Biochemistry of Fruits - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 41

3.3.3 Proteolysis and structure breakdown in chloroplasts
The major proteinaceous compartments in fruits are the chloroplasts that are distributed in
the epidermal and hypodermal layers of fruits. The chloroplasts are not very abundant in
fruits. During senescence, the chloroplast structure is gradually disassembled with a decline
in chlorophyll levels due to degradation and disorganization of the grana lamellar stacks
of the chloroplast. With the disorganization of the thylakoid, globular structures termed
as plastoglobuli accumulate within the chloroplast stroma and are rich in degraded lipids.
The degradation of chloroplasts and chlorophyll results in the unmasking of other colored
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 disassembly
during the latter part of ripening and senescence.
Chlorophyll degradation is initiated by the enzyme chlorophyllase that splits chloro-
phyll into chlorophyllide and the phytol chain. Phytol chain is made up of isoprenoid units
(methyl-1,3-butadiene) and its degradation products accumulate in the plastoglobuli. Fla-
vor components such as 6-methyl-5-heptene-2-one, a characteristic component of tomato
flavor, are also produced by the catabolism of phytol chain. The removal of magnesium
from chlorophyllide results in the formation of pheophorbide. Pheophorbide, which pos-
sesses a tetrapyrole structure, is converted to a straight-chain colorless tetrapyrrole by the
action of pheophorbide oxidase. Action of several other enzymes is necessary for the full
catabolism of chlorophyll. The protein complexes that organize the chlorophyll, the light-
harvesting complexes, are degraded by the action of several proteases. The enzyme ribulose-
bis-phosphate carboxylase/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 fraction with amino acids.
3.4 Secondary plant products
Secondary 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 obvi-
ous, but these compounds appear to have a role in the interaction of the plant with the
environment. The secondary plant products may include nonprotein amino acids, alka-
loids, isoprenoid components (terpenes, carotenoids, etc.), flavonoids and anthocyanins,
ester volatiles, and several other organic compounds with diverse structure. The number
and types of secondary plant products are enormous, but, with the perspective of fruit qual-
ity, the important secondary plant products include isoprenoids, anthocyanins, and ester
volatiles.
3.4.1 Isoprenoid biosynthesis
In general, isoprenoids possess a basic five-carbon skeleton in the form of 2-methyl-1,
3-butadiene (isoprene), which undergoes condensation to form larger molecules. There are
two distinct pathways for the formation of isoprenoids: the acetate/mevalonate pathway