MOLYBDENUM ACTIVE-CENTER OF DMSO REDUCTASE FROM RHODOBACTER-CAPSULATUS - CRYSTAL-STRUCTURE OF THE OXIDIZED ENZYME AT 1.82-ANGSTROM RESOLUTION AND THE DITHIONITE-REDUCED ENZYME AT 2.8-ANGSTROM RESOLUTION

The 1.82-Angstrom X-ray crystal structure of the oxidised (Mo-(VI)) fo rm of the enzyme dimethylsulfoxide reductase (DMSOR) isolated from Rho dobacter capsulatus is presented. The structure has been determined by building a partial model into a multiple isomorphous replacement map and fitting the crystal structure of DMSOR from Rhodobacter sphaeroide s to the partial model. The enzyme structure has been refined, at 1.82 -Angstrom resolution, to an R factor of 14.8% (R-free = 18.4%). The mo lybdenum is coordinated by seven ligands: four dithiolene sulfurs, O g amma of Ser147 and two oxo groups. The four sulfur ligands, at a metal -sulfur distance of 2.4 Angstrom or 2.5 Angstrom, are contributed by t he two molybdopterin guanine dinucleotide (MGD) cofactors. The coordin ation sphere of the molybdenum is different from that in previously re ported structures of DMSOR from R. sphaeroides and R. capsulatus. The 2.8-Angstrom structure of DMSOR, reduced by addition of sodium dithion ite, is also described and differs from the structure of the oxidised enzyme by the removal of a single oxo ligand from the molybdenum coord ination sphere. A structure, at 2.5-Angstrom resolution, has also been obtained from crystals soaked in mother liquor buffered at pH 7.0. No differences are observed in the structure at pH 7 when compared with the native crystal structure at pH 5.5.