Molecular cloning and characterization of a cytochrome P450 taxoid 2alpha-hydroxylase involved in Taxol biosynthesis

The Taxol biosynthetic pathway, arising from the primary isoprenoid precursors isopentenyl diphosphate and dimethylallyl diphosphate in yew (Taxus), consists of approximately twenty steps, at least nine of which are thought to be cytochrome P450-mediated oxygenations. Several oxygenases involved in the early hydroxylation steps of the pathway have been identified and the corresponding genes have been cloned; however, defining the enzymes and their genes responsible for oxygenations in the central portion of the pathway is more difficult because neither the exact sequence of reactions nor the relevant intermediates are known. A surrogate substrate, (+)-taxusin (taxa-4(20),11(12)-dien-5alpha,9alpha,10beta,13alpha-tetraol tetraacetate), that was previously employed in the isolation of a taxoid 7beta-hydroxylase, was used here to functionally screen a family of cytochrome P450 oxygenases originating from a Taxus cell EST library. This in vivo screen in yeast led to the identification of a 1488bp cDNA clone (encoding a 495 residue protein) that was capable of producing 2alpha-hydroxytaxusin from taxusin with a K(m) value of 10.5 +/- 2.7 microM and k(cat) of about 0.05 s(-1) for the surrogate substrate. This structurally typical cytochrome P450 resembles most closely the previously isolated taxoid 7beta-hydroxylase, which also uses taxusin as a substrate, and both 2alpha- and 7beta-hydroxylases are capable of the reciprocal conversion of their respective pentaol tetraacetate products to the common hexaol tetraacetate. This C2-hydroxylase would appear to mediate the mid-pathway functionalization of the C2-position of the taxane core that ultimately bears a benzoyl group as an important Taxol pharmacophore. Overexpression of this cytochrome P450 taxoid 2alpha-hydroxylase in Taxus cells may improve Taxol yields and could prove useful in the production of other 2alpha-hydroxy taxoids as starting materials for subsequent acylation at this position.     

Partial purification and characterization of acetyl coenzyme A: taxa-4(20),11(12)-dien-5alpha-ol O-acetyl transferase that catalyzes the first acylation step of taxol biosynthesis

The acetylation of taxa-4(20),11(12)-dien-5alpha-ol is considered to be the third specific step of Taxol biosynthesis that precedes further hydroxylation of the taxane nucleus. An operationally soluble acetyl CoA:taxadienol-O-acetyl transferase was demonstrated in extracts of Taxus canadensis and Taxus cuspidata cells induced with methyl jasmonate to produce Taxol. The reaction was dependent on both cosubstrates and active enzyme, and the product of this acetyl transferase was identified by radiochromatographic and GC-MS analysis. Following determination of the time course of acetyl transferase appearance in induced cell cultures, the operationally soluble enzyme was partially purified by a combination of anion exchange, hydrophobic interaction, and affinity chromatography on immobilized coenzyme A resin. This acetyl transferase has a pI and pH optimum of 4.7 and 9.0, respectively, and a molecular weight of about 50,000 as determined by gel permeation chromatography. The enzyme shows high selectivity and high affinity for both cosubstrates, with Km values of 4.2 and 5.5 microM for taxadienol and acetyl CoA, respectively. The enzyme does not acetylate the more advanced Taxol precursors, 10-deacetylbaccatin III or baccatin III. This acetyl transferase is insensitive to monovalent and divalent metal ions, is only weakly inhibited by p-hydroxymercuribenzoate, N-ethylmaleimide, and coenzyme A, and resembles in general properties the few other O-acetyl transferases of higher plant origin that have been examined.     

