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Genes encoding a branched-chain α-keto acid dehydrogenase fromEnterococcus faecalis 10C1, E1α (bkdA ), E1β (bkdB ), E2 (bkdC ), and E3 (bkdD ), were found to reside in the gene clusterptb-buk-bkdDABC . The predicted products ofptb andbuk exhibited significant homology to the phosphotransbutyrylase and butyrate kinase, respectively, fromClostridium acetobutylicum . Activity and redox properties of the purified recombinant enzyme encoded bybkdD indicate thatE. faecalis has a lipoamide dehydrogenase that is distinct from the lipoamide dehydrogenase associated with the pyruvate dehydrogenase complex. Specific activity of theptb gene product expressed inEscherichia coli was highest with the substrates valeryl-coenzyme A (CoA), isovaleryl-CoA, and isobutyryl-CoA. In cultures, a stoichiometric conversion of α-ketoisocaproate to isovalerate was observed, with a concomitant increase in biomass. We propose that α-ketoisocaproate is converted via the BKDH complex to isovaleryl-CoA and subsequently converted into isovalerate via the combined actions of theptb andbuk gene products with the concomitant phosphorylation of ADP. In contrast, anE. faecalis bkd mutant constructed by disruption of thebkdA gene did not benefit from having α-ketoisocaproate in the growth medium, and conversion to isovalerate was less than 2% of the wild-type conversion. It is concluded that thebkd gene cluster encodes the enzymes that constitute a catabolic pathway for branched-chain α-keto acids that was previously unidentified inE. faecalis.

Catabolism of Branched-Chain α-Keto Acids in Enterococcus faecalis : the bkd Gene Cluster, Enzymes, and Metabolic Route