Characterization and heterologous reconstitution of Taxus biosynthetic enzymes leading to baccatin III.

Paclitaxel is a well known anticancer compound. Its biosynthesis involves the formation of a highly functionalized diterpenoid core skeleton (baccatin III) and the subsequent assembly of a phenylisoserinoyl side chain. Despite intensive investigation for half a century, the complete biosynthetic pathway of baccatin III remains unknown. In this work, we identified a bifunctional cytochrome P450 enzyme [taxane oxetanase 1 (TOT1)] in Taxus mairei that catalyzes an oxidative rearrangement in paclitaxel oxetane formation, which represents a previously unknown enzyme mechanism for oxetane ring formation. We created a screening strategy based on the taxusin biosynthesis pathway and uncovered the enzyme responsible for the taxane oxidation of the C9 position (T9αH1). Finally, we artificially reconstituted a biosynthetic pathway for the production of baccatin III in tobacco.

Recent Research Progress in Taxol Biosynthetic Pathway and Acylation Reactions Mediated by Taxus Acyltransferases.

Taxol is one of the most effective anticancer drugs in the world that is widely used in the treatments of breast, lung and ovarian cancer. The elucidation of the taxol biosynthetic pathway is the key to solve the problem of taxol supply. So far, the taxol biosynthetic pathway has been reported to require an estimated 20 steps of enzymatic reactions, and sixteen enzymes involved in the taxol pathway have been well characterized, including a novel taxane-10ß-hydroxylase (T10ßOH) and a newly putative ß-phenylalanyl-CoA ligase (PCL). Moreover, the source and formation of the taxane core and the details of the downstream synthetic pathway have been basically depicted, while the modification of the core taxane skeleton has not been fully reported, mainly concerning the developments from diol intermediates to 2-debenzoyltaxane. The acylation reaction mediated by specialized Taxus BAHD family acyltransferases (ACTs) is recognized as one of the most important steps in the modification of core taxane skeleton that contribute to the increase of taxol yield. Recently, the influence of acylation on the functional and structural diversity of taxanes has also been continuously revealed. This review summarizes the latest research advances of the taxol biosynthetic pathway and systematically discusses the acylation reactions supported by Taxus ACTs. The underlying mechanism could improve the understanding of taxol biosynthesis, and provide a theoretical basis for the mass production of taxol.

Biochemical characterization of acyl activating enzymes for side chain moieties of Taxol and its analogs.

Taxol (paclitaxel) is a very widely used anticancer drug, but its commercial sources mainly consist of stripped bark or suspension cultures of members of the plant genus Taxus. Taxol accumulates as part of a complex mixture of chemical analogs, termed taxoids, which complicates its production in pure form, highlighting the need for metabolic engineering approaches for high-level Taxol production in cell cultures or microbial hosts. Here, we report on the characterization of acyl-activating enzymes (AAEs) that catalyze the formation of CoA esters of different organic acids relevant for the N-substitution of the 3-phenylisoserine side chain of taxoids. On the basis of similarities to AAE genes of known function from other organisms, we identified candidate genes in publicly available transcriptome data sets obtained with Taxus × media. We cloned 17 AAE genes, expressed them heterologously in Escherichia coli, purified the corresponding recombinant enzymes, and performed in vitro assays with 27 organic acids as potential substrates. We identified TmAAE1 and TmAAE5 as the most efficient enzymes for the activation of butyric acid (Taxol D side chain), TmAAE13 as the best candidate for generating a CoA ester of tiglic acid (Taxol B side chain), TmAAE3 and TmAAE13 as suitable for the activation of 4-methylbutyric acid (N-debenzoyl-N-(2-methylbutyryl)taxol side chain), TmAAE15 as a highly efficient candidate for hexanoic acid activation (Taxol C side chain), and TmAAE4 as suitable candidate for esterification of benzoic acid with CoA (Taxol side chain). This study lays important groundwork for metabolic engineering efforts aimed at improving Taxol production in cell cultures.

Identification of rate-limiting enzymes involved in paclitaxel biosynthesis pathway affected by coronatine and methyl-ß-cyclodextrin in Taxus baccata L. cell suspension cultures.

