175
11
Muhlemann et al. 2014). Therefore, some phenylpropanoids are part of essential oils in
plants (see 7 Box 10.2). Many phenylpropanoids can also chelate metal ions, which leads
to their aggregation and changes in colour or function. Furthermore, glycosylation by
UDP- glycosyltransferases (UGTs) enhances the diversity of phenylpropanoids but also
influences their compartmentalization, biological activity, solubility, stability and toxicity
(Le Roy et al. 2016). Many phenylpropanoids are toxic and unstable molecules and so
rarely accumulate in their aglycone form in plant cells. The alteration of natural compounds with acyl moieties is catalysed by acyltransferases (AT) and generates additional
variants. The interaction of the hydroxyl groups of phenolics with the p-electrons of the
benzene ring allows many phenolic compounds to generate relatively long-lived free
radicals able to interfere with oxidation processes. Phenolics that possess two orthopositioned hydroxyl groups are very good antioxidants (de Cassia da Silveira et al. 2014;
Carvalho et al. 2015).
The addition of 3-malonyl-CoA to coumaroyl-CoA catalysed by the chalcone synthase
(CHS) leads to chalcone. CHS, a polyketide synthase, provides the first committed step
in flavonoid biosynthesis by catalysing the sequential decarboxylative addition of three
acetate units from malonyl-CoA to p-coumaroyl-CoA (Austin and Noel 2003; Falcone
Ferreyra et al. 2012). In the same active site, CHS then forms chalcone via intramolecular
cyclization and aromatization of the linear phenylpropanoid tetraketide. The plants utilize
flavonoids for such diverse purposes as antimicrobial defence, flower pigmentation, UV
photoprotection, pollen fertility and interaction with the environment.
Chalcones can also provide the basis for the synthesis of aurones, such as hispidol,
which is a selective adenosine receptor antagonist. The chalcone isomerase (CHI) modifies
chalcone to naringenin (bitter taste of grapefruit), a flavonoid. Flavonoids are subclassified into several families including flavonol, flavone, flavanone, flavan-3-ol, isoflavone and
anthocyanidin according to the structure of and the modifications to the aromatic rings.
In the group of isoflavone, one class are the rotenones, which are found almost exclusively
in legumes (Papilionaceae, Mimosaceae, Caesalpiniaceae and Fabaceae). Rotenone was
first isolated from the Peruvian derris root (Lonchocarpus, Fabaceae) and is used as fish
poison but is also a naturally occurring insecticide. Isoflavones are otherwise known from
soy, especially genistein, which is an oestrogen receptor modulator. The flavone galangin
is found in some plants from the ginger family and the Helichrysum genus, whereas cirsimarin and its aglycone cirsimaritin are flavones from Microtea debilis (Caryophyllales).
The flavanone 3-β hydroxylase (F3H) can further modify naringenin to dihydroflavonols such as dihydrokaempferol, dihydroquercetin and dihydromyricetin. Dihydroflavonols are the precursors for catechin and epicatechin, which are formed via
leucocyanidin, but also the precursors for gallocatechin and galloepicatechin, which
are formed via leucodelfinidin. Catechins and epicatechin are odourless white powders, which can be found in high quantities in cocoa (Theobroma cacao, Malvaceae),
tea (Camellia sinensis, Theaceae) and grapes (Vitis vinifera, Vitaceae). These secondary metabolites are induced by stress end elevated UV light, resulting in higher levels
if the tea plants are grown in high altitudes. Whereas green teas contain high levels
of catechins, the fermentation to yield black tea destroys catechins. Thereby they are
enzymatically oxidized yielding a complex mixture of oxidation products, including
theaflavins and thearubigins (Tanaka et al. 2009). Dihydroflavonols can be transformed
into flavonols, such as quercetin or kaempferol. They are present in a wide variety of
fruits and vegetables and are potent antioxidants that serve to protect the plant from
reactive oxygen species (ROS).
