Everything about Glycosides totally explained
In
chemistry,
glycosides are certain
molecules in which a
sugar part is bound to some other part. Glycosides play numerous important roles in living organisms. Many plants store important chemicals in the form of inactive glycosides; if these chemicals are needed, the glycosides are brought in contact with water and an
enzyme, and the sugar part is 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 in order to remove them from the body.
Formally, a glycoside is any molecule in which a sugar group is bonded through its
anomeric carbon to another group via an
O-glycosidic bond or an
S-glycosidic bond; glycosides involving the latter are also called
thioglycosides. The given definition is the one used by
IUPAC. Many authors require in addition that the sugar be bonded to a
non-sugar for the molecule to qualify as a glycoside, thus excluding the
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).
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. Mutant 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.
By glycone
If the glycone group of a glycoside is
glucose, then the molecule is a
glucoside; if it's
fructose, then the molecule is a
fructoside; if it's
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 "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 hydrolize 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's a derivative of
anthraquinone. They are present in
senna,
rhubarb and
aloes; they've a
laxative effect.
Coumarin glycosides
Here the aglycone is
coumarin. An example is
apterin which is reported to dilate the
coronary arteries as well as block
calcium channels.those obtained from dried leaves of
Psoralia corylifolia have Main glycosides psoralin and corylifolin.
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. Cyanogenic glycosides can 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 glucosides 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.).
Flavonoid glycosides
Here the aglycone is a
flavonoid. This is a large group of flavonoid glycosides. Examples 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.
Rutin found in rooibos tea.
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.
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 for example
congestive heart failure (historically as now recognised doesn't 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, these compounds contain
sulfur. Examples include
sinigrin, found in
black mustard, and
sinalbin, found in
white mustard.
Further Information
Get more info on 'Glycosides'.
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