Enzymatic basis of "hybridity" in thiomarinol biosynthesis

Enzymatic Strategies for the Biosynthesis of N-Acyl Amino Acid Amides

Amide bond-containing biomolecules are functionally significant and useful compounds with diverse applications. For example, N-acyl amino acids (NAAAs) are an important class of lipoamino acid amides with extensive use in food, cosmetic and pharmaceutical industries. Their conventional chemical synthesis involves the use of toxic chlorinating agents for carboxylic acid activation. Enzyme-catalyzed biotransformation for the green synthesis of these amides is therefore highly desirable. Here, we review a range of enzymes suitable for the synthesis of NAAA amides and their strategies adopted in carboxylic acid activation. Generally, ATP-dependent enzymes for NAAA biosynthesis are acyl-adenylating enzymes that couple the hydrolysis of phosphoanhydride bond in ATP with the formation of an acyl-adenylate intermediate. In contrast, ATP-independent enzymes involve hydrolases such as lipases or aminoacylases, which rely on the transient activation of the carboxylic acid. This occurs either through an acyl-enzyme intermediate or by favorable interactions with surrounding residues to anchor the acyl donor in a suitable orientation for the incoming amine nucleophile. Recently, the development of an alternative pathway involving ester-amide interconversion has unraveled another possible strategy for amide formation through esterification-aminolysis cascade reactions, potentially expanding the substrate scope for enzymes to catalyze the synthesis of a diverse range of NAAA amides.

The Hybrid Antibiotic Thiomarinol A Overcomes Intrinsic Resistance in Escherichia coli Using a Privileged Dithiolopyrrolone Moiety

An impermeable outer membrane and multidrug efflux pumps work in concert to provide Gram-negative bacteria with intrinsic resistance against many antibiotics. These resistance mechanisms reduce the intracellular concentrations of antibiotics and render them ineffective. The natural product thiomarinol A combines holothin, a dithiolopyrrolone antibiotic, with marinolic acid A, a close analogue of mupirocin. The hybridity of thiomarinol A converts the mupirocin scaffold from inhibiting Gram-positive bacteria to inhibiting both Gram-positive and -negative bacteria. We found that thiomarinol A accumulates significantly more than mupirocin within the Gram-negative bacterium Escherichia coli, likely contributing to its broad-spectrum activity. Antibiotic susceptibility testing of E. coli mutants reveals that thiomarinol A overcomes the intrinsic resistance mechanisms that render mupirocin inactive. Structure-activity relationship studies suggest that the dithiolopyrrolone is a privileged moiety for improving the accumulation and antibiotic activity of the mupirocin scaffold without compromising binding to isoleucyl-tRNA synthetase. These studies also highlight that accumulation is required but not sufficient for antibiotic activity. Our work reveals a role of the dithiolopyrrolone moiety in overcoming intrinsic mupirocin resistance in E. coli and provides a starting point for designing dual-acting and high-accumulating hybrid antibiotics.

A Cysteine-Reloading Process Initiating the Biosynthesis of the Bicyclic Scaffold of Dithiolopyrrolones

Dithiolopyrrolone antibiotics are well known for their outstanding biological activities, and their biosynthesis has been studied vigorously. However, the biosynthesis mechanism of the characteristic bicyclic scaffold is still unknown after years of research. To uncover this mechanism, a multi-domain non-ribosomal peptide synthase DtpB from the biosynthetic gene cluster of thiolutin was selected as an object to study. We discovered that its adenylation domain not only recognized and adenylated cysteine, but also played an essential role in the formation of the peptide bond. Notably, an eight-membered ring compound was also discovered as an intermediate during the formation of the bicyclic structure. Based on these findings, we propose a new mechanism for the biosynthesis of the bicyclic scaffold of dithiolopyrrolones, and unveil additional functions of the adenylation domain.

Chemical Evolution of Natural Product Structure

Natural products are the result of Nature’s exploration of biologically relevant chemical space through evolution and an invaluable source of bioactive small molecules for chemical biology and medicinal chemistry. Novel concepts for the discovery of new bioactive compound classes based on natural product structure may enable exploration of wider biologically relevant chemical space. The pseudo-natural product concept merges the relevance of natural product structure with efficient exploration of chemical space by means of fragment-based compound development to inspire the discovery of new bioactive chemical matter through de novo combination of natural product fragments in unprecedented arrangements. The novel scaffolds retain the biological relevance of natural products but are not obtainable through known biosynthetic pathways which can lead to new chemotypes that may have unexpected or unprecedented bioactivities. Herein, we cover the workflow of pseudo-natural product design and development, highlight recent examples, and discuss a cheminformatic analysis in which a significant portion of biologically active synthetic compounds were found to be pseudo-natural products. We compare the concept to natural evolution and discuss pseudo-natural products as the human-made equivalent, i.e. the chemical evolution of natural product structure.

