An Allosteric Model for the Influence of ${\text{H}}^{+}$ and ${\text{CO}}_2$ on Oxygen-Hemoglobin Binding

Abstract

In the physiology of oxygen-hemoglobin binding, an important role is played by the influence of ${\text{H}}^{+}$ and ${\text{CO}}_2$ on the affinity of hemoglobin for ${\text{O}}_2$. Here we extend the allosteric model of hemoglobin to include these effects. We assume purely allosteric modulation, i.e., that the modulatory effects of ${\text{H}}^{+}$ and ${\text{CO}}_2$ on oxygen binding occur only because of their influence on the T $\leftrightarrow$ R transition, in which all four subunits of the hemoglobin molecule participate simultaneously. We assume, moreover, that these modulatory influences occur only through the interaction of ${\text{H}}^{+}$ and ${\text{CO}}_2$ with the amino group at the N-terminal of each of the four polypeptide chains of the hemoglobin molecule. We fit the model to experimental data and obtain reasonable agreement with the observed shifts in oxygen-hemoglobin binding that occur when the concentrations of ${\text{H}}^{+}$ and ${\text{CO}}_2$ are changed.