Discovery, Structure Elucidation, and Biological Characterization of Nannocystin A, a Macrocyclic Myxobacterial Metabolite with Potent Antiproliferative Properties

The genetic basis of predation by myxobacteria

Myxobacteria (phylum Myxococcota) are abundant and virtually ubiquitous microbial predators. Facultatively multicellular organisms, they are able to form multicellular fruiting bodies and swarm across surfaces, cooperatively hunting for prey. Myxobacterial communities are able to kill a wide range of prey microbes, assimilating their biomass to fuel population growth. Their mechanism of predation is exobiotic - hydrolytic enzymes and toxic metabolites are secreted into the extracellular environment, killing and digesting prey cells from without. However, recent observations of single-cell predation and contact-dependent prey killing challenge the dogma of myxobacterial predation being obligately cooperative. Regardless of their predatory mechanisms, myxobacteria have a broad prey range, which includes Gram-negative bacteria, Gram-positive bacteria and fungi. Pangenome analyses have shown that their extremely large genomes are mainly composed of accessory genes, which are not shared by all members of their species. It seems that the diversity of accessory genes in different strains provides the breadth of activity required to prey upon such a smorgasbord of microbes, and also explains the considerable strain-to-strain variation in predatory efficiency against specific prey. After providing a short introduction to general features of myxobacterial biology which are relevant to predation, this review brings together a rapidly growing body of work into the molecular mechanisms and genetic basis of predation, presenting a summary of current knowledge, highlighting trends in research and suggesting strategies by which we can potentially exploit myxobacterial predation in the future.

Discovery of a nitroaromatic nannocystin with potent in vivo anticancer activity against colorectal cancer by targeting AKT1

The development of targeted chemotherapeutic agents against colorectal cancer (CRC), one of the most common cancers with a high mortality rate, is in a constant need. Nannocystins are a family of myxobacterial secondary metabolites featuring a 21-membered depsipeptide ring. The in vitro anti-CRC activity of natural and synthetic nannocystins was well documented, but little is known about their in vivo efficacy and if positive, the underlying mechanism of action. In this study we synthesized a nitroaromatic nannocystin through improved preparation of a key fragment, and characterized its in vitro activity and in vivo efficacy against CRC. We first described the total synthesis of compounds 2-4 featuring Heck macrocyclization to forge their 21-membered macrocycle. In a panel of 7 cancer cell lines from different tissues, compound 4 inhibited the cell viability with IC values of 1-6 nM. In particular, compound 4 (1, 2, 4 nM) inhibited the proliferation of CRC cell lines (HCT8, HCT116 and LoVo) in both concentration and time dependent manners. Furthermore, compound 4 concentration-dependently inhibited the colony formation and migration of CRC cell lines. Moreover, compound 4 induced cell cycle arrest at sub-G1 phase, apoptosis and cellular senescence in CRC cell lines. In three patient-derived CRC organoids, compound 4 inhibited the PDO with IC values of 3.68, 28.93 and 11.81 nM, respectively. In a patient-derived xenograft mouse model, injection of compound 4 (4, 8 mg/kg, i.p.) every other day for 12 times dose-dependently inhibited the tumor growth without significant change in body weight. We conducted RNA-sequencing, molecular docking and cellular thermal shift assay to elucidate the anti-CRC mechanisms of compound 4, and revealed that it exerted its anti-CRC effect at least in part by targeting AKT1.

Enantioselective Synthesis of the 1,3-Dienyl-5-Alkyl-6-Oxy Motif: Method Development and Total Synthesis

The 1,3-dienyl-5-alkyl-6-oxy motif is widely found in various types of bioactive natural products. However, present synthesis is mainly non-asymmetric which relied upon different olefination or transition metal-catalyzed cross-coupling reactions using enantioenriched precursors. Herein, based upon a newly developed enantioselective α-alkylation of conjugated polyenoic acids, a variety of 1,3-dienyl-5-alkyl-6-oxy motif (with E-configured internal olefin) was generated as the corresponding α-adducts in a highly enantioselective and diastereoselective manner. Utilizing 1,3-dienyl-5-alkyl-6-oxy motif as key intermediates, we further demonstrated their synthetic potential by expedient total syntheses of three types of natural products (glutarimide antibiotics, α-pyrone polyketides and Lupin alkaloids) within 4-7 steps.

