Genomic and metabolomic diversity within a familial population of Aspergillus flavus.

Aspergillus flavus is an agriculturally significant micro-fungus having potential to contaminate food and feed crops with toxic secondary metabolites such as aflatoxin (AF) and cyclopiazonic acid (CPA). Research has shown A. flavus strains can overcome heterokaryon incompatibility and undergo meiotic recombination as teleomorphs. Although evidence of recombination in the AF gene cluster has been reported, the impacts of recombination on genotype and metabolomic phenotype in a single generation are lacking. In previous studies, we paired an aflatoxigenic MAT1-1 A. flavus strain with a non-aflatoxigenic MAT1-2 A. flavus strain that had been tagged with green fluorescent protein and then 10 F1 progenies (a mix of fluorescent and non-fluorescent) were randomly selected from single-ascospore colonies and broadly examined for evidence of recombination. In this study, we determined four of those 10 F1 progenies were recombinants because they were not vegetatively compatible with either parent or their siblings, and they exhibited other distinctive traits that could only result from meiotic recombination. The other six progenies examined shared genomic identity with the non-aflatoxigenic, fluorescent, and MAT1-2 parent, but were metabolically distinct. This study highlights phenotypic and genomic changes that may occur in a single generation from the outcrossing of sexually compatible strains of A. flavus.

The fungal natural product fusidic acid demonstrates potent activity against Mycoplasma genitalium.

Antimicrobial resistance is extremely common in Mycoplasma genitalium, a frequent cause of urethritis in men and cervicitis, vaginitis, and pelvic inflammatory disease in women. Treatment of M. genitalium infections is difficult due to intrinsic and acquired resistance to many antibiotic classes. We undertook a program to identify novel antimicrobials with activity against M. genitalium from fungal natural products. Extracts of Ramularia coccinea contained a molecule with potent activity that was subsequently identified as fusidic acid, a fusidane-type antibiotic that has been in clinical use for decades outside the United States. We found that minimum inhibitory concentrations of fusidic acid ranged from 0.31 to 4 µg/mL among 17 M. genitalium strains including laboratory-passaged and low-passage clinical isolates. Time-kill data indicate that bactericidal killing occurs when M. genitalium is exposed to ≥10 µg/mL for 48 h, comparing favorably to serum concentrations obtained from typical loading dose regimens. Resistance to fusidic acid was associated with mutations in fusA consistent with the known mechanism of action in which fusidic acid inhibits protein synthesis by binding to elongation factor G. Interestingly, no mutants resistant to >10 µg/mL fusidic acid were obtained and a resistant strain containing a F435Y mutation in FusA was impaired for growth in vitro. These data suggest that fusidic acid may be a promising option for the treatment of M. genitalium infections.

Iron(III) Binding Properties of PF1140, a Fungal N-Hydroxypyridone, and Activity against Mycoplasma genitalium.

Mycoplasma genitalium is a sexually transmitted bacterium associated with urogenital disease syndromes in the US and worldwide. The global rise in drug resistance in M. genitalium necessitates the development of novel drugs to treat this pathogen. To address this need, we have screened extracts from a library of fungal isolates assembled through the University of Oklahoma Citizen Science Soil Collection Program. Analysis of one of the bioactive extracts using bioassay-guided fractionation led to the purification of the compound PF1140 (1) along with a new and several other known pyridones. The N-hydroxy pyridones are generally regarded as siderophores with high binding affinity for iron(III) under physiological conditions. Results from UV-vis absorption spectroscopy-based titration experiments revealed that 1 complexes with Fe3+. As M. genitalium does not utilize iron, we propose that the PF1140-iron complex induces cytotoxicity by facilitating the cellular uptake of iron, which reacts with endogenous hydrogen peroxide to produce toxic hydroxyl radicals.

Chemoreactive 2,5-Diketopiperazines from a Penicillium sp., Structure Revision of Reported Analogues and Proposed Facile Transformation Pathways.

