The oxaloacetate decarboxylase (OAD) Na+ pump consists of subunits α, β, and γ, which are expressed from an oadGAB gene cluster present in various anaerobic bacteria. Vibrio cholerae has two copies of oad genes, which are termed oad-1 and oad-2. The oad-2 genes are part of the citrate fermentation operon, while the oad-1 genes are flanked by genes encoding products not involved in a catabolic pathway. The gene sequences of oad-1 and oad-2 of V. cholerae strain O395-N1 were determined. The apparent frameshift in the published sequence of the oadA-2 gene from V. cholerae El Tor N16961 was not present in strain O395-N1. Upon anaerobic growth of V. cholerae on citrate, exclusively the oad-2 genes are expressed. OAD was isolated from these cells by monomeric avidin-Sepharose affinity chromatography. The enzyme was of higher specific activity than that from Klebsiella pneumoniae and was significantly more stable. Decarboxylase activity was Na+ dependent, and the activation profile showed strong cooperativity with a Hill coefficient nH=1.8. Oxalate and oxomalonate inhibited the enzyme with half-maximal concentrations of 10μM and 200μM, respectively. After reconstitution into proteoliposomes, the enzyme acted as a Na+ pump. With size-exclusion chromatography, the enzyme eluted in a symmetrical peak at a retention volume corresponding to an apparent molecular mass of approximately 570kDa, suggesting a tetrameric structure for OAD-2. The two oad gene clusters were heterologously expressed in Escherichia coli, and the decarboxylases were isolated from the host cell
