Changes in Nutritional Quality of Fruits and Vegetables During Storage - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 447

Sucrose

Photosynthesis Glucose Glucose-1-phosphate Starch

Pentose phosphate
pathway

Glycolysis

ATP Hexokinase

NADP

NADPH

Glucose-6-phosphate 6-Phosphogluconate

NADP
NADPH

ADP

Glucose-6-phosphate
dehydrogenase

6-Phosphogluconate
dehydrogenase
CO 2

Ribulose-5-phosphate

Glyceraldehyde-3-phosphate

NADPH pool

Pyruvate

Glyceraldehyde-3-
phosphate

Sedoheptulose-7-
phosphate

NAD
NADH

Pyruvate
dehydrogenase

Isoprenoid pathway

Acetyl CoA

Transaldolase

Fructose-6-phosphate

Erythrose-4-phosphate

Citric acid cycle

Phenyl propanoid
pathway

Fig. 21.1 Carbohydrate metabolism in fruits. Starch is catabolized to sugars that undergo glycolysis and pentose
phosphate pathway. Pentose phosphate pathway provides carbon and reducing power (NADPH) for the synthesis
of flavonoids through phenyl propanoid pathway.

21.4.2 Biosynthesis of isoprenoids

Isoprenoids are secondary plant products that are derived from primary metabolites. Al-
though there are several kinds of secondary plant products, the levels of isoprenoids, ester
volatiles, and anthocyanin components determine the fruit quality to a large extent. Iso-
prenoids are synthesized from two pathways: classical mevalonate (MVA) pathway and
1-deoxyxylulose-5-phosphate (DXP) pathway. These two pathways provide precursors,
isopentenyl pyrophosphate (IPP) and its isomer dimethylallyl pyrophosphate (DMAPP), for
the synthesis of isoprene (Liu et al., 2005b; Paliyath and Murr, 2006). Mevalonate pathway is
localized in the cytosol, and DXP pathway occurs in the chloroplast. In mevalonate pathway,
three acetyl-CoA molecules undergo condensation to produce 3-hydroxy-3-methyl-glutaryl
CoA (HMG-CoA), which forms mevalonic acid (MVA) in the presence of the enzyme