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Salicin, a glycoside related to Aspirin
Example of the chemical structure of one of the cardiac glycosides.

In chemistry, glycosides are molecules in which a sugar is bound to a non-carbohydrate moiety, usually a small organic molecule. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. These can be activated by enzyme hydrolysis,[1] which causes the sugar part to be broken off, making the chemical available for use. Many such plant glycosides are used as medications. In animals (including humans), poisons are often bound to sugar molecules as part of their elimination from the body.

Formally, a glycoside is any molecule in which a sugar group is bonded through its anomeric carbon to another group via a glycosidic bond. Glycosides can be linked by an O- (an O-glycoside), N- (a glycosylamine), S-(a thioglycoside) or C- (a C-glycoside) glycosidic bond. The given definition is the one used by IUPAC.[2] Many authors require in addition that the sugar be bonded to a non-sugar for the molecule to qualify as a glycoside, thus excluding polysaccharides. The sugar group is then known as the glycone and the non-sugar group as the aglycone or genin part of the glycoside. The glycone can consist of a single sugar group (monosaccharide) or several sugar groups (oligosaccharide).

Contents

Related compounds

Molecules containing an N-glycosidic bond are known as glycosylamines and are not discussed in this article. (Many authors in biochemistry call these compounds N-glycosides and group them with the glycosides; this is considered a misnomer and discouraged by IUPAC.)

Chemistry

Much of the chemistry of glycosides is explained in the article on glycosidic bonds. For example, 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. Genetically altered enzymes termed glycosynthases have been developed that can form glycosidic bonds in excellent yield.

There are a great many ways to chemically synthesize glycosidic bonds. Fischer glycosidation refers to the synthesis of glycosides by the reaction of unprotected monosaccharides with alcohols (usually as solvent) in the presence of a strong acid catalyst. The Koenigs-Knorr reaction is the condensation of glycosyl halides and alcohols in the presence of metal salts such as silver carbonate or mercuric oxide.

Classification

We can classify glycosides by the glycone, by the type of glycosidic bond, and by the aglycone.

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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.

By type of glycosidic bond

Depending on whether the glycosidic bond lies "below" or "above" the plane of the cyclic sugar molecule, glycosides are classified as α-glycosides or β-glycosides. Some enzymes such as α-amylase can only hydrolyze α-linkages; others, such as emulsin, can only affect β-linkages.

By aglycone

Glycosides are also classified according to the chemical nature of the aglycone. For purposes of biochemistry and pharmacology, this is the most useful classification.

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.

Anthraquinone glycosides

These glycosides contain an aglycone group that is a derivative of anthraquinone. They have a laxative effect. They are mainly found in dicot plants except the Liliaceae family which are monocots. They are present in senna, rhubarb and Aloe species. Antron and anthranol are reduced forms of anthraquinone.

Coumarin glycosides

Here the agl is coumarin or a derivative. An example is apterin which is reported to dilate the coronary arteries as well as block calcium channels. Other coumarin glycosides are obtained from dried leaves of Psoralea corylifolia.

Cyanogenic glycosides

In this case, the aglycone contains a cyanide group. In many plants, these glycosides are stored in the vacuole but if the plant is attacked they are released and become activated by enzymes in the cytoplasm. These remove the sugar part of the molecule and release toxic hydrogen cyanide. Storing them in inactive forms in the cytoplasm prevents them from damaging the plant under normal conditions.

An example of these is amygdalin from almonds. They can also be found in the fruits (and wilting leaves) of the rose family (including cherries, apples, plums, almonds, peaches, apricots, raspberries, and crabapples). Cassava, an important food plant in Africa and South America, contains cyanogenic glycosides and therefore has to be washed and ground under running water prior to consumption. Sorghum (Sorghum bicolor) expresses cyanogenic glycosides in its roots and thus is resistant to pests such as rootworms (Diabrotica spp.) that plague its cousin maize (Zea mays L.). It was once thought that cyanogenic glycosides might have anti-cancer properties, but this idea was disproven, see Amygdalin. A recent study may also show that increasing CO2 levels, caused by anthropogenic emissions, may result in much higher levels of cyanogenic glycoside production in Sorghum and Cassava plants, making them highly toxic and inconsumable. A doubling of CO2 concentration was found to double the concentration of cyanogenic glycosides in the leaves.[3][4]

Flavonoid glycosides

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

Among the important effects of flavonoids are their antioxidant effect. They are also known to decrease capillary fragility.

Phenolic glycosides (simple)

Here the aglycone is a simple phenolic structure. An example is arbutin found in the Common Bearberry Arctostaphylos uva-ursi. It has a urinary antiseptic effect.

Saponins

These compounds give a permanent froth when shaken with water. They also cause hemolysis of red blood cells. Saponin glycosides are found in liquorice. Their medicinal value is due to their expectorant effect.

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 (historically as now recognised does not improve survivability; other agents are now preferred) and arrhythmia.

Steviol glycosides

These sweet glycosides found in the stevia plant Stevia rebaudiana Bertoni have 40-300 times the sweetness of sucrose. The two primary glycosides, stevioside and rebaudioside A, are used as natural sweeteners in many countries. These glycosides have steviol as the aglycone part. Glucose or rhamnose-glucose combinations are bound to the ends of the aglycone to form the different compounds.

Thioglycosides

As the name implies (q.v. thio-), these compounds contain sulfur. Examples include sinigrin, found in black mustard, and sinalbin, found in white mustard.

See also

References

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