Lectin

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Lectins are carbohydrate-binding proteins or glycoproteins which are highly specific for their sugar moieties. The name ‘lectin’ is derived from the Latin word legere, meaning ‘to select’. Although they were first discovered more than 100 years ago in plants, they are now known to be present throughout nature. It is generally believed that the earliest description of such a hemagglutinin was by Peter Hermann Stillmark in his doctoral thesis presented in 1888 to the University of Dorpat, (one of the oldest universities in czarist Russia). This hemagglutinin, which was also highly toxic, was isolated by Stillmark from seeds of the castor tree (Ricinus communis) and was named ricin. But most of the lectins are basically non-enzymic in action and non-immune in origin. Lectins occur ubiquitously in nature. They may bind carbohydrate moiety as such free in solution or carbohydrate moiety which is a part of protein/particulate body. They agglutinate cells and/or precipitates glycoconjugates.

An oligosaccharide (shown in grey) bound in the binding site of a plant lectin (Griffonia simplicifolia isolectin IV in complex with the Lewis b blood group determinant). Only a part of the oligosaccharide (central, in grey) is shown for clarity.

While the function of lectins in plants is believed to be the binding of glycoproteins on the surface of cells, their role in animals also includes the binding of soluble extracellular and intercellular glycoproteins. For example, there are lectins found on the surface of mammalian liver cells that specifically recognize galactose residues. It is believed that these cell-surface receptors are responsible for the removal of certain glycoproteins from the circulatory system. Another example is the mannose-6-phosphate receptor that recognizes hydrolytic enzymes containing this residue and subsequently targets these proteins for delivery to the lysosomes. They serve many different biological functions from the regulation of cell adhesion to glycoprotein synthesis and the control of protein levels in the blood. Lectins are also known to play important roles in the immune system by recognising carbohydrates that are found exclusively on pathogens, or that are inaccessible on host cells.

Purified lectins are important in a clinical setting because they are used for blood typing. Some of the glycolipids and glycoproteins on an individual's red blood cells can be identified by lectins. A lectin from Dolichos biflorus is used to identify cells that belong to the A1 blood group. A lectin from Ulex europaeus is used to identify the H blood group antigen, and a lectin from Vicia graminea is used to identify the N blood group antigen.

The real function of lectins in plants is still to be found - they are not necessary for rhizobia binding as mentioned above (ruled out with lectin-knockout transgene) and cell adhesion function as their sole purpose in plants also questionable. Large presence in seeds (from which lectins are usually isolated) decreases with growth is suggesting a great role in plant's germination and perhaps in the seed's survival itself.

Lectins are considered as direct predecessors to the immune system and they have still great role in it - lectin complement activation pathway, Mannose binding lectin, S,P,E lectins, etc.

"Higher" lectins like NK cell receptors are less specific to simple carbohydrates and show higher affinity to obscure oligosaccharide structures.

Lectins from legume plants have been widely used as model systems to understand the molecular basis of how proteins recognize carbohydrates, because they are relatively easy to obtain and have a wide variety of sugar specificities. The many crystal structures of legume lectins have led to a detailed insight of the atomic interactions between carbohydrates and proteins.

One example of the powerful biological attributes of lectins is the biochemical warfare agent ricin. Ricin is isolated from seeds of the castor oil plant and is a protein that is comprised of two domains, one is a lectin that binds cell surface galactosyl residues and enables the protein to enter cells. The second domain is an N-glycosidase that cleaves nucleobases from ribosomal RNA resulting in inhibition of protein synthesis and cell death.

[edit] See also

[edit] References

Loris, R., Hamelryck, T., Bouckaert, J. & Wyns, L. (1998) Legume Lectin Structure. Biochem. Biophys. Acta , 1383: 9-36 link (PDF)

Sharon, N., Lis, H. Lectins, Second Edition (2003) Kluwer Academic

[edit] External links

World of Lectin links maintained by Thorkild C. Bøg-Hansen
Ni Y, Tizard I (1996). "Lectin-carbohydrate interaction in the immune system". Vet Immunol Immunopathol 55 (1-3): 205-23. PMID 9014318.