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

542 Part 5: Fruits, Vegetables, and Cereals

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 favouring 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 ki-

nase 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 coen-

zyme A (acetyl CoA), which enters the citric acid cycle through

which it is completely oxidised to carbon dioxide (Fig. 27.3).

The conversion of pyruvate to acetyl CoA is mediated by the en-

zyme 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.).

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, oxaloac-

etate combines with acetyl CoA to form citrate, and is mediated

by citrate synthase (Fig. 27.3). In the next step, citrate is con-

verted to isocitrate by the action of aconitase. The next two steps

in the cycle involve oxidative decarboxylation. The conversion

of isocitrate toα-ketoglutarate involves the removal of a car-

bon dioxide molecule and reduction of NAD to NADH. This

step is catalysed by isocitrate dehydrogenase.α-ketoglutarate is

converted to succinyl-CoA byα-ketoglutarate dehydrogenase,

along with the removal of another molecule of carbon dioxide

and the conversion of NAD to NADH. Succinate, the next prod-

uct, is formed from succinyl CoA by the action of succinyl CoA

synthetase that involves the removal of the CoA moiety and the

conversion of ADP to ATP. Through these steps, the complete

oxidation of the acetyl CoA moiety has been achieved with the

removal of two molecules of carbon dioxide. Thus, succinate

is a four-carbon organic acid. Succinate is further converted to

fumarate and malate in the presence of succinate dehydrogenase

and fumarase, respectively. Malate is oxidised to oxaloacetate

by the enzyme malate dehydrogenase along with the conversion

of NAD to NADH. Oxaloacetate then can combine with another

molecule of acetyl CoA to repeat the cycle. The reducing power

generated in the form of NADH and FADH (succinate dehydro-

genation step) is used for the biosynthesis of ATP through the

electron transport chain in the mitochondria.

Gluconeogenesis

Several fruits store large amounts of organic acids in their vac-

uole and these acids are converted back to sugars during ripen-

ing, a process termed as gluconeogenesis. Several irreversible

steps in the glycolysis and citric acid cycle are bypassed dur-

ing gluconeogenesis. Malate and citrate are the major organic

acids present in fruits. In fruits such as grapes, where there is a

transition from a sour to a sweet stage during ripening, organic

acids content declines. Grape contains predominantly tartaric

acid along with malate, citrate, succinate, fumarate and several

organic acid intermediates of metabolism. The content of or-

ganic acids in berries can affect their suitability for processing.

High acid content coupled with low sugar content can result in

poor-quality wines. External warm growth conditions enhance

the metabolism of malic acid in grapes during ripening and could

result in a high tartarate/malate ratio, which is considered ideal

for vinification.

The metabolism of malate during ripening is mediated by the

malic enzyme, NADP-dependent malate dehydrogenase. Along

with a decline in malate content, there is a concomitant increase

in the sugars, suggesting a possible metabolic precursor prod-

uct relationship between these two events. Indeed, when grape

berries were fed with radiolabelled malate, the radiolabel could

be recovered in glucose. The metabolism of malate involves its

conversion to oxaloacetate mediated by malate dehydrogenase,

the decarboxylation of oxaloacetate to PEP catalysed by PEP-

carboxykinase, and a reversal of glycolytic pathway leading to

sugar formation (Ruffner et al. 1983). The gluconeogenic path-

way from malate may contribute only a small percentage (5%)

of the sugars, and a decrease in malate content could primarily

result from reduced synthesis and increased catabolism through

the citric acid cycle. The inhibition of malate synthesis by the in-

hibition of the glycolytic pathway could result in increased sugar

accumulation. Metabolism of malate in apple fruits is catalysed

by NADP-malic enzyme, which converts malate to pyruvate. In

apples, malate appears to be primarily oxidised through the cit-

ric acid cycle. Organic acids are important components of citrus

fruits. Citric acid is the major form of the acid followed by malic

acid and several less abundant acids such as acetate, pyruvate,

oxalate, glutarate, fumarate and so on. In oranges, the acidity

increases during maturation of the fruit and declines during the

ripening phase. Lemon fruits, by contrast, increase their acid

content through the accumulation of citrate. The citrate levels in

various citrus fruits range from 75% to 88%, and malate levels

range from 2% to 20%. Ascorbate is another major component of

citrus fruits. Ascorbate levels can range from 20 to 60 mg/100 g

juice in various citrus fruits. The orange skin may possess

150–340 mg/100 g fresh weight of ascorbate, which may not

be extracted into the juice.

Anaerobic Respiration

Anaerobic respiration is a common event in the respiration

of ripe fruits and especially becomes significant when fruits

are exposed to low temperature. Often, this may result from

oxygen-depriving conditions induced inside the fruit. Under

anoxia, ATP production through the citric acid cycle and mito-

chondrial electron transport chain is inhibited. Anaerobic res-

piration is a means of regenerating NAD, which can drive

the glycolyic pathway and produce minimal amounts of ATP

(Fig. 27.3). Under anoxia, pyruvate formed through glycolysis