taken a kind of a trash molecule, 564 00:29:33,544 --> 00:29:34,960 the erythrose 4-phosphate, that we 565 00:29:34,960 --> 00:29:36,814 don't have a lot of other uses for, 566 00:29:36,814 --> 00:29:38,230 and we've converted into something 567 00:29:38,230 --> 00:29:42,260 that we can use for energy production, just as an example. 568 00:29:42,260 --> 00:29:43,900 Now what I'd like to do is come back up 569 00:29:43,900 --> 00:29:47,080 to this dihydroxyacetone-like fragment 570 00:29:47,080 --> 00:29:50,350 that I talked about a little bit ago. 571 00:29:50,350 --> 00:29:55,340 In the glycolysis pathway, we start with the hexose glucose, 572 00:29:55,340 --> 00:29:57,340 and eventually, after about four or five steps, 573 00:29:57,340 --> 00:30:02,710 we get to another hexose, fructose 1,6-bisphosphate. 574 00:30:02,710 --> 00:30:06,660 Then the aldolase step takes and splits that in half 575 00:30:06,660 --> 00:30:11,380 into, on the one hand, a dihydroxyacetone-like fragment 576 00:30:11,380 --> 00:30:14,350 and, on the other hand, a glyceraldehyde 3-phosphate 577 00:30:14,350 --> 00:30:15,460 fragment. 578 00:30:15,460 --> 00:30:19,550 And the dihydroxyacetone-like fragment 579 00:30:19,550 --> 00:30:23,420 is covalently connected by way of its middle carbon, 580 00:30:23,420 --> 00:30:27,940 the carbonyl, to the enzyme aldolase. 581 00:30:27,940 --> 00:30:31,300 When we look at what's happening in the transaldolase reaction, 582 00:30:31,300 --> 00:30:32,710 it's really the same thing. 583 00:30:32,710 --> 00:30:35,290 We've taken the seven-carbon molecule, in this case, 584 00:30:35,290 --> 00:30:38,650 and we've split off this three-carbon intermediate. 585 00:30:38,650 --> 00:30:42,010 Again, it's chemically just about identical to what 586 00:30:42,010 --> 00:30:46,060 happened in glycolysis, when we pulled apart 587 00:30:46,060 --> 00:30:48,310 the six-carbon sugar and we've now 588 00:30:48,310 --> 00:30:51,490 got this thing on the right which we can do something with. 589 00:30:51,490 --> 00:30:55,930 The dihydroxyacetone phosphate is covalently connected 590 00:30:55,930 --> 00:30:58,980 to the enzyme transaldolase. 591 00:30:58,980 --> 00:31:00,840 It's a transaldolase. 592 00:31:00,840 --> 00:31:03,900 It can do the aldolase reaction, the splitting, 593 00:31:03,900 --> 00:31:07,080 but then it can take and it can transfer very easily. 594 00:31:07,080 --> 00:31:09,120 What are we going to transfer it to? 595 00:31:09,120 --> 00:31:12,360 Well the dihydroxyacetone phosphate chemically 596 00:31:12,360 --> 00:31:17,190 can be transferred very easily to glyceraldehyde 3-phosphate. 597 00:31:17,190 --> 00:31:21,210 So I'm going to find a way now to connect 598 00:31:21,210 --> 00:31:25,700 this dihydroxyacetone-like fragment 599 00:31:25,700 --> 00:31:28,130 to a glyceraldehyde 3-phosphate. 600 00:31:28,130 --> 00:31:30,410 And I find one right here. 601 00:31:30,410 --> 00:31:33,980 Let me go back a little bit and see where it came from. 602 00:31:33,980 --> 00:31:36,250 I had the xylulose 5-phosphate. 603 00:31:36,250 --> 00:31:38,810 I knocked off the top two carbons. 604 00:31:38,810 --> 00:31:43,430 I used those in order to make the sedoheptulose 7-phosphate. 605 00:31:43,430 --> 00:31:46,650 My residue was the three-carbon molecule 606 00:31:46,650 --> 00:31:48,020 glyceraldehyde 3-phosphate. 607 00:31:48,020 --> 00:31:51,320 But again, this could be a waste product, but instead 608 00:31:51,320 --> 00:31:54,080 of using it as a waste product, what I can do 609 00:31:54,080 --> 00:31:57,920 is to take the glyceraldehyde 3-phosphate that 610 00:31:57,920 --> 00:32:02,960 is a waste product of the transketolase reaction 611 00:32:02,960 --> 00:32:07,410 down here, and I can utilize it by combination 612 00:32:07,410 --> 00:32:10,470 with the dihydroxyacetone phosphate from up here, 613 00:32:10,470 --> 00:32:12,630 the transaldolase reaction. 614 00:32:12,630 --> 00:32:15,090 Transaldolase has the transfer ability 615 00:32:15,090 --> 00:32:18,300 to bring these molecules together covalently, plug them 616 00:32:18,300 --> 00:32:21,010 together, 3 and 3, to make 6. 617 00:32:21,010 --> 00:32:23,850 And that's your second molecule of fructose 6-phosphate. 618 00:32:28,540 --> 00:32:31,730 While I've still got this sheet with the whole pathway on it, 619 00:32:31,730 --> 00:32:34,460 I'm going to give you a little bit of a summary. 620 00:32:34,460 --> 00:32:38,460 I took three molecules of glucose 6-phosphate, 621 00:32:38,460 --> 00:32:43,930 oxidized it twice, generated two molecules for each molecule 622 00:32:43,930 --> 00:32:47,480 of glucose 6-phosphate. 623 00:32:47,480 --> 00:32:52,620 I got two molecules of NADPH, the reducing equivalents. 624 00:32:52,620 --> 00:32:56,280 The pathway up to that point created a beta ketoacid. 