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Answer to Question #87036 in Biochemistry for Arun Rawat

Question #87036
a) How is the enzyme pyruvate dehydrogenase complex different from other enzymes?
Explain how it functions in the conversion of pyruvate to acetyl- CoA?
Expert's answer

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Pyruvate dehydrogenase complex is a protein complex that performs the oxidative decarboxylation of pyruvate. The main difference of the complex is that includes three enzymes (pyruvate dehydrogenase (E1) dihydrolipoyl transacetylase (E2) and dihydrolipoamide dehydrogenase (E3)) and two auxiliary protein and for its operation requires five cofactors (CoA, NAD+, thiamine pyrophosphate (TPP), FAD and lipoic acid (lipoate )). Oxidative decarboxylation of pyruvate is a biochemical process consisting of the removal of one carbon dioxide (CO2) molecule from the pyruvate molecule and the addition of coenzyme A (CoA) to the decarboxylated pyruvate to form acetyl-CoA. Oxidative decarboxylation of pyruvate involves several stages. First, the first carbon atom (C-1) of pyruvate is detached as CO2, and C-2 (aldehyde form in pyruvate) is attached to the TPP as a hydroxyethyl group (-CHOH-CH3). Next, the hydroxyethyl group is oxidized to acetate that is associated with the release of two electrons used to restore the bond -S-S- of the E2 lipoyl group to two thiols groups (-SH). Then, the acetyl residue formed during the redox reaction in the second stage is boundto the lipoyl -SH-group, and is transferred to CoA producing acetyl-CoA. During the two last reactions, which are catalyzed by dihydrolipoamide dehydrogenase, the reduced lipoyllysine returns to its oxidized form. The electrons that were originally present in the hydroxyethyl group are transferred from the lipoyllysine to FAD to form FADH2, and then to NAD+ to form NADH+ H+.

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