Sulfoxidation mechanism of vanadium bromoperoxidase from Ascophyllum nodosum - Evidence for direct oxygen transfer catalysis

We have previously shown that vanadium bromoperoxidase from Ascophyllum nodosum mediates production of the (R)-enantiomer of methyl phenyl sulfoxide with 91% enantiomeric excess. Investigation of the intrinsic selectivity of vanadium bromoperoxidase reveals that the enzyme catalyzes the sulfoxidation of methyl phenyl sulfide in a purely enantioselective manner. The K-m of the enzyme for methyl phenyl sulfide was determined to be approximate to 3.5 mM in the presence of 25% methanol or tert-butanol, The selectivity of the sulfoxidation of methyl phenyl sulfide is optimal in the temperature range 25-30 degreesC and can be further optimized by increasing the enzyme concentration, yielding selectivities with up to 96% enantiomeric excess. Furthermore, we established for the first time that vanadium bromoperoxidase is functional at temperatures up to 70 degreesC. A detailed investigation of the sulfoxidation activity of this enzyme using O-18-labeled hydrogen peroxide shows that vanadium bromoperoxidase mediates the direct transfer of the peroxide oxygen to the sulfide. A schematic model of the vanadium haloperoxidase sulfoxidation mechanism is presented.