Quantitative description of the binding of GM1 oligosaccharide by cholera enterotoxin

We present a quantitative molecular interpretation of binding between the five B subunits of cholera enterotoxin and the oligosaccharide of ganglioside GM1, based on the currently accepted quaternary structure of the toxin and principles of multiple equilibria. A sequential binding equation is derived and fitted to published binding data obtained by equilibrium dialysis. In one study of binding to reduced toxin (I), intact toxin (II), and isolated B subunits (III) at low concentrations, analysis by the Hill equation suggested that binding was positively cooperative and that there were only four binding sites per toxin molecule; individual affinity constants could not be estimated because of the empirical nature of the Hill equation. Our analysis suggests that the evidence for positive cooperativity is stronger for I and III than for II. Affinity constants for the first binding step are about 2.0-2.1 microM-1 for I and 2.5-2.7 microM-1 for II and III; those for the second binding step are about 3.5-5.0 microM-1 and for I and III, but only 2.5 microM-1 for II. Constants for later binding steps are apparently within the range of 2-7 microM-1. Predictions of the sequential model at higher ligand concentrations diverge substantially from those of the Hill equation, and are supported by data obtained at higher protein and ligand concentrations. Thus all available equilibrium dialysis data are consistent with a single set of affinity constants and with the hypothesis of five equivalent binding sites.