Monoterpene cyclases. Stereoelectronic requirements for substrate binding and ionization

Chemical diversity in angiosperms - monoterpene synthases control complex reactions that provide the precursors for ecologically and commercially important monoterpenoids

Monoterpene synthases (MTSs) catalyze the first committed step in the biosynthesis of monoterpenoids, a class of specialized metabolites with particularly high chemical diversity in angiosperms. In addition to accomplishing a rate enhancement, these enzymes manage the formation and turnover of highly reactive carbocation intermediates formed from a prenyl diphosphate substrate. At each step along the reaction path, a cationic intermediate can be subject to cyclization, migration of a proton, hydride, or alkyl group, or quenching to terminate the sequence. However, enzymatic control of ligand folding, stabilization of specific intermediates, and defined quenching chemistry can maintain the specificity for forming a signature product. This review article will discuss our current understanding of how angiosperm MTSs control the reaction environment. Such knowledge allows inferences about the origin and regulation of chemical diversity, which is pertinent for appreciating the role of monoterpenoids in plant ecology but also for aiding commercial efforts that harness the accumulation of these specialized metabolites for the food, cosmetic, and pharmaceutical industries.

Enantiomeric natural products: occurrence and biogenesis

In nature, chiral natural products are usually produced in optically pure form-however, occasionally both enantiomers are formed. These enantiomeric natural products can arise from a single species or from different genera and/or species. Extensive research has been carried out over the years in an attempt to understand the biogenesis of naturally occurring enantiomers; however, many fascinating puzzles and stereochemical anomalies still remain.

A 1,6-ring closure mechanism for (+)-δ-cadinene synthase?

Recombinant (+)-δ-cadinene synthase (DCS) from Gossypium arboreum catalyzes the metal-dependent cyclization of (E,E)-farnesyl diphosphate (FDP) to the cadinane sesquiterpene δ-cadinene, the parent hydrocarbon of cotton phytoalexins such as gossypol. In contrast to some other sesquiterpene cyclases, DCS carries out this transformation with >98% fidelity but, as a consequence, leaves no mechanistic traces of its mode of action. The formation of (+)-δ-cadinene has been shown to occur via the enzyme-bound intermediate (3R)-nerolidyl diphosphate (NDP), which in turn has been postulated to be converted to cis-germacradienyl cation after a 1,10-cyclization. A subsequent 1,3-hydride shift would then relocate the carbocation within the transient macrocycle to expedite a second cyclization that yields the cadinenyl cation with the correct cis stereochemistry found in (+)-δ-cadinene. An elegant 1,10-mechanistic pathway that avoids the formation of (3R)-NDP has also been suggested. In this alternative scenario, the final cadinenyl cation is proposed to be formed through the intermediacy of trans, trans-germacradienyl cation and germacrene D. In addition, an alternative 1,6-ring closure mechanism via the bisabolyl cation has previously been envisioned. We report here a detailed investigation of the catalytic mechanism of DCS using a variety of mechanistic probes including, among others, deuterated and fluorinated FDPs. Farnesyl diphosphate analogues with fluorine at C2 and C10 acted as inhibitors of DCS, but intriguingly, after prolonged overnight incubations, they yielded 2F-germacrene(s) and a 10F-humulene, respectively. The observed 1,10-, and to a lesser extent, 1,11-cyclization activity of DCS with these fluorinated substrates is consistent with the postulated macrocyclization mechanism(s) en route to (+)-δ-cadinene. On the other hand, mechanistic results from incubations of DCS with 6F-FPP, (2Z,6E)-FDP, neryl diphosphate, 6,7-dihydro-FDP, and NDP seem to be in better agreement with the potential involvement of the alternative biosynthetic 1,6-ring closure pathway. In particular, the strong inhibition of DCS by 6F-FDP, coupled to the exclusive bisabolyl- and terpinyl-derived product profiles observed for the DCS-catalyzed turnover of (2Z,6E)-farnesyl and neryl diphosphates, suggested the intermediacy of α-bisabolyl cation. DCS incubations with enantiomerically pure [1-(2)H(1)](1R)-FDP revealed that the putative bisabolyl-derived 1,6-pathway proceeds through (3R)-nerolidyl diphosphate (NDP), is consistent with previous deuterium-labeling studies, and accounts for the cis stereochemistry characteristic of cadinenyl-derived sesquiterpenes. While the results reported here do not unambiguously rule in favor of 1,6- or 1,10-cyclization, they demonstrate the mechanistic versatility inherent to DCS and highlight the possible existence of multiple mechanistic pathways.