Mukaiyama aldol reaction: an effective asymmetric approach to access chiral natural products and their derivatives/analogues

The Mukaiyama aldol reaction is generally a Lewis-acid catalyzed cross-aldol reaction between an aldehyde or ketone and silyl enol ether. It was first described by Mukaiyama in 1973, almost 5 decades ago, to achieve the enantioselective synthesis of β-hydroxy carbonyl compounds in high percentage yields. Mukaiyama aldol adducts play a pivotal role in the synthesis of various naturally occurring and medicinally important organic compounds such as polyketides, alkaloids, macrolides, etc. This review highlights the significance of the Mukaiyama aldol reaction towards the asymmetric synthesis of a wide range of biologically active natural products reported recently (since 2020).

Identification of secondary metabolites containing a diketopiperazine core in extracts from myxobacterial strains with growth inhibition activity against a range of prey species

Myxobacteria produce a variety of bioactive secondary metabolites, and with a wealth of under-researched species they hold vast potential for undiscovered compounds. With the ever-increasing need for new antibiotics, the development of novel therapeutics is vitally important. Therefore, this study aimed to extract and elucidate antimicrobial metabolites from the following myxobacteria: Myxococcus xanthus CA010 and AB022; Corallococcus exiguus DSM 14696T; Myxococcus stipitatus DSM 14675T; and Corallococcus aberystwythensis AB050AT. Metabolite mixtures were extracted in acetone from XAD-16 resin incubated in liquid cultures and analysed using GC-MS. Bioactivity was identified using a growth inhibition assay against a panel of clinically relevant prey species including Gram-positive and Gram-negative bacteria and a fungus. Growth of Klebsiella pneumoniae and Enterococcus faecalis was most affected by the metabolite mixtures and the mixtures from AB022 and AB050AT were effective against the most prey. GC-MS analysis revealed metabolites with roles in the synthesis and degradation of amino acids and fatty acids, but also identified compounds A and B with a diketopiperazine (DKP) core. With previously confirmed bioactivity of compound A, it is suggested that these DKP compounds are contributing to the antimicrobial activity observed. Furthermore, many compounds could not be identified and so these unknowns present further potential for novel bioactive compounds.

Recent advances on cyclodepsipeptides: biologically active compounds for drug research

Cyclodepsipeptides are a large family of peptide-related natural products consisting of hydroxy and amino acids linked by amide and ester bonds. A number of cyclodepsipeptides have been isolated and characterized from fungi and bacteria. Most of them showed antitumor, antifungal, antiviral, antimalarial, and antitrypanosomal properties. Herein, this review summarizes the recent literatures (2010-2022) on the progress of cyclodepsipeptides from fungi and bacteria except for those of marine origin, in order to enrich our knowledge about their structural features and biological sources.

Repurposing approved non-oncology drugs for cancer therapy: a comprehensive review of mechanisms, efficacy, and clinical prospects

