Chemistry Reference
In-Depth Information
with classical and modern approaches of isolation, identification, and metabolite
profiling of terpenoids from plant tissue.
The combined genomics and chemical approaches to plant terpenoid research
are not restricted to the few plant species for which more or less complete genome
sequences are now available. The discovery of many of the genes and enzymes
for the formation of terpenoids such as menthol and related monoterpenes in
peppermint (
Mentha x piperita
) (15), artemisinin in
Artemisia annua
(16), Taxol
in the yew tree (
Taxus
) (17), or conifer diterpene resin acids in species of spruce
(
Picea
) and pine (
Pinus
) (18) have been possible on the foundation of highly
specialized efforts of EST and full-length cDNA sequencing combined with char-
acterization of recombinant enzymes and analysis of the terpenoid metabolome
of the target plant species.
5.2.1 Common Steps in Plant Terpenoid Biosynthesis
The universal precursors to terpenoids, the C
5
-compounds dimethylallyl
pyrophosphate (DMAPP) and isopentenyl pyrophosphate (IPP), originate
from two pathways in plants (Fig. 5.1). The mevalonate (MEV) pathway is
well described in many eukaryotic organisms. This pathway is present in the
cytosol/endoplasmic reticulum of plants. More recently, another pathway has
been described, the 2
C
-methyl-D-erythritol-4-phosphate (MEP) pathway, which
is found in the plastids of plants (19). The localization of the different pathways
and the plastid-directing transit peptides found in hemi-TPS, mono-TPS, and
di-TPS, but not in sesqui-TPS, result in the production of terpenoids from at
least two different precursors pools.
Hemiterpenoids are produced from the isoprenyl diphosphate DMAPP. All
other terpenoids are produced from DMAPP and IPP via longer-chain prenyl
diphosphate intermediates formed by prenyl transferases. Prenyl transferases
(20) catalyze the formation of geranyl diphosphate (GPP), farnesyl diphosphate
(FPP), and geranylgeranyl diphosphate (GGPP) from one molecule of DMAPP
and one, two, or three molecules of IPP, respectively (Fig. 5.1). Isoprenyl
diphosphates are the substrates for all TPS, which lead to the hemiterpenoids,
monoterpenoids, sesquiterpenoids, and diterpenoids, which will be highlighted
with selected examples in the following sections.
5.2.2 Isoprene (C
5
, Hemiterpene)
Isoprene (2-methyl 1,3-butadiene, Fig. 5.2) is the simplest terpenoid produced by
many plants, and is produced abundantly by some tree species such as poplars.
The mechanism of isoprene biosynthesis is a straightforward diphosphate ion-
ization of the DMAPP precursor followed by deprotonation of the allylic cation
(Fig. 5.2). Only a few isoprene synthases have been identified and characterized
in plants, from
Populus
spp. (poplar) (21, 22) and
Pueraria montana
(kudzu
vine) (23). These TPS contain a transit peptide that targets the plastids, and thus
isoprene biosynthesis is derived from products of the MEP pathway. Isoprene is
