163
10
lycopene. This is a branch point from which two different cyclases can modify the compound. One branch leads to the α-carotenes, which results in the formation of lutein,
whereas the other branch leads to the β-carotenes, resulting in the formation of zeaxanthin, violaxanthin and neoxanthin. Due to the high amount of double bonds, many of
these substances are colourful, notably the orange-red coloured carotenoids. Most of the
enzymes are attached to the envelope membranes of the chloroplasts. Carotenes and
xanthophylls (containing in contrast to carotenes oxygen atoms in form of hydroxyl
groups and/or epoxide bridges) are mainly known for their protective role in photosynthesis. β-carotin (or β-carotene) is also degraded into retinal and further to retinol, better
known as Vitamin A. Vitamin A is an essential vitamin for humans and animals because
they cannot produce β-carotin and need to take it up with the diet. β-carotin accumulates
in many fruits, vegetables and fungi, which can be recognized by their orange colour,
especially carrots (Daucus carota, Apiaceae). Additionally, carotenoid cleavage dioxygenases produce carotenoid cleavage products that have been shown to be important for the
formation of hormones such as abscisic acid (ABA) and strigolactones (McQuinn et al.
2015; Hou et al. 2016). Additionally these enzymes can produce cleavage products such as
bixin (from Annatto seeds), crocin and safranal in crocus (Crocus sativus, Iridaceae;
important for taste, colour and smell of saffron), or mycorradicin, which is responsible for
colouring of arbuscular mycorrhizal roots.
10.5 Sesquiterpenes (From MVA Pathway, C15, e.g. Valerenic
Acid, Anisatin and Tutin)
The first condensation product of sesquiterpenes in the cytosol consists of three isoprene
units and is called farnesyl pyrophosphate. It is the result of sequential head-to-tail condensations of two IPP and one DMAPP molecule catalyzed by farnesyl pyrophosphate
(FPP) synthase (. Fig. 10.5). Sesquiterpenoids are upregulated upon defense and emitted
from vegetative tissues in response to herbivore feeding and function as an indirect
defense by attracting natural enemies of herbivores. On the other hand, they can be volatile constituents of floral odours that attract pollinators (7 Box. 10.2).
The sesquiterpene pathway is based on TPS-dependent reactions and works along the
same mechanisms shown for monoterpenes (7 Box. 10.1). Deprotonation of the farnesylation leads to (E)-β-farnesene, or there is an additional isomerization via the tertiary
diphosphate intermediate nerolidyl diphosphate (. Fig. 10.5). The two main acyclic forms
are farnesene (green apple odour, aphid alarm pheromone) and nerolidol (in citrus, ginger, jasmine, lavender and lemon grass; woody aroma and reminiscent of fresh bark).
Caulerpenyne, a highly toxic acetylated linear sesquiterpenoid, is found in large
amounts in the green algae Caulerpa taxifolia (Brunelli et al. 2000; Jung et al. 2002). This is
a tropical seaweed and an invasive species in the Mediterranean Sea. Each alga is a single,
supersized cell up to several meters in length. When this cell is damaged, caulerpin is
metabolized by an enzyme to produce oxytoxin 2, which immediately cross-links proteins
forming a gummy gel inside the cell that can plug the break in just 30 s and hardens into
protective scar tissue.
The farnesylation is cyclized to yield two major monocyclic forms, which are humulane and bisabolane. Humulane has as an intermediate, the humulyl cation, giving rise
to the derivatives humulene and E-β-caryophyllene present in Syzygium aromaticum
10.5 · Sesquiterpenes (From MVA Pathway, C15, e.g. Valerenic Acid, Anisatin and Tutin)
10
lycopene. This is a branch point from which two different cyclases can modify the compound. One branch leads to the α-carotenes, which results in the formation of lutein,
whereas the other branch leads to the β-carotenes, resulting in the formation of zeaxanthin, violaxanthin and neoxanthin. Due to the high amount of double bonds, many of
these substances are colourful, notably the orange-red coloured carotenoids. Most of the
enzymes are attached to the envelope membranes of the chloroplasts. Carotenes and
xanthophylls (containing in contrast to carotenes oxygen atoms in form of hydroxyl
groups and/or epoxide bridges) are mainly known for their protective role in photosynthesis. β-carotin (or β-carotene) is also degraded into retinal and further to retinol, better
known as Vitamin A. Vitamin A is an essential vitamin for humans and animals because
they cannot produce β-carotin and need to take it up with the diet. β-carotin accumulates
in many fruits, vegetables and fungi, which can be recognized by their orange colour,
especially carrots (Daucus carota, Apiaceae). Additionally, carotenoid cleavage dioxygenases produce carotenoid cleavage products that have been shown to be important for the
formation of hormones such as abscisic acid (ABA) and strigolactones (McQuinn et al.
2015; Hou et al. 2016). Additionally these enzymes can produce cleavage products such as
bixin (from Annatto seeds), crocin and safranal in crocus (Crocus sativus, Iridaceae;
important for taste, colour and smell of saffron), or mycorradicin, which is responsible for
colouring of arbuscular mycorrhizal roots.
10.5 Sesquiterpenes (From MVA Pathway, C15, e.g. Valerenic
Acid, Anisatin and Tutin)
The first condensation product of sesquiterpenes in the cytosol consists of three isoprene
units and is called farnesyl pyrophosphate. It is the result of sequential head-to-tail condensations of two IPP and one DMAPP molecule catalyzed by farnesyl pyrophosphate
(FPP) synthase (. Fig. 10.5). Sesquiterpenoids are upregulated upon defense and emitted
from vegetative tissues in response to herbivore feeding and function as an indirect
defense by attracting natural enemies of herbivores. On the other hand, they can be volatile constituents of floral odours that attract pollinators (7 Box. 10.2).
The sesquiterpene pathway is based on TPS-dependent reactions and works along the
same mechanisms shown for monoterpenes (7 Box. 10.1). Deprotonation of the farnesylation leads to (E)-β-farnesene, or there is an additional isomerization via the tertiary
diphosphate intermediate nerolidyl diphosphate (. Fig. 10.5). The two main acyclic forms
are farnesene (green apple odour, aphid alarm pheromone) and nerolidol (in citrus, ginger, jasmine, lavender and lemon grass; woody aroma and reminiscent of fresh bark).
Caulerpenyne, a highly toxic acetylated linear sesquiterpenoid, is found in large
amounts in the green algae Caulerpa taxifolia (Brunelli et al. 2000; Jung et al. 2002). This is
a tropical seaweed and an invasive species in the Mediterranean Sea. Each alga is a single,
supersized cell up to several meters in length. When this cell is damaged, caulerpin is
metabolized by an enzyme to produce oxytoxin 2, which immediately cross-links proteins
forming a gummy gel inside the cell that can plug the break in just 30 s and hardens into
protective scar tissue.
The farnesylation is cyclized to yield two major monocyclic forms, which are humulane and bisabolane. Humulane has as an intermediate, the humulyl cation, giving rise
to the derivatives humulene and E-β-caryophyllene present in Syzygium aromaticum
10.5 · Sesquiterpenes (From MVA Pathway, C15, e.g. Valerenic Acid, Anisatin and Tutin)