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 --> 00:00:18,250 at ocw.mit.edu. 8 00:00:21,090 --> 00:00:22,980 JOHN ESSIGMANN: Let's Look at Storyboard 17, 9 00:00:22,980 --> 00:00:27,030 Panel A. So far in 5.07, we've looked in detail 10 00:00:27,030 --> 00:00:29,430 at carbohydrate catabolism, we've 11 00:00:29,430 --> 00:00:31,620 seen that the complete catabolism 12 00:00:31,620 --> 00:00:35,200 of a molecule of glucose by way of glycolysis, pyruvate, 13 00:00:35,200 --> 00:00:37,860 dehydrogenase, and the TCA cycle results 14 00:00:37,860 --> 00:00:42,324 in the generation of about 36 to 38 molecules of ATP. 15 00:00:42,324 --> 00:00:43,740 At this point, we're going to turn 16 00:00:43,740 --> 00:00:47,130 to catabolism of another metabolic fuel, lipids. 17 00:00:47,130 --> 00:00:48,900 As we'll see lipids, because they 18 00:00:48,900 --> 00:00:51,660 contain more energy per gram, because they're 19 00:00:51,660 --> 00:00:53,940 more highly reduced than carbohydrates, 20 00:00:53,940 --> 00:00:56,880 will produce much more ATP pundit weight 21 00:00:56,880 --> 00:00:58,320 than carbohydrates. 22 00:00:58,320 --> 00:01:01,160 For example, if we were to metabolize hexanoic acid, 23 00:01:01,160 --> 00:01:04,690 the six carbon hydrocarbon, the same number of carbons 24 00:01:04,690 --> 00:01:08,130 as glucose, we'd get over 50 ATPs rather than 25 00:01:08,130 --> 00:01:10,860 about 35 ATPs per molecule, which 26 00:01:10,860 --> 00:01:13,150 we would have got from glucose. 27 00:01:13,150 --> 00:01:15,000 Now let's look at Panel B. Lipids 28 00:01:15,000 --> 00:01:18,160 have many roles in biological systems. 29 00:01:18,160 --> 00:01:20,190 The first that will be relevant to this lecture 30 00:01:20,190 --> 00:01:22,740 is that there are primary energy reserve. 31 00:01:22,740 --> 00:01:27,480 In fact about 80% of our stored energy is in the form of lipid. 32 00:01:27,480 --> 00:01:29,940 Second, as JoAnne taught us, lipids 33 00:01:29,940 --> 00:01:32,540 are key components of biological membranes, 34 00:01:32,540 --> 00:01:34,710 and thus, they contribute in a major way 35 00:01:34,710 --> 00:01:36,780 to the compartmentalisation that's 36 00:01:36,780 --> 00:01:40,410 critical for normal biological functions. 37 00:01:40,410 --> 00:01:44,580 And the third role is that some lipids are signaling molecules. 38 00:01:44,580 --> 00:01:46,980 The one I've pictured here is Estradiol. 39 00:01:46,980 --> 00:01:50,220 While Estradiol may not look like a typical lipid 40 00:01:50,220 --> 00:01:51,030 it actually is. 41 00:01:51,030 --> 00:01:54,660 Indeed, it's made by a lipid biosynthetic pathway that 42 00:01:54,660 --> 00:01:57,570 starts with Acetyl Coenzyme A. And we'll 43 00:01:57,570 --> 00:01:59,940 see later that Acetyl CoA also serves 44 00:01:59,940 --> 00:02:04,850 as the precursor for the canonical lipid fatty acid. 45 00:02:04,850 --> 00:02:07,350 Fatty acids are the classic lipid. 46 00:02:07,350 --> 00:02:11,580 They're hydrocarbon chains that are fully saturated or contain 47 00:02:11,580 --> 00:02:15,800 a small number of double bonds and sometimes branches. 48 00:02:15,800 --> 00:02:18,380 Sometimes the fatty acid moiety is esterified 49 00:02:18,380 --> 00:02:20,360 to the backbone of glycerol. 50 00:02:20,360 --> 00:02:24,250 If you have three fatty acids on a glycerol backbone, one 51 00:02:24,250 --> 00:02:26,180 to each of the hydroxyl groups, that's 52 00:02:26,180 --> 00:02:28,160 called a triacylglyceride. 