Cancer poses a significant global health challenge, with predictions of increasing prevalence in the coming years due to limited prevention, late diagnosis, and inadequate success with current therapies. In addition, the high cost of new anti-cancer drugs creates barriers in meeting the medical needs of cancer patients, especially in developing countries. The lengthy and costly process of developing novel drugs further hinders drug discovery and clinical implementation. Therefore, there has been a growing interest in repurposing approved drugs for other diseases to address the urgent need for effective cancer treatments. The aim of this comprehensive review is to provide an overview of the potential of approved non-oncology drugs as therapeutic options for cancer treatment. These drugs come from various chemotherapeutic classes, including antimalarials, antibiotics, antivirals, anti-inflammatory drugs, and antifungals, and have demonstrated significant antiproliferative, pro-apoptotic, immunomodulatory, and antimetastatic properties. A systematic review of the literature was conducted to identify relevant studies on the repurposing of approved non-oncology drugs for cancer therapy. Various electronic databases, such as PubMed, Scopus, and Google Scholar, were searched using appropriate keywords. Studies focusing on the therapeutic potential, mechanisms of action, efficacy, and clinical prospects of repurposed drugs in cancer treatment were included in the analysis. The review highlights the promising outcomes of repurposing approved non-oncology drugs for cancer therapy. Drugs belonging to different therapeutic classes have demonstrated notable antitumor effects, including inhibiting cell proliferation, promoting apoptosis, modulating the immune response, and suppressing metastasis. These findings suggest the potential of these repurposed drugs as effective therapeutic approaches in cancer treatment. Repurposing approved non-oncology drugs provides a promising strategy for addressing the urgent need for effective and accessible cancer treatments. The diverse classes of repurposed drugs, with their demonstrated antiproliferative, pro-apoptotic, immunomodulatory, and antimetastatic properties, offer new avenues for cancer therapy. Further research and clinical trials are warranted to explore the full potential of these repurposed drugs and optimize their use in treating various cancer types. Repurposing approved drugs can significantly expedite the process of identifying effective treatments and improve patient outcomes in a cost-effective manner.

Myxobacteria: biology and bioactive secondary metabolites

Myxobacteria are Gram-negative eubacteria and they thrive in a variety of habitats including soil rich in organic matter, rotting wood, animal dung and marine environment. Myxobacteria are a promising source of new compounds associated with diverse bioactive spectrum and unique mode of action. The genome information of myxobacteria has revealed many orphan biosynthetic pathways indicating that these bacteria can be the source of several novel natural products. In this review, we highlight the biology of myxobacteria with emphasis on their habitat, life cycle, isolation methods and enlist all the bioactive secondary metabolites purified till date and their mode of action.

Solid-Phase Synthesis of Nannocystin Ax and Its Analogues

Solid-phase total synthesis of nannocystin Ax (1) was disclosed. A coupling reaction between a peptide and a polyketide moiety was conducted on a solid support, and macrocyclization was achieved by Mitsunobu cyclization. The established synthetic route was efficient to prepare its analogues, which contain different types of peptide moieties.

CuII-Catalyzed cis-Selective Synthesis of Ketoepoxides from Phenacyl Bromides and Water

A verity of α,β-ketoepoxides was synthesized using a Cu^II-catalyzed oxidative C-C/O-C coupled cyclization strategy with high yield and cis-selectivity. Water is used as the source of oxygen and phenacyl bromide as the carbon in the valuable epoxides. The self-coupling method was extended to cross-coupling between phenacyl bromides with benzyl bromides. A high cis-diastereoselectivity was observed in all the synthesized ketoepoxides. Control experiments and density functional theory (DFT) study were performed to understand the Cu^II-Cu^I transition mechanism.

Anticancer Small-Molecule Agents Targeting Eukaryotic Elongation Factor 1A: State of the Art

Eukaryotic elongation factor 1A (eEF1A) canonically delivers amino acyl tRNA to the ribosomal A site during the elongation stage of protein biosynthesis. Yet paradoxically, the oncogenic nature of this instrumental protein has long been recognized. Consistently, eEF1A has proven to be targeted by a wide assortment of small molecules with excellent anticancer activity, among which plitidepsin has been granted approval for the treatment of multiple myeloma. Meanwhile, metarrestin is currently under clinical development for metastatic cancers. Bearing these exciting advances in mind, it would be desirable to present a systematic up-to-date account of the title topic, which, to the best of our knowledge, has thus far been unavailable in the literature. The present review summarizes recent advances in eEF1A-targeting anticancer agents, both naturally occurring and synthetically crafted, with regard to their discovery or design, target identification, structure–activity relationship, and mode of action. Their structural diversity and differential eEF1A-targeting mechanisms warrant continuing research in pursuit of curing eEF1A-driven malignancy.

