Component of the branched-chain alpha-ketoacid dehydrogenase (BCKADH) complex, that catalyzes the overall conversion of branched-chain alpha-ketoacids to acyl-CoA and CO(2). (R)-N(6)-dihydrolipoyl-L-lysyl-[protein] + 2-methylpropanoyl-CoA = (R)-N(6)-(S(8)-2-methylpropanoyldihydrolipoyl)-L-lysyl-[protein] + CoA Binds 1 lipoyl cofactor covalently. Forms a 24-polypeptide structural core with octahedral symmetry (By similarity). Part of the BCKADH complex, consisting of multiple copies of BkdA/BkdB (E1), BkdC (E2) and Lpd (E3). Up-regulated upon nutrient starvation. Is also highly up-regulated in a DlaT-deficient strain. Part of the bkdABC operon. Belongs to the 2-oxoacid dehydrogenase family.