Merkel cell carcinoma (MCC) is a rare and aggressive cutaneous cancer. Two new prenylated indole 2,5-diketopiperazine alkaloids, brevianamides E1 (1) and E2 (2), were isolated from a Penicillium fungus. Both compounds showed moderate cytotoxic activity against select MCC cell lines (i.e., MCC13, MKL-1, UISO, and WaGa) in the low micromolar range. The relative and absolute configurations of 1 and 2 were determined by combined approaches, including NOESY spectroscopy, DFT ECD and DP4 plus calculations, and Marfey's reaction. Literature research and the comparison of NMR and ECD data led to the structure revision of three previously reported natural analogues, notoamides K and P and asperversiamide L. The structurally unstable 1 and 2 underwent steady interconversion under neutral aqueous conditions. Investigation of the degradation of 2 in acidic methanol solutions led to the identification of a new methoxylated derivative (6) and two new ring-opened products (7 and 8) with the rearranged, elongated, 4-methylpent-3-ene side chain. The facile transformation of 2 to 7 and 8 was promoted by the intrinsic impurity (i.e., formaldehyde) of HPLC-grade methanol through the aza-Cope rearrangement.

In Vitro Susceptibility and Resistance of Mycoplasma genitalium to Nitroimidazoles.

Mycoplasma genitalium is a sexually transmitted reproductive tract pathogen of men and women. M. genitalium infections are increasingly difficult to treat due to poor efficacy of doxycycline and acquired resistance to azithromycin and moxifloxacin. A recent clinical trial suggested that metronidazole may improve cure rates for women with pelvic inflammatory disease and reduced the detection of M. genitalium when included with standard doxycycline plus ceftriaxone treatment. As data regarding susceptibility of mycoplasmas to nitroimidazoles are lacking in the scientific literature, we determined the in vitro susceptibility of 10 M. genitalium strains to metronidazole, secnidazole, and tinidazole. MICs ranged from 1.6 to 12.5 µg/mL for metronidazole, 3.1 to 12.5 µg/mL for secnidazole, and 0.8 to 6.3 µg/mL for tinidazole. None of these agents was synergistic with doxycycline in checkerboard broth microdilution assays. Tinidazole was superior to metronidazole and secnidazole in terms of MIC and time-kill kinetics and was bactericidal (>99.9% killing) at concentrations below reported serum concentrations. Mutations associated with nitroimidazole resistance were identified by whole-genome sequencing of spontaneous resistant mutants, suggesting a mechanism for reductive activation of the nitroimidazole prodrug by a predicted NAD(P)H-dependent flavin mononucleotide (FMN) oxidoreductase. The presence of oxygen did not affect MICs of wild-type M. genitalium, but a nitroimidazole-resistant mutant was defective for growth under anaerobic conditions, suggesting that resistant mutants may have a fitness disadvantage in anaerobic genital sites. Clinical studies are needed to determine if nitroimidazoles, especially tinidazole, are effective for eradicating M. genitalium infections in men and women.

Resolving a Natural Product Cold Case: Elucidation of Fusapyrone Structure and Absolute Configuration and Demonstration of Their Fungal Biofilm Disrupting Properties.

Fusapyrones are fungal metabolites, which have been reported to have broad-spectrum antibacterial and antifungal properties. Despite the first members of this chemical class being described three decades prior, many aspects of their structures have remained unresolved, thereby constraining efforts to fully understand structure-activity relationships within this metabolite family and impeding the design of streamlined syntheses. Among the main challenges posed by fusapyrones is the incorporation of several single and groups of stereocenters separated by atoms with freely rotating bonds, which have proven unyielding to spectroscopic analyses. In this study, we obtained a series of new (2-5 and 7-9) and previously reported fusapyrones (1 and 6), which were subjected to a combination of spectroscopic, chemical, and computational techniques enabling us to offer proposals for their full structures, as well as provide a pathway to reinterpreting the absolute configurations of other published fusapyrone metabolites. Biological testing of the fusapyrones revealed their abilities to inhibit and disrupt biofilms made by the human fungal pathogen, Candida albicans. These results show that fusapyrones reduce hyphae formation in C. albicans, as well as decrease the surface adherence capabilities of planktonic cells and cells transitioning into early-stage biofilm formation.

Persephacin Is a Broad-Spectrum Antifungal Aureobasidin Metabolite That Overcomes Intrinsic Resistance in Aspergillus fumigatus.

