The carbon chains of methionine, isoleucine, threonine and valine are broken down to produce succinyl-CoA, which feeds into the Citric Acid Cycle. Methionine donates its methyl group via S-adenosylmethionine to one of several acceptors and the four remaining carbons are the backbone of pronionyl-CoA that is subsequently converted to succinyl-CoA. The degredation of threonine shown here occurs in humans. A different mechanism is used in other organisms. The transdeamination and decarboxylation of valine are followed by a series of steps resulting in the formation of propionyl-CoA. Isoleucine also undergoes transdeamination, follwed by oxidative decarboxylation. These steps yield propionyl-CoA as well as acetyl-CoA. Deficiency in the steps that convert isoleucine to propionyl-CoA causes sudden unexplained infant death, Reye-like syndrome, cardiomyopathy, or skeletal myopathy.
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