Carbohydrates are biological molecules which mainly provide energy for different metabolic activities to living cells. Some common examples of carbohydrates are glucose, fructose, maltose, sucrose, lactose etc. Carbohydrates can be classified in a different manner such as:
- On the basis of taste
- On the basis of hydrolysis product
- On the basis of origin
On the basis of origin, carbohydrates can be classified as plants and animal carbohydrates. On the basis of taste, carbohydrates can be classified sugar and non-sugar molecules. Sugar molecules are sweet molecules such as fructose, glucose etc. Fructose has maximum value of sweetness and can be considered as sweetest sugar. Sugar molecules are soluble in water. On the contrary, non-sugar molecules are not sweet and insoluble in water such as starch, cellulose etc.
On the basis of hydrolysis product, carbohydrates can be classified as a monosaccharide, oligosaccharide and polysaccharide. Monosaccharide units are the simplest carbohydrates which cannot be hydrolysis further into simple products such as glucose and fructose. Around 2-10 monosaccharide units are polymerised and bonded together by glycosidic linkages to form oligosaccharides such as maltose, lactose etc. Similarly polysaccharides are composed of more than 100 units of monosaccharide. Let’s discuss some common examples of oligosaccharides.
All carbohydrates are bio polymers which composed of certain number of monomer units known as monosaccharides
. Hence monosaccharides like glucose, galactose are the simplest units of carbohydrates. If a relatively small number of monosaccharide units generally from 2-10 units are bonded to form carbohydrates, they are called as oligosaccharides
Monosaccharide units are bonded with glycosidic linkage
by the elimination of water molecules, hence through condensation reactions. Oligosaccharide name derived from Greek word oligos; a few and sacchar stand for sugar. Oligosaccharides are commonly found on the plasma membrane of animal cells and play a role in cell–cell recognition. They are also found in proteins or to lipid moieties either O- or N-linked to compatible amino acid. Oligosaccharides mainly found in on the plasma membrane of animal cells and can play a role in cell-cell recognition.
In oligosaccharides, the monomer residues are bonded through glycosidic linkage and can be hydrolyzed by acid to form constituent monosaccharide units. Generally 2-10 monosaccharide units are bonded in oligosaccharide via condensation reaction. Some of the common examples of oligosaccharides are maltose, sucrose, and lactose. All these are trivially named but still used today.
- The condensation process takes place between carbonyl group (>C=O) of one monosaccharide unit and hydroxyl group(-OH) of another residue with the elimination of water molecule results to form glycosidic linkage.
- Glycosidic linkage can be locked alpha or beta configuration of monosaccharide residues.
- For the representation of glycosidic linkage, the carbon atoms which are involved in the bond are listed with the configuration of monosaccharide units.
- For example, If the carbon-1 (carbonyl group) of one α-monosaccharide molecule and carbon-4 of the other α-monosaccharide molecule are bonded to get α-1,4 (or α(1→4)) glycosidic bond.
- The presence of free carbonyl group makes the sugar reducing in nature. Monosaccharides are reducing sugar due the presence of free aldehyde or ketone group in molecule.
- The reducing end involves in the formation of glycosidic linkage with other monosaccharide units. Hence some of the oligosaccharides are reducing in nature while some are non-reducing in nature.
- For example; In sucrose both the aldehyde group of the glucose and the ketone group of the fructose are involved in the α(1→2) linkage, therefore the molecule is non-reducing in nature.
- The hydrolysis of sucrose in the presence of enzyme invertase form glucose and fructose which have free carbonyl group, hence reducing in nature.
On the other hand, Maltose and lactose are reducing sugars. Let’s discusses some of the oligosaccharides with their structure;
1. Maltose and Cellibiose
Both of these oligosaccharides are composed of two glucose units, hence they are disaccharides which are differ in glycosidic linkage. In maltose, there is a α(1→4)glycosidic linkage
and named as α-D-Glucopyranosyl-(1→4)-β-D-glucopyranose
. While in Cellibiose, both glucose units are bonded with β(1→4) link.
That is the reason, maltose contains one free carbonyl group and a reducing sugar while Cellibiose is a non-reducing one.
Sucrose is another example of oligosaccharide which composed of glucose and fructose connected together with an α(1→2) bond and named as α-D-glucopyranosyl-(1→2)-β-D-fructofuranosid.
In each unit of sucrose, the C-1 of one α-glucose bonded with C-2 of β-fructose unit through α(1→2) glycosidic linkage.
Since the carbonyl group of both monosaccharide involve in glycosidic linkage, sucrose is an example of non-reducing sugar.
