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.


Related Images


Miscellaneous monoterpenes

Biosynthesis of monoterpenes

Bornyl pyrophosphate biosynthesis

l-Isothujone biosynthesis

Menthol and related compounds

Catabolism of monoterpenes

Bergamottin

Formation of cannabinoids

Bioactive monoterpenes

More bioactive monoterpenes

Irregular monoterpenes


Lecture Slides

Plants with Monoterpenes




© 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.