154
10
What You Will Learn in This Chapter
Terpenes and their derivatives, the terpenoids, are synthetized from a five C atom isoprene
unit. These units are added to each other to give rise to compounds of different complexities. The synthesis takes place either in the cytosol (MVA pathway) or within the plastids
(MEP pathway). Monoterpenes, diterpenes and tetraterpenes are products from the MEP
pathway, whereas sesquiterpenes, sterols and triterpenes are derivatives from the MVA
pathway. Tailoring enzymes leads to the variations observed. End products are important
for plant growth such as hormones (gibberellin and brassinosteroids) or carotenoids. They
can be simpler structures, which are sometimes also volatile (such as menthol or
ß- caryophyllene) or more complex, even polycyclic (such as ryanodine or stigmasterol).
10.1 Introduction
The building units of terpenes are isoprene units (Dewick 2002; Vranova et al. 2012).
These precursors with five carbon atoms (C5) are present in the cell as diphosphates,
either isopentenyl pyrophosphate (IPP) or its allylic isomer dimethylallyl pyrophosphate
(DMAPP). Their fusion is mostly head-to-tail (a 1,4 link) but can also be head-to-head (a
1,1 link) or tail-to-tail (a 4,4 link) and leads to the formation of terpenes of different length
(. Fig. 10.1). These structures can be further modified at the methyl groups or by adding
oxygen atoms and are then called terpenoids or isoprenoids. It is estimated that between
20,000 and 40,000 different structures are present in plants.
Two main pathways exist for the formation of DMAPP and IPP, the mevalonate pathway (MVA) and a mevalonate-independent pathway called MEP (after the first component 2-C-methyl-D-erythritol 4-phosphate) or DOXP pathway (after 1-deoxy-D-xylulose
5-phosphate). The MVA pathway can be found in all organisms, and in plants, it takes
place mainly in the cytosol, whereas the MEP pathway resides in plastids (Tholl 2015).
Therefore, the MEP pathway and its end products are specific for plants but can also be
found in some bacteria. Due to the lack of the MEP pathway in humans, enzymes of this
pathway are good targets for the treatment of human pathogens. One example is fosmidomycin, which acts on the DXP reductoisomerase of bacteria and Plasmodium falciparum
and is used against infections and malaria (Rodriguez-Concepcion 2004).
In animals, the MVA pathway produces components such as cholesterol, dolichol
and ubiquinone, which are important for membrane integrity and electron transport. In
plants, six enzymes are needed for the MVA pathway (. Fig. 10.2). The first steps are the
condensation of two acetyl-CoA via acetoacetyl-CoA thiolase to form acetoacetyl-CoA
and the addition of another acetyl-CoA by the 3-hydroxy-3-methylglutaryl-coenzyme A
(HMG-CoA) synthase to form HMG-CoA. The HMG-CoA reductase then synthetizes
MVA, one of the control steps of the pathway. This enzyme consists of an endoplasmic
reticulum (ER) membrane anchor and a conserved cytosolic catalytic domain. It is tightly
regulated via phosphorylation and protein stability, stressing the importance of this
pathway (Leivar et al. 2011; Doblas et al. 2013). Two phosphorylation steps are needed
to produce mevalonate 5-pyrophosphate (MVAPP) and the diphosphomevalonate decarboxylase catalyzes the conversion of MVAPP to IPP. Components of the MVA pathway are
distributed between the cytosol and peroxisomes, especially the enzymes for the two last
steps could be localized to the peroxisome, while the first phosphorylation step is cytosolic
(Pulido et al. 2012).
Chapter 10 · Terpenes and Terpenoids
10
What You Will Learn in This Chapter
Terpenes and their derivatives, the terpenoids, are synthetized from a five C atom isoprene
unit. These units are added to each other to give rise to compounds of different complexities. The synthesis takes place either in the cytosol (MVA pathway) or within the plastids
(MEP pathway). Monoterpenes, diterpenes and tetraterpenes are products from the MEP
pathway, whereas sesquiterpenes, sterols and triterpenes are derivatives from the MVA
pathway. Tailoring enzymes leads to the variations observed. End products are important
for plant growth such as hormones (gibberellin and brassinosteroids) or carotenoids. They
can be simpler structures, which are sometimes also volatile (such as menthol or
ß- caryophyllene) or more complex, even polycyclic (such as ryanodine or stigmasterol).
10.1 Introduction
The building units of terpenes are isoprene units (Dewick 2002; Vranova et al. 2012).
These precursors with five carbon atoms (C5) are present in the cell as diphosphates,
either isopentenyl pyrophosphate (IPP) or its allylic isomer dimethylallyl pyrophosphate
(DMAPP). Their fusion is mostly head-to-tail (a 1,4 link) but can also be head-to-head (a
1,1 link) or tail-to-tail (a 4,4 link) and leads to the formation of terpenes of different length
(. Fig. 10.1). These structures can be further modified at the methyl groups or by adding
oxygen atoms and are then called terpenoids or isoprenoids. It is estimated that between
20,000 and 40,000 different structures are present in plants.
Two main pathways exist for the formation of DMAPP and IPP, the mevalonate pathway (MVA) and a mevalonate-independent pathway called MEP (after the first component 2-C-methyl-D-erythritol 4-phosphate) or DOXP pathway (after 1-deoxy-D-xylulose
5-phosphate). The MVA pathway can be found in all organisms, and in plants, it takes
place mainly in the cytosol, whereas the MEP pathway resides in plastids (Tholl 2015).
Therefore, the MEP pathway and its end products are specific for plants but can also be
found in some bacteria. Due to the lack of the MEP pathway in humans, enzymes of this
pathway are good targets for the treatment of human pathogens. One example is fosmidomycin, which acts on the DXP reductoisomerase of bacteria and Plasmodium falciparum
and is used against infections and malaria (Rodriguez-Concepcion 2004).
In animals, the MVA pathway produces components such as cholesterol, dolichol
and ubiquinone, which are important for membrane integrity and electron transport. In
plants, six enzymes are needed for the MVA pathway (. Fig. 10.2). The first steps are the
condensation of two acetyl-CoA via acetoacetyl-CoA thiolase to form acetoacetyl-CoA
and the addition of another acetyl-CoA by the 3-hydroxy-3-methylglutaryl-coenzyme A
(HMG-CoA) synthase to form HMG-CoA. The HMG-CoA reductase then synthetizes
MVA, one of the control steps of the pathway. This enzyme consists of an endoplasmic
reticulum (ER) membrane anchor and a conserved cytosolic catalytic domain. It is tightly
regulated via phosphorylation and protein stability, stressing the importance of this
pathway (Leivar et al. 2011; Doblas et al. 2013). Two phosphorylation steps are needed
to produce mevalonate 5-pyrophosphate (MVAPP) and the diphosphomevalonate decarboxylase catalyzes the conversion of MVAPP to IPP. Components of the MVA pathway are
distributed between the cytosol and peroxisomes, especially the enzymes for the two last
steps could be localized to the peroxisome, while the first phosphorylation step is cytosolic
(Pulido et al. 2012).
Chapter 10 · Terpenes and Terpenoids