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

Telescoping a Prenyltransferase and a Diterpene Synthase to Transform Unnatural FPP Derivatives to Diterpenoids

New diterpenoids are accessible from non-natural FPP derivatives as substrates for an enzymatic elongation cyclization cascade using the geranylgeranyl pyrophosphate synthase (GGPPS) from Streptomyces cyaneofuscatus and the spata-13,17-diene synthase (SpS) from Streptomyces xinghaiensis. This approach led to four new biotransformation products including three new cyclododecane cores and a macrocyclic ether. For the first time, a 1,12-terpene cyclization was observed when shifting the central olefinic double bond toward the geminial methyl groups creating a nonconjugated 1,4-diene.

Elimination of enzymes catalysis compartmentalization enhancing taxadiene production in Saccharomyces cerevisiae

Taxadiene is an important precursor in taxol biosynthesis pathway, but its biosynthesis in eukaryotic cell factories is limited, which seriously hinders the biosynthesis of taxol. In this study, it is found that there was the catalysis compartmentalization between two key exogenous enzymes of geranylgeranyl pyrophosphate synthase and taxadiene synthase (TS) for taxadiene synthesis progress, due to their different subcellular localization. Firstly, the enzyme-catalysis compartmentalization was overcome by means of the intracellular relocation strategies of taxadiene synthase, including N-terminal truncation of taxadiene synthase and enzyme fusion of GGPPS-TS. With the help of two strategies for enzyme relocation, the taxadiene yield was increased by 21% and 54% respectively, among them the GGPPS-TS fusion enzyme is more effective. Further, the expression of GGPPS-TS fusion enzyme was improved via the multi-copy plasmid, resulting that the taxadiene titer was increased by 38% to 21.8 mg/L at shake-flask level. Finally, the maximum taxadiene titer of 184.2 mg/L was achieved by optimization of the fed-batch fermentation conditions in 3 L bioreactor, which is the highest reported titer of taxadiene biosynthesis accomplished in eukaryotic microbes. This study provides a successful example for improving biosynthesis of complex natural products by solving the critical problem of multistep enzymes catalysis compartmentalization.

Purification and Biochemical Characterization of Taxadiene Synthase from Bacillus koreensis and Stenotrophomonas maltophilia

Taxadiene synthase (TDS) is the rate-limiting enzyme of Taxol biosynthesis that cyclizes the geranylgeranyl pyrophosphate into taxadiene. Attenuating Taxol productivity by fungi is the main challenge impeding its industrial application; it is possible that silencing the expression of TDS is the most noticeable genomic feature associated with Taxol-biosynthetic abolishing in fungi. As such, the characterization of TDS with unique biochemical properties and autonomous expression that is independent of transcriptional factors from the host is the main challenge. Thus, the objective of this study was to kinetically characterize TDS from endophytic bacteria isolated from different plants harboring Taxol-producing endophytic fungi. Among the recovered 23 isolates, Bacillus koreensis and Stenotrophomonas maltophilia achieved the highest TDS activity. Upon using the Plackett–Burman design, the TDS productivity achieved by B. koreensis (18.1 µmol/mg/min) and S. maltophilia (14.6 µmol/mg/min) increased by ~2.2-fold over the control. The enzyme was purified by gel-filtration and ion-exchange chromatography with ~15 overall folds and with molecular subunit structure 65 and 80 kDa from B. koreensis and S. maltophilia, respectively. The chemical identity of taxadiene was authenticated from the GC-MS analyses, which provided the same mass fragmentation pattern of authentic taxadiene. The tds gene was screened by PCR with nested primers of the conservative active site domains, and the amplicons were sequenced, displaying a higher similarity with tds from T. baccata and T. brevifolia. The highest TDS activity by both bacterial isolates was recorded at 37–40 °C. The Apo-TDSs retained ~50% of its initial holoenzyme activities, ensuring their metalloproteinic identity. The activity of purified TDS was completely restored upon the addition of Mg2+, confirming the identity of Mg2+ as a cofactor. The TDS activity was dramatically reduced upon the addition of DTNB and MBTH, ensuring the implementation of cysteine-reactive thiols and ammonia groups on their active site domains. This is the first report exploring the autonomous robust expression TDS from B. koreensis and S. maltophilia with a higher affinity to cyclize GGPP into taxadiene, which could be a novel platform for taxadiene production as intermediary metabolites of Taxol biosynthesis.

