Biosynthesis of Monoterpenes

Monoterpenes are synthesized and stored in specialized structures (Sangwan et al. 2001) whose cells express all the genes necessary for monoterpene biosynthesis. In conifers, specialized epithelial cells form resin ducts with large central storage cavities. In lemongrass, essential oil accumulates in cells inside the leaf blades, while in mints, the essential oil is generated in the secretory cells of hair-like structures called glandular trichomes, which 

The most well-known producers of essential oil monoterpenes are herbs such as the mint (Mentha) and sage (Salvia) families. Arabidopsis thaliana does not form glandular trichomes and has only recently been found to produce monoterpenes (Chen etal. 2003). Genes coding for monoterpene synthases in Arabidopsis have been cloned and overexpressed (Bohlmann et al. 2000 Bouvier et al. 2000 Chen et al. 2003), but the biological role of the monoterpene products is presently imclear. 

R = H iinaiooi (18) (coriander, nerofi, lavender oils) R = OCOCH3 linaiyi acetate (18) (lavender, bergamot) 

Thymol (26) and carvacrol (27), isolated from herbs such as thyme and savory, are members of the menthane family in which the cyclohexane structure has been oxidized to an aromatic (phenolic) ring. Oils containing these phenolic terpenes have been shown to be particularly effective as antibacterial agents (Kalemba and Kimicka 2003). 

The more complicated cyclic carbon skeletons of the bornanes, thujanes and pinanes are also derived from intramolecular cyclization of the linalyl cation. Members of all three families are present in the essential oil of cotrrmon sage (Salvia officinalis) which contains some 75 separate volatile compounds (Santos-Gomes and Fernandes-Ferreira 

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Davis EM, Croteau R (2000) Cyclization Enzymes in the Biosynthesis of Monoterpenes, Sesquiterpenes and Diterpenes. 209 53-95 Davies JA, see Schwert DD (2002) 221 165-200... 

The biosynthesis of monoterpenes, the major components of peppermint essential oils, can be divided into four stages (Fig. 9.4). Stage 1 includes the formation of isopentenyl diphosphate (IPP) and dimethylallyl alcohol (DMAPP). In plants, two separate pathways are utilized for the synthesis of these universal C5 intermediates, with the cytosolic mevalonate pathway being responsible for the formation of sterols and certain sesquiterpenes, and the plastidial mevalonate-independent pathway being involved in the biosynthesis of isoprene, monoterpenes, certain sesquiterpenes, diterpenes, tetraterpenes, as well as the side chains of chlorophyll and plastoquinone.16 In peppermint oil gland secretory cells, however, the mevalonate pathway is blocked and the biosynthesis of monoterpenoid essential... 

Davis EM, Croteau R, Leeper FJ, Vederas JC (2000) Cyclization enzymes in the biosynthesis of monoterpenes, sesquiterpenes, and diterpenes. In Topics in current chemistry biosynthesis-aromatic polyketides, isoprenoids, alkaloids. Springer, Heidelberg, pp 53-95... 

Croteau, R., Felton, M. and Ronald, R.C. (1980a) Biosynthesis of monoterpenes conversion of the acyclic precursors geranyl pyrophosphate and neryl pyrophosphate to the rearranged monoterpenes fenchol and fenchone by a soluble enzyme preparation from fennel (Foeniculum vulgare). Archives of Biochemistry and Biophysics 200(2), 524-533. 

Buriat, V., Oudin, A., Courtois, M., Rideau, M. and St. Pierre, B. (2004) Co-expression of three MEP pathway genes and geraniol-lO-hydroxylase in internal phloem parenchyma of Catharanthus roseus implicates multicellular location of intermediates during the biosynthesis of monoterpene indole alkaloids and isoprenoid-derived primary metabolites. Plant., 38,131 1. 

Mettal, U., Boland, W., Beyer, P. and Kleinig, H. (1988) Biosynthesis of monoterpene hydrocarbons by isolated chromoplasts from daffodil flowers. Eur.. Biochem., 170, 613-6. 

Oligomerization of isoprenoids under elimination of pyrophosphate affords the precursors for the biosynthesis of monoterpenes, sesquiterpenes, diterpenes, triterpenes, and tetra-terpenes (93). Long-chain oligomer pyrophosphates also supply the side chains of vitamin E (99, a-tocopherol. Fig. 11), heme a (18), chlorophyll (17, Fig. 2), and the quinone type cofactors, including vitamin K (menaquinone, 98) and coenzyme QIO (ubiquinone, 97). The quinone moieties are derived from hydroxybenzoate that is synthesized from tyrosine in animals or from chorismate in microorganisms (53, 54). 

The previous view on the biosynthesis of monoterpenes is that the cyclization of the geranyl pyrophosphate proceeds via a common cationic precursor, which in turn is further transformed into the various monoterpenes, as shown in Fig. (2). 

The book by Mann et al. (1994) on natural products provides a good introduction to the biogenesis of natural perfume ingredients and the review by Croteau (1987) gives further detail on the biosynthesis of monoterpenes. 

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