Fig. 2
Biochemistry of BCAAs. Plasma (brown), cytosolic (light blue) and mitochondrial (gray) compartments are depicted. Concentrations of branched-chain amino acids (BCAAs) in physiological and pathological conditions are reported in the table. BCAAs can both enter the cell from the plasma and be produced by protein breakdown. Intracellular BCAAs are transaminated in mitochondria by branched-chain aminotransferase (BCAT). The resulting branched-chain α-keto acids (BCKAs, especially α-keto acid from leucine) inhibit branched-chain α-keto acid dehydrogenase kinase, resulting in elevation of the active state of the rate limiting enzyme branched-chain α-keto acid dehydrogenase complex (BCKDH). BCAAs can be oxidized to generate ATP. Carbon originating from BCAAs enters the tricarboxylic acid (TCA) cycle as acetyl-CoA for complete disposal as CO2. Isoleucine and valine provide carbon for anaplerotic conversion of propionyl-CoA to succinyl-CoA. IB-CoA, isobutyryl-coenzyme A; IV-CoA, isovaleryl-coenzyme A; MB-CoA, α-methylbutyryl-coenzyme A; R-CoA, acyl-coenzyme A