Glycoside – Classification


Glycosides are certain molecules in which a sugar part is bound to some other part. Glycosides play numerous important roles in living organisms.
A glycoside is an organic compound, usually of plant origin, and comprising a sugar portion linked to a non-sugar moiety in a particular manner. 
The molecule from which the non-sugar moiety of a glycoside is derived is called the aglycones or genin. 
The linkage between the sugar and the aglycones is a hemiacetal linkage formed by the reducing group (usually aldehydes or keto group) of the sugar and an alcoholic or phenolic hydroxyl group of the aglycone.
Glycoside       ————->    Aglycone (genin) + Glycone (Sugar)
The glycone and aglycone portions can be chemically separated by hydrolysis in the presence of acid. 
There are also numerous enzymes that can form and break glycosidic bonds. 
The most important cleavage enzymes are the glycoside hydrolases, and the most important synthetic enzymes in nature are glycosyltransferases.
Mutant enzymes termed glycosynthases have been developed that can form glycosidic bonds. 
There are a great many ways to chemically synthesize glycosidic bonds.

Classification based on Linkages

i. C-glycoside: Some of the anthraquinone glycoside such as cascarosides from cascara & aloin from aloe show the presence of C-glycoside.
Glycone-OH + HC-Aglycone    ————->     glycone-C-aglycone + H2O
ii. O-glycoside: They are common in very higher plants like senna, rhubarb etc.
Glycone-OH + HO-Aglycone     ————->     glycone-O-aglycone + H2O
iii. S-glycoside: Their occurrence is restricted to isothiacynanate glycosides like sinigrin. They are formed by interaction of sulfydryl group of aglycone & hydroxyl group of glycone.
Glycone-OH + HS-Aglycone      ————->  glycone-S-aglycone + H2O
iv. N-glycoside: The most typical representive example of N-glycoside is nucleosides, where the amino group react with OH-group of ribose/deoxyribose & ultimately gives N-glucosidic form.
Glycone-OH + NH-Aglycone     ————->   glycone-N-aglycone + H2O

Classification of glycoside

1. By glycone: 

If the glycone group of a glycoside is glucose, then the molecule is a glucoside; 

if it is fructose, then the molecule is a fructoside; 

if it is glucuronic acid, then the molecule is a glucuronide; etc. 

In the body, toxic substances are often bonded to glucuronic acid to increase their water solubility; the resulting glucuronides are then excreted.

2. By type of glycosidic bond: 

Depending on whether the glycosidic bond lies “above” or “below” the plane of the cyclic sugar molecule, glycosides are classified as α-glycosides or β- glycosides. 

Some enzymes such as α-amylase can only hydrolyzed α-linkages; others, such as emulsin, can only affect β-linkages. 

Stereochemically these considered as a theoretical aspect, because the plants contain only β- glycosides.

3. By aglycone:

i.      Steroidal glycosides or cardiac glycosides: 

Here the aglycone part is a steroidal nucleus.

These glycosides are found in the plant genera Digitalis, Scilla, and Strophanthus. 

They are used in the treatment of heart diseases e.g. congestive heart failure and arrhythmia.

ii. Alcoholic glycosides:

 An example of an alcoholic glycoside is salicin which is found in the genus salix. 

Salicin is converted in the body into salicylic acid, which is closely related to aspirin and has analgesic, antipyretic and antiinflammatory effects. Example: Salicin

iii. Anthraquinone glycosides: 

These glycosides contain an aglycone group that is a derivative of anthraquinone. 

They are present in senna, rhubarb and aloes; they have a laxative effect. E.g. Barbaloin.

iv. Coumarin glycosides: 

Here the aglycone is coumarin. 

An example is apterin which is reported to dilate the coronary arteries as well as block channels those obtained from dried leaves of Psoralia corylifolia have Main glycosides psoralin and corylifolin.  [Coumarin (benzopyrone)] [Apterin]

v. Cyanogenic  glycosides: 

 In  this  case,  the  aglycone  contains  a  cyanide  group  and  the glycoside can release the poisonous hydrogen cyanide if acted upon by some enzyme. 

An example of these is amygdalin from almonds.

vi. Flavonoids glycosides: 

Here the aglycone is a flavonoid. 

This is a large group of flavonoid glycosides. E.g. – Hesperidin (aglycone: Hesperetin, glycone: Rutinose). 

Here, the aglycone is a flavonoid. Examples of this large group of glycosides include:

Hesperidin (aglycone: Hesperetin, glycone: Rutinose), 

Naringin (aglycone: Naringenin, glycone: Rutinose) Rutin (aglycone: Quercetin, glycone: Rutinose) Quercitrin (aglycone: Quercetin, glycone: Rhamnose)

vii.      Soponins: 

These compounds give a permanent froth when shaken with water. 

They also cause   hemolysis   of   red   blood   cells.   

Soponins   glycosides   are   found   in   liquorice (Glycyrrhizin). 

Their medicinal value is due to their expectorant effect. [Glycyrrhizin]

viii.     Thioglycosides: 

As the name implies, these compounds contain sulfur. E.g.- sinigrin, found in black mustard, and sinalbin, found in white mustard. Example: Sinigrin

ix. Phenolic glycosides: 

Here the aglycone is a simple Phenolic structure. E.g. – arbutin found in the Common Bearberry Arctostaphylos uva-ursi. 

It has a urinary antiseptic effect.

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