53 00:02:28,160 --> 00:02:32,030 That's our primary storage form of energy. 54 00:02:32,030 --> 00:02:34,670 With phospholipids, one of the hydroxyl groups 55 00:02:34,670 --> 00:02:38,330 of the glycerol backbone is esterified to either phosphate 56 00:02:38,330 --> 00:02:40,880 or some kind of decorated phosphate 57 00:02:40,880 --> 00:02:45,020 where the decoration could be a sugar or some other moiety. 58 00:02:45,020 --> 00:02:47,460 As was seen in JoAnne's lecture's, phospholipids 59 00:02:47,460 --> 00:02:51,320 are the key building blocks of biological membrane. 60 00:02:51,320 --> 00:02:53,600 Now take a look at Panel D. The rest of this lecture 61 00:02:53,600 --> 00:02:56,660 will deal with the details of how fatty acids are broken down 62 00:02:56,660 --> 00:02:59,630 to carbon dioxide with the intermediate production 63 00:02:59,630 --> 00:03:03,200 of reducing equivalents in the form of NADH and FADH-2 64 00:03:03,200 --> 00:03:06,890 and ultimately energy equivalence in the form of ATP. 65 00:03:06,890 --> 00:03:08,960 Cells can acquire lipids directly 66 00:03:08,960 --> 00:03:12,410 from the blood where typically they're transported by albumin. 67 00:03:12,410 --> 00:03:15,980 They can come directly from our diet or from other organs 68 00:03:15,980 --> 00:03:18,410 or from breakdown of triacylglycerides 69 00:03:18,410 --> 00:03:20,390 within our cells. 70 00:03:20,390 --> 00:03:22,160 We're going to be looking at four 71 00:03:22,160 --> 00:03:25,010 steps in fatty acid catabolism. 72 00:03:25,010 --> 00:03:28,010 The first step involves the appearance of the fatty acid 73 00:03:28,010 --> 00:03:29,790 in the cytoplasm of the cell. 74 00:03:29,790 --> 00:03:32,400 The fatty acid can come in either from breakdown 75 00:03:32,400 --> 00:03:35,450 of a triacylglyceride stored in the cytoplasm, 76 00:03:35,450 --> 00:03:37,940 or the fatty acid could appear from transport 77 00:03:37,940 --> 00:03:39,920 across the membrane from the blood. 78 00:03:39,920 --> 00:03:41,970 In the cytoplasm, the fatty acid, 79 00:03:41,970 --> 00:03:43,730 which is technically a carboxylic acid, 80 00:03:43,730 --> 00:03:48,960 will be thioesterified to form a fatty Acyl Coenzyme A. 81 00:03:48,960 --> 00:03:51,290 The second step of fatty acid catabolism 82 00:03:51,290 --> 00:03:55,490 involves the transport of the fatty Acyl Coenzyme A ester 83 00:03:55,490 --> 00:03:57,950 into the mitochondrion, which is the site 84 00:03:57,950 --> 00:03:59,990 of fatty acid oxidation. 85 00:03:59,990 --> 00:04:02,270 The third step in fatty acid catabolism 86 00:04:02,270 --> 00:04:05,750 involves the actual oxidation process itself. 87 00:04:05,750 --> 00:04:09,020 The series of reactions is called beta oxidation. 88 00:04:09,020 --> 00:04:11,480 Beta oxidation results in the conversion 89 00:04:11,480 --> 00:04:17,180 of the carboxylic acid starting material to Acetyl Coenzyme A. 90 00:04:17,180 --> 00:04:19,940 There can be several fates to the Acetyl 91 00:04:19,940 --> 00:04:22,490 CoA produced by beta oxidation. 92 00:04:22,490 --> 00:04:24,740 But the one we're going to be looking at 93 00:04:24,740 --> 00:04:28,430 is its entry into the TCA cycle, where the Acetyl CoA is 94 00:04:28,430 --> 00:04:31,460 metabolized to carbon dioxide with the generation of reducing 95 00:04:31,460 --> 00:04:32,480 equivalents. 