Multiple States of the Molybdenum Centre of Dimethylsulphoxide Reductase from Rhodobacter Capsulatus Revealed by EPR Spectroscopy

Abstract

The dimethylsulphoxide reductase of Rhodobacter capsulatus contains a pterin molybdenum cofactor molecule as its only prosthetic group. Kinetic studies were consistent with re-oxidation of the enzyme being rate limiting in the turnover of dimethylsulphoxide in the presence of the benzyl viologen radical. EPR spectra of molybdenum(V) were generated by reducing the highly purified enzyme under a variety of conditions, and with careful control it was possible to generate at least five clearly distinct EPR signals. These could be simulated, indicating that each corresponds to a single chemical species. Structures of the signal-giving species are discussed in light of the EPR parameters and of information from the literature. Three of the signals show coupling of molybdenum to an exchangeable proton and, in the corresponding species, the metal is presumed to bear a hydroxyl ligand. One signal with gav 1.96 shows a very strong similarity to a signal for the desulpho form of xanthine oxidase, while two others with gav values of 1.98 show a distinct similarity to signals from nitrate reductase of Escherichia coli. These data indicate an unusual flexibility in the active site of dimethylsulphoxide reductase, as well as emphasising structural similarities between molybdenum enzymes bearing different forms of the pterin cofactor. Interchange among the different species must involve either a change of coordination geometry, a ligand exchange, or both. The latter may involve replacement of an amino acid residue co-ordinating molybdenum via O or N, for a cysteine co-ordinating via S. Since the two signals with gav 1.96 were obtained only under specific conditions of reduction of the enzyme by dithionite, it is postulated that their generation may be triggered by reduction of the pteridine of the molybdenum cofactor from a dihydro state to the tetrahydro state.