Isoprenoid Biosynthesis in Fruits and Vegetables - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 286

Rohmer
pathway

Glucose-1-P

DXP
synthase

Glyceraldehyde-3-P

1-Deoxy- D -xylulose-5-P

Pyruvate

(DXP)

Classical
MVA
pathway

Acetyl CoA

Reductoisomerase

HMG CoA

2-C-Methyl- D -erythritol-4-P

HMGR

Mevalonate (MVA)

C 5 Isopentenyl-pp (IPP)

Dimethylallyl-PP (DMAPP)

Monoterpenes

C 10 Geranyl-PP (GPP)

C 15 Farnesyl-PP (FPP) Sesquiterpenes

Side chains of cytochromes,
ubiquinone
Iosoprenoid polymers
Squalene
Phytosterol
Abscisic acid
Triterpenoids

Diterpenes
GA
Side chains of chlorophylls,
tocopherols, and
phylloquinone
Phytoene
Carotenoids

C 20 Geranylgeranyl-PP (GGPP)

Interrelationship between the mevalonate pathway and the DXP pathway.

Fig. 13.3

sequential and linear addition of three molecules of IPP to one molecule of DMAPP. In the
next step, phytoene synthase (PSY) catalyzes the first committed step of carotenoid biosyn-
thesis: the condensation of two molecules of GGPP to form phytoene (C 40 ) (Cunningham
and Gantt, 1998). Phytoene is converted to lycopene via four desaturation reactions. The
first two are catalyzed by phytoene desaturase (PDS) and the second two are catalyzed by
ζ -carotene desaturase (ZDS).

In photosynthetically active tissue, lycopene does not accumulate because it serves as
an intermediate metabolite in the synthesis of the cyclic and oxygenated carotenoids that act
as photoprotectants and light receptors in chloroplast thylakoids. One of the major bicyclic
products of lycopene is -carotene. This compound is the second most abundant carotenoid

in ripe fruits because of b lycopene cyclase —a specific lycopene cyclase that is expressed
exclusively in the chromoplasts of flowers and fruits (Ronen et al., 2000). With b lycopene
cyclase as an exception, the activity of all lycopene cyclases is greatly reduced during fruit
ripening (Fraser et al., 1994; Ronen et al., 1999); this, in concert with increased expression
of DXS , PSY1 , and PDS genes (Ronen et al., 1999; Lois et al., 2000), allows lycopene to
accumulate.

β

13.6 Phytoene synthase

PSY catalyzes the condensation of two molecules of GGPP (C 20 ) to form phytoene: the
basic C 40 skeleton for the synthesis of all carotenoids (Cunningham and Gantt, 1998).