Diisonitrile Natural Product SF2768 Functions As a Chalkophore That Mediates Copper Acquisition in Streptomyces thioluteus

Analysis of Co-localized Biosynthetic Gene Clusters Identifies a Membrane-Permeabilizing Natural Product

Combination therapy is an effective strategy to combat antibiotic resistance. Multiple synergistic antimicrobial combinations are produced by enzymes encoded in biosynthetic gene clusters (BGCs) that co-localize on the bacterial genome. This phenomenon led to the hypothesis that mining co-localized BGCs will reveal new synergistic combinations of natural products. Here, we bioinformatically identified 38 pairs of co-localized BGCs, which we predict to produce natural products that are related to known compounds, including polycyclic tetramate macrolactams (PoTeMs). We further showed that ikarugamycin, a PoTeM, increases the membrane permeability of Acinetobacter baumannii and Staphylococcus aureus, which suggests that ikarugamycin might be an adjuvant that facilitates the entry of other natural products. Our work outlines a promising avenue to discover synergistic combinations of natural products by mining bacterial genomes.

Biosynthesis of isonitrile lipopeptides

Isonitrile lipopeptides discovered from Actinobacteria have attracted wide attention due to their fascinating biosynthetic pathways and relevance to the virulence of many human pathogens including Mycobacterium tuberculosis. Specifically, the identification of the new class of isonitrile-forming enzymes that belong to non-heme iron (II) and α-ketoglutarate dependent dioxygenases has intrigued several research groups to investigate their catalytic mechanism. Here we summarize the recent studies on the biosynthesis of isonitrile lipopeptides from Streptomyces and Mycobacterium. The latest research on the core and tailoring enzymes involved in the pathway as well as the isonitrile metabolic enzymes are discussed in this review.

Chalkophomycin Biosynthesis Revealing Unique Enzyme Architecture for a Hybrid Nonribosomal Peptide Synthetase and Polyketide Synthase

Chalkophomycin is a novel chalkophore with antibiotic activities isolated from Streptomyces sp. CB00271, while its potential in studying cellular copper homeostasis makes it an important probe and drug lead. The constellation of N-hydroxylpyrrole, 2H-oxazoline, diazeniumdiolate, and methoxypyrrolinone functional groups into one compact molecular architecture capable of coordinating cupric ions draws interest to unprecedented enzymology responsible for chalkophomycin biosynthesis. To elucidate the biosynthetic machinery for chalkophomycin production, the chm biosynthetic gene cluster from S. sp. CB00271 was identified, and its involvement in chalkophomycin biosynthesis was confirmed by gene replacement. The chm cluster was localized to a ~31 kb DNA region, consisting of 19 open reading frames that encode five nonribosomal peptide synthetases (ChmHIJLO), one modular polyketide synthase (ChmP), six tailoring enzymes (ChmFGMNQR), two regulatory proteins (ChmAB), and four resistance proteins (ChmA'CDE). A model for chalkophomycin biosynthesis is proposed based on functional assignments from sequence analysis and structure modelling, and is further supported by analogy to over 100 chm-type gene clusters in public databases. Our studies thus set the stage to fully investigate chalkophomycin biosynthesis and to engineer chalkophomycin analogues through a synthetic biology approach.

Chalkophore mediated respiratory oxidase flexibility controls M. tuberculosis virulence

Oxidative phosphorylation has emerged as a critical therapeutic vulnerability of M. tuberculosis, but it is unknown how M. tuberculosis and other pathogens maintain respiration during infection. M. tuberculosis synthesizes diisonitrile lipopeptide chalkophores that chelate copper tightly, but their role in host-pathogen interactions is also unknown. We demonstrate that M. tuberculosis chalkophores maintain the function of the heme-copper bcc:aa3 respiratory oxidase under copper limitation. Chalkophore deficient M. tuberculosis cannot survive, respire to oxygen, or produce ATP under copper deprivation in culture. M. tuberculosis lacking chalkophore biosynthesis is attenuated in mice, a phenotype that is severely exacerbated by loss of the CytBD alternative respiratory oxidase (encoded by cydAB), revealing a multilayered flexibility of the respiratory chain that maintains oxidative phosphorylation during infection. Taken together, these data demonstrate that chalkophores counter host inflicted copper deprivation and highlight that protection of cellular respiration is a critical virulence function in M. tuberculosis.

Methodology for awakening the potential secondary metabolic capacity in actinomycetes

Secondary metabolites produced by actinomycete strains undoubtedly have great potential for use in applied research areas such as drug discovery. However, it is becoming difficult to obtain novel compounds because of repeated isolation around the world. Therefore, a new strategy for discovering novel secondary metabolites is needed. Many researchers believe that actinomycetes have as yet unanalyzed secondary metabolic activities, and the associated undiscovered secondary metabolite biosynthesis genes are called "silent" genes. This review outlines several approaches to further activate the metabolic potential of actinomycetes.

Variation in biosynthesis and metal-binding properties of isonitrile-containing peptides produced by Mycobacteria versus Streptomyces

A number of bacteria are known to produce isonitrile-containing peptides (INPs) that facilitate metal transport and are important for cell survival; however, considerable structural variation is observed among INPs depending on the producing organism. While non-heme iron 2-oxoglutarate dependent isonitrilases catalyze isonitrile formation, how the natural variation in INP structure is controlled and its implications for INP bioactivity remain open questions. Herein, total chemical synthesis is utilized with X-Ray crystallographic analysis of mycobacterial isonitrilases to provide a structural model of substrate specificity that explains the longer alkyl chains observed in mycobacterial versus Streptomyces INPs. Moreover, proton NMR titration experiments demonstrate that INPs regardless of alkyl chain length are specific for binding copper instead of zinc. These results suggest that isonitrilases may act as gatekeepers in modulating the observed biological distribution of INP structures and this distribution may be primarily related to differing metal transport requirements among the producing strains.

Actinobacterial chalkophores: the biosynthesis of hazimycins

Copper is a transition metal element with significant effects on the morphological development and secondary metabolism of actinobacteria. In some microorganisms, copper-binding natural products are employed to modulate copper homeostasis, although their significance in actinobacteria remains largely unknown. Here, we identified the biosynthetic genes of the diisocyanide natural product hazimycin in Kitasatospora purpeofusca HV058, through gene knock-out and heterologous expression. Biochemical analyses revealed that hazimycin A specifically binds to copper, which diminishes its antimicrobial activity. The presence of a set of putative importer/exporter genes surrounding the biosynthetic genes suggested that hazimycin is a chalkophore that modulates the intracellular copper level. A bioinformatic survey of homologous gene cassettes, as well as the identification of two previously unknown hazimycin-producing Streptomyces strains, indicated that the isocyanide-based mechanism of copper homeostasis is prevalent in actinobacteria.

