Structure, dynamics and reactivity in hemoglobin.

The cooperative binding of O2 to hemoglobin (Hb) has been explored primarily within the framework of a two-state model. At low O2 pressure, the low affinity T state quaternary structure is favored whereas at high O2 pressures the equilibrium favors the higher affinity R state structure. In addition to R-T dependent changes in O2 affinity, the ligand binding properties within a given quaternary structure vary both with animal species and with solution conditions.