167
10
(Lindemann 2015; Gurel et al. 2017). Another toxic cardiac glycoside is oleandrin, which
is found in laticifers especially of oleander (Nerium oleander, Apocynaceae).
Triterpenes are folded into the chair-chair-chair conformation prior to cyclization,
leading to a dammarenyl carbocation intermediate (. Fig. 10.6). The triterpene synthases
catalyse the synthesis of tricyclic, tetracyclic and pentacyclic molecules by concerted reaction steps of single enzymes. As in the previous groups, Cyt P450 enzymes are essential in modifying the scaffolds further to give rise to more elaborate molecules (Ghosh
2017). Triterpenes are often found in plants in a glycosylated form, the so-called saponins.
Glycosylation increases the polarity and therefore the water solubility of the triterpenes,
which are per se hydrophobic. These amphiphilic properties of the compounds lead to
the formation of foam in water, which explains the name saponin (soap). Furthermore,
they allow the molecules to insert themselves into membranes, often due to interaction
with cholesterols (Lorent et al. 2014). This perturbs the membranes and induces tensions
leading to membrane perforation. When acting on red blood cells, which contain high
amounts of cholesterol, this can lead to haemolysis. Saponins can also influence the micelle
formation between sterols and bile acids, which is necessary for sterol absorption in the
intestine, thereby interfering with cholesterol uptake. Saponins are defensive secondary
metabolites that allow plants to cope with unfavourable environmental conditions (storing and conserving water, resisting predators and surviving severe weather conditions).
The dammarenyl carbocation can also give rise to protopanaxadiol and further the
saponin ginsenoside (from the root of ginseng (Panax ginseng, Araliaceae)). Ginsenosides
activate apoptosis in animals by triggering fast-mediated cell death through interference
with membrane lipid rafts. Moreover, their structures are similar to steroid hormones,
and they have been indicated as agonists for multiple steroidal receptors in mammalians
(Kennedy and Scholey 2003; Park et al. 2017). Other prominent triterpenes are amyrin
and lupeol. Lupeol gives rise to the potential anticancer agent betulinic acid, found in the
bark of several trees including birch. α-Amyrin gives rise to ursolic acid, which is found
in peels of fruits such as apples and could be acting on the growth-controlling pathways in
mammalians. β-Amyrin is transformed into oleanic acid, which possibly has anti-cancer
properties, and the saponines avenacin and glycyrrhizin. Avenacin is an antifungal component in the oat root (Avena sativa, Poaceae) and glycyrrhizin is the sweet-tasting constituent of liquorice (in Glycyrrhiza glabra, Fabaceae) that accumulates in root and stolon.
From the sap of the fire tree (Euphorbia tirucalli, Euphorbiaceae), the tetracyclic euphol,
an anti-inflammatory drug, can be extracted (Dutra et al. 2012).
Take-Home Messages
5 Terpenes and terpenoids are synthetized by two different pathways, the cytosolic MVA pathway and the plastid MEP pathway.
5 Terpene synthetases are involved in many enzymatic steps of biosynthesis.
5 Monoterpenes include menthol and thujone and provide the precursors for
indole alkaloids.
5 Diterpenes include salvinorin A, ryanodine and resiniferatoxin.
5 Sesquiterpenes include ß-caryophyllene and GABA antagonists such as picrotoxin
and tutin.
5 Especially monoterpenes and sesquiterpenes can be extracted as essential oils.
10.6 · Sterols and Triterpene (From MVA Pathway, C30)
10
(Lindemann 2015; Gurel et al. 2017). Another toxic cardiac glycoside is oleandrin, which
is found in laticifers especially of oleander (Nerium oleander, Apocynaceae).
Triterpenes are folded into the chair-chair-chair conformation prior to cyclization,
leading to a dammarenyl carbocation intermediate (. Fig. 10.6). The triterpene synthases
catalyse the synthesis of tricyclic, tetracyclic and pentacyclic molecules by concerted reaction steps of single enzymes. As in the previous groups, Cyt P450 enzymes are essential in modifying the scaffolds further to give rise to more elaborate molecules (Ghosh
2017). Triterpenes are often found in plants in a glycosylated form, the so-called saponins.
Glycosylation increases the polarity and therefore the water solubility of the triterpenes,
which are per se hydrophobic. These amphiphilic properties of the compounds lead to
the formation of foam in water, which explains the name saponin (soap). Furthermore,
they allow the molecules to insert themselves into membranes, often due to interaction
with cholesterols (Lorent et al. 2014). This perturbs the membranes and induces tensions
leading to membrane perforation. When acting on red blood cells, which contain high
amounts of cholesterol, this can lead to haemolysis. Saponins can also influence the micelle
formation between sterols and bile acids, which is necessary for sterol absorption in the
intestine, thereby interfering with cholesterol uptake. Saponins are defensive secondary
metabolites that allow plants to cope with unfavourable environmental conditions (storing and conserving water, resisting predators and surviving severe weather conditions).
The dammarenyl carbocation can also give rise to protopanaxadiol and further the
saponin ginsenoside (from the root of ginseng (Panax ginseng, Araliaceae)). Ginsenosides
activate apoptosis in animals by triggering fast-mediated cell death through interference
with membrane lipid rafts. Moreover, their structures are similar to steroid hormones,
and they have been indicated as agonists for multiple steroidal receptors in mammalians
(Kennedy and Scholey 2003; Park et al. 2017). Other prominent triterpenes are amyrin
and lupeol. Lupeol gives rise to the potential anticancer agent betulinic acid, found in the
bark of several trees including birch. α-Amyrin gives rise to ursolic acid, which is found
in peels of fruits such as apples and could be acting on the growth-controlling pathways in
mammalians. β-Amyrin is transformed into oleanic acid, which possibly has anti-cancer
properties, and the saponines avenacin and glycyrrhizin. Avenacin is an antifungal component in the oat root (Avena sativa, Poaceae) and glycyrrhizin is the sweet-tasting constituent of liquorice (in Glycyrrhiza glabra, Fabaceae) that accumulates in root and stolon.
From the sap of the fire tree (Euphorbia tirucalli, Euphorbiaceae), the tetracyclic euphol,
an anti-inflammatory drug, can be extracted (Dutra et al. 2012).
Take-Home Messages
5 Terpenes and terpenoids are synthetized by two different pathways, the cytosolic MVA pathway and the plastid MEP pathway.
5 Terpene synthetases are involved in many enzymatic steps of biosynthesis.
5 Monoterpenes include menthol and thujone and provide the precursors for
indole alkaloids.
5 Diterpenes include salvinorin A, ryanodine and resiniferatoxin.
5 Sesquiterpenes include ß-caryophyllene and GABA antagonists such as picrotoxin
and tutin.
5 Especially monoterpenes and sesquiterpenes can be extracted as essential oils.
10.6 · Sterols and Triterpene (From MVA Pathway, C30)