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Glycosidic Linkage

Carbohydrates are an ideal source of energy as they hydrolyzed in the presence of various enzymes to form ultimately glucose which further provides energy. The excess of carbohydrates in diet even can upset the balance of blood sugar level which results in fluctuations in energy and mood.

There is mainly two type of carbohydrates. 

1. Simple carbohydrates

Simple carbohydrates- are also called as sugar and exist in either a natural or refined form. Natural sugars are found in fruits and vegetables. While refined sugars are found in biscuits, honey and jams, chocolate, brown and white cane sugar, pizzas, jellies, prepared foods and sauces and soft drinks.

2. Complex carbohydrates

Complex carbohydrates- oxidized slowly to release energy while the natural form of carbohydrates contributes to long-term good health, sustained energy levels, and appetite control.Carbohydrates are biopolymers which formed by the condensation polymerization of monomer units monosaccharides.   Hence monosaccharide is called as simplest units carbohydrates consist of primary and secondary alcoholic group with a carbonyl group. 

During polymerization, the carbonyl group of one monosaccharide unit gets condenses with an alcoholic unit of another monosaccharide to form disaccharide unit. Further, as the number of monosaccharide units increases, the polymeric chain becomes longer. These polymeric chains are called as polysaccharides


Glycosidic Linkage Definition

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A covalent bond which bonded a carbohydrate molecule to another molecule is called as a glycosidic bond.

A glycosidic bond can be formed between the hemiacetal group of saccharides and the hydroxyl group of some organic compound like an alcohol. If the glycosidic linkage is in between a carbohydrate residue and another molecule but not saccharide, is called as an aglycone.

Whereas the presence of glycosidic bond between two saccharide unit results in a structure with reducing end. The bond of an amino group or other nitrogen-containing groups with the sugar is also called as a glycosidic bond. For example, n a nucleoside, the sugar- base bond is also a glycosidic bond and glycosides are a substance which contains a glycosidic bond.

  1. If glycosidic oxygen linked with aglycone or reducing end sugar, it is called as O-glycosidic bonds.
  2. Similarly, S-glycosidic bonds are found in thioglycosides, where the sulfur atom bonded in place of oxygen in glycosidic bond and in N-glycosidic bonds, nitrogen is bonded in place of nitrogen, such compounds are called as glycosylamines.
  3. C-glycosyl bonds have the glycosidic carbon in place of oxygen. The Glycosidic linkage is assigned on the basis position of carbon atom involves in linkage as it is an acetal linkage between carbon atoms of two monomer units.
  4. The most common monosaccharide is glucose which contains a carbonyl group (aldehyde group) with five hydroxyl groups. The open chain form of glucose contains carbonyl group at the C-1 position, four hydroxyl groups at C-2, 4, 5, 6 on the right side and on C-3 at left side.
  5. The C-1 of glucose is very close to C-5 in three-dimensional space arrangements, hence it can arrange in ring form instead of the open chain by the bond formation between H-OH of C-5 with C-1 by hemiacetal linkage and form a cyclic structure.
  6. In the cyclic structure of glucose, the OH groups on carbons 2, 4, and 5 which were on the right in open chain form, will be in the down positions, and the hydroxyl group (OH) lies above the plane of C-3 position.

On the basis of the position of OH group at C-1, there are two possible forms, in alpha form, the hydroxyl group (OH) on C-1 goes down and in beta form, it goes up.

Generally, glucose circulates in the bloodstream in the form of glucose or form disaccharides, in which two Monosaccharides linked with a glycosidic linkage which formed by a condensation reaction between the hydroxyl group of the first-position carbon and the hydroxyl group of the fourth-position carbon of each monosaccharide units.

A disaccharide can be composed of two same sugar molecules like maltose, which is composed of two D-glucose molecules linked through a glycosidic bond, or two different hexose sugars like lactose formed with D-glucose and D-galactose.

D-glucose and D-galactose

Alpha 1, 4 Glycosidic Linkage

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Glycosidic linkage named after the anomers of monosaccharide units (alpha and beta units) involve in this linkage. The position of alpha and beta glycosidic bonds can be identifying on the basis of the relative stereochemistry of the anomeric position and the stereo center furthest from C-1 in the saccharide. 

Hence the alpha and beta designations of the glycosidic bonds are based on the configuration of the first-position carbon of the monosaccharide which are linked in a glycosidic bond. For example, maltose is formed by an alpha glycosidic bond, whereas lactose consists of a beta glycosidic bond.

Glycosidic Bond
  1. In D-hexose sugars like D-glucose in their pyranose forms, glycopyranose, the $\alpha$-glycosidic bond is formed in an axial orientation and a $\alpha$-glycosidic bond is oriented equatorially.
  2. Pharmacologists often join substances to glucuronic acid thorugh the glycosidic bonds for increasing their water solubility, this is called as glucuronidation.
  3. If alpha-form of monosaccharides involves in glycosidic linkage, it is known as alpha-glycosidic linkage which can be represented by the position of carbon atoms involve in bond formation.
  4. For example in maltose sugar, two glucose units get condense and bonded with glycosidic linkage. The hydroxyl group on C-1 of one $\alpha$ -glucopyranose is bonded with the hydroxyl group of C-4 of another molecule with the elimination of water. Since C-1 and C-4 of two alpha units involve in glycosidic linkage, it termed as alpha1-4-glycosidic linkage.

1-4-Glycosidic Linkage

Similarly a Trisaccharide like Maltotriose composed of consisting of three glucose residues which are bonded with $\alpha$1-4 glycosidic bonds. Similarly in maltotetralose four glucose units linked through $\alpha$z4(1, 4) bond and in maltopentalose, five glucose molecules linked with an $\alpha$(1, 4) bond.


Polysaccharide like starch is composed of two type of polysaccharide chains, amylase and amylopectine. Out of these two units, Amylose unit is a linear, helical chain which consists of around 500 to 20,000 alpha-D-glucose monomers linked through alpha (1-4) glycosidic bonds. 


Alpha 1, 6 Glycosidic Linkage

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Similar to alpha1-4-glycosidic linkage, monosaccharide units can form alpha 1-6 linkage in which C-1 of one unit gets bonded with C-6 of another unit through glycosidic linkage. 

For example, in the amylopectin unit of starch is a huge branches polymeric chain of glucose containing numerous amylase-like chains which are bonded with alpha (1-4) bonds, and branch points are connected with alpha-1-6 glycosidic linkage.

 Alpha 1-6 Glycosidic Linkage

Another polysaccharide, Glycogen also contains both alpha glycosidic linkage but the frequency of alpha (1-6) linkage is more in glycogen compare to amylopectin.

Alpha(1-6) Linkage

Just like amylopectin, dextran a polysaccharide, also contains alpha 1-6 glycosidic linkage in main chain and side branches are bonded with alpha 1-3 and alpha 1-4 glycosidic linkage.
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