An analysis on the slope of Scatchard plots.

The Scatchard plot, [X]b/[E]t[X] versus [X]b/[EP]t where [X]b denotes the concentration of bound ligand, [E]t the total concentration of the binding protein and [X] the free ligand concentration, was designed originally for plotting data of ligand binding to a macromolecule possessing identical non-interacting ligand binding sites. However, the plot is used for describing cooperative binding processes. In such cases, the slope of the Scatchard plot is not then equal to minus the intrinsic association constant any more. The meaning of the slope in such cases is a complex function of the binding parameters and its exact interpretation depends on the particular model used to analyze the binding data. In this communication, the meaning of the slope of the Scatchard plot is analyzed in terms of the different allosteric models.