Oligosaccharide

Oligosaccharide
n., plural: oligosaccharides
[ɒlɪɡəʊˈsækəɹaɪd]
Definition: A carbohydrate made up of a small number of monosaccharide and thus smaller than a polysaccharide

Table of Contents

Carbohydrates are organic compounds consisting of carbon, hydrogen, and oxygen, usually in the ratio of 1:2:1. They are one of the major classes of biomolecules. They are an important source of energy. They also serve as structural components. As a nutrient, they can be classified into two major groups: simple carbohydrates and complex carbohydrates.

Simple carbohydrates — sometimes simply referred to as sugar — are those that are readily digested and serve as a rapid source of energy. Complex carbohydrates (saccharide polymers) are those that need more time to be digested and metabolized. They often are high in fiber and, unlike simple carbohydrates, they are less likely to cause spikes in blood sugar.

Oligosaccharide Definition

Etymologically, oligosaccharide means a few saccharides. A saccharide refers to the unit structure of carbohydrates. Thus, an oligosaccharide is a carbohydrate comprised of a few saccharides, i.e. about three to ten (mono)saccharide units. The number of monomer units in an oligosaccharide is not rigorously defined.

There are references that consider carbohydrates comprised of two to ten monosaccharide units as an oligosaccharide, thus, including disaccharides (which are saccharides made up of two monosaccharide units). There are other references that define an oligosaccharide as carbohydrates consisting of three to six monosaccharide constituents as oligosaccharides.

Etymology: Ancient Greek ὀλίγοι (olígoi, meaning “few”) + saccharide
Synonyms: double sugar; biose
Compare: monosaccharide, polysaccharide

Characteristics of Oligosaccharides

Similar to other carbohydrates, oligosaccharides are made up of hydrogen, carbon, and oxygen, and the ratio of hydrogen atoms to oxygen atoms is often 2:1, which explains why they are referred to as hydrates of carbon. And because of the presence of carbon and C-C and C-H covalent bonds, oligosaccharides, just as the other carbohydrates are organic compounds.

An oligosaccharide, though, has a longer chain of saccharide monomeric units than a monosaccharide (comprised of only one saccharide unit) or a disaccharide (comprised of two saccharides). Nonetheless, it is relatively smaller than a polysaccharide (which is comprised of more than ten saccharide units).

The chemical process of joining monosaccharide units is referred to as dehydration synthesis since it results in the release of water as a byproduct. The oligosaccharide is formed by the joining of monosaccharide units via glycosidic bonds. Glycosidic bonds are covalent bonds that may form between the hydroxyl groups of two monosaccharides.

Many of the naturally-occurring oligosaccharides are linked to other biomolecules, such as proteins, peptides, and lipids. Carbohydrates that are covalently linked to another biomolecule via glycosylation are referred to as glycoconjugates and the carbohydrate constituent of the complex is called a glycan. For instance, a glycolipid is a carbohydrate (e.g. certain oligosaccharides and polysaccharides) attached to a lipid. A glycoprotein is a carbohydrate attached to a protein.

Classifications of Oligosaccharides

Oligosaccharides may be classified based on the number of monosaccharides they contain. Some of them are described below.

Trisaccharides are oligosaccharides comprised of three monosaccharides. Examples include:

nigerotriose – 3 glucose units joined by α(1→3) glycosidic linkage
maltotriose – 3 glucose units joined by (1→4) glycosidic linkage
melezitose (glucose-fructose-glucose)
maltotriulose (glucose-glucose-fructose)
raffinose (galactose-glucose-fructose)
kestose (glucose-fructose-fructose).

Tetrasaccharides are oligosaccharides comprised of four monosaccharides. Examples are:

nigerotetraose 4 glucose units joined by α(1→3) glycosidic linkage
maltotetraose 4 glucose units joined by (1→4) glycosidic linkage
lychnose (galactose-glucose-fructose-galactose)
nystose (glucose-fructose-fructose-fructose)
sesamose (galactose-galactose-fructose-glucose)
stachyose (galactose-galactose-glucose-fructose)

Pentasaccharides are those comprised of five sugar units. N-linked oligosaccharides are mostly pentasaccharide.

Hexasaccharides are oligosaccharides comprised of six sugar units. α-Cyclodextrin is an example. It consists of six glucose units linked via α-1, 4 linkages.

Heptasaccharides are oligosaccharides containing seven sugar units, octasaccharides contain eight, nonasaccharides have nine, decasaccharides have ten, and so on.

Glycosylation

Glycosylation refers to a process where a glycan is bound to a protein, a lipid, or another organic molecule, especially through the action of certain enzymes. The step-by-step processes of glycosylation vary, depending on the form of glycosylation.

For instance, N-linked glycosylation is a form of glycosylation where the glycan is attached to a nitrogen atom of asparagine or arginine residue of a protein.