96 00:04:32,480 --> 00:04:35,240 Those reduced electron carriers have the potential 97 00:04:35,240 --> 00:04:39,340 to be converted into energy currency in the form of ATP. 98 00:04:39,340 --> 00:04:41,550 The fourth topic or stage in fatty acid 99 00:04:41,550 --> 00:04:44,540 catabolism that we'll deal with concerns specialized endings 100 00:04:44,540 --> 00:04:47,060 of the catabolic pathway. 101 00:04:47,060 --> 00:04:48,970 The first problem that we'll look at concerns 102 00:04:48,970 --> 00:04:52,330 the fact that some fatty acids have an odd number of carbons 103 00:04:52,330 --> 00:04:56,080 in them, whereas the classical fatty acid beta oxidation 104 00:04:56,080 --> 00:05:00,040 system was primarily designed to process fatty acids with even 105 00:05:00,040 --> 00:05:02,770 numbers of carbon units. 106 00:05:02,770 --> 00:05:04,360 The second problem that we'll look 107 00:05:04,360 --> 00:05:06,820 at as an ending of fatty acid oxidation 108 00:05:06,820 --> 00:05:10,000 concerns the fact that some of the fatty acids in our diet 109 00:05:10,000 --> 00:05:12,490 have a double bond that is either in the wrong stereo 110 00:05:12,490 --> 00:05:15,940 chemistry or is in the wrong place 111 00:05:15,940 --> 00:05:18,490 to enable easy metabolism. 112 00:05:18,490 --> 00:05:21,430 Nature has worked out ways to reposition the double bond 113 00:05:21,430 --> 00:05:23,740 to facilitate the entry of the molecule 114 00:05:23,740 --> 00:05:27,480 into classical beta oxidation schemes. 115 00:05:27,480 --> 00:05:30,390 Let's look now at Panel E. The first step 116 00:05:30,390 --> 00:05:33,990 in fatty acid catabolism involves thioesterification 117 00:05:33,990 --> 00:05:37,320 of the carboxylate residue of the fatty acid. 118 00:05:37,320 --> 00:05:39,390 We're going to see in a couple of minutes 119 00:05:39,390 --> 00:05:42,900 that placing a Coenzyme A moiety on the carboxylate 120 00:05:42,900 --> 00:05:45,750 is going to enable chemistry at the beta carbon. 121 00:05:45,750 --> 00:05:48,800 Without the Coenzyme A group, chemistry at the beta carbon 122 00:05:48,800 --> 00:05:50,960 would be impossible. 123 00:05:50,960 --> 00:05:54,470 The enzyme involved is called Fatty Acyl Coenzyme 124 00:05:54,470 --> 00:05:57,670 A synthetase, sometimes called ligase. 125 00:05:57,670 --> 00:06:01,490 And it additionally goes by the more common name Thiokinase. 126 00:06:01,490 --> 00:06:04,400 This enzyme uses ATP to adenalate 127 00:06:04,400 --> 00:06:06,740 the carboxalate residue. 128 00:06:06,740 --> 00:06:11,270 And then it allows Coenzyme A to replace the AMP residue 129 00:06:11,270 --> 00:06:14,240 with the resulting product being a fatty Acyl Coenzyme 130 00:06:14,240 --> 00:06:19,180 A. This reaction happens in the cytoplasm of the cell. 131 00:06:19,180 --> 00:06:21,790 Beta oxidation however, is going to occur 132 00:06:21,790 --> 00:06:23,210 in the mitochondrial matrix. 133 00:06:23,210 --> 00:06:27,220 So we have to find a way to get this fatty Acetyl Coenzyme A 134 00:06:27,220 --> 00:06:30,720 into the mitochondrial matrix. 135 00:06:30,720 --> 00:06:34,120 Let's go now to Storyboard 18, Panel A. Panel A 136 00:06:34,120 --> 00:06:37,650 shows the cytoplasm, the mitochondrial outer membrane, 137 00:06:37,650 --> 00:06:40,650 the intermembrane space, the inner membrane, 138 00:06:40,650 --> 00:06:42,240 and the mitochondrial matrix. 139 00:06:42,240 --> 00:06:43,890 As I just said, the matrix is going 140 00:06:43,890 --> 00:06:47,490 to be the site at which beta oxidation occurs. 141 00:06:47,490 --> 00:06:51,570 The intermembrane space contains a small alcohol 142 00:06:51,570 --> 00:06:53,220 called Carnitine. 