The effect of pH on the rate of dissociation of the oxygenated beta chain tetramer of Hb A.

The effect of pH on the overall assembly of oxyhemoglobin A following mixing of equivalent concentrations of alpha and beta heme subunits has been studied in 0.1 M potassium phosphate buffers at 20 degrees C. The resultant kinetic profiles monitored at 582.5 nm (the maximum of the oxyhemoglobin - oxy chain difference spectrum) were homogeneous and appeared to be first order. The rate of these exponential time courses, reflecting the rate of dissociation of the beta chain tetramer, increased from 0.013 min-1 at pH 6.4 to 0.30 min-1 at pH 8.0 and 1.0 min-1 at pH 8.5. Concurrent with this increased rate was a decrease in the overall color yield from the reaction. The absorbance changes, which involve a significant contribution from the beta chain tetramer to monomer dissociation step, changed three fold over the pH range studied. The findings indicate that protons enhance the stability of the beta chain tetramer.