160
10
important source of essential oils for the flavour, fragrance and aromatherapy industries.
Due to its binding to the cold-sensing ion channel TRPM8 menthol can trigger a cooling
sensation (Kamatou et al. 2013). The enzymatic reactions occur in specialized anatomical
structures of the epidermis called glandular trichomes. The limonene synthase of Mentha
× piperita has been localized only to the leucoplasts (colourless plastids) of gland secretory
cells, indicating that the essential oil production occurs within these cells. However, the
succeeding steps of monoterpene modification appear to occur outside of the leucoplasts
in mint (Turner et al. 1999).
Additional ring closure via the terpinyl cation leads to bicyclic compounds such as
careen (in pine, rosemary and cedar), camphene (in turpentine, cypress, camphor, ginger, valerian), 1,8-cineole (in eucalyptus, bay leaves, sage), pinene (in pine resin, pines,
spruces and firs, fruits of Pistacia terebinthus), sabinene (in Quercus ilex, Picea abies,
black pepper, nutmeg), fenchol (in rosemary, basil, hops) and camphor. Camphor is biosynthetically produced in plants by way of borneol (Croteau et al. 1981). Besides being
present in sage, camphor oil can be isolated by distillation from the wood of the camphor
tree (Cinnamomum camphora, Lauraceae) or is found in camphor basil (Ocimum kilimandscharicum, Lamiaceae). It has served as a fumigant against the Black Death and as
an embalming fluid (Chen et al. 2013). These days it is used as an insect repellent and as
fragrance, which can induce cold and warm sensations, although it can be toxic at higher
dosage. Pinene is discussed as a cannabis antidote as it forms the biosynthetic base for
CB2 ligands and has been suggested to act as an acetylcholinesterase inhibitor aiding
memory (Russo 2011). Sabinene can further be oxidized to form α-thujone (in Thuja,
Juniperus, sage, thyme, rosemary), which is known for its presence in the absinthe drink
and which acts as a GABA antagonist. All these aforementioned primary products can
then be further modified by secondary reactions such as hydroxylation, peroxidation,
methylation, acylation, glycosylation or cleavage (often by Cyt P450 enzymes), which
increases the diversity of these compounds.
Iridoids, characterized by their cyclopentane (C5) ring fused with a pyran (C6) ring,
are formed via a different cyclization step. Geraniol is first oxidized and then undergoes
cyclization by the iridoid synthase (De Luca et al. 2014; Ilc et al. 2016). Iridoids, such as
aucubin (in Plantago) and amarogentin (in gentian, Gentiana lutea), are usually stabilized by a sugar moiety. This sugar moiety also allows storage of a less toxic compound.
Amarogentin is known for its bitter taste. The iridoid loganic acid (which is also fused to
a glucose moiety) is converted to loganine and further secologanin, the last step being an
oxidative cleavage of the C5 ring. Secologanin reacts with tryptamine forming strictosidine, leading to indole alkaloids (see 7 Sect. 12.2 in 7 Chap. 12).
The fusion of two molecules of dimethylallyl pyrophosphate (DMAPP) occurs via c1′–
2–3 cyclopropanation by the enzyme chrysanthemyl diphosphate synthase yielding chrysanthemyl pyrophosphate (Yang et al. 2014). Chrysanthemyl pyrophosphate is further
converted into chrysanthemol by hydrolysis of the diphosphate moiety, is then oxidized
to chrysanthemic acid and further esterified to pyrethric acid. The acids are esterified
with one of three alcohols (pyrethrolone, cinerolone or jasmolone), which are probably
derived from jasmonate precursors through the oxylipin pathway. This yields the different
forms of pyrethroids. The chrysanthemyl diphosphate synthase can be found in glandular
trichomes, whereas the later synthesis steps take place in the pericarp. Pyrethrins can
be extracted from dried pyrethrum flowers (Chrysanthemum; Tanacetum cinerariifolium)
and constitute a well-known insecticide, which affects the Na + -channels of nerve cells
especially in insects.
