Glycoside
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[[Image:Salicin-perspective-2D-skeletal.png|thumb| [[Salicin]], a glycoside related to [[Aspirin]] ]]
In [[chemistry]], '''glycosides''' are certain [[molecule]]s 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 [[medication]]s. 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 [[glycosidic bond|O-glycosidic bond]] or an [[glycosidic bond|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 [[polysaccharide]]s. 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 [[glycosidic bond|N-glycosidic bond]] are known as [[glycosylamine]]s 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 bond]]s. For example, the glycone and aglycone portions can be chemically separated by [[hydrolysis]] in the presence of [[acid]]. There are also numerous [[enzyme]]s 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 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 "above" or "below" the plane of the cyclic sugar molecule, glycosides are classified as '''α-glycosides''' or '''β-glycosides'''. Some [[enzyme]]s such as [[amylase|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 [[alcohol]]ic 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 [[inflammation|antiinflammatory]] effects.
====Anthraquinone glycosides====
These glycosides contain an aglycone group that is a derivative of [[anthraquinone]]. They are present in [[senna (herb)|senna]], [[rhubarb]] and [[aloe]]s; they have a [[laxative]] effect.
====Coumarin glycosides====
Here the aglycone is [[coumarin]]. An example is [[apterin]] which is reported to dilate the [[coronary artery|coronary arteries]] as well as block [[calcium channel]]s.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 [[almond]]s. Cyanogenic glycosides can be found in the fruits (and wilting leaves) of the [[Rosaceae|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:
*[[Hesperidin]] (aglycone: [[Hesperetin]], glycone: [[Rutinose]])
*[[Naringin]] (aglycone: [[Naringenin]], glycone: [[Rutinose]])
*[[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 (simple)====
Here the aglycone is a simple [[phenol]]ic structure. An example is [[arbutin]] found in the [[Bearberry|Common Bearberry]] ''Arctostaphylos uva-ursi''. It has a urinary antiseptic effect.
Rutin found in rooibos tea.
====Saponins====
{{main|Saponin}}
These compounds give a permanent froth when shaken with water. They also cause [[hemolysis]] of [[red blood cell]]s. Saponin glycosides are found in [[liquorice]]. Their medicinal value is due to their [[expectorant]] effect.
====Steroidal glycosides or [[cardiac glycoside]]s====
Here the aglycone part is a [[steroid]]al nucleus. These glycosides are found in the plant genera ''[[Digitalis]]'', ''[[Scilla]]'', and ''[[Strophanthus]]''. They are used in the treatment of [[heart disease]]s e.g. [[congestive heart failure]] (historically as now recognised does not improve survivability; other agents are now preferred] and [[arrhythmia]].
====Steviol glycosides====
{{main|Steviol glycoside}}
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 [[sweetener]]s 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,{{vague|say what part of the name, as English readers can't guess it}}
these compounds contain [[sulfur]]. Examples include [[sinigrin]], found in [[black mustard]], and [[sinalbin]], found in [[white mustard]].
==References==
{{reflist}}
== External links==
* [http://www.iupac.org/goldbook/G02661.pdf Definition of glycosides], from the [[IUPAC]] Compendium of Chemical Terminology, the "[[Gold Book]]"
* [http://www.chem.qmul.ac.uk/iupac/2carb/33.html IUPAC naming rules for glycosides]
{{Glycosides}}
[[Category:Glycosides|*]]
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