BACKGROUND: Paclitaxel is a potent antitumor alkaloid widely used for the treatment of several cancer types. This valuable secondary metabolite naturally exists in the inner bark of Taxus species in very low amounts. The small-scale production of paclitaxel in Taxus cell cultures requires utilization of several elicitors. OBJECTIVE: The main objective of this work was to identify key genes that encode rate-limiting enzymes in paclitaxel biosynthesis pathway by investigating the possible relationship between paclitaxel production and a set of 13 involved genes' relative expression in Taxus baccata L. cell suspension cultures affected by coronatine and methyl-ß-cyclodextrin. METHODS: In the present research, the most important key genes were identified using gene expression profiling evaluation and paclitaxel production assessment in Taxus baccata L. cell cultures affected by mentioned elicitors. RESULTS AND CONCLUSION: Gene expression levels were variably increased using methyl-ß-cyclodextrin, and in some cases, a synergistic effect on transcript accumulation was observed when culture medium was supplemented with both elicitors. It was revealed that DBAT, BAPT, and DBTNBT are the most important rate-limiting enzymes in paclitaxel biosynthesis pathway in Taxus baccata L. cell suspension cultures under coronatine and methyl-ß-cyclodextrin elicitation condition. Moreover, PAM was identified as one of the important key genes especially in the absence of ß-phenylalanine. In cell cultures affected by these elicitors, paclitaxel was found largely in the culture media (more than 90%). The secretion of this secondary metabolite suggests a limited feedback inhibition and reduced paclitaxel toxicity for producer cells. It is the result of the ABC gene relative expression level increment under methyl-ß-cyclodextrin elicitation and highly depends on methyl-ß-cyclodextrin's special property (complex formation with hydrophobic compounds). Paclitaxel biosynthesis was obviously increased due to the effect of coronatine and methyl-ß-cyclodextrin elicitation, leading to the production level of 5.62 times higher than that of the untreated cultures. Graphical abstract Rate Limiting Enzymes in Paclitaxel Biosynthesis Pathway: DBAT, BAPT, DBTNBT and PAM.

The effects of salicylic acid and glucose on biochemical traits and taxane production in a Taxus baccata callus culture.

The combined use of elicitors can be an effective way to increase the production of secondary metabolites (SMs) in plant cell, tissue and organ cultures. This study investigated the effects of a salicylic acid (SA) pretreatment and different glucose levels on the growth, biochemical traits and taxane production in a Taxus baccata callus culture. For this purpose, after a pretreatment with SA (5 µM), three-month-old calli were cultured on B5 medium fortified with different concentrations of glucose (0, 0.5, 1, 2 and 3%), and they were compared with calli cultured on a B5 medium supplemented only with glucose. When the calli were harvested at 21 days, their fresh weight (g), dry weight (g) and cell viability (%) had decreased significantly (p < 0.05) with the higher glucose concentrations. The glucose treatment increased the hydrogen peroxide (H2O2) and malondialdehyde (MDA) content, and caused oxidative stress in treated tissues. The lower H2O2 content and oxidative stress was associated with an increased antioxidant enzyme activity in the SA-pretreated samples, which resulted in less membrane damage and improved growth and cell viability under the glucose treatment compared to the control. By reducing the activity of phenylalanine ammonia-lyase (PAL) and polyphenol oxidase (PPO), the SA pretreatment reduced the production and oxidation of phenolic compounds under the glucose treatment; this decrease was associated with less browning of tissues and higher viability. Increases in taxol (5.1-fold) and total taxanes (3.5-fold) in the SA-pretreated calli cultured on the medium containing 2% glucose, compared to the control, indicated that the two treatments had a significant effect on taxane production in the T. baccata callus culture.

Enhanced catalytic activities and modified substrate preferences for taxoid 10ß-O-acetyl transferase mutants by engineering catalytic histidine residues.

OBJECTIVES: Taxoid 10ß-O-acetyl transferase (DBAT) was redesigned to enhance its catalytic activity and substrate preference for baccatin III and taxol biosynthesis. RESULTS: Residues H162, D166 and R363 were determined as potential sites within the catalytic pocket of DBAT for molecular docking and site-directed mutagenesis to modify the activity of DBAT. Enzymatic activity assays revealed that the kcat/KM values of mutant H162A/R363H, D166H, R363H, D166H/R363H acting on 10-deacetylbaccatin III were about 3, 15, 26 and 60 times higher than that of the wild type of DBAT, respectively. Substrate preference assays indicated that these mutants (H162A/R363H, D166H, R363H, D166H/R363H) could transfer acetyl group from unnatural acetyl donor (e.g. vinyl acetate, sec-butyl acetate, isobutyl acetate, amyl acetate and isoamyl acetate) to 10-deacetylbaccatin III. CONCLUSION: Taxoid 10ß-O-acetyl transferase mutants with redesigned active sites displayed increased catalytic activities and modified substrate preferences, indicating their possible application in the enzymatic synthesis of baccatin III and taxol.

