020 CHAPTER 26 Lipids
Terpene Biosynthesis
The reaction of dimethylallyl pyrophosphate with isopentenyl pyrophosphate forms
geranyl pyrophosphate, a 10-carbon compound. In the first step of the reaction,
isopentenyl pyrophosphate acts as a nucleophile and displaces a pyrophosphate group
from dimethylallyl pyrophosphate. Pyrophosphate is an excellent leaving group: Its
four OH groups have values of 0.9, 2.0, 6.6, and 9.4. Therefore, three of the four
groups will be primarily in their basic forms at physiological pH A pro-
ton is removed in the next step, resulting in the formation of geranyl pyrophosphate.The following scheme shows how some of the many monoterpenes could be syn-
thesized from geranyl pyrophosphate:1 pH=7.3 2.pKadimethylallyl pyrophosphategeranyl pyrophosphatepyrophosphateisopentenyl pyrophosphateOOOPO−OO−PO−OOOPO−OO−PO−OOOPO−OO−PO−−OOOPO−OO−PO−
+ H+O +OOPO−OO−PO−++geranyl pyrophosphate-terpineol
in juniper oilO Ogeraniol
in rose and
geranium oilsOHcitronellol
in rose and
geranium oilsmenthol
in peppermint oilOHcitronellal
in lemon oilC
HOOPO−OO−PO−H 2 O H+
H 2 OH 2 OOH OHOOHreductionreductionoxidationoxidation+OHterpin hydrate
a common constituent
of cough medicinelimonene
in orange and
lemon oils