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.
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