Oligosaccharide

In biology, glycosylation is the process by which a carbohydrate is covalently attached to an organic molecule, creating structures such as glycoproteins and glycolipids. N-Linked oligosaccharides N-Linked glycosylation involves oligosaccharide attachment to asparagine via a beta linkage to the amine nitrogen of the side chain. For prokaryotes, this process occurs at the plasma membrane. In both cases, the acceptor substrate is an asparagine residue. The asparagine residue linked to an N-linked oligosaccharide usually occurs in the sequence Asn-X-Ser/Thr, Glycosylation sites in O-linked oligosaccharides are determined by the secondary and tertiary structures of the polypeptide, which dictate where glycosyltransferases will add sugars. ==Glycosylated biomolecules==

Glycoproteins and glycolipids are by definition covalently bonded to carbohydrates. They are very abundant on the surface of the cell, and their interactions contribute to the overall stability of the cell. Glycoproteins Glycoproteins have distinct Oligosaccharide structures which have significant effects on many of their properties, affecting critical functions such as antigenicity, solubility, and resistance to proteases. Glycoproteins are relevant as cell-surface receptors, cell-adhesion molecules, immunoglobulins, and tumor antigens. Glycolipids Glycolipids are important for cell recognition, and are important for modulating the function of membrane proteins that act as receptors. Glycolipids are lipid molecules bound to oligosaccharides, generally present in the lipid bilayer. Additionally, they can serve as receptors for cellular recognition and cell signaling. For example, the chaperone activity of glycolipids has been studied for its relevance to HIV infection. ==Functions==

Cell recognition All cells are coated in either glycoproteins or glycolipids, both of which help determine cell types. These can be visualized using mass spectrometry. The oligosaccharides found on the A, B, and H antigen occur on the non-reducing ends of the oligosaccharide. The H antigen (which indicates an O blood type) serves as a precursor for the A and B antigen. Selectins, a family of lectins, mediate certain cell–cell adhesion processes, including those of leukocytes to endothelial cells. In an immune response, endothelial cells can express certain selectins transiently in response to damage or injury to the cells. In response, a reciprocal selectin–oligosaccharide interaction will occur between the two molecules which allows the white blood cell to help eliminate the infection or damage. Protein-Carbohydrate bonding is often mediated by hydrogen bonding and van der Waals forces. ==Dietary oligosaccharides==

Fructo-oligosaccharides (FOS), which are found in many vegetables, are short chains of fructose molecules. They differ from fructans such as inulin, which as polysaccharides have a much higher degree of polymerization than FOS and other oligosaccharides, but like inulin and other fructans, they are considered soluble dietary fibre. FOS as a dietary supplementation was linked to glucose homeostasis. These FOS supplementations can be considered prebiotics which produce short-chain fructo-oligosaccharides (scFOS).'''''' Galacto-oligosaccharides (GOS) in particular are used to create a prebiotic effect for infants that are not being breastfed. Galactooligosaccharides (GOS), which also occur naturally, consist of short chains of galactose molecules. Human milk is an example of this and contains oligosaccharides, known as human milk oligosaccharides (HMOs), which are derived from lactose. These oligosaccharides have biological function in the development of the gut flora of infants. Examples include lacto-N-tetraose, lacto-N-neotetraose, and lacto-N-fucopentaose. HMOs can also protect infants by acting as decoy receptors against viral infection. HMOs accomplish this by mimicking viral receptors which draws the virus particles away from host cells. Experimentation has been done to determine how glycan-binding occurs between HMOs and many viruses such as influenza, rotavirus, human immunodeficiency virus (HIV), and respiratory syncytial virus (RSV). The strategy HMOs employ could be used to create new antiviral drugs. Sources Oligosaccharides are a component of fibre from plant tissue. FOS and inulin are present in Jerusalem artichoke, burdock, chicory, leeks, onions, and asparagus. Inulin is a significant part of the daily diet of most of the world's population. FOS can also be synthesized by enzymes of the fungus Aspergillus niger acting on sucrose. GOS is naturally found in soybeans and can be synthesized from lactose. FOS, GOS, and inulin are also sold as nutritional supplements. == See also ==