Terpenoid

Terpenoids, also known as isoprenoids, are a large and diverse class of natural products built from isoprene units (C5H8). They arise biosynthetically from isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP). Through successive condensations, these five-carbon units form larger prenyl pyrophosphates such as GPP (C10), FPP (C15), and GGPP (C20), which serve as precursors to a wide range of terpenoids.

Terpenoids include hydrocarbons called terpenes and a broad set of oxygenated or rearranged derivatives. They are commonly classified by size: monoterpenes (C10), sesquiterpenes (C15), diterpenes (C20), triterpenes (C30), and tetraterpenes (C40), among others. Their structures can be acyclic or cyclic, and many carry multiple functional groups such as alcohols, aldehydes, ketones, epoxides, or esters.

Biosynthesis occurs via two main metabolic routes. The mevalonate pathway operates in animals, fungi, and many bacteria, while the non-mevalonate (MEP/DOXP) pathway functions in most bacteria and plant plastids; plants often use both. IPP and DMAPP are the universal five-carbon intermediates that are assembled into larger prenyl chains and then converted by cyclization, oxidation, and rearrangement to give the final terpenoids.

Terpenoids fulfill diverse biological roles in plants and other organisms, including pigments (carotenoids), aromas and flavors (limonene, pinene, menthol), hormones and signaling molecules (gibberellins, abscisic acid; steroid terpenoids), and defense compounds. Notable examples include beta-carotene, lycopene, artemisinin, taxol, squalene, and menthol. They are widely used in flavors, fragrances, pharmaceuticals, and potential biofuels.

The term terpenoid is sometimes used interchangeably with terpenes, but is often reserved for terpenes that have been chemically modified. Terpenoids occur throughout the plant kingdom and in some fungi and bacteria, and are typically extracted from essential oils or synthesized for research and industrial use.