Tetramer-dimer dissociation in homoglobin and the Bohr effect

Abstract

The pH dependence of the apparent tetramer to dimer dissociation constant has been determined at 20 degrees for both oxy- and deoxyhemoglobins A and Kansas. These measurements were made by three different procedures: gel chromatography, sedimentation velocity, and kinetic methods in either of three buffer systems: 0.05 M cacodylate, Tris, or glycine with 1 mM EDTA and 0.1 M NaCl between pH 6.5 and 11. The tetramer-dimer dissociation constant of human oxyhemoglobin A decreases from about 3.2 X 10(-6) M at pH 6.0 to about 3.2 X 10(-8) M at pH 8.5. The slope of this line indicates that the dissociation of tetramer to dimer is accompanied by the uptake of about 0.6 protons per mol of tetramer in this region. The corresponding dissociation constant for deoxyhemoglobin in the same pH region increases apparently almost linearly from 1.0 x 10(-12) M at pH 6.5 to about 1.0 x 10(-5) M at pH 11. To dimer is associated with the release of about 1.6 protons per mol of tetramer. Comparison of these data with the known proton release accompanying the oxygenation of tetramers confirms that the pH dependence of oxygen binding by dimers must be very small. The present data predict that the overall proton release or uptake per oxygen bound by dimer should be less than 0.1. The tetramer-dimer dissociation equilibria of oxy- and deoxyhemoglobins above pH 8.5 have identical pH dependences. In this range the dissociation constant of deoxy-Hb is about one-tenth that of oxyhemoglobin. Human oxyhemoglobin Kansas is known to have an enhanced tetramer-dimer dissociation compared with that of hemoglobin A. Below pH 8.5 the tetramer-dimer dissociation constant of Hb Kansas is about 400 times greater than that of HbA in the absence of phosphate buffers. In contrast, the tetramer-dimer dissociation constants of deoxyhemoglobins A and Kansas appear to be identical. These findings are consistent with previous structural observations on these hemoglobins. The data on the tetramer-dimer dissociation of human hemoglobin were used to calculate the total free energy of binding of oxygen to the tetramer and the median oxygen pressure on the basis of fundamental linkage relations and a pH-independent estimate of the total free energy of binding oxygen to dimer. Simulated oxygen binding curves were generated with the equations of Ackers and Halvorson (Ackers, G. K., and Halvorson, H. (1974) Proc. Natl. Acad. Sci. U.S.A. 71, 4312-4316) by making two assumptions: (a) that the dimers are noncooperative and pH-independent in O2 binding and (b) that the distribution of cooperative energy in the oxygenation of tetramers is independent of pH. We have compared these simulations with experimental data obtained at low protein concentrations (30 to 124 muM heme) to show that the variation in oxygen affinity with pH can be described in terms of the subunit equilibria. We conclude that an accurate analysis of the contributions of individual oxygen binding steps to the Bohr effect cannot be made without considering the contributions of the dimers to oxygen binding...