Sphingolipids as coenzymes in anion transfer and tumor death

Many kinds of natural sphingolipids and their analogs stimulate or inhibit a wide assortment of biochemical phenomena and enzymes. The puzzle considered here is: how can these lipids control so many different kinds of processes? In almost every study in which a structural comparison was made, an allylic alcohol moiety [-CH=CH-CH(OH)-] was found to be an essential feature of the sphingolipid. Many of those stimulations lead to cell death, emphasizing the importance of allylic sphingolipid structure in the design of chemotherapeutic agents. The proposal offered here is that these lipids function as coenzymes, in which the allylic moiety acts as an anion transferring agent, forming transient phosphate or acyl or peptidyl esters for the synthesis or hydrolysis of phosphoproteins, proteins, and phospholipids. Sphingolipids that inhibit these reactions may simply displace the active sphingolipids from their sites in the enzymes' active regions, or bind to the enzymes' allosteric region. This kind of competition could act as a major homeostatic control mechanism. Some of the allylic sphingolipids also generate reactive oxygen, possibly by oxidation of the allylic alcohol group. This explains the need to control redox-controlling metabolites in sphingolipid-controlled processes (e.g., glutathione). Many anticancer drugs that produce apoptosis in tumors possess an allylic alcohol residue, affect protein phosphorylation, and produce reactive oxygen species. They may be therapeutically useful because they control the action of sphingolipids as anion transfer agonists or inhibitors.

Bornyl diphosphate synthase: structure and strategy for carbocation manipulation by a terpenoid cyclase

The x-ray crystal structure of dimeric (+)-bornyl diphosphate synthase, a metal-requiring monoterpene cyclase from Salvia officinalis, is reported at 2.0-A resolution. Each monomer contains two alpha-helical domains: the C-terminal domain catalyzes the cyclization of geranyl diphosphate, orienting and stabilizing multiple reactive carbocation intermediates; the N-terminal domain has no clearly defined function, although its N terminus caps the active site in the C-terminal domain during catalysis. Structures of complexes with aza analogues of substrate and carbocation intermediates, as well as complexes with pyrophosphate and bornyl diphosphate, provide "snapshots" of the terpene cyclization cascade.

Molecular cloning and characterization of a new linalool synthase

Mentha citrata Ehrh. (bergamot mint; Lamiaceae) produces an essential oil containing only the acyclic monoterpenol (-)-3R-linalool and its acetate ester. A cloning strategy based upon the assumption that the responsible monoterpene synthase would resemble, in sequence, monoterpene cyclases from this plant family yielded a cDNA encoding the (--)-3R-linalool synthase. The nucleotide sequence of this monoterpene synthase is similar to those of several monoterpene cyclases from the mint (Lamiaceae) family (62-72% identity), but differs substantially from that of 3S-linalool synthase from Clarkia (41% identity; this composite gene appears to be of recent origin) and from that of 3R-linalool synthase from Artemisia (52% identity; the functional role of this gene is uncertain). Heterologous expression in Escherichia coli of a truncated version of the cDNA (in which the plastidial transit peptide was deleted) allowed purification and characterization of the enzyme, which was shown to possess most properties similar to other known monoterpene cyclases, but with a K(m) value for the natural substrate, geranyl diphosphate, of 56 microM with k(cat) of 0.83 s(-1). These kinetic constants for this 3R-linalool synthase are higher than those of any defined monoterpene cyclase, but the kinetic efficiency does not approach that reported for the 3S-linalool synthase from Clarkia. Although linalyl diphosphate is an enzyme-bound intermediate of monoterpene cyclase reactions, this tertiary allylic isomer of the geranyl substrate is not an efficient precursor of linalool with the M. citrata synthase. Modeling of the active site of this linalool synthase from Mentha and comparison to the modeled active sites of phylogenetically related monoterpene cyclases revealed structural differences in the binding of the diphosphate moiety which initiates the ionization step of the electrophilic reaction sequence and in the access of water to the active site to permit stereoselective quenching of the initially formed carbocationic intermediate to produce 3R-linalool.

