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

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 ) catalyzes the reduction of organic acids not only with low-potential Eu(II) or Ti(III) complexes but also with H₂ as an electron donor. Additionally, AOR catalyzes the H₂-dependent reduction of NAD⁺ or benzyl viologen. The rate of H₂-dependent NAD⁺ reduction equals to 10% of that of aldehyde oxidation, representing the highest H₂ turnover rate observed among the Mo/W enzymes. As AOR 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 H₂ as the only reductant. The newly discovered W-hydrogenase side activity of AOR may find applications in either NADH recycling or conversion of carboxylic acids to more useful biochemicals.