Before we begin this article, it is important to draw your attention to the fact that many people often confuse glycosides with glucosides. While all glycosides are not glucosides, all glucosides are essentially glycosides. In fact, according to chemistry, glycosides are basically elements bound to a sugar, while in a glucoside, the sugar is actually glucose. Glycosides are soluble in water and alcohol.
In other words, glycosides are basically molecules wherein a sugar is attached to a non-carbohydrate element, generally a minor natural molecule. Glycosides have several important functions in all living organisms. A number of plants stock up compounds in the form of dormant glycosides. These chemicals may be set in motion by certain enzymes by means of hydrolysis that results in the sugar portion to be detached. The activation of these chemicals by the enzymes make them accessible for utilization. A number of such glycosides enclosed by the plants are of therapeutic value. However, in the case of animals and this includes humans, often venoms are attached to sugar molecules as a portion of their abolition from the body.
Precisely speaking, every molecule wherein a sugar group is attached by means of its anomeric carbon (C-1 carbon) to another group through an O-glycosidic attachment or an S-glycosidic link is called a glycoside. In fact, glycosides in which a sugar group is bound to another by means of an S-glycosidic attachment are also known as thioglycosides. In such cases, the sugar group is subsequently identified as glycone, while the non-sugar group is recognized as the genin or aglycone element of the glycoside. In fact, the glycone may comprise a solitary sugar group called monosaccharide or a number of sugar groups known as oligosaccharide.
Initially, glycosides were identified as combined acetals, or a molecule having two solitary connected oxygens bound to one carbon atom, that developed from a recurring variety of monosaccharides, in such a manner that there was a connection of a glycosyl group with a non-acyl group. The attachment between the -OR group and the glycosyl group is known as a glycosidic bond.
In glycosides that appear in nature, the inherent carbonate is expressed as the glycone, while the amalgam ROH, from which the carbonate reside is derived or split, is known as the aglycone or aglycon.
Hydrolysis and amalgamation
It is possible to isolate the glycone and aglycone segments of glycosides through the chemical process known as hydrolysis conducted in the presence of acid. In addition, several enzymes are also capable of forming as well as disintegrating the glycosidic attachments. While glycoside hydrolases are the most important enzymes that are effective in breaking the glycosidic bonds, glycosyltransferases are the key synthetic enzymes in nature that perform the task equally well. Transformed enzymes called glycosynthases have been developed so that they can materialize glycosidic attachments in exceptional yield.
In addition, there are several other effective chemical means by which glycosidic bonds can be developed artificially. Fischer glycosidation denotes the artificial process of amalgamation of glycosides by means of a reaction of unprotected monosaccharides with alcohols, generally in the solvent form, in the presence of a potent acid catalyst. On the other hand, the Koenigs-Knorr reaction involves the compression or condensation of glycosyl halides together with alcohols in the presence of metal salts like silver carbonate or mercuric oxide.
Glycosides may be categorized by different methods or according to different headings. For instance, glycosides may be classified according to the glycone, the nature of glycosidic bond as well as the aglycone.
- According to glycone
- When a glycoside’s glycone group is glucose, the molecule is called a glucoside, but if the glycone group of a glycoside is fructose, the molecule is called a fructose. Put in a different way, the concluding letter ‘e’ of the name of the analogous cyclic type of the monosaccharide is substituted by the suffix ‘ide’. In addition, in the event of a glucuronic acid, the molecule is known as a glucuronide and so on. In the human body, noxious materials are habitually attached to glucuronic acid to augment their solubility in water and the glucuronides formed subsequently are expelled from the system.
- According to the sort of glycosidic bond
- Glycosides are categorized as a-glycosides or ß-glycosides solely on the basis of the fact whether the glycosidic attachments are positioned ‘above’ or ‘below’ the level of the cyclic sugar molecule. While a number of enzymes like a-amylase are able to hydrolyze a-linkages, many others, for instance emulsin, are only able to have an influence on ß-linkages.
- According to aglycone
- Glycosides are also categorized depending on the chemical character of the aglycone. Categorization by this method is very helpful for the purposes of biochemistry as well as pharmacology.
- Alcoholic glycosides
- Salicin found in the genus Salix (willows) is a good instance of an alcoholic glycoside. Our body transforms salicin into salicylic acid that is intimately associated with aspirin and possesses analgesic (pain killing), anti-pyretic (alleviating fever) as well as anti-inflammatory properties.
