Tungsten Enzyme Using Hydrogen as an Electron Donor to Reduce Carboxylic Acids and NAD(+)

[Image: see text] Tungsten-dependent aldehyde oxidoreductases (AORs) catalyze the oxidation of aldehydes to acids and are the only known enzymes reducing non-activated acids using electron donors with low redox potentials. We report here that AOR from Aromatoleum aromaticum (AOR(Aa)) catalyzes the reduction of organic acids not only with low-potential Eu(II) or Ti(III) complexes but also with H(2) as an electron donor. Additionally, AOR(Aa) catalyzes the H(2)-dependent reduction of NAD(+) or benzyl viologen. The rate of H(2)-dependent NAD(+) reduction equals to 10% of that of aldehyde oxidation, representing the highest H(2) turnover rate observed among the Mo/W enzymes. As AOR(Aa) simultaneously catalyzes the reduction of acids and NAD(+), we designed a cascade reaction utilizing a NAD(P)H-dependent alcohol dehydrogenase to reduce organic acids to the corresponding alcohols with H(2) as the only reductant. The newly discovered W-hydrogenase side activity of AOR(Aa) may find applications in either NADH recycling or conversion of carboxylic acids to more useful biochemicals.