1 00:00:00,500 --> 00:00:02,840 The following content is provided under a Creative 2 00:00:02,840 --> 00:00:04,380 Commons license. 3 00:00:04,380 --> 00:00:06,680 Your support will help MIT OpenCourseWare 4 00:00:06,680 --> 00:00:11,070 continue to offer high-quality educational resources for free. 5 00:00:11,070 --> 00:00:13,670 To make a donation, or view additional materials 6 00:00:13,670 --> 00:00:17,630 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:17,630 --> 00:00:18,800 at ocw.mit.edu. 8 00:00:24,890 --> 00:00:27,360 BOGDAN FEDELES: Greetings, and welcome to 5.07 9 00:00:27,360 --> 00:00:29,300 Biochemistry online. 10 00:00:29,300 --> 00:00:31,115 I'm Dr. Bogden Fedeles. 11 00:00:31,115 --> 00:00:32,910 Let's metabolize some problems. 12 00:00:32,910 --> 00:00:35,320 I have a good problem for you today. 13 00:00:35,320 --> 00:00:38,430 This is problem one, from problem set nine. 14 00:00:38,430 --> 00:00:39,930 It is a problem in which we're going 15 00:00:39,930 --> 00:00:42,990 to calculate how much energy we get from metabolizing 16 00:00:42,990 --> 00:00:45,690 a molecule of fat, more specifically, 17 00:00:45,690 --> 00:00:48,450 a molecule of triacylglycerol. 18 00:00:48,450 --> 00:00:50,640 Here is a structural representation 19 00:00:50,640 --> 00:00:52,770 of the triacylglycerol. 20 00:00:52,770 --> 00:00:55,950 Recognize the glycerol molecule in the middle 21 00:00:55,950 --> 00:01:01,080 here, that it's holding together three fatty acids. 22 00:01:01,080 --> 00:01:03,820 Now notice, I picked a short-chain fatty acid, 23 00:01:03,820 --> 00:01:06,310 a long-chain fatty acid, and a fatty acid that 24 00:01:06,310 --> 00:01:09,430 actually has a double bond. 25 00:01:09,430 --> 00:01:11,810 Now when this molecule gets metabolized, 26 00:01:11,810 --> 00:01:15,970 it's going to be acted upon by an enzyme called the lipase. 27 00:01:15,970 --> 00:01:21,480 It's going to hydrolyze the molecule into its constituents. 28 00:01:21,480 --> 00:01:24,690 Obviously the lipase is going to use water molecules, 29 00:01:24,690 --> 00:01:28,490 and it's going to break it down into glycerol, shown here. 30 00:01:31,110 --> 00:01:31,980 Glycerol. 31 00:01:31,980 --> 00:01:36,710 Then, this fatty acid that has two, four, six, C6 fatty acid. 32 00:01:40,480 --> 00:01:44,705 This fatty acid has two, four, six, eight, 10, 12, 14, 16, 33 00:01:44,705 --> 00:01:48,730 C16 fatty acid. 34 00:01:48,730 --> 00:01:52,350 And this one, it's an unsaturated fatty acid, 35 00:01:52,350 --> 00:01:54,655 has a double bond, and if you count the carbons 36 00:01:54,655 --> 00:01:56,770 it should add up to 15 carbons. 37 00:01:59,300 --> 00:02:02,660 So not only it has a double bond, 38 00:02:02,660 --> 00:02:05,480 but also it's an odd-numbered fatty acid. 39 00:02:08,606 --> 00:02:11,230 In order to figure out how much energy we can get from this one 40 00:02:11,230 --> 00:02:15,100 molecule of fat, we will look at how much energy 41 00:02:15,100 --> 00:02:18,400 we get from each one of these constituents-- 42 00:02:18,400 --> 00:02:21,910 namely, the glycerol and the three fatty acids-- 43 00:02:21,910 --> 00:02:24,430 and calculate what is the maximum amount of ATP 44 00:02:24,430 --> 00:02:26,530 we can generate when we metabolize 45 00:02:26,530 --> 00:02:31,180 each one of these molecules completely to CO2 and water. 46 00:02:31,180 --> 00:02:33,220 In order to keep track of how much energy we 47 00:02:33,220 --> 00:02:35,380 get from each of the molecules, let's 48 00:02:35,380 --> 00:02:38,320 make a handy table right here. 49 00:02:38,320 --> 00:02:44,570 So we're going to put glycerol and the C6 fatty acid, 50 00:02:44,570 --> 00:02:49,090 and the C16 fatty acid, and C15 fatty acid. 51 00:02:53,790 --> 00:02:55,210 All right. 52 00:02:55,210 --> 00:02:59,140 And each one of these, we're going to follow the metabolism, 53 00:02:59,140 --> 00:03:06,610 and see how much ATP we're going to need to put in or generate. 54 00:03:06,610 --> 00:03:10,450 Also, the redox cofactors in ADH or FADH2. 55 00:03:13,270 --> 00:03:14,740 Also, for fatty acids, we're going 56 00:03:14,740 --> 00:03:18,700 to be dealing with beta-oxidation. 