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Oligosaccharides Definition
Oligosaccharides are monosaccharide carbohydrate is smaller than a polysaccharide because it contains fewer monosaccharides. The name oligosaccharide comes from the Greek word oligosaccharides, which means “a few saccharides.”
The unit structure of carbohydrates is referred to as a saccharide. An oligosaccharide is a carbohydrate made up of a small number of saccharides, usually three to ten (mono)saccharide units. An oligosaccharide’s number of monomer units is not precisely specified.
According to some sources, carbohydrates with two to ten monosaccharide units are considered oligosaccharides, which includes disaccharides (which are saccharides made up of two monosaccharide units). Other sources describe oligosaccharides as carbohydrates made up of three to six monosaccharide components.
What are Oligosaccharides?
Carbohydrates are organic molecules that are made up of carbon, hydrogen, and oxygen in a 1:2:1 ratio. They are a type of biomolecule that falls into one of the most important groups. They are a significant energy source. They’re also used as structural elements.
They are divided into two categories as nutrients: simple carbs and complex carbohydrates. Simple carbohydrates, often known as sugar, are carbohydrates that are easily digested and provide a quick source of energy.
Complex carbohydrates (saccharide polymers) take longer to digest and metabolise than simple carbs. They are frequently abundant in fibre and, unlike simple carbs, are less prone to induce blood sugar increases.
Polysaccharides, like other carbohydrates, are made up of hydrogen, carbon, and oxygen. The ratio of hydrogen to oxygen atoms in polysaccharides is generally 2:1, which is why they’re called carbon hydrates.
Oligosaccharides, like other carbohydrates, are organic molecules since they include carbon and C-C and C-H covalent bonds. In contrast to a monosaccharide (which has only one saccharide unit) or a disaccharide (which has just one saccharide unit), an oligosaccharide features a longer chain of saccharide monomeric units (comprised of two saccharides).
Despite this, it is a miniature version of a polysaccharide (which is comprised of more than ten saccharide units). Dehydration synthesis refers to the chemical process of connecting monosaccharide units that results in the release of water as a byproduct.
The oligosaccharide is made up of monosaccharide molecules joined together by glycosidic linkages. Covalent connections between the hydroxyl groups of two monosaccharides are known as glycosidic bonds. In nature, many oligosaccharides are linked to another biomolecule, such as proteins, peptides, or lipids.
Glyconjugates are carbohydrate complexes that are glycosylated and covalently linked to another biomolecule, with the carbohydrate component of the complex usually called a glycan. A glycolipid, for example, is a lipid with a carbohydrate linked to it (such as some oligosaccharides and polysaccharides). A glycoprotein is a protein that has a carbohydrate linked to it.
Oligosaccharides Classification
The number of monosaccharides in oligosaccharides can be used to classify them.
Three monosaccharides make up trisaccharides, which are oligosaccharides. Examples include nigerotriose, which has three glucose units linked together by α(1→3) glycosidic connection, and maltotriose, which has three glucose units linked together by a (1→4) glycosidic linkage, melezitose (glucose-fructose-glucose), maltotriulose (glucose-glucose-fructose), raffinose (galactose-glucose-fructose) and kestose(glucose-fructose-fructose).
Tetrasaccharides are four monosaccharide-based oligosaccharides. Maltotetraose 4 glucose units connected by (1→4) glycosidic linkage and nigerotetraose 4 glucose units joined by α(1→3) glycosidic linkage, lychnose (galactose-glucose-fructose-galactose), nystose (glucose-fructose-fructose-fructose), sesamose (galactose-galactose-fructose-glucose), and stachyose (galactose-galactose-glucose-fructose) are the examples.
Pentasaccharides are sugar molecules that have five sugar units. The majority of N-linked oligosaccharides are pentasaccharides. Six sugar units make up hexasaccharides, which are oligosaccharides. An example is α-cyclodextrin. It is made up of six glucose units connected by α-1, 4 connections. Heptasaccharides have seven sugar units, octasaccharides have eight, nonasaccharides have nine, decasaccharides have 10, and so on.
Oligosaccharides and Glycosylation
Glycosylation is a chemical reaction in which a glycan is attached to a protein, a lipid, or another organic molecule, usually through the action of enzymes. The process of glycosylation differs depending on the type of glycosylation.
N-linked glycosylation, for example, is a kind of glycosylation in which the glycan is connected to a nitrogen atom in a protein’s asparagine or arginine residue. The technique of attaching O-linked glycans to the hydroxyl oxygen of a protein’s serine, threonine, tyrosine, hydroxylysine, or hydroxyproline side chains is referred as O-linked glycosylation.
