Flavonoid biosynthesis

Flavonoids are synthesized by the phenylpropanoid metabolic pathway in which the amino acid phenylalanine is used to produce 4-coumaroyl-CoA. This can be combined with malonyl-CoA to yield the true backbone of flavonoids, a group of compounds called chalcones, which contain two phenyl rings. Conjugate ring-closure of chalcones results in the familiar form of flavonoids, the three-ringed structure of a flavone. The metabolic pathway continues through a series of enzymatic modifications to yield flavanones → dihydroflavonols → anthocyanins. Along this pathway, many products can be formed, including the flavonols, flavan-3-ols, proanthocyanidins (tannins) and a host of other various polyphenolics.

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Flavanoids can possess chiral carbons. Methods of analysis should take this element into account especially regarding bioactivity or enzyme stereospecificity.

Enzymes

The biosynthesis of flavonoids involves several enzymes.

Backbone

• 4-Coumaroyl-CoA chalcone (naringenin chalcone)
• Naringenin chalcone flavanone (naringenin).

Path to cyanidin:

• flavanone 3'-hydroxyflavonoid (eriodictyol)
• eriodictyol dihydroflavonol (taxifolin)
• taxifolin leucocyanidin
• leucocyanidin cyanidin

Path to (–)-epicatechin:

• cyanidin (–)-epicatechin

Path to (+)-catechin:

• leucocyanidin (+)-catechin

Path to flavones:

• Flavone synthase

Path to flavonols:

• dihydroflavonol flavonol

Path to 3-deoxyanthocyanidins:

• flavanone flavan-4-ol
• (Process analogous to taxifolin → cyanidin follows)

Methylation

• Apigenin 4'-O-methyltransferase
• Luteolin O-methyltransferase
• Quercetin 3-O-methyltransferase

Glycosylation

• Anthocyanidin 3-O-glucosyltransferase
• Flavone 7-O-beta-glucosyltransferase
• Flavone apiosyltransferase
• Flavonol-3-O-glucoside L-rhamnosyltransferase
• Flavonol 3-O-glucosyltransferase

Further acetylations

• Isoflavone-7-O-beta-glucoside 6"-O-malonyltransferase

References