ISOPRENOID

any of a class of organic compounds composed of two or more units consisting of five carbon atoms in a specific pattern. Isoprenoids play widely varying roles in the physiological processes of plants and animals. They also have a number of commercial uses. Isoprenoids in living organisms range in function from pigments and fragrances to vitamins and precursors of sex hormones. One of the most familiar natural substances, rubber, is a polyisoprene. Other commercially valuable isoprenoids are those used as flavourings, solvents, and raw materials for chemicals. The five-carbon unit that constitutes the basic building-block of isoprenoids is a hydrocarbon called isoprene. Isoprene (2-methyl-1,3-butadiene) is a branched-chain, unsaturated hydrocarbon, unsaturated meaning it contains one or more double bonds between carbon atoms. Isoprene has, in fact, two carboncarbon double bonds. Isoprenoids contain from two to many thousands of isoprene units. Most of the natural isoprenoids are composed of two, three, four, six, or eight isoprenes. Those containing two to four isoprenes are formed by simple addition of the five-carbon units. The six- and eight-unit molecules are made by linking together two three-unit and four-unit molecules, respectively. The carbon backbone can have one or more functional chemical groups, such as hydroxyl and carbonyl, attached to it. These help to give isoprenoids their diversity. The isoprenoids are also known as terpenes. The smallest terpene moleculesthose containing 10 carbon atomsare called monoterpenes. The larger molecules, which increase by five-carbon units at a time, are called sesquiterpenes, diterpenes, triterpenes, and tetraterpenes, respectively. The monoterpenes are mostly volatile, which accounts for their fragrances. Terpenes of higher molecular weight are less volatile, although sesquiterpenes contribute to the flavours of some foods. Adding water molecules to a pyrophosphate group connected to a monoterpene produces geraniol, a component of geranium oil and an important contributor to rose perfumes. Related molecules are menthol from peppermint oil; citral from lemongrass oil; and limonene from lemon and orange oils. Other monoterpenes include pinene, from turpentine, and camphor. Many of the larger terpenes are cyclici.e., they contain ring structures. However, the diterpene called phytol, an important precursor of the plant pigment chlorophyll, is acyclic. A related diterpene that contains one ring of six carbon atoms is vitamin A. One of the most important triterpenes is squalene. Although acyclic, it is a precursor of the four-ring steroids that play a key role in controlling many biological activities. Squalene is found widely in nature, notably in fish-liver oils. Five-ring triterpenes, which are derived from squalene, like steroids, are found in many plants. They are often linked chemically to sugars to form compounds called saponins. The tetraterpenes include many plant pigments such as lycopene, which accounts for the red of tomatoes, and -carotene, which accounts for the yellow in carrots. -carotene is a symmetrical molecule that the digestive process splits to produce vitamin A. Polyterpenes are polymers having numerous isoprene units. The best known is natural rubber, in which 4,000 to 5,000 five-carbon units are linked together. Another polymeric isoprene is gutta-percha, a tough plastic substance used in insulation and in dentistry.