The Effect of Inositol Hexaphosphate on the Allosteric Properties of Carp Hemoglobin

Abstract For carp hemoglobin in the presence of 0.7 and 7 mm inositol hexaphosphate (IHP) at 20° in the pH range below pH 6.2 and 6.4, respectively, oxygen binding is noncooperative. The absence of heme-heme interaction is indicated by a value of 0.75 for the Hill parameter n. The oxygen affinity and the oxygen dissociation rate, k, are pH- and phosphate-independent and are those of carp hemolysate below pH 5.6. Above pH 6.2 and 6.4, oxygen binding to the IHP-bound hemoglobin is cooperative and affected by changes in pH and organic phosphate conditions. The extent of heme-heme interaction is pH-dependent as indicated by an increase and subsequent decrease of the value of the Hill parameter n as pH is raised. Addition of 0.7 and 7 mm IHP to stripped carp hemoglobin has nearly the same effect on all aspects of ligand binding, affinity, kinetics, and cooperativity, as decreasing the pH of stripped hemoglobin by 1.6 and 1.8 pH units, respectively. In the case of CO binding, above pH 5.7, 0.7 mm IHP has nearly the same effect as decreasing the pH of stripped hemoglobin by 1.6 pH units. Below pH 5.7, CO affinity increases with decrease in pH. This acid Bohr effect is accompanied by a corresponding increase in the value of n. However, this is apparently not simply a reversal of the alkaline Bohr effect because the kinetic basis for the affinity change is very different.