Taxol biosynthesis: differential transformations of taxadien-5 alpha-ol and its acetate ester by cytochrome P450 hydroxylases from Taxus suspension cells

The biosynthesis of the diterpenoid antineoplastic drug Taxol in Taxus species involves the cyclization of the ubiquitous isoprenoid intermediate geranylgeranyl diphosphate to taxa-4(5),11(12)-diene followed by cytochrome P450-mediated hydroxylation (with allylic rearrangement) of this olefin precursor to taxa-4(20),11(12)-dien-5 alpha-ol, and further oxygenation and acylation reactions. Based on the abundances of naturally occurring taxoids, the subsequent order of oxygenation of the taxane core is considered to occur at C10, then C2 and C9, followed by C13, and finally C7 and C1. Circumstantial evidence suggests that the acetylation of taxadien-5 alpha-ol may constitute the third specific step of Taxol biosynthesis. To determine whether taxadienol or the corresponding acetate ester serves as the direct precursor of subsequent oxygenation reactions, microsomal preparations isolated from induced Taxus cells and optimized for cytochrome P450 catalysis were incubated with each potential substrate. Both taxadienol and taxadienyl acetate were oxygenated to the level of a diol and to higher polyols at comparable rates by cytochrome P450 enzymes of the microsomal preparation. Preparative-scale incubation allowed the isolation of sufficient quantities of the diol derived from taxadienol to permit the NMR-based structural elucidation of this metabolite as taxa-4(20),11(12)-dien-5 alpha,13 alpha-diol, which may represent an alternate route of taxoid metabolism in induced cells. GC-MS-based structural definition of the diol monoacetate derived in microsomes from taxadienyl acetate confirmed this metabolite as taxa-4(20),11(12)-dien-5 alpha-acetoxy-10 beta-ol, thereby indicating that acetylation at C5 of taxadienol precedes the cytochrome P450-mediated insertion of the C10-beta-hydroxyl group of Taxol.     

A common pharmacophore for Taxol and the epothilones based on the biological activity of a taxane molecule lacking a C-13 side chain

Extensive structure-activity studies done with Taxol have identified the side chain at C-13 as one of the requirements for biological activity. Baccatin III, an analogue of Taxol lacking the C-13 side chain, has none of the biological characteristics of Taxol. Since 2-m-azido Taxol, a Taxol derivative with a m-azido substituent in the C-2 benzoyl ring, has greater activity than Taxol, we questioned whether 2-m-azido baccatin III might be active. 2-m-Azido baccatin III inhibited the proliferation of human cancer cells at nanomolar concentrations, blocked cells at mitosis, and reorganized the interphase microtubules into distinct bundles, a typical morphological change induced by Taxol. In contrast to 2-m-azido baccatin III, 2-p-azido baccatin III was similar to baccatin III, having no Taxol-like activity, further indicating the specificity and significance of the 2-meta position substituent. Molecular modeling studies done with the C-2 benzoyl ring of Taxol indicated that it fits into a pocket formed by His227 and Asp224 on beta-tubulin and that the 2-m-azido, in contrast to the 2-p-azido substituent, is capable of enhancing the interaction between the benzoyl group and the side chain of Asp224. The observation that the C-13 side chain is not an absolute requirement for biological activity in a taxane molecule has enabled the development of a new common pharmacophore model between Taxol and the epothilones.     

Taxoid metabolism: Taxoid 14beta-hydroxylase is a cytochrome P450-dependent monooxygenase

The production of the anticancer drug Taxol in Taxus (yew) cell cultures is often accompanied by the formation of side-route polyoxygenated taxoid metabolites bearing a 14beta-hydroxyl group. The recent acquisition of several new semisynthetic taxoid intermediates enabled the screening of a family of Taxus cytochrome P450 cDNA clones for the 14beta-hydroxylase and additional taxoid oxygenases. The candidate cytochrome P450 clones were functionally expressed in yeast and tested by in vivo feeding of radiolabeled 5alpha-acetoxy-10beta-hydroxy taxadiene and 5alpha,13alpha-dihydroxy taxadiene. One clone efficiently and specifically transformed the 5alpha-acetoxy-10beta-ol, but not the 5alpha,13alpha-diol, to a more polar product with the chromatographic properties of a taxoid triol monoacetate, and the identity of this product was confirmed by spectroscopic means as 5alpha-acetoxy-10beta,14beta-dihydroxy taxadiene. Microsome preparation from the transformed yeast allowed characterization of this new hydroxylase, which was shown to resemble other cytochrome P450 taxoid hydroxylases with pH optimum at 7.5 and a K(m) value for the taxoid substrate of about 50 microM. Because Taxol is unsubstituted at C14, the 14beta-hydroxylase cannot reside on the pathway to the target drug but rather appears to be responsible for diversion of the pathway to 14-hydroxy taxoids that are prominent metabolites of Taxus cell cultures. Manipulation of this hydroxylase gene could permit redirection of the pathway to increase flux toward Taxol and could allow the preparation of 13alpha,14beta-hydroxy taxoids as new therapeutic agents.     