Changes in an enzyme ensemble during catalysis observed by high-resolution XFEL crystallography

Enzymes populate ensembles of structures necessary for catalysis that are difficult to experimentally characterize. We use time-resolved mix-and-inject serial crystallography at an x-ray free electron laser to observe catalysis in a designed mutant isocyanide hydratase (ICH) enzyme that enhances sampling of important minor conformations. The active site exists in a mixture of conformations, and formation of the thioimidate intermediate selects for catalytically competent substates. The influence of cysteine ionization on the ICH ensemble is validated by determining structures of the enzyme at multiple pH values. Large molecular dynamics simulations in crystallo and time-resolved electron density maps show that Asp17 ionizes during catalysis and causes conformational changes that propagate across the dimer, permitting water to enter the active site for intermediate hydrolysis. ICH exhibits a tight coupling between ionization of active site residues and catalysis-activated protein motions, exemplifying a mechanism of electrostatic control of enzyme dynamics.

Protein target highlights in CASP15: Analysis of models by structure providers

We present an in-depth analysis of selected CASP15 targets, focusing on their biological and functional significance. The authors of the structures identify and discuss key protein features and evaluate how effectively these aspects were captured in the submitted predictions. While the overall ability to predict three-dimensional protein structures continues to impress, reproducing uncommon features not previously observed in experimental structures is still a challenge. Furthermore, instances with conformational flexibility and large multimeric complexes highlight the need for novel scoring strategies to better emphasize biologically relevant structural regions. Looking ahead, closer integration of computational and experimental techniques will play a key role in determining the next challenges to be unraveled in the field of structural molecular biology.

Repurposing Iron- and 2-Oxoglutarate-Dependent Oxygenases to Catalyze Olefin Hydration

Mononuclear nonheme iron(II) and 2-oxoglutarate (Fe/2OG)-dependent oxygenases and halogenases are known to catalyze a diverse set of oxidative reactions, including hydroxylation, halogenation, epoxidation, and desaturation in primary metabolism and natural product maturation. However, their use in abiotic transformations has mainly been limited to C-H oxidation. Herein, we show that various enzymes of this family, when reconstituted with Fe(II) or Fe(III), can catalyze Mukaiyama hydration-a redox neutral transformation. Distinct from the native reactions of the Fe/2OG enzymes, wherein oxygen atom transfer (OAT) catalyzed by an iron-oxo species is involved, this nonnative transformation proceeds through a hydrogen atom transfer (HAT) pathway in a 2OG-independent manner. Additionally, in contrast to conventional inorganic catalysts, wherein a dinuclear iron species is responsible for HAT, the Fe/2OG enzymes exploit a mononuclear iron center to support this reaction. Collectively, our work demonstrates that Fe/2OG enzymes have utility in catalysis beyond the current scope of catalytic oxidation.

Aldehyde accumulation in Mycobacterium tuberculosis with defective proteasomal degradation results in copper sensitivity

Mycobacterium tuberculosis is a major human pathogen and the causative agent of tuberculosis disease. M. tuberculosis is able to persist in the face of host-derived antimicrobial molecules nitric oxide (NO) and copper (Cu). However, M. tuberculosis with defective proteasome activity is highly sensitive to NO and Cu, making the proteasome an attractive target for drug development. Previous work linked NO susceptibility with the accumulation of para-hydroxybenzaldehyde (pHBA) in M. tuberculosis mutants with defective proteasomal degradation. In this study, we found that pHBA accumulation was also responsible for Cu sensitivity in these strains. We showed that exogenous addition of pHBA to wild-type M. tuberculosis cultures sensitized bacteria to Cu to a degree similar to that of a proteasomal degradation mutant. We determined that pHBA reduced the production and function of critical Cu resistance proteins of the regulated in copper repressor (RicR) regulon. Furthermore, we extended these Cu-sensitizing effects to an aldehyde that M. tuberculosis may face within the macrophage. Collectively, this study is the first to mechanistically propose how aldehydes can render M. tuberculosis susceptible to an existing host defense and could support a broader role for aldehydes in controlling M. tuberculosis infections. IMPORTANCE M. tuberculosis is a leading cause of death by a single infectious agent, causing 1.5 million deaths annually. An effective vaccine for M. tuberculosis infections is currently lacking, and prior infection does not typically provide robust immunity to subsequent infections. Nonetheless, immunocompetent humans can control M. tuberculosis infections for decades. For these reasons, a clear understanding of how mammalian immunity inhibits mycobacterial growth is warranted. In this study, we show aldehydes can increase M. tuberculosis susceptibility to copper, an established antibacterial metal used by immune cells to control M. tuberculosis and other microbes. Given that activated macrophages produce increased amounts of aldehydes during infection, we propose host-derived aldehydes may help control bacterial infections, making aldehydes a previously unappreciated antimicrobial defense.

Changes in an Enzyme Ensemble During Catalysis Observed by High Resolution XFEL Crystallography

Enzymes populate ensembles of structures with intrinsically different catalytic proficiencies that are difficult to experimentally characterize. We use time-resolved mix-and-inject serial crystallography (MISC) at an X-ray free electron laser (XFEL) to observe catalysis in a designed mutant (G150T) isocyanide hydratase (ICH) enzyme that enhances sampling of important minor conformations. The active site exists in a mixture of conformations and formation of the thioimidate catalytic intermediate selects for catalytically competent substates. A prior proposal for active site cysteine charge-coupled conformational changes in ICH is validated by determining structures of the enzyme over a range of pH values. A combination of large molecular dynamics simulations of the enzyme in crystallo and time-resolved electron density maps shows that ionization of the general acid Asp17 during catalysis causes additional conformational changes that propagate across the dimer interface, connecting the two active sites. These ionization-linked changes in the ICH conformational ensemble permit water to enter the active site in a location that is poised for intermediate hydrolysis. ICH exhibits a tight coupling between ionization of active site residues and catalysis-activated protein motions, exemplifying a mechanism of electrostatic control of enzyme dynamics.

