Glycoproteins - Hughes R. 1985

Functions
Lectins: Carbohydrate-Binding Proteins
General Properties

The term "lectin" refers to a protein of non-immune origin capable of precipitating Glycoproteins and causing Cell agglutination, with this activity being inhibited by CARBOHYDRATES. This definition implies multiple valence: the protein must have at least two carbohydrate-binding sites in order to form cross-links between different glycoprotein molecules. The first Lectins to be studied were isolated from plants. Paul Ehrlich [1] used lectin from castor beans (Ricinus communis) in his work. The results of his studies on The activity of this lectin are discussed below. In 1919, Sumner obtained another lectin, concanavalin A, in crystalline form from the jack bean (Canavalia ensiformis). This lectin has been studied most thoroughly. A Brief Overview of the Structure and properties of plant lectins is given in Table 4.1 [55]. Recently, lectins have also been isolated from animal Tissues; research in this direction has played an exceptionally important role in elucidating the functional role of carbohydrate-containing macromolecules.

Class="center">Table 4.1. Properties of some purified lectins

The BIOLOGICAL Functions OF lectins in plants remain unclear. Among the hypotheses put forward in this regard, the following deserve mention. First, lectins may play a role analogous to The Immune System of immunocompetent organisms, i.e., binding pathogenic microorganisms and facilitating their destruction. Second, there is compelling evidence that lectins in legumes promote the colonization of plant roots by nitrogen-fixing microorganisms, such as Rhizobia. It is quite possible that lectins are also involved in other processes, such as seed germination; indeed, this may even be their most important function. Regardless of the role played by plant lectins, they serve as a very convenient tool for investigating such issues as the functions of glycoproteins in Cells and the specificity of carbohydrate-Protein Interactions. Such carbohydrate-protein interactions are of major biological importance for A number of well-studied systems described in subsequent sections. Purified plant lectins provide a highly convenient model for studying these interactions.