625 00:32:56,280 --> 00:32:58,770 Therefore there was this decarboxylation event. 626 00:32:58,770 --> 00:33:02,780 So what I ended up with is three five-carbon molecules, 627 00:33:02,780 --> 00:33:04,680 ribulose 5-phosphate. 628 00:33:04,680 --> 00:33:06,870 The ribulose 5-phosphate, depending 629 00:33:06,870 --> 00:33:09,180 upon the balance of activities of the next two enzymes, 630 00:33:09,180 --> 00:33:12,420 will partition at about a 2-to-1 ratio 631 00:33:12,420 --> 00:33:18,540 into xylulose 5-phosphate and ribose 5-phosphate. 632 00:33:18,540 --> 00:33:22,680 Then I explained how the xylulose 5-phosphate had 633 00:33:22,680 --> 00:33:24,780 this carbonyl at the second position, 634 00:33:24,780 --> 00:33:26,350 and that looked like the carbonyl 635 00:33:26,350 --> 00:33:28,170 in a lot of other biochemical reactions 636 00:33:28,170 --> 00:33:30,270 we've seen earlier in 5.07. 637 00:33:30,270 --> 00:33:35,930 And it's vulnerable to attack by the ylid carbanion of TPP, 638 00:33:35,930 --> 00:33:39,100 thiamine pyrophosphate. 639 00:33:39,100 --> 00:33:41,950 Applying that transketolase enzyme 640 00:33:41,950 --> 00:33:46,900 to xylulose 5-phosphate split off the top two carbons 641 00:33:46,900 --> 00:33:52,730 as this electrophilic chemical reagent that I call C2-TPP. 642 00:33:52,730 --> 00:33:57,670 The bottom three carbons became glyceraldehyde 3-phosphate. 643 00:33:57,670 --> 00:34:01,840 The C2-TPP got added onto ribose 5-phosphate, which 644 00:34:01,840 --> 00:34:07,570 is one of the molecules that was made from the early-on ribulose 645 00:34:07,570 --> 00:34:08,830 5-phosphate. 646 00:34:08,830 --> 00:34:10,690 That made the seven-carbon molecule 647 00:34:10,690 --> 00:34:12,520 sedoheptulose 7-phosphate. 648 00:34:12,520 --> 00:34:15,265 And again, I said that that's a precursor to other things, 649 00:34:15,265 --> 00:34:18,889 like aromatic amino acids, if you need them. 650 00:34:18,889 --> 00:34:21,610 The sedoheptulose 7-phosphate, however, 651 00:34:21,610 --> 00:34:24,460 has a structure that makes it amenable to having 652 00:34:24,460 --> 00:34:28,389 its top three carbons taken off by an aldolase-like enzyme, 653 00:34:28,389 --> 00:34:29,980 transaldolase. 654 00:34:29,980 --> 00:34:32,139 What makes this aldolase special is it 655 00:34:32,139 --> 00:34:38,989 will take the three carbons off as a protein three-carbon 656 00:34:38,989 --> 00:34:42,440 adduct and allow that three carbons 657 00:34:42,440 --> 00:34:46,429 to be added onto one of the glyceraldehyde 3-phosphates 658 00:34:46,429 --> 00:34:50,989 that was generated in the transketolase reaction. 659 00:34:50,989 --> 00:34:55,949 3 plus 3 is 6, and that becomes your fructose 6-phosphate. 660 00:34:55,949 --> 00:35:03,410 Overall, what happens is you put in three glucose 6-phosphates, 661 00:35:03,410 --> 00:35:06,470 you lose some CO2s, and you produce 662 00:35:06,470 --> 00:35:09,450 two fructose 6-phosphates and one 663 00:35:09,450 --> 00:35:12,770 leftover glyceraldehyde 3-phosphate. 664 00:35:12,770 --> 00:35:16,400 And these are in the mainstream of glycolysis 665 00:35:16,400 --> 00:35:18,180 and gluconeogenesis. 666 00:35:18,180 --> 00:35:21,740 So that's the details of the pathway. 667 00:35:21,740 --> 00:35:24,230 We start with a six-carbon compound, 668 00:35:24,230 --> 00:35:27,380 three molecules of it, and we generate 669 00:35:27,380 --> 00:35:29,630 three molecules of CO2. 670 00:35:29,630 --> 00:35:33,420 We're going to generate six molecules of NADPH. 671 00:35:33,420 --> 00:35:37,790 And at the beginning of the page that we're looking at here, 672 00:35:37,790 --> 00:35:40,160 we have three molecules of a five-carbon compound, 673 00:35:40,160 --> 00:35:41,630 this ribulose 5-phosphate. 674 00:35:41,630 --> 00:35:45,260 And again, at this point, the carbon-scrambling phase 675 00:35:45,260 --> 00:35:46,010 takes over. 676 00:35:46,010 --> 00:35:48,570 And at the end, we end up with taking 677 00:35:48,570 --> 00:35:54,230 the 15 carbons of the three 5 carbon ribulose 5-phosphates, 678 00:35:54,230 --> 00:35:58,380 and scrambling them into two six-carbon fructose 679 00:35:58,380 --> 00:36:01,040 6-phosphates and one three-carbon glyceraldehyde 680 00:36:01,040 --> 00:36:01,820 3-phosphate. 681 00:36:05,930 --> 00:36:08,570 We've now looked at the pentose phosphate 682 00:36:08,570 --> 00:36:10,590 pathway from two levels. 683 00:36:10,590 --> 00:36:16,010 Let's say initially, what I see right here is the pathway from, 684 00:36:16,010 --> 00:36:18,380 let's say, the 10,000-foot level. 685 00:36:18,380 --> 00:36:22,850 We see there's the mainstream of metabolism of glucose 686 00:36:22,850 --> 00:36:23,910 to pyruvate-- 687 00:36:23,910 --> 00:36:27,650 that's glycolysis-- or pyruvate to glucose, gluconeogenesis. 