Biochemical characterization of L-carnitine dehydrogenases from Rhizobium sp. and Xanthomonas translucens

Recently, we obtained two L-carnitine dehydrogenases (CDHs) from soil isolates, Rhizobium sp. (Rs-CDH) and Xanthomonas translucens (Xt-CDH). The respective molecular masses of Rs-CDH and Xt-CDH were approximately 50 kDa and 37 kDa. In this study, the genes encoding both enzymes were cloned. Their primary structures exhibited high identities with those of 3-hydroxyacyl-CoA dehydrogenases. In addition, Rs-CDH had a 180-residue long extra sequence in its C-terminal region. Except for the initial 20 residues, the extra sequence exhibited similarity to thioesterase. The activity of Rs-CDH was affected only slightly by deletion of thioesterase domain, but it was eliminated by the deletion of the whole C-terminal extra sequence. A further deletion experiment indicated that the region of Ala330-Pro335 of Rs-CDH has important functions in catalytic activity. Moreover, based on the deletion experiment on Xt-CDH, the five-residue tail is considered to have a function similar to Ala330-Pro335 of Rs-CDH.