How proteins bind carbohydrates: Lessons from legume lectins

The pioneering studies of Irvin Liener on soybean agglutinin (SBA) in the early 1950s served as the starting point of our involvement in lectin research during the past four decades, Initially we characterized SBA extensively as a glycoprotein and showed that its covalently linked glycan is an oligomannoside commonly present in animal glycoproteins. We have also introduced the use of the lectin to the study of normal and malignant cells and to the purging of bone marrow for transplantation. Our recent work focuses on the combining site of Erythrina corallodendron lectin, closely related to SIBA. In this legume lectin, as in essentially all other members of the same protein family, irrespective of their sugar specificity, interactions with a constellation of three invariant residues (aspartic acid, asparagine, and an aromatic residue) are essential for ligand binding. Lectins from other families, whether of plants or animals, also combine with carbohydrates by H-bonds and hydrophobic interactions, but the amino acids involved may differ even if the specificity of the lectins is the same. Therefore, nature finds diverse solutions for the design of binding sites for structurally similar ligands, such as mono- or oligosaccharides. This diversity strongly suggests that lectins are products of convergent evolution.