Conversely, O-linked glycosylation is a process where O-linked glycans are attached to the hydroxyl oxygen of serine, threonine, tyrosine, hydroxylysine, or hydroxyproline side chains of a protein. It may also be the process where the O-linked glycans attach to the oxygen on lipids.

Other forms of glycosylation exist, such as C-linked (i.e. glycan attached to carbon), P-linked (i.e. glycan, to phosphorus), and S-linked (glycan, to sulfur). Oligosaccharides may serve as glycans in certain glycoconjugates.

Two major types of oligosaccharides in this regard are:

(1) N-linked oligosaccharide, wherein the oligosaccharide is attached to the asparagine residue by a beta-linkage

and

(2) O-linked oligosaccharide, wherein the oligosaccharide is attached to the threonine or serine of a protein.

Common Oligosaccharides

Here are examples of common oligosaccharides:

Raffinose family of oligosaccharides

Raffinose is a trisaccharide formed from the combination of three monomers: galactose, glucose, and fructose. It has a chemical formula of C₁₈H₃₂O₁₆. Thus, it is a trisaccharide. When hydrolyzed with enzyme α-galactosidase, it yields D-galactose and sucrose.

Raffinose occurs in legumes, whole grains, cabbage, brussel sprouts, broccoli, cottonseed, molasses of beetroot, asparagus, etc. Raffinose is just one of the members of the RFOs (raffinose family of oligosaccharides). Other members include stachyose (a tetrasaccharide), verbascose (a pentasaccharide), etc.

They are formed from sucrose with the ensuing addition of galactose moieties donated from galactinol. RFOs are abundant in plants and serve as desiccation protectants in seeds and as storage sugars. It is also for transporting sugar in the phloem sap.¹

Fructooligosaccharides

Storage oligosaccharides, such as fructooligosaccharides, are common in plants. Fructooligosaccharides (also called oligofructans) are short chains of fructose residues found in many plants, especially in blue Agave plant, Jerusalem artichoke, and yacón. Commercially, they are used as sweeteners and food additives.

Galactooligosaccharides

While fructooligosaccharides are plant-derived, galactooligosaccharides and human milk oligosaccharides are dairy-derived. Galacto-oligosaccharides are oligosaccharides made up of a short chain of galactose molecules. Human milk oligosaccharides occur in breast milk and are mainly composed of 2′-fucosyllactose (a trisaccharide comprised of fucose, galactose, and glucose units).

Biological Importance

Fructooligosaccharides, galactooligosaccharides, and human milk oligosaccharides are examples of dietary oligosaccharides. They are integrated into the human diet for their prebiotic effect. Dietary oligosaccharides are an essential energy source. Fructooligosaccharides, in particular, are an important source of dietary fiber. They are extracted from plants that produce them (chiefly by blue Agave, Jerusalem artichoke, and yacón).

They are also present in certain edible fruits and vegetables, such as bananas, leeks, onions, garlic, and asparagus. In these plants, they serve as storage oligosaccharides. In humans, they are employed chiefly as prebiotics.

Dairy-derived dietary oligosaccharides, particularly galactooligosaccharides and human milk oligosaccharides, have also the same role. Human intestines cannot digest them. Rather, these oligosaccharides are passed through the large intestine where they promote the growth of colon bacteria, Bifidobacterium. These anaerobic microbes have fructose-6-phosphate phosphoketolase, which is the enzyme used to ferment them.

Many oligosaccharides serve as the glycan component of glycoproteins. Glycoproteins are proteins covalently attached to a carbohydrate, especially an oligosaccharide. They are formed from glycosylation. The carbohydrate constituent is attached to the protein via the -OH group of serine or threonine (i.e. O-glycosylated) or via the amide NH₂ of asparagine (i.e. N-glycosylated). Glycoproteins containing oligosaccharides are involved in different biological functions, such as antigenicity, solubility, cell adhesion, cell recognition, and immune function.

Glycolipids are biological structures comprised of a carbohydrate, usually an oligosaccharide, covalently linked to a lipid molecule. In the phospholipid bilayer of the cell membrane, the glycoproteins provide structural stability. Apart from that, glycoproteins are also involved in other biological functions, e.g. cell adhesion, cell recognition, cell signaling, and immune function.

Human blood types (A, B, AB, O) are based on the glycolipids on the surface of erythrocytes. The oligosaccharide component of the glycolipid determines the blood group antigen. For example, blood type A has N-acetylgalactosamine, and blood type B has a galactose. Blood type AB has both antigens whereas blood type O lacks these antigens.

Answer the quiz below to check what you have learned so far about oligosaccharides.

Reference

Sengupta,S., Mukherjee, S., Basak, P., and Majumder, A.L. (2015). Significance of galactinol and raffinose family oligosaccharide synthesis in plants. Front Plant Sci. 6: 656. doi: 10.3389/fpls.2015.00656