57 00:03:18,700 --> 00:03:22,400 And pretty much every single molecule, 58 00:03:22,400 --> 00:03:24,190 when it's going to be born completely, 59 00:03:24,190 --> 00:03:26,840 it's going to generate first acetyl-CoA. 60 00:03:29,380 --> 00:03:30,150 All right. 61 00:03:30,150 --> 00:03:37,300 And here, we're going to tally up the total amount of ATP 62 00:03:37,300 --> 00:03:38,770 for each one of them. 63 00:03:38,770 --> 00:03:41,720 And then we're going to tally up for the entire molecule. 64 00:03:41,720 --> 00:03:44,190 So let's start with the glycerol molecule. 65 00:03:44,190 --> 00:03:48,100 Now, if you have watched the problem set seven, 66 00:03:48,100 --> 00:03:51,250 you might recognize the following pathway. 67 00:03:51,250 --> 00:03:53,740 As we just discussed, triacylglyceride 68 00:03:53,740 --> 00:03:56,370 can be hydrolyzed to form glycerol. 69 00:03:56,370 --> 00:04:00,000 And the glycerol, then, is first phosphorylated 70 00:04:00,000 --> 00:04:04,550 by glycerol kinase, and oxidized by glycerol phosphate 71 00:04:04,550 --> 00:04:08,290 dehydrogenase, to generate dihydro acetyl phosphate, which 72 00:04:08,290 --> 00:04:11,470 can then enter the glycolysis, and follow 73 00:04:11,470 --> 00:04:15,100 glycolysis all the way to pyruvate, and then acetyl-CoA. 74 00:04:15,100 --> 00:04:19,740 From then, acetyl-CoA can go into the TCA cycle. 75 00:04:19,740 --> 00:04:22,230 So let's tally up how much energy 76 00:04:22,230 --> 00:04:25,140 we can get from one molecule of glycerol. 77 00:04:25,140 --> 00:04:27,630 Let's take a look specifically at the steps 78 00:04:27,630 --> 00:04:33,340 where we are generating ATP, or generating redox cofactors, 79 00:04:33,340 --> 00:04:37,240 such as NADH or FADH2. 80 00:04:37,240 --> 00:04:39,260 First, we need to put in ATP. 81 00:04:39,260 --> 00:04:41,830 Neglect glycerol kinase. 82 00:04:41,830 --> 00:04:44,780 But we're going to get back one ATP 83 00:04:44,780 --> 00:04:46,900 in the phosphoglycerate kinase step, 84 00:04:46,900 --> 00:04:50,360 and one ATP in the pyruvate kinase step. 85 00:04:50,360 --> 00:04:53,680 So the net ATP formation is one. 86 00:04:53,680 --> 00:04:56,790 Now in the glycolysis, we're also 87 00:04:56,790 --> 00:05:00,300 going to generate one NADH, in the pyruvate dehydrogenase 88 00:05:00,300 --> 00:05:04,140 another NADH, and the glycerol-3-phosphate 89 00:05:04,140 --> 00:05:06,600 dehydrogenase will generate also an NADH. 90 00:05:06,600 --> 00:05:08,730 Now, keep in mind, this NADH is going 91 00:05:08,730 --> 00:05:10,860 to be outside the mitochondria, so we're 92 00:05:10,860 --> 00:05:13,230 going to have to use a shuttle to bring it in. 93 00:05:13,230 --> 00:05:15,450 But we're considering that we're using 94 00:05:15,450 --> 00:05:18,060 an efficient shuttle, that gives us the full amount of energy 95 00:05:18,060 --> 00:05:19,410 for this NADH. 96 00:05:19,410 --> 00:05:21,270 So, once again, the total, it's going 97 00:05:21,270 --> 00:05:22,630 to be three molecules of NADH. 98 00:05:25,650 --> 00:05:28,890 So, going back to our table, we said 99 00:05:28,890 --> 00:05:33,150 the glycerol is going to give us a net one molecule of ATP, 100 00:05:33,150 --> 00:05:35,720 three molecules of NADH. 101 00:05:35,720 --> 00:05:39,570 There's going to be no FADH2, no beta-oxidation, 102 00:05:39,570 --> 00:05:42,480 and we're going to get one molecule of acetyl-CoA. 103 00:05:42,480 --> 00:05:44,820 As you've seen by this point many times, 104 00:05:44,820 --> 00:05:47,580 acetyl-CoA will enter the TCA cycle, 105 00:05:47,580 --> 00:05:50,370 where it's going to be completely oxidized to two CO2 106 00:05:50,370 --> 00:05:53,280 molecules, and in the process is going to generate 107 00:05:53,280 --> 00:05:56,070 the equivalent of 12 ATPs. 108 00:05:56,070 --> 00:05:59,800 Let's take a look at where those are coming from. 109 00:05:59,800 --> 00:06:02,350 Here is a schematic of the TCA cycle. 110 00:06:02,350 --> 00:06:05,710 And one acetyl-CoA molecule comes in, 111 00:06:05,710 --> 00:06:10,930 and it's going to generate one, two, three molecules of NADH, 112 00:06:10,930 --> 00:06:15,500 one molecule of FADH2, and one molecule of GTP. 113 00:06:15,500 --> 00:06:19,040 Now, if we keep in mind that for every FADH2 114 00:06:19,040 --> 00:06:22,720 we generate about two ATPs, and for every NADH 115 00:06:22,720 --> 00:06:26,870 we generate three ATPs, that's a total of, 3 times 3 is 9, 116 00:06:26,870 --> 00:06:30,880 plus 2 is 11, plus a GDP is equivalent to an ATP. 117 00:06:30,880 --> 00:06:35,110 That's about 12 molecules of ATP per molecule of acetyl-CoA 118 00:06:35,110 --> 00:06:36,920 that enters the TCA cycle. 