The 1.3 angstrom crystal structure of Rhodobacter sphaeroides dimethyl sulfoxide reductase reveals two distinct molybdenum coordination environments

During the past four years, a substantial amount of structural information has been accumulated on the molybdoenzyme dimethyl sulfoxide (DMSO) reducta se from purple bacteria. This enzyme contains a mononuclear Mo coordinated by two molybdopterin guanine dinucleotides as its single cofactor. Crystall ographic studies on the enzyme from Rhodobacter sphaeroides and Rhodobacter capsulatus revealed substantial differences in the Mo coordination environ ment in the oxidized Mo(VI) state, despite a close structural similarity in the overall fold of the protein. The crystal structure of DMSO reductase f rom R. sphaeroides identified a Mo environment with a mono-ore Ligation and an asymmetric coordination by the two molybdopterins, with three short and one very long Mo-S bond. In contrast, two independent crystallographic stu dies of the enzyme from R. capsulatus revealed two additional Mo coordinati on environments: a pentacoordinated dioxo metal ligation sphere in which on e molybdopterin is completely dissociated from the Mo and a heptacoordinate d environment with symmetrical metal coordination by both molybdopterins an d two oxo ligands. In all three structures the side chain of a serine was a ligand to the Mo. Adding to the controversy, EXAFS studies on the R. sphae roides enzyme suggested a hexacoordinated active site geometry, whereas the same technique indicated seven ligands for the R. capsulatus enzyme. The 1 .3 Angstrom resolution crystal structure of oxidized DMSOR from R. sphaeroi des presented here reveals plasticity at the active site. The Mo is discret ely disordered and exists in a hexacoordinated and a pentacoordinated ligat ion sphere. The hexacoordinated model reconciles the existing differences i n active site coordination of R. sphaeroides DMSO reductase as studied by c rystallographic and EXAFS techniques. In addition, the pentacoordinated str ucture closely resembles one of the reported R. capsulatus crystal structur es. In retrospect, the active site geometry in the previously reported 2.2 Angstrom crystal structure of R, sphaeroides DMSO reductase appears to repr esent an average of the two conformations described here. Thus, structural flexibility at the active site appears to give rise to the observed differe nces in the Mo coordination environment.