A tungsten enzyme using hydrogen as an electron donor to reduce carboxylic acids and NAD+

Abstract

Tungsten-dependent aldehyde oxidoreductases (AOR) catalyse 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) catalyses 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.

A bacterial tungsten-containing aldehyde oxidoreductase forms an enzymatic decorated protein nanowire

“…These masses correspond to sole AorAB protomers (86 kDa), a heterotrimeric type of complex (AorABC; 132 kDa), and three larger complexes containing subsequently more AorAB protomers: Aor(AB) 2 C (218 kDa), Aor(AB) 3 C (304 kDa), and Aor(AB) 4 C (392 kDa). In accordance with the previous data on AOR Aa (21), the five-subunit Aor(AB) 2 C complex appears to be the most abundant in the preparation studied. Therefore, AOR Aa apparently forms oligomeric structures of Aor(AB) n C composition.…”

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

“… 15 Therefore, AOR Aa is applicable for converting many different organic acids into biotechnologically interesting products ( e.g. , vanillin 50 ).…”

A bacterial tungsten-containing aldehyde oxidoreductase forms an enzymatic decorated protein nanowire

“…AOR from Aromatoleum aromaticum was produced by heterologous expression in Aromatoleum evansii with an added twin-Strep-tag® at the N-terminus of the small subunit AorA to facilitate the purification of the complex by affinity chromatography on a streptavidin column ( 18 ). Minor impurities in the obtained preparation were removed by a subsequent run of the enzyme over a size-exclusion column.…”

“…The reduction of carboxylic acids is catalysed by AOR Aa in the presence of either artificial low-potential electron donors or hydrogen, showing a substrate spectrum of acid reduction that is comparable with that of the aldehyde oxidation reactions ( 12 ). The unprecedented ability of this enzyme to use hydrogen as an electron donor for acid or NAD + reduction qualifies the enzyme as a new type of hydrogenase ( 18 ). In addition, the enzyme can also be driven towards acid reduction to aldehydes through coupling to an electrochemical cell [unpublished data].…”