119 00:06:36,920 --> 00:06:39,040 So now let's tally up how much energy 120 00:06:39,040 --> 00:06:41,570 we can get from one molecule of glycerol. 121 00:06:41,570 --> 00:06:44,590 So we know we get one ATP, three NADHs, 122 00:06:44,590 --> 00:06:48,860 now each NADH is going to give us three molecules of ATP. 123 00:06:48,860 --> 00:06:53,260 Now FADH2, we know these give two molecules of ATP. 124 00:06:53,260 --> 00:06:55,330 We don't have any beta-oxidation-- 125 00:06:55,330 --> 00:06:57,730 we're going to be talking more about fatty acids-- 126 00:06:57,730 --> 00:07:01,750 and acetyl-CoA we just talked about, we get 12 molecules 127 00:07:01,750 --> 00:07:03,910 of ATP per acetyl-CoA. 128 00:07:03,910 --> 00:07:07,840 So, the total here is 12 plus 9, plus 1, 129 00:07:07,840 --> 00:07:14,090 that's going to be 22 ATPs from one molecule of glycerol. 130 00:07:14,090 --> 00:07:16,100 Now let's talk about the fatty acids. 131 00:07:16,100 --> 00:07:17,870 In order to metabolize them, first we 132 00:07:17,870 --> 00:07:20,720 need to activate them into thioesters. 133 00:07:20,720 --> 00:07:25,100 These are going to be thioesters formed with coenzyme A, or CoA. 134 00:07:25,100 --> 00:07:27,530 Here's an overview of the activation process 135 00:07:27,530 --> 00:07:32,840 by which fatty acids become fatty acid thioesters. 136 00:07:32,840 --> 00:07:35,240 Of course, this is written for the C6 fatty acid, 137 00:07:35,240 --> 00:07:37,790 but would occur for any other fatty acid, 138 00:07:37,790 --> 00:07:41,590 regardless of the chain length. 139 00:07:41,590 --> 00:07:45,190 Now this process is catalyzed by acetyl-CoA synthetase, that 140 00:07:45,190 --> 00:07:52,930 uses ATP to first generate this mixed anhydride, with AMP. 141 00:07:52,930 --> 00:07:54,715 This process is called adenylation. 142 00:07:54,715 --> 00:07:57,910 So this activates the acid, which 143 00:07:57,910 --> 00:08:00,950 then reacts with coenzyme A shown here, 144 00:08:00,950 --> 00:08:05,710 HS-CoA, which will generate the thioester of the fatty acid. 145 00:08:05,710 --> 00:08:08,440 Now, here we are using one molecule of ATP, 146 00:08:08,440 --> 00:08:10,930 but we're breaking it into alpha-phosphate to generate 147 00:08:10,930 --> 00:08:14,860 pyrophosphate, which is then broken down into two 148 00:08:14,860 --> 00:08:16,420 inorganic phosphates. 149 00:08:16,420 --> 00:08:18,430 And the energy in this reaction, catalyzed 150 00:08:18,430 --> 00:08:22,450 by inorganic pyrophosphatase, drives 151 00:08:22,450 --> 00:08:25,330 the reaction towards the right. 152 00:08:25,330 --> 00:08:29,020 Now, since we're generating AMP in the second step, 153 00:08:29,020 --> 00:08:32,799 we need a second molecule of ATP to convert this AMP back 154 00:08:32,799 --> 00:08:34,909 to ADP. 155 00:08:34,909 --> 00:08:38,900 So, overall, this process requires two molecules of ATP 156 00:08:38,900 --> 00:08:43,039 to generate one molecule of the fatty acid thioester. 157 00:08:43,039 --> 00:08:46,160 Once a fatty acid is activated into a thioester 158 00:08:46,160 --> 00:08:50,580 with coenzyme A, it can now undergo beta-oxidation. 159 00:08:50,580 --> 00:08:53,750 This is a set of reactions in which the fatty acid is broken 160 00:08:53,750 --> 00:08:58,580 down into a shorter fatty acid and one molecule of acetyl-CoA. 161 00:08:58,580 --> 00:09:01,010 The process then can repeat over and over, 162 00:09:01,010 --> 00:09:03,560 until the entire fatty acid is broken down 163 00:09:03,560 --> 00:09:05,910 into acetyl-CoA molecules. 164 00:09:05,910 --> 00:09:08,660 So let's take a look at how beta-oxidation works. 165 00:09:08,660 --> 00:09:11,630 Here is an overview of beta-oxidation pathway. 166 00:09:11,630 --> 00:09:16,240 We're starting with a fatty acyl-CoA thioester that 167 00:09:16,240 --> 00:09:20,370 has "n" carbons, and by the end of the process, 168 00:09:20,370 --> 00:09:24,290 we're going to get a thioester that has "n" minus 2 carbons, 169 00:09:24,290 --> 00:09:26,540 and the remaining two carbons are 170 00:09:26,540 --> 00:09:29,690 going to be in the form of acetyl-CoA. 171 00:09:29,690 --> 00:09:31,930 Now, this beta-oxidation involves four steps. 