- Anthraquinone glycosides
- These glycosides enclose an aglycone group that is derived from anthraquinone. These glycosides are found in aloes, rhubarb and senna. They possess a laxative or purgative property.
- Coumarin glycosides
- In this instance coumarin is an aglycone and apterin is an example of coumarin glycoside. Apterin is said to expand the coronary arteries and also functions as a calcium channel blocker. Coumarin aglycones acquired from dehydrated Psoralea corylifolia leaves possess the principal glycosides called corylifolin and psoralin.
- Cyanogenic glycosides
- In this instance, aglycone encloses a cyanide group and the glycoside is able to discharge the toxic hydrogen cyanide when there is any reaction with a number of enzymes. Amygdalin obtained from almonds is a good example of cyanogenic glycosides. A number of fruits as well floppy leaves belonging to the rose family, inclusive of apples, cherries, plums, peaches, almonds, apricots, crabapples and raspberries enclose cyanogenic glycosides. Cassava, a significant food plant found in South America and Africa, encloses cyanogenic glycosides and hence it is essential to carefully wash it under running water and ground before it can be ingested. Even sorghum (sorghum bicolor) possesses cyanogenic glycosides in its roots and hence, it is resilient to insects like rootworms (Diabrotica spp.), which infect the associated maize (Zea mays L.).
- Flavonoid glycosides
- The aglycone in this instance is a flavonoid. In fact, this one is a vast cluster of flavonoid glycosides and some of the examples of flavonoid glycosides include naringin, hesperidin, quercitrin and rutin. Flavonoid glycosides possess antioxidant properties and this is very beneficial for our overall wellbeing. The antioxidant effect of flavonoid glycosides are said to lessen capillary weakness.
- Phenolic glycosides (Simple)
- In this case, a simple phenolic arrangement is the aglycone. Arbutin, present in common bearberry is an example of phenolic glycoside (simple). Rutin is present in rooibos tea. Phenolic glycosides (simple) possess antiseptic properties that are useful for the health of the urinary tract.
- Glycosides with foaming features are known as saponins. Saponins comprise polycyclic aglycones bound to one or many sugar side chains and the aglycone segment of the glycoside, also known as sapogenin, is a steroid or a triterpene. Saponins possess foaming aptitude and this is the outcome of the amalgamation of a hydrophilic (water soluble) sugar segment of the glycoside and a hydrophobic (fat soluble) sapogenin. Generally, saponins taste bitter and some of them are also poisonous. The toxic saponins are called sapotoxin. When wobbled with water, these compounds create permanent foam. The compounds also result in the hemolysis of the red blood cells (the breaking down of red blood cells with liberation of hemoglobin). Liquorice encloses saponin glycosides. Saponin glycosides have a prominent therapeutic benefit as they possess expectorant properties.
- Steroidal glycosides or cardiac glycosides
- In this case, the aglycone segment is a steroidal nucleus. Generally, steroidal glycosides or cardiac glycosides are present in the plant genera Digitalis, Strophanthus and Scilla. As the name of this glycoside suggests, they are used to treat heart conditions such as congestive heart failure and arrhythmia (any trouble in the rhythm of the heartbeat). However, presently medical practitioners prefer other agents than these glycosides to treat conditions like congestive heart failure.
- Steviol glycosides
- These glycosides have a sweet flavor and are present in the stevia plant (Stevia rebaudiana bertoni). Steviol glycosides present in the stevia plant possess as much as 40 to 300 times more sweetness in comparison to sucrose. In many countries, people use the two basic glycosides – stevioside and rebaudioside A – as accepted sweeteners. Steviol is the aglycone segment of steviol glycosides and rhamnose-glucose or glucose mishmashes are attached to the terminals of the aglycone to develop diverse compounds. It may be noted here that the steviol glycosides are stable to heat, pH changes and are also not subject to fermentation. When these glycosides are consumed, they also do not stimulate any glycemic response and as a result of this, steviol glycosides are preferred as natural sweeteners for diabetic patients as well as others who take low carbs diets.
- As the name of these glycosides suggest, these amalgams enclose sulfur. Sinigrin, present in black mustard, as well as sinalbin, present in white mustard, are good examples of thioglycosides.