Inchemistry, aglycoside/ˈɡlaɪkəsaɪd/ is amolecule in which asugar is bound to anotherfunctional group via aglycosidic bond. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. These can be activated byenzymehydrolysis,[1] which causes the sugar part to be broken off, making the chemical available for use. Many such plant glycosides are used asmedications. Several species ofHeliconius butterfly are capable of incorporating these plant compounds as a form of chemical defense against predators.[2] In animals and humans, poisons are often bound to sugar molecules as part of their elimination from the body.
In formal terms, a glycoside is any molecule in which a sugar group is bonded through itsanomeric carbon to another group via aglycosidic bond. Glycosides can be linked by an O- (anO-glycoside), N- (aglycosylamine), S-(athioglycoside), or C- (aC-glycoside) glycosidic bond. According to theIUPAC, the name "C-glycoside" is amisnomer; the preferred term is "C-glycosyl compound".[3] The given definition is the one used byIUPAC, which recommends theHaworth projection to correctly assignstereochemical configurations.[4]
Many authors require in addition that the sugar be bonded to anon-sugar for the molecule to qualify as a glycoside, thus excludingpolysaccharides. The sugar group is then known as theglycone and the non-sugar group as theaglycone orgenin part of the glycoside. The glycone can consist of a single sugar group (monosaccharide), two sugar groups (disaccharide), or several sugar groups (oligosaccharide).
The first glycoside ever identified wasamygdalin, by the French chemistsPierre Robiquet and Antoine Boutron-Charlard, in 1830.[5]
Molecules containing an N-glycosidic bond are known asglycosylamines. Many authors inbiochemistry call these compoundsN-glycosides and group them with the glycosides; this is considered a misnomer and is discouraged by theInternational Union of Pure and Applied Chemistry. Glycosylamines and glycosides are grouped together asglycoconjugates; other glycoconjugates includeglycoproteins,glycopeptides,peptidoglycans,glycolipids, andlipopolysaccharides.[citation needed]
Much of the chemistry of glycosides is explained in the article onglycosidic bonds. For example, the glycone and aglycone portions can be chemically separated byhydrolysis in the presence ofacid and can be hydrolyzed byalkali. There are also numerousenzymes that can form and break glycosidic bonds. The most important cleavage enzymes are theglycoside hydrolases, and the most important synthetic enzymes in nature areglycosyltransferases. Genetically altered enzymes termedglycosynthases have been developed that can form glycosidic bonds in excellent yield.[citation needed]
There are 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. TheKoenigs-Knorr reaction is the condensation of glycosyl halides and alcohols in the presence of metal salts such assilver carbonate ormercuric oxide.[citation needed]
 | This sectionneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources in this section. Unsourced material may be challenged and removed.(January 2021) (Learn how and when to remove this message) |
Glycosides can be classified by the glycone, by the type of glycosidic bond, and by the aglycone.
If the glycone group of a glycoside isglucose, then the molecule is aglucoside; if it isfructose, then the molecule is afructoside; if it isglucuronic acid, then the molecule is aglucuronide; 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.
Depending on whether the glycosidic bond lies "below" or "above" the plane of the cyclic sugar molecule, glycosides are classified asα-glycosides orβ-glycosides. Some enzymes such asα-amylase can only hydrolyze α-linkages; others, such asemulsin, can only affect β-linkages.
There are four type of linkages present between glycone and 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.
An example of analcoholic glycoside issalicin, which is found in the genusSalix. Salicin is converted in the body intosalicylic acid, which is closely related toaspirin and hasanalgesic,antipyretic, andanti-inflammatory effects.
These glycosides contain an aglycone group that is a derivative ofanthraquinone. They have alaxative effect. They are mainly found indicot plants except the familyLiliaceae which aremonocots. They are present insenna,rhubarb andAloe species. Anthron and anthranol are reduced forms of anthraquinone.
Here, the aglycone iscoumarin or a derivative. An example isapterin which is reported to dilate thecoronary arteries as well as blockcalcium channels. Other coumarin glycosides are obtained from dried leaves ofPsoralea corylifolia.