Coexpression in yeast of Taxus cytochrome P450 reductase with cytochrome P450 oxygenases involved in Taxol biosynthesis

To maximize redox coupling efficiency with recombinant cytochrome P450 hydroxylases from yew (Taxus) species installed in yeast for the production of the anticancer drug Taxol, a cDNA encoding NADPH:cytochrome P450 reductase from T. cuspidata was isolated. This single-copy gene (2,154 bp encoding a protein of 717 amino acids) resembles more closely other reductases from gymnosperms (approximately 90% similarity) than those from angiosperms (<80% similarity). The recombinant reductase was characterized and compared to other reductases by heterologous expression in insect cells and was shown to support reconstituted taxoid 10beta-hydroxylase activity with an efficiency comparable to that of other plant-derived reductases. Coexpression in yeast of the reductase along with T. cuspidata taxoid 10beta-hydroxylase, which catalyzes an early step of taxoid biosynthesis, demonstrated significant enhancement of hydroxylase activity compared to that supported by the endogenous yeast reductase alone. Functional transgenic coupling of the Taxus reductase with a homologous cytochrome P450 taxoid hydroxylase represents an important initial step in reconstructing Taxol biosynthesis in a microbial host.     

Specificity of the N-benzoyl transferase responsible for the last step of Taxol biosynthesis

The last few steps in the biosynthesis of the anticancer drug Taxol in yew (Taxus) species are thought to involve the attachment of β-phenylalanine to the C13-O-position of the advanced taxane diterpenoid intermediate baccatin III to yield N-debenzoyl-2′-deoxytaxol, followed by hydroxylation on the side chain at the C2′-position to afford N-debenzoyltaxol, and finally N-benzoylation to complete the pathway. A cDNA encoding the N-benzoyl transferase that catalyzes the terminal step of the reaction sequence was previously isolated from a family of transferase clones (derived from an induced Taxus cell cDNA library) by functional characterization of the corresponding recombinant enzyme using the available surrogate substrate N-debenzoyl-2′-deoxytaxol [K. Walker, R. Long, R. Croteau, Proc. Nat. Acad. Sci. USA 99 (2002) 9166–9171]. Semi-synthetic N-debenzoyltaxol was prepared by coupling of 7-triethylsilybaccatin III and (2R,3S)-β-phenylisoserine protected as the N-Boc N,O-isopropylidene derivative by means of carbodiimide activation and formic acid deprotections. The selectivity of the recombinant N-transferase for N-debenzoyltaxol was evaluated, and the enzyme was shown to prefer, by a catalytic efficiency factor of two, N-debenzoyltaxol over N-debenzoyl-2′-deoxytaxol as the taxoid co-substrate in the benzoyl transfer reaction, consistent with the assembly sequence involving 2′-hydroxylation prior to N-benzoylation. Selectivity for the acyl/aroyl-CoA co-substrate was also examined, and the enzyme was shown to prefer benzoyl-CoA. Transfer from tigloyl-CoA to N-debenzoyltaxol to afford cephalomannine (Taxol B) was not observed, nor was transfer observed from hexanoyl-CoA or butanoyl-CoA to yield Taxol C or Taxol D, respectively. These results support the proposed sequence of reactions for C13-O-side chain assembly in Taxol biosynthesis, and suggest that other N-transferases are responsible for the formation of related, late pathway, N-acylated taxoids.     

