Ferrocyanide − a novel catalyst for oxymyoglobin oxidation by molecular oxygen

Abstract

A comparative study of the rates of ferrocyanide-catalyzed oxidation of several oxymyoglobins by molecular oxygen is reported. Oxidation of the native oxymyoglobins from sperm whale, horse and pig, as well as the chemically modified (MbO(2)) sperm whale oxymyoglobin, with all accessible His residues alkylated by sodium bromoacetate (CM-MbO(2)), and the mutant sperm whale oxymyoglobin [MbO(2)(His119-->Asp)], was studied. The effect of pH, ionic strength and the concentration of anionic catalyst ferrocyanide, [Fe(CN)(6)](4-), on the oxidation rate is investigated, as well as the effect of MbO(2) complexing with redox-inactive Zn(2+), which forms the stable chelate complex with functional groups of His119, Lys16 and Asp122, all located nearby. The catalytic mechanism was demonstrated to involve specific [Fe(CN)(6)](4-) binding to the protein in the His119 region, which agrees with a high local positive electrostatic potential and the presence of a cavity large enough to accommodate [Fe(CN)(6)](4-) in that region. The protonation of the nearby His113 and especially His116 plays a very important role in the catalysis, accelerating the oxidation rate of bound [Fe(CN)(6)](4-) by dissolved oxygen. The simultaneous occurrence of both these factors (i.e. specific binding of [Fe(CN)(6)](4-) to the protein and its fast reoxidation by oxygen) is necessary for the efficient ferrocyanide-catalyzed oxidation of oxymyoglobin.