Specificity of concanavalin A binding to asparagine-linked glycopeptides. A nuclear magnetic relaxation dispersion study

We have investigated the binding of a series of high affinity asparagine-linked glycopeptides, including high mannose type and a bisected hybrid type, and several related synthetic oligosaccharides, to Ca²⁺-Mn²⁺-concanavalin A (ConA), using solvent proton nuclear relaxation dispersion (NMRD) measurements. We find that binding of the glycopeptides induces a common smaller decrease in the NMRD profile of ConA compared to that induced by monosaccharide binding. This effect is also observed with a synthetic analog of complex-type carbohydrates, hepta, which also shows enhanced affinity for the protein relative to monosaccharide binding. The high affinity of the glycopeptides and hepta, and their unique effects on the NMRD profile, are mimicked by binding of the trimannosyl oligosaccharide, 3,6-di-O-(α-D-mannopyranosyl)-D-mannose, which is present as a structural element in all of the glycopeptides and synthetic oligosaccharides. However, adding a so-called bisecting N-acetyl-D-glucosamine residue to the trimannosyl oligosaccharide greatly reduces its binding affinity and produces a decrease in the NMRD profile of the protein similar to that observed for monosaccharide binding. These results indicate that the trimannosyl oligosaccharide is a unique moiety recognized by the lectin for high affinity and extended site binding, and the presence of a bisecting N-acetyl-D-glucosamine residue in the trimannosyl oligosaccharide eliminates this type of interaction. The results also demonstrate that ConA primarily binds to the outer trimannosyl regions of high mannose and bisected hybrid-type glycopeptides compared to the central trimannosyl region of complex glycopeptides. Two mechanisms of enhanced affinity binding of saccharides and glycopeptides to ConA are discussed.