Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

ISOPRENOID BIOSYNTHESIS IN FRUITS AND VEGETABLES 285
COOH
CO
C 3 H
+
CH 2 OP
CH 2 OH
CHO
1-Deoxy-D-xylulose-
5-phosphate synthase
CH 2 OP
CHOH
CHOH
C
CH 3
O
Pyruvate
GAP
1-Deoxy-D-xylulose-
5-phosphate
DMAPP CH 2 OH
+
CH 2 OPP
3x
IPP
CH 2 OPP
2x
GGPP
Phytoene synthase
C 40 H 64 phytoene
C 40 H 56 lycopene
C 40 H 56 β-carotene
Fig. 13.2 Structure of major plastid-localized DXP/isoprenoid intermediates. Some key enzymes (bold face) and
intermediates involved in isoprenoid biosynthesis are abbreviated: DMAPP, dimethylallyl pyrophosphate; GAP,
glyceraldehyde-3-phosphate; IPP, isopentenyl pyrophosphate.
of plastid- and cytosolic-derived isoprenoid intermediates (Lichtenthaler, 1999; Rohmer,
1999; Eisenreich et al., 2001). This is an important point because it means that the entire
carotenoid biosynthetic process—from simple C 3 molecules to complete C 40 structures—
takes place within the plastids and is regulated by a set of plastid-localized enzymes (Fig.
13.3).
DXP is derived from glyceraldehyde-3-phosphate (GAP) and pyruvate through a reaction
catalyzed by the enzyme DXP synthase (DXS). The reaction involves the condensation
of hydroxyethyl thiamine (derived by the decarboxylation of pyruvate) with the aldehyde
group of GAP to form DXP (Lange et al., 1998; Rohmer, 1999). Through several subsequent
enzymatic steps, DXP is converted to isopentenyl pyrophosphate (IPP) and its isomer,
dimethylallyl pyrophosphate (DMAPP), both C 5 compounds serve as the basic building
blocks of the isoprenoid pathway (Charon et al., 2000; Rodríguez-Concepción et al.,
2000). Geranylgeranyl pyrophosphate (GGPP; C 20 ) is formed by GGPP synthase via the