Food Biochemistry and Food Processing

21 Biochemistry of Fruits 505

centrations over 10 M. Thus, if the cytosolic con-
centrations of these ions progressively increase dur-
ing ripening or senescence, the membranes are dam-
aged as a consequence. However, this is an inherent
feature of the ripening process in fruits and results in
the development of ideal organoleptic qualities that
makes them edible. However, uncontrolled mem-
brane deterioration can result in the loss of shelf life
and quality in fruits.
The properties and regulation of the membrane
degradation pathway are increasingly becoming
clear. Enzymes such as phospholipase D (PLD) and
lipoxygenase (LOX) are very well studied. There are
several isoforms of phospholipase D designated as
PLD-alpha, PLD-beta, PLD-gamma, and so on. The

expression and activity levels of PLD-alpha are much

higher than those of the other PLD isoforms. Thus,

PLD-alpha is considered a housekeeping enzyme.

The regulation of PLD activity is an interesting fea-

ture. PLD is normally a soluble enzyme. The second-

ary structure of PLD shows the presence of a segment

of around 130 amino acids at the N-terminal end,

designated as the C2 domain. This domain is charac-

teristic of several enzymes and proteins that are inte-

gral components of the hormone signal transduction

system. In response to hormonal and environmental

stimulation and the resulting increase in cytosolic

calcium concentration, C2 domain binds calcium and

transports PLD to the membrane where it can initiate

membrane lipid degradation. The precise relation

Figure 21.6.Phospholipid catabolic pathway and its relation to fruit ripening.