Tungsten aldehyde oxidoreductase - a new type hydrogenase 

Introduction

The incorporation of molybdenum or tungsten pterin complex into the active site of enzymes enormously expands the available repertoire of possible chemical transformations. The central metal facilitates two electron redox processes due to stable IV and VI oxidation states and acts as a Lewis acid (with its open ligation position), which can directly activate a bound reagent. Meanwhile, the other ligands of the Mo/W ions participate in the reactions by (i) providing additional means to activate recalcitrant bonds (e.g. in homo or heterolytic C–H cleavage), (ii) enabling hydroxylation without molecular oxygen (by O transfer to activated intermediates), (iii) introducing strong acid/base catalysis (accepting protons or enabling proton transfer electron transfer processes), (iv) tuning the Mo/W redox potentials the by varying their positions, or (v) even participate directly in the redox processes by providing a reservoir for additional electrons. Moreover, many Mo/W-enzymes contain additional redox-active cofactors assembling into “nanowire” structures, which connect the Mo/W-cofactors with other active sites within the enzyme. As a result, many chemically highly challenging reactions are catalysed by Mo/W-enzymes, such as direct reduction of aromatic rings, reduction of CO₂ to formate or of non-activated carboxylic acids to aldehydes, hydroxylation of alkylaromatic or heterocyclic compounds, hydration of acetylene to acetaldehyde, or oxidation of molecular hydrogen.

In this presentation we will show results of our mechanistic investigations conducted for tungsten aldehyde oxidoreductase from Aromatoleum aromaticum, which unexpectedly turned out to be able to oxidize molecular hydrogen. This so far unique feature of AORAa enables to revert its natural activity towards the reduction of acids, or to provide a cheap tool for NADH recycling. The reverse reaction provides a versatile tool in the reduction of various acids of interest to the corresponding aldehydes, especially in coupled reactions with other enzymes, like alcohol dehydrogenases or aminotransferases. We will present experimental evidence for the unusual activity along with QM modelling indicating possible mechanism for hydrogen activation and reduction of carboxylic acids by AOR.

(This contribution is co-authored by Agnieszka Winiarska (MPI for Terrestrial Microbiology, Marburg), Tobias Erb (MPI Marburg) and Johann Heider (Philipps-Universität Marburg))

Maciej Szaleniec

currently holds a professor position at the Institute of Catalysis and Surface Chemistry, PAS and is the head of the Joint Laboratory of Biotechnology and Enzyme Catalysis, ICSC PAS in Kraków, Poland. In his research, he is focused on both experimental and theoretical studies of enzymes from anaerobic bacteria, especially molybdenum and tungsten-containing enzymes. For over 20 years, he has worked in close cooperation with Professor Johann Heider from Marburg University, and he has explored the metabolic pathways of anaerobic bacteria with his former teacher and friend. His research interest includes kinetic studies of enzymes (steady and pre-steady state, kinetic isotope effect), development of bioreactor processes as well as multiscale theoretical modelling of enzyme reaction pathways (QM:MM and MD). He is especially interested in enzymes catalyzing regioselective/enantio-selective hydroxylation of hydrocarbons, reduction of ketones, dehydrogenation of sterols, radical formation of the C-C bond and recently oxidation of aldehydes and reduction of carboxylic acids. He has published over 70 papers and chapters, obtained several patents and implemented one biotechnological drug synthesis method in the industry.