A two-state thermodynamic and kinetic analysis of the allosteric functioning of the haemoglobin of an extreme poikilotherm.

The blood of the extreme poikilotherm Trematomus borchgrevinki contains one major haemoglobin component, which may be separated from the minor species by ion-exchange chromatography. This haemoglobin shows co-operative CO-binding isotherms at pH 8.2. An analysis of the temperature-dependence of the binding curves has allowed the thermodynamic constants associated with the two-state allosteric parameters L, KR and KT to be measured. The binding of CO at lower pH (6.2) is characterized by the maintenance of the T-state to relatively high degrees of saturation. Kinetic investigations with the use of flash photolysis of the haemoglobin-CO complex under various conditions has allowed the determination of the thermodynamic parameters association with the T-state and R-state rate constants. An amalgamation of these data has allowed the mathematical simulation of predicted time courses for CO binding to this haemoglobin, by using the two-state model, which very closely represents those obtained experimentally. The overall findings show that this haemoglobin closely follows the two-state model of co-operative interaction.