Text Box 1FIGURE 2123Practice Exam 3C BMB 401Q5-14 Using Columns 2 and 3 in the table above, choose the correct enzyme for each statement below. Some enzymes may be used more than once. Which enzyme, or enzyme complex…Match the correct hormone in Glucagon (g) or Insulin (i) with the correct downstream effect (hormones may be used more than once):Biochemistry 401 lecture 20. Today we're going to talk about the TCA cycle. This is a cycle that involves eight enzymes, and its purpose is the oxidation of fuel, and the formation of anabolic precursors. This cycle also produces one GTP, three NADH and one FADH2. We're going todiscuss key reactions of this cycle and its regulation. These notes are color-coded to make things easier for you. The enzymes are colored in blue andare italicized, inhibitors are red, positive effectors are green, andimportant anabolic products are pink. Some key thoughts regarding regulationare shown below. There are primarily three modes of regulation in the cycle.The first is the availability of substrate. As metabolites build up in the cell,this increase in substrate will drive reactions forward. When these substrates are scarce, the reactions will slow down. Some enzymes are also inhibited by their products. And finally there isnegative feedback by downstream intermediates.This is the TCA cycle. TCA stands for tri-carboxylic acid. This is another name for citrate, which appears at the top of the cycle. If this were a clock, citrate would be at twelve o'clock. Citrate's three carboxylic acid groups are highlighted in red. This cycle is also called the citric acid cycle, and the Krebs cycle, for Hans Krebs, who elucidated much of this cycle. Now in looking at this, it is important that you know the names of the enzymes, and the metabolites that they important that you know the names of the enzymes, and the metabolites that they form. It is also important that you know where NADH, FADH2, carbon dioxide and GTP are made. As we go further on into this lecture, you're going to see that many of the intermediates in this cycle can be used for other things besides just the production of energy, and it's importantthat you know those as well. One thing that I'd like to tell you, is that in thenext exam you will not be asked to discriminate between real names and fakenames. If you see an enzyme on the exam, it will be an actual enzyme.So you're not going to have to distinguish between isocitratedehydrogenase for instance, and isocitrate synthetase, or something like that. Pleasenote that there are two enzymes in this cycle that sound pretty similar –succinyl CoA synthetase and succinate dehydrogenase. Now the citric acid cycleis the hub of metabolism. This is where fuels go to be oxidized. A lot of theintermediates are also very important for other pathways. Alpha ketoglutarateand oxaloacetate are important, both for the synthesis of nucleotides, and aminoacids. Succinyl CoA in the production of porphyrins, and so truly the TCA cyclelies at the center of metabolism.So let's look at this as an overview, to really figure out what's going on here. We're going to startand end with a four-carbon molecule oxaloacetate. We're going to bring in two carbons in the form of an acetyl group to yield a six-carbon molecule. Then there are going to be twooxidative decarboxylations in succession. Each one will yield NADH andCO2. After the first one, we'll have five carbons. After the second one we'll havefour carbons. Following this, we are going to form GTP or ATP, depending on thetissue. FADH2 and the last NADH to end up with oxaloacetate. And so in this process,there are three molecules of NADH that are produced. Two molecules of carbondioxide, a molecule of either GTP or ATP, depending on the tissue, and one moleculeof FADH2, and this is for every complete turn of the cycle going from oxaloacetate at the start, and ending at oxaloacetate. Let's look at this a little more closely before we go into the individual reactions. And so oxaloacetate is going to combine with two carbons from acetyl CoA. This is going to occur through a condensation reaction, which produces the six-carbon molecule citrate. Then citrate is going to undergo an isomerization. Water is going to leave, and then water is going to come back again, to form isocitrate, and again this is an isomerization reaction that involves a dehydration and rehydration. The next thing that's going to happen is the first oxidative decarboxylation to produce alpha-ketoglutarate, and since this is a redox reaction, we have to have an electron carrier. The electron carrier is NAD+. NAD+ accepts electrons twoat a time as a hydride ion, and the extra proton is released into solution. And so,at the end of this reaction, we have five-carbon alpha-ketoglutarate carbondioxide and NADH plus a proton. The next thing that's going to happen is we'regoing to have a second oxidative decarboxylation. This happens in reactions that are a lotlike what happens in the pyruvate dehydrogenase complex. Very similar enzyme complex, conducting very similar reactions, and what we're left with at the end is succinyl CoA, carbon dioxide, NADH, and a proton. Again, this is a second oxidative decarboxylation, and we've now produced two molecules of NADH. We've released two molecules of carbon dioxide, and now we're left with a four-carbon intermediate succinyl CoA. Now this is an activated intermediate. The next thing that's going to happen is pretty big. We're going to make ATP. Andthis occurs through a substrate-level phosphorylation, and so we're going touse GDP or ADP – it depends on the tissue which one is used – and inorganicphosphate, to make GTP or ATP. And what we're left with is succinate. This is afour-carbon molecule, and we've produced also of course, the GTP or the ATP, andcoenzyme A is also released. And again this happens through substrate-levelphosphorylation. the next thing that's going to happen is we're going to take this succinate, and we're going to get all the energy that we can out of this. So we're going to make sure that this is oxidized as much as it can be. The next thing that's going to happen is we're going to in factoxidize this, with FAD as the electron carrier, and we're going to end up withFADH2 and fumarate, which is a four-carbon molecule. Now the next thing we'regoing to do, is we're going to hydrate this fumarate. We're going to addwater in order to form malate. Now we've seen malate before, and in order to
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