Alteration of product formation by directed mutagenesis and truncation of the multiple-product sesquiterpene synthases delta-selinene synthase and gamma-humulene synthase

Two recombinant sesquiterpene synthases from grand fir, delta-selinene synthase and gamma-humulene synthase, each produce more than 30 sesquiterpene olefins from the acyclic precursor farnesyl diphosphate. These enzymes contain a pair of DDxxD motifs, on opposite lips of the presumptive active site, which are thought to be involved in substrate binding and could promote multiple orientations of the substrate alkyl chain from which multiple families of cyclic olefins could derive. Mutagenesis of the first aspartate of either DDxxD motif resulted in depressed k(cat), with lesser effect on K(m), for delta-selinene synthase and afforded a much simpler product spectrum composed largely of monocyclic olefins. Identical alterations in gamma-humulene synthase produced similar kinetic effects with a simplified product spectrum of mostly acyclic and monocyclic olefins. Although impaired in product diversity, none of the mutant synthases lost entirely the capacity to generate complex structures. These results confirm the catalytic significance of the DDxxD motifs and imply that they also influence permitted modes of cyclization. Deletion of an N-terminal arginine pair in delta-selinene synthase (an element potentially involved in substrate isomerization) altered kinetics without substantially altering product outcome. Finally, mutation of an active-site tyrosine residue thought to play a role in proton exchange had little influence; however, substitution of a nearby active site aspartate dramatically altered kinetics and product outcome.

Molecular cloning, functional expression and characterization of d-limonene synthase from Agastache rugosa

We cloned the gene of d-limonene synthase (ArLMS) from Agastache rugosa (Labiatae). The function of ArLMS was elucidated by the preparation of recombinant protein and subsequent enzyme assay. ArLMS consisted of 2077 nucleotides including 1839 bp of coding sequence that encodes a protein of 613 amino acids. This protein has a 60 kDa molecular weight, which is identical to that of d-limonene synthase from Schizonepeta tenuifolia (Labiatae). The deduced amino acid sequence of ArLMS shows high homology with the known d- and l-limonene synthases from Labiatae plants. Here, we discussed the amino acid residues responsible for the stereochemical regulation in limonene biosynthesis.

Mechanism of monoterpene cyclization: stereochemical aspects of the transformation of noncyclizable substrate analogs by recombinant (-)-limonene synthase, (+)-bornyl diphosphate synthase, and (-)-pinene synthase

