Monoterpenoids, comprising two C5 isoprene units (C10H16), represent volatile secondary metabolites essential for plant defense, pollinator attraction, and pharmacological applications due to their structural diversity and membrane-disrupting properties.
Biosynthetic Pathways
Monoterpenoids derive exclusively from the plastidial MEP pathway: DMAPP + IPP → geranyl diphosphate (GPP, C10) via geranyl diphosphate synthase (GPPS), followed by ionization/cyclization by monoterpene synthases (TPS, EC 4.2.3.x). Linear precursors (myrcene, ocimene) arise via deprotonation; monocyclic (limonene, terpinene) via 4+2 cyclization; bicyclic (pinene, camphene, borneol) via subsequent 6+3 or 4+3 ring closure with Mg²⁺/Mn²⁺ cofactor. Oxygenated derivatives (menthol, linalool, geraniol) form via CYP450 oxidations.
Structural Diversity
Monoterpenoids display diverse structural classes:
- Acyclic: Myrcene, linalool, geraniol (allylic alcohols)
- Monocyclic: Limonene (endocyclic Δ), α-/β-phellandrene, terpinene, p-cymene
- Bicyclic: α-/β-pinene (pinane), camphor (camphane), menthol (menthane), thymol/carvacrol (thymol)
- Iridoids: Loganic acid, aucubin (iridane cyclopentanopyran)
