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 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.