180 Handbook of herbs and spices
mevalonic acid) and shikimate (based on shikimic acid) pathways. The terpenes are
wholly mevalonate derived whereas the phenylpropenes originate from shikimic acid.
Figure 11.3 illustrates the sequence of formation of methyl chavicol from phenylalanine
by the shikimic acid pathway. Figure 11.4(a)–(d) illustrates the general biosynthetic
pathway from mevalonic acid to sesquiterpenes.
11.3.1 Biosynthesis of monoterpenes and sesquiterpenes
Mevalonic acid is a chemical intermediate containing six carbons that is formed in
the plant by the combination of three molecules of acetate, which have, in turn, been
derived from acetyl coenzyme A. This is a universal process in all higher plants and
produces compounds vital to the life processes. The biosynthesis of mono- and
sesquiterpenes from mevalonic acid involves three steps: (i) conversion of mevalonic
acid to isopentenyl pyrophosphate (IPP) and 3,3-dimethyl allyl pyrophosphate
(DMAPP), (ii) combination of IPP and DMAPP to give geranyl pyrophosphate (GPP)
and (iii) combination of GPP with IPP to give farnesyl pyrophosphate (FPP). IPP is
the initial product formed from mevalonic acid and it is then converted into DMAPP
by the enzyme isopentenyl pyrophosphate isomerase (Gershenzon and Croteau 1990,
Waterman 1993).
One molecule of IPP and one molecule of DMAPP combine under the influence
of geranyl pyrophosphate synthase to give geranyl phyrophosphate (GPP), the first
recognizable monoterpene. This process is then continued by the addition of another
OH OHOH OH OH OMeOH OHCOOH CHOCOOH COOHNH 2Phenylalanine cinnamic acid p-coumaric acid p-coumaryl aldehydep-coumaryl alcohol p-dihydrocoumaryl alcohol chavicol methyl chavicol
Fig. 11.3 Formation of methyl chavicol from phenyl alanine by shikimic acid pathway.