Food Biochemistry and Food Processing (2 edition)

BLBS102-c27 BLBS102-Simpson March 21, 2012 13:25 Trim: 276mm X 219mm Printer Name: Yet to Come

540 Part 5: Fruits, Vegetables, and Cereals

cell wall components such as cellulose. UDP-glucose is con-

verted to sucrose-6-phosphate by the enzyme sucrose phosphate

synthase (SPS), which utilises fructose-6-phosphate during this

reaction. Finally, sucrose is formed from sucrose-6-phosphate

by the action of phosphatase with the liberation of the inorganic

phosphate.

Even though sucrose biosynthesis is an integral part of starch

metabolism, sucrose often is not the predominant sugar that ac-

cumulates in fruits. Sucrose is further converted into glucose

and fructose by the action of invertase, which are character-

istic to many ripe fruits. By the actions of sucrose synthase

and UDP-glucose pyrophosphorylase, glucose-1-phosphate can

be regenerated from sucrose. As well, sugar alcohols such

as sorbitol and mannitol formed during sugar metabolism are

major transport and storage components in apple and olive,

respectively.

Biosynthesis and catabolism of starch has been extensively

studied in banana, where prior to ripening, it can account for

20–25% by fresh weight of the pulp tissue. All the starch degrad-

ing enzymes,α-amylase,β-amylase,α-glucosidase and starch

phosphorylase, have been isolated from banana pulp. The ac-

tivities of these enzymes increase during ripening. Concomitant

with the catabolism of starch, there is an accumulation of the

sugars, primarily, sucrose, glucose and fructose. At the initiation

of ripening, sucrose appears to be the major sugar component,

which declines during the advancement of ripening with a si-

multaneous increase in glucose and fructose through the action

of invertase (Beaudry et al. 1989). Mango is another fruit which

stores large amounts of starch. The starch is degraded by the

activities of amylases during the ripening process. In mango,

glucose, fructose and sucrose are the major forms of simple sug-

ars (Selvaraj et al. 1989). The sugar content is generally very

high in ripe mangoes and can reach levels in excess of 90%

of the total soluble solids content. By contrast to the bananas,

the sucrose levels increase with the advancement of ripening in

mangoes, potentially due to gluconeogenesis from organic acids

(Kumar and Selvaraj 1990). As well, the levels of pentose sugars

increase during ripening, and could be related to an increase in

the activity of the PPP.

Glycolysis

The conversion of starch to sugars and their subsequent

metabolism occur in different compartments. During the de-

velopment of fruits, photosynthetically fixed carbon is utilised

for both respiration and biosynthesis. During this phase, the

biosynthetic processes dominate. As the fruit matures and be-

gin to ripen, the pattern of sugar utilisation changes. Ripening

is a highly energy-intensive process. And this is reflected in

the burst in respiratory carbon dioxide evolution during ripen-

ing. As mentioned earlier, the respiratory burst is characteristic

of some fruits which are designated as climacteric fruits. The

post-harvest shelf life of fruits can depend on their intensity of

respiration. Fruits such as mango and banana possess high level

of respiratory activity and are highly perishable. The application

of controlled atmosphere conditions having low oxygen levels

and low temperature have thus become a routine technology for

the long-term preservation of fruits.

The sugars and sugar phosphates generated during the

catabolism of starch are metabolised through the glycolysis and

citric acid cycle (Fig. 27.3). Sugar phosphates can also be chan-

nelled through the PPP, which is a major metabolic cycle that

provides reducing power for biosynthetic reactions in the form of

NADPH, as well as supplying carbon skeletons for the biosyn-

thesis of several secondary plant products. The organic acids

stored in the vacuole are metabolised through the functional re-

versal of respiratory pathway and is termed as gluconeogenesis.

Altogether, sugar metabolism is a key biochemical characteristic

of the fruits.

In the glycolytic steps of reactions (Fig. 27.3), glucose-6-

phosphate is isomerised to fructose-6-phosphate by the enzyme

hexose-phosphate isomerase. Glucose 6-phosphate is derived

from glucose-1-phosphate by the action of glucose phosphate

mutase. Fructose-6-phosphate is phosphorylated at the C1 posi-

tion, yielding fructose-1,6- bisphosphate. This reaction is catal-

ysed by the enzyme phosphofructokinase (PFK) in the pres-

ence of ATP. Fructose-1,6-bisphosphate is further cleaved into

two three carbon intermediates, dihydroxyacetone phosphate

and glyceraldehyde-3-phosphate, catalysed by the enzyme al-

dolase. These two compounds are interconvertible through an

isomerisation reaction mediated by triose phosphate isomerase.

Glyceraldehyde-3-phosphate is subsequently phosphorylated at

the C1 position using orthophosphate, as well as oxidised using

NAD, to generate 1,3-diphosphoglycerate and NADH. In the

next reaction, 1,3-diphosphoglycerate is dephosphorylated by

glycerate-3-phosphate kinase in the presence of ADP, along with

the formation of ATP. Glycerate-3-phosphate formed during this

reaction is further isomerised to 2-phosphoglycerate in the pres-

ence of phosphoglycerate mutase. In the presence of the enzyme

enolase, 2-phosphoglycerate is converted to phosphoenol pyru-

vate (PEP). Dephosphorylation of phosphoenolpyruvate in the

presence of ADP by pyruvate kinase yields pyruvate and ATP.

Metabolic fate of pyruvate is highly regulated. Under normal

conditions, it is converted to acetyl CoA, which then enters the

citric acid cycle. Under anaerobic conditions, pyruvate can be

metabolised to ethanol, which is a by-product in several ripening

fruits.

There are two key regulatory steps in glycolysis, one mediated

by PFK and the other by pyruvate kinase. In addition, there

are other types of modulation involving cofactors and enzyme

structural changes reported to be involved in glycolytic control.

ATP levels increase during ripening. However, in fruits, this

does not cause a feed back inhibition of PFK as observed in

animal systems. There are two isozymes of PFK in plants, one

localised in plastids and the other localised 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 PEP. Thus, any conditions that may

cause the accumulation of PEP 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