Biochemistry of Fruits - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 39

Ethylene

Ethylene receptor

Membrane

Phospholipid

Phospholipase D

Ester voatiles (flavor)

Alcohol acylCoA-
acyl transferase

Hexanol

Phosphatidic acid

Phosphatidic acid
phosphatase

Hexanal

Short-chain fatty acids

Diacylglycerol

Peroxidized fatty acids

Lipolytic acylhydrolase

Free fatty acids

Lipoxygenase

Phospholipid catabolic pathway and its relation to fruit ripening.

Fig. 3.6

and is a key enzyme of the pathway (Fig. 3.6). Phospholipase D acts on phospholipids,
liberating phosphatidic acid and the respective headgroup (choline, ethanolamine, glyc-
erol, inositol). Phosphatidic acid in turn is acted upon by phosphatidate phosphatase, which
removes the phosphate group from phosphatidic acid, with the liberation of diacylglyc-
erols (diglycerides). The acyl chains of diacylglycerols are then deesterified by the enzyme
lipolytic acyl hydrolase, liberating free fatty acids. Unsaturated fatty acids with a cis -1,4-
pentadiene structure (linoleic acid, linolenic acid) are acted upon by lipoxygenase, causing
the peroxidation of fatty acids. This step may also cause the production of activated oxygen
species such as singlet oxygen, superoxide, and peroxy radicals. The peroxidation products
of linolenic acid can be 9-hydroperoxy linoleic acid or 13-hydroperoxy linoleic acid. The
hydroperoxylinoleic acids undergo cleavage by hydroperoxide lyase resulting in several
products including hexanal, hexenal, and -keto fatty acids (keto group toward the methyl

end of the molecule). For example, hydroperoxide lyase action on 13-hydroperoxylinolenic
acid results in the formation of cis -3-hexenal and 12-keto -cis- 9-dodecenoic acid. Hexanal
and hexenal are important fruit volatiles. The short-chain fatty acids may feed into catabolic
pathway (

ω

-oxidation) that results in the formation of short-chain acyl CoAs, ranging from
acetyl CoA to dodecanoyl CoA. The short-chain acyl CoAs and alcohols (ethanol, propanol,
butanol, pentanol, hexanol, etc.) are esterified to form a variety of esters that constitute
components of flavor volatiles that are characteristic to fruits. The free fatty acids and their
catabolites (fatty aldehydes, fatty alcohols, alkanes, etc.) can accumulate in the membrane,
causing membrane destabilization (formation of gel-phase, nonbilayer structures, etc.). An
interesting regulatory feature of this pathway is the very low substrate specifity of enzymes

β