Monoterpene cyclases

A descriptor for an enzyme active site that permits binding of a family of related compounds (e.g., mimics of the reaction intermediate) that can be derived from the initial binding and conformational changes in the substrate. This concept arose from the observation that a number of monoterpene cyclases were incapable of discriminating between enantiomers of the reaction intermediate, even though the enzyme catalyzes the synthesis of an enantiomerically pure product from an achiral substrate. An example is trichodiene synthase which catalyzes the cyclization of farnesyl diphosphate to trichodiene. 

Gijzen, M., Lewinsohn, E. and Croteau, R. (1991) Characterization of the constitutive and wound-inducible monoterpene cyclases of grand fir Abies grandis). Arch. 

Rajaonarivony, J.I.M., Gershenzon, J. and Croteau, R. (1992) Characterization and mechanism of (4S)-limonene synthase, a monoterpene cyclase from the glandular trichomes of peppermint (Mentha x piperita). Arch. Biochem. Biophys., 296, 49-57. 

Wagschal, K., Savage, T.J. and Croteau, R. (1991) Isotopically sensitive branching as a tool for evaluating multiple product formation by monoterpene cyclases. Tetrahedron, 47, 5933 4. 

Biosynthetic and bloregulatory studies. In recent years substantial advances have been made In understanding the biosynthesis and catabolism of monoterpenes In plants (94). In the so called essential oil plants the monoterpenes are predominantly carbocyclic and hence monoterpene cyclases have been a primary focus of the research (94). 

LEWINSOHN, E., GIJZEN, M SAVAGE, T.J., CROTEAU, R., Defense-mechanisms of conifers - Relationship of monoterpene cyclase activity to anatomical specialization and oleoresin monoterpene content.. Plant Physiol., 1991, 96, 38-43. 

The variation in monoterpene skeletal types found in conifer oleoreslns is determined to a large degree by differential expression of the corresponding monoterpene cyclases. These... 

The cyclization of geranyl-OPP (3) to produce monocyclic and bicyclic monoterpenes proceeds via enzyme-bound intermediates that are probably similar to those postulated as intermediates in Wagner-Meerwein rearrangements. These reactions (catalyzed by strong acid) result in the formation of many types of monoterpenes when compounds similar to the precursors of cyclic monoterpenes are used as reactants. These enzymes, called monoterpene cyclases, largely deter-... 

The terpenoid fraction of many conifers consists of 20-50% volatile monoterpenes and sesquiterpenes and 50-80% nonvolatile diterpene acids. In general, there is little variation in the diterpene fraction among trees of a particular species, although the monoterpene fraction often varies greatly. Alterations in the monoterpene content often determine whether attack by bark beetles will occur and be successful. The monoterpenes may be attractants, repellents, or toxins of not only the insects, but also the pathogenic fungi involved in these systems. The conversion of geranyl pyrophosphate to the parent compounds of the various cyclic types is catalyzed by monoterpene cyclases (see the subsection biosynthesis, above). 

The conversion of 10-oxogeranial (10) (and 10-oxoneral) to iridodial (2) has been demonstrated a novel monoterpene cyclase from Rauvolfia serpentina that catalyzes formation of iridodial has been partially purified and characterized (Uesato et al, 1987). NADPH accelerates the cyclization. The molecular weight of the cyclase and its subunit are 118,000 and 28,700, respectively (Uesato et al., 1987). 

Uesato, S., H. Ikeda, T. Fujita, H. Inouye, and M. H. Zenk, Elucidation of iridodial formation mechanism—Partial purification and characterization of the novel monoterpene cyclase from Rauvolfia serpentina cell suspension cultures. Tetrahedron Lett., 28. 4431-4434 (1987). 

More than a dozen cDNAs encoding monoterpene synthases from a range of plant sources have now been isolated [22, 32-36]. The corresponding recombinant enzymes generate a diverse set of products and provide an exceptional set of catalysts for the detailed study of structure-function relationships in monoterpene cyclization. However, it was earher work with native synthases that first allowed definition of the cyclization reaction [ 15]. Thus, the stereochemical model for monoterpene cyclization was developed largely from studies with the bicyclic monoterpene cyclases (-i-)- and (-)-bornyl diphosphate synthases, (-1-)-and (-)-pinene synthases, and (-)-e do-fenchol synthase. It was also the purification of several native monoterpene synthases that gave the first indication that... 

Apparent from Scheme 4 is the necessity of the ionization-isomerization step to the cisoid linalyl intermediate for cyclization to the a-terpinyl cation, a reaction not directly possible from the geranyl substrate because of the frans-C2,C3 bond. A notable feature of all monoterpene cyclases is the abUity to utilize LPP... 

The nearly complete catalytic intolerance for glutamyl and alanyl substitutions in the DDxxD motif of limonene synthase is novel and unlike the much less pronounced effects of comparable substitutions in the sesquiterpene cyclase trichodiene synthase [97, 98]. However, pre-steady state kinetic analysis of trichodiene synthase [101] and several other sesquiterpene synthases [102] has recently shown that product release is rate limiting in these cases, and thus can mask the kinetic influence of the aspartate mutations on earlier steps in the catalytic cycle. In the instance of monoterpene cyclase catalysis, product release is not the slow step since comparison of k at values with GPP and LPP as substrate clearly reveals the initial ionization-isomerization to be rate limiting. Thus, perturbations that influence the first ionization step will be fully reflected in overall rate suppression for limonene synthase. This kinetic sensitivity at the initial steps of the reaction cycle does not, however, explain the near complete intolerance of limonene synthase to aspartate substitution in the DDxxD motif and it is thus tempting to speculate a more specific, but presently unidentified, influence on the requisite isomerization of GPP. 

One-year-old plants of slash pine had increased oleoresin production after flooding [63], Site fertilization increased constitutive flow of resin in 6-year-old loblolly pine [72], but reduced it in 11-year-old plants [22], Rates of monoterpenes emission increased exponentially with temperature in slash pine [70] and P. halepensis [66], However, variable emissirm responses were observed in Aleppo pine plants after exposure to different irradiance levels and limited water availability [66], Light and water stresses reduced the constitutive level of monoterpene cyclase activity in Grand fir saplings [62], Highest production of oleoresin in slash pine was observed in spring [54, 73] when compared to other seasons examined. 

Lewinsohn E, Worden E, Croteau R (1994) Monoterpene cyclases in Grand Fir callus-cultures - modulation by elicitors and growth-regulators. Phytochemistry 36(3) 651-656... 

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