Thermodynamic characterization of the allosteric transition in trout hemoglobin

The pH induced spectral changes in the Soret region occurring in the carbomonoxy derivative of trout HbIV have been measured under carefully controlled conditions of temperature and organic phosphate concentration. Parallel experiments on the kinetics of carbon monoxide dissociation by NO replacement have been performed over the same pH range. Both sets of results agree satisfactorily with a thermodynamic scheme independently drawn on the basis of functional data previously analyzed within the framework of a two state allosteric model. Thus, the whole body of data strongly supports the idea that the spectral changes are themselves an indication of the pH induced structural transition, thereby reflecting the stabilization of the liganded T-state of the molecule at low pH. This allows us to definitely conclude that the functional changes induced in trout HbIV-CO by protons are associated with a red shift of the Soret absorption band.