The tightly coupled nature of the reaction sequence catalyzed by monoterpene synthases has prevented direct observation of the topologically required isomerization step leading from geranyl diphosphate to the presumptive, enzyme-bound, tertiary allylic intermediate linalyl diphosphate, which ultimately cyclizes to the various monoterpene skeletons. Previous experimental approaches using the noncyclizable substrate analogs 6,7-dihydrogeranyl diphosphate and racemic methanogeranyl diphosphate, in attempts to dissect the cryptic isomerization step from the normally coupled reaction sequence, were thwarted by the limited product available from native monoterpene synthases and by the inability to resolve chiral monoterpene products at the microscale. These approaches were revisited using three recombinant monoterpene synthases and chiral phase capillary gas chromatographic methods to separate antipodal products of the substrate analogs. The recombinant monoterpene olefin synthases, (-)-limonene synthase from spearmint and (-)-pinene synthase from grand fir, yielded essentially only achiral, olefin products (corresponding to the respective analogs and homologs of myrcene, trans-ocimene and cis-ocimene) from 6,7-dihydrogeranyl diphosphate and (2S,3R)-methanogeranyl diphosphate; no significant amounts of terpenols or homoterpenols were formed, nor was direct evidence obtained for the formation of the anticipated analog and homolog of the tertiary intermediate linalyl diphosphate (i.e., 6,7-dihydrolinalyl diphosphate and homolinalyl diphosphate, respectively). In the case of recombinant (+)-bornyl diphosphate synthase from common sage, the achiral olefins were generated, as before, from 6,7-dihydrogeranyl diphosphate and (2R,3S)-methanogeranyl diphosphate, but 6,7-dihydrolinalool and homolinalool also comprised significant components of the respective product mixtures, indicating greater access of water to the active site of this enzyme compared to the olefin synthases; again, no direct evidence for the production of 6,7-dihydrolinalyl diphosphate or homolinalyl diphosphate was obtained. Resolution of the terpenol products of (+)-bornyl diphosphate synthase, by chiral phase separation, revealed the predominant formation of (3R)-dihydrolinalool from dihydrogeranyl diphosphate and of (4S)-homolinalool from (2R,3S)-methanogeranyl diphosphate. The opposite stereochemistries of these products indicates water trapping from opposite faces of the corresponding tertiary carbocationic intermediates of the respective reactions, a phenomenon that appears to result from the binding conformations of these substrate analogs. Although these experiments failed to provide direct evidence for the tertiary intermediate of the tightly coupled isomerization-cyclization sequence, they did reveal a mechanistic difference between the olefin synthases and bornyl diphosphate synthase involving access of water as a participant in the reaction.

Monoterpene synthases from gymnosperms and angiosperms: stereospecificity and inactivation by cysteinyl- and arginyl-directed modifying reagents

To 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 thiol-directed 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.

Biosynthesis of monoterpenes: inhibition of (+)-pinene and (-)-pinene cyclases by thia and aza analogs of the 4R- and 4S-alpha-terpinyl carbocation

(+)-Pinene cyclase (synthase) from Salvia officinalis leaf catalyzes the cyclization of geranyl pyrophosphate, via (3R)-linalyl pyrophosphate and the (4R)-alpha-terpinyl cation, to (+)-alpha-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)-alpha-terpinyl cation, to (-)-alpha-pinene and (-)-beta-pinene and to lesser amounts of related olefins. Racemic thia analogs of the linalyl and alpha-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 alpha-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 alpha-terpinyl carbocation. Enantioselectivity in the enzymatic conversion of (RS)-alpha-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 discriminate between enantiomers of alpha-terpinyl pyrophosphate in this unusual reaction. Either the alpha-terpinyl antipodes are too similar to allow differentiation by the pinene cyclases, or these enzymes lack an inherent requirement to distinguish the (4R)- and (4S)-forms because they encounter only one enantiomer in the course of the normal reaction from geranyl pyrophosphate.

Evidence for an essential histidine residue in 4S-limonene synthase and other terpene cyclases

(4S)-Limonene synthase, isolated from glandular trichome secretory cell preparations of Mentha x piperita (peppermint) leaves, catalyzes the metal ion-dependent cyclization of geranyl pyrophosphate, via 3S-linalyl pyrophosphate, to (-)-(4S)-limonene as the principal product. Treatment of this terpene cyclase with the histidine-directed reagent diethyl pyrocarbonate at a concentration of 0.25 mM resulted in 50% loss of enzyme activity, and this activity could be completely restored by treatment of the preparation with 5 mM hydroxylamine. Inhibition with diethyl pyrocarbonate was distinguished from inhibition with thiol-directed reagents by protection studies with histidine and cysteine carried out at varying pH. Inactivation of the cyclase by dye-sensitized photooxidation in the presence of rose bengal gave further indication of the presence of a readily modified histidine residue. Protection of the enzyme against inhibition with diethyl pyrocarbonate was afforded by the substrate geranyl pyrophosphate in the presence of Mn2+, and by the sulfonium ion analog of the linalyl carbocation intermediate of the reaction in the presence of inorganic pyrophosphate plus Mn2+, suggesting that an essential histidine residue is located at or near the active site. Similar studies on the inhibition of other monoterpene and sesquiterpene cyclases with diethyl pyrocarbonate suggest that a histidine residue (or residues) may play an important role in catalysis by this class of enzymes.

