Reduction of metmyoglobin by inorganic disulfide species

The mechanism of the metal centered reduction of metmyoglobin (MbFe^III) by inorganic disulfide species has been studied by combined spectroscopic and kinetic analyses, under argon atmosphere. The process is kinetically characterized by biexponential time traces, for variable ratios of excess disulfide to protein, in the pH interval 6.6-8.0. Using UV-vis and resonance Raman spectroscopies, we observed that MbFe^III is converted into a low spin hexacoordinated ferric complex, tentatively assigned as MbFe^III(HSS^-)/MbFe^III(SS^2-), in an initial fast step. The complex is slowly converted into a pentacoordinated ferrous form, assigned as MbFe^II according to the resonance Raman records. The reduction is a pH-dependent process, but independent of the initial disulfide concentration, suggesting the unimolecular decomposition of the intermediate complex following a reductive homolysis. We estimated the rate of the fast formation of the complex at pH 7.4 (k_on = 3.7 × 10³ M^-1 s^-1), and a pK_a2 = 7.5 for the equilibrium MbFe^III(HSS^-)/MbFe^III(SS^2-). Also, we estimated the rate for the slow reduction at the same pH (k_red = 10^-2 s^-1). A reaction mechanism compliant with the experimental results is proposed. This mechanistic study provides a differential kinetic signature for the reactions of disulfide compared to sulfide species on metmyoglobin, which may be considered in other hemeprotein systems.