A Sulfonium Cation Intermediate in the Mechanism of Methionine Sulfoxide Reductase B: A DFT Study

The hybrid density functional theory method B3LYP in combination with three systematically larger active site models has been used to investigate the substrate binding and catalytic mechanism by which Neisseria gonorrhoeae methionine sulfoxide reductase B (MsrB) reduces methionine-R-sulfoxide (Met-R-SO) to methionine. The first step in the overall mechanism is nucleophilic attack of an active site thiolate at the sulfur of Met-R-SO to form an enzyme–substrate sulfurane. This occurs with concomitant proton transfer from an active site histidine (His480) residue to the substrates oxygen center. The barrier for this step, calculated using our largest most complete active site model, is 17.2 kJ mol–1. A subsequent conformational rearrangement and intramolecular −OH transfer to form an enzyme-derived sulfenic acid (Cys495S–OH) is not enzymatically feasible. Instead, transfer of a second proton from a second histidyl active site residue (His477) to the sulfurane’s oxygen center to give water and a sulfonium cat...