Fungi pose a persistent threat to humankind with worrying indications that emerging and re-emerging pathogens (e.g., Candida auris, Coccidioides spp., drug-resistant Aspergilli, and more) exhibit resistance to the limited number of approved antifungals. To address this problem, our team is exploring endophytic fungi as a resource for the discovery of new antifungal natural products. The rationale behind this decision is based on evidence that endophytes engage with plants in mutualistic relationships wherein some fungi actively participate by producing chemical defense measures that suppress pathogenic microorganisms. To improve the odds of bioactive metabolite discovery, we developed a new hands-free laser-cutting system capable of generating >50 plant samples per minute that, in turn, enabled our team to prepare and screen large numbers of endophytic fungi. One of the fungal isolates obtained in this way was identified as an Elsinoë sp. that produced a unique aureobasidin analogue, persephacin (1). Some distinctive features of 1 are the absence of both phenylalanine residues combined with the incorporation of a novel amino acid residue, persephanine (9). Compound 1 exhibits potent antifungal effects against a large number of pathogenic yeast (including several clinical C. auris strains), as well as phylogenetically diverse filamentous fungi (e.g., Aspergillus fumigatus). In an ex vivo eye infection model, compound 1 outperformed standard-of-care treatments demonstrating the ability to suppress fluconazole-resistant Candida albicans and A. fumigatus at a concentration (0.1% solution) well below the clinically recommended levels used for fluconazole and natamycin (2% and 5% solutions, respectively). In 3D tissue models for acute dermal and ocular safety, 1 was found to be nontoxic and nonirritating at concentrations required to elicit antifungal activity. Natural product 1 appears to be a promising candidate for further investigation as a broad-spectrum antifungal capable of controlling a range of pathogens that negatively impact human, animal, and plant health.

Appraisal of Fungus-Derived Xanthoquinodins as Broad-Spectrum Anti-Infectives Targeting Phylogenetically Diverse Human Pathogens.

Xanthoquinodins make up a distinctive class of xanthone-anthraquinone heterodimers reported as secondary metabolites from several fungal species. Through a collaborative multi-institutional screening program, a fungal extract prepared from a Trichocladium sp. was identified that exhibited strong inhibitory effects against several human pathogens (Mycoplasma genitalium, Plasmodium falciparum, Cryptosporidium parvum, and Trichomonas vaginalis). This report focuses on one of the unique samples that exhibited a desirable combination of biological effects: namely, it inhibited all four test pathogens and demonstrated low levels of toxicity toward HepG2 (human liver) cells. Fractionation and purification of the bioactive components and their congeners led to the identification of six new compounds [xanthoquinodins NPDG A1-A5 (1-5) and B1 (6)] as well as several previously reported natural products (7-14). The chemical structures of 1-14 were determined based on interpretation of their 1D and 2D NMR, HRESIMS, and electronic circular dichroism (ECD) data. Biological testing of the purified metabolites revealed that they possessed widely varying levels of inhibitory activity against a panel of human pathogens. Xanthoquinodins A1 (7) and A2 (8) exhibited the most promising broad-spectrum inhibitory effects against M. genitalium (EC50 values: 0.13 and 0.12 µM, respectively), C. parvum (EC50 values: 5.2 and 3.5 µM, respectively), T. vaginalis (EC50 values: 3.9 and 6.8 µM, respectively), and P. falciparum (EC50 values: 0.29 and 0.50 µM, respectively) with no cytotoxicity detected at the highest concentration tested (HepG2 EC50 > 25 µM).

Assessing Microbial Metabolic and Biological Diversity to Inform Natural Product Library Assembly.

The pressing need for novel chemical matter to support bioactive compound discovery has led natural product researchers to explore a wide range of source organisms and environments. One of the implicit guiding principles behind those efforts is the notion that sampling different environments is critical to accessing unique natural products. This idea was tested by comparing fungi from disparate biomes: aquatic sediments from Lake Michigan (USA) and terrestrial samples taken from the surrounding soils. Matched sets of Penicillium brevicompactum, Penicillium expansum, and Penicillium oxalicum from the two source environments were compared, revealing modest differences in physiological performance and chemical output. Analysis of LC-MS/MS-derived molecular feature data showed no source-dependent differences in chemical richness. High levels of scaffold homogeneity were also observed with 78-83% of scaffolds shared among the terrestrial and aquatic Penicillium spp. isolates. A comparison of the culturable fungi from the two biomes indicated that certain genera were more strongly associated with aquatic sediments (e.g., Trichoderma, Pseudeurotium, Cladosporium, and Preussia) versus the surrounding terrestrial environment (e.g., Fusarium, Pseudogymnoascus, Humicola, and Acremonium). Taken together, these results suggest that focusing efforts on sampling the microbial resources that are unique to an environment may have a more pronounced effect on enhancing the sought-after natural product diversity needed for chemical discovery and screening collections.

