Lectin
From Free net encyclopedia
Lectins are a type of receptor proteins 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 int eh 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. While the function of lectins in plants is believed to be the binding of glycoproteins on teh surface of cells, the role in animals also includes the binding of soluble extracellular and interecellular 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 rsponsible for eth eremoval of certain glycoproteins from teh circulatory system. Another example is the mannose-6-phosphate receptor that recognizes hydrolytic eenzymes containing thise residue and subsequently targests theses 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 in that they are used for blood typing. an individual's blood type is determined by the make up of certain cell-surface glycolipids and glycoproteins. These sugar containing molecules are called blood antigens and are classified into three groups: A, B, and O. The three differ by the outer-most sugar residue in a chain of oligosaccharides. Since lectins are specific for sugar residues, they can be used to detemrine what blood type an individual is. If a person receives a blood transfusion with blood cells that differ in the glycosylated molecules on teh surface of the cells, then the recipient will have an immune reaction where antibodies are produced against these foreign carbohydrates. Thus, cell surface oligosaccharides play an important role in recognition of cells and how they interact with the surrounding environment.
See also
External links
- World of Lectin links maintained by Thorkild C. Bøg-Hansen