Subunit exchange and ligand binding. II. The mechanism of the allosteric effect in hemoglobin.

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Abstract
The previously proposed hypothesis that the allosteric effects which are involved in cooperative ligand binding by hemoglobin are brought about by [alpha][beta] subunit exchange has been corroborated by further experimental evidence. Oxygen binding he me-proteins which show no heme-heme interaction, i.e., myoglobin and the isolated [alpha] and [beta] chains of hemoglobin, completely fail to undergo subunit exchange. Hemoglobin variants with normal sigmoid oxygen dissociation curves, on the other hand, i.e., hemoglobins S, C, and F, exchange to the same extent as hemoglobin A. The implications of these results for the molecular mechanism of allosteric change are discussed.