Legume lectin

A typical legume lectin monomer (lentil lectin), complexed with a sugar (glucose). The four sugar binding loops are shown in different colors. The variable loop that confers monosaccharide specificity is shown in orange.

The legume lectins are a family of sugar binding proteins or lectins found in the seeds and, in smaller amounts, in the roots, stems, leaves and bark of plants belonging to the Fabaceae family. The exact function of the legume lectins in vivo is unknown but they are probably involved in the defense of plants against predators. Related proteins in other plant families and in animals have also been found. They have been used for decades as a model system for the study of protein-carbohydrate interactions, because they show an amazing variety of binding specificities and are easy to obtain and purify. Over the years, a quite impressive amount of structural data has been gathered. Well-studied members of this protein family include phytohemagglutinin and concanavalin A.

Sugar binding by legume lectins

The legume lectins use an ingenious framework for binding specific sugars. This framework consists of a conserved monosaccharide binding site in which four conserved residues from four separate regions in the protein confer affinity (see figure), a variable loop that confers monosaccharide specificity and a number of subsites around the monosaccharide binding site that harbor additional sugar residues or hydrophobic groups.