Site-directed late-stage diversification of macrocyclic nannocystins facilitating anticancer SAR and mode of action studies

Nannocystins are a family of 21-membered cyclodepsipeptides with excellent anticancer activity. However, their macrocyclic architecture poses a significant challenge to structure modification. Herein, this issue is addressed by leveraging the strategy of post-macrocyclization diversification. In particular, a novel serine-incorporating nannocystin was designed so that its appending hydroxyl group could diversify into a wide variety of side chain analogues. Such effort facilitated not only structure-activity correlation at the subdomain of interest, but also the development of a macrocyclic coumarin-labeled fluorescence probe. Uptake experiments indicated good cell permeability of the probe, and endoplasmic reticulum was identified as its subcellular localization site.

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

Natural products targeting the elongation phase of eukaryotic protein biosynthesis

Covering: 2000 to 2020 The translation of mRNA into proteins is a precisely regulated, complex process that can be divided into three main stages, i.e. initiation, elongation, termination, and recycling. This contribution is intended to highlight how natural products interfere with the elongation phase of eukaryotic protein biosynthesis. Cycloheximide, isolated from Streptomyces griseus, has long been the prototype inhibitor of eukaryotic translation elongation. In the last three decades, a variety of natural products from different origins were discovered to also address the elongation step in different manners, including interference with the elongation factors eEF1 and eEF2 as well as binding to A-, P- or E-sites of the ribosome itself. Recent advances in the crystallization of the ribosomal machinery together with natural product inhibitors allowed characterizing similarities as well as differences in their mode of action. Since aberrations in protein synthesis are commonly observed in tumors, and malfunction or overexpression of translation factors can cause cellular transformation, the protein synthesis machinery has been realized as an attractive target for anticancer drugs. The therapeutic use of the first natural products that reached market approval, plitidepsin (Aplidin®) and homoharringtonine (Synribo®), will be introduced. In addition, we will highlight two other potential indications for translation elongation inhibitors, i.e. viral infections and genetic disorders caused by premature termination of translation.

Myxobacteria as a Source of New Bioactive Compounds: A Perspective Study

Myxobacteria are unicellular, Gram-negative, soil-dwelling, gliding bacteria that belong to class δ-proteobacteria and order Myxococcales. They grow and proliferate by transverse fission under normal conditions, but form fruiting bodies which contain myxospores during unfavorable conditions. In view of the escalating problem of antibiotic resistance among disease-causing pathogens, it becomes mandatory to search for new antibiotics effective against such pathogens from natural sources. Among the different approaches, Myxobacteria, having a rich armor of secondary metabolites, preferably derivatives of polyketide synthases (PKSs) along with non-ribosomal peptide synthases (NRPSs) and their hybrids, are currently being explored as producers of new antibiotics. The Myxobacterial species are functionally characterized to assess their ability to produce antibacterial, antifungal, anticancer, antimalarial, immunosuppressive, cytotoxic and antioxidative bioactive compounds. In our study, we have found their compounds to be effective against a wide range of pathogens associated with the concurrence of different infectious diseases.

Generation and Biological Evaluation of Degraded Derivatives of the Three E/Z-Isomers of Yaku'amide B

Potently cytostatic yaku'amide B (1) is a highly unsaturated linear tridecapeptide. During our synthetic studies of the E/Z-isomers of the α,β-dehydroisoleucines of 1, an unexpected retro-aldol reaction proceeded to transform E/Z-isomers 2, 3, and 4 into 2a, 3a, and 4a/4b, respectively. Compounds 2a, 3a, and 4a have a glycine at residue-1 instead of β-hydroxyisoleucine, and the β-hydroxyvaline at residue-8 in 4a is further replaced by glycine in 4b. Evaluation of the growth inhibition activities against MCF-7 cells revealed that 4b was approximately 10-fold weaker than the equipotent 2-4 and 2a-4a, demonstrating the biological importance of a bulky side chain at residue-8.

