The strictly anaerobic pathogenic bacterium Clostridium difficile occurs in the human gut and is able to thrive from fermentation of leucine. Thereby the amino acid is both oxidized to isovalerate plus CO(2) and reduced to isocaproate. In the reductive branch of this pathway, the dehydration of (R)-2-hydroxyisocaproyl-coenzyme A (CoA) to (E)-2-isocaprenoyl-CoA is probably catalyzed via radical intermediates. The dehydratase requires activation by an ATP-dependent one-electron transfer (J. Kim, D. Darley, and W. Buckel, FEBS J. 272:550-561, 2005). Prior to the dehydration, a dehydrogenase and a CoA transferase are supposed to be involved in the formation of (R)-2-hydroxyisocaproyl-CoA. Deduced amino acid sequences of ldhA and hadA from the genome of C. difficile showed high identities to d-lactate dehydrogenase and family III CoA transferase, respectively. Both putative genes encoding the dehydrogenase and CoA transferase were cloned and overexpressed in Escherichia coli; the recombinant Strep tag II fusion proteins were purified to homogeneity and characterized. The substrate specificity of the monomeric LdhA (36.5 kDa) indicated that 2-oxoisocaproate (K(m) = 68 muM, k(cat) = 31 s(-1)) and NADH were the native substrates. For the reverse reaction, the enzyme accepted (R)- but not (S)-2-hydroxyisocaproate and therefore was named (R)-2-hydroxyisocaproate dehydrogenase. HadA showed CoA transferase activity with (R)-2-hydroxyisocaproyl-CoA as a donor and isocaproate or (E)-2-isocaprenoate as an acceptor. By site-directed mutagenesis, the conserved D171 was identified as an essential catalytic residue probably involved in the formation of a mixed anhydride with the acyl group of the thioester substrate. However, neither hydroxylamine nor sodium borohydride, both of which are inactivators of the CoA transferase, modified this residue. The dehydrogenase and the CoA transferase fit well into the proposed pathway of leucine reduction to isocaproate.