Multivalency effects in protein--carbohydrate interaction: the binding of the Shiga-like toxin 1 binding subunit to multivalent C-linked glycopeptides.

A series of monovalent and bivalent glycopeptides displaying a C-linked analogue of the Pk trisaccharide, the in vivo ligand for the pentavalent Shiga-like toxin binding subunit (SLT-1B), were prepared and evaluated as ligands for SLT-1B by isothermal titration microcalorimetry and competitive enzyme-linked immunosorbent assay (ELISA). Although none of the monovalent ligands showed any enhancement in affinity compared to O-methyl glycoside, two bivalent ligands show significant enhancements in affinity in assays. This observation represents the first calorimetric observation of an enhancement in affinity for this system. In contrast, only one of the two ligands shows an enhancement in the competitive ELISA. Together, these data signal a difference in the means by which the two ligands achieve affinity, apparently triggered by a change in the nature of the linker domain. These results provide a rationalization for apparently contradictory reports from the recent literature and again emphasize the importance of investigating complex binding phenomena by multiple techniques.