Interactions of Concanavalin A with Asparagine-Linked Glycopeptides: Formation of Homogeneous Cross-Linked Lattices in Mixed Precipitation Systems

We have previously shown that certain oligomannose and bisected hybrid type glycopeptides are bivalent for binding to concanavalin A (Con A) [Bhattacharyya, L., Ceccarini, C., Lorenzoni, P., & Brewer, C. F. (1987) J. Biol. Chem. 262, 1288-1293]. Each glycopeptide gives a quantitative precipitation profile with the protein which consists of a single peak that corresponds to the binding stoichiometry of glycopeptide to protein monomer (1:2). We have shown that the affinities of the primary and secondary sites of the glycopeptides influence their extent of precipitation with the lectin [Bhattacharyya, L., & Brewer, C. F. (1988) Eur. J. Biochem. (in press)]. In the present study, we demonstrate that equimolar mixtures of any two of the glycopeptides result in a quantitative precipitation profile which shows two protein peaks. Using radiolabeled glycopeptides, the precipitation profiles of the individual glycopeptides were determined. The results show that each glycopeptide forms its own precipitation profile with the protein which is independent of the profile of the other glycopeptide. For mixtures containing an equimolar ratio of two glycopeptides, the glycopeptide with lower affinity shows a precipitation maximum at a lower concentration than the one with higher affinity. However, this can be reversed by increasing the ratio of the lower affinity glycopeptide in the mixture. Thus, the relative precipitation maxima of the glycopeptides are determined by mass-action equilibria involving competitive binding of the two carbohydrates to the protein. These equilibria, in turn, are sensitive to the relative amounts and affinities of the carbohydrates at both their primary and secondary sites. These findings indicate that each glycopeptide forms a unique homogeneous cross-linked lattice with the lectin which excludes the lattice of another glycopeptide in a mixture. The results are discussed in terms of the structure—function properties of asparagine-linked carbohydrates and lectins, as well as other multivalent binding systems.