Arginine racemization by coupled catabolic and anabolic dehydrogenases

Abstract

D-amino acids exist in living organisms as specialized components of many different machineries. Biosynthesis of D-amino acids from racemization of predominant L-enantiomers is catalyzed by a single enzyme. Here, we report the finding of a novel 2-component amino acid racemase for D-to-L inversion in D-arginine metabolism of Pseudomonas aeruginosa. From DNA microarray analysis, the putative dauBAR operon (for D-arginine utilization) of unknown functions was found to be highly induced by D-arginine. The importance of the dau operon in D-arginine metabolism was demonstrated by the findings that strains with a lesion at dauA or dauB failed to use D-arginine as sole carbon source. Two lines of evidence suggest that DauA and DauB are required for D-to-L racemization of arginine. First, growth complementation of an L-arginine auxotroph by D-arginine was abolished by a lesion at dauA or dauB. Second, D-arginine induced L-arginine-specific genes in the parental strain PAO1 but not in its dauA or dauB mutants. This hypothesis was further supported by activity measurements of the purified enzymes: DauA catalyzes oxidative deamination of D-arginine into 2-ketoarginine and ammonia, and DauB is able to use 2-ketoarginine and ammonia as substrates and convert them into L-arginine in the presence of NADPH or NADH. Thus, we propose that DauA and DauB are coupled catabolic and anabolic dehydrogenases to perform D-to-L racemization of arginine, which serves as prerequisite of D-arginine utilization through L-arginine catabolic pathways.