Flavonoid biosynthesis

Flavonoids are synthesized by the phenylpropanoid metabolic pathway in which the amino acid phenylalanine is used to produce 4-coumaroyl-CoA.^[1] 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^[2] especially regarding bioactivity or enzyme stereospecificity.^[3]

Enzymes

The biosynthesis of flavonoids involves several enzymes.

• Anthocyanidin reductase
• Chalcone isomerase
• Dihydrokaempferol 4-reductase
• Flavone synthase
• Flavonoid 3'-monooxygenase
• Flavonol synthase
• Flavanone 3-dioxygenase
• Flavanone 4-reductase
• Leucoanthocyanidin reductase
• Leucocyanidin oxygenase
• Naringenin-chalcone synthase

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

1. Ververidis Filippos, F; Trantas Emmanouil; Douglas Carl; Vollmer Guenter; Kretzschmar Georg; Panopoulos Nickolas (October 2007). "Biotechnology of flavonoids and other phenylpropanoid-derived natural products. Part I: Chemical diversity, impacts on plant biology and human health". Biotechnology Journal. 2 (10): 1214–34. doi:10.1002/biot.200700084. PMID 17935117.
2. Methods of analysis and separation of chiral flavonoids. Jaime A. Yáñeza, Preston K. Andrewsb and Neal M. Journal of Chromatography B, Volume 848, Issue 2, 1 April 2007, Pages 159-181
3. A theoretical study of the conformational behavior and electronic structure of taxifolin correlated with the free radical-scavenging activity. Patrick Trouillas, Catherine Fagnère, Roberto Lazzaroni, Claude Calliste, Abdelghafour Marfak and Jean-Luc Duroux, Food Chemistry, Volume 88, Issue 4, December 2004, Pages 571-582