Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

266 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
caffeic acid (caffeate), ferulic acid (ferulate), 5-hydroxyferulic acid (5-hydroxyferulate),
and sinapic acid (sinapate). Most often, hydroxycinnamates exist as conjugates of esters
and amides. The reaction ofp-coumaric acid to form p-coumaroyl-CoA is catalyzed by
4-hydroxycinnamoyl CoA ligase (4CL).
12.5.2 Hydroxybenzoates
Hydroxybenzoate (C 6 −C 1 ), for example, gallic acid (gallate), is a common phenolic acid
that is derived from hydroxycinnamates. Several pathways of biosynthesis of individual
hydroxybenzoates have been proposed. Side-chain degradation of hydroxycinnamates is one
of the common routes of the formation of many hydroxybenzoates. Under some conditions,
gallate can also be synthesized from shikimic acid or can be a result of degradation of
flavonoids. Gallate is present in some fruits as gallotannins. p-Coumaric acid derivatives and
p-coumaroyl-CoA are also the precursors for lignans ([C 6 C 3 ]2) and lignins ([C 6 C 3 ]n),
dimeric and polymeric phenylpropanoids, which has a significant role in the structural
components of cell walls. Decarboxylation of benzoic acid and phenylpropanoid derivatives
can result in the formation of simple phenols such as catechol.
12.5.3 Chlorogenic acid
The exact biosynthetic pathway of chlorogenic acid (caffeoyl quinic acid) formation in
fruits is not very clear; however, biosynthesis diverges from the flavonoid biosynthetic
pathway downstream of PAL, but upstream of chalcone synthase (CHS) (Fig. 12.3). Three
different biosynthetic pathways of chlorogenic acid have been elucidated (Niggeweg et al.,
2004). The first route diverges from p-coumaroyl-CoA, by formation of an ester bond with
shikimic acid with the aid of the enzyme hydroxycinnamoyl transferase (HCT) to produce
p-coumaroyl shikimic acid. This molecule is converted to caffeoyl shikimic acid by the
enzyme p-coumarate 3 ′ -hydrolase (C3H) (Niggeweg et al., 2004). HCT is then used again
to convert caffeoyl shikimic acid to caffeoyl CoA and then to hydroxycinnamoyl CoA
by quinate hydroxycinnamoyl transferase (HQT), which replaces CoA with quinic acid
to produce chlorogenic acid (route 1) (Stockigt and Zenk, 1974; Niggeweg et al., 2004).
Alternatively, cinnamic acid can be bound to glucose via UDP-glucose/cinnamate glucosyl
transferase (UGCT) to produce cinnamoyl D-glucose. Two hydroxyl groups are then added
to the cinnamoyl D-glucose by yet unknown enzyme(s) to form caffeoyl D-glucose. The
glucose group is then replaced by quinic acid through the action of hydroxycinnamoyl
D-glucose/quinate hydroxycinnamoyl transferase (HCGQT) to produce chlorogenic acid
(Niggeweg et al., 2004) (route 2). A third theorized pathway branches off from p-coumaroyl-
CoA, creating an ester bond to quinic acid using the enzyme HCT to produce p-coumaroyl
quinic acid. An addition of a hydroxyl group at the carbon-3 position of p-coumaroyl quinic
acid by C3H completes the final step for the biosynthesis of chlorogenic acid of this putative
pathway (route 3).
12.5.4 Flavonoids
The step of the formation of the C 15 aglycone skeleton (C6 C3 C6) of flavonoids is the
condensation of one molecule of p-coumaroyl-CoA and three molecules of malonyl-CoA