Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

ISOPRENOID BIOSYNTHESIS IN FRUITS AND VEGETABLES 293
pathway that resulted in the formation of the C 5 monomer, isopentenyl pyrophosphate. In this
classic isoprenoid pathway, the precursor of isoprenoids, mevalonate, is synthesized from
and 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA), which in turn is formed from acetoacetyl-CoA
by the condensation of three acetyl-CoA units. HMG-CoA is converted to the
C 6 mevalonate in an irreversible reaction catalyzed by HMG-CoA reductase (HMGR; EC
1.1.1.34). This enzyme catalyzes two reduction steps, each requiring NADPH. Mevalonate
is sequentially phosphorylated by two separate soluble kinases, mevalonate kinase and
phosphomevalonate kinase, to form 5-pyrophosphomevalonate. Formation of the “active
C 5 isoprene unit,” isopentenyl pyrophosphate, is then catalyzed by pyrophosphomevalonate
decarboxylase (McGarvey and Croteau, 1995).
IPP, along with its isomerization product DMAPP, represents the “activated” monomer
building blocks for all isoprenoids. The first isomerization enzyme, IPP isomerase, requires
a divalent metal ion and operates through an unusual mechanism involving a carbocation
intermediate (McGarvey and Croteau, 1995). Isoprene, the simplest of the isoprenoids,
is synthesized directly from DMAPP by the enzyme isoprene synthase, eliminating the
diphosphate unit. Condensation of DMAPP with IPP in a head/tail fashion by various
prenyltransferases generates prenyl diphosphates of different chain lengths. The C 10 compound,
geranyl pyrophosphate (GPP), is catalyzed by GPP synthase. Addition of a second
IPP unit to GPP generates the C 15 compound FPP by FPP synthase; and addition
of a third IPP generates GGPP by GGPP synthase; and so on (McGarvey and Croteau,
1995). The families of enzymes responsible for the conversion of GPP, FPP, and GGPP
to the monoterpene, sesquiterpene, and diterpene classes, respectively, are referred to as
monoterpene, sesquiterpene, and diterpene synthases or cyclases, and represent reactions
committing carbon from the central isoprenoid pathway to the end products (Chappell,
1995).
In higher plants, at least three distinct semiautonomous subcellular compartments exist
that synthesize isoprenoids: cytoplasm/ER (sesquiterpenes and triterpenes, e.g., sterol),
plastids (monoterpenes and diterpenes, e.g., chlorophyll, carotenoids, and prenylquinones),
and mitochondria (ubiquinones). It is generally accepted that at least the final biosynthetic
steps are bound to these compartments. The biosynthesis of particular mono- and diterpenes
is generally attributed to the plastidic compartment, even if other subsequent biosynthetic
steps and accumulation of the final isoprenoid may occur in separate compartments
(Lichtenthaler et al., 1997).
13.11 HMG-CoA reductase as the key regulatory enzyme
HMGR, a highly conserved enzyme in eukaryotes, catalyzes the rate-limiting step of IPP
biosynthesis in animals and most of the isoprenoid biosynthesis in plants. In higher plants,
HMGR is encoded by a multigene family (Lichtenthaler et al., 1997) with the genes characteristically
distinguishable from each other by the sequence differences at the 3 ′ -untranslated
regions of the cDNAs (McCaskill and Croteau, 1997). As well, HMGR genes are nuclearencoded
(Lichtenthaler et al., 1997). HMGR is encoded by at least two distinctive genes in
A. thaliana (Caelles et al., 1989), cotton (Gossypium hirsutum L.) (Loguercio et al., 1999),
and rice (Oryza sativa) (Nelson et al., 1994); by three genes in rubber (Hevea brasiliensis)
(Chye et al., 1992), tomato (Lycopersicon esculentum) (Weissenborn et al., 1995), and
potato (Solanum tuberosum) (Yang et al., 1991; Choi et al., 1992); and an even larger