glycosides

Glycosides are a class of compounds in which a sugar moiety, called the glycone, is covalently bonded to a non-sugar moiety, the aglycone, through a glycosidic linkage. The linkage is most commonly O-glycosidic, but glycosidic bonds to nitrogen (N-glycosides) or sulfur (S-glycosides) also occur.

In nature they are widespread, especially in plants, where glycosides serve as storage forms of bioactive aglycones or as defense compounds. The sugar unit increases solubility and can mask or attenuate activity. Most glycosides are activated by hydrolysis, releasing the aglycone and a sugar residue, catalyzed by specific glycosidases or by acidic conditions in the digestive tract.

Examples include cardiac glycosides such as digoxin and digitoxin, whose aglycones are steroidal and which act by inhibiting Na+/K+-ATPase; cyanogenic glycosides like amygdalin, which can release hydrogen cyanide on hydrolysis; and various flavonoid and phenolic glycosides that contribute to plant color and solubility. Saponins, tannins, and many other natural products also occur as glycosides with diverse biological activities.

Biosynthesis mainly occurs through glycosyltransferases that attach sugar residues from activated donors (e.g., UDP-glucose) to acceptor molecules. In humans and other animals, dietary glycosides are often hydrolyzed in the gut to release active aglycones, which can be absorbed or detoxified. Glycosides also play roles in pharmacology, nutrition, and plant physiology, and are studied for their medicinal use and ecological function.