Bifunctional abietadiene synthase: free diffusive transfer of the (+)-copalyl diphosphate intermediate between two distinct active sites

Abietadiene synthase (AS) catalyzes two sequential, mechanistically distinct cyclizations in the conversion of geranylgeranyl diphosphate to a mixture of abietadiene double bond isomers as the initial step of resin acid biosynthesis in grand fir (Abies grandis). The first reaction converts geranylgeranyl diphosphate to the stable bicyclic intermediate (+)-copalyl diphosphate via protonation-initiated cyclization. In the second reaction, diphosphate ester ionization-initiated cyclization generates the tricyclic perhydrophenanthrene-type backbone, and is directly coupled to a 1,2-methyl migration that generates the C13 isopropyl group characteristic of the abietane family of diterpenes. Using the transition-state analogue inhibitor 14,15-dihydro-15-azageranylgeranyl diphosphate, it was demonstrated that each reaction of abietadiene synthase is carried out at a distinct active site. Mutations in two aspartate-rich motifs specifically delete one or the other activity and the location of these motifs suggests that the two active sites reside in separate domains. These mutants effectively complement each other, suggesting that the copalyl diphosphate intermediate diffuses between the two active sites in this monomeric enzyme. Free copalyl diphosphate was detected in steady-state kinetic reactions, thus conclusively demonstrating a free diffusion transfer mechanism. In addition, both mutant enzymes enhance the activity of wild-type abietadiene synthase with geranylgeranyl diphosphate as substrate. The implications of these results for the kinetic mechanism of abietadiene synthase are discussed.

Cloning, heterologous expression, and functional characterization of 5-epi-aristolochene-1,3-dihydroxylase from tobacco (Nicotiana tabacum)

Capsidiol is a bicyclic, dihydroxylated sesquiterpene produced by several solanaceous species in response to a variety of environmental stimuli. It is the primary antimicrobial compound produced by Nicotiana tabacum in response to fungal elicitation, and it is formed via the isoprenoid pathway from 5-epi-aristolochene. Much of the biosynthetic pathway for the formation of this compound has been elucidated, except for the enzyme(s) responsible for the conversion of 5-epi-aristolochene to its dihydroxylated form, capsidiol. Biochemical evidence from previous studies with N. tabacum (Whitehead, I. M., Threlfall, D. R., and Ewing, D. F., 1989, Phytochemistry 28, 775-779) and Capsicum annuum Hoshino, T., Yamaura, T., Imaishi, H., Chida, M., Yoshizawa, Y., Higashi, K., Ohkawa, H., Mizutani, J., 1995, Phytochemistry 38, 609-613. suggested that the oxidation of 5-epi-aristolochene to capsidiol was mediated by at least one elicitor-inducible cytochrome P450 hydroxylase. In extending these observations, we developed an in vivo assay for 5-epi-aristolochene hydroxylase activity and used it to demonstrate a dose-dependent inhibition of activity by ancymidol and ketoconazole, two well characterized inhibitors of cytochrome P450 enzymes. Using degenerate oligonucleotide primers designed to the well conserved domains found within most P450 enzymes, including the heme binding domain, cDNA fragments representing four distinct P450 families (CYP71, CYP73, CYP82, and CYP92) were amplified from a cDNA library prepared against mRNA from elicitor-treated cells using PCR. The PCR fragments were subsequently used to isolate full-length cDNAs (CYP71D20 and D21, CYP73A27 and A28, CYP82E1 and CYP92A5), and these in turn were used to demonstrate that the corresponding mRNAs were all induced in elicitor-treated cells, albeit with different induction patterns. Representative, full-length cDNAs for each of the P450s were engineered into a yeast expression system, and the recombinant yeast assessed for functional expression of P450 protein by measuring the CO difference spectra of the yeast microsomes. Only microsomal preparations from yeast expressing the CYP71D20 and CYP92A5 cDNAs exhibited significant CO difference absorbance spectra at 450 nm and were thus tested for their ability to hydroxylate 5-epi-aristolochene and 1-deoxycapsidiol, a putative mono-hydroxylated intermediate in capsidiol biosynthesis. Interestingly, the CYP71D20-encoded enzyme activity was capable of converting both 5-epi-aristolochene and 1-deoxycapsidiol to capsidiol in vitro, consistent with the notion that this P450 enzyme catalyzes both hydroxylations of its hydrocarbon substrate.