688 00:36:27,650 --> 00:36:31,850 And we have seen how in the pentose phosphate pathway, 689 00:36:31,850 --> 00:36:34,730 we go from glucose 6-phosphate, we 690 00:36:34,730 --> 00:36:37,610 do some oxidation-- that's why you get the NADPH. 691 00:36:37,610 --> 00:36:39,140 You lose some CO2. 692 00:36:39,140 --> 00:36:40,630 You get the pentoses. 693 00:36:40,630 --> 00:36:44,870 I've introduced you to ribose, ribulose, and xylulose. 694 00:36:44,870 --> 00:36:48,440 Then in order to get these pentoses back 695 00:36:48,440 --> 00:36:50,420 into mainstream metabolism, we do 696 00:36:50,420 --> 00:36:54,170 this complicated carbon-scrambling routine, 697 00:36:54,170 --> 00:36:58,820 and then get back to fructose 6-phosphate and GAP to be 698 00:36:58,820 --> 00:37:02,510 able to continue in mainstream metabolism. 699 00:37:02,510 --> 00:37:04,130 Nothing is wasted. 700 00:37:04,130 --> 00:37:06,140 There's the 10,000-foot level. 701 00:37:06,140 --> 00:37:08,090 And then I went, down here at the bottom, 702 00:37:08,090 --> 00:37:11,420 to sort of the 2-foot level, all the little details 703 00:37:11,420 --> 00:37:12,740 of the chemistry. 704 00:37:12,740 --> 00:37:17,040 So I've taught you this, the pentose phosphate pathway, 705 00:37:17,040 --> 00:37:19,820 in the beginning at the 10,000-foot level, 706 00:37:19,820 --> 00:37:22,670 and then got you down to the 2-foot level with all 707 00:37:22,670 --> 00:37:24,980 of the details of the chemical mechanisms. 708 00:37:24,980 --> 00:37:29,060 And our course, Biological Chemistry 5.07, 709 00:37:29,060 --> 00:37:32,150 is a chemistry course, so that's very appropriate. 710 00:37:32,150 --> 00:37:36,260 But 5.07 is also 25.07. 711 00:37:36,260 --> 00:37:38,420 20 is the Biological Engineering department. 712 00:37:38,420 --> 00:37:43,010 And the engineers actually think a lot about, let's say, 713 00:37:43,010 --> 00:37:45,770 mathematical equations more than chemical equations. 714 00:37:45,770 --> 00:37:47,810 And they think about schematics, written out 715 00:37:47,810 --> 00:37:50,510 like you'd look at an electrical schematic. 716 00:37:50,510 --> 00:37:53,900 So because the Biological Chemistry group 717 00:37:53,900 --> 00:37:56,630 we have is quite large, we have kind of an ending 718 00:37:56,630 --> 00:38:02,330 to this pathway that appeals, I think, to more the engineering 719 00:38:02,330 --> 00:38:03,200 mindset. 720 00:38:03,200 --> 00:38:06,440 So for the engineers, I'm going to move from the two-foot level 721 00:38:06,440 --> 00:38:08,840 up to about the 100-foot level. 722 00:38:08,840 --> 00:38:12,350 And I'm going to present it as a schematic. 723 00:38:12,350 --> 00:38:16,520 The shorthand that I use involves these little boxes. 724 00:38:16,520 --> 00:38:18,560 Each of these is a molecule. 725 00:38:18,560 --> 00:38:20,870 And the abbreviation is in the top-- 726 00:38:20,870 --> 00:38:23,780 G6P, glucose 6-phosphate. 727 00:38:23,780 --> 00:38:24,840 It has six carbons. 728 00:38:24,840 --> 00:38:27,620 That'll help me keep track of how many carbons are there. 729 00:38:27,620 --> 00:38:29,120 And there are three molecules of it. 730 00:38:29,120 --> 00:38:33,200 So we've got 6 times 3, or 18 carbons, that we start with. 731 00:38:33,200 --> 00:38:38,180 In the oxidative phase, we've seen that each molecule 732 00:38:38,180 --> 00:38:42,350 gives rise to two NADPHs. 733 00:38:42,350 --> 00:38:44,420 2 times 3 is 6. 734 00:38:44,420 --> 00:38:46,335 So we've got 6 NADPHs. 735 00:38:46,335 --> 00:38:48,710 There are three molecules that are starting, and remember 736 00:38:48,710 --> 00:38:52,060 that beta ketoacid that's formed? 737 00:38:52,060 --> 00:38:54,700 We have three of those, and they're going to carboxylate. 738 00:38:54,700 --> 00:38:57,290 So I get rid of three CO2s. 739 00:38:57,290 --> 00:39:02,000 So the broken line indicates the boundary 740 00:39:02,000 --> 00:39:05,870 between the irreversible and the reversible parts 741 00:39:05,870 --> 00:39:07,680 of the reaction. 742 00:39:07,680 --> 00:39:10,220 This gives us ribulose 5-phosphate. 743 00:39:10,220 --> 00:39:13,552 That's our initial product entering the pathway. 744 00:39:13,552 --> 00:39:15,800 It's a five-carbon molecule. 745 00:39:15,800 --> 00:39:19,820 There are three molecules of it, total of 15 carbons. 746 00:39:19,820 --> 00:39:23,940 Depending upon the activities of the epimerase in the bottom-- 747 00:39:23,940 --> 00:39:25,510 E-- or the isomerase-- 748 00:39:25,510 --> 00:39:32,050 I-- on the top, you either get more or less of the X5P, 749 00:39:32,050 --> 00:39:36,440 xylulose 5-phosphate, or ribose 5-phosphate, R5P. 750 00:39:36,440 --> 00:39:41,000 And as I said, typically the epimerase 751 00:39:41,000 --> 00:39:44,630 wins out by about 2-to-1 over the isomerase. 