Oligosaccharide

An oligosaccharide (

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 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]}

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]}

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]}

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.

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

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.

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]}

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]

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]

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