Structure and biosynthesis of sorangipyranone - a new γ-dihydropyrone from the myxobacterial strain MSr12020

Sorangipyranone was isolated as a novel natural product featuring a unique 2,3-dihydro-γ-4H-pyrone scaffold from cultures of the myxobacterial strain MSr12020. We report here the full structure elucidation of sorangipyranone by spectroscopic techniques including 2D NMR and high-resolution mass spectrometry together with the analysis of the biosynthetic pathway. Determination of the absolute configuration was performed by time-dependent density functional theory-electronic circular dichroism calculations and determination of the applicability of the Snatzke's helicity rule, to correlate the high-wavelength n→π* electronic circular dichroism (ECD) transition and the absolute configuration of the 2,3-dihydro-4H-γ-pyrone, was done by the analysis of low-energy conformers and the Kohn-Sham orbitals. Sorangipyranone outlines a new class of a γ-dihydropyrone-containing natural product comprised of malonyl-CoA-derived building blocks and features a unique polyketide scaffold. In silico analysis of the genome sequence of the myxobacterial strain MSr12020 complemented with feeding experiments employing stable isotope-labeled precursors allowed the identification and annotation of a candidate biosynthetic gene cluster that encodes a modular polyketide synthase assembly line. A model for the biosynthetic pathway leading to the formation of the γ-dihydropyrone scaffold is presented in this study.

Natural products as drugs and tools for influencing core processes of eukaryotic mRNA translation

Eukaryotic protein synthesis is the highly conserved, complex mechanism of translating genetic information into proteins. Although this process is essential for cellular homoeostasis, dysregulations are associated with cellular malfunctions and diseases including cancer and diabetes. In the challenging and ongoing search for adequate treatment possibilities, natural products represent excellent research tools and drug leads for new interactions with the translational machinery and for influencing mRNA translation. In this review, bacterial-, marine- and plant-derived natural compounds that interact with different steps of mRNA translation, comprising ribosomal assembly, translation initiation and elongation, are highlighted. Thereby, the exact binding and interacting partners are unveiled in order to accurately understand the mode of action of each natural product. The pharmacological relevance of these compounds is furthermore assessed by evaluating the observed biological activities in the light of translational inhibition and by enlightening potential obstacles and undesired side-effects, e.g. in clinical trials. As many of the natural products presented here possess the potential to serve as drug leads for synthetic derivatives, structural motifs, which are indispensable for both mode of action and biological activities, are discussed. Evaluating the natural products emphasises the strong diversity of their points of attack. Especially the fact that selected binding partners can be set in direct relation to different diseases emphasises the indispensability of natural products in the field of drug development. Discovery of new, unique and unusual interacting partners again renders them promising tools for future research in the field of eukaryotic mRNA translation.

Optimization of Two Steps in Scale-Up Synthesis of Nannocystin A

We have accomplished a 10-step (longest linear) total synthesis of nannocystin A on a four hundred milligram scale. The previously reported Kobayashi vinylogous Mukaiyama aldol reaction to connect C4 and C5 was unreproducible during the scaling up process. A more convenient and cost-efficient Keck asymmetric vinylogous aldol reaction was employed to improve this transformation.

Inhibitors of Eukaryotic Translational Machinery as Therapeutic Agents

Inhibiting eukaryotic protein translation with small molecules is emerging as a powerful therapeutic strategy. The advantage of targeting cellular translational machinery is that it is required for the highly proliferative state of many neoplastic cells, replication of certain viruses, and ultimately the expression of a wide variety of protein targets. Although, this approach has been exploited to develop clinical agents, such as homoharringtonine (HHT, 1), used to treat chronic myeloid leukemia (CML), inhibiting components of the translational machinery is often associated with cytotoxic phenotypes. However, recent studies have demonstrated that certain small molecules can inhibit the translation of specific subsets of proteins, leading to lower cytotoxicity, and opening-up therapeutic opportunities for translation inhibitors to be deployed in indications beyond oncology and infectious disease. This review summarizes efforts to develop inhibitors of the eukaryotic translational machinery as therapeutic agents and highlights emerging opportunities for translation inhibitors in the future.