Lysosomal prenylcysteine lyase is a FAD-dependent thioether oxidase

Prenylated proteins contain either a 15-carbon farnesyl or a 20-carbon geranylgeranyl isoprenoid covalently attached via a thioether bond to a cysteine residue at or near their C terminus. As prenylated proteins comprise up to 2% of the total protein in eukaryotic cells, and the thioether bond is a stable modification, their degradation raises a metabolic challenge to cells. A lysosomal enzyme termed prenylcysteine lyase has been identified that cleaves prenylcysteines to cysteine and an unidentified isoprenoid product. Here we show that the isoprenoid product of prenylcysteine lyase is the C-1 aldehyde of the isoprenoid moiety (farnesal in the case of C-15). The enzyme requires molecular oxygen as a cosubstrate and utilizes a noncovalently bound flavin cofactor in an NAD(P)H-independent manner. Additionally, a stoichiometric amount of hydrogen peroxide is produced during the reaction. These surprising findings indicate that prenylcysteine lyase utilizes a novel oxidative mechanism to cleave thioether bonds and provide insight into the unique role this enzyme plays in the cellular metabolism of prenylcysteines.

Abietadiene synthase from grand fir (Abies grandis): characterization and mechanism of action of the "pseudomature" recombinant enzyme

The oleoresin secreted by grand fir (Abies grandis) is composed of resin acids derived largely from the abietane family of diterpene olefins as precursors which undergo subsequent oxidation of the C18-methyl group to a carboxyl function, for example, in the conversion of abieta-7,13-diene to abietic acid. A cDNA encoding abietadiene synthase has been isolated from grand fir and the heterologously expressed bifunctional enzyme shown to catalyze both the protonation-initiated cyclization of geranylgeranyl diphosphate to the intermediate (+)-copalyl diphosphate and the ionization-dependent cyclization of (+)-copalyl diphosphate, via a pimarenyl intermediate, to the olefin end products. Abietadiene synthase is translated as a preprotein bearing an N-terminal plastidial targeting sequence, and this form of the recombinant protein expressed in Escherichia coli proved to be unsuitable for detailed structure-function studies. Since the transit peptide-mature protein cleavage site could not be determined directly, a truncation series was constructed to delete the targeting sequence and prepare a "pseudomature" form of the enzyme that resembled the native abietadiene synthase in kinetic properties. Both the native synthase and the pseudomature synthase having 84 residues deleted from the preprotein converted geranylgeranyl diphosphate and the intermediate (+)-copalyl diphosphate to a nearly equal mixture of abietadiene, levopimaradiene, and neoabietadiene, as well as to three minor products, indicating that this single enzyme accounts for production of all of the resin acid precursors of grand fir. Kinetic evaluation of abietadiene synthase with geranylgeranyl diphosphate and (+)-copalyl diphosphate provided evidence for two functionally distinct active sites, the first for the cyclization of geranylgeranyl diphosphate to (+)-copalyl diphosphate and the second for the cyclization of (+)-copalyl diphosphate to diterpene end products, and demonstrated that the rate-limiting step of the coupled reaction sequence resides in the second cyclization process. The structural implications of these findings are discussed in the context of primary sequence elements considered to be responsible for binding the substrate and intermediate and for initiating the respective cyclization steps.

Intramolecular proton transfer in the cyclization of geranylgeranyl diphosphate to the taxadiene precursor of taxol catalyzed by recombinant taxadiene synthase

BACKGROUND

The committed step in the biosynthesis of the anticancer drug taxol in yew (Taxus) species is the cyclization of geranylgeranyl diphosphate to taxa-4(5),11(12)-diene. The enzyme taxadiene synthase catalyzes this complex olefin cation cyclization cascade involving the formation of three rings and three stereogenic centers.

