Isoprenoids

Isoprenoids, also known as terpenoids, are a broad and diverse class of natural products derived from the five-carbon isoprene unit (C5H8). They range from simple hydrocarbons to complex polycyclic compounds and include essential pigments, hormones, vitamins, and industrial materials. The basic building blocks are isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP), produced by the mevalonate pathway in animals, fungi, and archaea, and by the MEP (non-mevalonate) pathway in many bacteria and plant plastids. Condensation of IPP with allylic diphosphates by prenyltransferases yields chain-elongated intermediates such as geranyl pyrophosphate (GPP, C10), farnesyl pyrophosphate (FPP, C15), and geranylgeranyl pyrophosphate (GGPP, C20), which then undergo cyclization and further modification to generate the vast diversity of isoprenoids.

The functions of isoprenoids span primary metabolism and secondary metabolism. In primary metabolism, they contribute to cofactors and membrane components, including ubiquinone (coenzyme Q), plastoquinone, tocopherols (vitamin E), and the side chains of chlorophyll and carotenoids. In secondary metabolism, they form monoterpenes, sesquiterpenes, diterpenes, triterpenes, and tetraterpenes such as carotenoids, as well as sterols (e.g., cholesterol and phytosterols) and polyisoprenoid polymers like natural rubber.

Isoprenoids are widespread in all domains of life and are of particular economic and ecological importance, providing flavors and fragrances, pharmaceuticals (for example, taxanes and antimalarials), vitamins, pigments, and industrial materials. The field continues to reveal new compounds and biosynthetic pathways, underscoring the versatility of the isoprenoid building block concept.