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11
Condensed tannins (proanthocyanidins) are formed via the condensation of two
derivatives of dihydroflavonones, Flavan-3,4-diols and anthocyanidins. The resulting high
molecular weight multimers can complex with carbohydrates and proteins. Tannins are
able to precipitate and denature proteins, which is where their name is derived from as
tannins from wood were used to tan animal hides into leather. Tannins are found in many
plants and their products including tea, red wine and fruits such as cranberries and apples,
where they contribute to the bitter, astringent taste.
Anthocyanidins are also the precursors for anthocyanins. Anthocyanin pigments are
transported to and accumulate in vacuoles, where the low pH leads to conjugation or
complexation (with metals, malonic acid or other flavones) and results in compounds
with enhanced or modified colours to yield pink, red, purple or blue pigmentation.
This pigmentation can best be seen in many flower petals (Grotewold 2006; Petroni and
Tonelli 2011).
Addition of 3-malonyl-CoA to coumaroyl-CoA with the help of the stilbene synthase
(STS) leads to the formation of stilbene and further stilbenoids. Due to their structure,
which is similar to human hormones, they are also called phytoestrogens. Resveratrol
is the most widespread stilbene, found especially in grapes (red wine), giant knotweed,
peanuts and mulberries. Many of its derivatives are also present in plants (e.g. rhaponticin
from rhubarb).
p-Coumaroyl-CoA and p-coumaric acid are the precursors of coumarins and furanocoumarins (Bourgaud et al. 2006). The committing enzyme in their synthesis is
hydroxycinnamoyl transferase (HCT), which leads to the formation of caffeoyl-CoA
and further feruloyl-CoA to yield scopoletin and esculatin. The third typical coumarin, umbelliferone, is synthetized via 2,4-dihydroxycinnamoyl-CoA. Coumarins,
characterized by their vanilla-like or fresh hay-like odour, are found in many plant
species, especially Fabaceae and Lauraceae (tonka bean, Dipteryx odorata; cinnamon,
Cinnamomum sp.). Coumarin is produced during wilting by enzymes, which split off
sugar residues. During the process of spoiling of sweet clover, the natural coumarins
are converted into toxic dicumarol. Dicumarol blocks blood clotting by inhibiting the
enzyme required for the synthesis of prothrombin and in addition antagonizes vitamin
K. This can lead to internal bleeding in animals (e.g. sweet clover disease in mammals).
Due to these properties, dicumarol is also used in rat poison (e.g. warfarin). Moreover,
clinically it serves as an anticoagulant for protection from thrombosis and heart attack
(e.g. Marcumar).
Dihydroisocoumarin-derivatives such as phyllodulcin, a high-intensity sweetener, are
extracted from hydrangea leaves (Hydrangea macrophylla var. thunbergii). Phyllodulcin
interacts with the human sweet taste receptor, a G protein-coupled receptor (GPCRs). A
prenyltransferase uses umbelliferone, a coumarin mostly found in Apiaceae, as a substrate,
which leads to the formation of furanocoumarins. Linear furocoumarins such as psoralen,
xanthotoxin, bergapten and isopimpinellin are mainly found in Apiaceae, Moraceae,
Rutaceae and Leguminosae, and the angular dihydro-furanocoumarins, such as angelicin,
sphondin, and pimpinellin, are confined to the Apiaceae and Leguminosae (Bourgaud
et al. 2006). Bergamottin (5-geranoxy-psoralen) from grapefruit has been shown to interfere with drug metabolism by inactivating intestinal Cyt P450 enzymes (Girennavar et al.
2007). Furocoumarin can also intercalate between the base pairs of the DNA, and after
UVA radiation, covalent complexes are formed (Gasparro 1996). Therefore, eating furocoumarins, which are, for example, present in celery, can lead to phototoxic reactions in
the skin after exposure to sun.
Chapter 11 · Phenylpropanoids
11
Condensed tannins (proanthocyanidins) are formed via the condensation of two
derivatives of dihydroflavonones, Flavan-3,4-diols and anthocyanidins. The resulting high
molecular weight multimers can complex with carbohydrates and proteins. Tannins are
able to precipitate and denature proteins, which is where their name is derived from as
tannins from wood were used to tan animal hides into leather. Tannins are found in many
plants and their products including tea, red wine and fruits such as cranberries and apples,
where they contribute to the bitter, astringent taste.
Anthocyanidins are also the precursors for anthocyanins. Anthocyanin pigments are
transported to and accumulate in vacuoles, where the low pH leads to conjugation or
complexation (with metals, malonic acid or other flavones) and results in compounds
with enhanced or modified colours to yield pink, red, purple or blue pigmentation.
This pigmentation can best be seen in many flower petals (Grotewold 2006; Petroni and
Tonelli 2011).
Addition of 3-malonyl-CoA to coumaroyl-CoA with the help of the stilbene synthase
(STS) leads to the formation of stilbene and further stilbenoids. Due to their structure,
which is similar to human hormones, they are also called phytoestrogens. Resveratrol
is the most widespread stilbene, found especially in grapes (red wine), giant knotweed,
peanuts and mulberries. Many of its derivatives are also present in plants (e.g. rhaponticin
from rhubarb).
p-Coumaroyl-CoA and p-coumaric acid are the precursors of coumarins and furanocoumarins (Bourgaud et al. 2006). The committing enzyme in their synthesis is
hydroxycinnamoyl transferase (HCT), which leads to the formation of caffeoyl-CoA
and further feruloyl-CoA to yield scopoletin and esculatin. The third typical coumarin, umbelliferone, is synthetized via 2,4-dihydroxycinnamoyl-CoA. Coumarins,
characterized by their vanilla-like or fresh hay-like odour, are found in many plant
species, especially Fabaceae and Lauraceae (tonka bean, Dipteryx odorata; cinnamon,
Cinnamomum sp.). Coumarin is produced during wilting by enzymes, which split off
sugar residues. During the process of spoiling of sweet clover, the natural coumarins
are converted into toxic dicumarol. Dicumarol blocks blood clotting by inhibiting the
enzyme required for the synthesis of prothrombin and in addition antagonizes vitamin
K. This can lead to internal bleeding in animals (e.g. sweet clover disease in mammals).
Due to these properties, dicumarol is also used in rat poison (e.g. warfarin). Moreover,
clinically it serves as an anticoagulant for protection from thrombosis and heart attack
(e.g. Marcumar).
Dihydroisocoumarin-derivatives such as phyllodulcin, a high-intensity sweetener, are
extracted from hydrangea leaves (Hydrangea macrophylla var. thunbergii). Phyllodulcin
interacts with the human sweet taste receptor, a G protein-coupled receptor (GPCRs). A
prenyltransferase uses umbelliferone, a coumarin mostly found in Apiaceae, as a substrate,
which leads to the formation of furanocoumarins. Linear furocoumarins such as psoralen,
xanthotoxin, bergapten and isopimpinellin are mainly found in Apiaceae, Moraceae,
Rutaceae and Leguminosae, and the angular dihydro-furanocoumarins, such as angelicin,
sphondin, and pimpinellin, are confined to the Apiaceae and Leguminosae (Bourgaud
et al. 2006). Bergamottin (5-geranoxy-psoralen) from grapefruit has been shown to interfere with drug metabolism by inactivating intestinal Cyt P450 enzymes (Girennavar et al.
2007). Furocoumarin can also intercalate between the base pairs of the DNA, and after
UVA radiation, covalent complexes are formed (Gasparro 1996). Therefore, eating furocoumarins, which are, for example, present in celery, can lead to phototoxic reactions in
the skin after exposure to sun.
Chapter 11 · Phenylpropanoids