157
10
which is further converted into IPP and DMAPP by the HMBPP reductase. All the
enzymes of the MEP pathway are localized in the plastid stroma.
The MVA and MEP pathways in the cytosol and plastids are not completely separated
since there is exchange via their common intermediates, especially IPP. IPP can also be
transferred to the peroxisomes and mitochondria. The isopentenyl diphosphate isomerase
(IDI) mediates the equilibrium and supply of IPP and DMAPP (Berthelot et al. 2012).
This enzyme appears to localize in the peroxisomes, mitochondria and plastids, although
alternative splice forms have also been found in the cytoplasm (Phillips et al. 2008; Clastre
et al. 2011; Guirimand et al. 2012). Besides the difference in the biosynthesis of IPP, there
is also a difference in the first condensation step between the MVA and the MEP pathways (. Fig. 10.2). Head-to-tail condensation of one DMAPP with one IPP molecule in
plastids results in geranyl pyrophosphate (GPP; C10, with 10 carbon atoms; . Fig. 10.1c)
formation, the precursor of monoterpenes, and is catalyzed by the GPP synthase in the
MEP pathway. For the MVA pathway, the first condensation product is farnesyl pyrophosphate (FPP; C15, . Fig. 10.1c), which is a sequential head-to-tail condensation of two IPPs
and one DMAPP molecule by the FPP synthase (FPS) in the cytosol or in mitochondria.
Knockout of all FPS genes is lethal for Arabidopsis, and embryo development is arrested
at the pre-globular stage, demonstrating that this pathway is essential (Closa et al. 2010).
In the next condensation step, two GPP or FPP fuse, leading to mainly C30 bodies in the
cytoplasm and C20 and C40 bodies in the plastids. Polyprenols comprising C50–C130
bodies are also present in plants; natural rubber can contain more than 10,000 1,4-linked
isoprene units.
The primary products of condensation are linear compounds. These are not only
precursors for further modifications but also have specific roles in plants. Nevertheless,
they also undergo alterations such as reduction, oxidation and side-chain modifications,
which lead to terpenoid hydrocarbons, alcohols, ethers, aldehydes, ketones or carboxylic
acids and their esters (Tholl 2015). Derivatization often leads to cyclization. Cyt P450
enzymes are involved in many of these steps, which take place mainly in the ER or cytosol.
End products from the different compartments vary: in the cytosol mainly sequiterpens
(C15), sterols (C30), triterpenes (C30) and polyterpens (>30) are produced, while in the
mitochondria especially the coenzyme Q 10 essential for electron transport is produced
and also diterpenes and sesquiterpenes. In plastids, monoterpenes (C10), diterpenes
(C20) and tetraterpenes (C40) are synthesized, but many of the end products are then
transferred into the cytoplasm.
Many plants produce terpenes in specialized cells or tissues such as the glandular
trichomes or the epithelial cells that surround the resin ducts of conifers. This defense
strategy allows concentrating terpenoids in areas most likely to be targeted by a predatory
organism, such as the surface of leaves (glandular trichomes), in resin ducts or laticifers
(see 7 Sect. 1.3.7 in 7 Chap. 1).
10.2 Monoterpenes (From MEP Pathway, C10, e.g. Menthol,
Camphor and Thujone)
Combining two C5 units leads to the most basic terpene, which is the linear monoterpene
precursor geranyl pyrophosphate (GPP, C10). This compound is then further processed
by monoterpene synthases/cyclases. These mostly belong to the enzyme group of terpene
synthases (TPS) (see 7 Box 10.1).
10.2 · Monoterpenes (From MEP Pathway, C10, e.g. Menthol, Camphor and Thujone)
10
which is further converted into IPP and DMAPP by the HMBPP reductase. All the
enzymes of the MEP pathway are localized in the plastid stroma.
The MVA and MEP pathways in the cytosol and plastids are not completely separated
since there is exchange via their common intermediates, especially IPP. IPP can also be
transferred to the peroxisomes and mitochondria. The isopentenyl diphosphate isomerase
(IDI) mediates the equilibrium and supply of IPP and DMAPP (Berthelot et al. 2012).
This enzyme appears to localize in the peroxisomes, mitochondria and plastids, although
alternative splice forms have also been found in the cytoplasm (Phillips et al. 2008; Clastre
et al. 2011; Guirimand et al. 2012). Besides the difference in the biosynthesis of IPP, there
is also a difference in the first condensation step between the MVA and the MEP pathways (. Fig. 10.2). Head-to-tail condensation of one DMAPP with one IPP molecule in
plastids results in geranyl pyrophosphate (GPP; C10, with 10 carbon atoms; . Fig. 10.1c)
formation, the precursor of monoterpenes, and is catalyzed by the GPP synthase in the
MEP pathway. For the MVA pathway, the first condensation product is farnesyl pyrophosphate (FPP; C15, . Fig. 10.1c), which is a sequential head-to-tail condensation of two IPPs
and one DMAPP molecule by the FPP synthase (FPS) in the cytosol or in mitochondria.
Knockout of all FPS genes is lethal for Arabidopsis, and embryo development is arrested
at the pre-globular stage, demonstrating that this pathway is essential (Closa et al. 2010).
In the next condensation step, two GPP or FPP fuse, leading to mainly C30 bodies in the
cytoplasm and C20 and C40 bodies in the plastids. Polyprenols comprising C50–C130
bodies are also present in plants; natural rubber can contain more than 10,000 1,4-linked
isoprene units.
The primary products of condensation are linear compounds. These are not only
precursors for further modifications but also have specific roles in plants. Nevertheless,
they also undergo alterations such as reduction, oxidation and side-chain modifications,
which lead to terpenoid hydrocarbons, alcohols, ethers, aldehydes, ketones or carboxylic
acids and their esters (Tholl 2015). Derivatization often leads to cyclization. Cyt P450
enzymes are involved in many of these steps, which take place mainly in the ER or cytosol.
End products from the different compartments vary: in the cytosol mainly sequiterpens
(C15), sterols (C30), triterpenes (C30) and polyterpens (>30) are produced, while in the
mitochondria especially the coenzyme Q 10 essential for electron transport is produced
and also diterpenes and sesquiterpenes. In plastids, monoterpenes (C10), diterpenes
(C20) and tetraterpenes (C40) are synthesized, but many of the end products are then
transferred into the cytoplasm.
Many plants produce terpenes in specialized cells or tissues such as the glandular
trichomes or the epithelial cells that surround the resin ducts of conifers. This defense
strategy allows concentrating terpenoids in areas most likely to be targeted by a predatory
organism, such as the surface of leaves (glandular trichomes), in resin ducts or laticifers
(see 7 Sect. 1.3.7 in 7 Chap. 1).
10.2 Monoterpenes (From MEP Pathway, C10, e.g. Menthol,
Camphor and Thujone)
Combining two C5 units leads to the most basic terpene, which is the linear monoterpene
precursor geranyl pyrophosphate (GPP, C10). This compound is then further processed
by monoterpene synthases/cyclases. These mostly belong to the enzyme group of terpene
synthases (TPS) (see 7 Box 10.1).
10.2 · Monoterpenes (From MEP Pathway, C10, e.g. Menthol, Camphor and Thujone)