Analysis of the binding specificities of oligomannoside-binding proteins using methylated monosaccharides

The binding specificities of the closely related lectins from Canavalia ensiformis and Dioclea grandiflora were examined using specifically O-alkylated mono- and disaccharides. Both lectins accept any substitution at the monosaccharide C2 hydroxyl group. The binding energy of C2-alkylated ligands-concanavalin A complexes increases by 1 kcal mol-1 for the C2-O-ethyl ligand, while the binding energies of the corresponding complexes with the Dioclea lectin are identical. Both lectins accept methyl, but not ethyl, substitution of the C3 hydroxyl, in contrast to earlier reports. The results are interpreted in terms of existing models of the concanavalin A binding site. While the results are consistent with a model of the concanavalin A extended binding site that places the non-reducing terminus of all disaccharides in the monosaccharide binding site, they point to the dangers of interpreting the binding behavior of unnatural saccharide ligands on the basis of crystallographic data obtained with native ligands.