172 00:09:31,930 --> 00:09:35,750 In the first step, we're going to use a dehydrogenase 173 00:09:35,750 --> 00:09:40,050 to oxidize this single bond between the alpha 174 00:09:40,050 --> 00:09:44,720 and beta carbons, and make a trans double bond. 175 00:09:44,720 --> 00:09:48,410 So this is the alpha carbon, this is the beta carbon. 176 00:09:48,410 --> 00:09:53,690 So these fatty acyl-CoA dehydrogenases, 177 00:09:53,690 --> 00:09:55,180 there are actually several of them, 178 00:09:55,180 --> 00:09:56,721 and they have different specificities 179 00:09:56,721 --> 00:09:58,730 for short, medium, long, and very 180 00:09:58,730 --> 00:10:01,730 long chain fatty acyl-CoAs. 181 00:10:01,730 --> 00:10:05,420 But regardless, there will be some dehydrogenase 182 00:10:05,420 --> 00:10:09,590 to act on any length fatty acyl-CoA, 183 00:10:09,590 --> 00:10:13,060 and introduce this trans double bond. 184 00:10:13,060 --> 00:10:16,040 In the next step, we add one water molecule 185 00:10:16,040 --> 00:10:19,670 to generate a beta-hydroxyacyl-CoA, which 186 00:10:19,670 --> 00:10:24,630 is subsequently oxidized to generate a beta-ketoacyl-CoA. 187 00:10:24,630 --> 00:10:27,020 In this oxidation, we're going to use NAD, 188 00:10:27,020 --> 00:10:29,030 generating one molecule NADH. 189 00:10:29,030 --> 00:10:35,090 Finally, the thiolase, or beta-ketoacyl thiolase, 190 00:10:35,090 --> 00:10:38,630 is going to break down this bond between the alpha and beta 191 00:10:38,630 --> 00:10:42,500 carbons, in a reverse Claisen reaction, 192 00:10:42,500 --> 00:10:44,570 to generate one molecule of acetyl-CoA. 193 00:10:44,570 --> 00:10:50,260 And the remainder of the fatty acid is another thioester. 194 00:10:50,260 --> 00:10:52,930 So one round of beta-oxidation is 195 00:10:52,930 --> 00:10:55,520 going to generate one molecule of FADH2, 196 00:10:55,520 --> 00:10:58,120 and one molecule of NADH, as well 197 00:10:58,120 --> 00:11:01,210 as one molecule of acetyl-CoA. 198 00:11:01,210 --> 00:11:05,260 Now let's update our table with the information we just 199 00:11:05,260 --> 00:11:05,980 learned. 200 00:11:05,980 --> 00:11:09,010 So, as we just said, for every fatty acid 201 00:11:09,010 --> 00:11:11,710 we need to expend two molecules of ATP, 202 00:11:11,710 --> 00:11:14,410 to transform them into the thioesters. 203 00:11:14,410 --> 00:11:17,590 That's why I put here minus 2 for each one of the three 204 00:11:17,590 --> 00:11:19,000 fatty acids. 205 00:11:19,000 --> 00:11:21,160 We also learned that in beta-oxidation, we 206 00:11:21,160 --> 00:11:26,720 generate one molecule of FADH2, and one molecule of NADH. 207 00:11:26,720 --> 00:11:29,650 So, for every beta-oxidation, we generate the equivalent 208 00:11:29,650 --> 00:11:31,120 of five ATP. 209 00:11:34,720 --> 00:11:36,940 Now we're ready to calculate how much energy we 210 00:11:36,940 --> 00:11:41,410 can get from each of the three fatty acids in our problem. 211 00:11:41,410 --> 00:11:50,520 Now, for the C6 fatty acid that's what's represented here, 212 00:11:50,520 --> 00:11:53,370 we discussed, we're going to activate it, 213 00:11:53,370 --> 00:11:57,100 we're going to need to use two ATP molecules 214 00:11:57,100 --> 00:12:03,580 and coenzyme A. We're going to form the thioester. 215 00:12:06,250 --> 00:12:09,841 And then we're going to do beta-oxidation 216 00:12:09,841 --> 00:12:13,650 Now, for a fatty acid that has six carbons, 217 00:12:13,650 --> 00:12:15,530 we're going to do the beta-oxidation, 218 00:12:15,530 --> 00:12:17,520 and the molecule is going to cleave there, 219 00:12:17,520 --> 00:12:19,440 and we're going to do it one more time, 220 00:12:19,440 --> 00:12:21,840 the molecule is going to be cleaved there. 221 00:12:21,840 --> 00:12:25,890 So we're going to do two rounds of beta-oxidation. 222 00:12:29,330 --> 00:12:31,955 And each one of these two carbons 223 00:12:31,955 --> 00:12:33,330 is going to become an acetyl-CoA. 224 00:12:33,330 --> 00:12:36,200 So we're going to generate three acetyl-CoAs. 225 00:12:39,220 --> 00:12:44,290 Now, similarly, for the C16 fatty acid-- 226 00:12:44,290 --> 00:12:50,020 two, four, six, eight, 10, 12, 14, 16. 