Overall structure of the [4FE-4S] cluster and the tungstoenzyme encompassed in the protein; the protein backbone is dark blue at the N-terminal end and green at the C-terminal end.grey = W, yellow = sulfur, red = O, blue = N. The octahedral active site of the tungstoenzyme coordinated by two molybdopterin co-factors, Cys141 and a water molecule. The active site residues are Asp13 and Cys12.
The mechanism is thought to involve attachment of acetylene to the metal followed bynucleophilic attack of water. Because acetylene binding to the Mo innitrogenase lends some support that the mechanism involves a Mo→CH2=CH2 bond. Acetylene inhibits several microbial transformations where it interacts with the active site of the metal-dependent enzymes includinghydrogenase and nitrogenase.[2] Thisenzyme relies ontungsten as the metal center and is the heaviest metal that plays a prominent part in thenitrogen,sulfur andcarbon metabolic processes.[3] The [4Fe-4S]cubane keeps the W in the reduced W(IV) state, the most stable reduced oxidation state, while W(VI) is the other stable oxidation state (2nd and 3rd row transition metals are usually most stable in their highest oxidation state).[2] Mo and W enzymes ubiquitously involve W(IV)/W(VI) in the catalysis, however AH is unique since the tungstoenzyme stays as W(IV) in the catalysis.[4] The tungstoenzyme stays as W(IV) throughout the catalysis because the enzyme catalyzes a non-redox reaction described as the hydration of acetylene to acetaldehyde.[5] Theactive site tungsten has a distorted octahedral geometry that is coordinated bymolybdopterin co-factors along with a cysteine residue coordinated by a water molecule as the sixth ligand.[4] The active site residues are Asp13, Cys12, Trp179, Arg606, Met140 and Ile142.[3] Asp13 plays an important role in assisting the catalysis where the active site residue deprotonates the water hydroxide making it a better nucleophile.[3]
^abcKroneck PM (March 2016). "Acetylene hydratase: a non-redox enzyme with tungsten and iron-sulfur centers at the active site".Journal of Biological Inorganic Chemistry.21 (1):29–38.doi:10.1007/s00775-015-1330-y.PMID26790879.S2CID254088239.
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