Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Oxidative Degradation of Amino Acids. The Urea Cycle
Three amino acids are converted into succinyl-CoA

The carbon skeletons of Methionine, isoleucine, and valine are degraded in reactions that ultimately yield succinyl-CoA, an intermediate of The Citric Acid Cycle (Fig. 19-12). The Catabolic pathways of isoleucine and valine proceed in a similar manner. Both Amino Acids undergo Transamination followed by Oxidative Decarboxylation of the resulting α-keto acids. Four of the five carbon atoms of valine and three of the six carbon atoms of isoleucine are incorporated into succinic acid.

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Fig. 19-12. Pathways leading from isoleucine, methionine, and valine to succinyl-CoA.

The oxidative decarboxylation of the three α-keto acids derived from the deamination of valine, isoleucine, and leucine is catalyzed by a single enzyme complex, the α-keto acid dehydrogenase. In some individuals, a genetic defect renders this enzyme inactive, causing α-keto acids to accumulate in the Blood and spill over into the urine, imparting a characteristic odor from which the condition derives its name: maple syrup urine disease. This rare metabolic disorder leads to severe impairment of normal Brain development and, if left untreated, to death in early childhood. Treatment requires a strict diet that restricts the intake of the three common amino acids—valine, isoleucine, and leucine—to the absolute minimum necessary. Such "treatment" is exceptionally costly.