Recent Advances in Metabolic Engineering, Protein Engineering, and Transcriptome-Guided Insights Toward Synthetic Production of Taxol

The diterpenoid paclitaxel (Taxol®) is a blockbuster anticancer agent that was originally isolated from the Pacific yew (Taxus brevifolia) five decades ago. Despite the wealth of information gained over the years on Taxol research, there still remains supply issues to meet increasing clinical demand. Although alternative Taxol production methods have been developed, they still face several drawbacks that cause supply shortages and high production costs. It is highly desired to develop biotechnological production platforms for Taxol, however, there are still gaps in our understanding of the biosynthetic pathway, catalytic enzymes, regulatory and control mechanisms that hamper production of this critical drug by synthetic biology approaches. Over the past 5 years, significant advances were made in metabolic engineering and optimization of the Taxol pathway in different hosts, leading to accumulation of taxane intermediates. Computational and experimental approaches were leveraged to gain mechanistic insights into the catalytic cycle of pathway enzymes and guide rational protein engineering efforts to improve catalytic fitness and substrate/product specificity, especially of the cytochrome P450s (CYP450s). Notable breakthroughs were also realized in engineering the pathway in plant hosts that are more promising in addressing the challenging CYP450 chemistry. Here, we review these recent advances and in addition, we summarize recent transcriptomic data sets of Taxus species and elicited culture cells, and give a bird's-eye view of the information that can be gleaned from these publicly available resources. Recent mining of transcriptome data sets led to discovery of two putative pathway enzymes, provided many lead candidates for the missing steps and provided new insights on the regulatory mechanisms governing Taxol biosynthesis. All these inferences are relevant to future biotechnological production of Taxol.

Nature-driven approaches to non-natural terpene analogues

The reactions catalysed by terpene synthases belong to the most complex and fascinating cascade-type transformations in Nature.

The verticillenes. Pivotal intermediates in the biosynthesis of the taxanes and the phomactins

Recent enzymatic studies, quantum chemical calculations and biomimetic conversions consolidate the role of verticillenes in the biosynthesis of taxanes and phomactins.

Comparative Transcriptome Analysis Revealed the Tissue-Specific Accumulations of Taxanes among Three Experimental Lines of Taxus yunnanensis

Taxus yunnanensis (Yew) is known for natural anticancer metabolite paclitaxel (Taxol) and its biosynthesis pathway in yew species still needs to be completely elucidated. In the current study, productions of paclitaxel and 10-DAB III from three different tissues (needle, branch, and root) of T. yunnanensis wild type (WT) and two new cultivars Zhongda-1 (Zd1) and Zhongda-2 (Zd2) were determined, and significant tissue differences in contents of the taxanes were observed among the three experimental lines. The much higher 10-DAB III and lower paclitaxel contents in needle of Zd2 when compared with that of Zd1 indicated the low conversion from 10-DAB III to paclitaxel in the needle of Zd2. In order to uncover the mechanisms of the tissue-specific biosynthesis of the taxanes, transcriptome analysis of cultivar Zd2 was conducted, and the previously reported transcriptome data of Zd1 and WT were used to perform a comparison. The enhancement of TDAT and T10βH side biosynthetic pathway in roots of Zd2 in early taxane synthesis might lead to the biosynthesis of other toxoids, while the preference of T13αH route in the needle and branch of Zd2 was mainly responsible for the tissue-specific reinforced biosynthesis of 10-DAB III and paclitaxel in Zd2. Different from Zd1, the tissue-specific pattern of paclitaxel biosynthesis genes in Zd2 was similar to WT. However, the lower transcript abundance of final steps genes (TBT, DBAT, BAPT, and DBTNBT) of the paclitaxel biosynthesis pathway in Zd2 than in Zd1 might further promote 10-DAB III accumulation in Zd2.

Environmental relevant levels of the cytotoxic drug cyclophosphamide produce harmful effects in the polychaete Nereis diversicolor

