The mechanism of oxymyoglobin oxidation by copper ions and complexes: myoglobins carboxymethylated and carboxyamidated at histidine residues
Shekhovtsova, E.A.; Postnikova, G.B.
Biofizika 53(4): 562-572
2008
ISSN/ISBN: 0006-3029 PMID: 18819271 Document Number: 615454
The oxidation of sperm whale oxymyoglobin (MbO2) and its chemically modified derivatives alkylated at solvent-accessible histidines by sodium bromoacetate (CM-MbO2) and iodoacetamide (CA-MbO2) in the presence of ions and glycine complexes of copper, Cu2+, Cu(Gly)+, and Cu(Gly)2, has been studied. The influence of the reagent concentration, pH, and ionic strenth of medium, and also of competitive redox-inactive zinc ions on the reaction was investigated. The localization of Cu(Gly)2 in native sperm whale met-Mb and CM-met-Mb was examined by the high-resolution NMR method. The data obtained confirm that the linkage of copper compounds to surface histidines (all of them are away from the heme, at a distance of 1.8-2.7 nm) has only a minor (no more than 35%) contribution to the overall reaction rate, in particular under conditions of a large, more than 8-10-fold, data, excess of the reagent. The noticeable contribution of His116(113), His48, and His81, which according to NMR are localized on the protein surface and have the greatest affinity to copper, is revealed only at small concentrations of copper, a less than 5-fold excess relative to the protein. This is supported by the sigmoidal pH-dependence curve with the transition pK 6.5 at the equimolar copper concentration. The main contribution to the rate of the reaction studied should involve a linkage of copper to internal histidines, His97(FG4), which is 0.66 nm apart from the heme, and to distal His64(E7). Both are hydrogen bonded, the first with carboxyl group of one of the heme propionates, and the second with liganded O2, have a much lower affinity to copper than surface histidines, and are inaccessible to the modification.