Inhibition of Isoleucyl-tRNA Synthetase by the Hybrid Antibiotic Thiomarinol

Hybrid antibiotics are an emerging antimicrobial strategy to overcome antibiotic resistance. The natural product thiomarinol A is a hybrid of two antibiotics: holothin, a dithiolopyrrolone (DTP), and marinolic acid, a close analogue of the drug mupirocin that is used to treat methicillin-resistant Staphylococcus aureus (MRSA). DTPs disrupt metal homeostasis by chelating metal ions in cells, whereas mupirocin targets the essential enzyme isoleucyl-tRNA synthetase (IleRS). Thiomarinol A is over 100-fold more potent than mupirocin against mupirocin-sensitive MRSA; however, its mode of action has been unknown. We show that thiomarinol A targets IleRS. A knockdown of the IleRS-encoding gene, ileS, exhibited sensitivity to a synthetic analogue of thiomarinol A in a chemical genomics screen. Thiomarinol A inhibits MRSA IleRS with a picomolar K_i and binds to IleRS with low femtomolar affinity, 1600 times more tightly than mupirocin. We find that thiomarinol A remains effective against high-level mupirocin-resistant MRSA and provide evidence to support a dual mode of action for thiomarinol A that may include both IleRS inhibition and metal chelation. We demonstrate that MRSA develops resistance to thiomarinol A to a substantially lesser degree than mupirocin and the potent activity of thiomarinol A requires hybridity between DTP and mupirocin. Our findings identify a mode of action of a natural hybrid antibiotic and demonstrate the potential of hybrid antibiotics to combat antibiotic resistance.

Recent highlights of biosynthetic studies on marine natural products

Marine bacteria are excellent yet often underexplored sources of structurally unique bioactive natural products. In this review we cover the diversity of marine bacterial biomolecules and highlight recent studies on structurally novel natural products. We include different compound classes and discuss the latest progress related to their biosynthetic pathway analysis and engineering: examples range from fatty acids over terpenes to PKS, NRPS and hybrid PKS-NRPS biomolecules.

Bacterial pathogens: threat or treat (a review on bioactive natural products from bacterial pathogens)

Covering: up to the second quarter of 2020 Threat or treat? While pathogenic bacteria pose significant threats, they also represent a huge reservoir of potential pharmaceuticals to treat various diseases. The alarming antimicrobial resistance crisis and the dwindling clinical pipeline urgently call for the discovery and development of new antibiotics. Pathogenic bacteria have an enormous potential for natural products drug discovery, yet they remained untapped and understudied. Herein, we review the specialised metabolites isolated from entomopathogenic, phytopathogenic, and human pathogenic bacteria with antibacterial and antifungal activities, highlighting those currently in pre-clinical trials or with potential for drug development. Selected unusual biosynthetic pathways, the key roles they play (where known) in various ecological niches are described. We also provide an overview of the mode of action (molecular target), activity, and minimum inhibitory concentration (MIC) towards bacteria and fungi. The exploitation of pathogenic bacteria as a rich source of antimicrobials, combined with the recent advances in genomics and natural products research methodology, could pave the way for a new golden age of antibiotic discovery. This review should serve as a compendium to communities of medicinal chemists, organic chemists, natural product chemists, biochemists, clinical researchers, and many others interested in the subject.

Genome Mining of Oxidation Modules in trans-Acyltransferase Polyketide Synthases Reveals a Culturable Source for Lobatamides

Bacterial trans-acyltransferase polyketide synthases (trans-AT PKSs) are multimodular megaenzymes that biosynthesize many bioactive natural products. They contain a remarkable range of domains and module types that introduce different substituents into growing polyketide chains. As one such modification, we recently reported Baeyer-Villiger-type oxygen insertion into nascent polyketide backbones, thereby generating malonyl thioester intermediates. In this work, genome mining focusing on architecturally diverse oxidation modules in trans-AT PKSs led us to the culturable plant symbiont Gynuella sunshinyii, which harbors two distinct modules in one orphan PKS. The PKS product was revealed to be lobatamide A, a potent cytotoxin previously only known from a marine tunicate. Biochemical studies show that one module generates glycolyl thioester intermediates, while the other is proposed to be involved in oxime formation. The data suggest varied roles of oxygenation modules in the biosynthesis of polyketide scaffolds and support the importance of trans-AT PKSs in the specialized metabolism of symbiotic bacteria.