Baccatin III induces assembly of purified tubulin into long microtubules

Baccatin III is widely considered to be an inactive derivative of Taxol. We have reexamined its effect on in vitro assembly of tubulin under a variety of conditions. We found baccatin III to be active in all circumstances in which Taxol is active: it assembled GTP-tubulin, GDP-tubulin, and microtubule protein into normal microtubules and stabilized these polymers against cold-induced disassembly. The effect of baccatin III on in vitro microtubule assembly was quantitatively assessed through determination of critical concentrations, which can be used to obtain the apparent equilibrium constants for the addition of tubulin subunits to growing microtubules. The apparent equilibrium constants for the growth reaction for baccatin III-induced GTP-tubulin and GDP-tubulin assembly measured at 37 degrees C were 4.2-4.6-fold less than those measured for Taxol-induced GTP-tubulin and GDP-tubulin assembly. These data indicate that the entire Taxol side chain contributes only about -1 kcal/mol to the apparent standard free energy of microtubule growth at 37 degrees C regardless of the nature of the E site nucleotide. These data also support the idea that the majority of the interactions between Taxol and tubulin that affect this equilibrium occur between the baccatin portion of the molecule and the binding site. We have also observed a structural difference in microtubules formed using baccatin III and Taxol. Baccatin III-induced microtubules were routinely much longer than those assembled by Taxol, even when very high concentrations of baccatin III were employed. One interpretation of these data is that baccatin III and Taxol differ in their abilities to nucleate GTP-tubulin. This difference in activity may have bearing on the large disparity in cytotoxicity of the two molecules.     

Genetic engineering of taxol biosynthetic genes in Saccharomyces cerevisiae

Baccatin III, an intermediate of Taxol biosynthesis and a useful precursor for semisynthesis of the anti-cancer drug, is produced in yew (Taxus) species by a sequence of 15 enzymatic steps from primary metabolism. Ten genes encoding enzymes of this extended pathway have been described, thereby permitting a preliminary attempt to reconstruct early steps of taxane diterpenoid (taxoid) metabolism in Saccharomyces cerevisiae as a microbial production host. Eight of these taxoid biosynthetic genes were functionally expressed in yeast from episomal vectors containing one or more gene cassettes incorporating various epitope tags to permit protein surveillance and differentiation of those pathway enzymes of similar size. All eight recombinant proteins were readily detected by immunoblotting using specific monoclonal antibodies and each expressed protein was determined to be functional by in vitro enzyme assay, although activity levels differed considerably between enzyme types. Using three plasmids carrying different promoters and selection markers, genes encoding five sequential pathway steps leading from primary isoprenoid metabolism to the intermediate taxadien-5alpha- acetoxy-10beta-ol were installed in a single yeast host. Metabolite analysis showed that yeast isoprenoid precursors could be utilized in the reconstituted pathway because products accumulated from the first two engineered pathway steps (leading to the committed intermediate taxadiene); however, a pathway restriction was encountered at the first cytochrome P450 hydroxylation step. The means of overcoming this limitation are described in the context of further development of this novel approach for production of Taxol precursors and related taxoids in yeast.     

Paclitaxel binding to human serum albumin--automated docking studies

A computational approach was used to study the interaction of the potent anticancer drug paclitaxel (Taxol) with human serum albumin. The primary and secondary binding sites were located at the interface of subdomains IIA and IIIA, and in the cleft between domains I and III of the protein, respectively. The C13 side chain and the baccatin core of paclitaxel were found to contribute approximately equally to the binding energy at the primary site, whereas the binding mode appears to be governed by the C13 side chain.     

Regioselectivity of taxoid-O-acetyltransferases: heterologous expression and characterization of a new taxadien-5alpha-ol-O-acetyltransferase

In addition to the anticancer drug Taxol, yew (Taxus) species produce a large variety of other taxane diterpenoids which differ mainly in the type of acyl and aroyl groups appended to the many hydroxyl functions on the taxane core; acetate esters are particularly common. Taxol bears an acetate at C10 and another at C4 thought to originate by intramolecular migration of a C5 acetate function in the process of oxetane ring formation, but many other naturally occurring taxoids bear acetate groups at C1, C2, C7, C9, and C13, in addition to C5 and C10. cDNAs encoding a taxoid 5alpha-O-acetyltransferase (taxadien-5alpha-ol as substrate) and a taxoid 10beta-O-acetyltransferase (10-deacetylbaccatin III as substrate) have been acquired from a recently isolated family of Taxus acyl/aroyltransferase clones. To explore the origins of other acetylated taxoids, the group of recombinant Taxus acyltransferases was investigated with a range of polyhydroxylated taxoids as substrates. From this survey, a new acetyltransferase clone (denoted TAX19) was identified that was capable of acetylating taxadien-5alpha-ol with activity comparable to that of the previously identified 5alpha-O-acetyltransferase (clone TAX1). However, when these two recombinant enzymes were presented with taxadien-triol and tetraol substrates, they exhibited different regiospecificities. The TAX1 enzyme preferentially acetylates the "northern" hemisphere hydroxyls at C9 and C10, whereas the TAX19 enzyme preferentially acetylates the "east-west" pole positions at C5 and C13. The TAX1 enzyme possesses the lowest KM value with taxadien-5alpha-ol (an early pathway metabolite) as substrate, with much higher KM values for the polyhydroxylated taxoid substrates, whereas the TAX19 enzyme possesses lower KM values (than the TAX1 transferase) for all taxoid substrates tested. These results suggest that both TAX1 and TAX19 acyltransferases may function at the early C5 acetylation step of taxoid metabolism, and that the TAX19 acyltransferase, because of its broader specificity for polyhydroxylated taxoids, may also function later in metabolism and be responsible for the production of many other acetylated taxoids.     

