How Do Oligosaccharides Function in Living Organisms?
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.
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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: Definition, Types, and Biological Role
1. What are oligosaccharides and how are they classified?
Oligosaccharides are carbohydrates formed when 2 to 10 monosaccharide units are joined together by glycosidic linkages. Upon hydrolysis, they yield their constituent monosaccharides. They are primarily classified based on the number of monosaccharide units they contain:
Disaccharides: Composed of two monosaccharide units (e.g., sucrose, lactose).
Trisaccharides: Composed of three monosaccharide units (e.g., raffinose).
Tetrasaccharides: Composed of four monosaccharide units (e.g., stachyose).
2. What is a glycosidic linkage in the context of oligosaccharides?
A glycosidic linkage is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group. In oligosaccharides, this bond forms between the anomeric carbon of one monosaccharide and a hydroxyl group of another. The formation of this linkage occurs through a dehydration reaction, involving the loss of a water molecule, and it is fundamental for connecting individual sugar units to build the oligosaccharide chain.
3. Describe the composition of sucrose, lactose, and maltose as per the CBSE syllabus.
Sucrose, lactose, and maltose are important disaccharides with the following compositions:
Sucrose: Commonly known as table sugar, it is composed of one unit of α-D-glucose and one unit of β-D-fructose linked together.
Lactose: Also known as milk sugar, it is made up of one unit of β-D-galactose and one unit of β-D-glucose.
Maltose: Referred to as malt sugar, it consists of two units of α-D-glucose.
4. What is the primary biological role of oligosaccharides?
The primary biological role of oligosaccharides is in cell recognition and cell adhesion. When attached to proteins and lipids on the outer surface of cell membranes, they form glycoproteins and glycolipids. These complex molecules act as unique cell markers or identifiers, allowing cells to recognise and interact with each other. This function is critical for processes like immune responses, fertilisation, and the organisation of cells into tissues.
5. Why is sucrose considered a non-reducing sugar while lactose and maltose are reducing sugars?
This distinction is based on the availability of a free hemiacetal group in the molecule's structure.
In lactose and maltose, one of the monosaccharide units retains a free hemiacetal group. This group can open up to form a reactive aldehyde group, which can be oxidised, thereby making them reducing sugars.
In sucrose, the anomeric carbons of both the glucose and fructose units are involved in forming the glycosidic bond. Consequently, there are no free hemiacetal groups available to open into an aldehyde form. Therefore, sucrose cannot be oxidised and is classified as a non-reducing sugar.
6. How do oligosaccharides contribute to determining a person's blood type?
Human blood types (A, B, AB, O) are determined by specific oligosaccharides attached to proteins and lipids on the surface of red blood cells, which function as antigens.
Type O has a fundamental core oligosaccharide structure.
Type A has an additional N-acetylgalactosamine unit attached to this core.
Type B has an additional galactose unit attached to the core.
Type AB has both the Type A and Type B oligosaccharide structures.
The immune system creates antibodies against the oligosaccharide antigens that are not present on its own cells.
7. Explain the importance of glycoproteins and glycolipids.
Glycoproteins and glycolipids are vital biomolecules formed when oligosaccharide chains are covalently bonded to proteins and lipids, respectively.
Glycoproteins are crucial for cell-to-cell communication, acting as cell surface receptors, immunoglobulins (antibodies), and some hormones. They also play a role in ensuring the correct folding and stability of proteins.
Glycolipids are key components of cell membranes, enhancing stability. Their most significant role is in cell recognition, where they function as surface markers that allow cells to identify each other, an essential process for tissue formation and immune responses.
8. Why are many oligosaccharides considered prebiotics?
Many oligosaccharides, such as Fructo-oligosaccharides (FOS), are classified as prebiotics because they resist digestion by human enzymes in the stomach and small intestine. They travel intact to the large intestine, where they selectively nourish and stimulate the growth of beneficial gut bacteria, like Bifidobacteria and Lactobacilli. The fermentation of these oligosaccharides by the gut microbiota helps maintain a healthy gut microbiome and produces beneficial short-chain fatty acids (SCFAs).
9. Why can consumption of certain oligosaccharides, like those in beans, cause bloating and gas?
This occurs because the human digestive system lacks the specific enzymes (e.g., α-galactosidase) needed to break down complex oligosaccharides like raffinose and stachyose, which are abundant in beans, lentils, and other legumes. When these undigested sugars reach the large intestine, they are fermented by resident gut bacteria. This bacterial fermentation process produces gases such as hydrogen, carbon dioxide, and methane, which can accumulate and cause symptoms like bloating, flatulence, and abdominal discomfort.
