Isocitric Lipoic acid

Lipoic acid and FAD serve as prosthetic groups in the enzyme isocitrate dehydrogenase. Describe their possible roles in the reaction catalyzed by the enzyme. 

Lipoic acid contains a sulfhydryl group that could act as an acceptor for electrons from isocitrate. Those electrons could then be transferred to NAD" via FAD. The roles of the prosthetic groups would be similar to those they play in the reaction catalyzed by pyruvate dehydrogenase. 

As pyruvic acid decarboxylation constitutes the link between glycolysis and the Krebs cycle, a-ketoglutaric decarboxylation divides the reactions involving 6-carbon acids (citrate, isocitrate, and oxalosuccinate) and those involving 4-carbon acids (succinate, fumarate, and malate). The analogy between the two reactions is not restricted to their role in intermediate metabolism, but extends also to the mechanism of action of the two multiple-enzyme systems. In a-ketoglutaric decarboxylation, the overall reaction leads to the formation of CO2 and succinate. CoA, NAD, thiamine, lipoic acid, and magnesium are requirements for this multiple-enzyme system activity. 

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