Chemistry and biology of monoterpene indole alkaloid biosynthesis

Virus-Induced Gene Silencing for Functional Genomics of Specialized Metabolism in Medicinal Plants

Virus-induced gene silencing (VIGS) is a functional genomics tool to transiently downregulate the expression of target gene(s) by exploiting the plant's innate defense mechanism against invading RNA viruses. VIGS is a rapid and efficient approach to analyze the gene function, particularly, in the plants that are not amenable to stable genetic transformation. This strategy has been successfully used to decipher the function of several genes and transcription factors involved in the biosynthesis of specialized metabolites and regulation of specialized metabolism, respectively, in different medicinal and aromatic plants. Here, we describe a detailed Tobacco rattle virus (TRV)-mediated VIGS protocol for silencing of the gene encoding Phytoene desaturase (PDS) in important medicinal plants Catharanthus roseus, Calotropis gigantean, Rauwolfia serpentina, and Ocimum basilicum. Our methods allow the study of gene function within 3-4 weeks after agro-inoculation, and can be an easy and efficient approach for future studies on understanding of the biosynthesis of specialized metabolites in these important medicinal plants.

Enantioselective Construction of cis-Hydroindole Scaffolds via Asymmetric Inverse-Electron-Demand Diels–Alder Reaction: Application to the Formal Total Synthesis of (+)-Minovincine

cis-Hydroindole scaffolds widely exist in a large number of natural products, pharmaceuticals, and organocatalysts. Therefore, the development of efficient and enantioselective methods for the construction of cis-hydroindoles is of great interest and importance. Herein, a novel approach for the enantioselective synthesis of cis-hydroindole scaffolds has been realized through a chiral N,N′-dioxide/Mg(OTf)₂ complex catalyzed asymmetric inverse-electron-demand Diels–Alder (IEDDA) reaction of 2-pyrones and cyclic enamines. A series of substituted cis-hydroindole derivatives bearing multiple contiguous stereocenters and functional groups were obtained in good to excellent yields and enantioselectivities (up to 99% yield, and 95% ee) under mild reaction conditions. Moreover, the enantioselective formal total synthesis of (+)-minovincine was concisely furnished with high efficiency and stereoselectivity to demonstrate the synthetic potential of this method.

An enantioselective IEDDA reaction between 2-pyrone and cyclic enamine was realized to construct cis-hydroindoles in high yield and ee, providing a concise route for formal total synthesis of (+)-minovincine.

Predicting Monoterpene Indole Alkaloid-Related Genes from Expression Data with Artificial Neural Networks

Elucidation of biological pathways leading to specialized metabolites remains a complex task. It is however a mandatory step to allow bioproduction into heterologous hosts. Many steps have already been identified using conventional approaches, enlarging the space of known possible chemical steps. In the recent past years, identification of missing steps has been fueled by the generation of genomic and transcriptomic data for nonmodel species. The analysis of gene expression profiles has revealed that in many cases, genes encoding enzymes involved in the same biosynthetic pathways are coexpressed across different tissue types and environmental conditions. Hence, coexpressed studies, either in the form of differential gene expression, gene coexpression network, or unsupervised clustering methods, have helped deciphering missing steps to complete knowledge on biosynthetic pathways. Already identified biosynthetic steps can be used as baits to capture the remaining unknown steps. The present protocol shows how supervised machine learning in the form of artificial neural networks (ANNs) can efficiently classify genes as specialized metabolism related or not according to their expression levels. Using Catharanthus roseus as an example, we show that ANN trained on a minimal set of bait genes results in many true positives (correctly predicted genes) while keeping false positives low (containing possible candidate genes).

Ancestral Sequence Reconstruction for Exploring Alkaloid Evolution

The complex and bioactive monoterpene indole alkaloids (MIAs) found in Catharanthus roseus and related species are the products of many millions of years of evolution through mutation and natural selection. Ancestral sequence reconstruction (ASR) is a method that combines phylogenetic analysis and experimental biochemistry to infer details about past events in protein evolution. Here, I propose that ASR could be leveraged to understand how enzymes catalyzing the formation of complex alkaloids arose over evolutionary time. I discuss the steps of ASR, including sequence selection, multiple sequence alignment, tree inference, and the generation and characterization of inferred ancestral enzymes.

Asymmetric synthesis of polycyclic spiroindolines via the Dy-catalyzed cascade reaction of 3-(2-isocyanoethyl)indoles with aziridines

An asymmetric cascade reaction catalyzed by a chiral N,N′-dioxide–Dy(iii) complex was realized to construct the valuable [6,5,5,6] tetracyclic spiroindolines with good yields and enantioselectivities by a concise and one-step protocol.

Pictet-Spengler Reaction for the Chemical Synthesis of Strictosidine

Strictosidine is the common biosynthetic precursor of Monoterpene Indole Alkaloids (MIA). A practical single-step procedure to assemble strictosidine from secologanin is described via a bioinspired Pictet-Spengler reaction. Mild conditions and purification by crystallization and flash chromatography allow access to the targeted product in fair yield.

