THE CITRIC ACID CYCLE

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1 THE CITRIC ACID CYCLE Introduction In this Cbse Biology Notes on the Citric Acid Cycle, also called the Krebs Cycle, we will pick up where we left off in the last section with the aerobic product of glycolysis, pyruvate. When oxygen is present, the pyruvate moves out of the cytosol in which glycolysis took place and crosses the membrane into the matrix of the mitochondria. There, before entering the citric acid cycle proper, the pyruvate undergoes a transition stage, in which the two pyruvates are converted into two acetyl-coenzyme A (acetyl-coa), two carbon dioxide molecules, and two NADH. Then, during the series of eight reactions that make up the citric acid cycle, the two acetyl-coa molecules are oxidized, yielding two more molecules of carbon dioxide and 2 ATP. The carbon dioxide generated in these two processes is the carbon dioxide we exhale when we breathe. The citric acid cycle, or Krebs cycle, is central to metabolism, since at this stage a large portion of carbohydrates, lipids, and proteins are degraded by oxidation. One characteristic that marks the citric acid cycle is that it does not only have degradative functions. A number of very important coenzymes are produced in the cycle's reactions. These coenzymes go on to oxidative phosphorylation, resulting in a huge payoff of 32 ATP. Another interesting aspect of the citric acid cycle is its status as a "cycle": the final productof the cycle, oxaloacetate, is a necessary molecule for the first reaction of the cycle with acetyl-coa. We will begin our discussion by looking at the conversion of pyruvate to acetyl-coa, the starting material of the citric acid cycle. Next, we will follow the eight reactions of the citric acid cycle that ultimately lead to the production of oxaloacetate and numerous coenzymes that go on to be used in oxidative phosphorylation. Before the Citric Acid Cycle After emerging from glycolysis, the two pyruvate are transported into the mitochondria. There, the pyruvate undergo a transition stage before entering the actual citric acid cycle. In this phase the pyruvate is transformed into acetyl-coenzyme A (acetyl-coa), the starting product in the citric acid cycle. ; 2 Pyruvate + 2 coenzyme A + 2NAD + -> 2 acetyl-coa +2CO NADH

2 Formation of Acetyl-CoA Acetyl-CoA is a common product of carbohydrate, lipid, and protein breakdown. It consists of an acetyl group attached to a coenzyme A molecule. Coenzyme A is a large molecule that contains a molecule of ADP with two side chain groups stemming from its phosphate arms. Acetyl groups attach to the end of these side chains. In this way, the coenzyme A acts as a carrier of acetyl groups. When it is broken down by water, large amounts of energy are released, which, as we shall see, drive the citric acid cycle. The most common way that acetyl-coa is derived in the metabolic pathway is with the help of the pyruvate dehydrogenase multienzyme complex. The pyruvate dehydrogenase multienzyme is a complex of three distinct enzymes that together convert pyruvate into acetyl-coa with the help of a molecule of coenzyme A and NAD. The mechanism for the formation of acetyl-coa is complex, as seen below. Generally, in reaction 1, the enzyme pyruvate dehydrogenase pulls a carbon dioxide molecule off the pyruvate. This is accomplished with the help of a molecule called TPP that forms a temporary bond with the pyruvate molecule. The carbon dioxide removal reaction is similar to that of the yeast pyruvate decarboxylase in alcoholic fermentation. Figure 1: Pyruvate Metabolism to form Acetyl-CoA. In reaction 2, the enzyme dihydrolipoyl transacetylase helps to attach another temporary molecule called a lipoamide. With this bond formation, the TPP molecule from the first step is released leading to the formation of an acetyl group. In the third step, this lipoamide group is reduced and released as a molecule of CoA attacks the acetyl group. We now have acetyl- CoA. The third enzyme, dihydrolipoyl dehydrogenase, is responsible for restoring the

3 lipoamide to its original, oxidized state so that it can be reused in the cycle in a fourth step. The molecule of NAD asserts itself at this point, helping to reoxidize the lipoamide. At this point, we have acetyl-coa and are ready to enter the citric acid cycle. THE CITRIC ACID CYCLE The Reactions of the Citric Acid Cycle We are now ready to begin going through the reactions of the citric acid cycle. The cycle begins with the reaction between acetyl-coa and the four-carbon oxaloacetate to form six-carbon citric acid. Through the next steps of the cycle, two of the six carbons of the citric acid leave as carbon dioxide to ultimately yield the four carbon product, oxaloacetate, which is used again in the first step of the next cycle. During the eight reactions that take place, for every molecule of acetyl- CoA the cycle produces three NADH and one flavin adenine dinucleotide (FAD/FADH2), along with one molecule of ATP. Figure 2: The Citric Acid Cycle (Krebs Cycle).

