Biosynthesis of monoterpenes: Inhibition of (+)-pinene and (−)-pinene cyclases by thia and aza analogs of the 4R- and 4S-α-terpinyl carbocation☆
Abstract
(+)-Pinene cyclase (synthase) from Salvia officinalis leaf catalyzes the cyclization of geranyl pyrophosphate, via (3R)-linalyl pyrophosphate and the (4R)-α-terpinyl cation, to (+)-α-pinene and to lesser quantities of stereochemically related monoterpene olefins, whereas (−)-pinene cyclase converts the same achiral precursor, via (3S)-linalyl pyrophosphate and the (4S)-α-terpinyl cation, to (−)-α-pinene and (−)-β-pinene and to lesser amounts of related olefins. Racemic thia analogs of the linalyl and α-terpinyl carbocation intermediates of the reaction sequence were previously shown to be good uncompetitive inhibitors of monoterpene cyclases, and inhibition was synergized by the presence of inorganic pyrophosphate. These results suggested that the normal reaction proceeds through a series of carbocation:pyrophosphate anion paired intermediates. Both the (4R)- and the (4S)-thia and -aza analogs of the α-terpinyl cation were prepared and tested as inhibitors with the antipodal pinene cyclases, both in the absence and in the presence of inorganic pyrophosphate. Although the inhibition kinetics were complex, cooperative binding of the analogs and inorganic pyrophosphate was demonstrated, consistent with ion pairing of intermediates in the course of the normal reaction. Based on the antipodal reactions catalyzed by the pinene cyclases, stereochemical differentiation between the (4R)- and the (4S)-analogs was anticipated; however, neither enzyme effectively distinguished between enantiomers of the thia and aza analogs of the α-terpinyl carbocation. Enantioselectivity in the enzymatic conversion of (RS)-α-terpinyl pyrophosphate to limonene by the pinene cyclases was also examined. Consistent with the results obtained with the thia and aza analogs, the pinene cyclases were unable to discrimintae between enantiomers of α-terpinyl pyrophosphate in this unusual reaction. Either the α-terpinyl antipodes are too similar to allow differentiation by the pinene cyclases, or these enzymes lack an inherent requirment to distinguish the (4R)- and (4S)-forms because they encounter only one enantiomer in the course of the normal reaction from geranyl pyrophosphate.
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Nature-driven approaches to non-natural terpene analogues
2020, Natural Product ReportsCovering: up to 2019
The reactions catalysed by terpene synthases belong to the most complex and fascinating cascade-type transformations in Nature. Although many accept only one natural terpene precursor and convert it with high selectivity into only one product, several of these remarkable biocatalysts were recently shown to have a surprising plasticity towards non-natural substrate analogues. For an easy access to the topic also for readers who are new to the field, this review will first briefly cover the principles of natural terpene biosynthesis. This is followed by a chapter that highlights purely chemical transformations mimicking terpene synthase catalysed reactions. Then, the main focus of this article will shed light on the recent advances of terpene synthase catalysed transformations of synthetic substrate analogues. As will be demonstrated, a simple conceptual approach extensively broadens the chemical space that can be reached with terpene synthases.
Monoterpene synthases from Gymnosperms and angiosperms: Stereospecificity and inactivation by cysteinyl- and arginyl-directed modifying reagents
1995, Archives of Biochemistry and BiophysicsTo further define specific structural and mechanistic differences among monoterpene synthases from divergent plant sources, the stereospecificity of the enzyme-catalyzed isomerization of geranyl pyrophosphate to linalyl pyrophosphate and the subsequent cyclization to monoterpene olefins (which have been well established for monoterpene synthases from herbaceous angiosperms) were examined for monoterpene synthases from a conifer, lodgepole pine (Pinus contorta). The chiral monoterpenes isolated from lodgepole pine oleoresin and the major chiral products from cell-free assays of each of the four lodgepole pine monoterpene synthases belonged to the stereochemical family related by the biosynthetic intermediacy of 3S-linalyl pyrophosphate. Furthermore, both the putative intermediate, 3S-linalyl pyrophosphate, and the natural substrate, geranyl pyrophosphate, were enzymatically converted to the same monoterpene enantiomers. Thus, like monoterpene synthases from herbaceous angiosperms, monoterpene synthases from lodgepole pine appear to catalyze both the stereospecific isomerization of geranyl pyrophosphate to linalyl pyrophosphate and the subsequent cyclization of this enzyme-bound intermediate to multiple, stereochemically related monoterpene olefin isomers. The susceptibility of monoterpene synthases to inactivation by cysteinyl- and arginyl-directed chemical modification reagents was also examined to identify specific structural differences between enzymes from conifers and angiosperms. Like monoterpene synthases from peppermint (Mentha x piperita) and culinary sage (Salvia officinalis), monoterpene synthases from lodgepole pine were inactivated by thioldirected reagents; however, unlike monoterpene synthases from these herbaceous angiosperms, monoterpene synthases from lodgepole pine were not protected against inactivation by coincubation with substrate and metal ion cofactor. Lodgepole pine monoterpene synthases were also inactivated by the arginyl-directed reagent phenylglyoxal, and coincubation with substrate and cofactor, to effect active-site protection, reduced the rate of inactivation 10-fold. (+)-Pinene synthase and (−)-pinene synthase from sage were also inactivated by phenylglyoxal, but no protection was afforded by coincubation with substrate and cofactor. Thus, monoterpene synthases of conifers appear to have catalytically important arginyl residues specifically located at or near the active site and have at least some catalytically important thiol residues at a non-substrate-protectable region of the enzyme, in contrast to monoterpene synthases from angiosperms which appear to have catalytically important cysteinyl residues at the active site and have catalytically important arginyl residues located at a non-substrate-protectable region of the enzyme.