Chapter 10 · Terpenes and Terpenoids
10
important source of essential oils for the flavour, fragrance and aromatherapy industries.
Due to its binding to the cold-sensing ion channel TRPM8 menthol can trigger a cooling
sensation (Kamatou et al. 2013). The enzymatic reactions occur in specialized anatomical
structures of the epidermis called glandular trichomes. The limonene synthase of Mentha
× piperita has been localized only to the leucoplasts (colourless plastids) of gland secretory
cells, indicating that the essential oil production occurs within these cells. However, the
succeeding steps of monoterpene modification appear to occur outside of the leucoplasts
in mint (Turner et al. 1999).
Additional ring closure via the terpinyl cation leads to bicyclic compounds such as
careen (in pine, rosemary and cedar), camphene (in turpentine, cypress, camphor, ginger, valerian), 1,8-cineole (in eucalyptus, bay leaves, sage), pinene (in pine resin, pines,
spruces and firs, fruits of Pistacia terebinthus), sabinene (in Quercus ilex, Picea abies,
black pepper, nutmeg), fenchol (in rosemary, basil, hops) and camphor. Camphor is biosynthetically produced in plants by way of borneol (Croteau et al. 1981). Besides being
present in sage, camphor oil can be isolated by distillation from the wood of the camphor
tree (Cinnamomum camphora, Lauraceae) or is found in camphor basil (Ocimum kilimandscharicum, Lamiaceae). It has served as a fumigant against the Black Death and as
an embalming fluid (Chen et al. 2013). These days it is used as an insect repellent and as
fragrance, which can induce cold and warm sensations, although it can be toxic at higher
dosage. Pinene is discussed as a cannabis antidote as it forms the biosynthetic base for
CB2 ligands and has been suggested to act as an acetylcholinesterase inhibitor aiding
memory (Russo 2011). Sabinene can further be oxidized to form α-thujone (in Thuja,
Juniperus, sage, thyme, rosemary), which is known for its presence in the absinthe drink
and which acts as a GABA antagonist. All these aforementioned primary products can
then be further modified by secondary reactions such as hydroxylation, peroxidation,
methylation, acylation, glycosylation or cleavage (often by Cyt P450 enzymes), which
increases the diversity of these compounds.
Iridoids, characterized by their cyclopentane (C5) ring fused with a pyran (C6) ring,
are formed via a different cyclization step. Geraniol is first oxidized and then undergoes
cyclization by the iridoid synthase (De Luca et al. 2014; Ilc et al. 2016). Iridoids, such as
aucubin (in Plantago) and amarogentin (in gentian, Gentiana lutea), are usually stabilized by a sugar moiety. This sugar moiety also allows storage of a less toxic compound.
Amarogentin is known for its bitter taste. The iridoid loganic acid (which is also fused to
a glucose moiety) is converted to loganine and further secologanin, the last step being an
oxidative cleavage of the C5 ring. Secologanin reacts with tryptamine forming strictosidine, leading to indole alkaloids (see 7 Sect. 12.2 in 7 Chap. 12).
The fusion of two molecules of dimethylallyl pyrophosphate (DMAPP) occurs via c1′–
2–3 cyclopropanation by the enzyme chrysanthemyl diphosphate synthase yielding chrysanthemyl pyrophosphate (Yang et al. 2014). Chrysanthemyl pyrophosphate is further
converted into chrysanthemol by hydrolysis of the diphosphate moiety, is then oxidized
to chrysanthemic acid and further esterified to pyrethric acid. The acids are esterified
with one of three alcohols (pyrethrolone, cinerolone or jasmolone), which are probably
derived from jasmonate precursors through the oxylipin pathway. This yields the different
forms of pyrethroids. The chrysanthemyl diphosphate synthase can be found in glandular
trichomes, whereas the later synthesis steps take place in the pericarp. Pyrethrins can
be extracted from dried pyrethrum flowers (Chrysanthemum; Tanacetum cinerariifolium)
and constitute a well-known insecticide, which affects the Na + -channels of nerve cells
especially in insects.
Chapter 10 · Terpenes and Terpenoids