4 Reaction 1: Citrate Synthase The first reaction of the citric acid cycle is catalyzed by the enzyme citrate synthase. In this step, oxaloacetate is joined with acetyl-coa to form citric acid. Once the two molecules are joined, a water molecule attacks the acetyl leading to the release of coenzyme A from the complex. Reaction 2: Acontinase Figure 3: Reaction 1. The next reaction of the citric acid cycle is catalyzed by the enzyme acontinase. In this reaction, a water molecule is removed from the citric acid and then put back on in another location. The overall effect of this conversion is that the OH group is moved from the 3' to the 4' position on the molecule. This transformation yields the molecule isocitrate. Figure 4: Reaction 2.

5 Reaction 3: Isocitrate Dehydrogenase Two events occur in reaction 3 of the citric acid cycle. In the first reaction, we see our first generation of NADH from NAD. The enzyme isocitrate dehydrogenase catalyzes the oxidation of the OH group at the 4' position of isocitrate to yield an intermediate which then has a carbon dioxide molecule removed from it to yield alpha-ketoglutarate. Figure 4: Reaction 3. Reaction 4: Alpha-ketoglutarate deydrogenase In reaction 4 of the citric acid cycle, alpha-ketoglutarate loses a carbon dioxide molecule and coenzyme A is added in its place. The decarboxylation occurs with the help of NAD, which is converted to NADH. The enzyme that catalyzes this reaction is alpha-ketoglutarate dehydrogenase. The mechanism of this conversion is very similar to what occurs in the first few steps of pyruvate metabolism. The resulting molecule is called succinyl-coa. Figure 5: Reaction 4.

6 Reaction 5: Succinyl-CoA Synthetase The enzyme succinyl-coa synthetase catalyzes the fifth reaction of the citric acid cycle. In this step a molecule of guanosine triphosphate (GTP) is synthesized. GTP is a molecule that is very similar in its structure and energetic properties to ATP and can be used in cells in much the same way. GTP synthesis occurs with the addition of a free phosphate group to a GDP molecule (similar to ATP synthesis from ADP). In this reaction, a free phosphate group first attacks the succinyl-coa molecule releasing the CoA. After the phosphate is attached to the molecule, it is transferred to the GDP to form GTP. The resulting product is the molecule succinate. Reaction 6: Succinate Dehydrogenase Figure 6: Reaction 5. The enzyme succinate dehydrogenase catalyzes the removal of two hydrogens from succinate in the sixth reaction of the citric acid cycle. In the reaction, a molecule of FAD, a coenzyme similar to NAD, is reduced to FADH2 as it takes the hydrogens from succinate. The product of this reaction is fumarate. Figure 7: Reaction 6. FAD, like NAD, is the oxidized form while FADH2 is the reduced form. Although FAD and NAD perform the same oxidative and reductive roles in reactions, FAD and NAD work on different classes of molecules. FAD oxidizes carbon-carbon double and triple bonds while NAD oxidizes mostly carbon-oxygen bonds.

7 Reaction 7: Fumarase In this reaction, the enzyme fumarase catalyzes the addition of a water molecule to the fumarate in the form of an OH group to yield the molecule L- malate. Reaction 8: Malate Dehydrogenase Figure 7: Reaction 7. In the final reaction of the citric acid cycle, we regenerate oxaloacetate by oxidizing L malate with a molecule of NAD to produce NADH. Conclusion Figure 8: Reaction 8. We have now concluded our discussion of the reactions that compose the citric acid cycle. It is helpful at this point to take a minute to take stock of what the citric acid cycle has generated from one acetyl-coa molecule. The acetyl-coa, has been oxidized to two molecules of carbon dioxide. Three molecules of NAD were reduced to NADH. One molecule of FAD was reduced to FADH2. One molecule of GTP (the equivalent of ATP) was produced. Keep in mind that a reduction is really a gain of electrons. In other words, NADH and FADH2 molecules act as electron carriers and are used to generate ATP in the next stage of glucose metabolism, oxidative phosphorylation.

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