Heck macrocyclization in natural product total synthesis

Covering: 1981-2020 Heck macrocyclization is a logical extension of the award-winning Mizoroki-Heck reaction. Through covalent linking of two otherwise discrete coupling partners, the resultant chimeric substrate is transformed into a large ring with enhanced rigidity and unique functional group disposition. Pioneered in the early 1980s, this methodology has evolved into a competent option for creating diverse macrocycles. Despite its growing influence, hitherto no systematic survey has ever appeared in the literature. The present review delineates the state-of-the-art of Heck macrocyclization in the context of natural product synthesis. Sixteen selected cases, each examined from a different perspective, coalesce into the view that the title reaction is a viable tool for synthesis-enabled macrocycle research.

From Target-Oriented to Motif-Oriented: A Case Study on Nannocystin Total Synthesis

Natural product total synthesis is in essence target-oriented in that a set of organic transformations are orchestrated into a workable process, leading ultimately to the target molecule with a predefined architecture. For a bioactive lead, proof of synthetic viability is merely the beginning. Ensuing effort repurposes the initial synthesis for structural diversification in order to probe structure-activity relationship (SAR). Yet accessibility is not equal to flexibility; moving from convergency to divergency, it is not always feasible to explore the chemical space around a particular substructure of interest simply by tweaking an established route. In this situation, the motif-oriented strategy becomes a superior choice, which gives priority to synthetic flexibility at the concerned site such that a route is adopted only if it is capable of implementing diversification therein. This strategy was recently devised by Fürstner et al., enabling them to achieve total synthesis of both natural and non-natural nannocystins varied at an otherwise challenging position. The present review examines seven distinctive nannocystin total syntheses reported thus far and showcases the merits of conventional (target-oriented) as well as motif-oriented strategies, concluding that these two approaches complement each other and are both indispensable for natural product based drug discovery.

Repurposing of drugs: An attractive pharmacological strategy for cancer therapeutics

Human malignancies are one of the major health-related issues though out the world and anticipated to rise in the future. The development of novel drugs/agents requires a huge amount of cost and time that represents a major challenge for drug discovery. In the last three decades, the number of FDA approved drugs has dropped down and this led to increasing interest in drug reposition or repurposing. The present review focuses on recent concepts and therapeutic opportunities for the utilization of antidiabetics, antibiotics, antifungal, anti-inflammatory, antipsychotic, PDE inhibitors and estrogen receptor antagonist, Antabuse, antiparasitic and cardiovascular agents/drugs as an alternative approach against human malignancies. The repurposing of approved non-cancerous drugs is an effective strategy to develop new therapeutic options for the treatment of cancer patients at an affordable cost in clinics. In the current scenario, most of the countries throughout the globe are unable to meet the medical needs of cancer patients because of the high cost of the available cancerous drugs. Some of these drugs displayed potential anti-cancer activity in preclinic and clinical studies by regulating several key molecular mechanisms and oncogenic pathways in human malignancies. The emerging pieces of evidence indicate that repurposing of drugs is crucial to the faster and cheaper discovery of anti-cancerous drugs.

A Decade of Antifungal Leads from Natural Products: 2010-2019

In this review, we discuss novel natural products discovered within the last decade that are reported to have antifungal activity against pathogenic species. Nearly a hundred natural products were identified that originate from bacteria, algae, fungi, sponges, and plants. Fungi were the most prolific source of antifungal compounds discovered during the period of review. The structural diversity of these antifungal leads encompasses all the major classes of natural products including polyketides, shikimate metabolites, terpenoids, alkaloids, and peptides.

