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An oligosaccharide is a saccharide polymer with a minimal number of monosaccharides.
Glycosidic linkages can connect monosaccharides because sugars have a lot of hydroxyl groups. Oligosaccharides consist of two or more monosaccharides linked together by O-glycosidic linkages.
Most oligosaccharides with three or more units do not exist as free molecules in cells. They are present as glycoconjugates with non-sugar molecules (lipids or proteins).
The standard nomenclature for disaccharides and oligosaccharides specify the following:
order of monosaccharide units,
the arrangement at each anomeric carbon, and
the carbon atoms involved in the glycosidic bond
Properties of Oligosaccharides
Oligosaccharides, like other carbohydrates, are organic substances since they include carbon and C-C and C-H covalent bonds. Unlike mono- or disaccharides, an oligosaccharide has a longer chain of saccharide monomeric units. Despite this, it is much smaller than a polysaccharide.
Many oligosaccharides found in nature are related to another biomolecule, such as proteins, peptides, or lipids. Glycoconjugates are carbohydrate complexes that have covalent bonds with another biomolecule via glycosylation. In such bonds, the carbohydrate component of the complex is called a glycan.
In this sense, there are two major types of oligosaccharides:
N-linked oligosaccharide, with a beta-linkage connecting the oligosaccharide to the asparagine residue.
O-linked oligosaccharide, in which the oligosaccharide links to a protein’s threonine or serine.
Types of Oligosaccharides
One can use the number of monosaccharides in oligosaccharides to classify them. For example:
When two monosaccharides bond by glycosidic linkage, they create a disaccharide. Disaccharides, like monosaccharides, are simple sugars that dissolve in water. Sucrose, lactose, and maltose are three frequent examples.
Three monosaccharides make up trisaccharides, which are oligosaccharides. nigerotriose, maltotriose, melezitose, maltotriose, raffinose.
Tetrasaccharides are four monosaccharide-based oligosaccharides, nigerotetraose, maltotetraose, lychnose, nystose, sesamose.
Sucrose
Sucrose, -D-glucopyranosyl -D-fructofuranoside is also called cane sugar.
Sucrose comprises six-carbon sugars D-glucose and D-fructose and is a common and abundant disaccharide. It is a glycosyl glycoside. An acetal oxygen bridge from glucose’s hemiacetal bonds to fructose’s hemiketal in sucrose.
Sucrose is an essential photosynthetic intermediate product; in many plants, it is the primary mode of sugar transfer from the leaves to other regions of the plant body. In the cytosol, two enzymes, sucrose-phosphate synthase (SPS) and sucrose phosphatase (SP), work together to produce sucrose (SPP).
Sucrose synthase (SucS) catalyses the synthesis and breakdown of sucrose and is present in soluble and membrane-bound forms. SucS in sink tissues is primarily responsible for sucrose degradation.
Sucrose is a non-reducing sugar because it has no free anomeric carbon atoms. The anomeric carbons of both monosaccharide units are involved in the glycosidic bond.
Lactose
Lactose is a disaccharide containing galactose and glucose subunits. Galactose and glucose combine to produce -14 glycosidic linkage. -D-galactopyranosyl -(14)-D-glucose is its scientific name.
-lactose and -lactose refers to the anomeric form of the glucopyranose ring alone. Glucose can be in either the -pyranose form or the -pyranose form, whereas galactose can only be in the -pyranose form.
Lactose is a key component of both human and animal milk. Lactose is an energy carrier in milk because it is a disaccharide of glucose and galactose.
E. coli’s lactose transporter (lactose permease) is a well-studied proton-driven cotransporter prototype. It belongs to the major facilitator superfamily (MFS).
This protein has a single polypeptide chain (417 residues) that works as a monomer to transport one proton and one lactose molecule into the cell, resulting in a net lactose buildup. By oxidizing fuels and utilizing the oxidation to push protons outward, E. coli develops a gradient of protons and charges across its plasma membrane.
The lipid bilayer is impervious to protons, yet the lactose transporter allows proton reentry, and lactose is transported into the cell via symport simultaneously. With a negative overall free-energy change, the endergonic buildup of lactose is related to the exergonic flow of protons into the cell.
The lactose operon (also known as the lac operon) is a group of genes present in E. coli and other bacteria responsible for the uptake and metabolism of lactose.
The genes for -galactosidase (Z), galactoside permease (Y), and thiogalactoside transacetylase (A) are all found in the lactose (lac) operon. A ribosome binding site precedes each of the three genes, which guides the translation of that gene individually. The lac repressor protein is responsible for lac operon regulation (Lac).
The absence of lactose inhibits the lac operon genes. Mutations in the operator or another gene, the I gene, causes constitutive synthesis of gene products. One can restore the repression when the I gene is deficient by inserting a functioning I gene on another DNA molecule into the cell.
This aspect suggests that the I gene encodes a diffusible chemical called the Lac repressor, which induces gene repression. This protein is a tetramer of identical monomers.
The Lac repressor appears to bind to both the main operator and one of the two secondary sites to repress the operon, with the intervening DNA looped out. Either binding arrangement prevents transcription initiation.
The lac operon is triggered when cells receive lactose. When a signal molecule binds to a specific location on the Lac repressor, it causes a conformational shift that causes the repressor to dissociate from the operator. The lac operon system’s inducer is allolactose, an isomer of lactose, rather than lactose itself.
Maltose
Malt sugar, also known as maltose, is a byproduct of starch hydrolysis catalyzed by the enzyme -amylase. Two D-glucose residues connect by a glycosidic bond between C-1 of one glucose residue and C-4 of the other in maltose. Its other name is -D- glucopyranosyl-(14- D-glucopyranose).
Lactase, maltase, sucrase-isomaltase, and trehalase are enzymes that break down disaccharides and other short glucose polymers into monosaccharides at the brush boundary of the small intestine. Maltase breaks down maltose into glucose monomers.
Brewing, baking, soft drink, canning, confectionery, and other culinary sectors employ a variety of maltose-containing syrups. High-maltose syrup (45% – 60%), extremely high-maltose syrup (70% – 85%), and high conversion syrup are the three varieties of maltose-containing syrups (30% – 47% ).
Fructo-oligosaccharide
Fructo-oligosaccharides are plant sugar chains derived from asparagus, Jerusalem artichokes, and soybeans. One can also produce fructo-oligosaccharides in the lab. Medicine manufacture employs these sugars. Drugs for treating constipation contain fructooligosaccharides.
Conclusion
A saccharide polymer with a minimal number of monosaccharides is known as an oligosaccharide. Cell recognition and cell binding are two roles that oligosaccharides can perform.