Cytotoxic drugs applied in chemotherapy enter the aquatic environment after patient's metabolism and excretion, in both main compounds and their respective metabolites. The increased consumption and discharge of these drugs raise concern on the genotoxic burden to non-target aquatic species, due to their unselective action on DNA. Settlement and adsorption of cytotoxic drugs to aquatic sediments pose risks to benthic species through chronic exposure. The aim of the present study was to assess the effects induced by the anticancer drug cyclophosphamide (CP) on the polychaete Nereis diversicolor, after 14 days of exposure to environmental relevant concentrations (10, 100, 500 and 1000 ng L^-1). Burrowing impairment, neurotoxicity (Acetylcholinesterase - AChE activity), oxidative stress (superoxide dismutase - SOD; catalase - CAT; glutathione peroxidases - GPXs activities), biotransformation (glutathione-S-transferases - GST), oxidative damage (lipid peroxidation - LPO) and genotoxicity (DNA damage) were assessed. Burrowing impairments were higher at the lowest CP concentrations tested. The higher CP levels tested (500 and 1000 ng L^-1) induced a significant inhibition on the enzymatic antioxidant system (SOD, GPx) and on GST activity. DNA damage was also significant at these concentrations as an outcome of CP metabolism, and high levels of oxidative damage occurred. The results showed that the prodrug CP was metabolically activated in the benthic biological model N. diversicolor. In addition to the potential cytotoxic impact likely to be caused in aquatic species with similar metabolism, N. diversicolor proved to be reliable and vulnerable to the cytotoxic mode of action of CP, even at the lower doses.

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

Taxoid 10β-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 DBAT^H162A 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.

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.

A Maze of Dyotropic Rearrangements and Triple Shifts: Carbocation Rearrangements Connecting Stemarene, Stemodene, Betaerdene, Aphidicolene, and Scopadulanol

Results of quantum chemical investigations shed new light on the mechanisms of formation of the stemarene, stemodene, betaerdene, aphidicolene, and scopadulanol diterpenes from syn-copalyl diphosphate ( syn-CPP). These terpenes are shown to be connected by a complex network of reaction pathways involving concerted but asynchronous dyotropic rearrangements and triple shift rearrangements. The interconnection of these pathways leads to multiple routes for formation of each diterpene, which could lead to different origins for some carbon atoms in a given diterpenes under different conditions.

Molecular dynamics study of taxadiene synthase catalysis

Molecular dynamics (MD) simulations have been performed to study the dynamic behavior of noncovalent enzyme carbocation complexes involved in the cyclization of geranylgeranyl diphosphate to taxadiene catalyzed by taxadiene synthase (TXS). Taxadiene and the observed four side products originate from the deprotonation of carbocation intermediates. The MD simulations of the TXS carbocation complexes provide insights into potential deprotonation mechanisms of such carbocations. The MD results do not support a previous hypothesis that carbocation tumbling is a key factor in the deprotonation of the carbocations by pyrophosphate. Instead water bridges are identified which may allow the formation of side products via multiple proton transfer reactions. A novel reaction path for taxadiene formation is proposed on the basis of the simulations. © 2018 Wiley Periodicals, Inc.

Structural and Chemical Biology of Terpenoid Cyclases

The year 2017 marks the twentieth anniversary of terpenoid cyclase structural biology: a trio of terpenoid cyclase structures reported together in 1997 were the first to set the foundation for understanding the enzymes largely responsible for the exquisite chemodiversity of more than 80000 terpenoid natural products. Terpenoid cyclases catalyze the most complex chemical reactions in biology, in that more than half of the substrate carbon atoms undergo changes in bonding and hybridization during a single enzyme-catalyzed cyclization reaction. The past two decades have witnessed structural, functional, and computational studies illuminating the modes of substrate activation that initiate the cyclization cascade, the management and manipulation of high-energy carbocation intermediates that propagate the cyclization cascade, and the chemical strategies that terminate the cyclization cascade. The role of the terpenoid cyclase as a template for catalysis is paramount to its function, and protein engineering can be used to reprogram the cyclization cascade to generate alternative and commercially important products. Here, I review key advances in terpenoid cyclase structural and chemical biology, focusing mainly on terpenoid cyclases and related prenyltransferases for which X-ray crystal structures have informed and advanced our understanding of enzyme structure and function.

Intramolecular Benzoin Reaction Catalyzed by Benzaldehyde Lyase from Pseudomonas Fluorescens Biovar I

Intramolecular benzoin reactions catalyzed by benzaldehyde lyase from Pseudomonas fluorescens biovar I (BAL) are reported. The structure of the substrates envisaged for this reaction consists of two benzaldehyde derivatives linked by an alkyl chain. The structural requirements needed to achieve the intramolecular carbon-carbon bond reaction catalyzed by BAL were established. Thus, a linker consisting of a linear alkyl chain of three carbon atoms connected through ether-type bonds to the 2 and 2' positions of two benzaldehyde moieties, which could be substituted with either Cl, Br, or OCH₃ at either the 3 and 3' or 5 and 5' positions, were suitable substrates for BAL. Reactions with 61-84 % yields of the intramolecular product and ee values between 64 and 98 %, were achieved.