Environmental and Genetic Factors Involved in Plant Protection-Associated Secondary Metabolite Biosynthesis Pathways

Plant specialized metabolites (PSMs) play essential roles in the adaptation to harsh environments and function in plant defense responses. PSMs act as key components of defense-related signaling pathways and trigger the extensive expression of defense-related genes. In addition, PSMs serve as antioxidants, participating in the scavenging of rapidly rising reactive oxygen species, and as chelators, participating in the chelation of toxins under stress conditions. PSMs include nitrogen-containing chemical compounds, terpenoids/isoprenoids, and phenolics. Each category of secondary metabolites has a specific biosynthetic pathway, including precursors, intermediates, and end products. The basic biosynthetic pathways of representative PSMs are summarized, providing potential target enzymes of stress-mediated regulation and responses. Multiple metabolic pathways share the same origin, and the common enzymes are frequently to be the targets of metabolic regulation. Most biosynthetic pathways are controlled by different environmental and genetic factors. Here, we summarized the effects of environmental factors, including abiotic and biotic stresses, on PSM biosynthesis in various plants. We also discuss the positive and negative transcription factors involved in various PSM biosynthetic pathways. The potential target genes of the stress-related transcription factors were also summarized. We further found that the downstream targets of these Transcription factors (TFs) are frequently enriched in the synthesis pathway of precursors, suggesting an effective role of precursors in enhancing of terminal products. The present review provides valuable insights regarding screening targets and regulators involved in PSM-mediated plant protection in non-model plants.

A Rapid and Efficient Vacuum-Based Agroinfiltration Protocol for Transient Gene Overexpression in Leaves of Catharanthus roseus

Functional genomics analyses in planta can be hampered in non-model plants that are recalcitrant to the genetic transformation such as the medicinal plant Catharanthus roseus (Apocynaceae). No stable transformation and regeneration of plantlets have been achieved with a high efficiency in this plant to date. In addition, while virus-mediated transient gene silencing has been reported a decade ago in C. roseus, tools for transient overexpression remain scarce. Here, we describe an efficient and reliable methodology for transiently overexpressing any gene of interest in C. roseus leaves. This protocol combines a vacuum-based Agroinfiltration approach and the high translational efficiency of a deconstructed virus-based binary vector (pEAQ-HT). The described methodology is robust, easy to perform, and results in high amount of transient expression in C. roseus. This protocol is expected to serve as valuable tool to enhance the in planta characterization of gene functions or even transiently knock-in novel enzymatic activities.

Visible-light-induced indole synthesis via intramolecular C–N bond formation: desulfonylative C(sp2)–H functionalization

LED visible-light-induced redox neutral desulfonylative C(sp2)–H functionalization for the synthesis of N-substituted indoles in the absence of any additional additive has been established on the basis of KIE, Hammett plotting and DFT calculations.

Saccharomyces cerevisiae as a Heterologous Host for Natural Products

Cell factories can provide a sustainable supply of natural products with applications as pharmaceuticals, food-additives or biofuels. Besides being an important model organism for eukaryotic systems, Saccharomyces cerevisiae is used as a chassis for the heterologous production of natural products. Its success as a cell factory can be attributed to the vast knowledge accumulated over decades of research, its overall ease of engineering and its robustness. Many methods and toolkits have been developed by the yeast metabolic engineering community with the aim of simplifying and accelerating the engineering process.In this chapter, a range of methodologies are highlighted, which can be used to develop novel natural product cell factories or to improve titer, rate and yields of an existing cell factory with the goal of developing an industrially relevant strain. The addressed topics are applicable for different stages of a cell factory engineering project and include the choice of a natural product platform strain, expression cassette design for heterologous or native genes, basic and advanced genetic engineering strategies, and library screening methods using biosensors. The many engineering methods available and the examples of yeast cell factories underline the importance and future potential of this host for industrial production of natural products.

Electrochemical oxidative rearrangement of tetrahydro-β-carbolines in a zero-gap flow cell

Oxidative rearrangement of tetrahydro-β-carbolines (THβCs) is one of the most efficient methods for the synthesis of biologically active spirooxindoles, including natural products and drug molecules. Here, we report the first electrochemical approach to achieve this important organic transformation in a flow cell. The key to the high efficiency was the use of a multifunctional LiBr electrolyte, where the bromide (Br⁻) ion acts as a mediator and catalyst and lithium ion (Li⁺) acts as a likely hydrophilic spectator, which might considerably reduce diffusion of THβCs into the double layer and thus prevent possible nonselective electrode oxidation of indoles. Additionally, we build a zero-gap flow cell to speed up mass transport and minimize concentration polarization, simultaneously achieving a high faradaic efficiency (FE) of 96% and an outstanding productivity of 0.144 mmol (h⁻¹ cm⁻²). This electrochemical method is demonstrated with twenty substrates, offering a general, green path towards bioactive spirooxindoles without using hazardous oxidants.

