Molybdenum active centre of DMSO reductase from Rhodobacter capsulatus:: crystal structure of the oxidised enzyme at 1.82-Å resolution and the dithionite-reduced enzyme at 2.8-Å resolution

The 1.82-Angstrom X-ray crystal structure of the oxidised (Mo-(VI)) form of the enzyme dimethylsulfoxide reductase (DMSOR) isolated from Rhodobacter 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 sphaeroides 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 molybdenum is coordinated by seven ligands: four dithiolene sulfurs, O gamma 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 the two molybdopterin guanine dinucleotide (MGD) cofactors. The coordination sphere of the molybdenum is different from that in previously reported structures of DMSOR from R. sphaeroides and R. capsulatus. The 2.8-Angstrom structure of DMSOR, reduced by addition of sodium dithionite, is also described and differs from the structure of the oxidised enzyme by the removal of a single oxo ligand from the molybdenum coordination 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.