Human Biochemistry, Volume 2 - Murray R. 1993

Special Topics
Glycoproteins and Proteoglycans
Glycoproteins – Lectins

Lectins are Plant-derived Proteins that specifically bind to one or more sugars. Many lectins have been purified, and over 40 are commercially available (Table 54.5). In biochemical research, lectins serve a variety of purposes, including:

1) the purification and analysis of Glycoproteins. Lectins such as concanavalin A (ConA) can be covalently attached to an inert support, such as Sepharose. The resulting Sepharose–ConA conjugate can then be used to isolate glycoproteins containing oligosaccharide chains that interact specifically with ConA;

2) probing Cell surfaces. Because lectins recognize specific sugars, they act as useful probes for identifying sugar residues on cell-surface membranes. Numerous studies have utilized lectins to compare the surfaces of normal and malignant Cells (see Chapter 57). For instance, certain lectins agglutinate Cancer cells at lower concentrations than normal cells, highlighting structural or organizational differences in specific cell-surface glycoproteins between normal and transformed cells;

3) selecting mutant cells lacking specific Enzymes involved in oligosaccharide synthesis. When mammalian cell cultures are treated with appropriate concentrations of certain lectins (e.g., ConA), the majority of the cells die, but many of the survivors lack specific enzymes required for oligosaccharide Biosynthesis. This resistance presumably stems from the absence of one or more surface glycoproteins capable of binding the lectin. Such mutant cell lines are invaluable tools for elucidating various aspects of glycoprotein biosynthesis.

Class="center">Table 54.5. Three lectins and the sugars with which they interact. In most cases, lectins exhibit Specificity for the anomeric glycosidic linkage (α or β); this is not specified in the table.