A zero-gap flow cell was designed for the first electro-oxidative rearrangement of tetrahydro-β-carbolines to spirooxindoles with high yield, faradaic efficiency and productivity when LiBr was discovered as a bi-functional mediator and catalyst.

Asymmetric synthesis of dihydrocarbazoles through a Friedel-Crafts alkylation/annulation sequential reaction of indoles

An enantioselective tandem Friedel-Crafts alkylation/annulation of indoles with diazoacetoacetate enones is realized in one pot. A series of dihydrocarbazoles were obtained in moderate yields with good to excellent ee values by using a Rh^II/Sc^III dual-metallic catalyst system. Control experiments revealed that Sc^III is critical to both the alkylation and annulation.

Asymmetric Synthesis of Bridged N-Heterocycles with Tertiary Carbon Center through Barbas Dienamine-Catalysis: Scope and Applications

A direct aza-Diels-Alder reaction between 2-aryl-3H-indolin-3-ones and cyclic-enones has been developed to access chiral indolin-3-one fused polycyclic bridged compounds. This method proceeds via proline-catalyzed Barbas-dienamine intermediate formation from various cyclic-enones such as 2-cyclopenten-1-one, 2-cyclohexene-1-one, and 2-cycloheptene-1-one, followed by a reaction with 2-aryl-3H-indol-3-ones. Several indolin-3-ones fusing [2.2.2], [2.2.1], and [3.2.1] skeletons decorated with a tertiary carbon chiral center have been prepared. Computational studies (DFT) supported the observed stereoselectivity in the method. The synthesized compounds have shown exciting photophysical activities and selective sensing of Pd²⁺ and Fe³⁺ ions through the fluorescence quenching "switch-off" mode.

Bioinspired Transformations Using Strictosidine Aglycones: Divergent Total Syntheses of Monoterpenoid Indole Alkaloids in the Early Stage of Biosynthesis

A series of bioinspired transformations that are applied to convert strictosidine aglycones into monoterpenoid indole alkaloids is reported. The highly reactive key intermediates, strictosidine aglycones, were prepared in situ by simple removal of a silyl protecting group from the silyl ether derivatives, and converted selectively via bioinspired transformations under substrate control into heteroyohimbine- and corynantheine-type, and akagerine and naucleaoral related alkaloids. Thus, concise, divergent total syntheses of 13 monoterpenoid indole alkaloids, (-)-cathenamine, (-)-tetrahydroalstonine, (+)-dihydrocorynantheine, (-)-corynantheidine, (-)-akagerine, (-)-dihydrocycloakagerine, (-)-naucleaoral B, (+)-naucleidinal, (-)-naucleofficines D and III, (-)-nauclefiline, and (-)-naucleamides A and E, were accomplished in fewer than 13 steps.

Synthesis of tetracyclic spiroindolines by an interrupted Bischler-Napieralski reaction: total synthesis of akuammicine

Judicious substrate design allows interruption of the classical Bischler–Napieralski reaction, providing access to a range of diversely substituted tetracyclic spiroindolines. These complex polycyclic scaffolds are valuable building blocks for the construction of indole alkaloids, as showcased in a concise total synthesis of (±)-akuammicine.

The interrupted Bischler–Napieralski reaction of β,γ-unsaturated tryptamides affords tetracyclic spiro pyrroloindolines, which can be used in the total synthesis of the Strychnos alkaloid, akuammicine.

Catalytic Asymmetric Alkynylation of 3,4-Dihydro-β-carbolinium Ions Enables Collective Total Syntheses of Indole Alkaloids

Chiral tetrahydro-β-carboline (THβC) is not only a prevailing structural feature of many natural alkaloids but also a versatile synthetic precursor for a vast array of monoterpenoid indole alkaloids. Asymmetric synthesis of C1-alkynyl THβCs remains rarely explored and challenging. Herein, we describe the development of two complementary approaches for the catalytic asymmetric alkynylation of 3,4-dihydro-β-carbolinium ions with up to 96 % yield and 99 % ee. The utility of chiral C1-alkynyl THβCs was demonstrated by the collective total syntheses of seven indole alkaloids: harmicine, eburnamonine, desethyleburnamonine, larutensine, geissoschizol, geissochizine, and akuammicine.

Secoiridoid dimers and their biogenetic precursors from the fruits of Cornus officinalis with potential therapeutic effects on type 2 diabetes

Cornusdiridoid A-F (1-6), six unusual cornuside-morroniside secoiridoid dimers, and their possible new biogenetic precursor, 3″,5″-dehydroxycornuside (7), together with four known secoiridoids (8-11), were obtained from the fruits of Cornus officinalis. Their structures were elucidated on the basis of various spectroscopic and chemical methods. A plausible biosynthetic pathway of compounds 1-11 was proposed. The α-glucosidase inhibitory, antioxidant and anti-inflammatory activities of these isolates were evaluated. Some of them emerged out as potent antidiabetic, anti-inflammatory and free radical scavenging agents. Molecular docking was also carried out for antidiabetic target α-glucosidase to investigate the possible binding modes of the most potent α-glucosidase inhibitor, vincosamide (9). These results revealed that the secoiridoids from C. officinalis fruits may be served as new potential antidiabetic agents to prevent and treat type 2 diabetes.