143 00:06:53,220 --> 00:06:54,900 And the mitochondrial outer membrane 144 00:06:54,900 --> 00:06:58,260 contains an enzyme called Carnitine Acyl Transferase I 145 00:06:58,260 --> 00:07:00,090 or CAT-I. 146 00:07:00,090 --> 00:07:04,470 CAT-I removes the Acyl group from the Fatty Acyl Coenzyme 147 00:07:04,470 --> 00:07:07,050 A in the cytoplasm and transfers it 148 00:07:07,050 --> 00:07:09,450 to the alcoholic residue in the center 149 00:07:09,450 --> 00:07:11,850 of the carnitine molecule forming 150 00:07:11,850 --> 00:07:14,820 an ester of the fatty acid with carnitine. 151 00:07:14,820 --> 00:07:17,850 This ester is delivered to CAT-II, 152 00:07:17,850 --> 00:07:22,750 which is embedded in the inner membrane on the matrix side. 153 00:07:22,750 --> 00:07:26,210 CAT-II will then transfer the Acyl functionality 154 00:07:26,210 --> 00:07:31,370 to a Coenzyme A, restoring the fatty Acyl Coenzyme A molecule. 155 00:07:31,370 --> 00:07:35,000 Thus, CAT-I and CAT-II working in a concerted way, 156 00:07:35,000 --> 00:07:38,930 result in the effective transfer of a fatty Acyl Coenzyme A 157 00:07:38,930 --> 00:07:42,200 from the cytoplasm into the mitochondrial matrix, 158 00:07:42,200 --> 00:07:44,700 the site of beta oxidation, which will be our next step. 159 00:07:49,190 --> 00:07:53,480 Let's now look at Panel B. This panel shows an inset 160 00:07:53,480 --> 00:07:56,480 with the mitochondrial inner membrane, the electron transfer 161 00:07:56,480 --> 00:08:00,110 complex, and ETFP, the electron transferring 162 00:08:00,110 --> 00:08:03,200 flavor protein, which is going to be the entry 163 00:08:03,200 --> 00:08:06,770 point of electrons from the initial step of oxidation 164 00:08:06,770 --> 00:08:11,640 of the Fatty Acyl CoA into the electron transport chain. 165 00:08:11,640 --> 00:08:14,060 We also see in this panel, the fatty acid 166 00:08:14,060 --> 00:08:18,500 polmitate the C-16 Straight Chain Carboxylic Acid. 167 00:08:18,500 --> 00:08:22,010 The hydrogen beta to the Coenzyme A ester 168 00:08:22,010 --> 00:08:25,070 is relatively acidic, therefore this hydrogen 169 00:08:25,070 --> 00:08:27,680 will be taken off to form an alkene. 170 00:08:27,680 --> 00:08:30,080 And the hydride will be transferred 171 00:08:30,080 --> 00:08:34,490 from the beta carbon, the third carbon from the right. 172 00:08:34,490 --> 00:08:36,500 Those electrons are transferred to a flavin 173 00:08:36,500 --> 00:08:40,970 in the electron transferring flavor protein, ETFP. 174 00:08:40,970 --> 00:08:43,820 Eventually, those electrons are transferred to Coenzyme Q 175 00:08:43,820 --> 00:08:47,660 to form the reduced form of Coenzyme Q. Those electrons 176 00:08:47,660 --> 00:08:50,410 then travel along through the electron transport chain. 177 00:08:50,410 --> 00:08:52,520 The Organic product of this reaction 178 00:08:52,520 --> 00:08:56,050 is a trans enoyl Coenzyme A. 179 00:08:56,050 --> 00:08:59,500 Now let's take a look at Panel C. In the next step, 180 00:08:59,500 --> 00:09:03,580 water is added to the 3 Carbon of the enoyl Coenzyme A. 181 00:09:03,580 --> 00:09:07,675 Resulting product is a 3 Hydroxy Fatty Acyl Coenzyme 182 00:09:07,675 --> 00:09:11,530 A. We've seen oxidation of alcohols that looks something 183 00:09:11,530 --> 00:09:12,850 like this many times. 184 00:09:12,850 --> 00:09:15,860 For example, malate being oxidized by malate 185 00:09:15,860 --> 00:09:19,150 dehydrogenase to oxaloacetate. 