Chemistry Reference
In-Depth Information
Figure 5.2
Pathway of isoprene biosynthesis.
a major biogenic volatile organic compound (VOC; rivalling methane in global
production) with emission from plants estimated in the order of more than 10
12
kg
per year and therefore has been well studied for its role in atmospheric chemistry
(24). Isoprene is also important in the context of global cycles of carbon fixa-
tion versus carbon emission from natural sources. Although isoprene is emitted
in large amounts from poplars, which are actively being promoted as plantation
species for biofuel (ethanol) production, its physiological function in plants is
somewhat unclear. The protection from thermal and oxidative stress as well as
release of excess carbon flux and photosynthetic energy are thought to be the
main functions of isoprene in plants (21, 22).
Very recently, new molecular approaches have established a function of iso-
prene in thermotolerance through the use of under- and over-producing transgenic
lines of poplar (25) or Arabidopsis (which normally produces no isoprene) using
the poplar isoprene synthase (26). Other possible functions of isoprene remain
to be tested using similar molecular approaches. The effect of down-regulation
of isoprene emission in poplars remains to be tested in the field to explore
whether it is viable to reduce emission of this biogenic VOC in plantation
forests for biofuels production to maximize carbon fixation and minimize carbon
emission. A closely related hemiterpene of plant origin is 2-methyl-3-buten-
2-ol. It is produced abundantly in needles of conifers and is emitted into the
atmosphere (27).
5.2.3 (-)-Menthol (C
10
, Monoterpenoid)
(-)-Menthol is a well-known terpenoid from the essential oil of mint (
Mentha
spp.) (15), and is described here as a representative of the different acyclic and
cyclic plant monoterpenoids. Because of its pleasant odor, taste, and anesthetic
and antimicrobial effects, (-)-menthol is an industrially important terpenoid and
is produced commercially in large scale both from the essential oils of
Mentha
spp. and by asymmetric synthesis. The essential oil is produced in glandular
trichomes, which are secretory cells that number in the thousands on
Mentha
leaves. The presence of these specialized cells, which easily can be separated
physically from other cell types, has greatly facilitated studying (-)-menthol
biosynthesis.
(-)-Menthol biosynthesis involves a series of enzymes (Fig. 5.3) that first
generate a cyclic monoterpene and then functionalize it. The biochemistry of
this pathway has been elucidated by substrate feeding, cell-free enzyme assays
with plant extracts, and characterization of cloned and recombinantly expressed
