An oligosaccharide (; ) is a saccharide polymer containing a small number (typically three to ten) of monosaccharides (simple sugars). Oligosaccharides can have many functions including cell recognition and cell adhesion.
They are normally present as glycans: oligosaccharide chains are linked to lipids or to compatible amino acid side chains in proteins, by N- or O-glycosidic bonds. N-Linked oligosaccharides are always pentasaccharides attached to asparagine via a beta linkage to the amine nitrogen of the side chain. Alternately, O-linked oligosaccharides are generally attached to threonine or serine on the alcohol group of the side chain. Not all natural oligosaccharides occur as components of glycoproteins or glycolipids. Some, such as the raffinose series, occur as storage or transport carbohydrates in plants. Others, such as maltodextrins or cellodextrins, result from the microbial breakdown of larger polysaccharides such as starch or cellulose.
thumb|The structure of [[fructooligosaccharide]]
Glycosylation
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
thumb|An example of an N-linked oligosaccharide, shown here with GlcNAc. X is any amino acid except proline.
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,, although it is rare to see Asp, Glu, Leu, or Trp in this position.
O-Linked oligosaccharides
thumb|An example of an O-linked oligosaccharide with β-Galactosyl-(1n3)-α-N-acetylgalactosaminyl-Ser/Thr.
Oligosaccharides that participate in O-linked glycosylation are attached to threonine or serine on the hydroxyl group of the side chain. 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. 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.