Oligosaccharide

An oligosaccharide (/ˌɒlɪɡoʊˈsækəˌraɪd/;^[1] from Ancient Greek ὀλίγος (olígos) 'few' and σάκχαρ (sákkhar) 'sugar') is a saccharide polymer containing a small number (typically three to ten^[2][3][4][5]) of monosaccharides (simple sugars). Oligosaccharides can have many functions including cell recognition and cell adhesion.^[6]

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.^[7] 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.

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.^[8]

N-Linked oligosaccharides

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.^[7] The process of N-linked glycosylation occurs cotranslationally, or concurrently while the proteins are being translated. Since it is added cotranslationally, it is believed that N-linked glycosylation helps determine the folding of polypeptides due to the hydrophilic nature of sugars. All N-linked oligosaccharides are pentasaccharides: five monosaccharides long.^{[citation needed]}

In N-glycosylation for eukaryotes, the oligosaccharide substrate is assembled right at the membrane of the endoplasmatic reticulum.^[9] 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,^[7] where X can be any amino acid except for proline, although it is rare to see Asp, Glu, Leu, or Trp in this position.^{[citation needed]}

O-Linked oligosaccharides

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.^[7] O-linked glycosylation occurs in the Golgi apparatus, where monosaccharide units are added to a complete polypeptide chain. Cell surface proteins and extracellular proteins are O-glycosylated.^[10] Glycosylation sites in O-linked oligosaccharides are determined by the secondary and tertiary structures of the polypeptide, which dictate where glycosyltransferases will add sugars.^{[citation needed]}

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.^{[citation needed]}

Glycoproteins

Glycoproteins have distinct Oligosaccharide structures which have significant effects on many of their properties,^[11] 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.^[12]

Glycolipids

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 the lipid bilayer. Additionally, they can serve as receptors for cellular recognition and cell signaling.^[13] The head of the oligosaccharide serves as a binding partner in receptor 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, 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.^[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 determining blood 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 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.^[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]}

1. The sorting signals encoded in the amino acid sequence of the proteins.
2. The Oligosaccharide attached to the protein.

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.

Cell adhesion

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 by hydrogen bonding and van der Waals forces.^{[citation needed]}

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. Using fructo-oligosaccharides (FOS) as fiber supplementations is shown to have an effect on glucose homeostasis quite similar to insulin.^[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 of galactose molecules. Human milk is an example of this and contains oligosaccharides, known as human milk oligosaccharides (HMOs), which are derived from lactose.^[21][22] 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.^[21][22] These compounds cannot be digested in the human small intestine, and instead pass through to the large intestine, where they promote the growth of Bifidobacteria, 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 in animal feed to improve gastrointestinal health. They are normally obtained from the yeast cell walls of Saccharomyces 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]

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.^{[citation needed]}

See also

• Carbohydrate synthesis – Sub-field of organic chemistry
• Oligosaccharide nomenclature – Devising names for a class of carbohydrates
• Isomaltooligosaccharide – Mixture of short-chain carbohydrates

References

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2. Oligosaccharides at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
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17. Le Bourgot, Cindy; Apper, Emmanuelle; Blat, Sophie; Respondek, Frédérique (2018-01-25). "Fructo-oligosaccharides and glucose homeostasis: a systematic review and meta-analysis in animal models". Nutrition & Metabolism. 15 (1): 9. doi:10.1186/s12986-018-0245-3. ISSN 1743-7075. PMC 5785862. PMID 29416552.
18. Davani-Davari, Dorna; Negahdaripour, Manica; Karimzadeh, Iman; Seifan, Mostafa; Mohkam, Milad; Masoumi, Seyed Jalil; Berenjian, Aydin; Ghasemi, Younes (2019-03-09). "Prebiotics: Definition, Types, Sources, Mechanisms, and Clinical Applications". Foods. 8 (3): E92. doi:10.3390/foods8030092. ISSN 2304-8158. PMC 6463098. PMID 30857316.
19. Respondek, F.; Myers, K.; Smith, T. L.; Wagner, A.; Geor, R. J. (2011). "Dietary supplementation with short-chain fructo-oligosaccharides improves insulin sensitivity in obese horses". Journal of Animal Science. 89 (1): 77–83. doi:10.2527/jas.2010-3108. ISSN 1525-3163. PMID 20870952.
20. Mei, Zhaojun; Yuan, Jiaqin; Li, Dandan (2022). "Biological activity of galacto-oligosaccharides: A review". Frontiers in Microbiology. 13: 993052. doi:10.3389/fmicb.2022.993052. ISSN 1664-302X. PMC 9485631. PMID 36147858.
21. Miesfeld, Roger L. (July 2017). Biochemistry. McEvoy, Megan M. (First ed.). New York, NY. ISBN 978-0-393-61402-2. OCLC 952277065.{{cite book}}: CS1 maint: location missing publisher (link)
22. "Human Milk Oligosaccharides". NNI Global Website. Retrieved 2020-12-04.
23. Moise AM (2017-10-31). The Gut Microbiome: Exploring the Connection between Microbes, Diet, and Health. ABC-CLIO. p. 58. ISBN 978-1-4408-4265-8.
24. Moore, Rebecca E.; Xu, Lianyan L.; Townsend, Steven D. (2021-02-12). "Prospecting Human Milk Oligosaccharides as a Defense Against Viral Infections". ACS Infectious Diseases. 7 (2): 254–263. doi:10.1021/acsinfecdis.0c00807. ISSN 2373-8227. PMC 7890562. PMID 33470804.
25. Morozov, Vasily; Hansman, Grant; Hanisch, Franz-Georg; Schroten, Horst; Kunz, Clemens (2018). "Human Milk Oligosaccharides as Promising Antivirals". Molecular Nutrition & Food Research. 62 (6): 1700679. doi:10.1002/mnfr.201700679. PMID 29336526.
26. Moore, Rebecca E.; Xu, Lianyan L.; Townsend, Steven D. (2021-02-12). "Prospecting Human Milk Oligosaccharides as a Defense Against Viral Infections". ACS Infectious Diseases. 7 (2): 254–263. doi:10.1021/acsinfecdis.0c00807. ISSN 2373-8227. PMC 7890562. PMID 33470804.
27. Smiricky-Tjardes MR, Flickinger EA, Grieshop CM, Bauer LL, Murphy MR, Fahey GC (October 2003). "In vitro fermentation characteristics of selected oligosaccharides by swine fecal microflora". Journal of Animal Science. 81 (10): 2505–14. doi:10.2527/2003.81102505x. PMID 14552378.

External links

• Media related to Oligosaccharides at Wikimedia Commons