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Review
. 2010 Feb 4;11(2):595-621.
doi: 10.3390/ijms11020595.

The creation and physiological relevance of divergent hydroxylation patterns in the flavonoid pathway

Heidi Halbwirth  1
Affiliations
Review

The creation and physiological relevance of divergent hydroxylation patterns in the flavonoid pathway

Heidi Halbwirth. Int J Mol Sci. .

Abstract

Flavonoids and biochemically-related chalcones are important secondary metabolites, which are ubiquitously present in plants and therefore also in human food. They fulfill a broad range of physiological functions in planta and there are numerous reports about their physiological relevance for humans. Flavonoids have in common a basic C(6)-C(3)-C(6) skeleton structure consisting of two aromatic rings (A and B) and a heterocyclic ring (C) containing one oxygen atom, whereas chalcones, as the intermediates in the formation of flavonoids, have not yet established the heterocyclic C-ring. Flavonoids are grouped into eight different classes, according to the oxidative status of the C-ring. The large number of divergent chalcones and flavonoid structures is from the extensive modification of the basic molecules. The hydroxylation pattern influences physiological properties such as light absorption and antioxidative activity, which is the base for many beneficial health effects of flavonoids. In some cases antiinfective properties are also effected.

Keywords: 2-oxoglutarate-Fe(II)-dependent dioxygenase; chalcone; cytochrome P450 dependent monooxygenase; flavonoid; hydroxylation; oxidoreductase; plant.

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Figures

Figure 1.
Figure 1.
The basic structures of chalcone, aurone and flavan.
Figure 2.
Figure 2.
The basic structures of flavonoid classes.
Figure 3.
Figure 3.
The flavonoid pathway. Abbrev: ANR: anthocyanidine reductase, ANS: anthocyanidine synthase, AUS: aurone synthase, CHI: chalcone isomerase, CHS: chalcone synthase, DFR: dihydroflavonol 4-reductase, FHT: flavanone 3-hydroxylase, FLS: flavonol synthase, FNS: flavone synthase, IDH: 2-hydroxyisoflavanone dehydratase IFS: 2-hydroxyisoflavanone synthase, LAR: leucoanthocyanidin reductase.
Figure 4.
Figure 4.
Simplified C6-C3-C6 structure (left) of flavonoids (centre) and chalcones (right).
Figure 5.
Figure 5.
A-ring formation by CHS and creation of different hydroxylation pattern in ring A (modified from [56]). * all known CHIs catalyze this reaction, ** only specific CHIs catalyze this reaction.
Figure 6.
Figure 6.
Introduction of additional hydroxyl groups in the A-ring of flavonoids.
Figure 7.
Figure 7.
Two different possibilities to introduce the hydroxyl group in position 3′of flavonoids.
Figure 8.
Figure 8.
Examples of the introduction of hydroxyl groups in positions 3′ and 3′,5′ of flavonoids.
Figure 9.
Figure 9.
Formation of flavonoids with and without hydroxyl groups in position 3.
Figure 10.
Figure 10.
Structural base for flavonoid light absorbance.
Figure 11.
Figure 11.
Part of the absorption spectra of anthocyanidins (a–f) with divergent hydroxylation patterns. With the exception of pelargonidin (a) and robinetinidin (f), structures of cyanidin (b), delphinidin (c), luteolinidin (d) and apigeninidin (e) are only partially shown.
Figure 12.
Figure 12.
Chemical structures of cyanidin and luteolinidin and tissue colours of Zea mays silks accumulating the respective anthocyanidin, left: red silks accumulating common 3-hydroxyanthocyanins, right: salmon silks accumulating the 3-deoxyanthocyanins (derivatives of luteolinidin).
Figure 13.
Figure 13.
Chemical structures of the most common anthocyanidins, their spots on TLC (cellulose with H2O/HCl/Hac = 82/3/15 as solvent system) and the colour of plant tissues accumulating primarily the respective anthocyanidin derivatives (orange petunia is genetically modified).
Figure 14.
Figure 14.
The three main structural prerequisites of flavonoids for high antioxidant potential (modified from [127]).
Figure 15.
Figure 15.
Possible metal binding sites of flavonoids modified from [130].
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