What is Oligosaccharide?
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.
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 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.
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.
(Image will be uploaded soon)
An O-linked oligosaccharide with 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 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."
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.
FAQs on Oligosaccharide
1. What is the importance of oligosaccharides?
Oligosaccharides are used to recognize a cell type. The nature of the oligosaccharide which is attached to the cell protein determines the nature and recognition of the cell. The transporting vesicles transport the proteins based on this recognition. It thus plays a key role in determining the blood type in individuals based on the type of oligosaccharide present on the surface of the RBC or the antigen present in the individual. It also helps in cell adhesion via the formation of lectins which is a type of carbohydrate that can bind proteins.
2. What are the oligosaccharides included in our diets?
A variety of oligosaccharides find a place in our regular diet since they have a high nutritional value. They are also considered prebiotics which are responsible for the improvement of gut health and linings of the colon. They can be listed as follows:
Fructo-oligosaccharides are found in many vegetables and fruits which we include in our regular diet.
Galacto-oligosaccharides is an essential component of human milk on which infants mainly survive. It is also found in legumes.
Fructan type oligosaccharide has high nutritional value and is included in our regular foods like wheat, oats, banana, garlic, onion, asparagus and many other food items.
Oligosaccharides are in fact much better than simple, short-chained carbohydrates which often result in the induction of irritable bowel syndrome in the human body.
3. Why are Oligosaccharides not as effective as they should be for the human body?
Oligosaccharides have immense nutritional value and are good for the human diet when the simple sugars cause irritable bowel syndrome and other problems. However, some limitations remain in the intake of oligosaccharides in the human body. Oligosaccharides consist of many units of sugar making them a complex structure and not easily digestible in the human body. The human body lacks such enzymes which can break the units of the oligosaccharides in the intestine and digest them completely. Only in the large intestine where they promote the growth of some beneficial bacteria called bifidobacteria can break them to absorb the nutrients. This results in only absorption of very low calories from the oligosaccharides. However, they improve gut health by generating beneficial bacteria in the large intestine.
4. What is the meaning of the term Glycosylation?
Glycosylation is an important process in biology that deals with the linking of carbohydrates to organic molecules resulting in the formation of glycoproteins and glycolipids. Glycosylation can result in the formation of two types of oligosaccharides:
N-linked oligosaccharides are formed from N-linked glycosylation which occurs co-transitionally when the proteins are translated. It involves the attachment of oligosaccharides to asparagine using the Nitrogen bond.
O-linked oligosaccharides involve the participation of O-linked glycosylation attached to serine or threonine. This type of glycosylation usually takes place in the Golgi apparatus.
Thus the monosaccharide units combine or unite to form the complete polypeptide chains.
5. Define glycolipids and glycoproteins in the context of oligosaccharides.
Glycolipids and glycoproteins are basically lipid and protein molecules covalently attached to oligosaccharides. The oligosaccharides are attached to the proteins or lipids either via the Oxygen atom or the N-atom of serine or asparagine respectively. Glycolipids are a composition of a fatty acid with a carbohydrate moiety while glycoprotein is the union of a protein molecule with a carbohydrate moiety which may be fructose, glucose or galactose. They have different behaviors which affect their solubility and immunogenicity to the enzymes called proteases. Glycoproteins are essential adhesive aids in keeping together tissues in order to arrange them in the form of a proper system. The role of glycoprotein in the immunity of the body is also indispensable due to its contribution to cell surface receptors and immunoglobulins.
6. What are Glycolipids?
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.
7. Give the Side Effects of Oligosaccharides?
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.
8. What are Glycoproteins?
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.
9. Explain the role of Oligosaccharides as Prebiotics?
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).