Competition in oxygen-linked anion binding to normal and variant human hemoglobins

The two-state model of Monod, Wyman and Changeux has been used extensively in analysis of the functional behavior of human hemoglobin. Within the context of this model, the cooperativity, pH dependence of oxygen binding, and anionic regulation of oxygen affinity are all considered to be due to shifts in the equilibrium between low affinity (T) conformational states and high affinity (R) conformational states. The heterotropic effectors, such as protons, carbon dioxide, inorganic anions, and organic polyphosphates, are considered to be principally active in causing shifts in the allosteric equilibrium constant L. For both simple anions, such as chloride, and strong effectors, such as inositol hexaphosphate, the allosteric equilibrium constant L increases with increasing concentrations of the effector. Detailed examinations of the functional properties of human hemoglobin have revealed in recent years that the two-state model is a beautiful and elegant format for discussing conformational changes within a given condition, but that an infinite number of "T" and "R" states are, in fact, possible. In this paper, the consequences of the occurrence of specific oxygen-linked anion binding sites on the alpha and beta chains of the human hemoglobin tetramer are discussed in terms of the generalized two-state model. In particular, attention is directed to the interesting fact that the allosteric equilibrium constant can be decreased by addition of chloride instead of increased if the ligand binding behavior is analyzed in the presence of inositol hexaphosphate. The structural basis for competitive anionic effects involves the binding sites for anions on both alpha and beta chains. Analysis of the distinct effects of small and large anionic effectors is facilitated through studies of specific human hemoglobin variants and hemoglobins where the C terminal arginines of the alpha chains have been removed by ezymatic digestion.