Citric acid cycle reactions isocitrate dehydrogenase

There are two different forms of isocitrate dehydrogenase in all cells, one requiring NAD+ as electron acceptor and the other requiring NADP+. The overall reactions are otherwise identical. In eukaryotic cells, the NAD-dependent enzyme occurs in the mitochondrial matrix and serves in the citric acid cycle. The main function of the NADP-dependent enzyme, found in both the... 

Three modifications of the conventional oxidative citric acid cycle are needed, which substitute irreversible enzyme steps. Succinate dehydrogenase is replaced by fumarate reductase, 2-oxoglutarate dehydrogenase by ferredoxin-dependent 2-oxoglutarate oxidoreductase (2-oxoglutarate synthase), and citrate synthase by ATP-citrate lyase [3, 16] it should be noted that the carboxylases of the cycle catalyze the reductive carboxylation reactions. There are variants of the ATP-driven cleavage of citrate as well as of isocitrate formation [7]. The reductive citric acid... 

In animals the acetyl CoA produced from fatty acid degradation cannot be converted into pyruvate or oxaloacetate. Although the two carbon atoms from acetyl CoA enter the citric acid cycle, they are both oxidized to C02 in the reactions catalyzed by isocitrate dehydrogenase and a-ketoglutarate dehydrogenase (see... 

The first enzyme of the citric acid cycle to catalyze both the release of one carbon dioxide and the reduction of NAD+ is isocitrate dehydrogenase. The overall reaction of this step is as follows ... 

To measure the activity of an enzyme of the citric acid cycle, isocitrate dehydrogenase, and the effect of enzyme concentration on the rate of reaction. 

Isocitrate Dehydrogenase Reaction What type of chemical reaction is involved in the conversion of isocitrate to a-ketoglutarate Name and describe the role of any cofactors. What other reaction(s) of the citric acid cycle are of this same type ... 

Answer Anaplerotic reactions replenish intermediates in the citric acid cycle. Net synthesis of a-ketoglutarate from pyruvate occurs by the sequential actions of (1) pyruvate carboxylase (which makes extra molecules of oxaloacetate), (2) pyruvate dehydrogenase, and the citric acid cycle enzymes (3) citrate synthase, (4) aconitase, and (5) isocitrate dehydrogenase ... 

We come now to the first of four oxidation-reduction reactions in the citric acid cycle. The oxidative decarboxylation of isocitrate is catalyzed by isocitrate dehydrogenase. 

It is important to note that animals are unable to effect the net synthesis o/glu-cose from fatty acids. Specifically, acetyl Go A cannot be converted into pyruvate or oxaloacetate in animals. Recall that the reaction that generates acetyl CoA from pyruvate is irreversible (p. 477). The two carbon atoms of the acetyl group of acetyl CoA enter the citric acid cycle, but two carbon atoms leave the cycle in the decarboxylations catalyzed by isocitrate dehydrogenase and a-ke-toglutarate dehydrogenase. Consequently, oxaloacetate is regenerated, hut it is not formed de novo when the acetyl unit of acetyl CoA is oxidized by the citric acid cycle. In contrast, plants have two additional enzymes enabling them to convert the carbon atoms of acetyl CoA into oxaloacetate (Section 18.4.). 

Within the citric acid cycle itself, the three control points are the reactions catalyzed by citrate synthase, isocitrate dehydrogenase, and the a-ketoglutarate dehydrogenase complex. We have already mentioned that the first reaction of the cycle is one in which regulatory control appears, as is to be expected in the first reaction of any pathway. Gitrate synthase is an allosteric enzyme inhibited by ATP, NADH, succinyl-GoA, and its own product, citrate. 

How is control exerted within the citric acid cycle Within the citric acid cycle, the three control points are the reactions catalyzed by citrate synthase, isocitrate dehydrogenase, and the a-ketoglutarate dehydrogenase complex. 

If the amount of ADP in a cell increases relative to the amount of ATP, the cell needs energy (ATP). This situation not only favors the reactions of the citric acid cycle, which release energy, activating isocitrate dehydrogenase, but also stimulates the formation of NADH and FADH for ATP production by electron transport and oxidative phosphorylation. 

Figure 2.4. Citric acid cycle. This series of reactions is catalyzed by the following enzymes as numbered in the diagram (1) Citrate synthase, (2) Aconitase, (3) Aconitase, (4) Isocitrate dehydrogenase, (5) a-Ketoglutarate dehydrogenase compiex, 6) Succinyl CoA syntetase, (7) Succinate dehydrogenase, (8) Fumarase and (9) Malate dehydrogenase. Adapted from L. Stryerl .

The citric acid cycle is the series of enzyme-catalyzed reactions responsible for the oxidation of the acetyl group of acetyl-CoA to two molecules of CO2. The enzymes that catalyze the reactions are 1. citrate synthase 2. aconitase 3. isocitrate dehydrogenase 4. a-ketoglutarate dehydrogenase 5. succinyl-CoA synthetase 6. succinate dehydrogenase 7. fumarase and 8. malate dehydrogenase. 

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