Mutual effects of protons, NaCl, and oxygen on the dimer-tetramer assembly of human hemoglobin. The dimer Bohr effect.

The dimer-tetramer equilibrium constants of human oxyhemoglobin (4K2) and deoxyhemoglobin (0K2) have been determined at 21.5 degrees C as a function of pH and chloride concentration. In buffers containing 0.1 M NaCl, 1 mM EDTA, the apparent numbers of protons released upon assembly of dimers into tetramers were determined from the pH dependencies of 4K2 and 0K2. At pH 7.4, the assembly of unliganded tetramers is accompanied by the absorption of 0.9 +/- 0.1 mol of H+. For assenbly of oxyhemoglobin, there are 0.8 +/- 0.1 mol of H+ released per mol of tetramer formed. From these results and the value of 2.1 mol of H+/402 for the tetramer Bohr effect, a Bohr effect for dimers is determined as 0.2 +/- .08 mol of H+ released upon binding 2 mol of 02. Thus, the dimer Bohr effect is approximately 20% as large as the tetramer Bohr effect. At pH 7.4, the value of 0K2 is insensitive to [Cl-], whereas 4K2 varies inversely with [Cl-]. At pH 8.95, both 4K2 and 0K2 decrease with increasing [Cl-]. These and previous results indicate that salt bridges are not the dominant energetic factor in stabilizing the deoxy quaternary structure of hemoglobin. In buffer conditions of 0.1 M Tris-HCl, 0.1 M NaCl, 1 mM EDTA, pH 7.4, we estimate 1.8 mol of Cl- bound upon dissociation of 1 mol of oxy tetramers into oxy dimers, whereas the deoxy molecules dissociate without any change in bound chloride. From the [Cl-] dependence of oxygenation curves, we estimate 1.8 mol of [Cl-] released upon binding 4 mol of O2 to tetramers. Thus, oxygenation of dimers at pH 7.4 apparently involves no change in bound chloride.