Anoligosaccharide (/ˌɒlɪɡoʊˈsækəˌraɪd/;[1] from Ancient Greek ὀλίγος (olígos) 'few' and σάκχαρ (sákkhar) 'sugar') is asaccharidepolymer containing a small number (typically three to ten[2][3][4][5]) ofmonosaccharides (simple sugars). Oligosaccharides can have many functions includingcell recognition andcell adhesion.[6]
They are normally present asglycans: oligosaccharide chains are linked tolipids or to compatibleamino acid side chains inproteins, byN- orO-glycosidic bonds.N-Linked oligosaccharides are always pentasaccharides attached toasparagine via a beta linkage to the amine nitrogen of the side chain.[7] Alternately,O-linked oligosaccharides are generally attached tothreonine orserine on the alcohol group of the side chain. Not all natural oligosaccharides occur as components of glycoproteins or glycolipids. Some, such as theraffinose series, occur as storage or transportcarbohydrates in plants. Others, such asmaltodextrins orcellodextrins, result from the microbial breakdown of largerpolysaccharides such asstarch orcellulose.
In biology,glycosylation is the process by which a carbohydrate is covalently attached to an organic molecule, creating structures such as glycoproteins and glycolipids.[8]
N-Linked glycosylation involves oligosaccharide attachment toasparagine via a beta linkage to the amine nitrogen of the side chain.[7] The process ofN-linked glycosylation occurs cotranslationally, or concurrently while the proteins are being translated. Since it is added cotranslationally, it is believed thatN-linked glycosylation helps determine the folding of polypeptides due to the hydrophilic nature of sugars. AllN-linked oligosaccharides are pentasaccharides: five monosaccharides long.[citation needed]
InN-glycosylation for eukaryotes, the oligosaccharide substrate is assembled right at the membrane of theendoplasmatic reticulum.[9] Forprokaryotes, this process occurs at theplasma membrane. In both cases, the acceptor substrate is anasparagine residue. The asparagine residue linked to anN-linked oligosaccharide usually occurs in the sequence Asn-X-Ser/Thr,[7] where X can be any amino acid except forproline, although it is rare to see Asp, Glu, Leu, or Trp in this position.[citation needed]
Oligosaccharides that participate inO-linked glycosylation are attached tothreonine orserine on thehydroxyl group of the side chain.[7]O-linked glycosylation occurs in theGolgi apparatus, where monosaccharide units are added to a complete polypeptide chain. Cell surface proteins and extracellular proteins areO-glycosylated.[10] Glycosylation sites inO-linked oligosaccharides are determined by thesecondary andtertiary structures of the polypeptide, which dictate whereglycosyltransferases will add sugars.[citation needed]
Glycoproteins and glycolipids are by definitioncovalently bonded to carbohydrates. They are very abundant on the surface of the cell, and their interactions contribute to the overall stability of the cell.[citation needed]
Glycoproteins have distinct Oligosaccharide structures which have significant effects on many of their properties,[11] affecting critical functions such asantigenicity,solubility, and resistance toproteases. Glycoproteins are relevant ascell-surface receptors, cell-adhesion molecules,immunoglobulins, and tumor antigens.[12]
Glycolipids are important for cell recognition, and are important for modulating the function of membrane proteins that act as receptors.[13] Glycolipids are lipid molecules bound to oligosaccharides, generally present in thelipid bilayer. Additionally, they can serve as receptors for cellular recognition and cell signaling.[13] The head of the oligosaccharide serves as a binding partner inreceptor activity. The binding mechanisms of receptors to the oligosaccharides depends on the composition of the oligosaccharides that are exposed or presented above the surface of the membrane. There is great diversity in the binding mechanisms of glycolipids, which is what makes them such an important target for pathogens as a site for interaction and entrance.[14] For example, thechaperone activity of glycolipids has been studied for its relevance to HIV infection.
All cells are coated in either glycoproteins or glycolipids, both of which help determine cell types.[7]Lectins, or proteins that bind carbohydrates, can recognize specific oligosaccharides and provide useful information for cell recognition based on oligosaccharide binding.[citation needed]
An important example of oligosaccharide cell recognition is the role of glycolipids in determiningblood types. The various blood types are distinguished by the glycan modification present on the surface of blood cells.[15] These can be visualized using mass spectrometry. The oligosaccharides found on the A, B, and Hantigen occur on thenon-reducing ends of the oligosaccharide. The H antigen (which indicates an O blood type) serves as a precursor for the A and B antigen.[7] Therefore, a person with A blood type will have the A antigen and H antigen present on the glycolipids of the red blood cell plasma membrane. A person with B blood type will have the B and H antigen present. A person with AB blood type will have A, B, and H antigens present. And finally, a person with O blood type will only have the H antigen present. This means all blood types have the H antigen, which explains why the O blood type is known as the "universal donor".[citation needed]
Vesicles are directed by many ways, but the two main ways are:[citation needed]
The sorting signals are recognised by specific receptors that reside in the membranes or surface coats of budding vesicles, ensuring that the protein is transported to the appropriate destination.
Many cells produce specific carbohydrate-binding proteins known as lectins, which mediate cell adhesion with oligosaccharides.[16]Selectins, a family of lectins, mediate certain cell–cell adhesion processes, including those of leukocytes to endothelial cells.[7] 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 byhydrogen bonding andvan der Waals forces.[citation needed]
Fructo-oligosaccharides (FOS), which are found in many vegetables, are short chains offructose molecules. They differ fromfructans such asinulin, which as polysaccharides have a much higherdegree of polymerization than FOS and other oligosaccharides, but like inulin and other fructans, they are considered solubledietary fibre. FOS as a dietary supplementation was linked to glucose homeostasis.[17] These FOS supplementations can be considered prebiotics[18] which produce short-chain fructo-oligosaccharides (scFOS).[19] Galacto-oligosaccharides (GOS) in particular are used to create a prebiotic effect for infants that are not being breastfed.[20]
Galactooligosaccharides (GOS), which also occur naturally, consist of short chains ofgalactose molecules.Human milk is an example of this and contains oligosaccharides, known ashuman milk oligosaccharides (HMOs), which are derived fromlactose.[21][22] These oligosaccharides havebiological function in the development of thegut flora ofinfants. Examples includelacto-N-tetraose, lacto-N-neotetraose, and lacto-N-fucopentaose.[21][22] These compounds cannot bedigested in the humansmall intestine, and instead pass through to thelarge intestine, where they promote the growth ofBifidobacteria, which are beneficial to gut health.[23]
HMOs can also protect infants by acting as decoy receptors against viral infection.[24] HMOs accomplish this by mimicking viral receptors which draws the virus particles away from host cells.[25] 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).[26] The strategy HMOs employ could be used to create new antiviral drugs.[25]
Mannan oligosaccharides (MOS) are widely used inanimal feed to improve gastrointestinal health. They are normally obtained from the yeast cell walls ofSaccharomyces cerevisiae. Mannan oligosaccharides differ from other oligosaccharides in that they are not fermentable and their primary mode of action includes agglutination of type-1 fimbria pathogens and immunomodulation.[27]
Oligosaccharides are a component offibre from plant tissue. FOS and inulin are present inJerusalem artichoke,burdock,chicory,leeks,onions, andasparagus. 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 fungusAspergillus niger acting onsucrose. GOS is naturally found insoybeans and can be synthesized fromlactose. FOS, GOS, and inulin are also sold as nutritional supplements.[citation needed]
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