Mechanism of cooperative oxygen binding to hemoglobin (spin-labeled triphosphate-concerted transition...

Evidence is presented that a generalized concerted transition model provides a quantitative understanding of (a) the molecular species that are present in solutions of partially liganded hemoglobin and (b) the macromolecular mechanism of cooperativity. Model parameters for hemoglobin A and for hemoglobin Chesapeake were determined from studies of the binding of spin-label triphosphates to ligand-free and partially liganded hemoglobin solutions, and to the hybrids alpha(2) (+CN)beta(2) and alpha(2)beta(2) (+CN). This model is the same as that proposed originally by Monod, Wyman, and Changeux [J. Mol. Biol. (1965) 12, 88] for hemoglobin, except that the alpha-subunits are treated as nonequivalent to the beta-subunits.