Multi-domain terpenoid cyclase architecture and prospects for proximity in bifunctional catalysis

Crystal structures of terpenoid cyclases reveal assemblies of three basic domains designated α, β, and γ. While the biosynthesis of cyclic monoterpenes (C₁₀) and sesquiterpenes (C₁₅) most often involves enzymes with α or αβ domain architecture, the biosynthesis of cyclic diterpenes (C₂₀), sesterterpenes (C₂₅), and triterpenes (C₃₀) can involve enzymes with α, αα, βγ, or αβγ domain architecture. Indeed, some enzymes of terpenoid biosynthesis are bifunctional, with distinct active sites that catalyze sequential reactions. Interestingly, some of these enzymes oligomerize to form dimers, tetramers, and hexamers. Not only can such assemblies enable enzyme regulation by allostery, but they can also provide a modest enhancement of terpenoid product flux through proximity channeling or cluster channeling. The mixing and matching of functional terpenoid cyclase domains through tertiary and/or quaternary structure may also comprise an evolutionary strategy for facile product diversification.

Identification of amino acid networks governing catalysis in the closed complex of class I terpene synthases

Class I terpene synthases generate the structural core of bioactive terpenoids. Deciphering structure-function relationships in the reactive closed complex and targeted engineering is hampered by highly dynamic carbocation rearrangements during catalysis. Available crystal structures, however, represent the open, catalytically inactive form or harbor nonproductive substrate analogs. Here, we present a catalytically relevant, closed conformation of taxadiene synthase (TXS), the model class I terpene synthase, which simulates the initial catalytic time point. In silico modeling of subsequent catalytic steps allowed unprecedented insights into the dynamic reaction cascades and promiscuity mechanisms of class I terpene synthases. This generally applicable methodology enables the active-site localization of carbocations and demonstrates the presence of an active-site base motif and its dominating role during catalysis. It additionally allowed in silico-designed targeted protein engineering that unlocked the path to alternate monocyclic and bicyclic synthons representing the basis of a myriad of bioactive terpenoids.

Natural and engineered production of taxadiene with taxadiene synthase

Taxadiene synthase (TXS) is the rate-limiting enzyme in the biosynthesis of paclitaxel, an important anticancer compound. TXS catalyzes the conversion of the diterpene precursor geranylgeranyl pyrophosphate (GGPP) into the diterpene taxadiene. Due to the importance of taxadiene in the overall biosynthetic pathway of paclitaxel biosynthesis, the enzyme TXS has been the subject of intense scientific and engineering investigations. The crystal structure of TXS was recently elucidated, thereby providing an atomic blueprint for future protein engineering efforts. Metabolic engineering of TXS for taxadiene product in different microbial and plant organisms have also been extensively performed, culminating in the high-titer production in Escherichia coli. Additional aspects of taxadiene production by TXS will be discussed in the review, including metabolic regulation in native host and possible production by endophytic fungal hosts.

Caryolene-forming carbocation rearrangements

Density functional theory calculations on mechanisms of the formation of caryolene, a putative biosynthetic precursor to caryol-1(11)-en-10-ol, reveal two mechanisms for caryolene formation: one involves a base-catalyzed deprotonation/reprotonation sequence and tertiary carbocation minimum, whereas the other (with a higher energy barrier) involves intramolecular proton transfer and the generation of a secondary carbocation minimum and a hydrogen-bridged minimum. Both mechanisms are predicted to involve concerted suprafacial/suprafacial [2 + 2] cycloadditions, whose asynchronicity allows them to avoid the constraints of orbital symmetry.

The early stages of taxol biosynthesis: an interim report on the synthesis and identification of early pathway metabolites

The biosynthesis of the anti-cancer drug taxol (paclitaxel) has required the collaborative efforts of several research groups to tackle the synthesis and labeling of putative biosynthetic intermediates, in concert with the identification, cloning and functional expression of the biosynthetic genes responsible for the construction of this complex natural product. Based on a combination of precursor labeling and incorporation experiments, and metabolite isolation from Taxus spp., a picture of the complex matrix of pathway oxygenation reactions following formation of the first committed intermediate, taxa-4(5),11(12)-diene, is beginning to emerge. An overview of the current state of knowledge on the early-stages of taxol biosynthesis is presented.