Tolypocladamides A-G: Cytotoxic Peptaibols from Tolypocladium inflatum.

Seven new peptaibols named tolypocladamides A-G have been isolated from an extract of the fungus Tolypocladium inflatum, which inhibits the interaction between Raf and oncogenic Ras in a cell-based high-throughput screening assay. Each peptaibol contains 11 amino acid residues, an octanoyl or decanoyl fatty acid chain at the N-terminus, and a leucinol moiety at the C-terminus. The peptaibol sequences were elucidated on the basis of 2D NMR and mass spectral fragmentation analyses. Amino acid configurations were determined by advanced Marfey's analyses. Tolypocladamides A-G caused significant inhibition of Ras/Raf interactions with IC50 values ranging from 0.5 to 5.0 µM in a nanobioluminescence resonance energy transfer (NanoBRET) assay; however, no interactions were observed in a surface plasmon resonance assay for binding of the compounds to wild type or G12D mutant Ras constructs or to the Ras binding domain of Raf. NCI 60 cell line testing was also conducted, and little panel selectivity was observed.

Leveraging Peptaibol Biosynthetic Promiscuity for Next-Generation Antiplasmodial Therapeutics.

Malaria remains a worldwide threat, afflicting over 200 million people each year. The emergence of drug resistance against existing therapeutics threatens to destabilize global efforts aimed at controlling Plasmodium spp. parasites, which is expected to leave vast portions of humanity unprotected against the disease. To address this need, systematic testing of a fungal natural product extract library assembled through the University of Oklahoma Citizen Science Soil Collection Program has generated an initial set of bioactive extracts that exhibit potent antiplasmodial activity (EC50 < 0.30 µg/mL) and low levels of toxicity against human cells (less than 50% reduction in HepG2 growth at 25 µg/mL). Analysis of the two top-performing extracts from Trichoderma sp. and Hypocrea sp. isolates revealed both contained chemically diverse assemblages of putative peptaibol-like compounds that were responsible for their antiplasmodial actions. Purification and structure determination efforts yielded 30 new peptaibols and lipopeptaibols (1-14 and 28-43), along with 22 known metabolites (15-27 and 44-52). While several compounds displayed promising activity profiles, one of the new metabolites, harzianin NPDG I (14), stood out from the others due to its noteworthy potency (EC50 = 0.10 µM against multi-drug-resistant P. falciparum line Dd2) and absence of gross toxicity toward HepG2 at the highest concentrations tested (HepG2 EC50 > 25 µM, selectivity index > 250). The unique chemodiversity afforded by these fungal isolates serves to unlock new opportunities for translating peptaibols into a bioactive scaffold worthy of further development.

An Integrated Strategy for the Detection, Dereplication, and Identification of DNA-Binding Biomolecules from Complex Natural Product Mixtures.

A fundamental factor in natural product drug discovery programs is the necessity to identify the active component(s) from complex chemical mixtures. Whereas this has traditionally been accomplished using bioassay-guided fractionation, we questioned whether alternative techniques could supplement and, in some cases, even supplant this approach. We speculated that a combination of ligand-fishing methods and modern analytical tools (e.g., LC-MS and online natural product databases) offered a route to enhance natural product drug discovery. Herein, a candidate solution referred to as the lickety-split ligand-affinity-based molecular angling system (LLAMAS) is described. This approach utilizes an ultrafiltration-based LC-PDA-MS/MS-guided DNA-binding assay in combination with the (i) Global Natural Products Social Molecular Networking, (ii) Dictionary of Natural Products, and (iii) SciFinder platforms to identify DNA binders in complex chemical mixtures. LLAMAS was initially vetted in tests using known small-molecule DNA binders and then optimized to a 96-well plate-based format. A set of 332 plant samples used in traditional Chinese medicine was screened for DNA-binding activity with LLAMAS, resulting in the identification of seven DNA-binding molecules, including berberine (12), palmatine (13), coptisine (14), fangchinoline (15), tetrandrine (16), daurisoline (17), and dauricine (18). These results demonstrate that LLAMAS is an effective natural product discovery platform for the efficient identification and dereplication of DNA-binding molecules from complex mixtures.

Roseabol A, a New Peptaibol from the Fungus Clonostachys rosea.

