Hydrophobic Agaroses: Basis for a Model of Multivalent Effector-Receptor Interactions

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Abstract
An increase in the density of butyl residues bound to Sepharose 4 B leads to an enhancement of the affinity of these gels for phosphorylase b EC (2.4.1.1) in the presence of 1.1 M ammonium sulfate. A Hill coefficient of 2.9 indicates that a minimum of about 3 binding sites is involved in the positive cooperative adsorption of this enzyme. Binding studies of phosphorylase b on butyl-Sepharose of a specific degree of substitution demonstrate that the affinity of the gel for this ligand decreases as a function of fractional saturation. A Hill coefficient of 0.44 indicates negative cooperativity as a result of multivalent binding. A multivalent, mobile receptor model is derived which can explain such characteristics of effector-receptor interactions as: positive and negative cooperativity, high binding constants and low dissociation rate constants. The application of this model to experiments taken from the literature on the binding of the multivalent effectors concanavalin A and cholera toxin to fat cells shows that the postulated mode of interactions is probably realized in nature.