Branched-chain amino acid
A branched-chain amino acid (BCAA) is an amino acid having aliphatic side-chains with a branch (a carbon atom bound to more than two other carbon atoms). Among the proteinogenic amino acids, there are three BCAAs: leucine, isoleucine and valine. Non-proteinogenic BCAAs include norvaline and 2-aminoisobutyric acid.
The three proteinogenic BCAAs are among the nine essential amino acids for humans, accounting for 35% of the essential amino acids in muscle proteins and 40% of the preformed amino acids required by mammals.
Dietary BCAA supplementation has been used clinically to aid in the recovery of burn victims. A 2006 paper suggests that the concept of nutrition supplemented with all BCAAs for burns, trauma, and sepsis should be abandoned for a more promising leucine-only-supplemented nutrition that requires further evaluation. 
Degradation of branched-chain amino acids involves the branched-chain alpha-keto acid dehydrogenase complex (BCKDH). A deficiency of this complex leads to a buildup of the branched-chain amino acids (leucine, isoleucine, and valine) and their toxic by-products in the blood and urine, giving the condition the name maple syrup urine disease.
The BCKDH complex converts branched-chain amino acids into Acyl-CoA derivatives, which after subsequent reactions are converted either into acetyl-CoA or succinyl-CoA that enter the citric acid cycle.
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- Shimomura Y, Murakami T, Naoya Nakai N, Nagasaki M, Harris RA (2004). "Exercise Promotes BCAA Catabolism: Effects of BCAA Supplementation on Skeletal Muscle during Exercise". J. Nutr. 134 (6): 1583S–1587S. Retrieved 22 March 2011.
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- Abstract on sciencemag.org
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- Blomstrand E, Eliasson J, Karlsson HK, Köhnke R (2006). "Branched-chain amino acids activate key enzymes in protein synthesis after physical exercise". J. Nutr. 136 (1 Suppl): 269S–73S. PMID 16365096.
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- Branched-chain amino acids at the US National Library of Medicine Medical Subject Headings (MeSH)
- branched-chain amino acid degradation pathway