Analysis of proton release in oxygen binding by hemoglobin: implications for the cooperative mechanism

The relationship in hemoglobin between cooperativity (dependence of the Hill constant on pH0 and the Bohr effect (dependence of the mean oxygen affinity on pH) can be described by a statistical thermodynamic model [Szabo, A., & Karplus, M. (1972) J. Mol. Biol. 72, 163-197; Lee, A., & Karplus, M. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 7055-759]. In this model, salt bridges and other interactions serve to couple tertiary and quaternary structural changes. To test and refine the model, it is applied to the analysis of the pH dependence of the tetramer Adair constants corrected for statistical factors (K4i', i = 1-4). Attention is focused on the proton release of the first (delta H1+ = alpha log K41'/alpha pH) and last (delta H4+ = alpha log K44'/alpha pH) oxygenation steps, where K4i' are the Adair constants corrected for statistical factors. Measurements of delta H1+ and delta H4+ under carefully controlled conditions are reported, and good agreement between the model calculation and these experimental results is obtained. The salt bridges are found to be partially coupled to the ligation state in the deoxy quaternary structure; it is shown that a Monod-Wyman-Changeux-type model, in which the salt bridges are coupled only to quaternary structural change, is inconsistent with the data for delta H1. The significance of the present analysis for an evaluation of the Perutz mechanism [Perutz, M.F. (1970) Nature (London) 228, 726-734, 734-739] and other models for hemoglobin cooperativity is discussed.