Studies on taxadiene synthase: interception of the cyclization cascade at the verticillene stage and rearrangement to phomactatriene

Evaluation of the anticancer activity of enzymatically synthesized Baccatin III: an intermediate precursor of Taxol®

Baccatin III is an important precursor for the synthesis of clinically important anticancer drug Taxol. Previously, we have characterized a key enzyme of 10-deacetylbaccatin III-10-β-O-acetyltransferase (DBAT) which catalyses the 10-deacetylbaccatin III into baccatin III in taxol biosynthesis. Here, in the present study, we have evaluated and compared the cytotoxic properties of the enzymatically synthesized baccatin III (ESB III) with standard baccatin III in different human cancer cell lines, namely human cervical cancer (HeLa), human lung cancer (A549), human skin cancer (A431) and human liver cancer cells (HepG2). Among the various cancer lines tested, HeLa was more susceptible to ESB III with IC₅₀ of 4.30 µM while IC₅₀ values for A549, A431 and HepG2 ranged from 4 to 7.81 µM. Further, it showed G2/M phase cell cycle arrest, production of reactive oxygen species and depolarised mitochondrial membrane potential. In addition, annexin V-FITC staining was performed which showed the apoptotic cell death of HeLa cells, when treated with ESB III. Hence, ESB III was capable to show anticancer activities by inducing apoptotic cell death which could further be used for the semisynthesis of taxol in future.

C-C Bond Cleavage Approach to Complex Terpenoids: Development of a Unified Total Synthesis of the Phomactins

The rearrangement of carbon-carbon (C-C) single bonds in readily available carbocyclic scaffolds can yield uniquely substituted carbocycles that would be challenging to construct otherwise. This is a powerful and often non-intuitive approach for complex molecule synthesis. The transition-metal-mediated cleavage of C-C bonds has the potential to broaden the scope of this type of skeletal remodeling by providing orthogonal selectivities compared to more traditional pericyclic and carbocation-based rearrangements. To highlight this emerging technology, a unified, asymmetric, total synthesis of the phomactin terpenoids was developed, enabled by the selective C-C bond cleavage of hydroxylated pinene derivatives obtained from carvone. In this full account, the challenges, solutions, and intricacies of Rh(I)-catalyzed cyclobutanol C-C cleavage in a complex molecule setting are described. In addition, details of the evolution of strategies that ultimately led to the total synthesis of phomactins A, K, P, R, and T, as well as the synthesis and structural reassignment of Sch 49027, are given.

Absolute configuration of phomactatriene diterpenoids obtained by Wagner-Meerwein rearrangement of epimeric verticillols

The epimeric diterpenes (+)-(1S,3E,7E,11S,12S)-verticilla-3,7-dien-12-ol (1), isolated from Bursera suntui, and (+)-(1S,3E,7E,11S,12R)-verticilla-3,7-dien-12-ol (2), isolated from Bursera kerberi, gave the same Wagner-Meerwein rearrangement product (-)-(1E,4Z,8Z,11S,12R)-phomacta-1,(15)4,8-triene (3). The Et₂ O:BF₃ -induced transformations evidence that verticillenes and phomactanes, both containing the bicyclo[9.3.1]pentadecane skeleton, are biogenetically related through the verticillen-12-yl cation (A⁺ ), which also is a key intermediate in the biosynthetic pathways to generate antitumor taxanes. Molecular modeling using the Monte Carlo protocol, followed by density functional theory (DFT) geometry optimization employing the hybrid functionals B3LYP and B3PW91, both with the DGDZVP basis set, secured the configuration of 3 as followed from the good agreement between the calculated and experimental vibrational circular dichroism spectra. Similar DFT calculations allowed determining the absolute configuration of (+)-(1R,4R,5R,8S,9S,11S,12R,15R)-1,15:4,5:8,9-triepoxyphomactane (9), which surprisingly derives from epoxidation of the second minimum energy conformer of 3.

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.

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.

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.

Bicyclic and tetracyclic diterpenes from a Trichoderma symbiont of Taxus baccata

Trichoderma atroviridae UB-LMA is an endophytic fungus isolated from Taxus baccata trees. Liquid-state fermentation coupled to in situ solid phase extraction (SPE) was applied, and four compounds were discovered. Compounds 2-4 belong to the harziane tetracyclic diterpene family. Bicylic compound 1 may represent the biosynthetic precursor of this scarce family of compounds.

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.

Terpenes from marine-derived fungi

Terpenes from marine-derived fungi show a pronounced degree of structural diversity, and due to their interesting biological and pharmacological properties many of them have aroused interest from synthetic chemists and the pharmaceutical industry alike. The aim of this paper is to give an overview of the structural diversity of terpenes from marine-derived fungi, highlighting individual examples of chemical structures and placing them in a context of other terpenes of fungal origin. Wherever possible, information regarding the biological activity is presented.

Allylic and allenic halide synthesis via NbCl(5)- and NbBr(5)-mediated alkoxide rearrangements

Addition of NbCl(5) or NbBr(5) to a series of magnesium, lithium, or potassium allylic or propargylic alkoxides directly provides allylic or allenic halides. Halogenation formally occurs through a metalla-halo-[3,3] rearrangement, although concerted, ionic, and direct displacement mechanisms appear to operate competitively. Transposition of the olefin is equally effective for allylic alkoxides prepared by nucleophilic addition, deprotonation, or reduction. Experimentally, the niobium pentahalide halogenations are rapid, afford essentially pure (E)-allylic or -allenic halides after extraction, and are applicable to a range of aliphatic and aromatic alcohols, aldehydes, and ketones.