752 00:39:44,630 --> 00:39:48,710 So we've got a simple isomerization 753 00:39:48,710 --> 00:39:53,540 to take the ribulose 5-phosphate to ribose 5-phosphate. 754 00:39:53,540 --> 00:39:54,560 It's a C5 molecule. 755 00:39:54,560 --> 00:39:55,730 You get one of those. 756 00:39:55,730 --> 00:39:59,990 So we've got five of our carbons over here. 757 00:39:59,990 --> 00:40:02,150 If we look at the epimerase reactions, 758 00:40:02,150 --> 00:40:06,980 we get two boxes, one for each molecule 759 00:40:06,980 --> 00:40:08,690 of xylulose 5-phosphate. 760 00:40:08,690 --> 00:40:10,220 Each of them are C51. 761 00:40:10,220 --> 00:40:12,440 So those are our two molecules. 762 00:40:12,440 --> 00:40:16,270 Now let's take our first molecule of xylulose 763 00:40:16,270 --> 00:40:19,970 5-phosphate, the one in the top X5P box. 764 00:40:19,970 --> 00:40:23,675 And again, keep in mind that these arrowheads 765 00:40:23,675 --> 00:40:25,040 are going both ways. 766 00:40:25,040 --> 00:40:29,070 Because the carbon can flow both ways in these reactions. 767 00:40:29,070 --> 00:40:31,290 These are equilibrium reactions. 768 00:40:31,290 --> 00:40:36,310 We can take the top two carbons off of the X5P. 769 00:40:36,310 --> 00:40:41,030 This is the thiamine pyrophosphate reaction. 770 00:40:41,030 --> 00:40:44,150 And what that does is it gives us that chemically reactive 771 00:40:44,150 --> 00:40:46,340 C2-TPP fragment. 772 00:40:46,340 --> 00:40:48,920 Carbon 2, two carbons, one of those. 773 00:40:48,920 --> 00:40:49,520 OK? 774 00:40:49,520 --> 00:40:52,240 And the bottom half is glyceraldehyde 3-phosphate-- 775 00:40:52,240 --> 00:40:55,440 a three-carbon molecule, one of those. 776 00:40:55,440 --> 00:41:01,160 The ribose 5-phosphate, the five-carbon aldehyde, 777 00:41:01,160 --> 00:41:08,320 can have the C2-TPP add to it's one carbon. 778 00:41:08,320 --> 00:41:10,700 2 carbons plus 5 carbons give 7. 779 00:41:10,700 --> 00:41:14,580 Sedoheptulose 7-phosphate is the product-- 780 00:41:14,580 --> 00:41:16,560 one molecule of those. 781 00:41:16,560 --> 00:41:21,270 Transaldolase can split off the top three carbons 782 00:41:21,270 --> 00:41:24,690 to make that dihydroacetone-like fragment. 783 00:41:24,690 --> 00:41:26,670 This is the one that's covalently 784 00:41:26,670 --> 00:41:33,080 attached by this imine-like bond to the triose sugar. 785 00:41:33,080 --> 00:41:36,690 Three-carbon sugar-- one molecule of that. 786 00:41:36,690 --> 00:41:40,190 And after you do this transaldolase reaction 787 00:41:40,190 --> 00:41:42,120 and you've taken off the top three carbons, 788 00:41:42,120 --> 00:41:43,770 you've got a residue of four carbons. 789 00:41:43,770 --> 00:41:49,980 So that's the erythrose 4-phosphate, C4, one molecule. 790 00:41:49,980 --> 00:41:54,350 Now, the transaldolase reaction can do a transfer. 791 00:41:54,350 --> 00:41:58,450 So you'll take this DHA, dihydroxyacetone-like fragment, 792 00:41:58,450 --> 00:42:02,770 and combine it with glyceraldehyde 3-phosphate 793 00:42:02,770 --> 00:42:07,300 that it got from the molecule 1 of xylulose 5-phosphate. 794 00:42:07,300 --> 00:42:08,980 So we can follow the schematic. 795 00:42:08,980 --> 00:42:12,190 Xylulose 5-phosphate gives us this gap molecule, 796 00:42:12,190 --> 00:42:14,020 glyceraldehyde 3phosphate. 797 00:42:14,020 --> 00:42:15,740 It can combine with the molecule, 798 00:42:15,740 --> 00:42:22,120 the DHA-like fragment, to form for our first molecule 799 00:42:22,120 --> 00:42:24,310 of fructose 6-phosphate, which will then 800 00:42:24,310 --> 00:42:27,820 go off into glycolysis or gluconeogenesis. 801 00:42:27,820 --> 00:42:32,050 Now, the residue of this was this erythrose 4-phosphate 802 00:42:32,050 --> 00:42:32,830 product. 803 00:42:32,830 --> 00:42:34,230 What can we do with it? 804 00:42:34,230 --> 00:42:37,300 For again, this is an aldehyde. 805 00:42:37,300 --> 00:42:39,520 It can react with a C2 fragment from the C2-TPP. 806 00:42:42,520 --> 00:42:46,690 So where we get that C2-TPP is from our second molecule 807 00:42:46,690 --> 00:42:48,700 of xylulose 5-phosphate. 808 00:42:48,700 --> 00:42:52,990 Transketolase takes off the top two carbons 809 00:42:52,990 --> 00:42:56,140 and plunks it onto the erythrose 4-phosphate to form 810 00:42:56,140 --> 00:43:00,550 our second molecule of two of fructose 6-phosphate. 811 00:43:00,550 --> 00:43:02,270 C6, one molecule. 812 00:43:02,270 --> 00:43:05,140 And what's left over after all is said and done 813 00:43:05,140 --> 00:43:08,470 is this fragment right here, glyceraldehyde 3-phosphate. 814 00:43:08,470 --> 00:43:11,640 C3 molecule, one molecule. 