An Overview of Saturated Cyclic Ethers: Biological Profiles and Synthetic Strategies

Saturated oxygen heterocycles are widely found in a broad array of natural products and other biologically active molecules. In medicinal chemistry, small and medium rings are also important synthetic intermediates since they can undergo ring-opening and -expansion reactions. These applications have driven numerous studies on the synthesis of oxygen-containing heterocycles and considerable effort has been devoted toward the development of methods for the construction of saturated oxygen heterocycles. This paper provides an overview of the biological roles and synthetic strategies of saturated cyclic ethers, covering some of the most studied and newly discovered related natural products in recent years. This paper also reports several promising and newly developed synthetic methods, emphasizing 3-7 membered rings.

(Z)-α-Boryl-crotylboron reagents via Z-selective alkene isomerization and application to stereoselective syntheses of (E)-δ-boryl-syn-homoallylic alcohols

Stereoselective synthesis of (Z)-α-boryl-crotylboronate is developed. Ni-catalyzed Z-selective alkene isomerization of α-boryl substituted homoallylboronate provided the targeted (Z)-crotylboronate with high selectivity. Stereoselective addition of the novel crotylboron reagent to aldehydes gave (E)-δ-boryl-substituted syn-homoallylic alcohols with excellent diastereoselectivities. The vinyl boronate unit in the products can be directly used for a subsequent C-C bond-forming transformation as illustrated in the synthesis of the C1-7 fragment of the natural products nannocystin A and nannocystin Ax.

Myxobacterial natural products: An under-valued source of products for drug discovery for neurological disorders

Age-related disorders impose noticeable financial and emotional burdens on society. This impact is becoming more prevalent with the increasing incidence of neurodegenerative diseases and is causing critical concerns for treatment of patients worldwide. Parkinson's disease, Alzheimer's disease, multiple sclerosis and motor neuron disease are the most prevalent and the most expensive to treat neurodegenerative diseases globally. Therefore, exploring effective therapies to overcome these disorders is a necessity. Natural products and their derivatives have increasingly attracted attention in drug discovery programs that have identified microorganisms which produce a large range of metabolites with bioactive properties. Myxobacteria, a group of Gram-negative bacteria with large genome size, produce a wide range of secondary metabolites with significant chemical structures and a variety of biological effects. They are potent natural product producers. In this review paper, we attempt to overview some secondary metabolites synthesized by myxobacteria with neuroprotective activity through known mechanisms including production of polyunsaturated fatty acids, reduction of apoptosis, immunomodulation, stress reduction of endoplasmic reticulum, stabilization of microtubules, enzyme inhibition and serotonin receptor modulation.

Negishi cross-couplings in the synthesis of amino acids

The Negishi cross-coupling is a powerful C-C bond-forming reaction widely utilised in many areas of organic synthesis. This review details the use of Negishi cross-couplings in the synthesis of unnatural amino acids. The application of this reaction in the preparation of aromatic, heteroaromatic, and, complex amino acid derivatives are reviewed and presented herein.

Antimicrobial discovery inspired by ecological interactions

Bacteria represent an unparalleled source of antibiotics used to treat infectious diseases. Yet, genome analyses have revealed that their full biosynthetic potential is much larger than expected. Valuable strategies to unearth hidden antibiotics are genome mining, pathway engineering and triggering, as well as co-cultivation approaches. Nevertheless, there is growing understanding that it is often essential to consider the ecological context and that there is a great potential for antimicrobial discovery from bacteria engaged in well-defined interactions with other organisms. Various ecological scenarios involving antimicrobial agents are outlined in this review: predator-prey and pathogenic interactions, the protection of insect assets such as offspring and cultivars, as well as host protection in symbiotic relationships with plants, invertebrates and animals/humans. The illustrative examples given reinforce the idea that examination of interactions between organisms can yield new antimicrobial compounds, and ultimately further our understanding of the function of these molecules in the environment.

