STRUCTURE OF THE MOLYBDOENZYME DMSO REDUCTASE IN COMPLEX WITH DMSO
S. Bailey1, A.S. McAlpine1 & A.G. McEwan2
1Daresbury Laboratory, Daresbury,
Warrington, Cheshire, WA4 4AD, UK;
2Department of Microbiology,
University of Queensland, Brisbane, Qld. 4072, Australia
Keywords: dimethyl sulfoxide reductase; dimethyl sulfide; molybdenum; molybdopterin; crystal structure
DMSO reductase is an oxotransferase with a molybdopterin cofacter at the catalytic site. It catalyses the reduction of dimethylsulfoxide (DMSO) to dimethylsulfide (DMS) in a two stage reaction which is linked to oxygen atom transfer and electron transfer. In the first step, DMSO binds to reduced (Mo(IV)) enzyme, the enzyme is oxidised to Mo(VI) with an extra oxygen ligand and the DMS is released. Regeneration of reduced enzyme is achieved by transfer of two electrons, and release of oxygen as water. The enzyme, purified under aerobic conditions, is in the oxidised (Mo(VI)) state.
The crystal structure of DMSO reductase from Rhodobacter capsulatus has been determined in three states: as the oxidised (Mo(VI)) enzyme, as the dithionite reduced (MoIV) enzyme, and as the DMS-reduced (Mo(IV)) enzyme. The model of the protein in the oxidised form has been refined against data to 1.82 A resolution [1]. The protein contains four domains formed from non-contiguous regions of the polypeptide chain. The molybdenum sits in the centre of the protein at the base of a large depression on one side of the molecule. The Mo cofacter comprises one molybdenum ion and two pterin guanine dinucleotide molecules. The Mo is coordinated by seven ligands: two oxo groups, four dithiolene sulfur ligands (from the pterin cofacters), and a serine oxygen ligand. Two independant structures of the oxidised form of DMSO reducatse [2,3] show significant differences in the molybdenum coordination sphere.
Addition of excess DMS to the oxidised enzyme causes a change in the absorption spectrum. The same reaction occurs within the crystals of the enzyme and the crystal structure at 1.9 A reveals a complex with DMSO bound to the molybdenum via its oxygen atom [4]. X-ray edge data indicate that the metal is in the Mo(IV) state [5]. The DMSO ligand replaces one of the two oxo groups which ligate the oxidised form of the enzyme, suggesting very strongly that this is the oxygen which is transferred during catalysis. Residues 384-390 which are disordered in the oxidised structure are now clearly seen in the cleft leading to the active site. The side-chain of Trp388 forms a lid trapping the substrate/product.