11.2 · Phenylpropanoids (Derived from Phenylalanine)
11
Muhlemann et al. 2014). Therefore, some phenylpropanoids are part of essential oils in
plants (see 7 Box 10.2). Many phenylpropanoids can also chelate metal ions, which leads
to their aggregation and changes in colour or function. Furthermore, glycosylation by
UDP- glycosyltransferases (UGTs) enhances the diversity of phenylpropanoids but also
influences their compartmentalization, biological activity, solubility, stability and toxicity
(Le Roy et al. 2016). Many phenylpropanoids are toxic and unstable molecules and so
rarely accumulate in their aglycone form in plant cells. The alteration of natural compounds with acyl moieties is catalysed by acyltransferases (AT) and generates additional
variants. The interaction of the hydroxyl groups of phenolics with the p-electrons of the
benzene ring allows many phenolic compounds to generate relatively long-lived free
radicals able to interfere with oxidation processes. Phenolics that possess two orthopositioned hydroxyl groups are very good antioxidants (de Cassia da Silveira et al. 2014;
Carvalho et al. 2015).
The addition of 3-malonyl-CoA to coumaroyl-CoA catalysed by the chalcone synthase
(CHS) leads to chalcone. CHS, a polyketide synthase, provides the first committed step
in flavonoid biosynthesis by catalysing the sequential decarboxylative addition of three
acetate units from malonyl-CoA to p-coumaroyl-CoA (Austin and Noel 2003; Falcone
Ferreyra et al. 2012). In the same active site, CHS then forms chalcone via intramolecular
cyclization and aromatization of the linear phenylpropanoid tetraketide. The plants utilize
flavonoids for such diverse purposes as antimicrobial defence, flower pigmentation, UV
photoprotection, pollen fertility and interaction with the environment.
Chalcones can also provide the basis for the synthesis of aurones, such as hispidol,
which is a selective adenosine receptor antagonist. The chalcone isomerase (CHI) modifies
chalcone to naringenin (bitter taste of grapefruit), a flavonoid. Flavonoids are subclassified into several families including flavonol, flavone, flavanone, flavan-3-ol, isoflavone and
anthocyanidin according to the structure of and the modifications to the aromatic rings.
In the group of isoflavone, one class are the rotenones, which are found almost exclusively
in legumes (Papilionaceae, Mimosaceae, Caesalpiniaceae and Fabaceae). Rotenone was
first isolated from the Peruvian derris root (Lonchocarpus, Fabaceae) and is used as fish
poison but is also a naturally occurring insecticide. Isoflavones are otherwise known from
soy, especially genistein, which is an oestrogen receptor modulator. The flavone galangin
is found in some plants from the ginger family and the Helichrysum genus, whereas cirsimarin and its aglycone cirsimaritin are flavones from Microtea debilis (Caryophyllales).
The flavanone 3-β hydroxylase (F3H) can further modify naringenin to dihydroflavonols such as dihydrokaempferol, dihydroquercetin and dihydromyricetin. Dihydroflavonols are the precursors for catechin and epicatechin, which are formed via
leucocyanidin, but also the precursors for gallocatechin and galloepicatechin, which
are formed via leucodelfinidin. Catechins and epicatechin are odourless white powders, which can be found in high quantities in cocoa (Theobroma cacao, Malvaceae),
tea (Camellia sinensis, Theaceae) and grapes (Vitis vinifera, Vitaceae). These secondary metabolites are induced by stress end elevated UV light, resulting in higher levels
if the tea plants are grown in high altitudes. Whereas green teas contain high levels
of catechins, the fermentation to yield black tea destroys catechins. Thereby they are
enzymatically oxidized yielding a complex mixture of oxidation products, including
theaflavins and thearubigins (Tanaka et al. 2009). Dihydroflavonols can be transformed
into flavonols, such as quercetin or kaempferol. They are present in a wide variety of
fruits and vegetables and are potent antioxidants that serve to protect the plant from
reactive oxygen species (ROS).
11.2 · Phenylpropanoids (Derived from Phenylalanine)