Megalochelin, a Tridecapeptide Siderophore from a Talented Streptomycete

Streptomycetes are bacteria known for their extraordinary biosynthetic capabilities. Herein, we describe the genome and metabolome of a particularly talented strain, Streptomyces ID71268. Its 8.4-Mbp genome harbors 32 bioinformatically predicted biosynthetic gene clusters (BGCs), out of which 10 are expressed under a single experimental condition. In addition to five families of known metabolites with previously assigned BGCs (nigericin, azalomycin F, ectoine, SF2766, and piericidin), we were able to predict BGCs for three additional metabolites: streptochlorin, serpetene, and marinomycin. The strain also produced two families of presumably novel metabolites, one of which was associated with growth inhibitory activity against the human opportunistic pathogen Acinetobacter baumannii in an iron-dependent manner. Bioassay-guided fractionation, followed by extensive liquid chromatography-mass spectrometry (LC-MS) and NMR analyses, established that the molecule responsible for the observed antibacterial activity is an unusual tridecapeptide siderophore with a ring-and-tail structure: the heptapeptide ring is formed through a C-C bond between a 2,3-dihydroxybenzoate (DHB) cap on Gly1 and the imidazole moiety of His7, while the hexapeptide tail is sufficient for binding iron. This molecule, named megalochelin, is the largest known siderophore. The megalochelin BGC encodes a 13-module nonribosomal peptide synthetase for the synthesis of the tridecapeptide, and a copper-dependent oxidase, likely responsible for the DHB-imidazole cross-link, whereas the genes for synthesis of the DHB starter unit are apparently specified in trans by a different BGC. Our results suggest that prolific producers of specialized metabolites may conceal hidden treasures within a background of known compounds.

Crystallographic Deciphering of Spontaneous Self-Assembly of Achiral Calciphores to Chiral Complexes

Thiapyricins (TPC-A/B, 1 and 2), which are new metallophore scaffolds exhibiting selective divalent cation binding property, were produced in response to metal-deprived conditions by Saccharothrix sp. TRM_47004 isolated from the Lop Nor Salt Lake. TPCs represent a thiazolyl-pyridine skeleton of a calcium-binding natural product, calciphore, owing to the selectivity to calcium ions among diverse metal ions. The thiapyricins exhibited notable co-crystalline characteristics of the apo- and holo-forms with racemic enantiomers comprising a pair of space isomers in a Δ/Λ-form. Therefore, we postulated a mechanism for the four-hierarchical self-assembly of achiral natural products into chiral complexes. Furthermore, their metal-chelating trait aided the adaptation of the host during metal starvation by increasing the production of TPCs. This study presents a structural paradigm of a new calciphore, provides insight into the mechanism of natural product assembly, and highlights the causality between the production of the metallophore and metallic habitats.

Biosynthesis of Isonitrile Lipopeptide Metallophores from Pathogenic Mycobacteria

Isonitrile lipopeptides (INLPs) are known to be related to the virulence of pathogenic mycobacteria by mediating metal transport, but their biosynthesis remains obscure. In this work, we use in vitro biochemical assays, site-directed mutagenesis, chemical synthesis, and spectroscopy techniques to scrutinize the activity of core enzymes required for INLP biosynthesis in mycobacteria. Compared to environmental Streptomyces, pathogenic Mycobacterium employ a similar chemical logic and enzymatic machinery in INLP biosynthesis, differing mainly in the fatty-acyl chain length, which is controlled by multiple enzymes in the pathway. Our in-depth study on the non-heme iron(II) and α-ketoglutarate-dependent dioxygenase for isonitrile generation, including Rv0097 from Mycobacterium tuberculosis (Mtb), demonstrates that it recognizes a free-standing small molecule substrate, different from the recent hypothesis that a carrier protein is required for Rv0097 in Mtb. A key residue in Rv0097 is further identified to dictate the varied fatty-acyl chain length specificity between Streptomyces and Mycobacterium.

Diisonitrile Lipopeptides Mediate Resistance to Copper Starvation in Pathogenic Mycobacteria

Bacterial pathogens and their hosts engage in intense competition for critical nutrients during infection, including metals such as iron, copper, and zinc. Some metals are limited by the host, and some are deployed by the host as antimicrobials. To counter metal limitation, pathogens deploy high-affinity metal acquisition systems, best exemplified by siderophores to acquire iron. Although pathogen strategies to resist the toxic effects of high Cu have been elucidated, the role of Cu starvation and the existence of Cu acquisition systems are less well characterized. In this study, we examined the role of diisonitrile chalkophores of pathogenic mycobacteria, synthesized by the enzymes encoded by the virulence-associated nrp gene cluster, in metal acquisition. nrp gene cluster expression is strongly induced by starvation or chelation of Cu but not starvation of Zn or excess Cu. Mycobacterium tuberculosis and Mycobacterium marinum strains lacking the nrp-encoded nonribosomal peptide sythetase, the fadD10 adenylate-forming enzyme, or the uncharacterized upstream gene ppe1 are all sensitized to Cu, but not Zn, starvation. This low Cu sensitivity is rescued by genetic complementation or by provision of a synthetic diisonitrile chalkophore. These data demonstrate that diisonitrile lipopeptides in mycobacteria are chalkophores that facilitate survival under Cu-limiting conditions and suggest that Cu starvation is a relevant stress for M. tuberculosis in the host.

Biosynthesis of Isonitrile- and Alkyne-Containing Natural Products

Natural products are a diverse class of biologically produced compounds that participate in fundamental biological processes such as cell signaling, nutrient acquisition, and interference competition. Unique triple-bond functionalities like isonitriles and alkynes often drive bioactivity and may serve as indicators of novel chemical logic and enzymatic machinery. Yet, the biosynthetic underpinnings of these groups remain only partially understood, constraining the opportunity to rationally engineer biomolecules with these functionalities for applications in pharmaceuticals, bioorthogonal chemistry, and other value-added chemical processes. Here, we focus our review on characterized biosynthetic pathways for isonitrile and alkyne functionalities, their bioorthogonal transformations, and prospects for engineering their biosynthetic machinery for biotechnological applications.

