An oligosaccharide (from the Greek ὀλίγος olígos, "a few", and σάκχαρ sácchar, "sugar") is a saccharide polymer containing a small number (typically two to ten) of simple sugars (monosaccharides). Oligosaccharides can have many functions including cell recognition and cell binding. For example, glycolipids have an important role in the immune response.
In general, they are found either N- or O-linked to compatible amino acid side-chains in proteins or to lipid moieties (see glycans). N-linked oligosaccharides are found 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.
In biology, glycosylation is the co-translational process by which a carbohydrate is covalently attached to an organic molecule – creating structures such as glycoproteins and glycolipids.
N-linked glycosylation involves oligosaccharide attachment to asparagine via a beta linkage to the amine nitrogen of the side chain. The process of N-linked glycosylation occurs cotranslationally, or concurrently while the proteins is 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 composed of a core of five sugars - known as a pentasaccharide.
In N-glycosylation for eukaryotes, the oligosaccharide substrate is assembled right at the membrane of the ER. For prokaryotes, this process occurs at the plasma membrane. In both cases, the acceptor substrate is an asparagine residue. This asparagine has increased nucleophilicity in the amide group. The unique arrangement of N-linked oligosaccharides usually has the oligosaccharide linked to the amide nitrogen of the Asn residue, in the sequence Asn-X-Ser/Thr. X can be any amino acid except for proline (though it is rare to see Asp, Glu, Leu, or Trp).
Oligosaccharides that participate in O-linked glycosylation are attached to threonine or serine on the alcohol group of the side chain. O-linked glycosylation occurs in the golgi apparatus, in which monosaccharide units are added to a complete polypeptide chain. Cell surface proteins and extracellular proteins are O-glycosylated. Glycosylation sites in O-linked oligosaccharides are specified only in the secondary and tertiary structures of the polypeptide, which will dictate where glycosyltransferases will add sugars.
Both glycoproteins and glycolipids have a covalently attached carbohydrate attached to their respective molecule. They are very abundant on the surface of the cell, and their interactions contribute to the overall stability of the cell.
Glycoproteins have distinct Oligosaccharide structures which contribute greatly to various properties of the glycoproteins. It is these properties that become important for 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 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. 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 oligosaccharide sugar are exposed/presented out of the membrane. There is great biodiversity in the binding mechanisms of glycolipids, which is what makes them such a target for pathogens as a site for interaction and entrance. For example, the chaperone activity of glycolipids has been studied for its relevance to HIV infection. The exposure of these glycolipids can also serve as a portal for other viral and toxic substances, which bind to certain receptors on the cell surface and initiate an invasion of the toxin into the cell.
All cells are coated in either glycoproteins or glycolipids, both of which help determine cell types. Lectins, or proteins that bind carbohydrates, which can recognize very specific oligosaccharides and provide useful information for cell recognition due to oligosaccharide binding.
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. 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. 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".
Many cells produce specific carbohydrate-binding ligands, known as lectins, which mediate cell-adhesion with oligosaccharides. 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.
Role in mother-to-child transmission of HIV-1
Mixed-feeding (breast-milk and formula or other non-human feed) is the highest prevalence factor of postnatal transmission of HIV-1. However, most exclusively breast-fed infants do not contract the virus from the infected mother, despite the continuous exposure. The specific glycans that are present in human breast milk can actually inhibit HIV-1 transfer by competing with the HIV-1 surface glycoprotein gp120 for binding to dendritic ICAM3-grabbing non-integrin (DC-SIGN). Human milk contains a high quantity of complex oligosaccharides that carry multiple Lewis antigen glycans. These human milk oligosaccharides (HMOs) reduce gp120 binding by more than 80%, which proves in inhibitory effects of human milk on HIV-1 mother-to-child transmission.
Fructo-oligosaccharides (FOS), which are found in many vegetables, consist of short chains of fructose molecules. (Inulin has a much higher degree of polymerization than FOS and is a polysaccharide.) Galactooligosaccharides (GOS), which also occur naturally, consist of short chains of galactose molecules. These compounds can be only partially digested by humans.
Mannan oligosaccharides (MOS) are widely used in animal feed to improve gastrointestinal health, energy levels and performance. They are normally obtained from the yeast cell walls of Saccharomyces cerevisiae. Research at the University of Illinois has demonstrated that mannan oligosaccharides differ from other oligosaccharides in that they are not fermentable and their primary mode of actions include agglutination of type-1 fimbrae pathogens and immunomodulation
Oligosaccharides are one of the components of fibre, found in plants. FOS and inulin are found naturally in Jerusalem artichoke, burdock, chicory, leeks, onions, and asparagus. FOS products derived from chicory root contain significant quantities of inulin, a fiber widely distributed in fruits, vegetables and plants. 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 (milk sugar). FOS, GOS, and inulin are available as nutritional supplements in capsules, tablets, and as a powder.
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.
- Oligosaccharide synthesis
- Oligosaccharide nomenclature
- Mannan Oligosaccharides, MOS
- Isomaltooligosaccharide, IMO
- Oligosaccharides at the US National Library of Medicine Medical Subject Headings (MeSH)
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