The biosynthesis of flavonoids is a
very complex process and involves series of reactions. The basic pathways to
the core flavonoid skeletons have been established both enzymatically and
genetically.7,8 The flavonoid biosynthesis, starts with general phenylpropanoid
pathway. The synthesis of phenylpropanoids starts with the removal of the amino
group of phenylalanine by phenyl ammonium lyase (PAL) to produce trans-cinnamic
acid. The aromatic ring of trans-cinnamic acid is then hydroxylated to produce
p-coumaric acid by the enzyme cinnamate 4-hydroxylate (C4H). Coumaric acid can
then be ligated to coenzyme A by a ligase [ 4-coumaroyl-CoA ligase (4CL) ].
Coumaroyl-CoA, an intermediate
formed by the phenylpropanoid pathway is a substrate for the enzyme chalcone
synthase (CHS) and stilbene synthase (STS). Coumaroyl-CoA alongwith malonyl-CoA
results in the formation of Chalcones and stilbene as shown below:
Ring closure of chalcone to produce
a pyran ring in naringenin occurs spontaneously. This step is catalysed by a
chalcone isomerase (CHI).
Flavones and flavonols can be made
from flavanone (naringenin). Flavanone 3-hydroxylase (F3H) catalyzes the
hydroxylation of C-3 position of C-ring. Then the introduction of a double bond
between C-2 and C-3, results in the formation of flavonol. The desaturation of
C-ring is calatyzed by flavonol synthases (FLS).
Further, the reduction of keto group
in the C-ring is reduced by the action of dihydroflavonol reductase (DFR), and
then anthocyanidin synthase (ANS) introduces two double bonds in the C-ring
forming anthocyanidin. flavonoid 3-O-glucosyltransferase (F3GT) transfers the
glucose residue from nucleotide sugar to 3-OH position forming anthocyanin.
(http://202.141.40.218/wiki/index.php/Flavonoids)