Probing the Mechanism of Isonitrile Formation by a Non-Heme Iron(II)-Dependent Oxidase/Decarboxylase

The isonitrile moiety is an electron-rich functionality that decorates various bioactive natural products isolated from diverse kingdoms of life. Isonitrile biosynthesis was restricted for over a decade to isonitrile synthases, a family of enzymes catalyzing a condensation reaction between l-Trp/l-Tyr and ribulose-5-phosphate. The discovery of ScoE, a non-heme iron(II) and α-ketoglutarate-dependent dioxygenase, demonstrated an alternative pathway employed by nature for isonitrile installation. Biochemical, crystallographic, and computational investigations of ScoE have previously been reported, yet the isonitrile formation mechanism remains obscure. In the present work, we employed in vitro biochemistry, chemical synthesis, spectroscopy techniques, and computational simulations that enabled us to propose a plausible molecular mechanism for isonitrile formation. Our findings demonstrate that the ScoE reaction initiates with C5 hydroxylation of (R)-3-((carboxymethyl)amino)butanoic acid to generate 1, which undergoes dehydration, presumably mediated by Tyr96 to synthesize 2 in a trans configuration. (R)-3-isocyanobutanoic acid is finally generated through radical-based decarboxylation of 2, instead of the common hydroxylation pathway employed by this enzyme superfamily.

Halogenated Metal-Binding Compounds from Shipworm Symbionts

Bacteria use small molecules to impose strict regulation over the acquisition, uptake, and sequestration of transition metal ions. Low-abundance nutrient metals, such as Fe(III), need to be scavenged from the environment by high-affinity chelating molecules called siderophores. Conversely, metal ions that become toxic at high concentrations need to be sequestered and detoxified. Often, bacteria produce a suite of compounds that bind various metal ions at different affinities in order to maintain homeostasis. Turnerbactin, a triscatecholate siderophore isolated from the intracellular shipworm symbiont Teredinibacter turnerae T7901, is responsible for iron regulation and uptake. Herein, another series of compounds are described that complex with iron, copper, and molybdenum in solution. Teredinibactins belong to a class of metal-binding molecules that utilize a phenolate-thiazoline moiety in the coordination of metal ions. In contrast to other compounds in this class, such as yersiniabactin, the phenyl ring is decorated with a 2,4-dihydroxy-3-halo substitution pattern. UV-vis absorption spectroscopy based titration experiments with CuCl₂ show the formation of an intermediate complex at substoichiometric concentrations and conversion to a copper-bound complex at 1:1 molar equiv.

Kupyaphores are zinc homeostatic metallophores required for colonization of Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is the etiological agent of human tuberculosis (TB). Mtb can persist inside host macrophages by successfully adapting to intracellular conditions. Acquisition of balanced amounts of essential micronutrients is one such important process. Our studies have identified a metallophore produced on demand to restore Mtb zinc metabolic imbalance. These diacyl-diisonitrile lipopeptides, named kupyaphores, are specifically induced during infection and move in and out of cells to protect bacteria from host-mediated nutritional deprivation and intoxication. Furthermore, we identify an Mtb isonitrile hydratase homolog, expressed in low-zinc conditions, which probably facilitates zinc release from kupyaphores. Identification of this zinc acquisition strategy could provide opportunities in future to understand systemic zinc dysbiosis and associated manifestations in TB patients.

Mycobacterium tuberculosis (Mtb) endures a combination of metal scarcity and toxicity throughout the human infection cycle, contributing to complex clinical manifestations. Pathogens counteract this paradoxical dysmetallostasis by producing specialized metal trafficking systems. Capture of extracellular metal by siderophores is a widely accepted mode of iron acquisition, and Mtb iron-chelating siderophores, mycobactin, have been known since 1965. Currently, it is not known whether Mtb produces zinc scavenging molecules. Here, we characterize low-molecular-weight zinc-binding compounds secreted and imported by Mtb for zinc acquisition. These molecules, termed kupyaphores, are produced by a 10.8 kbp biosynthetic cluster and consists of a dipeptide core of ornithine and phenylalaninol, where amino groups are acylated with isonitrile-containing fatty acyl chains. Kupyaphores are stringently regulated and support Mtb survival under both nutritional deprivation and intoxication conditions. A kupyaphore-deficient Mtb strain is unable to mobilize sufficient zinc and shows reduced fitness upon infection. We observed early induction of kupyaphores in Mtb-infected mice lungs after infection, and these metabolites disappeared after 2 wk. Furthermore, we identify an Mtb-encoded isonitrile hydratase, which can possibly mediate intracellular zinc release through covalent modification of the isonitrile group of kupyaphores. Mtb clinical strains also produce kupyaphores during early passages. Our study thus uncovers a previously unknown zinc acquisition strategy of Mtb that could modulate host–pathogen interactions and disease outcome.

Deciphering the Reaction Pathway of Mononuclear Iron Enzyme-Catalyzed N≡C Triple Bond Formation in Isocyanide Lipopeptide and Polyketide Biosynthesis

Despite the diversity of reactions catalyzed by 2-oxoglutarate-dependent nonheme iron (Fe/2OG) enzymes identified in recent years, only a limited number of these enzymes have been investigated in mechanistic detail. In particular, several Fe/2OG-dependent enzymes capable of catalyzing isocyanide formation have been reported. While the glycine moiety has been identified as a biosynthon for the isocyanide group, how the actual conversion is effected remains obscure. To elucidate the catalytic mechanism, we characterized two previously unidentified (AecA and AmcA) along with two known (ScoE and SfaA) Fe/2OG-dependent enzymes that catalyze N≡C triple bond installation using synthesized substrate analogues and potential intermediates. Our results indicate that isocyanide formation likely entails a two-step sequence involving an imine intermediate that undergoes decarboxylation-assisted desaturation to yield the isocyanide product. Results obtained from the in vitro experiments are further supported by mutagenesis, the product-bound enzyme structure, and in silico analysis.

Native mass spectrometry-based metabolomics identifies metal-binding compounds

Although metals are essential for the molecular machineries of life, systematic methods for discovering metal-small molecule complexes from biological samples are limited. Here, we describe a two-step native electrospray ionization-mass spectrometry method, in which post-column pH adjustment and metal infusion are combined with ion identity molecular networking, a rule-based data analysis workflow. This method enabled the identification of metal-binding compounds in complex samples based on defined mass (m/z) offsets of ion species with the same chromatographic profiles. As this native electrospray metabolomics approach is suited to the use of any liquid chromatography-mass spectrometry system to explore the binding of any metal, this method has the potential to become an essential strategy for elucidating metal-binding molecules in biology.