The taxadiene-forming carbocation cascade

A complete pathway (structures and energies of intermediates and transition state structures connecting them) from geranylgeranyl diphosphate to taxadiene, obtained using quantum chemical calculations, is described. This pathway is fully consistent with previous labeling experiments, despite differing in several subtle ways (in terms of conformations of certain carbocation intermediates and in the concertedness and synchronicity of certain bond-forming events) from previous mechanistic proposals. Also, on the basis of the theoretical results, it is proposed that the 2-fluoro-geranylgeranyl diphosphate substrate analogue in the recently reported X-ray crystal structure of taxadiene synthase is bound in a nonproductive orientation.

Natural products synthesis: enabling tools to penetrate Nature's secrets of biogenesis and biomechanism

Selected examples from our laboratory of how synthetic technology platforms developed for the total synthesis of several disparate families of natural products was harnessed to penetrate biomechanistic and/or biosynthetic queries is discussed. Unexpected discoveries of biomechanistic reactivity and/or penetrating the biogenesis of naturally occurring substances were made possible through access to substances available only through chemical synthesis. Hypothesis-driven total synthesis programs are emerging as very useful conceptual templates for penetrating and exploiting the inherent reactivity of biologically active natural substances. In many instances, new enabling synthetic technologies were required to be developed. The examples demonstrate the often untapped richness of complex molecule synthesis to provide powerful tools to understand, manipulate and exploit Nature's vast and creative palette of secondary metabolites.

Biosynthesis via carbocations: theoretical studies on terpene formation

This review describes applications of quantum chemical calculations in the field of terpene biosynthesis, with a focus on insights into the mechanisms of terpene-forming carbocation rearrangements arising from theoretical studies.

Taxadiene synthase structure and evolution of modular architecture in terpene biosynthesis

With more than 55,000 members identified to date in all forms of life, the family of terpene or terpenoid natural products represents the epitome of molecular biodiversity. A particularly eminent member of this family is the polycyclic diterpenoid Taxol (paclitaxel), which promotes tubulin polymerization1 and exhibits remarkable efficacy in cancer chemotherapy2. The first committed step of Taxol biosynthesis in the Pacific yew (Taxus brevifolia)3 is the cyclization of the linear isoprenoid substrate geranylgeranyl diphosphate (GGPP) to form taxa-4(5),11(12)diene4, which is catalyzed by taxadiene synthase5. The full-length form of this diterpene cyclase contains 862-residues, but an ~80-residue N-terminal transit sequence is cleaved upon maturation in plastids6. We now report the X-ray crystal structure of a truncation variant lacking the transit sequence and an additional 27 residues at the N-terminus, henceforth designated TXS. Specifically, we have determined structures of TXS complexed with 13-aza-13,14-dihydrocopalyl diphosphate (ACP, 1.82 Å resolution) and 2-fluorogeranylgeranyl diphosphate (FGP, 2.25 Å resolution). The TXS structure is the first of a diterpene cyclase and reveals a modular assembly of three α-helical domains. The C-terminal catalytic domain is a class I terpenoid cyclase, which binds and activates substrate GGPP with a three-metal ion cluster. Surprisingly, the N-terminal domain and a third "insertion" domain together adopt the fold of a vestigial class II terpenoid cyclase. A class II cyclase activates the isoprenoid substrate by protonation instead of ionization, and the TXS structure reveals a definitive connection between the two distinct cyclase classes in the evolution of terpenoid biosynthesis.

Trading molecules and tracking targets in symbiotic interactions

It is probable that nearly every natural product structure results from interactions between organisms. Symbiosis, a subset of inter-organism interactions involving closely associated partners, has recently provided new and interesting experimental systems for the study of these interactions. This review discusses new observations about natural product function and structural evolution that emerge from the study of symbiotic systems. In particular, these advances directly address long-standing 'how' and 'why' questions about natural products, providing fundamental insights about the evolution, origin and purpose of natural products that are inaccessible by other methods.

Studies on Taxol® Biosynthesis. Preparation and Tritium Labeling of Biosynthetic intermediates by Deoxygenation of a Taxadiene Tetra-acetate Obtained from Japanese Yew

Taxa-4(20),11(12)-diene-5α-acetate 5 and Taxa-4(20), 11(12)-diene-5α-acetate, 10β-ol 6, have been identified as early stage intermediates involved in the biosynthesis of Taxol® (paclitaxel). Tritium-labeled 5 and 6 were successfully prepared by Barton deoxygenation using tri-n-butyltintritiide of the C-14-hydroxyl group of a taxoid obtained from Japanese Yew.

