Studies of ligand binding to cholera toxin, III. Cooperativity of oligosaccharide binding

Binding parameters for the interaction of cholera toxin and choleragenoid with monosialo-gangliotetraose, the oligosaccharide moiety of ganglioside II3 NeuAc-GgOse4-Cer, have been measured by equilibrium displacement dialysis. The experimental data were evaluated by a curve fitting computer program. The binding curves obtained at 6 degrees C reflected positive cooperativity with average Hill coefficients in the range of 1.16 to 1.25. The cholera toxin as well as its ganglioside-binding protomer B-protein bind 4 mol of monosialogangliotetraitol per mol of protein. The presence of disulfide-cleaving agents like 2-mercaptoethanol did not eliminate the cooperativity of the binding characteristics, but increased the Hill coefficient.