RESULTS

Recombinant taxadiene synthase was incubated with specifically deuterated substrates, and the mechanism of cyclization was probed using MS and NMR analyses of the products to define the crucial hydrogen migration and terminating deprotonation steps. The electrophilic cyclization involves the ionization of the diphosphate with closure of the A-ring, followed by a unique intramolecular transfer of the C11 proton to the re-face of C7 to promote closure of the B/C-ring juncture, and cascade termination by proton elimination from the beta-face of C5.

CONCLUSIONS

These findings provide insight into the molecular architecture of the first dedicated step of taxol biosynthesis that creates the taxane carbon skeleton, and they have broad implications for the general mechanistic capability of the large family of terpenoid cyclization enzymes.

Total synthesis of (+/-)-cameroonanol

[reaction: see text]The structure and relative stereochemistry of the novel silphiperfolane-type sesquiterpene cameroonan-7alpha-ol (1) were confirmed by a total synthesis of (+/-)-1 from 3,3,5-trimethylbicyclo[3.3.0]oct-1(8)-en-2-one (6) by means of a Sakurai reaction with (Z)-crotylsilane, free radical hydrobromination, base-induced cyclization, and LiAlH4 reduction.

Heterologous expression and characterization of a "Pseudomature" form of taxadiene synthase involved in paclitaxel (Taxol) biosynthesis and evaluation of a potential intermediate and inhibitors of the multistep diterpene cyclization reaction

The diterpene cyclase taxadiene synthase from yew (Taxus) species transforms geranylgeranyl diphosphate to taxa-4(5),11(12)-diene as the first committed step in the biosynthesis of the anti-cancer drug Taxol. Taxadiene synthase is translated as a preprotein bearing an N-terminal targeting sequence for localization to and processing in the plastids. Overexpression of the full-length preprotein in Escherichia coli and purification are compromised by host codon usage, inclusion body formation, and association with host chaperones, and the preprotein is catalytically impaired. Since the transit peptide-mature enzyme cleavage site could not be determined directly, a series of N-terminally truncated enzymes was created by expression of the corresponding cDNAs from a suitable vector, and each was purified and kinetically evaluated. Deletion of up to 79 residues yielded functional protein; however, deletion of 93 or more amino acids resulted in complete elimination of activity, implying a structural or catalytic role for the amino terminus. The pseudomature form of taxadiene synthase having 60 amino acids deleted from the preprotein was found to be superior with respect to level of expression, ease of purification, solubility, stability, and catalytic activity with kinetics comparable to the native enzyme. In addition to the major product, taxa-4(5),11(12)-diene (94%), this enzyme produces a small amount of the isomeric taxa-4(20), 11(12)-diene ( approximately 5%), and a product tentatively identified as verticillene ( approximately 1%). Isotopically sensitive branching experiments utilizing (4R)-[4-(2)H(1)]geranylgeranyl diphosphate confirmed that the two taxadiene isomers, and a third (taxa-3(4),11(12)-diene), are derived from the same intermediate taxenyl C4-carbocation. These results, along with the failure of the enzyme to utilize 2, 7-cyclogeranylgeranyl diphosphate as an alternate substrate, indicate that the reaction proceeds by initial ionization of the diphosphate ester and macrocyclization to the verticillyl intermediate, followed by a secondary cyclization to the taxenyl cation and deprotonation (i.e., formation of the A-ring prior to B/C-ring closure). Two potential mechanism-based inhibitors were tested with recombinant taxadiene synthase but neither provided time-dependent inactivation nor afforded more than modest competitive inhibition.