Unearthing the roots of the terpenome

Although terpenoid synthases catalyze the most complex reactions in biology, these enzymes appear to play little role in the chemistry of catalysis other than to trigger the ionization and chaperone the conformation of flexible isoprenoid substrates and carbocation intermediates through multistep reaction cascades. Fidelity and promiscuity in this chemistry (whether a terpenoid synthase generates one or several products), depends on the permissiveness of the active site template in chaperoning each step of an isoprenoid coupling or cyclization reaction. Structure-guided mutagenesis studies of terpenoid synthases such as farnesyl diphosphate synthase, 5-epi-aristolochene synthase, and gamma-humulene synthase suggest that the vast diversity of terpenoid natural products is rooted in the facile evolution of alpha-helical folds shared by terpenoid synthases in all forms of life.

Synthetic efficiency in enzyme mechanisms involving carbocations: aristolochene synthase

An intramolecular proton-transfer mechanism has been proposed for the carbocationic cyclization of farnesyl pyrophosphate (FPP) to (+)-aristolochene catalyzed by aristolochene synthase. This novel mechanism, which is based on results obtained by high-level ab initio molecular orbital and density functional theory calculations, differs from the previous proposal in the key step of carbocation propagation prior to the formation of the bicyclic carbon skeleton. Previously, germacrene A was proposed to be generated as an intermediate by deprotonation of germacryl cation followed by reprotonation of the C6-C7 double bond to yield eudesmane cation. In the mechanism proposed here the direct intramolecular proton transfer has a computed barrier of about 22 kcal/mol, which is further lowered to 16-20 kcal/mol by aristolochene synthase. An alternative pathway is also possible through a proton shuttle via a pyrophosphate-bound water molecule. The mechanism proposed here is consistent with the observation that germacrene A is not a substrate of aristolochene synthase. Furthermore, the modeled substrate-enzyme complex suggests that Trp 334 and Phe 178 play key roles in positioning the substrate in the reactive orientation in the binding pocket. This is consistent with experimental findings that mutations of either residue lead to pronounced generation of aborted cyclization products.

A Promiscuous Proton in Taxadiene Biosynthesis?

Herein we describe quantum chemical calculations on a two-step proton-transfer pathway that interconverts key intermediates in the biosynthesis of taxa-4,11-diene, a precursor of Taxol, that provides an energetically more favorable alternative to the usually proposed direct proton-transfer pathway. In effect, the bicyclic diterpene skeleton involved in this rearrangement provides a cage of three pi-bonds that surrounds a locally mobile proton. [reaction: see text]

Biosynthesis and engineering of isoprenoid small molecules

Isoprenoid secondary metabolites are a rich source of commercial products that have not been fully explored. At present, there are isoprenoid products used in cancer therapy, the treatment of infectious diseases, and crop protection. All isoprenoids share universal prenyl diphosphate precursors synthesized via two distinct pathways. From these universal precursors, the biosynthetic pathways to specific isoprenoids diverge resulting in a staggering array of products. Taking advantage of this diversity has been the focus of much effort in metabolic engineering heterologous hosts. In addition, the engineering of the mevalonate pathway has increased levels of the universal precursors available for heterologous production. Finally, we will describe the efforts to produce to commercial terpenoids, paclitaxel and artemisinin.

Which Is More Likely in Trichodiene Biosynthesis: Hydride or Proton Transfer?

The mechanisms proposed for enzyme-catalyzed formation of the sesquiterpene natural product trichodiene consistently include a step involving a 1,4-hydride transfer. Using quantum chemical methods (B3LYP/6-31+G(d,p) and mPW1PW91/6-31+G(d,p)), we discovered two alternative pathways for transformation of the intermediate bisabolyl cation to the cuprenyl cation, one of which--a proton-transfer pathway--appears to be much more energetically favorable (by more than 10 kcal/mol) than the hydride transfer pathways usually proposed.

The phomactins. A novel group of terpenoid platelet activating factor antagonists related biogenetically to the taxanes

A description of the structurally unusual "phomactin" family of platelet activating factor antagonists recently found in the marine fungus Phoma sp. is presented. The phomactins show an interesting structural and biosynthetic relationship with the more familiar taxane group of antitumor compounds isolated from yew trees. The Account highlights and discusses this unique relationship and also presents a cogent picture of plausible biogenetic interrelationships within the family of phomactins. Complementary synthetic endeavors with the phomactins are interwoven in the discussions, alongside contemporaneous biosynthetic studies with both the phomactins and the taxanes.