Food Biochemistry and Food Processing

21 Biochemistry of Fruits 507

pathway is acetyl coenzyme A (acetyl-CoA).
Through the condensation of three acetyl-CoA mol-
ecules, a key component of the pathway, 3-hydroxy-
3-methyl-glutaryl CoA (HMG-CoA) is generated.
HMG-CoA undergoes reduction 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 isopentenyl pyrophosphate
(IPP), the basic five-carbon condensational unit of
several terpenes. IPP is isomerized to dimethyl-
allylpyrophosphate (DMAPP), mediated by the en-
zyme IPP isomerase. Condensation of these two
components results in the synthesis of C 10 (geranyl),
C 15 (farnesyl), and C 20 (geranylgeranyl) pyrophos-
phates. The C 10 pyrophosphates give rise to mono-

terpenes, C 15 pyrophosphates give rise to sesquiter-

penes, and C 20 pyrophosphates give rise to diter-

penes. Monoterpenes are major volatile components

of fruits. In citrus fruits, these include components

such as limonene, myrcene, pinene, and others, oc-

curring in various proportions. Derivatives of mono-

terpenes such as geranial, neral (aldehydes), geran-

iol, linalool, terpineol (alcohols), geranyl acetate,

neryl acetate (esters), and so on, are also ingredients

of the volatiles of citrus fruits. Citrus fruits are espe-

cially rich in monoterpenes and derivatives. Alpha-

farnesene is a major sesquiterpene (C 15 ) component

evolved by apples. The catabolism of alpha-farne-

sene in the presence of oxygen into oxidized forms

has been implicated as a causative feature in the

development of the physiological disorder superfi-

cial scald (a type of superficial browning) in certain

Figure 21.7.Isoprenoid biosynthetic pathway in plants.