227 00:12:53,030 --> 00:12:58,930 All right, it's going to first activate two molecules of ATP 228 00:12:58,930 --> 00:13:14,020 and coenzyme A to form the thioester, with 16 carbons. 229 00:13:14,020 --> 00:13:18,690 And this will undergo beta-oxidation. 230 00:13:18,690 --> 00:13:22,420 And we're going to do it one, two, three, four, five, six, 231 00:13:22,420 --> 00:13:25,040 seven times. 232 00:13:25,040 --> 00:13:34,740 OK, so seven rounds of beta-oxidation 233 00:13:34,740 --> 00:13:39,630 is going to generate eight molecules of acetyl-CoA. 234 00:13:39,630 --> 00:13:41,940 Now let's go back and put in all this information 235 00:13:41,940 --> 00:13:43,080 into our table. 236 00:13:43,080 --> 00:13:47,040 So for the C6 fatty acid, we expended two molecules of ATP 237 00:13:47,040 --> 00:13:53,320 to activate it, and then we did two rounds of beta-oxidation, 238 00:13:53,320 --> 00:13:57,650 and we generated three molecules of acetyl-CoA. 239 00:13:57,650 --> 00:14:00,020 For the C16 fatty acid, again, we 240 00:14:00,020 --> 00:14:04,670 activated then we did seven rounds beta-oxidation, 241 00:14:04,670 --> 00:14:09,290 and we generated eight molecules of acetyl-CoA. 242 00:14:09,290 --> 00:14:12,740 So for the C6 fatty acid, we have a grand total of, 243 00:14:12,740 --> 00:14:17,740 3 times 12 is 36, plus 2 times 5, 10, is 46, minus 2, 244 00:14:17,740 --> 00:14:21,600 is 44 molecules of ATP. 245 00:14:21,600 --> 00:14:25,740 For the C16 fatty acid, well, 8 times 12 246 00:14:25,740 --> 00:14:36,590 is 96, plus 35, minus 2, that's 129 molecules of ATP. 247 00:14:36,590 --> 00:14:38,930 Now the C15 fatty acid is going to be a little bit more 248 00:14:38,930 --> 00:14:42,600 complicated, because on one hand, it has a double bond, 249 00:14:42,600 --> 00:14:44,930 so we need to figure out how to deal with that. 250 00:14:44,930 --> 00:14:48,350 On the other hand, it's an odd chain fatty acid, 251 00:14:48,350 --> 00:14:50,420 and as you imagine, the beta-oxidation breaks off 252 00:14:50,420 --> 00:14:52,310 two carbons at a time. 253 00:14:52,310 --> 00:14:54,980 So the last time we do beta-oxidation, 254 00:14:54,980 --> 00:14:57,350 we're going to be left with three carbons. 255 00:14:57,350 --> 00:14:59,120 That's called propionyl-CoA, and we'll 256 00:14:59,120 --> 00:15:01,202 have to figure out what to do with that. 257 00:15:01,202 --> 00:15:02,660 Just as with the other fatty acids, 258 00:15:02,660 --> 00:15:04,490 the C15 fatty acid is going to need 259 00:15:04,490 --> 00:15:07,640 to be activated into a thioester with CoA. 260 00:15:07,640 --> 00:15:12,600 So this is going to cost two ATP molecules, and, of course, 261 00:15:12,600 --> 00:15:15,600 we need to add the CoA. 262 00:15:15,600 --> 00:15:20,930 And now, this is the thioester of our C15 fatty acid. 263 00:15:20,930 --> 00:15:24,810 Now, since the double bond is pretty far away 264 00:15:24,810 --> 00:15:26,550 from the business-end of the molecule, 265 00:15:26,550 --> 00:15:30,730 we can do a number of rounds of beta-oxidation. 266 00:15:30,730 --> 00:15:36,230 In fact, we can do beta-oxidation once, twice. 267 00:15:36,230 --> 00:15:39,480 So two rounds of beta-oxidation is 268 00:15:39,480 --> 00:15:44,790 going to give us this molecule. 269 00:15:44,790 --> 00:15:50,710 Two rounds beta-oxidation. 270 00:15:50,710 --> 00:15:52,510 In each one of these rounds we're 271 00:15:52,510 --> 00:15:55,360 going to generate one molecule of acetyl-CoA, 272 00:15:55,360 --> 00:15:57,010 so two acetyl-CoA. 273 00:16:02,670 --> 00:16:07,410 So we get this fatty acid taiyo thioester, 274 00:16:07,410 --> 00:16:10,860 which contains a beta-gamma double bond. 275 00:16:10,860 --> 00:16:12,720 Now, it turns out there is an enzyme that 276 00:16:12,720 --> 00:16:17,130 can isomerize this double bond into an alpha-beta double bond. 277 00:16:17,130 --> 00:16:19,440 So this is what is going to happen next. 278 00:16:19,440 --> 00:16:22,830 The double bond moves from the beta-gamma to the alpha-beta. 279 00:16:22,830 --> 00:16:26,160 Now, this looks a lot like an intermediate 280 00:16:26,160 --> 00:16:28,290 in the beta-oxidation. 