Kinetic studies on the cooperative ligand binding by valency hybrid hemoglobins. Hemoglobin M Saskatoon and cyanomet hybrids.

where H and L represent the hybrid Hb and ligand, respectively. O2 binding features of Kb M Saskatoon were similar to those of Hb M Milwaukee (Makino, N., Sugita, Y., and Nakamura, T. (1979) J. Biol. Chem. 254,2353-2357), although the rate constants of OZ dissociation were smaller for Hb M Saskatoon. In CO binding, considerable differences were observed between the two kinds of Hbs M in the pH dependence of rate constants. In the hybrid aZ+CN p2, which binds O2 cooperatively in the presence of IHP, the affinity increase toward O2 was ascribed to a decrease in the rate of ligand dissociation, similarly to Hbs M. As to the Bohr effect on O2 binding of the cyanomet hybrids, pH change had a marked effect upon on rate constants when the binding is noncooperative, whereas only the off rate of species HL was sensitive to pH when the binding is cooperative. The results indicate that the alkaline Bohr effect on the kinetic process varies with the Hb structure. In the two types of Hbs M, and presumably also in Hb A, at least three functional states seem to participate in the ligand binding. The ligand equilibrium and kinetic properties of the valency hybrid Hbs were interpreted as that they reflect the different equilibrium states of conformation.