The molybdoenzyme dimethylsulfoxide (DMSO) reductase contributes to the release of dimethylsulfide, a compound that has been implicated in cloud nucleation and global climate regulation. The crystal structure of DMSO reductase from Rhodobacter sphaeroides reveals a monooxo molybdenum cofactor containing two molybdopterin guanine dinucleotides that asymmetrically coordinate the molybdenum through their dithiolene groups. One of the pterins exhibits different coordination modes to the molybdenum between the oxidized and reduced states, whereas the side chain oxygen of Ser147 coordinates the metal in both states. The change in pterin coordination between the Mo(VI) and Mo(IV) forms suggests a mechanism for substrate binding and reduction by this enzyme. Sequence comparisons of DMSO reductase with a family of bacterial oxotransferases containing molybdopterin guanine dinucleotide indicate a similar polypeptide fold and active site with two molybdopterins within this family.

Molybdenum K-edge X-ray absorption and Mo(V) electron paramagentic resonance (EPR) spectroscopies have been used to probe the metal coordination in oxidized and reduced forms of both wild-type and a site-directed mutant of Rhodobacter sphaeroides dimethyl sulfoxide (DMSO) reductase. We confirm our earlier findings (George, G. N.; Hilton, J.; Rajagopalan, K. V. J. Am. Chem. Soc. 1996, 118, 1113−1117) that the molybdenum site of the oxidized Mo(VI) enzyme possesses one terminal oxygen ligand (MoO) at 1.68 Å, four thiolate ligands at 2.44 Å, and one oxygen at 1.92 Å and that the dithionite-reduced Mo(IV) enzyme possesses a desoxo species with three or four Mo−S at 2.33 Å and two different Mo−O ligands at 2.16 and 1.92 Å. Mo(V) EPR indicates the presence of one exchangeable oxygen ligand, most likely an Mo−OH, in the signal-giving species, probably originating from the MoO of the oxidized enzyme (E m8.5(IV/V) = +37 mV, E m8.5(V/VI) = +83 mV). The addition of dimethyl sulfide, in the reverse of the physiological reaction, reduces the enzyme. In this form, the enzyme contains a desoxo active site with four Mo−S at 2.36 Å and two different Mo−O ligands at 1.94 and 2.14 Å. Recombinant wild-type R. sphaeroides DMSO reductase expressed in Escherichia coli initially has a dioxo structure (two MoO at 1.72 Å and four Mo−S at 2.48 Å) but assumes the wild-type Mo(VI) structure after a cycle of reduction and reoxidation. The site-directed Ser147→Cys mutant possesses a monooxo active site in the oxidized state (MoO at 1.70 Å) with five sulfur ligands (at 2.40 Å), consistent with cysteine 147 coordination to Mo. The dithionite reduced form of the mutant possesses a desoxo site also with five Mo−S ligands (at 2.37 Å) and one Mo−O at 2.12 Å. The variant has substantially different Mo(V) EPR and electrochemistry (E m8.5(IV/V) = −43 mV, E m8.5(V/VI) = +106 mV). The active-site structure and catalytic mechanism of DMSO reductase are discussed in the light of these results.