Isoprenoid Biosynthesis in Fruits and Vegetables - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 285

1-Deoxy- D -xylulose-
5-phosphate synthase

CH 2 OP

CH 2 OP

COOH

CHOH
CHOH

CO

+ +

CH 2 OH

CO

C 3 H

CHO

CH 3

1-Deoxy- D -xylulose-
5-phosphate

Pyruvate GAP

CH 2 OH

+ + CH 2 OPP

DMAPP IPP

3x

CH CH 2 OPP

GGPP

2x

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 reac-
tion 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 subse-
quent enzymatic steps, DXP is converted to isopentenyl pyrophosphate (IPP) and its iso-
mer, dimethylallyl pyrophosphate (DMAPP), both C 5 compounds serve as the basic build-
ing blocks of the isoprenoid pathway (Charon et al., 2000; Rodrıguez-Concepcion et al.,
2000). Geranylgeranyl pyrophosphate (GGPP; C 20 ) is formed by GGPP synthase via the