DMSO reductase is a molybdenum-containing enzyme that catalyzes reduction of dimethyl sulfoxide (DMSO) to dimethyl sulfide (DMS). This enzyme serves as the terminal reductase under anaerobic conditions in some bacteria, with DMSO being the terminal electron acceptor. During the course of the reaction, the oxygen atom in DMSO is transferred to molybdenum, and then reduced to water.

DMSO reductase (DMSOR) and other members of the DMSO reductase family are unique to bacteria and archaea. Enzymes of this family in anaerobic oxidative phosphorylation and inorganic-donor-based lithotrophic respiration. These enzymes have been engineered to degrade oxoanions. DMSOR catalyzes the transfer of two electrons and one oxygen atom in the reaction: The active site of DMSOR contains molybdenum, which is otherwise rare in biology.

As for other members of DMSO reductase family, the tertiary structure of DMSOR is composed of Mo-surrounding domains I-IV, with domain IV heavily interacting with pyranopterindithiolene Mo-cofactor(s) (P- and Q-pterin) of the active site. Members of the DMSO reductase family differ in terms of their active sites. In the case of DMSOR, the Mo center is found to two dithiolene provided by two pyranopterin cofactors. These organic cofactors, called molybdopterins, are linked to GMP to create a dinucleotide form. An additional fifth cap-like ligand is the side-chain O of serine-147 residue, further classifying the enzyme as Type III DMSO reductase. InType I and II serine is replaced by cysteine and aspartate residues, respectively. Depending on the redox state of the Mo, which fluctuates between IV, V, or VI as the reaction progresses, the active site Mo core can also be ligated to an oxygen atom of an aqua-, hydroxo-, or oxo-group, respectively. Studies have shown that the particular identity of the amino-acid used to coordinate the Mo core greatly influences Mo redox midpoint potential and protonation state of the oxygen-group ligation, which are key determinants in the enzyme’s mechanism for catalysis.

Initial isotopic DMSO¹⁸ studies established a double-oxotransferase mechanism for DMSOR of R. sphaeroides. In this mechanism the labeled O¹⁸ is transferred from substrate to Mo, which then transfers the O¹⁸ to 1,3,5-triaza-7-phosphaadamantane (PTA) to yield PTAO¹⁸. In an analogous mechanism, DMSO transfers O to Mo, and the resulting Mo(VI)O center is reduced, yielding water.

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