Terpenoids and Terpenes

Terpenoids, the largest and most structurally diverse class of plant natural products, are built from repeating five-carbon isoprene units. They span volatile monoterpenes of essential oils, sesquiterpenes, the antimalarial sesquiterpene lactone artemisinin, diterpenes such as the anticancer scaffold of paclitaxel, triterpenes and sterols, and the carotenoid pigments.

Definition

Terpenoids (terpenes and their oxygenated and modified derivatives) are isoprenoid compounds assembled from five-carbon isoprene units and classified by the number of those units, ranging from monoterpenes (C10) through sesquiterpenes, diterpenes, and triterpenes to polymeric isoprenoids.

Scope

This topic covers the isoprene 'building block' rule, the mevalonate and methylerythritol-phosphate pathways that supply terpenoid precursors, the classification by isoprene-unit count, and the pharmacognostic importance of the class. It is reference chemistry and does not provide treatment guidance for any terpenoid drug.

Core questions

• What is the isoprene rule and how does it organise the terpenoid class?
• How do the mevalonate and MEP pathways supply terpenoid precursors?
• How are terpenoids classified, and which subclasses are pharmacologically important?

Key concepts

• Isoprene rule and C5 building blocks
• Mevalonate (MVA) and methylerythritol-phosphate (MEP) pathways
• IPP and DMAPP precursors
• Monoterpenes, sesquiterpenes, diterpenes, triterpenes
• Sterols and saponins
• Carotenoids
• Essential oils

Mechanisms

All terpenoids derive from the universal five-carbon precursors isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), supplied by the cytosolic mevalonate pathway and the plastidial methylerythritol-phosphate pathway. Prenyltransferases condense these units into geranyl, farnesyl, and geranylgeranyl diphosphates, which terpene synthases cyclise into the carbon skeletons of mono-, sesqui-, di-, and triterpenes; subsequent oxidation, glycosylation, and rearrangement generate the immense diversity of the class, including sterols, saponins, and carotenoids.

Clinical relevance

Terpenoids include important drug scaffolds — the antimalarial artemisinin and the anticancer taxane and triterpenoid leads among them — and the essential-oil constituents widely used in traditional and complementary practice, making the class central to pharmacognosy. This entry is descriptive reference chemistry and is not a basis for dosing or individual treatment decisions.

Evidence & guidelines

The relevant evidence is chemical and biosynthetic — pathway elucidation, structure studies, and reviews of terpenoid pharmacology such as surveys of triterpenoids and carotenoids — rather than clinical-trial evidence for the class as a whole. Individual terpenoid medicines are governed by their own clinical and regulatory monographs.

History

The recognition that many fragrant plant oils share a common five-carbon repeating unit gave rise to the isoprene rule, which organised an otherwise bewildering diversity of structures. Twentieth-century work established the mevalonate pathway and later the independent methylerythritol-phosphate route, clarifying how plants supply the universal terpenoid precursors.

Seminal works

• dzubak-2006
• dewick-2009

Frequently asked questions

What is the difference between a terpene and a terpenoid?

Terpenes are pure hydrocarbons built from isoprene units, while terpenoids are their oxygenated or otherwise chemically modified derivatives; in common usage the term terpenoid covers the whole class.

How are terpenoids classified?

They are grouped by the number of isoprene (C5) units: monoterpenes (C10), sesquiterpenes (C15), diterpenes (C20), triterpenes (C30), tetraterpenes such as carotenoids (C40), and larger polyterpenes.