Characterization and mechanism of (4S)-limonene synthase, a monoterpene cyclase from the glandular trichomes of peppermint (Mentha x piperita)

(4S)-Limonene synthase, a monoterpene cyclase isolated from the secretory cells of the glandular trichomes of Mentha x piperita (peppermint), catalyzes the cyclization of geranyl pyrophosphate to (4S)-limonene, a key intermediate in the biosynthesis of p-menthane monoterpenes in Mentha species. The enzyme synthesizes principally (-)-(4S)-limonene (greater than 94% of the total products), plus several other monoterpene olefins. The general properties of (4S)-limonene synthase resemble those of other monoterpene cyclases. The enzyme shows a pH optimum near 6.7, an isoelectric point of 4.35, and requires a divalent metal ion for catalysis, either Mg2+ or Mn2+, with Mn2+ preferred. The Km value measured for geranyl pyrophosphate was 1.8 microM. The activity of (4S)-limonene synthase was inhibited by sodium phosphate, sodium pyrophosphate, and reagents directed against the amino acids cysteine, methionine, and histidine. In the presence of Mn2+, geranyl pyrophosphate protected against cysteine-directed inhibition, suggesting that at least one cysteine residue is located at or near the active site. Experiments with alternate substrates and substrate analogs confirmed many elements of the proposed reaction mechanism, including the binding of geranyl pyrophosphate in the form of a complex with the divalent metal ion, the preliminary isomerization of geranyl pyrophosphate to linalyl pyrophosphate (a bound intermediate capable of cyclization), and the participation of a series of carbocation:pyrophosphate anion pairs in the reaction sequence.

Uncompetitive inhibition of monoterpene cyclases by an analog of the substrate geranyl pyrophosphate and inhibition of monoterpene biosynthesis in vivo by an analog of geraniol

Monoterpene cyclases catalyze the divalent metal ion-dependent conversion of the acyclic precursor geranyl pyrophosphate to a variety of monocyclic and bicyclic monoterpene skeletons. Examination of the kinetics of inhibition of cyclization by the pyrophosphate ester of (E)-4-[2-diazo-3-trifluoropropionyloxy]-3-methyl-2-buten-1-o l, a photolabile structural analog of the substrate, using a partially purified preparation of geranyl pyrophosphate:(+)-pinene cyclase and geranyl pyrophosphate:(+)-bornyl pyrophosphate cyclase from common sage (Salvia officinalis) evidenced (under dark conditions) strictly uncompetitive inhibition with K'i values of 3.2 and 4.7 microM, respectively. These values are close to the corresponding Km values for the substrate with these two enzymes. This novel property of the substrate analog was also examined in the presence of two other inhibitors which bind to different domains of the cyclase active site (inorganic pyrophosphate and a sulfonium ion analog of a cyclic carbocationic intermediate of the reaction sequence (dimethyl-(4-methylcyclohex-3-en-1-yl)sulfonium iodide)) in order to address the mechanistic origins of the uncompetitive inhibition of cyclization. It was not possible, however, to rule out either an induced-fit mechanism or a sequential binding mechanism since the substrate is recognized by at least two binding domains and because direct examination of the effects of binding on cyclase conformation is currently not feasible. The substrate analog, although photoactive, did not give rise to light-dependent enzyme inactivation of greater magnitude than that obtained from ultraviolet light alone. The unusual behavior of the analog was attributed to intramolecular interaction of the electron-rich carbonyl group of the diazoester with the required divalent metal ion that is chelated by the pyrophosphate group. A photostable analog of geraniol that resembled the photoactive substrate analog in bearing a carbonyl function at C6 (6-oxo-3,7-dimethyloct-2(trans)en-1-ol) was prepared. Following foliar application to rapidly growing sage plants, this analog was seemingly activated to the corresponding pyrophosphate ester in vivo and selectively inhibited the activity of several cyclases in this tissue as evidenced by diminished production of the corresponding monoterpene end products.

