Monoterpenes
Monoterpenes
At least 1000 monoterpenes are known. They belong to more than 38 major structural types
This group of compounds is widespread in plants, from algae to monocots and dicots.
Monoterpenes also found in fungi, some are encountered in bacteria, but they are usually not accumulated in bacteria
Monoterpenes often produce odor, they are common common essential oil components
Monoterpenes are often accumulated in epitheleal cells, living cavities, in the plants.
The production of monoterpenes is usually associated with plastids in plant cells.
They are rarely accumulated in tissue cultures.
The production of monoterpenes is linked to morphological differentiation.
Synthesis and degradation of monoterpenes or "turnover" does not seem to be as important as thought by early investigators. This was in part an artifact of the biosynthetic systems studied.
Isolation
They are usually isolated by steam distillation, solvent extraction, or expression.
Biosynthesis
Monoterpenes are synthesized in higher plants from the DOXP pathway. This explains, in part, very low rates of incorporation of mevalonic acid in many previous studies.
Introduction of label from DOXP precursors is consistent with that origin.
Geranyl pyrophosphate or diphosphate (GPP) is the key intermediate in the synthesis of most monoterpenes.
GPP is synthesized from DMAPP and IPP, the IPP portion is usually labelled more effectively than the DMAPP portion.
Neryl pyrophosphate or diphosphate (NPP) also incorporated, but this may be because of interconversion of GPP and NPP. Studies by Croteau demonstrate that GPP is the actual precursor.
Linalyl-OPP also can be incorporated under many circumstances, but probably is not a real precursor in the plant either.
Incorporation of glycosides into monoterpenes occurs more readily than for usual monoterpenes. These glycosides work better for biosynthetic studies, but may
Acyclic compounds
Cyclic compounds
The origin of many skeletal types of monoterpenes can be rationalized by Wagner-Meerwein rearrangements. In classical organic chemistry, these were catalyzed by strong acids, such as sulfuric acid under non-biological conditions. They involve transer of hydride ions and other rearrangements.
In plants, monoterpene cyclases are key regulating enzymes for the synthesis of the major skeletal types of monoterpenes.
The end products are frequently hydrocarbons or alcohols that can be secondarily converted to other monoterpene types.
Intermediates seldom isolated in the syntheses of the products of monoterpene cyclases.
These can now be carried out in cell-free systems, several of which have now been investigated
These initial steps are followed by a series of secondary transformations.
(+)-Bornyl pyrophosphate cyclase has been isolated from sage. This enzyme makes (+)-bornyl pyrophosphate. It was used by Croteau to establish that geranyl-OPP is the usual precursor of monoterpenes, not NPP or LPP.
Another preparation from sage gives 1,8-cineole, limonene, terpinolene, α-terpineol, ... all by different routes, no free intermediates
GPP ----> single protein ----> 1,8-cineole (a chanelled process involving the enzyme 1,8-cineole cyclase?)
Other cyclases include: α-terpinene cyclase,
endo-fenchol cyclase, bornyl PP cyclase, (+)-pinene cyclase, (-)-pinene cyclase.
α-pinenes
pinene cyclase I
pinene cyclase II
β-pinenes
mint Mentha piperita
GPP ----> limonene
This is a soluble enzyme from microsomal preps from oil glands of this plant.
The initial steps are followed by secondary changes.
Cannabis compounds
Systematics
Allomones/kairomones/synomones
Monoterpenes are repellent or toxic to many animals.
Many monoterpenes are allelopathic. They are involved in plant-plant interactions.
Salvia leucophylla
Artemisia californica
Monoterpenes are attractive to other animals. Kairomones.
They are involved in feeding by Papilio spp. carvone and methyl chavicol
Insects also make/sequester monoterpenes and use them for pheromones.
boll weevils
bark beetles
aggregation pheromones
Monoterpenes are synomones in many flowers and fruits.
They are involved in the interactions of orchids and euglossine bees.
Irregular monoterpenes
There are 5 basic structural types.
almost all in Asteraceae
Pyrethrins, from Chrysanthemum species, are derived from one of these types.
Bornyl pyrophosphate biosynthesis
© David S. Seigler, Integrative Biology 425, Plant Secondary Metabolism, Department of Plant
Biology, 265 Morrill Hall, 505 S. Goodwin Ave., University of Illinois, Urbana, Illinois 61801, USA.
217-333-7577. seigler@life.uiuc.edu.
