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 cation intermediate is found to be greatly preferred, occurring with a quite low barrier of just 1.2 kJ mol–1. Formation of the final product complex in which an intraprotein disulfide bond is formed with generation of methionine preferably occurs in one step via nucleophilic attack of the sulfur of a second enzyme thiolate (Cys440S–) at the SCys495 center of the sulfonium intermediate with a barrier of 23.8 kJ mol–1. An alternate pathway for formation of the products via a sulfenic acid intermediate involves enzymatically feasible, but higher energy barriers. The role and impact of hydrogen bonding and active site residues on the properties and stability of substrate and mechanism intermediates and the affects of mutating His477 are also examined and discussed.