Voatriafricanines A and B, Trimeric Vobasine-Aspidosperma-Aspidosperma Alkaloids from Voacanga africana

Voatriafricanines A and B (1 and 2), the first examples of vobasine-aspidosperma-aspidosperma monoterpene trisindole alkaloids, were isolated from the stem barks of Voacanga africana, guided by a molecular networking strategy. Their structures, including absolute configurations, were elucidated by spectroscopic methods and ECD calculations. Compounds 1 and 2 possess intramolecular hydrogen bonding, sufficiently robust to transfer homonuclear and heteronuclear magnetizations. Compound 1 exhibited potent antimycobacterial activity with no discernible cytotoxic activity.

Divergent Synthesis of Skeletally Distinct Arboridinine and Arborisidine

A divergent synthesis of skeletally distinct arboridinine and arborisidine was achieved. The central divergent strategy was inspired by the divergent biosynthetic cyclization mode of arboridinine and arborisidine and their hidden topological connection. The branch point was reached through a Michael and Mannich cascade process. A site-selective intramolecular Mannich reaction was developed to construct the tetracyclic core of arboridinine, while a site-selective intramolecular α-amination of ketone was used to access the tetracyclic core of arborisidine. A strategic Peterson olefination through intramolecular nucleophile delivery was able to set up the exocyclic olefin of arboridinine.

Asymmetric C-H and N-H functionalization of Indoles involving Central Chirality via Chiral Phosphoric Acid Catalysis

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The C-H and N-H functionalization of indoles is an interesting area of research that has a useful impact on organic synthesis due to the availability of chiral indole scaffolds in the discovery of drugs, synthetic bioactive compounds, and natural products. The chiral phosphoric acid catalysts (CPAs) have proven to be a powerful and versatile class of enantioselective organocatalysts. Many asymmetric syntheses of organic compounds have been carried out with these catalysts in C–C and C-N bond formation reactions, and great progress has been reported. By 2011, several reviews were published covering some important topics and recent achievements in this field. Therefore, in this review, the most recent advances, research breakthroughs with key examples involving mechanisms of CPA-catalyzed C-H and N-H functionalization of indoles to form central chirality via Friedel Crafts, Michael type, and rearrangement reactions were reviewed and reported.

Improved virus-induced gene silencing allows discovery of a serpentine synthase gene in Catharanthus roseus

Specialized metabolites are chemically complex small molecules with a myriad of biological functions. To investigate plant-specialized metabolite biosynthesis more effectively, we developed an improved method for virus-induced gene silencing (VIGS). We designed a plasmid that incorporates fragments of both the target gene and knockdown marker gene (phytoene desaturase, PDS), which identifies tissues that have been successfully silenced in planta. To demonstrate the utility of this method, we used the terpenoid indole alkaloid (TIA) pathway in Madagascar periwinkle (Catharanthus roseus) as a model system. Catharanthus roseus is a medicinal plant well known for producing many bioactive compounds, such as vinblastine and vincristine. Our VIGS method enabled the discovery of a previously unknown biosynthetic enzyme, serpentine synthase (SS). This enzyme is a cytochrome P450 (CYP) that produces the β-carboline alkaloids serpentine and alstonine, compounds with strong blue autofluorescence and potential pharmacological activity. The discovery of this enzyme highlights the complexity of TIA biosynthesis and demonstrates the utility of this improved VIGS method for discovering unidentified metabolic enzymes in plants.

Aspidosperma and Strychnos alkaloids: Chemistry and biology

Of Nature's nearly 3000 unique monoterpene indole alkaloids derived from tryptophan, those members belonging to the Aspidosperma and Strychnos families continue to impact the fields of natural products (i.e., isolation, structure determination, biosynthesis) and organic chemistry (i.e., chemical synthesis, methodology development) among others. This review covers the biological activity (Section 2), biosynthesis (Section 3), and synthesis of both classical and novel Aspidosperma (Section 4), Strychnos (Section 5), and selected bis-indole (Section 6) alkaloids. Technological advancements in genetic sequencing and bioinformatics have deepened our understanding of how Nature assembles these intriguing molecules. The proliferation of innovative synthetic strategies and tactics for the synthesis of the alkaloids covered in this review, which include contributions from over fifty research groups from around the world, are a testament to the creative power and technical skills of synthetic organic chemists. To be sure, Nature-the Supreme molecular architect and source of a dazzling array of irresistible chemical logic puzzles-continues to inspire scientists across multiple disciplines and will certainly continue to do so for the foreseeable future.

