Terpenoids: Lower

Abstract

Lower terpenoids (monoterpenes and sesquiterpenes) are volatile components of the scents and fragrances of aromatic plants. They are frequently found in glandular hairs on leaves, in the scent glands of flowers or in specialized resin ducts in conifers. They are synthesized through stepwise condensations of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), the universal intermediates of isoprenoid metabolism. The precursors of monoterpenes are mainly supplied by the 2‐methylerythritol 4‐phosphate (MEP) pathway in plastids, whereas most sesquiterpenes are produced from acetate via the cytosolic mevalonate pathway. Subtle variations of basic terpene carbon skeletons have resulted in the appearance of tens of thousands of unique structures in nature, most of whose functions are currently unknown. As volatile, organic compounds, many participate in plant–plant or plant–insect chemical communications, as pollination vector attractants, feeding deterrents or in direct plant defence. Many have potent biological activities and are used in pharmaceutical and industrial applications.

Key concepts

  • Lower terpenoids (monoterpenes and sesquiterpenes) are volatile C10 and C15 compounds frequently present in floral emissions. Lower terpenoids emitted from flowers may attract symbiotic insects which assist the plant in pollination.

  • When some plants are attacked by insects or nematodes, they release volatile terpenoids which summon parasitic wasps who indirectly provide plants with protection by preying on their attackers.

  • Most plants have highly specialized structures such as trichomes where terpenoid biosynthesis and storage take place.

  • Lower terpenoids share a common biosynthetic origin with all other terpenoids through IPP and DMAPP, the universal C5 metabolic precursors of all isoprenoids.

  • Two parallel pathways exist for the formation of IPP and DMAPP in plants, the methylerythritol phosphate pathway in plastids and the mevalonate pathway in the cytosol.

  • Lower terpenoids are frequently chemically modified after their initial biosynthesis to give them highly specific biological activities.

  • In a limited number of plant families, the monoterpene geraniol is used in the biosynthesis of monoterpene indole alkalkoids, a highly specialized family of natural products with potent pharmacological properties in humans.

  • C15 sesquiterpenes are structurally diverse and are often modified through the addition of a lactone ring, which generally makes them extremely toxic to animals.

Keywords: plant volatiles; essential oils; natural products; chemical ecology; specialized metabolism; plant‐insect interactions

Figure 2.

Commonly occurring monoterpenes.

Figure 4.

Representative iridoid monoterpene structures. 20, loganin; 21, seco‐loganin; 22, strictosidine; 23, nepetalactone; 24, catalpol and 25, aucubin. Glc indicates a glucose moiety and Me a methyl group.

Figure 6.

Representative sesquiterpene structures. 26, α‐farnesene; 27, β‐farnesene; 28, α‐bisabolol; 29, δ‐cadinene; 30, β‐caryophyllene; 31, carotol; 32, rishitin; 33, juvabione; 34, α‐bisabolene; 35, (E)‐nerolidol; 36, DMNT; 37, α‐bergamotene and 38, cantharidin (C10 but thought to be derived from FPP). OMe denotes an O‐linked methyl group. Double arrows indicate multiple biosynthetic steps.

Figure 7.

Representative examples of biologically active sesquiterpene lactones. 39, alatolide; 40, alantolactone; 41, artabsin; 42, ambrosin; 43, 8‐deoxylactucin; 44, geigerin; 45, parthenin and 46, parthenolide.

Figure 1.

Formation of the branch point intermediates of the principal terpenoid biosynthetic pathways from central metabolic intermediates. The universal isoprenoid intermediates IPP (1) and DMAPP (2) are derived from pyruvate (3) and glyceraldehyde 3‐phosphate (4) in the plastidic MEP pathway. In the cytosol, IPP and DMAPP required for sesquiterpene and triterpene formation are derived from acetyl‐CoA (10) via the mevalonate pathway. OP denotes the phosphate moiety and OPP the diphosphate moiety.

Figure 3.

The reaction mechanism for (−)‐limonene synthase. Beginning with GPP, limonene synthase is produced without release of detectable‐free intermediates. However, mechanistic studies have confirmed LPP (11) as a veritable enzyme‐bound intermediate en route to formation of the final cyclized product. The initial ionization/isomeration step is considered to be the rate‐limiting step of the reaction. OPP denotes the diphosphate moiety.

Figure 5.

Biosynthesis of loganin from GPP.

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Phillips, Michael A, and Rodríguez Concepción, Manuel(Mar 2010) Terpenoids: Lower. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001915.pub2]