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Phenylalanine
PAL
Cinnamic acid
C4H
UGCT
p
-Coumaric acid
4CL
Cinnamoyl
D
-glucose
p
-Coumaroyl CoA
HCT
HCT
Route 2
p
-Coumaroyl shikimic acid
C3H
Route 1
p
-Coumaroyl
quinic acid
Caffeoyl
D
-glucose
Caffeoyl shikimic acid
HCT
C3H
HCGQT
Caffeoyl CoA
HQT
Route 3
+ Quinic
acid
+
Quinic
acid
+ Quinic
acid
Chlorogenic acid
(Caffeoyl quinic acid)
Fig. 12.3
Proposed three biosynthetic pathways for chlorogenic acid formation in plants. Enzymes as follows:
PAL, phenylalanine ammonia-lyase; C4H, cinnamate 4-hydroxylase; 4CL, 4-hydroxycinnamoyl CoA ligase; HCT,
hydroxycinnamoyl transferase; C3H,
p
-coumarate 3
-hydrolase; HQT, hydroxycinnamoyl CoA quinate hydrox-
ycinnamoyl transferase; UGCT, UDP-glucose/cinnamate glucosyl transferase; HCGQT, hydroxycinnamoyl
D
-
glucose/quinate hydroxycinnamoyl transferase. (Adapted from Niggeweg et al., 2004.)
to yield naringenin chalcone (2
,4,4
,6
-tetrahydroxychalcone) (Crozier et al., 2000) (Fig.
12.4). This rate-limiting step of the flavonoid biosynthesis is catalyzed by CHS, a dimeric
protein of 78-88 kDa. Similar to the stepwise addition of acetate from malonyl-CoA to
p
-
coumaroyl-CoA to form naringenin chalcone, stilbenes (C
6
C
2
C
6
) such as resveratrol can
be synthesized. Naringenin chalcone is isomerized to a flavanone by the enzyme chalcone
isomerase (CHI). From these central intermediates, the pathway diverges into several side
branches, each resulting in a different class of flavonoids: flavone, flanonol, flavanol, and
anthocyanins. Once the basic C
6
C
3
C
6
-carbon flavan nucleus is synthesized, numerous
hydroxylation, methoxylation, and/or glycosylation reactions can occur, ultimately resulting
in the synthesis of a water-soluble compound that is subsequently transported to the vacuole
for deposition (Marrs et al., 1995).
Formation of flavone from flavanone by the introduction of a double bond between C-2
and C-3 is a two-step reaction catalyzed by flavone synthase (FNS) I and II. Formation of
flavonols occurs from flavanone as well, through dihydroflavonol. Two enzymes involved
in the two-step conversion are flavanone 3-hydroxylase (F3H) and flavonol synthase (FLS),
respectively. Biosynthesis of flavan-3-ols and anthocyanidins requires the formation of
an intermediate flavan-3,4-
cis
-diol (leucoanthocyanin) from dihydroflavonol catalyzed by
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