Myxadazoles, Myxobacterium-Derived Isoxazole-Benzimidazole Hybrids with Cardiovascular Activities

There is a continuous need for novel microbial natural products to fill the drying-up drug development pipeline. Herein, we report myxadazoles from Myxococcus sp. SDU36, a family of novel chimeric small molecules that consist of N-ribityl 5,6-dimethylbenzimidazole and a linear fatty acid chain endowed with an isoxazole ring. The experiments of genome sequencing, gene insertion mutation, isotope labelling, and precursor feeding demonstrated that the fatty acid chain was encoded by a non-canonical PKS/NRPS gene cluster, whereas the origin of N-ribityl 5,6-dimethylbenzimidazole was related to the vitamin B₁₂ metabolism. The convergence of these two distinct biosynthetic pathways through a C-N coupling led to the unique chemical framework of myxadazoles, which is an unprecedented hybridization mode in the paradigm of natural products. Myxadazoles exhibited potent vasculogenesis promotion effect and moderate antithrombotic activity, underscoring their potential usage for the treatment of cardiovascular diseases.

A multimodal metabolomics approach using imaging mass spectrometry and liquid chromatography-tandem mass spectrometry for spatially characterizing monoterpene indole alkaloids secreted from roots

Plants release specialized (secondary) metabolites from their roots to communicate with other organisms, including soil microorganisms. The spatial behavior of such metabolites around these roots can help us understand roles for the communication; however, currently, they are unclear because soil-based studies are complex. Here, we established a multimodal metabolomics approach using imaging mass spectrometry (IMS) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) to spatially assign metabolites under laboratory conditions using agar. In a case study using Catharanthus roseus, we showed that 58 nitrogen (N)-containing metabolites are released from the roots into the agar. For the metabolite assignment, we used ¹⁵N-labeled and non-labeled LC-MS/MS data, previously reported. Four metabolite ions were identified using authentic standard compounds as derived from monoterpene indole alkaloids (MIAs) such as ajmalicine, catharanthine, serpentine, and yohimbine. An alkaloid network analysis using dot products and spinglass methods characterized five clusters to which the 58 ions belong. The analysis clustered ions from the indolic skeleton-type MIAs to a cluster, suggesting that other communities may represent distinct metabolite groups. For future chemical assignments of the serpentine community, key fragmentation patterns were characterized using the ¹⁵N-labeled and non-labeled MS/MS spectra.

Stereoselective Late-Stage Transformations of Indolo[2,3-a]quinolizines Skeleta to Nature-Inspired Scaffolds

The indolo[2,3-a]quinolizines, canthines, and arborescidines natural products exhibit a wide range of bioactivities including anticancer, antiviral, antibacterial, and anti-inflammatory, among others. Therefore, the development of modular and efficient strategies to access the core scaffolds of these classes of natural products is a remarkable achievement. The Complexity-to-Diversity (CtD) strategy has become a powerful tool that transforms natural products into skeletal and stereochemical diversity. However, many of the reactions that could be utilized in this process are limited by the type of functional groups present in the starting material, which restrict transformations into a variety of products to achieve the desired diversity. In the course of employing a (CtD) strategy en route to the synthesis of nature-inspired compounds, unexpected stereoelectronic-driven rearrangement reactions have been discovered. These reactions provided a rapid access to indolo[2,3-a]quinolizines-, canthines-, and arborescidines-inspired alkaloids in a modular and diastereoselective manner. The disclosed strategies will be widely applicable to other late-stage natural product transformation programs and drug discovery initiatives.

A Radical-Initiated Fragmentary Rearrangement Cascade of Ene-Ynamides to [1,2]-Annulated Indoles via Site-Selective Cyclization

Straightforward access to [1,2]-annulated indoles, key substructures in natural products, is highly desirable yet challenging. Herein, a radical triggered fragmentary cyclization cascade reaction of ene-ynamides is presented, providing a rapid access into [1,2]-annulated indoles by an intermolecular radical addition, intramolecular cyclization, desulfonylative aryl migration, and site-selective C(sp²)-N cyclization sequence. DFT calculations support oxidation of N-centered radical species to cations prior to the C-N bond formation, followed by an unusual aza-Nazarov cyclization.

Efficient production of vindoline from tabersonine by metabolically engineered Saccharomyces cerevisiae

Vindoline is a plant derived monoterpene indole alkaloid (MIA) with potential therapeutic applications and more importantly serves as the precursor to vinblastine and vincristine. To obtain a yeast strain for high yield production of vindoline from tabersonine, multiple metabolic engineering strategies were employed via the CRISPR/Cas9 mediated multiplex genome integration technology in the present study. Through increasing and tuning the copy numbers of the pathway genes, pairing cytochrome P450 enzymes (CYPs) with appropriate cytochrome P450 reductases (CPRs), engineering the microenvironment for functional expression of CYPs, enhancing cofactor supply, and optimizing fermentation conditions, the production of vindoline was increased to a final titer as high as ∼16.5 mg/L, which is more than 3,800,000-fold higher than the parent strain and the highest tabersonine to vindoline conversion yield ever reported. This work represents a key step of the engineering efforts to establish de novo biosynthetic pathways for vindoline, vinblastine, and vincristine.