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.

Activity Essential Residue Analysis of Taxoid 10ß-O-Acetyl Transferase for Enzymatic Synthesis of Baccatin.

Taxoid 10ß-O-acetyl transferase (DBAT) is a key enzyme in the biosynthesis of the famous anticancer drug paclitaxel, which catalyses the formation of baccatin III from 10-deacetylbaccatin III (10-DAB). However, the activity essential residues of the enzyme are still unknown, and the acylation mechanism from its natural substrate 10-deacetylbaccatin III and acetyl CoA to baccatin III remains unclear. In this study, the homology modelling, molecular docking, site-directed mutagenesis, and kinetic parameter determination of the enzyme were carried out. The results showed that the enzyme mutant DBATH162A resulted in complete loss of enzymatic activity, suggesting that the residue histidine at 162 was essential to DBAT activity. Residues D166 and R363 which were located in the pocket of the enzyme by homology modelling and molecular docking were also important for DBAT activity through the site-directed mutations. Furthermore, four amino acid residues including S31 and D34 from motif SXXD, D372 and G376 from motif DFGWG also played important roles on acylation. This was the first report of the elucidation of the activity essential residues of DBAT, making it possible for the further structural-based re-design of the enzyme for efficient biotransformation of baccatin III and paclitaxel.

One step synthesis of unnatural ß-arylalanines using mutant phenylalanine aminomutase from Taxus chinensis with high ß-regioselectivity.

Phenylalanine aminomutase (TcPAM) from Taxus chinensis catalyzes the regioselective hydroamination of trans-cinnamic acid (t-CA) to yield ß-phe. However, the final product mixture consists of both α- and ß-phe owing to low regioselectivity, which is still a challenge to synthesize highly pure ß-phe. Therefore, a modified TcPAM with high ß-selectivity is expected. Based on the catalytic mechanism and structure, two amino acid residues (Asn458 and Leu108) in active sites were identified as the key residues for controlling the regioselective hydroamination of t-CA and as promising candidates for mutagenesis to enhance ß-selectivity and decrease α-selectivity. The Asn458 and Leu108 residues were mutated to yield variant TcPAM-Asn458Phe/Leu108Glu, and the ß-selectivity was approximately 5.2-fold higher than that of wild-type TcPAM, while α-selectivity decreased to 68%, and the percentage of ß-phe in the product mixture increased from 42% to 83%. In addition, the mutant was applied to synthesize ß-arylalanines using substituent t-CA as a substrate. The regioselectivity was also affected by the substituent groups at the phenyl ring of t-CA with respect to their electronic properties and position, and the 4-methoxy and methyl substituent t-CA were transferred into ß-arylalanines. The conversion rate also exceeded 90%. In summary, the engineered TcPAM proved to be useful for one-step asymmetric amination of t-CA and its derivatives to synthesize highly pure ß-arylalanines.

Taxadiene-5α-ol is a minor product of CYP725A4 when expressed in Escherichia coli.

CYP725A4 is a P450 enzyme from Taxus cuspidata that catalyzes the formation of taxadiene-5α-ol (T5α-ol) from taxadiene in paclitaxel biosynthesis. Past attempts expressing CYP725A4 in heterologous hosts reported the formation of 5(12)-oxa-3(11)-cyclotaxane (OCT) and/or 5(11)-oxa-3(11)-cyclotaxane (iso-OCT) instead of, or in addition to, T5α-ol. Here, we report that T5α-ol is produced as a minor product by Escherichia coli expressing both taxadiene synthase and CYP725A4. The major products were OCT and iso-OCT, while trace amounts of unidentified monooxygenated taxanes were also detected by gas chromatography-mass spectrometry. Since OCT and iso-OCT had not been found in nature, we tested the hypothesis that protein-protein interaction of CYP725A4 with redox partners, such as cytochrome P450 reductase (CPR) and cytochrome b5, may affect the products formed by CYP725A4, possibly favoring the formation of T5α-ol over OCT and iso-OCT. Our results show that coexpression of CYP725A4 with CPR from different organisms did not change the relative ratios of OCT, iso-OCT, and T5α-ol, while cytochrome b5 decreased overall levels of the products formed. Although unsuccessful in finding conditions that promote T5α-ol formation over other products, we used our results to clarify conflicting claims in the literature and discuss other possible approaches to produce paclitaxel via metabolic and enzyme engineering.

