The transition between ‘acid’ and ‘alkaline’ ferric heme proteins

Abstract

We have studied the transition between the ‘acid’ and the ‘alkaline’ forms in several ferriheme proteins, which is generally attributed to the ionization of the water molecule coordinated with the heme iron in the sixth position. The different heme proteins studied (sperm whale and Aplysia myoglobins, Chironomus and human hemoglobins) have different dissociation constants, which arise mainly from heat effects. These differences are tentatively correlated to some differences in the general structure of the globin. Alkylation of sperm whale myoglobin or human hemoglobin, obtained by reaction with acetic or succinic anhydrides, leads to large changes in the p K′ of the transition. The profile of the curve relating the Δp K′ to the percent alkylation suggests the possibility of dividing the different reactive groups of the protein into different classes on the basis of their effects on the ionization process. The substitution of hemes differing from protoheme in sperm whale myoglobin leads to significant changes in the equilibrium constant of the reaction, the p K′ ncreasing in the order proto-deutero-meso-myoglobins.