The case of tryptamine and serotonin in plants: a mysterious precursor for an illustrious metabolite

Indolamines are tryptophan-derived specialized metabolites belonging to the huge and ubiquitous indole alkaloids group. Serotonin and melatonin are the best-characterized members of this family, given their many hormonal and physiological roles in animals. Following their discovery in plants, the study of plant indolamines has flourished and their involvement in important processes, including stress responses, growth and development, and reproduction, has been proposed, leading to their classification as a new category of phytohormones. However, the complex indolamine puzzle is far from resolved, particularly the biological roles of tryptamine, the early serotonin precursor representing the central hub of many downstream indole alkaloids. Tryptophan decarboxylase, which catalyzes the synthesis of tryptamine, strictly regulates the flux of carbon and nitrogen from the tryptophan pool into the indolamine pathway. Furthermore, tryptamine accumulates to high levels in the reproductive organs of many plant species and therefore cannot be classed as a mere intermediate but rather as an end product with potentially important functions in fruits and seeds. This review summarizes current knowledge on the role of tryptamine and its close relative serotonin, emphasizing the need for a clear understanding of the functions of, and mutual relations between, these indolamines and their biosynthesis pathways in plants.

High-Throughput Detection of an Alkaloidal Plant Metabolome in Plant Extracts Using LC-ESI-QTOF-MS

Plant alkaloids represent a diverse group of nitrogen-containing natural products. These compounds are considered valuable in drug discovery and development. High-throughput identification of such plant secondary metabolites in complex plant extracts is essential for drug discovery, lead optimization, and understanding the biological pathway. The present study aims to rapidly identify different classes of alkaloids in plant extracts through the liquid chromatography with electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) approach using 161 isolated and purified alkaloids. These are biologically important unique alkaloids belonging to different sub-classes such as isoquinoline, quinoline, indole, tropane, pyridine, piperidine, quinolizidine, aporphine, steroidal, and terpenoid. The majority of these are not available commercially and are known to manifest valuable biological activities. Four pools of a maximum of 50 phytostandards each were prepared, based on their log P value to minimize co-elution for rapid and cost-effective analyses. MS/MS spectra were acquired in the positive ionization mode by using their [M + H]⁺ and/or [M + Na]⁺ with both the average collisional energy (25.5-62 eV) and individual collisional energies (10, 20, 30, and 40 eV). Accurate mass, high-resolution mass spectrometry (HR-MS) data, MS/MS data, and retention times were curated for each compound. The developed LC-MS/MS method was successfully used to interrogate and fast dereplicate alkaloids in 13 medicinal plant extracts and a herbal formulation. A total of 56 alkaloids were identified based on the reference standard retention times (RTs), HR-MS spectra, and/or MS/MS spectra. The MS data have been submitted to the MetaboLights online database (MTBLS2914). The mass spectrometric and chromatographic data will be useful for the discovery of new congeners and the study of biological pathways of alkaloids in the plant kingdom.

Limonin as a Starting Point for the Construction of Compounds with High Scaffold Diversity

Structurally complex natural products have been a fruitful source for the discovery and development of new drugs. In an effort to construct a compound collection populated by architecturally complex members with unique scaffolds, we have used the natural product limonin as a starting point. Limonin is an abundant triterpenoid natural product and, through alteration of its heptacyclic core ring system using short synthetic sequences, a collection of 98 compounds was created, including multiple members with novel ring systems. The reactions leveraged in the construction of these compounds include novel ring cleavage, rearrangements, and cyclizations, and this work is highlighted by the discovery of a novel B-ring cleavage reaction, a unique B/C-ring rearrangement, an atypical D-ring cyclization, among others. Computational analysis shows that 52 different scaffolds/ring systems were produced during the course of this work, of which 36 are unprecedented. Phenotypic screening and structure-activity relationships identified compounds with activity against a panel of cancer cell lines.

Synthesis of a Seco iso-Secologanin Aglycone Analogue of Interest toward Secoiridoids and Monoterpene Indole Alkaloids

We report the access to an acyclic iso-secologanin aglycone analogue relevant to secoiridoids and monoterpene indole alkaloids. Its synthesis involved the regioselective allylic alkylation of a linear dienyl carbonate with dimethyl malonate, which was catalyzed by an iridium complex, and an anti-Markovnikov Wacker-type oxidation of the terminal alkene of the branched product that was obtained. The thus-formed aldehyde was engaged in a Pictet-Spengler reaction with tryptamine toward monoterpene indole alkaloids.

