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.

Flavonoid Biosynthesis Diagrams

File:Biosynthesis of catechin.png

Biosynthesis of catechin

File:Flavonoids Biochemistry.png

A biochemical diagram showing the class of flavonoids and their source in nature through various inter-related plant species.

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-oumaroyl-CoA chalcone synthase⟶3 malonyl-CoA chalcone (naringenin-chalcone)
• Naringenin-chalcone chalcone isomerase⇄  flavanone (naringenin).

Path to cyanidin:

• flavanone flavonoid 3'-monooxygenase⟶[O] 3'-hydroxyflavonoid (eriodictyol)
• eriodictyol flavanone 3-dioxygenase⟶2-oxoglutarate, [O] dihydroflavonol (taxifolin)
• taxifolin dihydrokaempferol 4-reductase⟶NADPH leucocyanidin
• leucocyanidin leucocyanidin oxygenase⟶2-oxoglutarate, [O] cyanidin

Path to (–)-epicatechin:

• cyanidin anthocyanidin reductase⟶NADPH (–)-epicatechin

Path to (+)-catechin:

• leucocyanidin leucoanthocyanidin reductase⟶NADPH (+)-catechin

Path to flavones:

• Flavone synthase

Path to flavonols:

• dihydroflavonol flavonol synthase⇄  flavonol

Path to 3-deoxyanthocyanidins:

• flavanone flavanone 4-reductase ⇄  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