A kinetic and equilibrium study of ligand binding to a Root-effect haemoglobin

The blood of the striped marlin (Tetrapturus audax) contains one major Root-effect haemoglobin. Titrations of this haemoglobin with CO show that at high pH the molecule is highly co-operative (Hill coefficient 2.8) whereas at low pH the titration data can best be described as the sum of contributions from non-co-operating subunits of different affinity. In terms of the two-state model the R-state affinity constant is much more sensitive to pH than is that of the T state. Flash-photolysis studies were used to characterize the kinetics of ligand binding to this haemoglobin. Both T and R states show kinetic heterogeneity in their recombination time courses, associated with the alpha- and beta-chains of the molecule. The rate constants for ligand binding to each chain, in each quaternary state, were determined, and in conjunction with the allosteric equilibrium parameters determined at pH8.0 were used in the two-state analysis of reaction curves, over a range of ligand concentration. The two-state model, extended to take account of chain difference, adequately fits the homotrophic effects observed for this haemoglobin. The two-state model is, however, inadequate in its description of the heterotropic effects produced by protons.