The mitochondrial branched-chain alpha-ketoacid dehydrogenase (BKCDH) complex catalyzes the reactions of alpha-ketoisocaproate, alpha-keto beta-methylvalerate, or alpha-ketoisovalerate with CoA and NAD+ to form isovaleryl-CoA, a-methylbutyryl-CoA, or isobutyryl-CoA, respectively, and CO2 and NADH (Chuang and Shih 2001). While bovine and microbial BCKD complexes have been characterized most extensively (Reed and Hackert 1990), structural studies of individual components and subcomplexes of human BKCD have confirmed their structures and roles in the overall oxidative carboxylation process, and have related these features to the disruptive effects of mutations on branched-chain amino acid metabolism in vivo: E1a and E1b components - AEvarsson et al. 2000; E2 - Chang et al. 2002; E3- Brautigam et al. 2005. In addition, structural studies have confirmed the lipoylation of lysine residue 44 in E2 protein (Chang et al. 2002) and the loss of an aminoterminal mitochondrial transport sequence from mature E3 protein (Bruatigam et al. 2005). Loss of mitochondrial transport sequences from proteins E1a, E1b, and E2 has been domstrated by sequence analysis (Wynn et al. 1999).