Glycosides can be classified by the glycone, by the type of glycosidic bond, and by the aglycone.
By glycone/presence of sugar 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. Compounds can also be generally defined based on the class of glycone; for example, biosides are glycosides with a disaccharide (biose) glycone.
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. There are four type of linkages present between glycone and aglycone: • C-linkage/glycosidic bond, "nonhydrolysable by acids or enzymes" • O-linkage/glycosidic bond • N-linkage/glycosidic bond • S-linkage/glycosidic bond
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
anti-inflammatory 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 family
Liliaceae which are
monocots. They are present in
senna,
rhubarb and
Aloe species. Anthron and anthranol are reduced forms of anthraquinone.
Coumarin glycosides Here, the aglycone 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.
Chromone glycosides In this case, the aglycone is called benzo-gamma-pyrone.
Cyanogenic glycosides In this case, the aglycone contains a
cyanohydrin group. Plants that make cyanogenic glycosides store them 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, allowing the cyanohydrin structure to collapse and release toxic
hydrogen cyanide. Storing them in inactive forms in the vacuole prevents them from damaging the plant under normal conditions. Along with playing a role in deterring herbivores, in some plants they control germination,
bud formation, carbon and nitrogen transport, and possibly act as antioxidants. Some butterfly species, such as the
Dryas iulia and
Parnassius smintheus, have evolved to use the cyanogenic glycosides found in their host plants as a form of protection against predators through their unpalatability.
Flavonoid glycosides Here, the aglycone is a
flavonoid. Examples of this large group of glycosides include: •
Hesperidin (aglycone:
hesperetin, glycone: elakkia
rutinose) •
Naringin (aglycone:
naringenin, glycone: neohesperidose) •
Rutin (aglycone:
quercetin, glycone: rutinose) •
Quercitrin (aglycone: quercetin, glycone:
rhamnose) Among the important effects of flavonoids are their
antioxidant effect. They are also known to decrease
capillary fragility.
Phenolic glycosides 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,
corticoid and anti-inflammatory effects. Steroid saponins are important starting material for the production of semi-synthetic
glucocorticoids and other
steroid hormones such as
progesterone; for example in
Dioscorea wild yam the
sapogenin diosgenin, in the form of its glycoside dioscin. The
ginsenosides are
triterpene glycosides and ginseng saponins from
Panax ginseng (Chinese ginseng) and
Panax quinquefolius (
American ginseng). In general, the use of the term saponin in organic chemistry is discouraged, because many plant constituents can produce
foam, and many
triterpene-glycosides are amphipolar under certain conditions, acting as a
surfactant. More modern uses of saponins in biotechnology are as
adjuvants in
vaccines:
Quil A and its derivative
QS-21, isolated from the bark of
Quillaja saponaria Molina, to stimulate both the Th1 immune response and the production of
cytotoxic T-lymphocytes (CTLs) against exogenous antigens make them ideal for use in
subunit vaccines and vaccines directed against intracellular pathogens as well as for therapeutic
cancer vaccines but with the aforementioned side-effect of
hemolysis. Saponins are also natural ruminal antiprotozoal agents that are potential to improve ruminal microbial fermentation reducing ammonia concentrations and methane production in ruminant
animals.
Steroid glycosides (cardiac glycosides) In these glycosides, the aglycone part is a
steroid 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.
Iridoid glycosides These contain an
iridoid group; e.g.
aucubin,
geniposidic acid, theviridoside,
loganin,
catalpol.
Thioglycosides As the name contains the prefix
thio-, these compounds contain
sulfur. Examples include
sinigrin, found in
black mustard, and
sinalbin, found in
white mustard. ==See also==