Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

BIOCHEMISTRY OF FRUITS 33
control. ATP levels increase during ripening. However, in fruits, this does not cause a
feedback inhibition of phosphofructokinase as observed in animal systems. There are two
isozymes of PFK in plants: one localized in plastids and the other localized in the cytoplasm.
These isozymes regulate the flow of carbon from the hexose phosphate pool to the pentose
phosphate pool. PFK isozymes are strongly inhibited by phosphoenol pyruvate. Thus, any
conditions that may cause the accumulation of phosphoenol pyruvate will tend to reduce
the carbon flow through glycolysis. By contrast, inorganic phosphate is a strong activator of
PFK. Thus, the ratio of PEP to inorganic phosphate would appear to be the major factor that
regulates the activity of PFK and carbon flux through glycolysis. Structural alteration of
phosphofructokinase, which increases the efficiency of utilization of fructose-6-phosphate,
is another means of regulation that can activate the carbon flow through the glycolytic
pathway.
Other enzymes of the glycolytic pathway are involved in the regulation of starch/sucrose
biosynthesis (Figs 3.2 and 3.3). Fructose-1,6-bisphosphate is converted back to fructose-6-
phosphate by the enzyme fructose-1,6-bisphosphatase, also releasing inorganic phosphate.
This enzyme is localized in the cytosol and chloroplast. Fructose-6-phosphate is converted
to fructose-2,6-bisphosphate by fructose-6-phosphate 2-kinase, which can be dephosphorylated
at the 2-position by fructose-2,6-bisphosphatase. Fructose-6-phosphate is an intermediary
in sucrose biosynthesis (Fig. 3.3). Sucrose phosphate synthase (SPS) is regulated by
reversible phosphorylation (a form of posttranslational modification that involves addition
of a phosphate moiety from ATP to an OH amino acid residue in the protein, such as serine or
threonine, mediated by a kinase, and dephosphorylation mediated by a phosphatase) by SPS
kinase and SPS phosphatase. Phosphorylation of the enzyme makes it less active. Glucose-
6-phosphate is an allosteric activator (a molecule that can bind to an enzyme and increase
its activity through enzyme subunit association) of the active form of SPS (dephosphorylated).
Glucose-6-phosphate is an inhibitor of SPS kinase, and inorganic phosphate is an
inhibitor of SPS phosphatase. Thus, under conditions when glucose-6-phosphate/inorganic
phosphate ratio is high, the active form of SPS will dominate, favoring sucrose phosphate
biosynthesis. These regulations are highly complex and may be regulated by the flux of
other sugars in several pathways.
The conversion of PEP to pyruvate mediated by pyruvate kinase is another key metabolic
step in the glycolytic pathway and is irreversible. Pyruvate is used in several metabolic
reactions. During respiration, pyruvate is further converted to acetyl coenzyme A (acetyl
CoA), which enters the citric acid cycle through which it is completely oxidized to carbon
dioxide (Fig. 3.3). The conversion of pyruvate to acetyl CoA is mediated by the enzyme
complex pyruvate dehydrogenase and is an oxidative step that involves the formation of
NADH from NAD. Acetyl CoA is a key metabolite and starting point for several biosynthetic
reactions (fatty acids, isoprenoids, phenylpropanoids, etc.).
3.3.1.4 Citric acid cycle
The citric acid cycle involves the biosynthesis of several organic acids, many of which serve
as precursors for the biosynthesis of several groups of amino acids. In the first reaction,
oxaloacetate combines with acetyl CoA to form citrate and is mediated by citrate synthase
(Fig. 3.4). In the next step, citrate is converted to isocitrate by the action of aconitase. The
next two steps in the cycle involve oxidative decarboxylation. The conversion of isocitrate
to α-ketoglutarate involves the removal of a carbon dioxide molecule and reduction of