Fig. 5. Shikimate pathway
The biosynthesis of flavonoids starts with the condensation of one
p-coumaroyl-CoA molecule (shikimate-derived, B ring) with three
molecules of malonyl-CoA (polyketid origin, A ring) to give chalcone
(2’4’6’4-tetrahydroxychalcone) catalyzed by chalcone synthase (CHS)
enzymes. Chalcones are subsequently isomerized to flavanone by chalcone
flavanone isomerase (CHI). The pathway diverges from these central
intermediates into several side branches, each providing a different
class of flavonoids.
The initial steps in this pathway are referred to as the general
phenylpropanoid pathway [26]. In phenylpropanoid pathway, an
aromatic amino acid, phenylalanine first converted to p-coumaroyl-CoA
through the activity of phenylalanine ammonia lyase (PAL), cinnamic acid
4-hydroxylase (C4H), and 4-coumarate-CoA ligase (4CL). PAL catalyzed the
deamination of phenylalanine to trans-cinnamic acid in the first
committed step called as deamination of phenylalanine to trans-cinnamic
acid [27]. In the phenylpropanoid pathway, the second step involves
the activity of C4H, a cytochrome P450 monooxygenase, in plants. This
enzyme catalyzes the hydroxylation of trans-cinnamic acid and generates
p-coumaric acid. This was the first oxidation reaction [28]. The
third step in the general phenylpropanoid pathway involves the
activation of p-coumaroyl-CoA by the action of 4-courmarate: CoA ligase.
In plants, the activity of 4CL is positively correlated with anthocyanin
and flavonol content in response to stress [29], whereas PAL, C4H,
and 4CL are generally coordinately expressed [30]. Chalcone synthase
(CHS) catalyzes the stepwise condensation of three acetate units to
yield naringenin chalcones. In vitro, naringenin chalcone is transformed
into naringenin by chalcone isomerase (CHI) [31]. Naringenin is a
very important flavonoid frame catalyzed by FNS I and FNS II (flavone
synthases I and II) and IFS (isoflavone synthase) to form flavones and
isoflavones respectively [32]. Naringen can also be catalyzed by
flavanone-3-hydroxylase (F3H), flavonol-3’-hydroxylase (F3’H) and
flavonol-3’5’-hydroxylase (F3’5’H) to synthesized dihydro-myricetin,
dihydro-kaempferol and dihydro-quercetin, respectively [33]. The
flavonol synthase (FLS) converted dihydro flavonols into flavonols (like
kaempferol, quercetin and myricetin), which further was catalyzed by
dihydroflavonol 4-reductase (DFR) to generate leucoanthocyanidins
[34], further catalyzed by leucoanthocyanidin dioxygenase (LDOX) to
produce anthocyanidins [35]. anthocyanidins and leucoanthcyanidins
were again converted to proanthocyanidins catalyzed by
leucoanthocyanidin reductase (LAR) and anthocyanidine reductase (ANR),
repectively [36]. variations in anthocyanins is then responsible for
the stabilization of vacuolar anthocyanins which includes glycosylation,
methylation and acylation [37].
In general phenylpropanoid pathway shown in figure 6 is common to all
the downstream metabolites like flavonoids and lignin. The metabolic
pathway continues through a series of enzymatic modifications to yield
flavonols, dihydroflavonols and anthocyanins.