Characterization of Chalkophomycin, a Copper(II) Metallophore with an Unprecedented Molecular Architecture

Metals play essential roles in life by coordination with small molecules, proteins, and nucleic acids. Although the coordination of copper ions in many proteins and methanobactins is known, the coordination chemistry of Cu(II) in natural products and their biological functions remain underexplored. Herein, we report the discovery of a Cu(II)-binding natural product, chalkophomycin (CHM, 1), from Streptomyces sp. CB00271, featuring an asymmetric square-coordination system of a bidentate diazeniumdiolate and a conjugated 1H-pyrrole 1-oxide-oxazoline. The structure of 1 may inspire the synthesis of Cu(II) chelators against neurodegenerative diseases or Cu(II)-based antitumor therapeutics.

Biosynthesis of fluopsin C, a copper-containing antibiotic from Pseudomonas aeruginosa

Metal-binding natural products contribute to metal acquisition and bacterial virulence, but their roles in metal stress response are underexplored. We show that a five-enzyme pathway in Pseudomonas aeruginosa synthesizes a small-molecule copper complex, fluopsin C, in response to elevated copper concentrations. Fluopsin C is a broad-spectrum antibiotic that contains a copper ion chelated by two minimal thiohydroxamates. Biosynthesis of the thiohydroxamate begins with cysteine and requires two lyases, two iron-dependent enzymes, and a methyltransferase. The iron-dependent enzymes remove the carboxyl group and the α carbon from cysteine through decarboxylation, N-hydroxylation, and methylene excision. Conservation of the pathway in P. aeruginosa and other bacteria suggests a common role for fluopsin C in the copper stress response, which involves fusing copper into an antibiotic against other microbes.

Genome mining of biosynthetic gene clusters intended for secondary metabolites conservation in actinobacteria

Evolution of genome sequencing technology, on the one hand, and advancement of computational genome mining tools, on the other hand, paves way for improvement in predicting secondary metabolites. In past, numerous efforts were made concerning genome mining for recognizing secondary metabolites within the genus, but only a negligible quantity of comparative genomic reports had carried out among species of different genera. In this study, we explored potential of 24 actinobacteria species belonging to the genera, including Streptomyces, Nocardia, Micromonospora, and Saccharomonospora, to traverse diversity and distribution of Biosynthetic Gene Clusters (BGCs). Investigating results obtained from antiSMASH (Antibiotics and Secondary Metabolites Analysis Shell), NaPDoS (Natural Product Domain Seeker), and NP.searcher revealed conservation of genus-specific gene clusters among various species. E.g., NAGGN (n-acetyl glutaminyl glutamine amide) is present in Micromonospora, furan in Nocardia, melanin, and lassopeptide occur in Streptomyces. Bioactive compounds like alkyl-O-dihydro geranyl methoxy hydroquinone, SapB, desferrioxamine E, 2-Methylisoborneol, mayamycin, cyclodipeptide synthase, diisonitrile, salinichelin, hopene, ectoine and isorenieratene are highly conserved among diverse genera. Furthermore, pharmacological activity of actinobacterial derived metabolites against bacterial and fungal pathogens were illustrated. We need to accomplish large-scale analysis of natural products, including various genera of actinobacteria to deliver comprehensive intuition to overcome antibiotic resistance.

Medicinal Chemistry of Isocyanides

In eons of evolution, isocyanides carved out a niche in the ecological systems probably thanks to their metal coordinating properties. In 1859 the first isocyanide was synthesized by humans and in 1950 the first natural isocyanide was discovered. Now, at the beginning of XXI century, hundreds of isocyanides have been isolated both in prokaryotes and eukaryotes and thousands have been synthesized in the laboratory. For some of them their ecological role is known, and their potent biological activity as antibacterial, antifungal, antimalarial, antifouling, and antitumoral compounds has been described. Notwithstanding, the isocyanides have not gained a good reputation among medicinal chemists who have erroneously considered them either too reactive or metabolically unstable, and this has restricted their main use to technical applications as ligands in coordination chemistry. The aim of this review is therefore to show the richness in biological activity of the isocyanide-containing molecules, to support the idea of using the isocyanide functional group as an unconventional pharmacophore especially useful as a metal coordinating warhead. The unhidden hope is to convince the skeptical medicinal chemists of the isocyanide potential in many areas of drug discovery and considering them in the design of future drugs.

Dual-purpose isocyanides produced by Aspergillus fumigatus contribute to cellular copper sufficiency and exhibit antimicrobial activity

The maintenance of sufficient but nontoxic pools of metal micronutrients is accomplished through diverse homeostasis mechanisms in fungi. Siderophores play a well established role for iron homeostasis; however, no copper-binding analogs have been found in fungi. Here we demonstrate that, in Aspergillus fumigatus, xanthocillin and other isocyanides derived from the xan biosynthetic gene cluster (BGC) bind copper, impact cellular copper content, and have significant metal-dependent antimicrobial properties. xan BGC-derived isocyanides are secreted and bind copper as visualized by a chrome azurol S (CAS) assay, and inductively coupled plasma mass spectrometry analysis of A. fumigatus intracellular copper pools demonstrated a role for xan cluster metabolites in the accumulation of copper. A. fumigatus coculture with a variety of human pathogenic fungi and bacteria established copper-dependent antimicrobial properties of xan BGC metabolites, including inhibition of laccase activity. Remediation of xanthocillin-treated Pseudomonas aeruginosa growth by copper supported the copper-chelating properties of xan BGC isocyanide products. The existence of the xan BGC in several filamentous fungi suggests a heretofore unknown role of eukaryotic natural products in copper homeostasis and mediation of interactions with competing microbes.

The Genome Analysis of the Human Lung-Associated Streptomyces sp. TR1341 Revealed the Presence of Beneficial Genes for Opportunistic Colonization of Human Tissues

Streptomyces sp. TR1341 was isolated from the sputum of a man with a history of lung and kidney tuberculosis, recurrent respiratory infections, and COPD. It produces secondary metabolites associated with cytotoxicity and immune response modulation. In this study, we complement our previous results by identifying the genetic features associated with the production of these secondary metabolites and other characteristics that could benefit the strain during its colonization of human tissues (virulence factors, modification of the host immune response, or the production of siderophores). We performed a comparative phylogenetic analysis to identify the genetic features that are shared by environmental isolates and human respiratory pathogens. The results showed a high genomic similarity of Streptomyces sp. TR1341 to the plant-associated Streptomyces sp. endophyte_N2, inferring a soil origin of the strain. Putative virulence genes, such as mammalian cell entry (mce) genes were not detected in the TR1341’s genome. The presence of a type VII secretion system, distinct from the ones found in Mycobacterium species, suggests a different colonization strategy than the one used by other actinomycete lung pathogens. We identified a higher diversity of genes related to iron acquisition and demonstrated that the strain produces ferrioxamine B in vitro. These results indicate that TR1341 may have an advantage in colonizing environments that are low in iron, such as human tissue.

