Kinetics of the alkaline tetramer → dimer dissociation in liganded human hemoglobin: A laser light-scattering stopped-flow study

The first-order dissociation of tetrameric HbCO to the dimer has been studied over the pH range 10.30-11.57 in a light-scattering stopped-flow apparatus using argon-ion laser excitation. The first-order dissociation rate constant varies from 0.25 sec^-1 to 24.0 sec^-1 over this pH interval. A semilogarithmic plot of k versus pH has a slope of 2.56 at pH 11.07, the midpoint. The pH dependence of the dissociation of the tetramer is consistent with progressive titration of α₁-α₂ and β₁-β₂ salt bridges. At pH 10.66 the dissociation rates of HbO₂, HbCO, methemoglobin, and HbCN vary less than 20% from their mean value. A study of the dissociation kinetics as a function of protein concentration allows one to obtain both association and dissociation rate constants, and hence equilibrium constants, for the tetramer ⇆ dimer reaction. In this manner, equilibrium constants were obtained on protein solutions with less than 15 sec of exposure to dissociating conditions.