Transcriptional Factor-Mediated Regulation of Active Component Biosynthesis in Medicinal Plants

Plants produce thousands of chemically diverse secondary metabolites, many of which have valuable pharmaceutical properties. There is much interest in the synthesis of these pharmaceuticallyvaluable compounds, including the key enzymes and the transcription factors involved. The function and regulatory mechanism of transcription factors in biotic and abiotic stresses have been studied in depth. However, their regulatory roles in the biosynthesis of bioactive compounds, especially in medicinal plants, have only begun. Here, we review what is currently known about how transcription factors contribute to the synthesis of bioactive compounds (alkaloids, terpenoids, flavonoids, and phenolic acids) in medicinal plants. Recent progress has been made in the cloning and characterization of transcription factors in medicinal plants on the genome scale. So far, several large transcription factors have been identified in MYB, WRKY, bHLH, ZIP, AP2/ERF transcription factors. These transcription factors have been predicted to regulate bioactive compound production. These transcription factors positively or negatively regulate the expression of multiple genes encoding key enzymes, and thereby control the metabolic flow through the biosynthetic pathway. Although the research addressing this niche topic is in its infancy, significant progress has been made, and advances in high-throughput sequencing technology are expected to accelerate the discovery of key regulatory transcription factors in medicinal plants. This review is likely to be useful for those interested in the synthesis of pharmaceutically- valuable plant compounds, especially those aiming to breed or engineer plants that produce greater yields of these compounds.

Biosynthesis and synthetic biology of psychoactive natural products

Psychoactive natural products play an integral role in the modern world. The tremendous structural complexity displayed by such molecules confers diverse biological activities of significant medicinal value and sociocultural impact. Accordingly, in the last two centuries, immense effort has been devoted towards establishing how plants, animals, and fungi synthesize complex natural products from simple metabolic precursors. The recent explosion of genomics data and molecular biology tools has enabled the identification of genes encoding proteins that catalyze individual biosynthetic steps. Once fully elucidated, the "biosynthetic pathways" are often comparable to organic syntheses in elegance and yield. Additionally, the discovery of biosynthetic enzymes provides powerful catalysts which may be repurposed for synthetic biology applications, or implemented with chemoenzymatic synthetic approaches. In this review, we discuss the progress that has been made toward biosynthetic pathway elucidation amongst four classes of psychoactive natural products: hallucinogens, stimulants, cannabinoids, and opioids. Compounds of diverse biosynthetic origin - terpene, amino acid, polyketide - are identified, and notable mechanisms of key scaffold transforming steps are highlighted. We also provide a description of subsequent applications of the biosynthetic machinery, with an emphasis placed on the synthetic biology and metabolic engineering strategies enabling heterologous production.

Divergent camptothecin biosynthetic pathway in Ophiorrhiza pumila

BACKGROUND

The anticancer drug camptothecin (CPT), first isolated from Camptotheca acuminata, was subsequently discovered in unrelated plants, including Ophiorrhiza pumila. Unlike known monoterpene indole alkaloids, CPT in C. acuminata is biosynthesized via the key intermediate strictosidinic acid, but how O. pumila synthesizes CPT has not been determined.

RESULTS

In this study, we used nontargeted metabolite profiling to show that 3α-(S)-strictosidine and 3-(S), 21-(S)-strictosidinic acid coexist in O. pumila. After identifying the enzymes OpLAMT, OpSLS, and OpSTR as participants in CPT biosynthesis, we compared these enzymes to their homologues from two other representative CPT-producing plants, C. acuminata and Nothapodytes nimmoniana, to elucidate their phylogenetic relationship. Finally, using labelled intermediates to resolve the CPT biosynthesis pathway in O. pumila, we showed that 3α-(S)-strictosidine, not 3-(S), 21-(S)-strictosidinic acid, is the exclusive intermediate in CPT biosynthesis.

CONCLUSIONS

In our study, we found that O. pumila, another representative CPT-producing plant, exhibits metabolite diversity in its central intermediates consisting of both 3-(S), 21-(S)-strictosidinic acid and 3α-(S)-strictosidine and utilizes 3α-(S)-strictosidine as the exclusive intermediate in the CPT biosynthetic pathway, which differs from C. acuminata. Our results show that enzymes likely to be involved in CPT biosynthesis in O. pumila, C. acuminata, and N. nimmoniana have evolved divergently. Overall, our new data regarding CPT biosynthesis in O. pumila suggest evolutionary divergence in CPT-producing plants. These results shed new light on CPT biosynthesis and pave the way towards its industrial production through enzymatic or metabolic engineering approaches.

