Lectin

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Lectins are a type of receptor protein of non-immune origin that specifically interact with sugar molecules (carbohydrates) without modifying them. These proteins recognize and bind specifically to monosaccharides and are classified by which sugar they recognize. Most lectins recognize either N-acetylneuraminic acid, N-Acetylglucosamine, N-Acetylgalactosamine, galactose, mannose, or fucose. Lectins are found in a variety of species from plants to insects to man. There are two proposed biological roles for lectins in plants. The first is as an intermediary in the symbiosis between some plants and bacteria by aiding in the attachment of nitrogen-fixing bacteria to legumes. The second proposed role is in the protection of plant seedlings against pathogens such as fungi, viruses, and bacteria by binding to the surface of the microorganisms via sugar residues and inhibiting their growth.

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 is shown for clarity.

While the function of lectins in plants is believed to be the binding of glycoproteins on the surface of cells, the 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.

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Loris, R., Hamelryck, T., Bouckaert, J. & Wyns, L. (1998) Legume Lectin Structure. Biochem. Biophys. Acta , 1383: 9-36
Sharon, N., Lis, H. Lectins, Second Edition (2003) Kluwer Academic