Stereochemistry of the Proton Elimination in the Formation of (+)- and (-)-α-Pinene by Monoterpene Cyclases from Sage (Salvia officinalis)
1994, Archives of Biochemistry and BiophysicsThe three pinene synthases (cyclases) from common sage (Salvia officinalis) catalyze the conversion of geranyl pyrophosphate to the bicyclic olefins (+)-α-pinene and (+)-camphene (cyclase I), (−)-α-pinene, (−)-β-pinene, and (−)-camphene (cyclase II), and (+)-α-pinene and (+)-β-pinene (cyclase III), in addition to smaller amounts of monocyclic and acyclic monoterpene olefins. (1R)-4-2H1-and (1S)-4-2H1-labeled geranyl pyrophosphates were prepared and used to examine the stereochemistry of the C3-proton elimination from the pinyl cation intermediates in the formation of the α-pinene enantiomers. Mass spectrometric analysis of the biosynthetic products derived from the chirally deuterated substrates revealed that cyclase I and cyclase III removed the C4-proR-hydrogen of the substrate (C3 proton trans to the dimethyl bridge of the pinyl nucleus) with a stereoselectivity exceeding 94% in the formation of (+)-α-pinene. Similarly, cyclase II removed the C4-proS-hydrogen of the substrate (C3-trans proton of the corresponding pinyl cation) with a stereoselectivity exceeding 78% in the formation of (−)-α-pinene. The stereoselectivity of these C3-axial hydrogen eliminations is rationalized on the basis of a stereochemical model for the electrophilic isomerization-cyclization reaction sequence catalyzed by the pinene cyclases. The changes in the overall rates of olefin biosynthesis by these enzymes and in the product ratios resulting from deuterium substitution also permitted confirmation of isotopically sensitive branching in pinene biosynthesis and allowed the observation of primary kinetic isotope effects in isolation.
Biosynthesis of Monoterpenes: Partial Purification, Characterization, and Mechanism of Action of 1,8-Cineole Synthase
1994, Archives of Biochemistry and BiophysicsGeranyl pyrophosphate:1,8-cineole cyclase (cineole synthase) catalyzes the conversion of geranyl pyrophosphate to the symmetrical monoterpene ether 1,8-cineole (1,3,3-trimethyl-2-oxabicyclo[2.2.2]octane) by a process thought to involve the initial isomerization of the substrate to the tertiary allylic isomer, linalyl pyrophosphate, and cyclization of this bound intermediate to the α-terpinyl carbocation that is subsequently captured by water and undergoes heterocyclization to the remaining double bond. The enzyme was isolated from the secretory cells of the glandular trichomes of Salvia officinalis (garden sage) and partially purified, and the properties of this monoterpene cyclase, previously determined in crude cell-free extracts, were reexamined. These properties (pH optimum, divalent metal ion requirement, molecular weight, pI) were similar to those determined previously with the exception of substrate utilization; geranyl pyrophosphate was shown to be a more efficient substrate than the cis-isomer, neryl pyrophosphate, in the absence of competing phosphatase activity that contaminated earlier preparations of this enzyme. As with other monoterpene cyclases of herbaceous species, cineole synthase was inhibited by cysteine- and histidine-directed reagents, and protection against inactivation was provided by the substrate-metal ion complex. Studies with 18O-labeled acyclic precursors and H218O, followed by mass spectrometric analysis of the product, confirmed that water was the sole source of the ether oxygen atom of 1,8-cineole. The electrophilic nature of the coupled isomerization-cyclization reaction was examined with a series of substrate and intermediate analogues. The overall stereochemistry of the cyclization of geranyl pyrophosphate to the symmetrical monoterpene was established by determining the enantioselectivity for (3R)- or (3S)-linalyl pyrophosphate as an alternative substrate and by oxidation of [3-3H]1,8-cineole, derived from [1-3H]geranyl pyrophosphate, to (±)-3-keto-1,8-cineole and radio-GLC separation of diastereomeric ketal derivatives to determine the labeled enantiomer.