281 00:16:28,290 --> 00:16:31,650 Once again, this reaction is catalyzed by am isomerase. 282 00:16:35,490 --> 00:16:38,970 And this alpha-beta unsaturated thioester 283 00:16:38,970 --> 00:16:44,650 can continue in a manner similar to beta-oxidation. 284 00:16:44,650 --> 00:16:50,860 So, first it's going to add water to form a hydroxyl here 285 00:16:50,860 --> 00:16:51,860 at the beta position. 286 00:16:51,860 --> 00:16:55,550 Then that hydroxyl is getting oxidized to form a keto group. 287 00:16:55,550 --> 00:16:58,670 And the thiolase is going to generate acetyl-CoA, 288 00:16:58,670 --> 00:17:00,960 and another thioester shown here. 289 00:17:00,960 --> 00:17:06,550 So, from here, we're going to generate one molecule of NADH, 290 00:17:06,550 --> 00:17:09,140 and one molecule of acetyl-CoA. 291 00:17:13,569 --> 00:17:17,710 All right, now this is a thioester 292 00:17:17,710 --> 00:17:20,800 of a completely saturated fatty acid. 293 00:17:20,800 --> 00:17:22,900 Now of course, it's still odd chained, 294 00:17:22,900 --> 00:17:26,380 so we have one, six, seven, eight, nine carbons. 295 00:17:26,380 --> 00:17:30,380 So we can do beta-oxidation actually three times. 296 00:17:32,970 --> 00:17:34,930 So, three rounds of beta-oxidation. 297 00:17:37,900 --> 00:17:42,960 And it's going to take us to three molecules of acetyl-CoA. 298 00:17:42,960 --> 00:17:44,910 And the last portion of the molecule 299 00:17:44,910 --> 00:17:49,340 is going to be this molecule, which we call propionyl-CoA, 300 00:17:49,340 --> 00:17:51,630 is a three carbon thioester. 301 00:17:55,960 --> 00:18:02,290 So far we have generated two, three, and another three 302 00:18:02,290 --> 00:18:05,370 here, six molecule of acetyl-CoA. 303 00:18:05,370 --> 00:18:07,950 And we've done, two, another three, 304 00:18:07,950 --> 00:18:10,480 five rounds of beta-oxidation. 305 00:18:10,480 --> 00:18:14,850 And we also generated an additional NADH molecule. 306 00:18:14,850 --> 00:18:17,134 So now let's update our table with this information, 307 00:18:17,134 --> 00:18:18,550 and then we're going to figure out 308 00:18:18,550 --> 00:18:20,590 what happens to propionyl-CoA. 309 00:18:20,590 --> 00:18:23,720 As we just discussed, the C15 fatty acid 310 00:18:23,720 --> 00:18:26,900 is going to get activated, so we need to ATPs there. 311 00:18:26,900 --> 00:18:32,420 Then it's going to undergo five rounds of beta-oxidation. 312 00:18:32,420 --> 00:18:36,980 And we generated a total of six molecules of acetyl-CoA, 313 00:18:36,980 --> 00:18:40,440 and one additional molecule of NADH. 314 00:18:40,440 --> 00:18:42,830 Let's now take a look at propionyl-CoA, 315 00:18:42,830 --> 00:18:45,170 and see how we metabolize it, and how much energy 316 00:18:45,170 --> 00:18:46,370 we can generate from it. 317 00:18:46,370 --> 00:18:49,760 It turns out the first step is to expand from a three carbon 318 00:18:49,760 --> 00:18:52,530 molecule to a four carbon molecule. 319 00:18:52,530 --> 00:18:58,460 This happens by adding one CO2. 320 00:18:58,460 --> 00:19:00,050 Of course, this process will require 321 00:19:00,050 --> 00:19:02,510 the expense of an ATP molecule. 322 00:19:02,510 --> 00:19:05,240 We generate this methylmalonyl-CoA, 323 00:19:05,240 --> 00:19:08,690 which is a branched four carbon chain molecule. 324 00:19:08,690 --> 00:19:12,100 And another enzyme, racemase, is going 325 00:19:12,100 --> 00:19:14,170 to interconvert this stereocenter 326 00:19:14,170 --> 00:19:17,110 from the S-configuaration to the R-configuration. 327 00:19:17,110 --> 00:19:20,890 Finally, this methylmalonyl-CoA is 328 00:19:20,890 --> 00:19:23,980 going to undergo a rearrangement of the groups 329 00:19:23,980 --> 00:19:27,070 to generate a linear molecule, succinyl-CoA. 330 00:19:27,070 --> 00:19:30,220 Now, this is one of the most fascinating transformations 331 00:19:30,220 --> 00:19:33,070 in the whole biochemistry, and involves 332 00:19:33,070 --> 00:19:36,460 an enzyme called methylmalonyl-CoA mutase, which 333 00:19:36,460 --> 00:19:40,890 requires cobalamin, or the coenzyme derived 334 00:19:40,890 --> 00:19:41,860 from vitamin B12. 