Acetylene hydratase: a non-redox enzyme with tungsten and iron–sulfur centers at the active site

Abstract

In living systems, tungsten is exclusively found in microbial enzymes coordinated by the pyranopterin cofactor, with additional metal coordination provided by oxygen and/or sulfur, and/or selenium atoms in diverse arrangements. Prominent examples are formate dehydrogenase, formylmethanofuran dehydrogenase, and aldehyde oxidoreductase all of which catalyze redox reactions. The bacterial enzyme acetylene hydratase (AH) stands out of its class as it catalyzes the conversion of acetylene to acetaldehyde, clearly a non-redox reaction and a reaction distinct from the reduction of acetylene to ethylene by nitrogenase. AH harbors two pyranopterins bound to W, and a [4Fe-4S] cluster. W is coordinated by four dithiolene sulfur atoms, one cysteine sulfur, and one oxygen ligand. AH activity requires a strong reductant suggesting W(IV) as the active oxidation state. Two different types of reaction pathways have been proposed. The 1.26 Å structure reveals a water molecule coordinated to W which could gain a partially positive net charge by the adjacent protonated Asp-13, enabling a direct attack of C2H2. To access the W-Asp site, a substrate channel was evolved distant from where it is found in other members of the DMSOR family. Computational studies of this second shell mechanism led to unrealistically high energy barriers, and alternative pathways were proposed where C2H2 binds directly to W. The architecture of the catalytic cavity, the specificity for C2H2 and the results from site-directed mutagenesis do not support this first shell mechanism. More investigations including structural information on the binding of C2H2 are needed to present a conclusive answer.

Synthesis and Reactivity of a Bioinspired Molybdenum(IV) Acetylene Complex

“… 9 − 13 The molybdenum variant of AH was reported to be 10 times less active in C 2 H 2 hydration than the tungsten analogue despite exhibiting the same active site architecture as well as a similar protein fold. 14 − 16 In contrast to experimental findings, a recent theoretical study suggests the utilization of molybdenum instead of tungsten in bioinspired complexes to be energetically more favorable when a mechanism is considered where C 2 H 2 is bound to the metal center and subsequently attacked by a hydroxide. 17 , 18 As Mo-dependent nitrogenase is known to accept C 2 H 2 as a substrate, the coordination of C 2 H 2 to a molybdenum(IV) center was investigated already in the late 1970s.…”

Syntrophotalea acetylenivorans sp. nov., a diazotrophic, acetylenotrophic anaerobe isolated from intertidal sediments

“…S. acetylenica and its acetylene hydrating acetylene hydratase (AH) enzyme have been studied extensively [13][14][15][16][17][18][19][20]. In addition there are a number of synthesis reviews on the evolutionary and astrobiological implications of acetylenotrophy and the unusual, tungsten-centred AH enzyme of S. acetylenica [10,[21][22][23][24]. We note that in 2020 members of the genus Pelobacter were reclassified [25], e.g.…”

“…In addition there are a number of synthesis reviews on the evolutionary and astrobiological implications of acetylenotrophy and the unusual, tungsten-centred AH enzyme of S. acetylenica [10, 21–24]. We note that in 2020 members of the genus Pelobacter were reclassified [25], e.g.…”

Acetylene-Fueled Trichloroethene Reductive Dechlorination in a Groundwater Enrichment Culture

“…As R. opacus is an aerobic acetylenotroph, the results from NCBI BLAST indicate that enzymes using acetylene as the substrate have conserved regions, even between aerobic and anaerobic bacteria. It is important to note that the only acetylene hydratase enzyme isolated and characterized to date is that of P. acetylenicus (52,53). Further research is required to biochemically verify the acetylene hydratases identified in this study and in other previous work.…”