Monoterpene synthetase

Preliminary characterization of the partially purified cineole synthetase indicated that the enzyme, like other monoterpene synthetases, exhibited a pH optimum on the acid side of neutrality, was inhibited by thiol-directed reagents, and required a divalent cation for catalysis (Mn + was preferred) (Croteau and Karp, 1977a). A single protein appeared to be involved in conversion of the acyclic precursor to 1,8-cineole, and no free intermediates could be discerned in the reaction. A mechanism for the formation of 1,8-cineole was proposed that involved the hydration of a double bond of an enzyme-bound monocyclic intermediate, followed by displacement of the enzyme by the hydroxyl group to form the heterocyclic ring and simultaneously release the product (Croteau and Karp, 1977a). 

The substrate specificity of 1,8-cineole, a-terpineol, limonene, and terpin-olene synthetases from sage has not yet been determined however, it is quite clear that the sage preparations, and similar preparations from Citrus (George-Nascimento and Cori, 1971 Chayet et al., 1977), can utilize both GPP and NPP for the biosynthesis of monocyclic monoterpenes. Only when such cell-free preparations are free of all competing enzyme activities can the true specificities of the monoterpene synthetases be accurately determined. 

Romagni, J. G., Duke, S. O. and Dayan, F. E. 2000. Inhibition of plant asparagine synthetase by monoterpene cineoles. Plant Physiol. 123, 303-313... 

Croteau, R., Felton, M. and Ronald, R.C. (1980b) Biosynthesis of monoterpenes preliminary characterization of /-encfo-fenchol synthetase from fennel (Foeniculum vulgare) and evidence that no free intermediate is involved in the cyclization of geranyl pyrophosphate to the rearranged product. Archives of Biochemistry and Biophysics 200(2), 534-546. 

There are a number of allelochemicals among the mono-, sesqui-, and diterpenoids. In particular, plants in arid and semiarid regions produce diverse volatile terpenoids with allelopathic activity.5 Among the volatile monoterpenes, 1,8-cineole (4) and camphor (5) exhibit strong growth inhibitory effects on plants and are considered to be involved in plant competition. 1,4-Cineole (6), a minor isomer of 1,8-cineole, is a potent inhibitor of asparagine synthetase.6 -Menthane-3,8-diols (fir 7 and frw .r8), -menth-2-en-l-ols (cis 9 and trans 10), thymol (11), carvacrol (12), 1,8-cineole, cr-pinene (13), and /3-pinene (14) were isolated as allelopathic monoterpenes from Eucalyptus species.7 Eucalyptus trees also produce allelopathic sesquiterpenes including spathulenol (15), and a-, (3-, and 7-eudesmols (16-18).7... 

The biosynthesis of sesquiterpenes in plants appears to be isolated from that of either monoterpenes or diterpenes. A prenyl transferase isolated from pumpkin Cucurbita pepo, Cucurbitaceae) converts C5 units into FPP, but not into gera-nylgeranyl pyrophosphate (the precursor to diterpenes). Another enzyme from the same source forms C20 terpenoids from C5 units, but does not accumulate lower homologs (also see Chapters 19 and 22). However, because famesyl pyrophosphate synthetase is a branch-point metabolite for the synthesis of sesquiterpenes, triterpenes, and sterols, this enzyme is ubiquitous in plants (Croteau and Johnson, 1985). 

Croteau R, Karp F 1979 Biosynthesis of monoterpenes preliminary characterization of bornyl pyrophosphate synthetase from sage Salvia officinalis) and demonstration that geranyl pyrophosphate is the preferred substrate for cyclization. Arch Biochem Biophys 198 512-522... 

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