Structure. Isoprenoids (terpenoids) belong to the prenol lipids category. Their structure is based on five-carbon isoprene units, the building blocks of prenol lipids. They can be categorized by the number of isoprene units, such as triterpenes which are isoprenoids consisting of six isoprene units. The isoprene chain can be linear, cyclic, or branched and may feature hydrocarbon and oxygen containing substituents, such as hydroxyl or carboxyl groups.
Function. Isoprenoids are present in all living organisms where they serve important physiological roles. Squalenes and oxidized squalenes are precursors and intermediates in the biosynthesis of sterol lipids in eukaryotes and hopanoid lipids in prokaryotes as well as important sebum constituents. Retinoids, such as the vitamin A series, are involved in cellular growth and cell cohesion, and immunomodulatory effects.
Various oily and waxy substances such as fish liver oil, wool wax, and egg yolk contain appreciable amounts of isoprenoids. In plants, some isoprenoids serve as growth hormones such as gibberellins, pigments like the carotenoids or as essential oils, e.g. limonene. Their wide use in medical and industrial applications is compromised by the very limited amount of isoprenoids in living organisms.