Evidence for a kinetic heterogeneity in ligand binding to R-state haemoglobin Kempsey [Asp-G1(99) beta----Asn]

Abstract

Thermodynamic and kinetic properties of O2 and CO binding to haemoglobin (Hb) Kempsey [Asp-G1(99) beta----Asn] were investigated and the activation parameters for the two ligands were determined. At every temperature the O2-binding isotherms display a weak co-operativity, n ranging between 1.1 and 1.2, and dissociation kinetics show a single-exponential behaviour. O2-binding kinetics were studied at 25 degrees C by temperature jump and are characterized at each saturation (from Y = 0.31 to Y = 1.0) by two processes, a fast bimolecular one and a slow monomolecular one (tau -1 = 20 s-1), which contributes to approx. 30% of the whole relaxation amplitude at every Y. CO-binding kinetics to Hb Kempsey were followed at several temperatures by flash photolysis and stopped flow. The process is biphasic, as reported elsewhere [Bunn, Wohl, Bradley, Cooley & Gibson (1974) J. Biol. Chem. 249, 7402-7409], and the relative contributions of the two bimolecular rates to the whole process are only slightly affected by temperature. On taking account for the fraction of dimers at every protein concentration, the slow phase corresponds to approx. 50% of the ligand binding to tetramers. Correlation of these results with previous spectroscopic data leads to the hypothesis that the biphasic time course of CO binding may be attributed to alpha/beta heterogeneity of the R-state of tetrameric Hb Kempsey.