SPECIFICITY OF C-GLYCOSIDE COMPLEXATION BY MANNOSE GLUCOSE SPECIFIC LECTINS

The binding of the mannose/glucose specific lectins from Canavalia ens iformis (concanavalin A) and Dioclea grandiflora to a series of C-gluc osides and mannosides was studied by titration microcalorimetry and fl uorescence anisotropy titration. These closely related lectins share a specificity for the trimannoside methyl -(alpha-D-mannopyranosyl)-alp ha-D-mannopyranoside, and are a useful model system for addressing the feasibility of differentiating between lectins with overlapping carbo hydrate specificities. The ligands were designed to address two issues : (1) how the recognition properties of non-hydrolyzable C-glycoside a nalogues compare with those of the corresponding O-glycosides and (2) the effect of presentation of more than one saccharide recognition epi tope on both affinity and specificity. Both lectins bind the C-glycosi des with affinities comparable to those of the O-glycoside analogues; however, the ability of both lectins to differentiate between gluco an d manno diastereomers was diminished in the C-glycoside series. Bivale nt norbornyl C-glycoside esters were bound by the lectin from Canavali a but only weakly by the lectin from Dioclea. In addition to binding t he bivalent ligands, concanavalin A discriminated between C-2 epimers, with the manno configuration binding more tightly than the gluco. The stoichiometry of binding of the bivalent ligands to both di- and tetr americ lectin was two binding sites per ligand, rather than the expect ed 1:1 stoichiometry. Together, these results suggest that concanavali n A may possess more than one class of carbohydrate binding sites and that these additional sites show stereochemical discrimination similar to that of the previously identified monosaccharide binding site. The implications of these findings for possible in vivo roles of plant le ctins and for the use of concanavalin A as a research tool are discuss ed.