Glycoclusters as lectin inhibitors: Comparative analysis on two plant agglutinins with different folding as a step towards rules for selectivity

The emerging physiological significance of carbohydrate (glycan)-protein (lectin) recognition engenders the interest to design synthetic inhibitors with a high level of selectivity among natural sugar receptors. Plant agglutinins are common models to determine structure-activity relationships. Focussing on the contribution of valency towards selectivity, copper-catalysed azide (sugar derivative)-alkyne (scaffold) cycloaddition yielded a panel of 10 bi- to tetravalent glycoclusters with N-acetylglucosamine as the bioactive headgroup. They were introduced into assays using (neo)glycoproteins and cell surfaces as platforms to study carbohydrate-dependent lectin binding. The ability of the bivalent compounds, which exhibit a distance profile of the sugar headgroups of about 16-21 Å, for intramolecular bridging of two contact sites from the eight hevein domains of wheat germ agglutinin led to comparatively high enhancements of inhibitory potency relative to a tetrameric leguminous lectin (distance profile of 50-70 Å between sugar-specific sites), especially for a β-S-glycoside. The extent of inhibition at fixed concentrations of the sugar depended on the type of matrix used for the assay. Increases to tri- and tetravalency played a less important role than the anomeric position to keep cross-reactivity low, these tested topologies enabling cross-linking for both lectins. The potential for cis-interactions (intramolecular interactions), with glycoclusters serving as molecular rulers, is suggested to help designing selective blocking reagents.