Category: catabolism
Valine, leucine, and isoleucine share the first two catabolic steps. (1) BCAT (branched-chain aminotransferase, tissue-distributed): reversible transamination → α-keto acids (KIC from leucine, KIV from valine, KMV from isoleucine). (2) BCKDH (branched-chain α-ketoacid dehydrogenase complex, mitochondrial; analogous to PDH + α-KGDH): oxidative decarboxylation. After that, each BCAA diverges to a different end product: valine + isoleucine are glucogenic (→ succinyl-CoA); leucine is purely ketogenic (→ acetoacetate + acetyl-CoA). BCKDH deficiency causes maple syrup urine disease (MSUD) — accumulated α-keto acids produce the characteristic odor + acute encephalopathy. Leucine specifically activates mTORC1 (see mtor_signaling); explains the BCAA-supplementation rationale for muscle hypertrophy.