Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

BIOCHEMISTRY OF FRUITS 37
six (glucose-6-phosphate, fructose-6-phosphate), and seven (sedoheptulose-7-phosphate)
carbons.
PPP involves the oxidation of glucose-6-phosphate, and the sugar phosphate intermediates
formed are recycled. The first two reactions of PPP are oxidative reactions mediated
by the enzymes glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase
(Fig. 3.5). In the first step, glucose-6-phosphate is converted to 6-phosphogluconate
by the removal of two hydrogen atoms by NADP to form NADPH. In the next step, 6-
phosphogluconate, a six-carbon sugar acid phosphate, is converted to ribulose-5-phosphate,
a five-carbon sugar phosphate. This reaction involves the removal of a carbon dioxide
molecule along with the formation of NADPH. Ribulose-5-phosphate undergoes several
metabolic conversions to yield fructose-6-phosphate. Fructose-6-phosphate can then be
converted back to glucose-6-phosphate by the enzyme glucose-6-phosphate isomerase and
the cycle repeated. Thus, six complete turns of the cycle can result in the complete oxidation
of a glucose molecule.
Despite the differences in the reaction sequences, the glycolytic pathway and the PPP
intermediates can interact with one another and share common intermediates. Intermediates
of both the pathways are localized in plastids as well as the cytoplasm, and intermediates can
be transferred across the plastid membrane into the cytoplasm and back into the chloroplast.
Glucose-6-phosphate dehydrogenase is localized in the both chloroplast and cytoplasm.
Cytosolic glucose-6-phosphate dehydrogenase activity is strongly inhibited by NADPH.
Thus, the ratio of NADP to NADPH could be the regulatory control point for the enzyme
function. The chloroplastic enzyme is regulated differently through oxidation and reduction,
and related to the photosynthetic process. 6-Phosphogluconate dehydrogenase exists as
distinct cytosol- and plastid-localized isozymes.
PPP is a key metabolic pathway related to biosynthetic reactions, antioxidant enzyme
function, and general stress tolerance of the fruits. Ribose-5-phosphate is used in the biosynthesis
of nucleic acids, and erythrose-4-phosphate is channeled into phenyl propanoid pathway
leading to the biosynthesis of the amino acids phenylalanine and tryptophan. Phenylalanine
is the metabolic starting point for the biosynthesis of flavonoids and anthocyanins
in fruits. Glyceraldehyde-3-phosphate and pyruvate serve as the starting intermediates for
the isoprenoid pathway localized in the chloroplast. Accumulation of sugars in fruits during
ripening has been related to the function of PPP. In mangoes, increase in the levels
of pentose sugars observed during ripening has been related to increased activity of PPP.
Increases in glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase
activities were observed during ripening of mango.
NADPH is a key component required for the proper functioning of the antioxidant enzyme
system (Fig. 3.5). During growth, stress conditions, fruit ripening, and senescence,
free radicals are generated within the cell. Activated forms of oxygen, such as superoxide,
hydroxyl, and peroxy radicals can attack enzymes and proteins, nucleic acids, lipids in the
biomembrane, etc., causing structural and functional alterations of these molecules. Under
most conditions, these are deleterious changes, which are nullified by the action of antioxidants
and antioxidant enzymes. Simple antioxidants such as ascorbate and vitamin E can
scavenge the free radicals and protect the tissue. Anthocyanins and other polyphenols may
also serve as simple antioxidants. In addition, the antioxidant enzyme system involves the
integrated function of several enzymes. The key antioxidant enzymes are superoxide dismutase
(SOD), catalase, ascorbate peroxidase, and peroxidase. SOD converts superoxide into