The interaction of baccatin III with the taxol binding site of microtubules determined by a homogeneous assay with fluorescent taxoid

The ubiquitous Taxol binding site of microtubules also binds newly discovered ligands. We have designed a homogeneous assay for the high throughput detection of Taxol biomimetics, based on the displacement of 7-O-[N-(2,7-difluoro-4'-fluoresceincarbonyl)-L-alanyl]Taxol from its binding site in diluted solutions of preserved microtubules. The state of this reference ligand is measured by fluorescence anisotropy in a microplate reader, with varying concentrations of nonfluorescent competitors. The binding equilibrium constant of Taxol has a value K(b) = 3.7 x 10(7) M(-1). We have found that baccatin III, an analogue of Taxol without the C-13 side chain, binds with K(b) = 1.5 x 10(5) M(-1), whereas the side chain methyl ester is inactive. This was unexpected from the structure-activity relationship of taxoids but compatible with models of Taxol docked at the microtubule site. Baccatin III binding has been confirmed by displacement of [(3)H]Taxol and by direct HPLC measurements of its cosedimentation with microtubules, among other methods. Consequently, baccatin III induces microtubule bundles and multipolar spindles in PtK2 and U937 cells, and mitotic arrest and apoptotic death of the U937 cells, at concentrations 200-500-fold larger than Taxol. The simplest analysis of these results strongly suggests that the interaction of the C-2 C-4 substituted taxane ring system with the microtubule binding site provides most (ca. 75%) of the free energy change of Taxol binding and is sufficient to activate microtubule stabilization and transmit the antitumor effects of Taxol, whereas the C-13 side chain provides a weak specific anchor.     

The kinetics of taxoid accumulation in cell suspension cultures of Taxus following elicitation with methyl jasmonate

Cell suspension cultures of Taxus canadensis and Taxus cuspidata rapidly produced paclitaxel (Taxol) and other taxoids in response to elicitation with methyl jasmonate. By optimizing the concentration of the elicitor, and the timing of elicitation, we have achieved the most rapid accumulation of paclitaxel in a plant cell culture, yet reported. The greatest accumulation of paclitaxel occurred when methyl jasmonate was added to cultures at a final concentration of 200 microM on day 7 of the culture cycle. The concentration of paclitaxel increased in the extracellular (cell-free) medium to 117 mg/day within 5 days following elicitation, equivalent to a rate of 23.4 mg/L per day. Paclitaxel was only one of many taxoids whose concentrations increased significantly in response to elicitation. Despite the rapid accumulation and high concentration of paclitaxel, its concentration never exceeded 20% of the total taxoids produced in the elicited culture. Two other taxoids, 13-acetyl-9-dihydrobaccatin III and baccatin VI, accounted for 39% to 62% of the total taxoids in elicited cultures. The accumulation of baccatin III did not parallel the pattern of accumulation for paclitaxel. Baccatin III continued to accumulate until the end of the culture cycle, at which point most of the cells in the culture were dead, implying a possible role as a degradation product of taxoid biosynthesis, rather than as a precursor.     