In this case, the aglycone is called benzo-gamma-pyrone.
In this case, the aglycone contains acyanohydrin group. Plants that make cyanogenic glycosides store them in thevacuole, but, if the plant is attacked, they are released and become activated by enzymes in thecytoplasm. These remove the sugar part of the molecule, allowing the cyanohydrin structure to collapse and release toxichydrogen cyanide. Storing them in inactive forms in the vacuole prevents them from damaging the plant under normal conditions.[6]
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.[6] The production of cyanogenic glycosides is an evolutionarily conserved function, appearing in species as old asferns and as recent asangiosperms.[6] These compounds are made by around 3,000 species. In screens they are found in about 11% of cultivated plants but only 5% of plants overall; humans seem to have selected for them.[6]
Examples includeamygdalin andprunasin which are made by thebitter almond tree; other species that produce cyanogenic glycosides aresorghum (from whichdhurrin, the first cyanogenic glycoside to be identified, was first isolated),barley,flax,white clover, andcassava, which produceslinamarin andlotaustralin.[6]
Amygdalin and a synthetic derivative,laetrile, were investigated as potential drugs to treat cancer and were heavily promoted asalternative medicine; they are ineffective and dangerous.[7]
Some butterfly species, such as theDryas iulia andParnassius smintheus, have evolved to use the cyanogenic glycosides found in their host plants as a form of protection against predators through their unpalatability.[8][9]
Here, the aglycone is aflavonoid. Examples of this large group of glycosides include:
Among the important effects of flavonoids are theirantioxidant effect. They are also known to decreasecapillary fragility.
Here, the aglycone is a simplephenolic structure. An example isarbutin found in theCommon BearberryArctostaphylos uva-ursi. It has a urinary antiseptic effect.
These compounds give a permanent froth when shaken with water. They also causehemolysis ofred blood cells. Saponin glycosides are found inliquorice. Their medicinal value is due to theirexpectorant,corticoid and anti-inflammatory effects. Steroid saponins are important starting material for the production of semi-syntheticglucocorticoids and othersteroid hormones such asprogesterone; for example inDioscoreawild yam thesapogenindiosgenin, in the form of its glycoside dioscin. Theginsenosides aretriterpene glycosides and ginseng saponins fromPanax ginseng (Chinese ginseng) andPanax quinquefolius (American ginseng). In general, the use of the term saponin in organic chemistry is discouraged, because many plant constituents can producefoam, and manytriterpene-glycosides are amphipolar under certain conditions, acting as asurfactant. More modern uses of saponins in biotechnology are asadjuvants invaccines:Quil A and its derivativeQS-21, isolated from the bark ofQuillaja saponaria Molina, to stimulate both the Th1 immune response and the production ofcytotoxic T-lymphocytes (CTLs) against exogenous antigens make them ideal for use insubunit vaccines and vaccines directed against intracellular pathogens as well as for therapeuticcancer vaccines but with the aforementioned side-effect ofhemolysis.[10] Saponins are also natural ruminal antiprotozoal agents that are potential to improve ruminal microbial fermentation reducing ammonia concentrations and methane production in ruminantanimals.[11]
In these glycosides, the aglycone part is asteroid nucleus. These glycosides are found in the plant generaDigitalis,Scilla, andStrophanthus. They are used in the treatment ofheart diseases, e.g.,congestive heart failure (historically as now recognised does not improve survivability; other agents[example needed] are now preferred[medical citation needed]) andarrhythmia.
These sweet glycosides found in thestevia plantStevia rebaudiana Bertoni have 40–300 times the sweetness ofsucrose. The two primary glycosides, stevioside and rebaudioside A, are used as naturalsweeteners in many countries. These glycosides havesteviol as the aglycone part.Glucose orrhamnose-glucose combinations are bound to the ends of the aglycone to form the different compounds.
These contain aniridoid group; e.g.aucubin,geniposidic acid, theviridoside,loganin,catalpol.
As the name contains the prefixthio-, these compounds containsulfur. Examples includesinigrin, found inblack mustard, andsinalbin, found inwhite mustard.
