21 Biochemistry of Fruits 501range 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 res-
piration of ripe fruits and especially becomes signif-
icant when fruits are exposed to low temperatures.
Often, this may result from oxygen-depriving condi-
tions induced inside the fruit. Under anoxia, ATP
production through the citric acid cycle and the
mitochondrial electron transport chain is inhibited.
Anaerobic respiration is a means of regenerating
NAD, which can drive the glycolyic pathway and
produce minimal amounts of ATP (Fig. 21.4). Under
anoxia, pyruvate formed through glycolysis is con-
verted by lactate dehydrogenase to lactate, using
NADH as the reducing factor and generating NAD.
Accumulation of lactate in the cytosol could cause
acidification, and under these low pH conditions,
lactate dehydrogenase is inhibited. The formation of
acetaldehyde by the decarboxylation of pyruvate is
stimulated by the activation of pyruvate decarboxy-
lase under low pH conditions in the cytosol. It is also
likely that the increase in concentration of pyruvate
in the cytoplasm may stimulate pyruvate decarboxy-
lase directly. Acetaldehyde is reduced to ethanol by
alcohol dehydrogenase using NADH as the reducing
power. Thus, acetaldehyde and ethanol are common
volatile components observed in the headspace of
fruits, indicative of the occurrence of anaerobic res-
piration. Cytosolic acidification is a condition that
stimulates deteriorative reactions. By removing lac-
tate through efflux and converting pyruvate to eth-
anol, cytosolic acidification can be avoided.
Anaerobic respiration plays a significant role in
the respiration of citrus fruits. During early stages of
growth, respiratory activity predominantly occurs in
the skin tissue. Oxygen uptake by the skin tissue
was much higher than that of the juice vesicles
(Purvis 1985). With advancing maturity, a decline
in aerobic respiration and an increase in anaerobic
respiration was observed in Hamlin orange skin
(Bruemmer 1989). In parallel with this, the levels of
ethanol and acetaldehyde increased. As well, a de-
crease in the organic acid substrates pyruvate and
oxaloacetate was detectable in Hamlin orange juice.
An increase in the activity levels of pyruvate decar-
boxylase, alcohol dehydrogenase, and malic enzyme
was noticed in parallel with the decline in pyruvate
and the accumulation of ethanol. In apple fruits,
malic acid is converted to pyruvate by the action of
NADP-malic enzyme, and pyruvate is subsequently
converted to ethanol by the action of pyruvate decar-
boxylase and alcohol dehydrogenase. The alcohol
dehydrogenase in apple can use NADPH as a cofac-
tor, and NADP is regenerated during ethanol pro-
duction, thus driving malate utilization. Ethanol is
either released as a volatile or can be used for the
biosynthesis of ethyl esters of volatiles.Pentose Phosphate PathwayOxidative pentose phosphate pathway (PPP) is a key
metabolic pathway that provides reducing power
(NADPH) for biosynthetic reactions as well as car-
bon precursors for the biosynthesis of amino acids,
nucleic acids, secondary plant products, and so forth.
The PPP shares many of its sugar phosphate inter-
mediates with the glycolytic pathway (Fig. 21.5).
The PPP is characterized by the interconversion
of sugar phosphates with three (glyceraldehyde-
3-phosphate), four (erythrose-4-phosphate), five (rib-
ulose, ribose, and xylulose phosphates), six (glucose-
6-phosphate, fructose-6-phosphate), and seven (sed-
oheptulose-7-phosphate) carbons.
PPP involves the oxidation of glucose-6-phos-
phate, and the sugar phosphate intermediates formed
are recycled. The first two reactions of PPP are oxi-
dative reactions mediated by the enzymes glucose-6-
phosphate dehydrogenase and 6-phosphogluconate
dehydrogenase (Fig. 21.5). 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 diox-
ide 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 can be 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