Subunit hybridization studies of partially ligated cyanomethemoglobins using a cryogenic method. Evidence for three allosteric states

Reaction of tetrameric hemoglobin with ligands at the four heme sites yields nine species that have structurally unique combinations of ligated and unligated subunits. Using hemoglobins where the ligated subunits contain cyanomethemoglobin, Smith and Ackers studied the dimer-tetramer assembly reactions in all nine of the partially ligated species (F. R. Smith and G. K. Ackers, Proc. Natl. Acad. Sci. U.S.A. 82 (1985) 5347). They found a third assembly free energy in addition to those of unligated hemoglobin and fully ligated cyanomethemoglobin. The observed distribution of the three assembly free energies among the ten species was found to be incompatible with the two-state mechanism of allosteric control (J. Monod, J. Wyman and J. P. Changeaux, J. Mol. Biol. 12 (1965) 81). The results indicated a mechanism of 'combinatorial switching' in which the binding free energies per site change with configuration of occupied sites and not just their number. In this study, we have confirmed the existence of three assembly free energies among the ten ligation species using a cryogenic method (M. Perrella and L. Rossi-Bernardi, Methods Enzymol. 76 (1981) 133). For one of the species we find a different free energy assignment from that reported by Smith and Ackers; for all other species we observe the same assignments as in earlier work. The revised distribution also requires a 'combinatorial' mechanism of allosteric switching among the three states.