Biocatalytic Synthesis of Moclobemide Using the Amide Bond Synthetase McbA Coupled with an ATP Recycling System

The biocatalytic synthesis of amides from carboxylic acids and primary amines in aqueous media can be achieved using the ATP-dependent amide bond synthetase McbA, via an adenylate intermediate, using only 1.5 equiv of the amine nucleophile. Following earlier studies that characterized the broad carboxylic acid specificity of McbA, we now show that, in addition to the natural amine substrate 2-phenylethylamine, a range of simple aliphatic amines, including methylamine, butylamine, and hexylamine, and propargylamine are coupled efficiently to the native carboxylic acid substrate 1-acetyl-9H-β-carboline-3-carboxylic acid by the enzyme, to give amide products with up to >99% conversion. The structure of wild-type McbA in its amidation conformation, coupled with modeling and mutational studies, reveal an amine access tunnel and a possible role for residue D201 in amine activation. Amide couplings were slower with anilines and alicyclic secondary amines such as pyrrolidine and piperidine. The broader substrate specificity of McbA was exploited in the synthesis of the monoamine oxidase A inhibitor moclobemide, through the reaction of 4-chlorobenzoic acid with 1.5 equiv of 4-(2-aminoethyl)morpholine, and utilizing polyphosphate kinases SmPPK and AjPPK in the presence of polyphosphoric acid and 0.1 equiv of ATP, required for recycling of the cofactor.

The Isolation of Pyrroloformamide Congeners and Characterization of Their Biosynthetic Gene Cluster

Dithiolopyrrolones are microbial natural products containing a disulfide or thiosulfonate bridge embedded in a unique bicyclic structure. By interfering with zinc ion homeostasis in living cells, they show strong antibacterial activity against a variety of bacterial pathogens, as well as potent cytotoxicity against human cancer cells. In the current study, two new dithiolopyrrolones, pyrroloformamide C (3) and pyrroloformamide D (4), were isolated from Streptomyces sp. CB02980, together with the known pyrroloformamides 1 and 2. The biosynthetic gene cluster for pyrroloformamides was identified from Streptomyces sp. CB02980, which shared high sequence similarity with those of dithiolopyrrolones, including holomycin and thiolutin. Gene replacement of pyfE, which encodes a nonribosomal peptide synthetase (NRPS), abolished the production of 1-4. Overexpression of pyfN, a type II thioesterase gene, increased the production of 1 and 2. Genome neighborhood network analysis of the characterized and orphan gene clusters of dithiolopyrrolones revealed a unified mechanism for their biosynthesis, involving an iterative-acting NRPS and a set of conserved tailoring enzymes for the bicyclic core formation.

Crossroads of Antibiotic Resistance and Biosynthesis

The biosynthesis of antibiotics and self-protection mechanisms employed by antibiotic producers are an integral part of the growing antibiotic resistance threat. The origins of clinically relevant antibiotic resistance genes found in human pathogens have been traced to ancient microbial producers of antibiotics in natural environments. Widespread and frequent antibiotic use amplifies environmental pools of antibiotic resistance genes and increases the likelihood for the selection of a resistance event in human pathogens. This perspective will provide an overview of the origins of antibiotic resistance to highlight the crossroads of antibiotic biosynthesis and producer self-protection that result in clinically relevant resistance mechanisms. Some case studies of synergistic antibiotic combinations, adjuvants, and hybrid antibiotics will also be presented to show how native antibiotic producers manage the emergence of antibiotic resistance.

