Effects of hemoglobin symmetry in a statistical equilibrium model for oxygen binding

The effect of hemoglobin symmetry on the statistical mechanics of its motions is considered. Hemoglobin binding equilibrium constants are presented in which symmetry factors appear that differ from one binding step to another. Inclusion of the symmetry factors improves the fit of a symmetry-modified Koshland, Némethy, Filmer expression (1966, Biochemistry, 5:365-385) with tetrahedral oxy-oxyhemoglobin subunit interactions to the high-ionic-strength binding curve of Rossi-Fanelli, Antonini and Caputo (1961, J. Biol. Chem., 236:397-400).