Oxygen binding and subunit interaction of hemoglobin in relation to the two-state model

Mills and Ackers (Mills, F.C., and Ackers, G.K. (1979) J. Biol. Chem. 254, 2881-2887) have reported the subunit interactions of hemoglobin to decrease on binding of the fourth molecule of oxygen to hemoglobin. This effect, which they called quaternary enhancement, is incompatible with the two-state Monod, Wyman, and Changeux allosteric model. Their free energy of binding of the fourth molecule (-9.3 kcal/mol) has been compared with independent kinetic estimates which give -8.6 kcal/mol. This smaller value is consistent with literature values and allows reasonable representation of the equilibrium curve using the two-state model without invoking quaternary enhancement.