Proton-dependent dissociation equilibrium of hemoglobin. 1. A 700-nanometer light-scattering study on horse methemoglobin in the pH range 4.8 to 7.2

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Abstract
The effect of proton concentration on the subunit dissociation of horse methemoglobin was investigated at 2 ionic strengths by light scattering photometry at 700 nm. Differential refractometry revealed a slight but systematic decrease of the specific refractive index increment with decreasing protein concentration for solutions in dialytic equilibrium with the solvent. In the pH range 4.8-7.2, the dissociation can be described by a simple equilibrium between tetramers and dimers. The dissociation constant Kd of the met derivative is similar to those of the O2- and CO-ligated states. From the slope of a plot of log Kd vs. pH, the number of protons bound is n = 1.3 .+-. 0.1 resulting from an increase in the pK values of 2 groups upon dissociation. These 2 groups must be identical because the dissociation is symmetrical.