Stereochemistry of the cyclization-rearrangement of (+)-copalyl diphosphate to (-)-abietadiene catalyzed by recombinant abietadiene synthase from Abies grandis

[reaction: see text] Syntheses and enzymatic cyclizations of 8alpha-hydroxy-17-nor copalyl diphosphate (8a), (15R)-[15-2H1] 8b, and (15R,17E)-[15-3H1,17-2H1] copalyl diphosphate ([2H,3H] 2) catalyzed by recombinant abietadiene synthase (rAS) gave 17-nor manoyl oxide (9a), (16E)-[16-2H1] 9b, and (15S,16R)-[16-2H1,16-3H1] abietadiene ([2H1,3H1] 4), respectively. These and other results indicate that conversion of CPP (2) to abietadiene (4) occurs by anti S(N)' cyclization to a sandaracopimar-15-en-8-yl carbocation intermediate (13+, 13beta-methyl) followed by hydrogen transfer and methyl migration suprafacially on the si face of the vinyl group.

Biosynthesis of cyclic diterpene hydrocarbons in rice cell suspensions: conversion of 9,10-syn-labda-8(17),13-dienyl diphosphate to 9beta-pimara-7,15-diene and stemar-13-ene

The biosynthesis of diterpene hydrocarbons with enzyme extracts from rice cell suspension cultures was investigated to verify proposed pathways and intermediates in the production of the momilactone and oryzalexin phytoalexins. Diterpene synthase activity in cells treated with chitin to elicit the phytoalexin response was compared with the activity in untreated cells using the acyclic substrates [1-3H](E,E,E)- and [1-3H] (E,Z,E)-geranylgeranyl diphosphates (GGPPs 4-OPP and 11-OPP) as well as the bicyclic substrates [15-3H]ent-copalyl and [15-3H] syn-copalyl diphosphates (CPPs, 5-OPP, and 6-OPP). ent-kaurene (7), ent-sanda, racopimaradiene (8), 9 beta H-pimara-7,15-diene (9), and stemar-13-ene (10) were identified as major products by comparisons with authentic standards. Marked increases in diterpene synthase activities were observed with enzyme from chitin-treated cells: (E,E,E)-GGPP (approximately 100 fold), ent-CPP (approximately 3 fold), and syn-CPP (approximately 60 fold). The very low conversions of (E,Z,E)-GGPP to hydrocarbon products excludes its role in the biosynthesis of 9,10-syn-diterpenes in rice cells. ent-Kaurene was the major diterpene formed from ent-CPP with enzyme from unelicited cells. In contrast the enzyme from chitin-treated cells converted ent-CPP to a mixture of ent-kaurene, ent-sandaracopimaradiene, and a third unidentified diterpene. With syn-CPP as substrate the induced syntheses afforded a mixture of 9 beta-pimaradiene, stemarene, and a third, unidentified syn-diterpene. Overall the results are consistent with the hypothesis that rice cells respond to treatment with chitin fragments by producing new diterpene synthases not present in the untreated cells. These induced cyclases initiate phytoalexin biosynthesis by diverting (E,E,E)-GGPP into new cyclization modes that produce ent-sandaracopimaradiene, stemarene, and 9 beta-pimaradiene, the presumed precursors to oryzalexins A-F, oryzalexin S, and momilactones A-C, respectively. The intermediate role of 9,10-syn-CPP in syn diterpene biosynthesis is verified.

Stereochemistry of the proton elimination in the formation of (+)- and (-)-alpha-pinene by monoterpene cyclases from sage (Salvia officinalis)

The three pinene synthases (cyclases) from common sage (Salvia officinalis) catalyze the conversion of geranyl pyrophosphate to the bicyclic olefins (+)-alpha-pinene and (+)-camphene (cyclase I), (-)-alpha-pinene, (-)-beta-pinene, and (-)-camphene (cyclase II), and (+)-alpha-pinene and (+)-beta-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 alpha-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 (+)-alpha-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 (-)-alpha-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.

