On the analysis of competitive binding of various ligands to cooperative and independent binding sites of macromolecules.

The paper deals with the practical evaluation of multiple binding equilibria of macromolecules and different ligands competing for the same binding sites. The necessary formalism is reviewed and set up for the equilibria involving a macromolecule with various classes of independent binding sites and/or a class of cooperative sites and up to three different ligands in competition for them. In particular, it was necessary to extend the Hill approximation to treat simultaneous competition for cooperative as well as independent binding sites, while earlier attempts are shown to be inadequate. Criteria are developed for a qualitative analysis of complex binding patterns using the Scatchard-plot of the experimental data in order to obtain a model of the binding structure and an adequate set of input parameters for the numerical analysis. Numerical examples refer to the binding of calcium and magnesium to the sarcoplasmic reticulum as studied by competitive replacement of manganese ions [3].