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

Abstract

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 (AORAa) 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, AORAa 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 AORAa 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 AORAa may find applications in either NADH recycling or conversion of carboxylic acids to more useful biochemicals.