186 00:09:19,150 --> 00:09:22,090 And as we have seen before, the hydroxyl group 187 00:09:22,090 --> 00:09:24,930 is converted to a keto functionality. 188 00:09:24,930 --> 00:09:29,740 Hydride transfer goes to NAD+ to form NADH. 189 00:09:29,740 --> 00:09:31,420 The enzyme that does this conversion 190 00:09:31,420 --> 00:09:36,310 is 3 Hydroxy Fatty Acyl Coenzyme A Dehydrogenase. 191 00:09:36,310 --> 00:09:40,360 At this point, we have generated one FADH-2 and one NADH 192 00:09:40,360 --> 00:09:44,320 in the overall process of the beta oxidation scheme. 193 00:09:44,320 --> 00:09:46,960 The 3 Keto Acyl Coenzyme A is now 194 00:09:46,960 --> 00:09:50,560 set up to release a first molecule of Acetyl Coenzyme 195 00:09:50,560 --> 00:09:55,510 A. The enzyme beta ketothiolase has a cystine on it. 196 00:09:55,510 --> 00:09:57,880 The thiol of the cystine will attack 197 00:09:57,880 --> 00:09:59,830 the carbon that has the keto group 198 00:09:59,830 --> 00:10:03,580 and release Acetyl Coenzyme A. The residue is 199 00:10:03,580 --> 00:10:06,460 a thioester in which the residual 14 200 00:10:06,460 --> 00:10:08,830 carbons of the polmitate that we started with 201 00:10:08,830 --> 00:10:12,880 are now connected to beta-ketothiolase. 202 00:10:12,880 --> 00:10:18,190 Lastly, beta-ketothiolase will transfer this residual 14 203 00:10:18,190 --> 00:10:20,950 carbons to a Coenzyme A molecule, 204 00:10:20,950 --> 00:10:23,290 forming the Fatty Acyl Coenzyme A 205 00:10:23,290 --> 00:10:26,040 that will be 14 carbons long, that is, 206 00:10:26,040 --> 00:10:29,320 it's two carbons shorter than the 16 carbons of polmitate 207 00:10:29,320 --> 00:10:30,580 that we started with. 208 00:10:30,580 --> 00:10:34,330 Overall, this process is called beta oxidation. 209 00:10:34,330 --> 00:10:37,660 The system is now set up to allow the 14 carbon 210 00:10:37,660 --> 00:10:42,040 molecule to go to 12 to 10 and so on, until the entire 16 211 00:10:42,040 --> 00:10:44,980 carbon hydrocarbon has been reduced through seven 212 00:10:44,980 --> 00:10:48,250 rounds of beta oxidation to eight molecules of Acetyl 213 00:10:48,250 --> 00:10:52,870 Coenzyme A. If these eight molecules of Acetyl CoA 214 00:10:52,870 --> 00:10:55,150 are further oxidized by the TCA cycle, 215 00:10:55,150 --> 00:10:58,670 you'll get 96 ATP molecules. 216 00:10:58,670 --> 00:11:02,320 And of course, along the way, in each round of beta oxidation, 217 00:11:02,320 --> 00:11:06,110 you will also produce seven FADH-2s. 218 00:11:06,110 --> 00:11:10,300 The seven FADH-2s will be converted into 14 ATPs. 219 00:11:10,300 --> 00:11:13,300 You will also get seven NADHs, and they 220 00:11:13,300 --> 00:11:16,210 will be converted into 21 ATPs. 221 00:11:16,210 --> 00:11:21,400 So the full conversion of the 16 carbon hydrocarbon polmitate 222 00:11:21,400 --> 00:11:26,640 will result in a total of 131 ATPs. 223 00:11:26,640 --> 00:11:28,900 In order to put together a full balance sheet, 224 00:11:28,900 --> 00:11:30,810 however, keep in mind that we needed 225 00:11:30,810 --> 00:11:33,990 to use several ATPs early in the process in order 226 00:11:33,990 --> 00:11:35,520 to prime the system. 227 00:11:35,520 --> 00:11:40,110 That is Fatty Acyl Coenzyme A synthetase used 2 high energy 228 00:11:40,110 --> 00:11:43,650 phosphate bonds in order to prime the fatty acid 229 00:11:43,650 --> 00:11:46,470 for production of the Coenzyme A intermediate 230 00:11:46,470 --> 00:11:49,077 that's necessary for beta oxidation.