High Local Ligand Concentration Contributes to ConA Binding

Despite the importance of the chelate effect in the increased functional affinities of ConA for the multidentate ligands described above, the chelate effect is not the only contributing mechanism. If chelation was the only important process, oligomers composed of more residues than are required to span the binding sites should display decreased potencies when evaluated on a saccharide residue basis because noninter-acting residues would detract from the observed affinity. This was not the case, however, inasmuch as potency did not decrease. Therefore, the potencies of the longer polymers are greater than the chelation model predicts (Fig. 14). We suspect that the high local concentration of saccharide residues available with these polymers decreases the rate of dissociation of the multivalent ligands, making rebinding more favorable. This statistical enhancement mode of multivalent binding also contributes to the increase in the observed affinity of ligands unable to span two sites (i.e., <25 residues in length). Moreover, the large number of mannose residues available for binding with these ligands suggests that a number of different lectin-ligand complexes can form. Finally, ConA clustering and steric stabilization (see Section VI.C) may also contribute to the efficacies of the longer polymers (Fig. 5).

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