Any of a group of organic compounds, occurring abundantly in plants, that yield a sugar and one or more nonsugar substances on hydrolysis.
[glycose, a monosaccharide (variant of GLUCOSE) + –IDE.]
glycosidic gly'co·sid'ic (-sĭd'ĭk) adj.
glycoside
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gly·co·side (glī'kə-sīd')  |
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A large important class of sugar derivatives in which the sugar is combined with a nonsugar. In their cyclic forms, monosaccharides (simple sugars) possess one carbon (C) atom (the anomeric carbon) that is bonded to two oxygen (O) atoms; one oxygen atom forms a part of the ring, whereas the other is outside the ring (exocyclic) and is part of a hydroxyl (OH) group. If the oxygen atom of the anomeric hydroxyl group becomes bonded to a carbon atom, other than that of a carbonyl (C &dbnd; O) group, the resulting compound is a glycoside. A glycoside thus consists of two parts (see illustration): the sugar (glycosyl) unit, which provides the anomeric carbon, and the moiety (the aglycon), which is the source of the exocyclic oxygen and carbon atoms of the glycosidic linkage. Such compounds frequently are referred to as O-glycosides to distinguish them from analogs having a sulfur (thio- or S-glycosides), nitrogen (amino- or N-glycosides), or carbon (anomalously called C-glycosides) as the exocyclic atom on the anomeric carbon. See also Hydroxyl;
wavy bond indicates that the group may have various orientations in space.">
Structural formulas of two glycosides. (a) Methyl β-D-glucopyranoside. (b) Lactose, 4-O-β-D-galactopyranosyl-D-glucopyranose; the wavy bond indicates that the group may have various orientations in space.
The formation of glycosides is the principal manner in which monosaccharides are incorporated into more complex molecules. For example, lactose (illustration b), the most abundant disaccharide in mammalian milk, has a glycosidic bond involving the anomeric carbon of D-galactose and the C-4 hydroxyl of D-glucose. The anomeric carbon atom can exist in either of two stereoisomeric configurations, a fact which is of immense importance to the chemistry and biochemistry of glycosides. For example, the principal structural difference between cellulose and amylose is that cellulose is β-glycosidically linked whereas amylose is α-linked. Humans are able to digest amylose but are unable to utilize cellulose for food. See also Cellulose; Lactose; Stereochemistry.
A very large number of glycosides exist in nature, many of which possess important biological functions. In many of these biologically important compounds the carbohydrate portion is essential for cell recognition, the terminal sugar units being able to interact with specific receptor sites on the cell surface.
One class of naturally occurring glycosides is called the cardiac glycosides because they exhibit the ability to strengthen the contraction of heart muscles. These cardiotonic agents are found in both plants and animals and contain complex aglycons, which are responsible for most of the drug action; however, the glycoside may modify the biological activity. The best-known cardiac glycosides come from digitalis and include the drug digoxin. See also Digitalis.
Glycosidic units frequently are found in antibiotics. For example, the important drug erythromycin A possesses two glycosidically linked sugar units. See also Antibiotic.
Perhaps the most ubiquitous group of glycosides in nature is the glycoproteins; in many of them carbohydrates are linked to a protein by O-glycosidic bonds. These glycoproteins include many enzymes, hormones, such antiviral compounds as interleukin-2, and the so-called antifreeze glycoproteins found in the sera of fish from very cold marine environments. See also Amino acids; Antifreeze (biology); Carbohydrate; Enzyme; Glycoprotein; Hormone.
Glycolipids are a very large class of natural glycosides having a lipid aglycon. These complex glycosides are present in the cell membranes of microbes, plants, and animals. See also Glycolipid; Lipid.
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Compounds of a sugar attached to another molecule. When glucose is the sugar, they are called glucosides. A wide variety occur in plants.
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(gli′kōsīd)
n
n
A compound that contains a sugar as part of the molecule.
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Any compound containing a carbohydrate moiety (sugar), particularly any such natural product in plants, convertible, by hydrolytic cleavage, into a sugar and a nonsugar component (aglycone), and named specifically after the sugar contained, as glucoside (glucose), pentoside (pentose), fructoside (fructose), etc.
- cardiac g. — any one of a group of glycosides occurring in certain plants (e.g. Digitalis) having a characteristic action on the contractile force of the heart muscle. See also cardenolide, bufadienolide.
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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; which can be activated with water and an enzyme. This 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 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).
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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.
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 are present in senna, rhubarb and aloes; they have 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. They 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.). Some cyanogenic glycosides may have anti-cancer properties. See Amygdalin[1] A recent study may also show that increasing CO2 levels, caused by pollution and global warming, may result in much higher levels of cyanogenic glycoside production in Sorghum and Cassava plants, making them highly toxic and inconsumable.[2]
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 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.
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.
References
- Brito-Arias, Marco (2007). Synthesis and Characterization of Glycosides. Springer. ISBN 978-0-387-26251-2.
- ^ Chang HK, Shin MS, Yang HY, et al (August 2006). "Amygdalin induces apoptosis through regulation of Bax and Bcl-2 expressions in human DU145 and LNCaP prostate cancer cells". Biol. Pharm. Bull. 29 (8): 1597–602. PMID 16880611. http://joi.jlc.jst.go.jp/JST.JSTAGE/bpb/29.1597?from=PubMed.
- ^ [[:Template:Cite website et al]]
External links
- Definition of glycosides, from the IUPAC Compendium of Chemical Terminology, the "Gold Book"
- IUPAC naming rules for glycosides
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