Defining the genes for the final steps in biosynthesis of the complex polyketide antibiotic mupirocin by Pseudomonas fluorescens NCIMB10586

The mupirocin trans-AT polyketide synthase pathway, provides a model system for manipulation of antibiotic biosynthesis. Its final phase involves removal of the tertiary hydroxyl group from pseudomonic acid B, PA-B, producing the fully active PA-A in a complex series of steps. To further clarify requirements for this conversion, we fed extracts containing PA-B to mutants of the producer strain singly deficient in each mup gene. This additionally identified mupM and mupN as required plus the sequence but not enzymic activity of mupL and ruled out need for other mup genes. A plasmid expressing mupLMNOPVCFU + macpE together with a derivative of the producer P. fluorescens strain NCIMB10586 lacking the mup cluster allowed conversion of PA-B to PA-A. MupN converts apo-mAcpE to holo-form while MupM is a mupirocin-resistant isoleucyl tRNA synthase, preventing self-poisoning. Surprisingly, the expression plasmid failed to allow the closely related P. fluorescens strain SBW25 to convert PA-B to PA-A.

Culturing marine bacteria from the genus Pseudoalteromonas on a cotton scaffold alters secondary metabolite production

The discovery of secondary metabolites from marine microorganisms is beset by numerous challenges including difficulties cultivating and subsequently eliciting expression of biosynthetic genes from marine microbes in the laboratory. In this paper, we describe a method of culturing three species from the marine bacterial genus Pseudoalteromonas using cotton scaffold supplemented liquid media. This simple cultivation method was designed to mimic the natural behavior of some members of the genus wherein they form epibiotic/symbiotic associations with higher organisms such as sponges and corals or attach to solid structures as a biofilm. Our scaffolded cultivation is highly effective at stimulating an attachment/biofilm phenotype and causes large changes to metabolite profiles for the microbes investigated. Metabolite changes include alteration to the production levels of known molecules such as violacein, thiomarinol A, and the alterochromide and prodiginine families of molecules. Finally and critically, our technique stimulates the production of unknown compounds that will serve as leads for future natural product discovery. These results suggest our cultivation approach could potentially be used as a general strategy for the activation of silent gene clusters in marine microbes to facilitate access to their full natural product biosynthetic capacity.

Investigating the Biosynthesis of Natural Products from Marine Proteobacteria: A Survey of Molecules and Strategies

The phylum proteobacteria contains a wide array of Gram-negative marine bacteria. With recent advances in genomic sequencing, genome analysis, and analytical chemistry techniques, a whole host of information is being revealed about the primary and secondary metabolism of marine proteobacteria. This has led to the discovery of a growing number of medically relevant natural products, including novel leads for the treatment of multidrug-resistant Staphylococcus aureus (MRSA) and cancer. Of equal interest, marine proteobacteria produce natural products whose structure and biosynthetic mechanisms differ from those of their terrestrial and actinobacterial counterparts. Notable features of secondary metabolites produced by marine proteobacteria include halogenation, sulfur-containing heterocycles, non-ribosomal peptides, and polyketides with unusual biosynthetic logic. As advances are made in the technology associated with functional genomics, such as computational sequence analysis, targeted DNA manipulation, and heterologous expression, it has become easier to probe the mechanisms for natural product biosynthesis. This review will focus on genomics driven approaches to understanding the biosynthetic mechanisms for natural products produced by marine proteobacteria.

Unified Biosynthetic Origin of the Benzodipyrrole Subunits in CC-1065

CC-1065 is the first characterized member of a family of naturally occurring antibiotics including yatakemycin and duocarmycins with exceptionally potent antitumor activity. CC-1065 contains three benzodipyrroles (1a-, 1b-, and 1c-) of which the 1a-subunit is remarkable by being composed of a cyclopropane ring, and the mechanism for the biological formation of benzodipyrrole rings remains elusive. Previously, biosynthetic studies of CC-1065 were limited to radioactively labeled precursor feeding experiments, which showed that tyrosine (Tyr) and serine (Ser) were incorporated into the two benzodipyrrole (1b- and 1c-) subunits via the same mode but that this was different from the key cyclopropabenzodipyrrole (1a-) subunit with N1-C2-C3 derived from Ser. Herein, the biosynthetic gene cluster of CC-1065 has been cloned, analyzed, and characterized by a series of gene inactivations. Significantly, a key intermediate bearing a C7-OH group derived from a Δc10C mutant exhibited improved cytotoxicity. Moreover, this data inspired us to suspect that the 1a-subunit might employ the same precursor incorporation mode as the 1b- and 1c-subunits. Subsequently, ¹³C-labeled Tyr feeding experiments confirmed that the N1-C2-C3 is originated from Tyr via DOPA as an intermediate. Collectively, a biosynthetic pathway of benzodipyrrole is proposed featuring a revised and unified precursor incorporation mode, which implicates an oxidative cyclization strategy for the assembly of benzodipyrrole. This work sets the stage for further study of enzymatic mechanisms and combinatorial biosynthesis for new DNA alkylating analogues.

