Catabolism and synthesis of leucine, isoleucine and valine are finely balanced, allowing the body to make the most of dietary input but removing excesses to prevent toxic build-up of their corresponding keto-acids. This is the butyryl-CoA dehydrogenase, subunit A domain 3, a largely alpha-helical bundle of the enzyme BCDHK. This enzyme is the regulator of the dehydrogenase complex that breaks branched-chain amino-acids down, by phosphorylating and thereby inactivating it when synthesis is required. The domain is associated with family HATPase_c Pfam:PF02518 which is towards the C-terminal.

1: Branched-chain alpha-ketoacid dehydrogenase kinase. Molecular cloning, expression, and sequence similarity with histidine protein kinases. Popov KM, Zhao Y, Shimomura Y, Kuntz MJ, Harris RA; J Biol Chem. 1992;267:13127-13130. PMID:1377677