BLBS102-c27 BLBS102-Simpson March 21, 2012 13:25 Trim: 276mm X 219mm Printer Name: Yet to Come
27 Biochemistry of Fruits 547coloured pigments and is a prelude to the state of ripening and
development of organoleptic qualities. Mitochondria, which are
also rich in protein, are relatively stable and undergo disassem-
bly during the latter part of ripening and senescence.
Chlorophyll degradation is initiated by the enzyme chloro-
phyllase which splits chlorophyll into chlorophyllide and the
phytol chain. Phytol chain is made up of isoprenoid units
(methyl-1,3-butadiene), and its degradation products accumu-
late in the plastoglobuli. Flavour components such as 6-methyl-
5-heptene-2-one, a characteristic component of tomato flavour,
are also produced by the catabolism of phytol chain. The
removal of magnesium from chlorophyllide results in the forma-
tion of pheophorbide. Pheophorbide, which possesses a tetrapy-
role structure, is converted to a straight chain colourless tetrapyr-
role by the action of pheophorbide oxidase. Action of several
other enzymes is necessary for the full catabolism of chloro-
phyll. The protein complexes that organise the chlorophyll,
the light-harvesting complexes, are degraded by the action of
several proteases. The enzyme ribulose-bis-phosphate carboxy-
lase/oxygenase (Rubisco), the key enzyme in photosynthetic
carbon fixation, is the most abundant protein in chloroplast.
Rubisco levels also decline during ripening/senescence due to
proteolysis. The amino acids resulting from the catabolism of
proteins may be translocated to regions where they are needed
for biosynthesis. In fruits, they may just enrich the soluble frac-
tion with amino acids.SECONDARY PLANT PRODUCTS
AND FLAVOUR COMPONENTSSecondary plant products are regarded as metabolites that are
derived from primary metabolic intermediates through well-
defined biosynthetic pathways. The importance of the secondary
plant products to the plant or organ in question may not readily
be obvious, but these compounds appear to have a role in the
interaction of the plant with the environment. The secondary
plant products may include non-protein amino acids, alkaloids,
isoprenoid components (terpenes, carotenoids, etc.), flavonoids
and anthocyanins, ester volatiles and several other organic com-
pounds with diverse structure. The number and types of sec-
ondary plant products are enormous, but, with the perspective
of fruit quality, the important secondary plant products include
isoprenoids, anthocyanins and ester volatiles.Isoprenoid BiosynthesisIn general, isoprenoids possess a basic five-carbon skeleton in
the form of 2-methyl-1,3-butadiene (isoprene), which under-
goes condensation to form larger molecules. There are two
distinct pathways for the formation of isoprenoids: the ac-
etate/mevalonate pathway (Bach et al. 1999) localised in the
cytosol and the DOXP pathway (Rohmer pathway, Rohmer et al.
1993) localised in the chloroplast (Fig. 27.6). The metabolic pre-
cursor for the acetate/mevalonate pathway is acetyl Coenzyme
A. Through the condensation of three acetyl CoA molecules,
a key component of the pathway, 3-hydroxy-3-methyl-glutaryl
CoA (HMG CoA) is generated. HMG-CoA undergoes reduc-tion in the presence of NADPH mediated by the key regulatory
enzyme of the pathway HMG CoA reductase (HMGR), to form
mevalonate. Mevalonate undergoes a two-step phosphorylation
in the presence of ATP, mediated by kinases, to form isopen-
tenyl pyrophosphate (IPP), the basic five carbon condensational
unit of several terpenes. IPP is isomerised to dimethylallylpy-
rophosphate (DMAPP) mediated by the enzyme IPP isomerase.
Condensation of these two components results in the synthe-
sis of C10 (geranyl), C15 (farnesyl) and C20 (geranylgeranyl)
pyrophosphates. The C10 pyrophosphates give rise to monoter-
penes, C15 pyrophosphates give rise to sesquiterpenes and C20
pyrophosphates give rise to diterpenes. Monoterpenes are ma-
jor volatile components of fruits. In citrus fruits, these include
components such as limonene, myrcene, pinene and so on occur-
ring in various proportions. Derivatives of monoterpenes such
as geranial, neral (aldehydes), geraniol, linalool, terpineol (al-
cohols), geranyl acetate, neryl acetate (esters) and so on are also
ingredients of the volatiles of citrus fruits. Citrus fruits are es-
pecially rich in monoterpenes and derivatives. Alpha-farnesene
is a major sesquiterpene (C15) component evolved by apples.
The catabolism of alpha-farnesene in the presence of oxygen
into oxidised forms has been implicated as a causative feature in
the development of the physiological disorder superficial scald
(a type of superficial browning) in certain varieties of ap-
ples such as red Delicious, McIntosh, Cortland and so on
(Rupasinghe et al. 2000, 2003).
HMGR is a highly conserved enzyme in plants and is encoded
by a multigene family (Lichtenthaler et al. 1997). The HMGR
genes (hmg1, hmg2, hmg3, etc.) are nuclear encoded and can
be differentiated from each other by the sequence differences
at the 3′-untranslated regions of the cDNAs. There are three
distinct genes for HMGR in tomato and two in apples. The dif-
ferent HMGR end products may be localised in different cellular
compartments and are synthesised differentially in response to
hormones, environmental signals, pathogen infection and so on.
In tomato fruits, the level ofhmg1expression is high during
early stage of fruit development when cell division and expan-
sion processes are rapid, when it requires high levels of sterols
for incorporation into the expanding membrane compartments.
The expression ofhmg2which is not detectable in young fruits
increases during the latter part of fruit maturation and ripening.
HMGR activity can be detected in both membranous and cy-
tosolic fractions of apple fruit skin tissue extract. HMGR is
a membrane-localised enzyme, and the activity is detectable
in the endoplasmic reticulum, plastid and mitochondrial mem-
branes. It is likely that HMGR may have undergone proteolytic
cleavage releasing a fragment into the cytosol, which also pos-
sesses enzyme activity. There is a considerable degree of interac-
tion between the different enzymes responsible for the biosyn-
thesis of isoprenoids, which may exist as multienzyme com-
plexes. The enzyme Farnesyl pyrophosphate synthase, respon-
sible for the synthesis of farnesyl pyrophosphate is a cytosolic
enzyme. Similarly, farnesene synthase, the enzyme which con-
verts farnesyl pyrophosphate to alpha-farnesene in apples, is a
cytosolic enzyme. Thus, several enzymes may act in concert
at the cytoplasm/endoplasmic reticulum boundary to synthesise
isoprenoids.