335 00:19:46,240 --> 00:19:48,010 This unusual transformation catalyzed 336 00:19:48,010 --> 00:19:50,560 by the methylmalonyl-CoA mutase, the enzyme that 337 00:19:50,560 --> 00:19:54,490 requires vitamin B12 cofactor, it's 338 00:19:54,490 --> 00:19:59,680 fascinating because it involves a carbon skeletal 339 00:19:59,680 --> 00:20:02,000 rearrangement of the molecule. 340 00:20:02,000 --> 00:20:05,782 And this reaction occurs via a radical mechanism. 341 00:20:05,782 --> 00:20:10,850 The radical is obtained by breaking a carbon metal bond. 342 00:20:10,850 --> 00:20:13,730 In this case, it's a carbon-cobalt bond. 343 00:20:13,730 --> 00:20:16,400 Now succinyl-CoA is a familiar molecule, 344 00:20:16,400 --> 00:20:18,840 you've encountered it in the TCA cycle. 345 00:20:18,840 --> 00:20:23,720 However, we cannot use the TCA cycle directly to completely 346 00:20:23,720 --> 00:20:26,870 metabolize succinyl-CoA, as all the intermediates in the TCA 347 00:20:26,870 --> 00:20:30,080 cycle are in fact in catalytic amounts. 348 00:20:30,080 --> 00:20:32,720 So, we're going to use just part of the TCA cycle, 349 00:20:32,720 --> 00:20:36,230 to generate a molecule, malate, which then 350 00:20:36,230 --> 00:20:38,200 can be converted into pyruvate. 351 00:20:38,200 --> 00:20:40,730 And pyruvate can then generate acetyl-CoA 352 00:20:40,730 --> 00:20:45,410 to re-enter the TCA cycle and be completely metabolized. 353 00:20:45,410 --> 00:20:49,160 Here is the TCA cycle, to refresh your memory. 354 00:20:49,160 --> 00:20:51,440 And this is succinyl-CoA that we can 355 00:20:51,440 --> 00:20:55,220 generate from the methylmalonyl-CoA mutase. 356 00:20:55,220 --> 00:20:57,680 Now, as we said, succinyl-CoA is going 357 00:20:57,680 --> 00:20:59,720 to be converted to malate. 358 00:20:59,720 --> 00:21:02,660 Malate can then escape the mitochondria 359 00:21:02,660 --> 00:21:05,570 and continue its transformation towards pyruvate. 360 00:21:05,570 --> 00:21:07,150 So, in this process, succinyl-CoA 361 00:21:07,150 --> 00:21:10,070 is going to generate one molecule of GTP 362 00:21:10,070 --> 00:21:10,950 to form succinate. 363 00:21:10,950 --> 00:21:13,490 And succinate to fumartate is going to give us 364 00:21:13,490 --> 00:21:16,770 one more molecule of FADH2. 365 00:21:16,770 --> 00:21:20,990 Then malate will escape the mitochondria. 366 00:21:20,990 --> 00:21:27,570 So to summarize what happens in the TCA cycle, 367 00:21:27,570 --> 00:21:31,580 succinyl-CoA is going to give us a molecule of GTP, 368 00:21:31,580 --> 00:21:35,270 and then one more molecule of FADH2. 369 00:21:35,270 --> 00:21:37,130 And it's going to make it to malate, 370 00:21:37,130 --> 00:21:40,670 and then malate is going to be converted to pyruvate 371 00:21:40,670 --> 00:21:42,590 using the malic enzyme. 372 00:21:42,590 --> 00:21:46,040 Now, this is an oxidation and decarboxylation 373 00:21:46,040 --> 00:21:47,960 that happens in one step. 374 00:21:47,960 --> 00:21:49,550 For the oxidation, we're going to need 375 00:21:49,550 --> 00:21:53,420 NADP, instead of the usual NAD. 376 00:21:53,420 --> 00:21:57,381 So we're going to generate one molecule of NADPH. 377 00:21:57,381 --> 00:21:58,880 Now for the purpose of this problem, 378 00:21:58,880 --> 00:22:03,120 we're going to treat NADPH as equivalent to NADH. 379 00:22:03,120 --> 00:22:07,290 So malate, via the malic enzyme, is going to form pyruvate. 380 00:22:07,290 --> 00:22:10,035 And then pyruvate, in the pyruvate dehydrogenase, 381 00:22:10,035 --> 00:22:14,620 is going to lose one CO2 and form acetyl-CoA. 382 00:22:14,620 --> 00:22:17,890 In the process we'll also generate one more NADH, 383 00:22:17,890 --> 00:22:22,330 and now acetyl-CoA can re-enter the TCA cycle 384 00:22:22,330 --> 00:22:26,470 and be completely metabolized, generating 385 00:22:26,470 --> 00:22:31,080 in the process about 12 ATP equivalents. 386 00:22:31,080 --> 00:22:33,519 So now let's go back, and update our table 387 00:22:33,519 --> 00:22:35,310 with all this information that we found out 388 00:22:35,310 --> 00:22:36,720 about propionyl-CoA. 389 00:22:36,720 --> 00:22:42,070 So as we said, propionyl-CoA required first the loss 390 00:22:42,070 --> 00:22:46,780 of another ATP to activate it, to form the methylmalonyl-CoA. 391 00:22:46,780 --> 00:22:51,180 But then methylmalonyl-CoA converted to succinyl-CoA. 392 00:22:51,180 --> 00:22:54,850 Succinyl-CoA generated one molecule of GTP, 393 00:22:54,850 --> 00:22:58,290 so we're going to put plus 1 back here. 394 00:22:58,290 --> 00:23:01,480 Then we're going to get one molecule of FADH2 395 00:23:01,480 --> 00:23:04,240 the generating succinate, from going 396 00:23:04,240 --> 00:23:06,460 from succinate to fumarate. 