Biochemistry of Fruits - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 42

Glucose-1-P

Rohmer
pathway

DOXP
synthase

Glyceraldehyde-3-P

1-Deoxy- D -xylulose-5-P
(DOXP)

Pyruvate

DOXP
reductoisomerase

Acetyl CoA

Classical
MVA
pathway

HMG CoA
HMGR

2-C-methyl- D -erythritol-4-P

Mevalonate (MVA)

Dimethylallyl-PP (DMAPP)

C 5 Isopentenyl-PP (IPP)

Monoterpenes

C 10 Geranyl-PP (GPP)

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

C 15 Farnesyl-PP (FPP)

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

C 20 Geranylgeranyl-PP (GGPP)

Isoprenoid biosynthetic pathway in plants.

Fig. 3.7

(Bach et al., 1999) localized in the cytosol and the DOXP pathway (Rohmer pathway;
Rohmer et al., 1993) localized in the chloroplast (Fig. 3.7). The metabolic precursor for the
acetate/mevalonate pathway is acetyl coenzyme A. Through the condensation of three acetyl
CoA molecules, a key component of the pathway, 3-hydroxy-3-methyl-glutaryl CoA (HMG
CoA), is generated. HMG-CoA undergoes reduction in the presence of NADPH mediated
by the key regulatory enzyme of the pathway HMG CoA reductase (HMGR) to form meval-
onate. Mevalonate undergoes a two-step phosphorylation in the presence of ATP, mediated
by kinases, to form isopentenyl pyrophosphate (IPP), the basic five-carbon condensational
unit of several terpenes. IPP is isomerized to dimethylallylpyrophosphate (DMAPP) me-
diated by the enzyme IPP isomerase. Condensation of these two components results in the
synthesis of C10 (geranyl), C15 (farnesyl), and C20 (geranylgeranyl) pyrophosphates. The
C10 pyrophosphates give rise to monoterpenes, C15 pyrophosphates give rise to sesquiter-
penes, and C20 pyrophosphates give rise to diterpenes. Monoterpenes are major volatile