815 00:43:11,640 --> 00:43:13,510 So if I did this correctly, we should 816 00:43:13,510 --> 00:43:15,670 have 15 carbons over here-- 817 00:43:15,670 --> 00:43:17,560 6, 6, and 3-- 818 00:43:17,560 --> 00:43:18,820 and we do. 819 00:43:18,820 --> 00:43:21,915 So these are the molecules that will flow into glycolysis 820 00:43:21,915 --> 00:43:24,280 or gluconeogenesis. 821 00:43:24,280 --> 00:43:30,010 Now because most of these arrows are double-headed, 822 00:43:30,010 --> 00:43:32,560 you can also imagine that fructose 823 00:43:32,560 --> 00:43:35,800 6-phosphate and glyceraldehyde 3-phosphate, 824 00:43:35,800 --> 00:43:41,140 in the mainstream metabolism, in glycolysis and gluconeogenesis, 825 00:43:41,140 --> 00:43:47,810 can backflow right to left and end up with these pentoses. 826 00:43:47,810 --> 00:43:52,190 So for example, if I wanted to make ribose 5-phosphate 827 00:43:52,190 --> 00:43:55,250 in order to make ATP, and I didn't 828 00:43:55,250 --> 00:43:57,410 need any reducing equivalents, all I 829 00:43:57,410 --> 00:44:01,340 would do is run all of this in reverse, get up to this point, 830 00:44:01,340 --> 00:44:03,830 ribose 5-phosphate, and then take this off 831 00:44:03,830 --> 00:44:05,630 to make nucleotides. 832 00:44:05,630 --> 00:44:07,250 On the other hand, if I'd like to make 833 00:44:07,250 --> 00:44:13,010 just NADPH for biosynthesis, what I would do 834 00:44:13,010 --> 00:44:14,330 is the following. 835 00:44:14,330 --> 00:44:16,820 I'd start with my glucose-6 phosphate. 836 00:44:16,820 --> 00:44:22,670 Early on, yield my 6 NADPHs. 837 00:44:22,670 --> 00:44:27,050 Go from left to right across the pathway, 838 00:44:27,050 --> 00:44:30,680 all the way to produce fructose 6-phosphate and GAP. 839 00:44:30,680 --> 00:44:33,950 And then what I'd do is I'd use gluconeogenesis in order 840 00:44:33,950 --> 00:44:38,670 to convert fructose 6-phosphate and GAP back 841 00:44:38,670 --> 00:44:42,870 into glucose 6-phosphate that then would be able to re-enter 842 00:44:42,870 --> 00:44:44,670 the pathway once again. 843 00:44:44,670 --> 00:44:46,410 Now, what I want to do is to show you 844 00:44:46,410 --> 00:44:50,790 that, I want to go back to the original drawing that I had, 845 00:44:50,790 --> 00:44:53,560 to show you how that would work. 846 00:44:53,560 --> 00:44:58,050 So what I just said was that if I were in a situation 847 00:44:58,050 --> 00:45:02,200 where all I wanted was NADPH, I could do the following. 848 00:45:02,200 --> 00:45:06,360 I could enter the carbon from glucose 6-phosphate, 849 00:45:06,360 --> 00:45:11,130 make the pentoses, get my NADPH, and then take the carbon 850 00:45:11,130 --> 00:45:14,490 and flush it back through the scrambling phase 851 00:45:14,490 --> 00:45:17,700 to produce fructose 6-phosphate and GAP. 852 00:45:17,700 --> 00:45:20,240 Now here's the important part, is 853 00:45:20,240 --> 00:45:22,410 I can now, through gluconeogenesis, 854 00:45:22,410 --> 00:45:25,020 move the carbon back up to glucose 6-phosphate 855 00:45:25,020 --> 00:45:28,530 and do it again, and again, and again. 856 00:45:28,530 --> 00:45:30,300 In other words, recursively, I can 857 00:45:30,300 --> 00:45:36,060 metabolize all of the carbon from glucose 6-phosphate. 858 00:45:36,060 --> 00:45:40,920 All six carbons can eventually become converted into CO2 859 00:45:40,920 --> 00:45:43,740 with copious amounts of NADPH. 860 00:45:43,740 --> 00:45:46,590 And that's part of the power of this pathway. 861 00:45:46,590 --> 00:45:47,860 It's very versatile. 862 00:45:47,860 --> 00:45:49,980 You can use it to make reducing equivalents. 863 00:45:49,980 --> 00:45:52,110 You can use it to make sugars. 864 00:45:52,110 --> 00:45:55,110 You can use it to make sedoheptulose 7-phosphate 865 00:45:55,110 --> 00:45:56,610 to make aromatic amino acids. 866 00:45:56,610 --> 00:45:58,170 It's a very powerful pathway. 867 00:46:03,310 --> 00:46:08,330 The top part of this picture shows us 868 00:46:08,330 --> 00:46:09,574 the schematic of the pathway. 869 00:46:09,574 --> 00:46:11,240 And I think with the schematic up there, 870 00:46:11,240 --> 00:46:15,770 I think this is a good time to make some summary comments. 871 00:46:15,770 --> 00:46:17,930 So one thing I mentioned at the very beginning 872 00:46:17,930 --> 00:46:23,150 is that we need NADPH in order to finance biosynthesis 873 00:46:23,150 --> 00:46:24,230 of lipids-- 874 00:46:24,230 --> 00:46:25,910 in particular for making membranes. 875 00:46:25,910 --> 00:46:27,360 So these are needed for growth. 876 00:46:27,360 --> 00:46:30,890 So I think now you can see why this pathway is turned 877 00:46:30,890 --> 00:46:34,610 on in cells that are making new tissue, that 878 00:46:34,610 --> 00:46:36,470 are making new cells. 