Oligosaccharides are basically carbohydrates formed by the union of three to six units of simple sugars or monosaccharides. However, in rare cases, as many as ten units of sugars have been seen to form an Oligosaccharide. They are either formed by combining molecules of monosaccharides or are formed from the breaking of complex sugars called polysaccharides. Very few oligosaccharides are found in nature from plants. To mention a few; Raffinose is formed of 3 molecules of monosaccharides, melibiose, gentianose and fructose. One oligosaccharide is obtained from arthropod’s blood and in a few plants which are Maltotriose which consists of three molecules of glucose. The molecular formula of Oligosaccharide is C37H62N2O29. It has a calorific va;ue of 1.5-2 cal/gm and is usually found in legumes, garlic, pear, watermelon and white onion. Many fruits also contain fructo-oligosaccharide.
In biology, glycosylation is explained as the process where a carbohydrate is covalently attached to an organic molecule by creating structures such as glycolipids and glycoproteins.
N-Linked glycosylation involves the attachment of an oligosaccharide to asparagine via a beta linkage to the side chain's amine nitrogen. The N-linked glycosylation process takes place cotranslationally or concurrently while the proteins are being translated. Since it can be added cotranslationally, it is believed that the N-linked glycosylation helps to determine the polypeptides folding because of the hydrophilic nature of sugars. All the N-linked oligosaccharides are said to be pentasaccharides: with five monosaccharides long.
For eukaryotes in N-glycosylation, the oligosaccharide substrate is assembled right at the endoplasmic reticulum membrane. For prokaryotes, this process takes place at the plasma membrane. In both cases, asparagine residue is an acceptor substrate. The asparagine residue that is linked to an N-linked oligosaccharide usually takes place in the sequence of Asn-X-Ser/Thr, where X can be any amino acid except for proline, although it is very rare to see Asp, Glu, Leu, or Trp in this particular position.
An N-linked oligosaccharide example is given above with GlcNAc, where X is any amino acid except proline.
Oligosaccharides, which participate in the O-linked glycosylation, are attached either to serine or threonine on the hydroxyl group of the side chain. The O-linked glycosylation takes place in the Golgi apparatus, where monosaccharide units can be added to a complete polypeptide chain. Extracellular and cell surface proteins are O-glycosylated. Glycosylation sites in the O-linked oligosaccharides can be determined by both the secondary and tertiary structures of the polypeptide that dictate where glycosyltransferases will add sugars.
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An O-linked oligosaccharide with example β-Galactosyl-(1n3)-α-N-acetylgalactosaminyl-Ser/Thr is given in the above diagram.
All the cells are coated either in glycolipids or glycoproteins, both of which help determine the cell types. Proteins or lectins that bind carbohydrates may recognize the particular oligosaccharides and provide some useful information for cell recognition depending on the oligosaccharide binding.
An important example of oligosaccharide cell recognition is given as the role of glycolipids in blood type determining. Various blood types are distinguished by the modification of glycan that is present on the surface of blood cells. These may be visualized using mass spectrometry. The oligosaccharides that are found on the A, B, and H antigens take place on the non-reducing ends of the oligosaccharide. The H antigen (that indicates an O blood type) serves as a precursor for both the A and B antigens.
Thus, a person with blood type A will have both the A antigen and H antigen present on the glycolipids of the membrane of red blood cell plasma. A person with blood type B will have both the B and H antigens present. And a person with blood type AB will have the three antigens A, B, and H. And finally, a person having blood type O will have only the H antigen. This means that all the blood types contain the H antigen which explains why the blood type O is called the "universal donor."
Several cells produce particular carbohydrate-binding proteins called lectins that mediate cell adhesion with the oligosaccharides. Selectins, which are a family of lectins, mediate certain cell-cell adhesion processes, including those of leukocytes to the endothelial cells. In the immune response, endothelial cells may express certain selectins transiently in response to the injury or damage to the cells.
Also, in response, a reciprocal selectin–oligosaccharide interaction will take place between the two molecules that allow the white blood cell to help to eliminate the damage or infection. Often, Protein-Carbohydrate bonding is mediated by the van der Waals forces and hydrogen bonding.
Types of Oligosaccharides
Let us look at the types of oligosaccharides in detail.
Oligosaccharides are classified into three types according to their number of monosaccharide units.
Disaccharides - These are the sugars having two monomeric units, and thus it is called di_saccharide. Some examples are maltose, sucrose, and lactose. Maltose is the action of an enzyme and it gives glucose +glucose; Sucrose (or cane sugar) in the action of invertase produces fructose + glucose, and Lactose(milk sugar) in the action of enzyme lactase produces galactose + glucose.
Trisaccharides - These contain three monomers like raffinose.
Tetrasaccharides - These contain four monomeric units like stachyose.