ESTIMATION OF PARAMETERS IN A MULTI-AFFINITY-STATE MODEL FOR HEMOGLOBIN FROM OXYGEN BINDING DATA IN WHOLE-BLOOD AND IN CONCENTRATED HEMOGLOBIN SOLUTIONS

Oxygen binding data on whole blood and concentrated haemoglobin solutions at different pH, pCO2 and 2,3-bis-phosphoglycerate concentrations were analysed with the use of a two-quaternary-state model regarding P2-Gri† † P2-Gri, 2,3-bis-phosphoglycerate purely as a quaternary effector (Herzfeld & Stanley, 1972). The model was applied as a multi-affinity-state model and was extended on a molecular mechanistic level in terms of the so-called Perutz mechanisms. Binding of oxygen to a subunit contained in the oxy quaternary state was found to be insensitive to changes in pH and pCO2 (Koxy = 0.71 ± 0.05 mm Hg-1). At 37 °C, pH 7.2 and pCO2 = 22 mm Hg, Kdeoxy was estimated to be lower than Koxy by a factor of 130, the allosteric constant was (1.5 ± 0.4)104 and the binding constant of P2-Gri for the deoxy quaternary conformation was estimated to be 25 times higher than that for the oxy quaternary conformation (130 ± 50 1 mol-1). With regard to the influence of protons, carbon dioxide and P2-Gri, the results are consistent with the concepts that (1) P2-Gri acts primarily as a quaternary effector, that (2) protons act as constraint effectors and second-order effectors, and that (3) carbon dioxide acts as a first and second-order quaternary effector. The calculations further indicate that (1) quaternary transition most likely takes place after oxygenation of the second subunit, that (2) the probabilities of intermediary states of oxygenation are surprisingly low, and that (3) the difference in total conformational energy between the two quaternary ligand-free states is almost exclusively confined to molecular constraints, and very little to the difference in quaternary conformational energy. © 1978.