879 00:46:36,470 --> 00:46:38,390 If you run it in the oxidative mode-- 880 00:46:38,390 --> 00:46:40,600 that's the one that I just described, 881 00:46:40,600 --> 00:46:43,970 where you purely use it to make NADPH-- 882 00:46:43,970 --> 00:46:49,640 you can oxidize all the carbons of your glucose 6-phosphate 883 00:46:49,640 --> 00:46:50,900 into CO2. 884 00:46:50,900 --> 00:46:53,900 That's the scenario that I just ran. 885 00:46:53,900 --> 00:46:56,930 I didn't emphasize enough that if, for example, you 886 00:46:56,930 --> 00:47:00,620 eat food that contains nucleic acid, which of course 887 00:47:00,620 --> 00:47:04,540 we all do, that nucleic acid contains ribose. 888 00:47:04,540 --> 00:47:06,860 That ribose, we want to be able to do something with. 889 00:47:06,860 --> 00:47:10,610 And what happens is it enters right here into the pathway. 890 00:47:10,610 --> 00:47:12,750 In other words, if I wanted to take that ribose 891 00:47:12,750 --> 00:47:15,110 and if I wanted to make energy out of it, 892 00:47:15,110 --> 00:47:17,690 the ribose would go from left to right, 893 00:47:17,690 --> 00:47:20,630 and I would end up with fructose 6-phosphate, 894 00:47:20,630 --> 00:47:21,820 glyceraldehyde 3-phosphate. 895 00:47:21,820 --> 00:47:24,500 And then I could use that, putting it through glycolysis 896 00:47:24,500 --> 00:47:26,380 and then the TCA cycle to make energy. 897 00:47:31,460 --> 00:47:34,300 Point 4 is one that I mentioned earlier. 898 00:47:34,300 --> 00:47:36,950 I'm a toxicologist, and point 4 shows 899 00:47:36,950 --> 00:47:41,180 how NADPH helps us defend against oxidative stress. 900 00:47:41,180 --> 00:47:45,200 And I have this little cartoon at the bottom. 901 00:47:45,200 --> 00:47:52,310 Free radicals are a consequence of living 902 00:47:52,310 --> 00:47:54,170 in an aerobic environment. 903 00:47:54,170 --> 00:48:01,310 We breathe oxygen. It can be incompletely reduced 904 00:48:01,310 --> 00:48:04,910 to produce things like superoxide and hydrogen 905 00:48:04,910 --> 00:48:08,420 peroxide and even hydroxyl radical. 906 00:48:08,420 --> 00:48:10,700 These are chemically very dangerous, 907 00:48:10,700 --> 00:48:13,820 and they'll form peroxides and other molecules that 908 00:48:13,820 --> 00:48:16,970 can cause disease. 909 00:48:16,970 --> 00:48:21,260 So we have protective systems based on a variety 910 00:48:21,260 --> 00:48:23,360 of small molecules in the cell. 911 00:48:23,360 --> 00:48:25,750 One of them is called glutathione. 912 00:48:25,750 --> 00:48:28,040 Millimolar concentrations of glutathione 913 00:48:28,040 --> 00:48:30,400 exist in our liver cells, and they exist in order 914 00:48:30,400 --> 00:48:32,150 to be able to do the kind of chemistry I'm 915 00:48:32,150 --> 00:48:33,890 going to show you right here. 916 00:48:33,890 --> 00:48:36,410 So a peroxide that I'm pointing to, 917 00:48:36,410 --> 00:48:39,470 ROOH, is dangerous because it can give rise 918 00:48:39,470 --> 00:48:43,400 to these free radicals that can be damaging to us. 919 00:48:43,400 --> 00:48:47,570 What we can do is take two molecules of this 920 00:48:47,570 --> 00:48:51,980 sulfur-containing glutathione-- it's a tripeptide-- 921 00:48:51,980 --> 00:48:59,140 and they will give up their H dot dot, their hydrides, 922 00:48:59,140 --> 00:49:04,930 in order to be able to basically break this ROOH into ROH 923 00:49:04,930 --> 00:49:08,830 and alcohol and HOH, water-- 924 00:49:08,830 --> 00:49:13,000 in other words, relatively harmless products. 925 00:49:13,000 --> 00:49:16,570 Now, glutathione, the RSH-- 926 00:49:16,570 --> 00:49:20,650 sometimes called GSH-- sacrifices itself and becomes 927 00:49:20,650 --> 00:49:24,235 this oxidized form, RSSR. 928 00:49:24,235 --> 00:49:26,110 Now, the reaction that does this first part's 929 00:49:26,110 --> 00:49:28,510 called glutathione peroxidase. 930 00:49:28,510 --> 00:49:34,810 In order to restore our cellular stores of reduced glutathione, 931 00:49:34,810 --> 00:49:39,820 RSH, we have to take the oxidized form, RSSR, 932 00:49:39,820 --> 00:49:41,230 and we have to reduce it. 933 00:49:41,230 --> 00:49:43,240 That's where the NADPH comes in. 934 00:49:43,240 --> 00:49:47,800 NADPH is the cofactor used by glutathione reductase. 935 00:49:47,800 --> 00:49:50,320 And what it does is puts in the reducing equivalents 936 00:49:50,320 --> 00:49:54,400 to basically convert oxidized glutathione back 937 00:49:54,400 --> 00:49:57,640 to what we want, which is the reduced glutathione, that 938 00:49:57,640 --> 00:50:01,640 can then go on to protect us against free radicals. 939 00:50:01,640 --> 00:50:06,580 NADPH has a very special role in protecting any cell 940 00:50:06,580 --> 00:50:08,380 from oxidative stress. 941 00:50:08,380 --> 00:50:10,570 It has a very special role, however, 942 00:50:10,570 --> 00:50:12,310 when you deal with red blood cells. 