397 00:23:06,460 --> 00:23:07,960 And then, we're going to generate 398 00:23:07,960 --> 00:23:11,890 two more molecules of NADH, one at the malate enzyme step, 399 00:23:11,890 --> 00:23:14,020 and one at the pyruvate dehydrogenase step, 400 00:23:14,020 --> 00:23:16,120 so we have plus 2 here. 401 00:23:16,120 --> 00:23:19,400 And, of course, in the end, we get one more molecule 402 00:23:19,400 --> 00:23:22,400 of acetyl-CoA as well. 403 00:23:22,400 --> 00:23:28,740 So now we have a total of seven molecules of acetyl-CoA, 404 00:23:28,740 --> 00:23:32,770 five rounds of beta-oxidation, one FADH2, three NADHs, 405 00:23:32,770 --> 00:23:37,250 and at a loss of two ATPs. 406 00:23:37,250 --> 00:23:44,580 So this totals up to 118 ATPs for the C15 fatty acid. 407 00:23:44,580 --> 00:23:51,810 Now we can tally the entire ATP yield of a molecule of fat, 408 00:23:51,810 --> 00:23:54,420 of this triacylglyceride, and that's 409 00:23:54,420 --> 00:24:01,180 going to be 313 molecules of ATP. 410 00:24:01,180 --> 00:24:04,696 Now that's a lot of energy from one single molecule of fat. 411 00:24:08,100 --> 00:24:10,570 Now, this problem has an additional question, 412 00:24:10,570 --> 00:24:14,160 and it asked us to contrast how much energy we get from a six 413 00:24:14,160 --> 00:24:17,870 carbon fatty acid and compare that 414 00:24:17,870 --> 00:24:20,610 to how much energy would get from one molecule of glucose, 415 00:24:20,610 --> 00:24:22,581 which also has six carbons. 416 00:24:22,581 --> 00:24:24,080 Let's take a look at our table here. 417 00:24:24,080 --> 00:24:29,240 The six carbon fatty acid generates about 44 molecules 418 00:24:29,240 --> 00:24:31,940 of ATP when completely oxidized. 419 00:24:31,940 --> 00:24:35,240 By contrast, one molecule of glucose 420 00:24:35,240 --> 00:24:41,340 would generate only about 34 to 36 molecules of ATP. 421 00:24:41,340 --> 00:24:48,450 If you want to follow the same analysis that we did here-- 422 00:24:48,450 --> 00:24:54,450 remember glucose, well, we need to spend two molecules of ATP 423 00:24:54,450 --> 00:24:56,130 to activate it, but then we're going 424 00:24:56,130 --> 00:25:00,460 to generate four molecules of ATP going to pyruvate. 425 00:25:00,460 --> 00:25:05,910 And then there's going to be two molecules of NADH generated, 426 00:25:05,910 --> 00:25:08,540 one at the GAPDH step, one at the pyruvate dehydrogenase 427 00:25:08,540 --> 00:25:11,550 step, and finally, we're going to generate 428 00:25:11,550 --> 00:25:14,280 two molecules of acetyl-CoA. 429 00:25:14,280 --> 00:25:19,370 So the total is going to be, 2 times 12 is 24, plus 4 times 3 430 00:25:19,370 --> 00:25:23,200 is 12, is 36, plus 2, is 38. 431 00:25:27,120 --> 00:25:30,980 So as you guys can see, the C6 fatty acid 432 00:25:30,980 --> 00:25:35,090 generates actually more ATP than one molecule of glucose. 433 00:25:35,090 --> 00:25:39,440 And that's probably reasonable, because the C6 fatty acids 434 00:25:39,440 --> 00:25:42,030 have a lot more C-H bonds. 435 00:25:42,030 --> 00:25:45,260 In other words, the carbons are more reduced. 436 00:25:45,260 --> 00:25:47,240 In glucose, we have a lot of hydroxyls, 437 00:25:47,240 --> 00:25:50,790 so the carbons are in a slightly higher oxidation state. 438 00:25:50,790 --> 00:25:54,770 Therefore, there's less energy generated total. 439 00:25:54,770 --> 00:25:57,440 Of course, one molecule of glucose 440 00:25:57,440 --> 00:26:00,080 pales in comparison with one molecule 441 00:26:00,080 --> 00:26:04,370 of fat, which has hundreds of ATP generated, 442 00:26:04,370 --> 00:26:06,590 as we thought in part A of this problem. 443 00:26:06,590 --> 00:26:08,510 Well, that sums up this problem. 444 00:26:08,510 --> 00:26:13,850 I hope it helped you realize why fats, or triacylglycerides, are 445 00:26:13,850 --> 00:26:17,600 so much more energy dense than other nutrients, such as sugars 446 00:26:17,600 --> 00:26:19,384 or amino acids.