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

Role of dimerization in the control of the functioning of the human haemoglobin mutant haemoglobin Howick (beta 37 Trp-->Gly)

Haemoglobin Howick shows a high oxygen affinity (p50 = 1 mmHg) and a low co-operativity (n = 1.3). Equilibrium studies show the protein to be essentially totally dimeric in the oxygenated form. A wide range of rapid kinetic experiments indicate that the deoxygenated form of the protein exists in a tetramer<-->dimer equilibrium with an associated equilibrium constant of 3 microM. These kinetic data also indicate that the oxygenated form of the protein exists in a tetramer<-->dimer equilibrium with an associated equilibrium constant of 35 mM, and furthermore clearly identifies a large increase in the rate of the tetramer-to-dimer dissociation process as the origin of the vastly increased dissociation equilibrium constants. Simulations of the protein-concentration-dependence of the oxygen-binding properties of haemoglobin Howick, based on the measured equilibrium parameters, closely fits the experimental data. The change in dimerization constant for the deoxygenated form of the protein corresponds remarkably well to the free-energy change predicted for the simple transfer of the amino acid side chain at position beta 37 from a hydrophobic to a hydrophilic environment during the dimerization process.