Comparison of the applicability of several allosteric models to the pH and 2,3-bis(phospho)glycerate dependence of oxygen binding by human blood

Oxygen saturation curves of blood were measured by the mixing method at several concentrations of protons and bis(phospho)glycerate. Various theoretical models for the co-operativity of oxygen binding by haemoglobin were then tested for their ability to fit the experimental curves. The effects of pH on oxygen binding could be described by both the Monod, Wyman & Changeux, and the Herzfeld & Stanley models, with most success when protons were assumed to affect oxygen affinity directly rather than through effects on the quaternary-state equilibrium. When the combined effects of pH and bis(phospho)glycerate were considered, however, all the versions of the Monod model that were used, including the three-state version, were unsuccessful. The best fit to the saturation curves was obtained with the Herzfeld & Stanley model, with protons acting as a direct effector of oxygen affinity, and bis(phospho)glycerate acting to lower oxygen affinity as well as influencing the quaternary-state equilibrium.