Cloning, expression, characterization and homology modeling of a novel water-forming NADH oxidase from Streptococcus mutans ATCC 25175

A novel nicotinamide adenine dinucleotide (NADH) oxidase from Streptococcus mutans ATCC 25175 (SmNox) was cloned and overexpressed in Escherichia coli BL21 (DE3). Sequence analysis revealed an open reading frame of 1374 bp, capable of encoding a polypeptide of 457 amino acid residues. The molecular mass of the purified SmNox was estimated to be ∼49.9 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purified SmNox had the highest specific activity of 281.2 U·mg−1 at optimal pH and temperature of 7.0 and 35 °C, with a Km of 57.7 μM and a Vmax of 154.3 U·mg−1. The good stability at room temperature was observed. Homology modeling and substrate docking were performed to evaluate the catalytic characteristics. The results indicated that Nicotinamide ring of NADH extends vertically toward to re-face of coenzyme (FAD), and the specific conformation of NADH suggested that the charges transfer in SmNox complex could be easier than in its homologous enzyme (LbNox) under alkaline environment. The characterization of the SmNox indicated it has potential in industrial regeneration of coenzyme NAD+ for coupling with dehydrogenases.