Bifunctional antimicrobial conjugates and hybrid antimicrobials

Covering: up to the end of 2016Novel antimicrobial drugs are continuously needed to counteract bacterial resistance development. An innovative molecular design strategy for novel antibiotic drugs is based on the hybridization of an antibiotic with a second functional entity. Such conjugates can be grouped into two major categories. In the first category (antimicrobial hybrids), both functional elements of the hybrid exert antimicrobial activity. Due to the dual targeting, resistance development can be significantly impaired, the pharmacokinetic properties can be superior compared to combination therapies with the single antibiotics, and the antibacterial potency is often enhanced in a synergistic manner. In the second category (antimicrobial conjugates), one functional moiety controls the accumulation of the other part of the conjugate, e.g. by mediating an active transport into the bacterial cell or blocking the efflux. This approach is mostly applied to translocate compounds across the cell envelope of Gram-negative bacteria through membrane-embedded transporters (e.g. siderophore transporters) that provide nutrition and signalling compounds to the cell. Such 'Trojan Horse' approaches can expand the antibacterial activity of compounds against Gram-negative pathogens, or offer new options for natural products that could not be developed as standalone antibiotics, e.g. due to their toxicity.

Role for dithiolopyrrolones in disrupting bacterial metal homeostasis

Natural products harbor unique and complex structures that provide valuable antibiotic scaffolds. With an increase in antibiotic resistance, natural products once again hold promise for new antimicrobial therapies, especially those with unique scaffolds that have been overlooked due to a lack of understanding of how they function. Dithiolopyrrolones (DTPs) are an underexplored class of disulfide-containing natural products, which exhibit potent antimicrobial activities against multidrug-resistant pathogens. DTPs were thought to target RNA polymerase, but conflicting observations leave the mechanisms elusive. Using a chemical genomics screen in Escherichia coli, we uncover a mode of action for DTPs-the disruption of metal homeostasis. We show that holomycin, a prototypical DTP, is reductively activated, and reduced holomycin chelates zinc with high affinity. Examination of reduced holomycin against zinc-dependent metalloenzymes revealed that it inhibits E. coli class II fructose bisphosphate aldolase, but not RNA polymerase. Reduced holomycin also strongly inhibits metallo-β-lactamases in vitro, major contributors to clinical carbapenem resistance, by removing active site zinc. These results indicate that holomycin is an intracellular metal-chelating antibiotic that inhibits a subset of metalloenzymes and that RNA polymerase is unlikely to be the primary target. Our work establishes a link between the chemical structures of DTPs and their antimicrobial action; the ene-dithiol group of DTPs enables high-affinity metal binding as a central mechanism to inhibit metabolic processes. Our study also validates the use of chemical genomics in characterizing modes of actions of antibiotics and emphasizes the potential of metal-chelating natural products in antimicrobial therapy.

Selected Mutations Reveal New Intermediates in the Biosynthesis of Mupirocin and the Thiomarinol Antibiotics

Thiomarinol and mupirocin are assembled on similar polyketide/fatty acid backbones and exhibit potent antibiotic activity against methicillin-resistant Staphylococcus aureus (MRSA). They both contain a tetrasubstituted tetrahydropyran (THP) ring that is essential for biological activity. Mupirocin is a mixture of pseudomonic acids (PAs). Isolation of the novel compound mupirocin P, which contains a 7-hydroxy-6-keto-substituted THP, from a ΔmupP strain and chemical complementation experiments confirm that the first step in the conversion of PA-B into the major product PA-A is oxidation at the C6 position. In addition, nine novel thiomarinol (TM) derivatives with different oxidation patterns decorating the central THP core were isolated after gene deletion (tmlF). These metabolites are in accord with the THP ring formation and elaboration in thiomarinol following a similar order to that found in mupirocin biosynthesis, despite the lack of some of the equivalent genes. Novel mupirocin-thiomarinol hybrids were also synthesized by mutasynthesis.

Biosynthesis of polyketides by trans-AT polyketide synthases

This review discusses the biosynthesis of natural products that are generated bytrans-AT polyketide synthases, a family of catalytically versatile enzymes that represents one of the major group of proteins involved in the production of bioactive polyketides.