943 00:50:12,310 --> 00:50:13,810 Red blood cells lack a nucleus. 944 00:50:13,810 --> 00:50:15,500 They lack a mitochondrion. 945 00:50:15,500 --> 00:50:19,090 They're pretty much entirely cytosol. 946 00:50:19,090 --> 00:50:21,712 Cytosol has a glycolysis pathway. 947 00:50:21,712 --> 00:50:23,170 That's where they get their energy. 948 00:50:23,170 --> 00:50:25,222 And the pentose phosphate pathway, 949 00:50:25,222 --> 00:50:27,430 which is where they get the reducing equivalents that 950 00:50:27,430 --> 00:50:30,310 are necessary to help maintain the red blood 951 00:50:30,310 --> 00:50:32,350 cells' integrity-- if you think about it, 952 00:50:32,350 --> 00:50:34,540 you know, a red blood cell's carrying a lot oxygen. 953 00:50:34,540 --> 00:50:36,619 Oxygen itself can be very damaging. 954 00:50:36,619 --> 00:50:38,410 Red blood cells are damaged, and you really 955 00:50:38,410 --> 00:50:41,770 need to have these reducing equivalents available in order 956 00:50:41,770 --> 00:50:45,130 to maintain the structural and functional integrity 957 00:50:45,130 --> 00:50:46,550 of a red blood cell. 958 00:50:46,550 --> 00:50:52,810 So anything that disrupts the NADPH pool in a red blood cell 959 00:50:52,810 --> 00:50:54,760 ultimately causes a kind of anemia, 960 00:50:54,760 --> 00:50:57,850 because that means the red blood cell can't persist 961 00:50:57,850 --> 00:51:00,100 as long as it would otherwise. 962 00:51:00,100 --> 00:51:04,850 People who have defects in NADPH metabolism-- for example, 963 00:51:04,850 --> 00:51:10,690 a person who might have a defect or a sluggish first enzyme 964 00:51:10,690 --> 00:51:14,620 in the pathway, the glucose 6-phosphate dehydrogenase-- 965 00:51:14,620 --> 00:51:17,170 the red blood cells, because of this NADPH problem, 966 00:51:17,170 --> 00:51:19,330 don't last as long. 967 00:51:19,330 --> 00:51:21,850 Clinically, it's observed that these people 968 00:51:21,850 --> 00:51:24,130 have somewhat of a resistance against diseases 969 00:51:24,130 --> 00:51:26,310 such as malaria. 970 00:51:26,310 --> 00:51:28,320 That might be because the red blood 971 00:51:28,320 --> 00:51:31,620 cells don't last long enough for the organism that 972 00:51:31,620 --> 00:51:34,770 causes malaria, Plasmodium falciparum, 973 00:51:34,770 --> 00:51:37,200 to be able to complete its life cycle. 974 00:51:37,200 --> 00:51:41,330 So sometimes a genetic defect can actually provide an asset. 975 00:51:41,330 --> 00:51:45,350 And in some places in the world where people have evolved 976 00:51:45,350 --> 00:51:48,930 in the presence of the malaria Plasmodium organism, 977 00:51:48,930 --> 00:51:51,060 they have defects in the pathway that 978 00:51:51,060 --> 00:51:53,460 are maintained, from generation to generation, 979 00:51:53,460 --> 00:51:56,095 in order to be resistant to the environment. 980 00:51:56,095 --> 00:51:57,720 The sixth point that I want to bring up 981 00:51:57,720 --> 00:52:01,110 with regard to this pathway has to do 982 00:52:01,110 --> 00:52:03,240 with the enzyme at the top of the pathway, 983 00:52:03,240 --> 00:52:07,909 once again, the glucose 6-phosphate dehydrogenase. 984 00:52:07,909 --> 00:52:09,450 I mentioned it at the very beginning, 985 00:52:09,450 --> 00:52:11,367 but I want to reiterate that enzymes 986 00:52:11,367 --> 00:52:13,200 that are at the top of a pathway where there 987 00:52:13,200 --> 00:52:15,300 is thermodynamically reversible steps, 988 00:52:15,300 --> 00:52:20,430 they are places where you have the ability to control entry 989 00:52:20,430 --> 00:52:21,340 into the pathway. 990 00:52:21,340 --> 00:52:23,480 So these are the rate-determining steps. 991 00:52:23,480 --> 00:52:27,180 And in this case, the presence of NAD+-- 992 00:52:27,180 --> 00:52:28,950 that is, the product-- 993 00:52:28,950 --> 00:52:35,250 determines the rate of passage through the pathway. 994 00:52:35,250 --> 00:52:38,850 So for example, if a cell has been doing a lot 995 00:52:38,850 --> 00:52:43,320 of biosynthesis, the NADP+ levels go up. 996 00:52:43,320 --> 00:52:47,152 That interacts allosterically in order 997 00:52:47,152 --> 00:52:49,360 to be able to increase the throughput of the pathway. 998 00:52:49,360 --> 00:52:52,350 So you increase the amount of NADPH 999 00:52:52,350 --> 00:52:55,680 biosynthesis to be able to let it match 1000 00:52:55,680 --> 00:52:58,620 the biosynthetic needs associated with doing 1001 00:52:58,620 --> 00:53:00,430 a lot of lipid biosynthesis. 1002 00:53:00,430 --> 00:53:01,800 So this is the regulated step. 1003 00:53:06,580 --> 00:53:10,660 The last thing I'm going to say about the pentose phosphate 1004 00:53:10,660 --> 00:53:15,760 pathway is that it's strikingly similar to the pathway called 1005 00:53:15,760 --> 00:53:18,160 the Calvin cycle, which are the dark reactions 1006 00:53:18,160 --> 00:53:20,380 of photosynthesis. 