Glycoside

Glycosides are a class of molecules in which, a sugar molecule is bonded to a “non-sugar” molecule. Glycosides play important roles in our lives. Many plants store medicinally important chemicals in the form of inactive glycosides. The non-sugar portion contains the biochemically active properties of medical interest. Once the glycoside is split into its two components (sugar and non-sugar parts), the non-sugar component is now free to exert its chemical effects on the body. For example, digitalis is a glycoside that when ingested, causes the heart to contract (pump) more forcefully. This is useful in medicine, where heart failure is present.

In chemical nomenclature (description), a glycoside is a molecule in which, a sugar group is bonded to another group through an O-glycosidic bond (a bond containing oxygen) or an S-glycosidic bond (a sulfur-containing bond); Glycosides with sulfur-containing bonds are also referred to as thioglycosides. Using the definition in which, the sugar molecule must be bonded to a non-sugar for the molecule to qualify as a glycoside, we can refer to each unit as the glycone (sugar) and the aglycone (non-sugar or genin part) of the glycoside. The glycone portion can be a single sugar group (monosaccharide) or several sugar groups (oligosaccharide). Also note, that like S-glycosidic and O-glycosidic bonds, there are also N-glycosidic bonds in nature. We bring this to your attention, because some sources do not consider molecules with N-glycosidic bonds as glycosides, but instead, classify them as glycosylamines. Those who do consider them glycosides, call them N-glycosides. We only elaborate upon this, to prevent confusion in the case you see these terms while using other sources.

Classification

Glycosides can be classified by the aglycone, glycone, or glycosidic bond.

Classification by aglycone

We can also use the chemical structure of the aglycone in order to classify glycosides. This is probably the most fun and useful way to classify glycosides. After all, the aglycone is what gives the glycoside its chemical, and thus, pharmaceutical (medicinal) properties.

Classification by glycone

If the glycone portion of a glycoside is glucose, then we refer to the molecule as a glucoside. 
If the glycone portion of the glycoside is fructose, then we refer to the molecule as fructoside. 
If the glycone portion of the glycoside is glururonic acid then we refer to the glycoside as a glucuronide. In the human, toxic substances are often bonded to glucuronic acid. The glucuronic acid functions to increase the water solubility of the glycoside, which can be more easily secreted.

Classification by glycosidic bond

Glycosides can be classified as alpha-glycosides or beta-glycosides as well. This classification depends on whether the glycosidic bond lies “above” or “below” the plane of the sugar part of the glycoside. You may think, “Why all the fuss over classifications?” These systems for classification have relevance in certain ways, and that’s why it’s helpful to know them. For example, using classification by type of bond, to categorize a glycoside gives us a clue towards what enzymes can actually work on them (cleave them into their subunits). Enzymes such as alpha-amylase can only hydrolize a-linkages. We have amylase in our saliva. This enzyme can begin breaking down carbohydrates as we chew them. Emulsin (another enzyme), only breaks beta-linkages.

Glycosides continued

Alcoholic glycosides

Salicin is an example of an alcholic glycoside. Salicin is found in the genus of plants known as Salix. In the human and other animals, salicin is converted into salicylic acid (related to aspirin). and has analgesic (pain relieving), antipyretic (reduces fever) and antiinflammatory (reduces pain, swelling and redness, if present) effects.

Anthraquinone glycosides

These glycosides contain an aglycone group that is a derivative of anthraquinone. Anthraquinone glycosides are found in senna, aloe and rhubarb and they have a laxative effect.

Coumarin glycosides

Coumarin glycosides are medically important. In these glycosides, coumarin is the aglycone (non-sugar portion). Apterin is a good example of a coumarin glycoside. Apterin is found in the leaves of Psoralia corylifolia, and has the ability to dilate coronary arteries (arteries of the heart). This is important because the coronary arteries supply the heart with freshly oxygentated blood. These arteries have a tendency to accumlate cholesterol plaques, resulting in “clogging” or narrowing of the arteries. This narrowing allows less blood through when the heart requires it, resulting in angina (narrowing of arteries with associated pain) and possiblly a heart attack (myocardial infarction: myo=muscle, cardial=referring to heart, infarction=death of tissue). Coumarin glycosides dilate the arteries, and can help when symptoms arise due to narrowing, until surgical intervention can be implemented. Psoralin, apterin, corylifolin and other glycosides found in Psoralia species are thus, very useful medically.

Steroidal glycosides or cardiac glycosides

In these glycosides, the aglycone is a steroidal nucleus. Cardiac or steroidal glycosides are found in the plant genera Digitalis, Strophanthus, and Scilla. As the name implies, “cardiac” glycosides have a strong effect on cardiac tissue (myocardiu, the contractile tissue of the heart). Cardiac glycosides are prescribed in the treatment of some types of heart disease (e.g., heart failure).

Cyanogenic glycosides

Cyanogenic glycosides are simply glycosides, in which 
1) the aglycone contains a cyanide group and 
2) poisonous hydrogen cyanide can be released from the glycoside if acted upon by an appropriate enzyme. 
Cyanogenic glycosides can be found in quite a few plants and their structures. Cassava, cherries, almonds, apples, plums, apricots, rasberries, peaches, and crab apples contain cyanogenic glycosides. Amygdalin is the cyanogenic glycoside found in almonds. In peach pits, cyanogenic glycosides have been used to make some of the older anti-neoplastic drugs. Although these glycosides can be harmful, they can also help us when we are ill.

Flavonoid glycosides

These glycosides have an aglycone that is referred to as a flavonoid. Flavinoid glycosides perform many useful functions, such as exerting antioxidant activity and preserving the strength of capillaries (tiny blood vessels). 
Examples of flavonoid glycosides include: 
Rutin [(aglycone: Quercetin), (glycone: Rutinose)]
Hesperidin [(aglycone: Hesperetin), (glycone: Rutinose)] 
Quercitrin [(aglycone: Quercetin), (glycone: Rhamnose)]
Naringin [(aglycone: Naringenin), (glycone: Rutinose)]

Thioglycosides

Thio refers to “sulfur” and thus, thioglycosides are ones, which contain sulfur. 
Examples
Sinalbin- a thioglycoside found in white mustard. Sinigrin, the thioglycoside found in black mustard.

Saponin glycosides

saponin means “soap.” Saponin glycosids form a “soapy froth” when combined with water and shaken. These glycosides can be found in licorice, where their medicinal importance is because of their expectorant (causing the “expelling” of phlegm) ability. Saponin glycosides can also cause hemolysis of red blood cells. what else can cause red blood cell pathologies? click here to learn about jequirity beans, castor beans, and fava beans.

Steviol glycosides

These are sweet glycosides found in the plant Stevia rebaudiana bertoni. The steviol glycosides are up to several hundred times sweeter than sucrose The two main stevial glycosides are rebaudioside A and stevioside. They are considered safe and are used as natural sweeteners around the world. These Steviol glycosides contain steviol as the aglycone (non-sugar portion), where glucose or rhamnose-glucose combinations the aglycone to form different types of glycosides.

Phenolic glycosides

These are glycosides, in which the aglycone is a phenolic structure. 
Example 
Arbutin-a glycoside found in Arctostaphylos (common bearberry), which has an antiseptic (against microbes) effect. Rooibos teas are consumed because of the rutin they contain. As the rutin is consumed, it passes from the digestive tract, into the blood stream. The kidneys, while filtering the blood, remove the rutin and incorporate it into urine. The rutin then acts as a urinary antiseptic en route to the bladder and urethra. Hence, its notoriety for treating and preventing urinary tract infections.