The Role of Polyphenols in Quality, Postharvest Handling, and Processing of Fruits - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 265

Glucose-1-P

Pentose phosphate
pathway

Erythrose-4-P Glucose-6-P

Deoxy-arabino-heptulosonate-7-P

Shikimic acid

Phosphoenolphyruvate (PEP)

5-Enol-pyruvyl-shikimate-3-phosphate

Pyruvate

Chorismic acid

Tryptophan

Prephenic acid

Acetyl CoA

Arogenic acid Tyrosine

Malonic
acid
pathway

Shikimic
acid
pathway

Phenylalanine

Malonyl CoA

PAL

trans -Cinnamic acid

C4H

p -Coumaric acid

4CL

p -Coumaroyl CoA
CHS

Hydroxycinnamates
Hydroxybenzoates
Coumarins
Lignans
Lignins
Hydrolysable tannins

Naringenin Chalcones

Flavonoids
Biflavonoids
Condensed tannins

Fig. 12.2 Biosynthetic pathways of major polyphenolic groups present in fruits. Indicated enzymes are as
follows: PAL, phenylalanine ammonia-lyase; C4H, cinnamate 4-hydroxylase; 4CL, 4-hydroxycinnamoyl CoA
ligase; CHS, chalcone synthase.

produce deoxy-arabino-heptulosonate-7-phosphate, which then converts to shikimic acid
(shikimate). Condensation of shikimic acid with another molecule of PEP results in the
formation of 5-enol-pyruvyl-shikimate-3-phosphate (EPSP). Conversion of EPSP to cho-
rismic acid (chorismate) is catalyzed by chorismate synthase by elimination of phosphate
from EPSP. Chorismate mutase catalyzes the next step, conversion of chorismic acid to
prephenic acid (prehenate), giving the basic phenyl-propanoid skeleton. Prephenic acid acts
as the precursor to three aromatic amino acids: arogenic acid, tyrosine, and phenylalanine.
The conversion of prehenate to arogenate is catalyzed by pyridoxal 5 -phosphate-dependent
prephenate aminotransferase, and arogenate dehydratase catalyzes the subsequent step of
formation of L -phenylalanine from arogenate.

12.5.1 Hydroxycinnamates

Phenylalanine is subsequently deaminated by the action of phenylalanine ammonia-lyase
(PAL) producing trans -cinnamic acid. PAL, a tetrameric protein of 270-330 kDa, is con-
sidered as the key regulatory enzyme of the phenylpropanoid pathway.
trans -Cinnamic acid is subsequently hydroxylated by cinnamate 4-hydroxylase (C4H)
to form p -coumaric acid. p -Coumaric acid further goes through a series of hydroxyla-
tion and methylation reactions to form many other hydroxycinnamates (C 6

C 3 ) including