Acetylene hydratase: from physiology to structure to function

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To date, acetylene is the only hydrocarbon known to be metabolized in the absence and presence of dioxygen in the same manner. The novel tungsten-[4Fe-4S]-enzyme acetylene hydratase (WAH) is the first enzyme involved in fermentative conversion of C2H2 to acetate and ethanol by the strict anaerobe Pelobacter acetylenicus. Growth with acetylene depends on the supply of tungstate. WAH converts C2H2 to acetaldehyde, a reaction distinct from the reduction of acetylene to ethylene by nitrogenase. WAH belongs to the superfamily of molybdopterin-dependent enzymes. The addition of one H2O molecule to the C≡C bond - formally a non-redox reaction - requires a strong reductant and the presence of chemically complex metal centers. When P. acetylenicus grew at elevated levels of molybdate, the less active variant MoAH could be isolated. WAH is highly specific for acetylene. Starting from the 1.26 Å 3D structure of native WAH, we will unravel its mechanism of action and explore the distinct role of the metal sites through biochemical, spectroscopic, computational, and crystallographic methods. Variants of AH will be obtained by site-directed mutagenesis which will be key to identify the essential amino acids at the catalytic site, and which will help to understand the substrate specificity of this unusual enzyme. In addition, they might help to enlarge the substrate spectrum of the enzyme in the future. Note that acetylene and derivatives represent valuable chemicals for the synthesis of important industrial building blocks.