PH dependence of the Adair constants of human hemoglobin. Nonuniform contribution of successive oxygen bindings to the alkaline Bohr effect

Abstract

In order to solve the problem of an apparent discrepancy between the pH variance of oxygen equilibrium curve and the linear relation between the number of released Bohr protons and the degree of ligation, precise oxygen equilibrium curves of human hemoglobin were determined at a number of pH values from 6.5 to 8.8. From the equilibrium data individual steps (Adair constants), ki (i equals 1, 2, 3, 4), were obtained and the number of Bohr protons (deltaHi+) released on the ith stage of oxygenation was estimated. The pH dependence of k4 was very small, while the other ks strongly depended on pH over the pH range examined. As a consequence, the contribution of each step of oxygen binding to the alkaline Bohr effect nonuniform: deltaH4 was very small compared with deltaH1+, deltaH2+, and deltaH3+. In spite of this, calcuation has shown that the fractional number of released protons is essentially proportional to fractional oxygen saturation because of cooperative effects in hemoglobin. Thus, the present study indicates that the linear relationship between the fractional number of released protons and the degree of ligation, as obtained from titration experiments, is not necessarily incompatible with the pH variance of the shape of the oxygen equilibrium curve. The nonuniform pH depencence of the Adair constants implies that the two-state allosteric model of Monod, J., Wyman, J., and Changeus, J.P. (1965) J. Mol. Biol. 12, 88-118 is not adequate to describe the heterotropic effect caused by protons.