An oligosaccharide (from the Greek ολίγος oligos, a few, and σάκχαρ sacchar, sugar) is a saccharide polymer containing a small number (typically three to nine) of simple sugars (monosaccharides). Oligosaccharides can have many functions; for example, they are commonly found on the plasma membrane of animal cells where they can play a role in cell-to-cell recognition.
In general, they are found either O- or 'N-linked to compatible amino acid side-chains in proteins or to lipid moieties (see glycans). N-linked Oligosaccharides are found attached to asparagine via beta linkage to the amine nitrogen of the side chain. Alternately, O-linked oligosaccharides are generally attached to threonine or serine on the OH group of the side chain.
Role in Glycosylation
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.
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 pentasaccharide.
O-linked glycosylation occurs in the golgi apparatus, in which monosaccharide units are added to a complete polysaccharide chain. 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.
Glycoproteins and Glycolipids
Both glycoproteins and glycolipids have a covalently attached carbohydrate attached to their respective molecule. Each plays an important role for the overall mechanisms involved within the organism. Glycoproteins have distinct Oligosaccharide structures which contribute greatly to various properties of the glycoproteins. It is these properties that become important to the overall development of glycoprotein products for critical functions such as antigenicity, solubility, and resistance to proteases.
Glycolipids are used for energy as well as 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. Their physical properties dictate the joint activity of the protein membrane and receptor. 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 what elements of an oligosaccharide sugar are exposed/presented out of the membrane. The exposure of these glycolipids can also serve as a portal for viral and toxic substances, which bind to certain receptors on the cell surface and initiate an invasion of the toxin into the cell.
Many cells produce specific carbohydrate-binding ligands, known as lectins, which mediate cell-adhesion with oligosaccharides. Selectins 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.
All cells are coated in either glycoproteins or glycolipids, both of which help determine cell types. Lectins, or proteins that bind carbohydrates, 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. This means all blood types have the H antigen, and explains why the O blood type is known as the “universal donor”.
Role in mother-to-child transmission of HIV-1
Breast-feeding is the highest prevalence factor of postnatal transmission of HIV-1. However, most breast-fed infants do not contract the virus from the infected mother despite the continuous exposure. The Lewis antigen glycans in human milk compete with HIV-1 glycoprotein-120 and binds to the dendritic cell-specific ICAM3-grabbing non-integrin (DC-SIGN) on human dendritic cells. The glycans inhibit the transfer of the virus to the CD4 T lymphocytes. Because the human milk oligosaccharides carry one more of the Lewis antigen glycan, it's hypothesized that they do compete with gp120 for binding.
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.
Oligosaccharides are often found as a component of glycoproteins or glycolipids and as such are often used as chemical markers, often for cell recognition. An example is ABO blood type specificity. A and B blood types have two different oligosaccharide glycolipids embedded in the cell membranes of the red blood cells, AB-type blood has both, while O blood type has neither.
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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