Selective biotransformations. Patents and literature

Stereo- and regioselective biotransformations is an area of intense research interest. The ability to take advantage of microorganisms and isolated enzyme systems to perform selective organic syntheses is well known. In recent years, several new methodologies have caused a rapid growth in the area of selective biotransformations. These novel techniques include genetic and protein engineering, biocatalysis in organic media, improved screening procedures, as well as an increased awareness of the value of biotechnology to solve important problems in organic synthesis. Recent US patents and scientific literature on stereo- and regioselective biotransformations are surveyed. Patent abstracts are summarized individually and a list of literature references are given.

Monoterpene biosynthesis: mechanistic evaluation of the geranyl pyrophosphate:(-)-endo-fenchol cyclase from fennel (Foeniculum vulgare)

Geranyl pyrophosphate:(-)-endo-fenchol cyclase catalyzes the conversion of geranyl pyrophosphate to (-)-endo-fenchol by a process thought to involve the initial isomerization of the substrate to the tertiary allylic isomer, linalyl pyrophosphate, and the subsequent cyclization of this bound intermediate. Studies with 18O-labeled acyclic precursors and H2(18)O, followed by mass spectrometric analysis of the cyclic product, confirmed that water was the sole source of the carbinol oxygen atom of endo-fenchol, thus indicating the participation of the solvent in terminating this presumptive carbocationic reaction. The isomerization component of the normally coupled reaction sequence was demonstrated directly using the substrate analog 2,3-cyclopropylgeranyl pyrosphosphate and by isolating the corresponding homoallylic analog of linalyl pyrophosphate as a major reaction product. The cyclization component of the reaction sequence was effectively dissected using linalyl pyrophosphate as substrate, and both isomerization and cyclization steps were shown to take place at the same active site of the cyclase, an observation consistent with the efficient coupling of these processes. 2-Fluorogeranyl pyrophosphate and 2-fluorolinalyl pyrophosphate were shown to be effective inhibitors of the cyclase, and the electron-withdrawing substituent was shown to greatly suppress the rate of cyclization of these labeled analogs, indicating that both steps of the coupled isomerization-cyclization sequence are initiated by ionization of an allylic pyrophosphate. Additional evidence for the electrophilic nature of the reaction was obtained by demonstrating the ability of the cyclase to solvolyze other substrate analogs which bear an allylic pyrophosphate, and by showing that cyclization was strongly inhibited by sulfonium analogs of presumptive carbocationic intermediates of the reaction sequence, especially in the presence of inorganic pyrophosphate as counterion. In spite of the fact that the fenchol cyclase terminates the cyclization with an external nucleophile (H2O), the primary mechanistic features of this isomerization-cyclization reaction are similar to those catalyzed by other cyclases that terminate the reaction by deprotonation or cation capture by the pyrophosphate moiety of the substrate.

Monoterpene biosynthesis: mechanism and stereochemistry of the enzymatic cyclization of geranyl pyrophosphate to (+)-cis- and (+)-trans-sabinene hydrate