Biosynthesis of Monoterpenes: Regio- and Stereochemistry of (+)-3-Carene Biosynthesis
1993, Archives of Biochemistry and BiophysicsIncubation of [1-3H1]geraniol with stem disks of Douglas fir (Pseudotsuga menziesii) and incubation of [1-3H1]geranyl pyrophosphate with both a soluble enzyme extract from Douglas fir and a partially purified preparation of (+)-3-carene synthase from lodgepole pine (Pinus contorta) resulted in the production of (+)-3-[3H] carene. Subsequent conversion of the product to car-3-en-5-one and to 4-isocaranone followed by base-catalyzed exchange of the α-hydrogens established that the 3H located at C1 in the geranyl substrate resided at C5 of (+)-3-carene. Incubation of the (+)-3-carene synthase preparation with (S)-[5-3H1, 4-14C]geranyl pyrophosphate resulted in the production of (+)-3-carene without loss of tritium, indicating that the 5-proR hydrogen is eliminated during cyclopropyl ring closure. Analysis of the conformational requirements for this 1,3 elimination involving the 5-proR hydrogen suggested that cyclopropyl ring formation occurs via a (4S)-α-terpinyl cation derived from the anti-endo cyclization of a (3S)-linalyl pyrophosphate intermediate. Kinetic analyses of the conversion of (1Z, 3R)-[1-3H1]linalyl pyrophosphate, (1Z, 3S)-[1-3H1]linalyl pyrophosphate and [1-3H1]geranyl pyrophosphate by (+)-3-carene synthase revealed that the velocity of the reaction with the (3S)-linalyl enantiomer was 25-fold greater than the velocity with the (3R)-enantiomer and twice that of the natural substrate, geranyl pyrophosphate, thereby confirming this stereo-chemical prediction and also indicating that the cyclization of the linalyl intermediate is faster than the coupled isomerization and cyclization of the geranyl substrate. From these results, a model that details the regio- and stereochemistry of the enzymatic conversion of geranyl pyrophosphate to (+)-3-carene is proposed.
Irreversible Inactivation of Monoterpene Cyclases by a Mechanism-Based Inhibitor
1993, Archives of Biochemistry and BiophysicsMonoterpene synthases (cyclases) catalyze the divalent metal ion-dependent transformation of geranyl pyrophosphate to representatives of the various monocyclic and bicyclic skeletal types by an electrophilic reaction mechanism involving coupled isomerization and cyclization steps. An analogue of the geranyl substrate, in which the terminal gem-dimethyl groups were joined to form a cyclopropyl function (6-cyclopropylidene-3E-methyl-hex-2-en-1-yl pyrophosphate) was shown to be a potent inhibitor of (−)-4S-limonene synthase from Mentha spicata and of several other monoterpene cyclases from diverse plant species. Inhibition was concentration and time dependent (pseudo-first-order kinetics), as well as absolutely contingent on the presence of the divalent metal ion cofactor. A double reciprocal plot of kinactivation versus inhibitor concentration gave an apparent Ki of approximately 0.3 μM and a maximum rate of inactivation of about 0.3 min−1 with limonene synthase. As expected for an active-site-directed process, the natural substrate, geranyl pyrophosphate, afforded protection against inactivation by the cyclopropylidene analogue. Selectivity of the inhibition was demonstrated with [1-3H]6-cyclopropylidene-3E-methyl-hex-2-en-1-yl pyrophosphate by specific labeling of limonene synthase in crude enzyme extracts as evidenced by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, radio-fluorography, and immunoblotting. The radioactive cyclase-inactivator complex was formed with 1:1 stoichiometry and was stable to extended dialysis and boiling in 2% sodium dodecyl sulfate, suggesting irreversible covalent modification of the enzyme involving a chemical reaction between cyclase and inhibitor. Thermally denatured limonene synthase and synthase that had been inactivated with the histidine-directed reagent diethylpyrocarbonate or the cysteine-directed reagent p-hydroxymercuribenzoate (two reagents known to modify the active site of the enzyme and inhibit catalysis) were not labeled when treated with the [1-3H]-analogue, indicating that the functional enzyme was necessary to effect complex formation. All of the evidence is consistent with the analogue serving as a mechanism-based inactivator that must undergo both ionization-dependent isomerization and cyclization steps to reveal an allylic cation which alkylates the protein. In addition to furnishing supporting evidence for the electrophilic reaction sequence, this mechanism-based inactivator provides a powerful new approach for the examination of cyclase active sites.
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This investigation was supported in part by U.S. National Institutes of Health Grants GM-13956 (to R.M.C.) and GM-31354 (to R.B.C.) and by Project 0268 from the Washington State University Agricultural Research Center.