Antifungal Activity and Biosynthetic Potential of New Streptomyces sp. MW-W600-10 Strain Isolated from Coal Mine Water

Crop infections by fungi lead to severe losses in food production and pose risks for human health. The increasing resistance of pathogens to fungicides has led to the higher usage of these chemicals, which burdens the environment and highlights the need to find novel natural biocontrol agents. Members of the genus Streptomyces are known to produce a plethora of bioactive compounds. Recently, researchers have turned to extreme and previously unexplored niches in the search for new strains with antimicrobial activities. One such niche are underground coal mine environments. We isolated the new Streptomyces sp. MW-W600-10 strain from coal mine water samples collected at 665 m below ground level. We examined the antifungal activity of the strain against plant pathogens Fusarium culmorum DSM62188 and Nigrospora oryzae roseF7. Furthermore, we analyzed the strain’s biosynthetic potential with the antiSMASH tool. The strain showed inhibitory activity against both fungi strains. Genome mining revealed that it has 39 BGCs, among which 13 did not show similarity to those in databases. Additionally, we examined the activity of the Streptomyces sp. S-2 strain isolated from black soot against F. culmorum DSM62188. These results show that coal-related strains could be a source of novel bioactive compounds. Future studies will elucidate their full biotechnological potential.

Integrated sensing of host stresses by inhibition of a cytoplasmic two-component system controls M. tuberculosis acute lung infection

Bacterial pathogens that infect phagocytic cells must deploy mechanisms that sense and neutralize host microbicidal effectors. For Mycobacterium tuberculosis, the causative agent of tuberculosis, these mechanisms allow the bacterium to rapidly adapt from aerosol transmission to initial growth in the lung alveolar macrophage. Here, we identify a branched signaling circuit in M. tuberculosis that controls growth in the lung through integrated direct sensing of copper ions and nitric oxide by coupled activity of the Rip1 intramembrane protease and the PdtaS/R two-component system. This circuit uses a two-signal mechanism to inactivate the PdtaS/PdtaR two-component system, which constitutively represses virulence gene expression. Cu and NO inhibit the PdtaS sensor kinase through a dicysteine motif in the N-terminal GAF domain. The NO arm of the pathway is further controlled by sequestration of the PdtaR RNA binding response regulator by an NO-induced small RNA, controlled by the Rip1 intramembrane protease. This coupled Rip1/PdtaS/PdtaR circuit controls NO resistance and acute lung infection in mice by relieving PdtaS/R-mediated repression of isonitrile chalkophore biosynthesis. These studies identify an integrated mechanism by which M. tuberculosis senses and resists macrophage chemical effectors to achieve pathogenesis.

Broad Spectrum Antibiotic Xanthocillin X Effectively Kills Acinetobacter baumannii via Dysregulation of Heme Biosynthesis

Isonitrile natural products exhibit promising antibacterial activities. However, their mechanism of action (MoA) remains largely unknown. Based on the nanomolar potency of xanthocillin X (Xan) against diverse difficult-to-treat Gram-negative bacteria, including the critical priority pathogen Acinetobacter baumannii, we performed in-depth studies to decipher its MoA. While neither metal binding nor cellular protein targets were detected as relevant for Xan's antibiotic effects, sequencing of resistant strains revealed a conserved mutation in the heme biosynthesis enzyme porphobilinogen synthase (PbgS). This mutation caused impaired enzymatic efficiency indicative of reduced heme production. This discovery led to the validation of an untapped mechanism, by which direct heme sequestration of Xan prevents its binding into cognate enzyme pockets resulting in uncontrolled cofactor biosynthesis, accumulation of porphyrins, and corresponding stress with deleterious effects for bacterial viability. Thus, Xan represents a promising antibiotic displaying activity even against multidrug resistant strains, while exhibiting low toxicity to human cells.

Chemical labeling strategies for small molecule natural product detection and isolation

Covering: Up to 2020.It is widely accepted that small molecule natural products (NPs) evolved to carry out a particular ecological function and that these finely-tuned molecules can sometimes be appropriated for the treatment of disease in humans. Unfortunately, for the natural products chemist, NPs did not evolve to possess favorable physicochemical properties needed for HPLC-MS analysis. The process known as derivatization, whereby an NP in a complex mixture is decorated with a nonnatural moiety using a derivatizing agent (DA), arose from this sad state of affairs. Here, NPs are freed from the limitations of natural functionality and endowed, usually with some degree of chemoselectivity, with additional structural features that make HPLC-MS analysis more informative. DAs that selectively label amines, carboxylic acids, alcohols, phenols, thiols, ketones, and aldehydes, terminal alkynes, electrophiles, conjugated alkenes, and isocyanides have been developed and will be discussed here in detail. Although usually employed for targeted metabolomics, chemical labeling strategies have been effectively applied to uncharacterized NP extracts and may play an increasing role in the detection and isolation of certain classes of NPs in the future.

A Role for Mycobacterium tuberculosis Sigma Factor C in Copper Nutritional Immunity

Sigma factor C (SigC) contributes to Mycobacterium tuberculosis virulence in various animal models, but the stress response coordinated by this transcription factor was undefined. The results presented here indicate that SigC prevents copper starvation. Whole genome expression studies demonstrate short-term (4-h) induction of sigC, controlled from a tetracycline-inducible promoter, upregulates ctpB and genes in the nonribosomal peptide synthase (nrp) operon. These genes are expressed at higher levels after 48-h sigC induction, but also elevated are genes encoding copper-responsive regulator RicR and RicR-regulated copper toxicity response operon genes rv0846-rv0850, suggesting prolonged sigC induction results in excessive copper uptake. No growth and global transcriptional differences are observed between a sigC null mutant relative to its parent strain in 7H9 medium. In a copper-deficient medium, however, growth of the sigC deletion strain lags the parent, and 40 genes (including those in the nrp operon) are differentially expressed. Copper supplementation reverses the growth defect and silences most transcriptional differences. Together, these data support SigC as a transcriptional regulator of copper acquisition when the metal is scarce. Attenuation of sigC mutants in severe combined immunodeficient mice is consistent with an inability to overcome innate host defenses that sequester copper ions to deprive invading microbes of this essential micronutrient.

