Tricarboxylic acid cycle 2-oxoglutarate dehydrogenase

The intermediary metabolism has multienzyme complexes which, in a complex reaction, catalyze the oxidative decarboxylation of 2-oxoacids and the transfer to coenzyme A of the acyl residue produced. NAD" acts as the electron acceptor. In addition, thiamine diphosphate, lipoamide, and FAD are also involved in the reaction. The oxoacid dehydrogenases include a) the pyruvate dehydrogenase complex (PDH, pyruvate acetyl CoA), b) the 2-oxoglutarate dehydrogenase complex of the tricarboxylic acid cycle (ODH, 2-oxoglutarate succinyl CoA), and c) the branched chain dehydrogenase complex, which is involved in the catabolism of valine, leucine, and isoleucine (see p. 414). 

Glutamate is synthesized from 2-oxoglutarate in the tricarboxylic acid (TCA) cycle by glutamate dehydrogenase (GDH) reaction, which is the main pathway for glutamate biosynthesis when nitrogen supply is sufficient. 

Oxaloacetate may also be produced by amino transfer reactions involving aspartate or indirectly from pyruvate through the concerted action of two malate dehydrogenase enzymes, EC 1.1.1.40 and EC 1.1.1.37. However, the amino transfer reaction is not anaplerotic since it does not accomplish net synthesis of a tricarboxylate cycle intermediate as it employs 2-oxoglutarate. Some glucogenic amino acids (Table 16.4) may contribute to anaplerosis (Section 16.3). 

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