Monoterpene biosynthesis: isotope effects associated with bicyclic olefin formation catalyzed by pinene synthases from sage (Salvia officinalis)

The three pinene synthases (cyclases) from common sage (Salvia officinalis) catalyze the conversion of geranyl pyrophosphate to the bicyclic olefins (+)-alpha-pinene and (+)-camphene (cyclase I), (-)-alpha-pinene, (-)-beta-pinene, and (-)-camphene (cyclase II), and (+)-alpha-pinene and (+)-beta-pinene (cyclase III), in addition to smaller amounts of monocyclic and acyclic monoterpene olefins. [1-3H,4-2H2]- and [10-2H2]-geranyl pyrophosphates were prepared and used in conjunction with 1-3H- and 1-3H,10-2H3-labeled geranyl precursors to examine the isotope effects attending the C4- and C10-deprotonation steps in the enzymatic synthesis of the pinenes. The observation of isotopically sensitive branching within each set of stereochemically related bicyclic olefins confirmed that each product set was synthesized by the respective pinene synthase by partitioning of common carbocationic intermediates along different reaction channels at the active site. The changes in product distribution resulting from deuterium substitution at C4 and C10 of the substrate were used to determine kinetic isotope effects (KIEs) for the terminating deprotonations; these observed KIEs represent the lower limits of the intrinsic isotope effects. The intramolecular isotope effects for the methyl-methylene elimination in beta-pinene formation by cyclases II and III were also evaluated with [10-2H2]geranyl pyrophosphate as substrate and by MS analysis of the olefin products. The intramolecular KIEs (kH/kD = 3.0 and 3.5) were significantly higher than the observed KIEs determined from product ratios (kH/kD = 1.7 and 2.6) since the former involves considerably less masking of the intrinsic isotope effects.

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.

Studies on geranylgeranyl diphosphate synthase from rat liver: specific inhibition by 3-azageranylgeranyl diphosphate

Geranylgeranyl diphosphate synthase from rat liver was separated from farnesyl diphosphate synthase, the most abundant and widely occurring prenyltransferase, by DEAE-Toyopearl column chromatography. The enzyme catalyzed the formation of E,E,E-geranylgeranyl diphosphate (V) from isopentenyl diphosphate (II) and dimethylallyl diphosphate (I), geranyl diphosphate (III), or farnesyl diphosphate (IV) with relative velocities of 0.09:0.15:1. 3-Azageranylgeranyl diphosphate (VII), designed as a transition-state analog for the geranylgeranyl diphosphate synthase reaction, was synthesized and found to act as a specific inhibitor for this synthase, but not for farnesyl diphosphate synthase. Diphosphate V and its Z,E,E-isomer (VI) also inhibited geranylgeranyl diphosphate synthase, but the effect was not as striking as that of the aza analog VII. Specific inhibition of geranylgeranyl diphosphate synthase by VII was also observed in experiments with 100,000g supernatants of rat brain and liver homogenates which contained isopentenyl diphosphate isomerase and prenyltransferases including farnesyl diphosphate synthase as well as geranylgeranyl diphosphate synthase. For farnesyl:protein transferase from rat brain, however, the aza compound did not show a stronger inhibitory effect than E,E,E-geranylgeranyl diphosphate.

Synthesis of brominated heptanones and bromoform by a bromoperoxidase of marine origin

The presence of naturally occurring volatile halohydrocarbons in marine organisms, seawater, and the upper atmosphere has prompted a serach for their biosynthetic origin. An earlier report documented the preparation of an enzyme extract from a marine algae which catalyzed the formation of dibromomethane, tribromomethane, and 1-bromopentane from 3-oxooctanoic acid. This report did not establish a pathway nor did it examine potential intermediates involved in the synthesis of the halometabolites (Theiler, R., Cook, J., Hager, L., & Siuda, J. (1978) Science (Washington, D.C.) 202, 1094-1096). This paper shows that an extract of the green marine algae, Penicillus capitatus, which contains a potent bromoperoxidase activity, is capable of catalyzing the incorporation of bromide ion into organic combination in the presence of 3-oxooctanoic acid. By use of gas chromatography and mass spectroscopy, it has been possible to identify tribromomethane, 1-bromo-2-heptanone, 1,1-dibromo-2-heptanone, and 1,1,1-tribromo-2-heptanone as products of this reaction. The properties of the enzymatically synthesized products have been compared to authentic compounds and found to be identical. The mono- and dibromoheptanones can be utilized as precursors for the enzymatic formation of tribromoheptanone, but the final hydrolysis of the tribromoheptanone to bromoform appears to be a nonenzymatic reaction with the P. capitatus extracts.