Mechanisms of sulfoxidation catalyzed by high-valent intermediates of hemeenzymes: Electron-transfer vs oxygen-transfer mechanism

Mechanisms of sulfoxidation catalyzed by high-valent intermediates of heme enzymes have been investigated by direct observation of sulfide-induced red uction of three different compound I species including HRP (horseradish per oxidase), the His64Ser myoglobin (Mb) mutant, and O=(FeMP+.)-M-IV (1) (TMP 5,10,15,20-tetramesitylporphyrin dianion). The reaction of thioanisole and compound I of HRP (10 mu M, pH 7.0, 298 K) gives the resting state of HRP w ith accumulation of compound II as an intermediate. The yield of sulfoxide by a stoichiometric reaction of HRP compound I with thioanisole was only 25 % +/- 5% On the other hand, the same sulfoxidation by both 1 and His64Ser M b compound I exclusively exhibited a two-electron process, resulting in qua ntitative formation of sulfoxide. When 1,5-dithiacyclooctane (DTCO) is empl oyed as a substrate, the reaction of His64Ser Mb compound I with DTCO exhib its rapid formation of compound II, which decays to the ferric state due to the low oxidation potential of DTCO. The observed rate constants (log k(ob s)) Of the reactions of 1 and compounds I of HRP and His64Ser Mb with a ser ies of p-substituted thioanisoles correlate with the one-electron oxidation potentials (EO,,) of the sulfides, A comparison of these correlations with the established correlation between log kobs and EO,, for the correspondin g electron-transfer reactions of substituted N,N-dimethylanilines has revea led that the sulfoxidation reactions of compound I of HRP with the sulfides proceed via electron transfer while the sulfoxidations catalyzed by 1 and compound I of His64Ser Mb occur via direct oxygen transfer.