The three-state model: a minimal allosteric description of homotropic and heterotropic effects in the binding of ligands to hemoglobin.

The effect of hydrogen ion, chloride, and 2,3-diphosphoglycerate upon the oxygen binding of hemoglobin may be satisfactorily described by an allosteric model employing three affinity states. The allosteric two-state model does not provide even an approximate description except under defined extreme conditions. In particular, the two-state model cannot provide a correct interpretation of recently reported electron spin resonance and nuclear magnetic resonance data.