Glycosides are unique molecules used by plants to safely store potentially dangerous compounds, and by animals to eliminate toxins. They consist of a sugar group, called a glycone, chemically attached to a non-carbohydrate compound. The non-carbohydrate is referred to as an aglycone, or a genin. These molecules have a variety of natural uses, such as defense mechanisms, as well as commercial ones, like medications, cleaning agents, and natural sweeteners.
The sugar group can be a single sugar, like glucose, or a group of sugars. Enzymes known as glycosyltransferases create bonds between the glycone and the aglycone in a process called glycosylation. The glycosidic bond can be broken by other enzymes called glycoside hydrolases.
Plants use glycosyltransferases to make a variety of glycoside compounds. These compounds often consist of potent chemicals, some of which include known medications and poisons. The glycoside structure renders the chemical inert until the plant must use it. Animals that have ingested these compounds may use their own enzymes to sequester it until it can be eliminated.
Some medications, like the steroids found in Digitalis plants, are present as glycosides, which in turn form the basis for heart medications. Other steroid-based glycosides can be used as the starting point to manufacture semi-synthetic drugs like glucocorticoids. Often the glycone groups are removed from the glycosides before their commercial use, but not in all cases. Glycosides found in some stevia plants have sweeter tastes than sugar, and are used as flavoring agents with the glycone kept intact.
Amygdalin, found in almonds, is an example of a poisonous aglycone. This contains cyanide that would normally harm the plant. The plant stores amygdalin as a glycoside to avoid harming itself. If the plant is damaged by another organism, enzymes break the glycoside bond and release the amygdalin. This process allows the plant to defend itself under the right conditions.
The pharmaceutical industry is making use of glycosides to manipulate the properties of drugs that contain natural sugars. Changing sugars can often change the pharmacological effects or properties of drugs. Chemicals or enzymes can be used for glycorandomization, which creates an array of natural product derivatives with different bound sugars.
Glycosylation generally occurs via enzyme catalysts in nature. A synthetic method known as chemical glycosylation allows this process to be performed in laboratories and manufacturing plants without requiring enzymes. A chemical known as an activator removes a group from a sugar, allowing it to accept a bond with the aglycone.