Current Understanding toward Isonitrile Group Biosynthesis and Mechanism

Isonitrile group has been identified in many natural products. Due to the broad reactivity of N≡C triple bond, these natural products have valuable pharmaceutical potentials. This review summarizes the current biosynthetic pathways and the corresponding enzymes that are responsible for isonitrile-containing natural product generation. Based on the strategies utilized, two fundamentally distinctive approaches are discussed. In addition, recent progress in elucidating isonitrile group formation mechanisms is also presented.

Exploration of Iron- and a-Ketoglutarate-Dependent Dioxygenases as Practical Biocatalysts in Natural Product Synthesis

Catalytic C─H oxidation is a powerful transformation with enormous promise to streamline access to complex molecules. In recent years, biocatalytic C─H oxidation strategies have received tremendous attention due to their potential to address unmet regio- and stereoselectivity challenges that are often encountered with the use of small-molecule-based catalysts. This Account provides an overview of recent contributions from our laboratory in this area, specifically in the use of iron- and α-ketoglutarate-dependent dioxygenases in the chemoenzymatic synthesis of complex natural products.

Repurposing the 3-Isocyanobutanoic Acid Adenylation Enzyme SfaB for Versatile Amidation and Thioesterification

Genome mining of microbial natural products enables chemists not only to discover the bioactive molecules with novel skeletons, but also to identify the enzymes that catalyze diverse chemical reactions. Exploring the substrate promiscuity and catalytic mechanism of those biosynthetic enzymes facilitates the development of potential biocatalysts. SfaB is an acyl adenylate-forming enzyme that adenylates a unique building block, 3-isocyanobutanoic acid, in the biosynthetic pathway of the diisonitrile natural product SF2768 produced by Streptomyces thioluteus, and this AMP-ligase was demonstrated to accept a broad range of short-chain fatty acids (SCFAs). Herein, we repurpose SfaB to catalyze amidation or thioesterification between those SCFAs and various amine or thiol nucleophiles, thereby providing an alternative enzymatic approach to prepare the corresponding amides and thioesters in vitro.

Complete genome sequence of Micromonospora craniellae LHW63014T, a potential metal ion-chelating agent producer

Micromonospora craniellae LHW63014^T is a novel marine Micromonospora, isolated from a Craniella species sponge collected in the South China Sea. In this study, we report the complete genome sequence of M. craniellae LHW63014^T, which is comprised of a circular chromosome of 6,839,926 bp with the G + C content of 70.9 mol%. The complete genome contained 6572 protein-coding genes, 48 tRNA genes, and 9 rRNA genes. Genomic annotations revealed that 79.09% of the protein-coding genes were assigned to the COG database, among which, the abundant genes were predicted to be involved in transcription, replication, recombination and repair, and amino acid transport and metabolism. Secondary metabolites prediction using antiSMASH revealed that 22 biosynthetic gene clusters (BGC) of secondary metabolites were located in the genome of M. craniellae LHW63014^T, 19 of which showed low similarity (<50%) to known BGCs and 5 of which showed the closest homology with BGCs encoding metal ion-chelating agents, indicating the immense potential of M. craniellae LHW63014^T to produce a wide variety of novel antibiotics, especially for metal ion-chelating agents.

Reclassification of Streptomyces diastaticus subsp. ardesiacus, Streptomyces gougerotii and Streptomyces rutgersensis

We investigated the taxonomic relationships among Streptomyces diastaticus subsp. ardesiacus, Streptomyces diastaticus subsp. diastaticus, Streptomyces gougerotii and Streptomyces rutgersensis. The 16S rRNA gene sequence similarity between S. diastaticus subsp. ardesiacus and S. diastaticus subsp. diastaticus was 97.7 %, whereas S. diastaticus subsp. diastaticus, S. gougerotii and S. rutgersensis showed 100 % nucleotide sequence identity. In addition, S. diastaticus subsp. diastaticus, S. gougerotii and S. rutgersensis formed a single clade in the phylogenetic tree. Digital DNA-DNA relatedness between S. diastaticus subsp. diastaticus and S. diastaticus subsp. ardesiacus was only 22.8%, indicative of different species. In comparison, DNA-DNA relatedness values for S. diastaticus subsp. diastaticus, S. gougerotii and S. rutgersensis ranged from 95.8 to 97.2 %, suggesting the three taxa belong to the same genomospecies. Previously reported phenotypic data also supported synonymy. Therefore, we propose that S. diastaticus subsp. ardesiacus should be classified as an independent species, Streptomyces ardesiacus sp. nov. The type strain is NBRC 13412^T (=ATCC 3315^T=CBS 713.72^T=DSM 40496^T=ISP 5496^T=JCM 4745^T=NBRC 3714^T=NRRL B-1241^T=RIA 1373^T). Our data also suggests that S. rutgersensis and S. gougerotii should be reclassified as later heterotypic synonyms of S. diastaticus.

Phylogenomic Classification and Biosynthetic Potential of the Fossil Fuel-Biodesulfurizing Rhodococcus Strain IGTS8

Rhodococcus strain IGTS8 is the most extensively studied model bacterium for biodesulfurization of fossil fuels via the non–destructive sulfur–specific 4S pathway. This strain was initially assigned to Rhodococcus rhodochrous and later to Rhodococcus erythropolis thus making its taxonomic status debatable and reflecting the limited resolution of methods available at the time. In this study, phylogenomic analyses of the whole genome sequences of strain IGTS8 and closely related rhodococci showed that R. erythropolis and Rhodococcus qingshengii are very closely related species, that Rhodococcus strain IGTS8 is a R. qingshengii strain and that several strains identified as R. erythropolis should be re-classified as R. qingshengii. The genomes of strains assigned to these species contain potentially novel biosynthetic gene clusters showing that members of these taxa should be given greater importance in the search for new antimicrobials and other industrially important biomolecules. The plasmid-borne dsz operon encoding fossil fuel desulfurization enzymes was present in R. qingshengii IGTS8 and R. erythropolis XP suggesting that it might be transferable between members of these species.

