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Catabolic Pathways for Methionine, Isoleucine, Threonine and Valine

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Description

The carbon skeletons of Methionine, Isoleucine, Threonine and Valine are degraded by pathways that yield Succinyl-CoA (Succinyl-Coenzyme A), an intermediate of the Citric Acid Cycle. Methionine donates its Methyl group to one of several possible acceptors through S-adenosylmethionine and three of its four remaining carbon atoms are converted to the Propionate of Propionyl-CoA, a precursor of Succinyl-CoA. Isoleucine undergoes transamination, followed by oxidative decarboxylation of the resulting Alpha-Keto Acid. The remaining five-carbon skeleton is further oxidized to Acetyl-CoA and Propionyl-CoA. Valine undergoes transamination and decarboxylation and then a series of oxidation reactions convert the remaining four carbons to Propionyl-CoA (Ref.1). In human tissues, Threonine is also converted in two steps to Propionyl-CoA. This is the primary pathway for Threonine degradation [...]

References:

1.How coenzyme B12 radicals are generated: the crystal structure of methylmalonyl-coenzyme A mutase at 2 A resolution.
Mancia F, Keep NH, Nakagawa A, Leadlay PF, McSweeney S, Rasmussen B, Bosecke P, Diat O, Evans PR.
Structure. 1996 Mar 15; 4(3):339-50.
2.Pyridoxal enzymes: mechanistic diversity and uniformity.
Hayashi H.
J. Biochem. 1995 Sep; 118(3):463-73.
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