Supercritical fluid extraction of taxol and baccatin III from needles of Taxus cuspidata

Summary Taxol and baccatin III were extracted from the ground needles of Taxus cuspidata using supercritical carbon dioxide mixed with 3 wt % ethanol as a cosolvent. The pressure and temperature ranges used to attain supercritical fluid condition are 100300 bar and 4070 °C, respectively. However, the amount of taxol and baccatin III in the extract obtained at 100 bar was not noticeable, while the major portion of extract was found to be the waxy compounds. The highest selectivity of taxol and baccatin III were about 0.094 and 0.158 wt %, respectively, at 40 °C and 300 bar. At the same pressure and temperature condition, taxol and baccatin III selectivities in the extract obtained from the ground seeds of Taxus cuspidata was about 0.198 and 0.157 wt %, respectively.     

Kaurenolides and fujenoic acids are side products of the gibberellin P450-1 monooxygenase in Gibberella fujikuroi

The steps involved in kaurenolide and fujenoic acids biosynthesis, from ent-kauradienoic acid and ent-6alpha,7alpha-dihydroxykaurenoic acid, respectively, are demonstrated in the gibberellin (GA)-deficient Gibberella fujikuroi mutant SG139, which lacks the entire GA-biosynthesis gene cluster, complemented with the P450-1 gene of GA biosynthesis (SG139-P450-1). ent-[2H]Kauradienoic acid was efficiently converted into 7beta-hydroxy[2H]kaurenolide and 7beta,18-dihydroxy[2H]kaurenolide by the cultures while 7beta-hydroxy[2H]kaurenolide was transformed into 7beta,18-dihydroxy[2H]kaurenolide. The limiting step was found to be hydroxylation at C-18. In addition, SG139-P450-1 transformed ent-6alpha,7alpha-dihydroxy[14C4]kaurenoic acid into [14C4]fujenoic acid and [14C4]fujenoic triacid. Fujenal was also converted into the same products but was demonstrated not to be an intermediate in this sequence. All the above reactions were absent in the mutant SG139 and were suppressed in the wild-type strain ACC917 by disruption of the P450-1 gene. Kaurenolide and fujenoic acids synthesis were associated with the microsomal fraction and showed an absolute requirement for NADPH or NADH, all properties of cytochrome P450 monooxygenases. Only 7beta-hydroxy[14C4]kaurenolide synthesis and not further 18-hydroxylation was detected in the microsomal fraction. The substrates for the P450-1 monooxygenase, ent-kaurenoic acid and [2H]GA12, efficiently inhibited kaurenolide synthesis with I50 values of 3 and 6 microM, respectively. Both substrates also inhibited ent-6alpha,7alpha-dihydroxy[14C4]kaurenoic acid metabolism by SG139-P450-1. Conversely, [14C4]GA14 synthesis from [14C4]GA12-aldehyde was inhibited by ent-[2H]kauradienoic acid and fujenal with I50 values of 10 and 30 microM, respectively. These results demonstrate that kaurenolides and seco-ring B kaurenoids are formed by the P450-1 monooxygenase (GA14 synthase) of G. fujikuroi and are thus side products that probably result from stabilization of radical intermediates involved in GA14 synthesis.     

Bio-production of Baccatin III,an Important Precursor of Paclitaxel by a Cost-Effective Approach

Natural production of anti-cancer drug taxol from Taxus has proved to be environmentally unsustainable and economically unfeasible. Currently, bioengineering the biosynthetic pathway of taxol is an attractive alternative production approach. 10-deacetylbaccatin III-10-O-acetyl transferase (DBAT) was previously characterized as an acyltransferase, using 10-deacetylbaccatin III (10-DAB) and acetyl CoA as natural substrates, to form baccatin III in the taxol biosynthesis. Here, we report that other than the natural acetyl CoA (Ac-CoA) substrate, DBAT can also utilize vinyl acetate (VA), which is commercially available at very low cost, acylate quickly and irreversibly, as acetyl donor in the acyl transfer reaction to produce baccatin III. Furthermore, mutants were prepared via a semi-rational design in this work. A double mutant, I43S/D390R was constructed to combine the positive effects of the different single mutations on catalytic activity, and its catalytic efficiency towards 10-DAB and VA was successfully improved by 3.30-fold, compared to that of wild-type DBAT, while 2.99-fold higher than the catalytic efficiency of WT DBAT towards 10-DAB and Ac-CoA. These findings can provide a promising economically and environmentally friendly method for exploring novel acyl donors to engineer natural product pathways.