Structural characterisation of the Mo(V) high-g unsplit species from Rhodobacter capsulatus dimethylsulfoxide reductase

Abstract

Dimethyl sulfoxide reductase of Rhodobacter capsulatus contains a bis(molybdopterin guanine dinucleotide molybdenum cofactor (bis-MGD-Mo) and can reduce dimethylsulfoxide to dimethylsulfide and trimethylamine-N-oxide to trimethylamine. Tryptophan-I 16 forms a hydrogen bond with the single 0x0 ligand coordinated to the molybdenum ion. Optical and EPR redox potentiometric titrations identified a single MO(V) species, the high-g unsplit species with Mo(VUV) and Mo(V/IV) redox potentials of +155, +60 mV at pH 8.0 with respect to the standard hydrogen elecrode. Multifrequency EPR studies ofthe 95Mo enriched enzyme allowed the accurate determination of the g5Mo hyperfme matrix which in conjunction with the g matrix from computer simulation of the EPR spectra of the native enzyme, shows that the unpaired electron is in a predominantly dZ based molecular orbital. Orientation selective HYSCORE measurements reveal the presence of proton and nitrogen hyperfine coupling arising from coupling of the unpaired electron with the proton and nitrogen on tryptophan-116.