The critical role of tryptophan-116 in the catalytic cycle of dimethylsulfoxide reductase from Rhodobacter capsulatus.

In dimethylsulfoxide reductase of Rhodobacter capsulatus tryptophan-116 forms a hydrogen bond with a single oxo ligand bound to the molybdenum ion. Mutation of this residue to phenylalanine affected the UV/visible spectrum of the purified Mo(VI) form of dimethylsulfoxide reductase resulting in the loss of the characteristic transition at 720 nm. Results of steady-state kinetic analysis and electrochemical studies suggest that tryptophan 116 plays a critical role in stabilizing the hexacoordinate monooxo Mo(VI) form of the enzyme and prevents the formation of a dioxo pentacoordinate Mo(VI) species, generated as a consequence of the dissociation of one of the dithiolene ligands of the molybdopterin cofactor from the Mo ion.