Exogenous and endogenous increase in fungal GGPP increased fungal Taxol production.

Taxol is an anticancer identified in both endophytic fungus and its host plant. Plant Taxol is a diterpenoid with geranylgeranyl diphosphate (GGPP) mediates the biosynthesis of its terpenoid moiety. Previous report has suggested that fungal Taxol may require terpenoid pathway for its biosynthesis. Here in this study, feeding a Taxol-producing endophytic fungus (Paraconiothyrium SSM001) with terpenoid precursors including isopentenyl pyrophosphate (IPP, isoprene) and GGPP enhanced Taxol production threefold and fivefold, respectively, compared to the control. Thus, we assumed that increasing the terpenoid pool size in particular GGPP by introducing a new copy number of GGPPS particularly from a Taxol-producing plant might increase the production level of fungal Taxol. Agrobacterium-mediated integration of Taxus canadensis geranylgeranyl diphosphate synthase (GGPPS) gene into the Paraconiothyrium SSM001 genome was successful and increased the terpenoid pool size indicated by an increase in carotenoid level and orange to red coloration of some GGPPS-transformed SSM001 colonies. Furthermore, the integration improved the level of Taxol production threefold. Feeding a GGPPS-transformed SSM001 fungus with a GGPP precursor increased the expression level of GGPPS transcript and Taxol production. The successful increase in both terpenoid and Taxol production levels due to GGPPS gene integration into the fungal genome might be a step forward in manipulating Taxol-producing endophytic fungi. Future control of the transformation time and the manipulation of the phenolic pathway could maximize the production level.

CYP79 P450 monooxygenases in gymnosperms: CYP79A118 is associated with the formation of taxiphyllin in Taxus baccata.

KEY MESSAGE: Conifers contain P450 enzymes from the CYP79 family that are involved in cyanogenic glycoside biosynthesis. Cyanogenic glycosides are secondary plant compounds that are widespread in the plant kingdom. Their biosynthesis starts with the conversion of aromatic or aliphatic amino acids into their respective aldoximes, catalysed by N-hydroxylating cytochrome P450 monooxygenases (CYP) of the CYP79 family. While CYP79s are well known in angiosperms, their occurrence in gymnosperms and other plant divisions containing cyanogenic glycoside-producing plants has not been reported so far. We screened the transcriptomes of 72 conifer species to identify putative CYP79 genes in this plant division. From the seven resulting full-length genes, CYP79A118 from European yew (Taxus baccata) was chosen for further characterization. Recombinant CYP79A118 produced in yeast was able to convert L-tyrosine, L-tryptophan, and L-phenylalanine into p-hydroxyphenylacetaldoxime, indole-3-acetaldoxime, and phenylacetaldoxime, respectively. However, the kinetic parameters of the enzyme and transient expression of CYP79A118 in Nicotiana benthamiana indicate that L-tyrosine is the preferred substrate in vivo. Consistent with these findings, taxiphyllin, which is derived from L-tyrosine, was the only cyanogenic glycoside found in the different organs of T. baccata. Taxiphyllin showed highest accumulation in leaves and twigs, moderate accumulation in roots, and only trace accumulation in seeds and the aril. Quantitative real-time PCR revealed that CYP79A118 was expressed in plant organs rich in taxiphyllin. Our data show that CYP79s represent an ancient family of plant P450s that evolved prior to the separation of gymnosperms and angiosperms. CYP79A118 from T. baccata has typical CYP79 properties and its substrate specificity and spatial gene expression pattern suggest that the enzyme contributes to the formation of taxiphyllin in this plant species.

Improving 10-deacetylbaccatin III-10-ß-O-acetyltransferase catalytic fitness for Taxol production.

The natural concentration of the anticancer drug Taxol is about 0.02% in yew trees, whereas that of its analogue 7-ß-xylosyl-10-deacetyltaxol is up to 0.5%. While this compound is not an intermediate in Taxol biosynthetic route, it can be converted into Taxol by de-glycosylation and acetylation. Here, we improve the catalytic efficiency of 10-deacetylbaccatin III-10-O-acetyltransferase (DBAT) of Taxus towards 10-deacetyltaxol, a de-glycosylated derivative of 7-ß-xylosyl-10-deacetyltaxol to generate Taxol using mutagenesis. We generate a three-dimensional structure of DBAT and identify its active site using alanine scanning and design a double DBAT mutant (DBATG38R/F301V) with a catalytic efficiency approximately six times higher than that of the wild-type. We combine this mutant with a ß-xylosidase to obtain an in vitro one-pot conversion of 7-ß-xylosyl-10-deacetyltaxol to Taxol yielding 0.64 mg ml-1 Taxol in 50 ml at 15 h. This approach represents a promising environmentally friendly alternative for Taxol production from an abundant analogue.