Diisonitrile-Mediated Reactive Oxygen Species Accumulation Leads to Bacterial Growth Inhibition

The diisonitrile copper chelator SF2768 biosynthesized by Streptomyces thioluteus functions as a chalkophore that transports extracellular copper into producer cells in a complexed form. It was demonstrated that the treatment of eight bacteria, including Bacillus subtilis and Acinetobacter baumannii, with SF2768 led to a moderate growth inhibition which is associated with an increased level of reactive oxygen species (ROS). In addition, SF2768 and its diisonitrile analogues proved to be effective tyrosinase inhibitors. Three new analogues, SF2768 I, K, and L, were identified by detailed spectroscopic analysis.

Pathway from N-Alkylglycine to Alkylisonitrile Catalyzed by Iron(II) and 2-Oxoglutarate-Dependent Oxygenases

N-alkylisonitrile, a precursor to isonitrile-containing lipopeptides, is biosynthesized by decarboxylation-assisted -N≡C group (isonitrile) formation by using N-alkylglycine as the substrate. This reaction is catalyzed by iron(II) and 2-oxoglutarate (Fe/2OG) dependent enzymes. Distinct from typical oxygenation or halogenation reactions catalyzed by this class of enzymes, installation of the isonitrile group represents a novel reaction type for Fe/2OG enzymes that involves a four-electron oxidative process. Reported here is a plausible mechanism of three Fe/2OG enzymes, Sav607, ScoE and SfaA, which catalyze isonitrile formation. The X-ray structures of iron-loaded ScoE in complex with its substrate and the intermediate, along with biochemical and biophysical data reveal that -N≡C bond formation involves two cycles of Fe/2OG enzyme catalysis. The reaction starts with an Fe^IV -oxo-catalyzed hydroxylation. It is likely followed by decarboxylation-assisted desaturation to complete isonitrile installation.

Facile Discovery and Quantification of Isonitrile Natural Products via Tetrazine-Based Click Reactions

A facile method for the quick discovery and quantification of isonitrile compounds from microbial cultures was established based on the isonitrile-tetrazine click reaction. This method was successfully applied to the rediscovery of diisonitrile antibotic SF2768 from an unknown strain Streptomyces tsukubensis. Finally, an in situ reduction further enabled bioorthogonal ligation of primary and secondary isonitriles for the first time.

Total Synthesis of the Bacterial Diisonitrile Chalkophore SF2768

Chalkophores are bacterial natural products that chelate and transport extracellular copper. The diisonitrile natural product SF2768 was first isolated from a Streptomyces species as an antifungal antibiotic and has more recently been characterized as a bacterial chalkophore and potential virulence factor. Herein, we report a modular synthesis of SF2768 and related acyclic analogues, allowing assignment of syn-stereochemistry across the central lactol ring. The copper-binding properties of these diisonitriles have also been studied.

Adaptation of Mycobacterium tuberculosis to Biofilm Growth Is Genetically Linked to Drug Tolerance

Mycobacterium tuberculosis spontaneously grows at the air-medium interface, forming pellicle biofilms, which harbor more drug-tolerant persisters than planktonic cultures. The underlying basis for increased persisters in M. tuberculosis biofilms is unknown. Using a transposon sequencing (Tn-seq) approach, we show here that multiple genes that are necessary for fitness of M. tuberculosis cells within biofilms, but not in planktonic cultures, are also implicated in tolerance of bacilli to a diverse set of stressors and antibiotics. Thus, development of M. tuberculosis biofilms appears to be associated with an enrichment of population, in which challenging growth conditions within biofilm architecture select for cells that maintain intrinsic tolerance to exogenous stresses, including antibiotic exposure. We further observed that the intrinsic drug tolerance of constituent cells of a biofilm determines the frequency of persisters. These findings together allow us to propose that the selection of elite cells during biofilm development promotes the frequency of persisters. Furthermore, probing the possibility that the population enrichment is an outcome of unique environment within biofilms, we demonstrate biofilm-specific induction in the synthesis of isonitrile lipopeptide (INLP). Mutation analysis indicates that INLP is necessary for the architecture development of M. tuberculosis biofilms. In summary, this study offers an insight into persistence of M. tuberculosis biofilms under antibiotic exposure, while identifying INLP as a potential biomarker for further investigation of this phenomenon.

A Histidine Residue and a Tetranuclear Cuprous-thiolate Cluster Dominate the Copper Loading Landscape of a Copper Storage Protein from Streptomyces lividans

The chemical basis for protecting organisms against the toxic effect imposed by excess cuprous ions is to constrain this through high-affinity binding sites that use cuprous-thiolate coordination chemistry. In bacteria, a family of cysteine rich four-helix bundle proteins utilise thiolate chemistry to bind up to 80 cuprous ions. These proteins have been termed copper storage proteins (Csp). The present study investigates cuprous ion loading to the Csp from Streptomyces lividans (SlCsp) using a combination of X-ray crystallography, site-directed mutagenesis and stopped-flow reaction kinetics with either aquatic cuprous ions or a chelating donor. We illustrate that at low cuprous ion concentrations, copper is loaded exclusively into an outer core region of SlCsp via one end of the four-helix bundle, facilitated by a set of three histidine residues. X-ray crystallography reveals the existence of polynuclear cuprous-thiolate clusters culminating in the assembly of a tetranuclear [Cu₄ (μ₂ -S-Cys)₄ (Ν^δ1 -His)] cluster in the outer core. As more cuprous ions are loaded, the cysteine lined inner core of SlCsp fills with cuprous ions but in a fluxional and dynamic manner with no evidence for the assembly of further intermediate polynuclear cuprous-thiolate clusters as observed in the outer core. Using site-directed mutagenesis a key role for His107 in the efficient loading of cuprous ions from a donor is established. A model of copper loading to SlCsp is proposed and discussed.

Emerging Opportunities To Manipulate Metal Trafficking for Therapeutic Benefit

The indispensable requirement for metals in life processes has led to the evolution of sophisticated mechanisms that allow organisms to maintain dynamic equilibria of these ions. This dynamic control of the level, speciation, and availability of a variety of metal ions allows organisms to sustain biological processes while avoiding toxicity. When functioning properly, these mechanisms allow cells to return to their metal homeostatic set points following shifts in the metal availability or other stressors. These periods of transition, when cells are in a state of flux in which they work to regain homeostasis, present windows of opportunity to pharmacologically manipulate targets associated with metal-trafficking pathways in ways that could either facilitate a return to homeostasis and the recovery of cellular function or further push cells outside of homeostasis and into cellular distress. The purpose of this Viewpoint is to highlight emerging opportunities for chemists and chemical biologists to develop compounds to manipulate metal-trafficking processes for therapeutic benefit.