The conversion of geranyl pyrophosphate to (+)-cis- and (+)-trans-sabinene hydrate by a partially purified cyclase from sweet marjoram (Majorana hortensis) is considered to proceed by the initial ionization and isomerization of the substrate to (-)-(3R)-linalyl pyrophosphate and the subsequent cyclization of this enzyme-bound tertiary allylic intermediate to the monocyclic (+)-(4R)-alpha-terpinyl cation. A 1,2-hydride shift and a second cyclization with water capture of the resulting cation complete the reaction sequence. [6-3H, 14C]Geranyl pyrophosphate, coupled with selective chemical degradation of the resulting sabinene hydrate products, was employed to demonstrate the hydride shift, while separate testing of the linalyl pyrophosphate enantiomers confirmed the involvement of the (3R)-antipode in the cyclization and indicated the cyclization of linalyl pyrophosphate to be faster than the coupled isomerization-cyclization of the geranyl substrate. (1R)- and (1S)-[1-3H, 14C]geranyl pyrophosphates, in conjunction with stereoselective degradations of the biosynthetic products to locate the 3H, were exploited to deduce that configuration at C1 of the substrate was retained in the reaction. These findings suggest the isomerization of the geranyl substrate to be a suprafacial process and the cyclization of the (3R)-linalyl intermediate to proceed via the anti,endo-conformation consistent with the stereo-chemistry of other monoterpene cyclizations and with chemical model studies. Sulfonium ion analogs of the presumptive linalyl and alpha-terpinyl cationic intermediates of the isomerization-cyclization sequence were shown to be potent inhibitors of the enzymatic reaction (Ki = 0.3 and 2.8 microM, respectively), and inhibition was synergized by the presence of inorganic pyrophosphate, indicating that the enzyme recognized and bound more tightly to these ion-paired species than to either cationic or anionic partner alone. Additionally, the enzyme was capable of ionizing (solvolyzing) the noncyclizable substrate analogs 6,7-dihydrogeranyl pyrophosphate and 2,3-methanogeranyl pyrophosphate. These results define the overall stereochemistry of the coupled isomerization-cyclization to sabinene hydrate, demonstrate the 1,2-hydride shift, and confirm the electrophilic nature of this enzymatic reaction type.

Monoterpene cyclases: physicochemical features required for pyrophosphate binding determined from inhibition by structural analogs

Monoterpene cyclases catalyze the divalent metal ion-dependent conversion of geranyl pyrophosphate, the ubiquitous C10 intermediate of isoprenoid biosynthesis, to a variety of monoterpene skeletons, and the pyrophosphoryl moiety is a primary determinant for substrate binding by these enzymes. To determine what specific features of this functional group are critical for enzymatic recognition, inorganic pyrophosphate and a series of structurally related analogs were examined as inhibitors of geranyl pyrophosphate:(+)-alpha-pinene cyclase and geranyl pyrophosphate:(+)-bornyl pyrophosphate cyclase from sage (Salvia officinalis). Analysis of trends in the magnitude of inhibition by the analogs relative to inorganic pyrophosphate indicated that the combination of ionization state (formal charge) at the enzymatic pH optimum, ability to chelate divalent metal ions, and intramolecular flexibility is required for effective interaction with both cyclases. Only when all of these criteria are met is inhibition of cyclization comparable to that observed with inorganic pyrophosphate.

Isotopically sensitive branching in the formation of cyclic monoterpenes: proof that (-)-alpha-pinene and (-)-beta-pinene are synthesized by the same monoterpene cyclase via deprotonation of a common intermediate

To determine whether the bicyclic monoterpene olefins (-)-alpha-pinene and (-)-beta-pinene arise biosynthetically from the same monoterpene cyclase by alternate deprotonations of a common carbocationic intermediate, the product distributions arising from the acyclic precursor [10-2H3,1-3H]geranyl pyrophosphate were compared with those resulting from incubation of [1-3H]geranyl pyrophosphate with (-)-pinene cyclase from Salvia officinalis. Alteration in proportions of the olefinic products generated by the partially purified pinene cyclase resulted from the suppression of the formation of (-)-beta-pinene (C10 deprotonation) by a primary deuterium isotope effect with a compensating stimulation of the formation of (-)-alpha-pinene (C4 deprotonation). (-)-Pinene cyclase as well as (+)-pinene cyclase also exhibited a decrease in the proportion of the acyclic olefin myrcene generated from the deuteriated substrate, accompanied by a corresponding increase in the commitment to cyclized products. The observation of isotopically sensitive branching, in conjunction with quantitation of the magnitude of the secondary deuterium isotope effect on the overall rate of product formation by the (+)- and (-)-pinene cyclases as well as two other monoterpene cyclases from the same tissue, supports the biosynthetic origin of (-)-alpha-pinene and (-)-beta-pinene by alternative deprotonations of a common enzymatic intermediate. A biogenetic scheme consistent with these results is presented, and alternate proposals for the origin of the pinenes are addressed.