Exploring the Influence of Domain Architecture on the Catalytic Function of Diterpene Synthases.

Terpenoid synthases catalyze isoprenoid cyclization reactions underlying the generation of more than 80,000 natural products. Such dramatic chemodiversity belies the fact that these enzymes generally consist of only three domain folds designated as α, ß, and γ. Catalysis by class I terpenoid synthases occurs exclusively in the α domain, which is found with α, αα, αß, and αßγ domain architectures. Here, we explore the influence of domain architecture on catalysis by taxadiene synthase from Taxus brevifolia (TbTS, αßγ), fusicoccadiene synthase from Phomopsis amygdali (PaFS, (αα)6), and ophiobolin F synthase from Aspergillus clavatus (AcOS, αα). We show that the cyclization fidelity and catalytic efficiency of the α domain of TbTS are severely compromised by deletion of the ßγ domains; however, retention of the ß domain preserves significant cyclization fidelity. In PaFS, we previously demonstrated that one α domain similarly influences catalysis by the other α domain [ Chen , M. , Chou , W. K. W. , Toyomasu , T. , Cane , D. E. , and Christianson , D. W. ( 2016 ) ACS Chem. Biol. 11 , 889 - 899 ]. Here, we show that the hexameric quaternary structure of PaFS enables cluster channeling. We also show that the α domains of PaFS and AcOS can be swapped so as to make functional chimeric αα synthases. Notably, both cyclization fidelity and catalytic efficiency are altered in all chimeric synthases. Twelve newly formed and uncharacterized C20 diterpene products and three C25 sesterterpene products are generated by these chimeras. Thus, engineered αßγ and αα terpenoid cyclases promise to generate chemodiversity in the greater family of terpenoid natural products.

Paclitaxel Biosynthesis: Adenylation and Thiolation Domains of an NRPS TycA PheAT Module Produce Various Arylisoserine CoA Thioesters.

Structure-activity relationship studies show that the phenylisoserinyl moiety of paclitaxel (Taxol) is largely necessary for the effective anticancer activity. Several paclitaxel analogues with a variant isoserinyl side chain have improved pharmaceutical properties versus those of the parent drug. To produce the isoserinyl CoAs as intermediates needed for enzyme catalysis on a semibiosynthetic pathway to paclitaxel analogues, we repurposed the adenylation and thiolation domains (Phe-AT) of a nonribosomal peptide synthetase (TycA) so that they would function as a CoA ligase. Twenty-eight isoserine analogue racemates were synthesized by an established procedure based on the Staudinger [2+2] cycloaddition reaction. Phe-AT converted 16 substituted phenylisoserines, one ß-(heteroaryl)isoserine, and one ß-(cyclohexyl)isoserine to their corresponding isoserinyl CoAs. We imagine that these CoA thioesters can likely serve as linchpin biosynthetic acyl donors transferred by a 13-O-acyltransferase to a paclitaxel precursor baccatin III to make drug analogues with better efficacy. It was also interesting to find that an active site mutant [Phe-AT (W227S)] turned over 2-pyridylisoserine and the sterically demanding p-methoxyphenylisoserine substrates to their CoA thioesters, while Phe-AT did not. This mutant is promising for further development to make 3-fluoro-2-pyridylisoserinyl CoA, a biosynthetic precursor of the oral pharmaceutical tesetaxel used for gastric cancers.

Heterologous expression and characterization of plant Taxadiene-5α-Hydroxylase (CYP725A4) in Escherichia coli.

Taxadiene-5α-Hydroxylase (CYP725A4) is a membrane-bound plant cytochrome P450 that catalyzes the oxidation of taxadiene to taxadiene-5α-ol. This oxidation is a key step in the production of the valuable cancer therapeutic and natural plant product, taxol. In this work, we report the bacterial expression and purification of six different constructs of CYP725A4. All six of these constructs are N-terminally modified and three of them are fused to cytochrome P450 reductase to form a chimera construct. The construct with the highest yield of CYP725A4 protein was then selected for substrate binding and kinetic analysis. Taxadiene binding followed type-1 substrate patterns with an observed KD of 2.1 ± 0.4 µM. CYP725A4 was further incorporated into nanoscale lipid bilayers (nanodiscs) and taxadiene metabolism was measured. Taxadiene metabolism followed Michaelis-Menten kinetics with an observed Vmax of 30 ± 8 pmol/min/nmolCYP725A4 and a KM of 123 ± 52 µM. Additionally, molecular operating environment (MOE) modeling was performed in order to gain insight into the interactions of taxadiene with CYP725A4 active site. Taken together, we demonstrate the successful expression and purification of the functional membrane-bound plant CYP, CYP725A4, in E. coli.

