Terpenoids

Terpenoids, also known as isoprenoids, are a large and diverse class of natural products derived from isoprene units. They include hydrocarbons called terpenes as well as their oxygenated and rearranged derivatives. Terpenoids span a wide range of structures, from simple monoterpenes to complex polycyclic systems, and they are key constituents of essential oils and resins. Examples include menthol, limonene, farnesol, and carotenoids.

Biosynthesis of terpenoids proceeds from the building blocks isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). These units are produced via two main pathways: the mevalonate (MVA) pathway and the methylerythritol phosphate (MEP) pathway. In plants, both pathways operate in different cellular compartments. Terpene synthases convert IPP/DMAPP assemblies into diverse carbon skeletons, which are then modified by oxidation, glycosylation, and other enzymatic steps to yield the final terpenoid structures.

Classification commonly follows carbon count: monoterpenoids (C10), sesquiterpenoids (C15), diterpenoids (C20), sesterterpenoids (C25), triterpenoids (C30), and tetraterpenoids (C40). Notable examples include monoterpenoids such as limonene and menthol, sesquiterpenoids like farnesol, diterpenoids such as taxol, triterpenoids including sterols and squalene, and tetraterpenoids such as carotenoids.

Terpenoids serve ecological roles in plants and microbes, including defense, attraction of pollinators, and signaling. They have broad commercial importance in fragrances, flavors, medicines (artemisinin, paclitaxel), and nutrition, and are produced by extraction, microbial synthesis, or chemical modification.