Asymmetric Total Synthesis of Nannocystin A

Nannocystin A is a novel 21-membered macrolactone isolated from myxobacterium Nanocystis sp. It is a potent elongation factor 1 inhibitor and inhibits cancer cell line growth at nanomolar concentrations. In this work, a concise asymmetric total synthesis of nannocystin A has been developed, which features Sharpless epoxidation, Stille coupling, and final macrolactamization.

Bioactive Peptide Natural Products as Lead Structures for Medicinal Use

The need for new drugs for the treatment of various diseases is enormous. From the previous century until the present, numerous peptide and peptide-derived natural products have been isolated from bacteria and fungi. Hence, microorganisms play a pivotal role as sources for novel drugs with an emphasis on anti-infective agents. Various disciplines from biology, chemistry, and medicine are involved in early stages of the search for peptide natural products including taxonomy, microbiology, bioanalytics, bioinformatics, and medicinal chemistry. Under biochemical aspects, small peptide drugs are basically either ribosomally synthesized and post-translationally modified (RiPPs) or synthesized by multimodular nonribosomal peptide synthetases (NRPSs). Within the context of current developments on bioactive peptide natural products, this Account predominantly highlights recent discoveries, approaches, and research from our laboratory on RiPPs and NRPSs from bacteria and fungi. In our search for peptides showing bioactivities of interest, different approaches were applied: classical screening, in silico prediction, in vitro reconstitution, site-directed mutagenesis, chemoenzymatics, heterologous expression, and total synthesis including structure-activity relationship (SAR) studies in the research on the labyrinthopeptins, albicidin, and the cyclodepsipeptides (CDPs). The ribosomally synthesized labyrinthopeptins, class III lanthipeptides, which have been discovered in a classical screening campaign, display highly attractive antiallodynic (against neuropathic pain caused by dysfunction of the nervous system) and antiviral activities. Therefore, the biosynthetic assembly was investigated by extensive enzymatic studies of the modifying enzymes, and site-directed mutagenesis was performed for the generation of analogs. By genome mining, other class III lanthipeptides have been uncovered, while synthetic access proved to be an unmet challenge for the labyrinthopeptins. In contrast, for the gyrase inhibitor albicidin, the establishment of a chemical synthesis followed by medicinal chemistry studies was the only viable option to gain access to derivatives. Albicidin, which has been discovered investigating plant host-pathogen interactions, has a strong activity against Gram-negative bacteria, for example, Escherichia coli and Pseudomonas aeruginosa, and a future synthetic derivative may become a lead structure for development of an anti-Gram-negative drug. The compound class of the cyclodepsipeptides contributes already two marketed drugs, enniatin (fusafungine) and emodepside. Cyclodepsipeptides show general antibacterial and antifungal effects, whereas specific insecticidal and anthelmintic activities provide lead structures for drug development. Hence, exploiting the chances of reprogramming NRPSs, the generation of chimeric or otherwise designed synthetases could render a new untapped structural space and thus novel bioactivities. While current developments in the fields of genomics, bioinformatics, and molecular biology facilitate the search for new natural products and the design of new peptide structures, the next decade will show which compounds have been carried on further applications and whether current developments have led to an increase in drug candidates.

Antibiotics from Gram-negative bacteria: a comprehensive overview and selected biosynthetic highlights

Covering: up to 2017The overwhelming majority of antibiotics in clinical use originate from Gram-positive Actinobacteria. In recent years, however, Gram-negative bacteria have become increasingly recognised as a rich yet underexplored source of novel antimicrobials, with the potential to combat the looming health threat posed by antibiotic resistance. In this article, we have compiled a comprehensive list of natural products with antimicrobial activity from Gram-negative bacteria, including information on their biosynthetic origin(s) and molecular target(s), where known. We also provide a detailed discussion of several unusual pathways for antibiotic biosynthesis in Gram-negative bacteria, serving to highlight the exceptional biocatalytic repertoire of this group of microorganisms.