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Oligosaccharide

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What is Oligosaccharide?

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Oligosaccharides are defined as any carbohydrate of from 3-6 units of simple sugars (called monosaccharides). A huge count of oligosaccharides has been partially prepared by breaking down several complex carbohydrates (called polysaccharides). Many few naturally occurring oligosaccharides can be found in plants. Raffinose, which is a trisaccharide, can be found in several plants that consist of melibiose (glucose and galactose) and fructose. Gentianose is the other plant trisaccharide. Maltotriose, which is a trisaccharide of glucose, takes place in some plants and certain arthropod's blood.

Glycosylation

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 endoplasmatic 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.

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An N-linked oligosaccharide example is given above with GlcNAc, where X is any amino acid except proline.

O-Linked Oligosaccharides

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 the example β-Galactosyl-(1n3)-α-N-acetylgalactosaminyl-Ser/Thr is given in the above diagram.

Functions

Cell Recognition

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 antigen 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 antigen 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 that explains why the blood type O is called the "universal donor."

Cell Adhesion

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.

They are:

  • Disaccharides,

  • Trisaccharides, and

  • Trisaccharides.

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.

FAQ (Frequently Asked Questions)

1. What are Glycolipids?

Answer: Glycolipids are essential for cell recognition and for modulating the function of membrane proteins, which act as receptors. Generally, glycolipids are the lipid molecules bound to oligosaccharides, which present in the lipid bilayer. In addition, they may serve as receptors for cell signalling and cellular recognition. The oligosaccharide head serves as a binding partner in the receptor activity. The receptors' binding mechanisms to the oligosaccharides depend on the oligosaccharide composition presented or exposed above the membrane surface.

2. Give the Side Effects of Oligosaccharides?

Answer: Since maltotriose oligosaccharides are indigestible, humans lack the enzymes necessary to break them down in the small intestine. So, they reach the large intestine, where the beneficial colonic bacteria break them down (into ferment) to the absorbable nutrients that provide some energy–about 2 Calories (kilocalories) per one gram on average. Certain breakdown products of the oligosaccharides–namely short-chain fatty acids (SCFAs) – can have a beneficial effect on the large intestinal lining.

3. What are Glycoproteins?

Answer: Glycoproteins have distinct Oligosaccharide structures that have significant effects on several of their properties, affecting the critical functions such as solubility, resistance, and antigenicity to proteases. Also, glycoproteins are relevant as cell-adhesion molecules, cell-surface receptors, tumour antigens, and immunoglobulins.

4. Explain the role of Oligosaccharides as Prebiotics?

Answer: Prebiotics are non-digestible nutrients, which selectively promote the normal intestinal bacteria growth that may have beneficial effects on the large intestinal lining. Currently, oligosaccharides considered as prebiotics include oligofructose or fructooligosaccharides or fructooligosaccharide (FOS) and trans-galactooligosaccharides (TOS).