Orthogonal Assays Clarify the Oxidative Biochemistry of Taxol P450 CYP725A4.

Natural product metabolic engineering potentially offers sustainable and affordable access to numerous valuable molecules. However, challenges in characterizing and assembling complex biosynthetic pathways have prevented more rapid progress in this field. The anticancer agent Taxol represents an excellent case study. Assembly of a biosynthetic pathway for Taxol has long been stalled at its first functionalization, putatively an oxygenation performed by the cytochrome P450 CYP725A4, due to confounding characterizations. Here, through combined in vivo (Escherichia coli), in vitro (lipid nanodisc), and metabolite stability assays, we verify the presence and likely cause of this enzyme's inherent promiscuity. Thereby, we remove the possibility that promiscuity simply existed as an artifact of previous metabolic engineering approaches. Further, spontaneous rearrangement and the stabilizing effect of a hydrophobic overlay suggest a potential role for nonenzymatic chemistry in Taxol's biosynthesis. Taken together, this work confirms taxadiene-5α-ol as a primary enzymatic product of CYP725A4 and provides direction for future Taxol metabolic and protein engineering efforts.

Mechanisms and effective control of physiological browning phenomena in plant cell cultures.

Browning phenomena are ubiquitous in plant cell cultures that severely hamper scientific research and widespread application of plant cell cultures. Up to now, this problem still has not been well controlled due to the unclear browning mechanisms in plant cell cultures. In this paper, the mechanisms were investigated using two typical materials with severe browning phenomena, Taxus chinensis and Glycyrrhiza inflata cells. Our results illustrated that the browning is attributed to a physiological enzymatic reaction, and phenolic biosynthesis regulated by sugar plays a decisive role in the browning. Furthermore, to confirm the specific compounds which participate in the enzymatic browning reaction, transcriptional profile and metabolites of T. chinensis cells, and UV scanning and high-performance liquid chromatography-mass spectrometry (HPLC-MS) profile of the browning compounds extracted from the brown-turned medium were analyzed, flavonoids derived from phenylpropanoid pathway were found to be the main compounds, and myricetin and quercetin were deduced to be the main substrates of the browning reaction. Inhibition of flavonoid biosynthesis can prevent the browning occurrence, and the browning is effectively controlled via blocking flavonoid biosynthesis by gibberellic acid (GA3 ) as an inhibitor, which further confirms that flavonoids mainly contribute to the browning. On the basis above, a model elucidating enzymatic browning mechanisms in plant cell cultures was put forward, and effective control approaches were presented.

Mechanistic Insights into Taxadiene Epoxidation by Taxadiene-5α-Hydroxylase.

The anticancer molecule taxol (Paclitaxel) stands as one of the most medically and economically important natural products. However, despite decades of extensive study, its biosynthesis remains poorly understood. Unpredictable behavior of the first oxygenation enzyme, taxadiene-5α-hydroxylase, which produces a range of undesired products, currently stands as a key bottleneck to improved taxol production. We herein present chemical and biological evidence of an unreported epoxidase activity of taxadiene-5α-hydroxylase that puts into question the previously proposed radical-rebound mechanism. We demonstrate that the poor selectivity of taxadiene-5α-hydroxylase arises from nonselective degradation of an epoxide intermediate produced via a selective oxidation step, rather than from promiscuous oxidation, as previously proposed. We support these conclusions by demonstrating variable enzyme behavior in differing hosts and conditions, similarity of products and product ratios generated from chemical epoxidation, and taxadiene-5α-hydroxylase, and differing enzymatic activity on alternative taxadiene isomers. Additionally, we use directed mutagenesis to describe the oxidizing species of the P450, demonstrate that further in vivo functionalization of oxidized taxadiene is unable to improve selectivity of the oxidation, and show that multiple products are produced in the Taxus cuspidata and are not simply an artifact of heterologous expression. Our results highlight an important, and previously unknown, obstacle to improved taxol production. We further offer insights to overcome the challenges posed by an epoxide-mediated reaction, which sets the basis for further engineering of taxol biosynthesis.