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Biophys Chem. 2007 Sep;129(2-3):284-8. Epub 2007 Jun 30.

Cooperative equilibrium curves generated by ordered ligand binding to multi-site molecules.

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Université de Rennes1 UMR6026 CNRS IFR140 Campus de Beaulieu, 35042 Rennes cedex France.


The sigmoid shape of equilibrium curves in normal axes and Hill coefficients higher than unity, are indexes of cooperativity or homotropic allostery where the affinity for the ligand increases as saturation progresses. The mathematical transformation of the Adair scheme of equilibria in the Hill plot, reveals that sigmoid binding curves can also be generated by ordered ligand binding to a receptor with multiple binding sites of identical microscopic association constants. This mechanism only based on the law of mass action, could participate to some extent to certain cooperative effects observed in non-biological systems and perhaps in the physiological binding of oxygen to heme proteins.

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