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Second, the hydroxyethyl group attached to TPP is oxidized to form an acetyl group and concomitantly transferred to lipoamide, a derivative of lipoic acid that is linked to the side chain of a lysine residue by an amide linkage. Thus, citrate is isomerized into isocitrate to enable the six-carbon unit to undergo oxidative decarboxylation. Pyruvate produced by glycolysis is converted into acetyl CoA, the biochfmia of the citric acid cycle.

At the top centerthe enzyme represented by a yellow, a blue, and two red spheres is unmodified and ready for a catalytic cycle. Because this reaction initiates the cycle, it is very important that side reactions be minimized.

The hydroxyl group adds boochemia only one side of the double bond of stryed hence, only the l isomer of malate is formed.

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Examination of the three-dimensional structure biocchemia succinyl CoA synthetase shows that each subunit comprises two domains Figure National Center for Biotechnology InformationU. Citrate Is Isomerized into Isocitrate The tertiary hydroxyl group is not properly located in the citrate molecule for the oxidative decarboxylations that follow.

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The cleavage of the thioester bond of succinyl CoA is coupled to the phosphorylation of a purine nucleoside diphosphate, usually GDP. Finally, malate is oxidized to form ibochemia. E 2 inserts the lipoyl-lysine arm of the lipoamide domain into the channel in E 1. Although the structure of an intact member of the pyruvate dehydrogenase complex family has not yet been determined in atomic detail, the structures of all of the component enzymes are now known, albeit from different complexes and species.

The formation of acetyl CoA from carbohydrates is less direct than from fat. Reactions of the Pyruvate Dehydrogenase Complex.

The structural integration of three kinds of enzymes makes the coordinated catalysis of a complex reaction possible. Oxaloacetate Is Regenerated by the Oxidation of Succinate Reactions of four-carbon compounds constitute the final stage of the citric acid cycle: In dramatic contrast, only 2 molecules of ATP are generated per molecule of glucose which generates 2 molecules of acetyl CoA by anaerobic glycolysis.

One of the histidine residues His donates a proton to the carbonyl oxygen of acetyl CoA to promote the removal of a methyl proton by Asp Succinyl CoA synthetase has evolved by adopting these domains and harnessing them to allow the capture of the energy associated with succinyl CoA cleavage to drive the generation of a nucleoside triphosphate. In fact, succinate dehydrogenase is directly associated with the electron-transport chain, the link between the citric acid cycle and ATP formation.

In the mitochondrial matrix, pyruvate is oxidatively decarboxylated by the pyruvate dehydrogenase complex to form acetyl CoA.

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The transacetylase core E 2 is shown in red, the pyruvate dehydrogenase component E 1 in yellow, and the dihydrolipoyl dehydrogenase E 3 in green. The citric acid cycle begins with the condensation of a four-carbon unit, oxaloacetate, and a two-carbon unit, the acetyl group of acetyl CoA. The positively charged ring of TPP acts as an electron sink that stabilizes the negative charge that is transferred to the ring as part of the decarboxylation.


The lipoamide domain is followed by a small domain that interacts with E 3 within the complex. Thus, 1 acetate unit generates approximately 10 molecules of ATP.

The enzyme catalyzing both steps is called aconitase because cis -aconitate is an intermediate. At the E 3 active site, the lipoamide acid is oxidized by coenzyme FAD.

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Twenty-four copies of E 1 and 12 copies of E 3 surround the E 2 core. Fumarase catalyzes a stereospecific trans addition of a hydrogen atom and a hydroxyl group. Support Center Support Center.

The pyruvate dehydrogenase complex cannot complete another catalytic cycle until the dihydrolipoamide is oxidized to lipoamide. Rather, two electrons are transferred from FADH 2 directly to iron-sulfur clusters of the enzyme. Aconitase is an iron-sulfur proteinor nonheme iron protein.


Structure of Succinyl CoA Synthetase. Citrate synthase is well suited to hydrolyze citryl CoA but not acetyl CoA.

This is the only step in the citric acid cycle that directly yields a compound with high phosphoryl transfer potential through a substrate-level phosphorylation. W H Freeman ;