Characteristics of a water-forming NADH oxidase from Methanobrevibacter smithii, an archaeon in the human gut

NADH oxidases (NOXs) catalysing the oxidation of NADH to yield NAD+ and H2O, H2O2, or both play an important role in protecting organisms from oxidative stress and maintaining the balance of NAD+/NADH. A gene encoding NOX was identified from Methanobrevibacter smithii (NOX-ms), the predominant archaeon in the human gut ecosystem. Subsequent analyses showed that it is an FAD-containing protein with a subunit molecular mass of 48 kDa. NOX-ms was purified to homogeneity after expression in Escherichia coli NOX-ms catalysed the oxidization of NADH and converted O2 to H2O with an optimal pH of 7.5 and a temperature optimum of approximately 37°C. The Vmax and Km values were 42.6-44.1 unit/mg and 47.8-54.6 μM for NADH. The apparent Vmax and Km for oxygen were 189.5-196.1 unit/mg and 14.6-16.8 μM. The mutation analysis suggests that Cys42 in NOX-ms plays a key role in the four-electron reduction of O2 to H2O. Quantitative reverse transcription-PCR (RT-qPCR) revealed that transcription of NOX-ms was also up-regulated after exposing the cells to oxidative stress and glucose. Finally, the potential of NOX-ms as a target to control colonization of M. smithii and its possible applications are discussed.