Lactose is a disaccharide composed of galactose and glucose, which are bonded through β-1→4 glycosidic linkage
, hence the systematic name of lactose is β-D-galactopyranosyl-(1→4)-D-glucose
. The glucose residue can be in any configuration; alpha or beta, whereas the galactose can only have the β-pyranose form. Hence the presence of alpha or beta-glucopyranose makes two different configurations of lactose, α-lactose and β-lactose
Another example of oligosaccharide is raffinose which is a trisaccharide
composed of galactose, glucose and fructose
residues. In each unit of reffinose, galactose connected to glucose unit through a 1α→6 glycosidic linkage
Other example of Trisaccharide is Maltotriose composed of three glucose molecules which are linked with α-1,4 glycosidic bonds
Melibiose is a disaccharide composed of galactose and glucose residue which are linked together with a β(1→6) glycosidic bond((D-Gal-α(1→6)-D-Glc).
The hydrolysis of raffinose produced melibiose and fructose in the presence of invertase. Enzyme Alpha-galactosidase
involves in the hydrolysis of melibiose into its component saccharides, glucose and galactose.
Some other examples of oligosaccharides are as follow.
- Maltotriose: Trisaccharide with three glucose molecules linked together with an α(1→4) glycosidic linkage.
- Maltotetralose: Tetrasaccharide with four glucose molecules linked with an α(1→4) glycosidic linkage.
- Maltopentalose: Pentasaccharide with five glucose molecules linked with an α(1→4) glycosidic linkage.
- Linear dextrins: Hexasaccharide with six or more glucose molecules linked with α(1→4) glycosidic linkage.
- Limit dextrins: Consist of small branched glucose chains linked with α(1→6) glycosidic linkage.
Some of the oligosaccharides with their glycosidic linkage are as follow.
The oligosaccharide chains which are also called as glycans have many biological functions. They attached to cell surface and extracellular proteins and lipids are known to mediate many important biological roles. The well known biological functions of glycans are overlapping and numerous which based on the structural and physical effects of the molecule.
||Common table sugar
||Glucose and fructose
||Product of starch hydrolysis
||Two glucose units
||Found in fungi
||Two glucose units
||Main sugar in milk
||Galactose and glucose
||Found in legumes
|| Galactose and glucose
|Cellibiose ||Building block of cellulose ||Two glucose units ||glucose-1β-4-glucose |
|Laltotriose ||Produced by alpha amylase (a common enzyme in human saliva) on amylose in starch ||Three glucose units ||α-1,4-glycosidic bonds |
|Raffinose ||Found in beans and vegetables like cabbage, brussel, sprouts, broccoli, asparagus ||Glucose, galactose and fructose ||1α-6-glycosidic linkage |
|Melibiose ||Produced by hydrolysis of raffinose ||Galactose and glucose residue ||β(1,6) glycosidic bond |
|Maltotriose ||Can be fermented by both alge and lager yeast ||Three glucose units ||α(1,4) glycosidic linkage |
|Maltotetralose ||Cannot be fermented by alge or lager yeast ||Four glucose units ||α(1,4) glycosidic linkage|
|Maltopentalose ||Cannot be fermented by alge or larger yeast ||Five glucose units ||α(1,4) glycosidic linkage |
Oligosaccharide boned with eukaryotic membrane to form glycoproteins and the attached oligosaccharide molecules are found extracellular side of membrane which makes intuitive sense as the oligosaccharides are added to these proteins within the lumen of the endomembrane system.
Oligosaccharides play an important role in cell-cell recognition process
. Oligosaccharides or glycans are located on the exterior surface of the cell membrane and involves in two very important functions.
1. GatekeeperGlycans regulate the transportation of foreign substances through cell membrane.
For example, during fertilization, only one sperm can penetrate egg and that sperm right matching is done by glycans on the exterior of the egg. When oligosaccharides that, the egg’s oligosaccharide allows the sperm to pass through. As the chemical signal is sent out across the surface of the egg, it instructs the remaining oligosaccharides to block any further penetration of other sperm.
2. Cell-to-cell communication
For the proper function of cell, the need to be able to communicate with of the other cells around them. Oligosaccharides help in communication between cells for proper functions. Oligosaccharides also involve in treatment of diseases caused by viruses like common cold, viral pneumonia and influenza. The attacking virus has to be recognized by one or more oligosaccharide bonded on cell membrane. If we can identify that oligosaccharide successfully, it may be possible to create some type of vaccine which would block that specific recognition signal. As that recognition signal blocked, the virus would no longer be a threat to the body.