Catalyzes the phosphorylation and inactivation of the branched-chain alpha-ketoacid dehydrogenase complex, the key regulatory enzyme of the valine, leucine and isoleucine catabolic pathways. Key enzyme that regulate the activity state of the BCKD complex. ATP + L-seryl-[3-methyl-2-oxobutanoate dehydrogenase] = ADP + H(+) + O-phospho-L-seryl-[3-methyl-2-oxobutanoate dehydrogenase] Monomer. Expressed in heart and liver. Autophosphorylated. Belongs to the PDK/BCKDK protein kinase family.