It might also be the method through which O-linked glycans bind to lipid oxygen. Other types of glycosylation include C-linked (glycan connected to carbon), P-linked (glycan hooked to phosphorus), and S-linked (glycan attached to sulphur) (glycan, to sulfur).
In certain glycoconjugates, oligosaccharides can act as glycans. In this regard, there are two forms of oligosaccharides: (1) N-linked oligosaccharide, which is joined to the asparagine residue by a beta-linkage, and (2) O-linked oligosaccharide, which is coupled to the threonine or serine of a protein.
Oligosaccharides Examples
Raffinose and stachyose are two prevalent oligosaccharides.
Galactose, glucose, and fructose are the three monomers that make up this trisaccharide. C18H32O16 is its chemical formula. As a result, it’s a trisaccharide. It produces D-galactose and sucrose when hydrolyzed by the enzyme -galactosidase. Raffinose can sometimes be abundant in legumes, whole grains, cabbage, brussel sprouts, broccoli, cotton seed, beet root molasses, asparagus, as well as other foods.
Raffinose is just one of the RFOs’ members (raffinose family of oligosaccharides). Other members include tetrasaccharide stachyose, pentasaccharide verbascose, and others. They’re made from sucrose, with galactose moieties from galactinol thrown in for good measure. RFOs are common in plants and act as a desiccant in seeds, a transport sugar in phloem sap, and storage sugars.
Plants have a lot of storage oligosaccharides, such fructooligosaccharides. Fructooligosaccharides (also known as oligofructans) are short chains of fructose residues found in a variety of plants, particularly the blue Agave, the Jerusalem artichoke, and yacón. They’re utilised as sweeteners and food additives in the food industry.
Galacto-oligosaccharides and human milk oligosaccharides are dairy-derived, whereas fructooligosaccharides are plant-derived. Galacto-oligosaccharides are oligosaccharides composed up of galactose molecules in a short chain. Human milk oligosaccharides are primarily made up of 2′-fucosyllactose and are found in breast milk (a trisaccharide composed of fucose, galactose, and glucose units).
Dietary oligosaccharides include fructo-oligosaccharides, galacto-oligosaccharides, and human milk oligosaccharides. They are included in the human diet because of their prebiotic properties.
Biological Importance of Oligosaccharides
Dietary oligosaccharides are an important source of energy. Fructooligosaccharides are a good source of dietary fibre in particular. Plants that generate them (most notably blue agave, Jerusalem artichoke, and yacón) are used to extract them.
Bananas, leeks, onions, garlic, and asparagus are among the edible fruits and vegetables that contain them. They act as storage oligosaccharides in these plants.
They are mostly used as prebiotics in humans. Dietary oligosaccharides produced from dairy, notably galacto-oligosaccharides and human milk oligosaccharides, play a similar effect.
They can’t be digested by humans’ intestines. Instead, these oligosaccharides travel through the large intestine, where they aid in the formation of Bifidobacterium, a kind of colon bacteria. The enzyme fructose-6-phosphate phosphoketolase is present in these anaerobic microorganisms, and it is utilised to ferment them.
Glycan components of glycoproteins are made up of several oligosaccharides. Proteins that are covalently linked to a carbohydrate, particularly an oligosaccharide, are known as glycoproteins. They’re made up of glycosylation. The -OH group of serine or threonine (i.e. O-glycosylated) or the amide NH2 of asparagine attaches the carbohydrate component to the protein (i.e. N-glycosylated). Antigenicity, solubility, cell adhesion, cell recognition, and immunological function are all activities performed by glycoproteins containing oligosaccharides.
Glycolipids are biological entities made up of a covalently bonded carbohydrate (typically an oligosaccharide) and a lipid molecule. The glycoproteins in the cell membrane’s phospholipid bilayer offer structural stability.
Glycoproteins are also involved in cell adhesion, cell recognition, cell communication, and immunological function, among other biological roles. The glycolipids on the surface of erythrocytes determine human blood types (A, B, AB, O).
The blood group antigen is determined by the oligosaccharide component of the glycolipid. N-acetylgalactosamine is found in blood type A, while galactose is found in blood type B. Both antigens are present in blood type AB, but not in blood type O.
Oligosaccharides Citations
- Conformational Studies of Oligosaccharides. Chemistry . 2020 Aug 6;26(44):9814-9825.
- Oligosaccharides and Complex Carbohydrates: A New Paradigm for Cranberry Bioactivity. Molecules . 2020 Feb 17;25(4):881.
- Human milk oligosaccharides: every baby needs a sugar mama. Glycobiology . 2012 Sep;22(9):1147-62.
- Figures are created with BioRender.com
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