1007 00:53:20,380 --> 00:53:24,460 Photosynthesis involves the capture of one CO2 1008 00:53:24,460 --> 00:53:29,230 into a five-carbon scaffold to make a six-carbon compound. 1009 00:53:29,230 --> 00:53:31,390 Then that six-carbon compounds splits 1010 00:53:31,390 --> 00:53:33,160 into three-carbon compounds. 1011 00:53:33,160 --> 00:53:36,370 It turns out if you work with enough molecules 1012 00:53:36,370 --> 00:53:39,190 of five-carbon compound, what you're able to do 1013 00:53:39,190 --> 00:53:42,340 is go through the carbon-scrambling phase 1014 00:53:42,340 --> 00:53:46,050 and work all your way back to the hexose glucose. 1015 00:53:46,050 --> 00:53:48,040 The carbon-scrambling intermediates 1016 00:53:48,040 --> 00:53:51,400 are almost identical chemically to the ones 1017 00:53:51,400 --> 00:53:54,350 that we have just looked at in the pentose phosphate pathway. 1018 00:53:54,350 --> 00:53:56,410 So if you look at the Calvin cycle, 1019 00:53:56,410 --> 00:54:00,130 you'll see three-carbon and four-carbon and five-carbon and 1020 00:54:00,130 --> 00:54:03,920 six-carbon and even seven-carbon intermediates, identical. 1021 00:54:03,920 --> 00:54:07,570 So nature, again, didn't reinvent the wheel twice. 1022 00:54:07,570 --> 00:54:09,490 What it did was at some point in time, 1023 00:54:09,490 --> 00:54:11,750 it took the pentose phosphate pathway 1024 00:54:11,750 --> 00:54:14,950 and it converted it into the photosynthetic dark reactions, 1025 00:54:14,950 --> 00:54:17,612 or perhaps vice versa. 1026 00:54:17,612 --> 00:54:18,820 They have different purposes. 1027 00:54:18,820 --> 00:54:23,230 The photosynthetic pathway involves capturing carbon 1028 00:54:23,230 --> 00:54:25,630 in order to make hexoses that we can 1029 00:54:25,630 --> 00:54:29,470 use to store for energy, future energy use, 1030 00:54:29,470 --> 00:54:32,080 or to make hexoses you could use to make cellulose, 1031 00:54:32,080 --> 00:54:34,150 if you're a plant. 1032 00:54:34,150 --> 00:54:36,040 On the other hand, the pentose phosphate 1033 00:54:36,040 --> 00:54:37,720 pathway-- again, the same chemistry. 1034 00:54:37,720 --> 00:54:40,300 But in this case, the chemistry is all 1035 00:54:40,300 --> 00:54:44,710 aimed at taking the hexose, the six-carbon compound, 1036 00:54:44,710 --> 00:54:46,750 and using it to make reducing equivalents 1037 00:54:46,750 --> 00:54:48,400 that we can use for biosynthesis, 1038 00:54:48,400 --> 00:54:51,580 or to protect us from oxidative stress, 1039 00:54:51,580 --> 00:54:54,520 or to make riboses that we need when we're growing, in order 1040 00:54:54,520 --> 00:54:58,130 to be able to make nucleic acids, nucleotides, ATP, 1041 00:54:58,130 --> 00:54:59,200 and so on, that we need. 1042 00:55:03,480 --> 00:55:07,109 The pentose phosphate pathway may seem a little bit 1043 00:55:07,109 --> 00:55:07,900 difficult to learn. 1044 00:55:07,900 --> 00:55:09,940 But I think it's very worthwhile. 1045 00:55:09,940 --> 00:55:17,530 It is something that provides us with amazing versatility. 1046 00:55:17,530 --> 00:55:20,800 It gives us compounds that are chemically 1047 00:55:20,800 --> 00:55:23,800 reactive at the level of two carbons, three carbons, four 1048 00:55:23,800 --> 00:55:27,460 carbons, five carbons, six carbons, and seven carbons. 1049 00:55:27,460 --> 00:55:28,990 You can take these pieces and you 1050 00:55:28,990 --> 00:55:32,200 can make a wide array of molecules that are absolutely 1051 00:55:32,200 --> 00:55:34,060 essential for life. 1052 00:55:34,060 --> 00:55:37,450 Moreover, it provides us with reducing equivalents, 1053 00:55:37,450 --> 00:55:38,830 which we need to grow-- 1054 00:55:38,830 --> 00:55:40,200 biosynthesis. 1055 00:55:40,200 --> 00:55:43,090 Moreover, those reducing equivalents also 1056 00:55:43,090 --> 00:55:45,580 help protect us from oxidative stress. 1057 00:55:45,580 --> 00:55:48,700 That helps keep us alive and healthy longer. 1058 00:55:48,700 --> 00:55:53,290 So the pentose phosphate pathway is complicated 1059 00:55:53,290 --> 00:55:55,700 because it does so many different things. 1060 00:55:55,700 --> 00:55:58,300 But any one of those things, I think, in itself 1061 00:55:58,300 --> 00:56:01,840 makes it worthwhile learning all of this complicated chemistry. 1062 00:56:01,840 --> 00:56:05,020 So that's the pentose phosphate pathway from the 10,000-foot 1063 00:56:05,020 --> 00:56:07,600 level down to the 2-foot level. 1064 00:56:07,600 --> 00:56:09,880 Incredibly versatile pathway, and I 1065 00:56:09,880 --> 00:56:11,770 hope that this has helped you understand it. 1066 00:56:11,770 --> 00:56:14,220 Thank you very much.