Optimization of Tabersonine Methoxylation to Increase Vindoline Precursor Synthesis in Yeast Cell Factories

Plant specialized metabolites are widely used in the pharmaceutical industry, including the monoterpene indole alkaloids (MIAs) vinblastine and vincristine, which both display anticancer activity. Both compounds can be obtained through the chemical condensation of their precursors vindoline and catharanthine extracted from leaves of the Madagascar periwinkle. However, the extensive use of these molecules in chemotherapy increases precursor demand and results in recurrent shortages, explaining why the development of alternative production approaches, such microbial cell factories, is mandatory. In this context, the precursor-directed biosynthesis of vindoline from tabersonine in yeast-expressing heterologous biosynthetic genes is of particular interest but has not reached high production scales to date. To circumvent production bottlenecks, the metabolic flux was channeled towards the MIA of interest by modulating the copy number of the first two genes of the vindoline biosynthetic pathway, namely tabersonine 16-hydroxylase and tabersonine-16-O-methyltransferase. Increasing gene copies resulted in an optimized methoxylation of tabersonine and overcame the competition for tabersonine access with the third enzyme of the pathway, tabersonine 3-oxygenase, which exhibits a high substrate promiscuity. Through this approach, we successfully created a yeast strain that produces the fourth biosynthetic intermediate of vindoline without accumulation of other intermediates or undesired side-products. This optimization will probably pave the way towards the future development of yeast cell factories to produce vindoline at an industrial scale.

Natural spirocyclic alkaloids and polyphenols as multi target dementia leads

The U rhynchophylla, U tomentosa, Isatis indigotica Fortune, Voacanga Africana, herbal constituents, fungal extracts from Aspergillus duricaulis culture media, include spirooxindoles, polyphenols or bridged spirocyclic alkaloids. Their constituents exhibit specific and synergistic multiple neuroprotective properties including inhibiting of Aβ fibril induced cytotoxicity, NMDA receptor inhibition in mice models of Alzheimer's disease (AD). The pioneering research from Woodward to Waldmann has advanced the synthesis of spirocyclic alkaloids. Furthermore, the elucidation of the genetic analysis, biochemical pathways that links strictosidine to the alkaloids akuammicine, stemmadenine, tabersonine, catharanthine, will now enable the biotechnological generation, also stimulate synthesis of related bridged spirocyclic alkaloids for medicinal investigations. From the value of spirocyclic structures as multi target dementia leads, we hypothesise that simpler Lipinski-like natural/synthetic alkaloid analogues may likewise be discovered that provide neurocognitive enhancing activities against dementia and AD.

Diversity in Chemical Structures and Biological Properties of Plant Alkaloids

Phytochemicals belonging to the group of alkaloids are signature specialized metabolites endowed with countless biological activities. Plants are armored with these naturally produced nitrogenous compounds to combat numerous challenging environmental stress conditions. Traditional and modern healthcare systems have harnessed the potential of these organic compounds for the treatment of many ailments. Various chemical entities (functional groups) attached to the central moiety are responsible for their diverse range of biological properties. The development of the characterization of these plant metabolites and the enzymes involved in their biosynthesis is of an utmost priority to deliver enhanced advantages in terms of biological properties and productivity. Further, the incorporation of whole/partial metabolic pathways in the heterologous system and/or the overexpression of biosynthetic steps in homologous systems have both become alternative and lucrative methods over chemical synthesis in recent times. Moreover, in-depth research on alkaloid biosynthetic pathways has revealed numerous chemical modifications that occur during alkaloidal conversions. These chemical reactions involve glycosylation, acylation, reduction, oxidation, and methylation steps, and they are usually responsible for conferring the biological activities possessed by alkaloids. In this review, we aim to discuss the alkaloidal group of plant specialized metabolites and their brief classification covering major categories. We also emphasize the diversity in the basic structures of plant alkaloids arising through enzymatically catalyzed structural modifications in certain plant species, as well as their emerging diverse biological activities. The role of alkaloids in plant defense and their mechanisms of action are also briefly discussed. Moreover, the commercial utilization of plant alkaloids in the marketplace displaying various applications has been enumerated.

Enzymatic reactions in teleocidin B biosynthesis

The teleocidin B family members are terpene indole compounds isolated from Streptomyces bacteria, and they strongly activate protein kinase C (PKC). Their unique structures have attracted many researchers in the natural product chemistry and pharmacology fields, and numerous isolation and bioactivity studies have been conducted. The accumulated information has facilitated the identification of the enzymatic reactions in teleocidin biosynthesis, and new developments in structural biology have strongly aided efforts to clarify the finer points of these reactions. This review describes the recent biochemical and structural biological studies to reveal their reaction mechanisms, with a primary focus on the terpene cyclization triggered by the C-N bond formation by P450 oxygenase (TleB), the prenyltransferase (TleC), and the methyltransferase (TleD). This new knowledge will benefit future engineering studies to create unnatural PKC activators.

Synthesis of tetrahydro-β-carbolines from 2-indolylmethyl azides and propargylic alcohols

A facile and efficient route to tetrahydro-β-carbolines from 2-indolylmethyl azides and propargylic alcohols via acid-catalyzed dehydrative annulation reactions is described. This reaction proceeds through a cascade sequence of Friedel–Crafts-type alkylation followed by intramolecular “Click” reaction, involving the formation of multiple chemical bonds in a single operation with excellent atom-economy and broad functional group tolerance.

The synthesis of tetrahydro-β-carbolines from 2-indolylmethyl azides and propargylic alcohols via acid-catalyzed dehydrative [4 + 2]-annulation reaction is described.