A new 11 amino acid linear peptide named roseabol A (1) and the known compound 13-oxo-trans-9,10-epoxy-11(E)-octadecenoic acid (2) were isolated from the fungus Clonostachys rosea. Combined NMR and MS analysis revealed that roseabol A (1) contained amino acid residues characteristic of the peptaibol family of peptides such as isovaline, α-aminoisobutyric acid, hydroxyproline, leucinol, and an N-terminal isovaleric acid moiety. The amino acid sequence was established by a combination of NMR studies and tandem MS fragmentation analyses, and the absolute configurations of the constituent amino acids of 1 were determined by the advanced Marfey's method. Compound 2 showed inhibitory activity against Merkel cell carcinoma, a rare and difficult-to-treat type of skin cancer, with an IC50 value of 16.5 µM.

Leucinostatins from Ophiocordyceps spp. and Purpureocillium spp. Demonstrate Selective Antiproliferative Effects in Cells Representing the Luminal Androgen Receptor Subtype of Triple Negative Breast Cancer.

The structures of four leucinostatin analogues (1-4) from Ophiocordyceps spp. and Purpureocillium spp. were determined together with six known leucinostatins [leucinostatins B (5), A (6), B2 (7), A2 (8), F (9), and D (10)]. The structures of the metabolites were established using a combination of analytical methods including HRESIMS and MS/MS experiments, 1D and 2D NMR spectroscopy, chiral HPLC, and advanced Marfey's analysis of the acid hydrolysate, as well as additional empirical and chemical methods. Compounds 1-10 were evaluated for their biological effects on triple negative breast cancer (TNBC) cells. Leucinostatins 1-10 showed selective cytostatic activities in MDA-MB-453 and SUM185PE cells representing the luminal androgen receptor subtype of TNBC. This selective activity motivated further investigation into the mechanism of action of leucinostatin B (5). The results demonstrate that this peptidic fungal metabolite rapidly inhibits mTORC1 signaling in leucinostatin-sensitive TNBC cell lines, but not in leucinostatin-resistant cells. Leucinostatins have been shown to repress mitochondrial respiration through inhibition of the ATP synthase, and we demonstrated that both the mTORC1 signaling and LAR-selective activities of 5 were recapitulated by oligomycin. Thus, inhibition of the ATP synthase with either leucinostatin B or oligomycin is sufficient to selectively impede mTORC1 signaling and inhibit the growth of LAR-subtype cells.

Triple-Negative Breast Cancer Cells Exhibit Differential Sensitivity to Cardenolides from Calotropis gigantea.

Triple-negative breast cancers (TNBC) are aggressive and heterogeneous cancers that lack targeted therapies. We implemented a screening program to identify new leads for subgroups of TNBC using diverse cell lines with different molecular drivers. Through this program, we identified an extract from Calotropis gigantea that caused selective cytotoxicity in BT-549 cells as compared to four other TNBC cell lines. Bioassay-guided fractionation of the BT-549 selective extract yielded nine cardenolides responsible for the selective activity. These included eight known cardenolides and a new cardenolide glycoside. Structure-activity relationships among the cardenolides demonstrated a correlation between their relative potencies toward BT-549 cells and Na+/K+ ATPase inhibition. Calotropin, the compound with the highest degree of selectivity for BT-549 cells, increased intracellular Ca2+ in sensitive cells to a greater extent than in the resistant MDA-MB-231 cells. Further studies identified a second TNBC cell line, Hs578T, that is also highly sensitive to the cardenolides, and mechanistic studies were conducted to identify commonalities among the sensitive cell lines. Experiments showed that both cardenolide-sensitive cell lines expressed higher mRNA levels of the Na+/Ca2+ exchanger NCX1 than resistant TNBC cells. This suggests that NCX1 could be a biomarker to identify TNBC patients that might benefit from the clinical administration of a cardiac glycoside for anticancer indications.

Using the Cancer Dependency Map to Identify the Mechanism of Action of a Cytotoxic Alkenyl Derivative from the Fruit of Choerospondias axillaris.

An extract prepared from the fruit of Choerospondias axillaris exhibited differential cytotoxic effects when tested in a panel of pediatric cancer cell lines [Ewing sarcoma (A-673), rhabdomyosarcoma (SJCRH30), medulloblastoma (D283), and hepatoblastoma (Hep293TT)]. Bioassay-guided fractionation led to the purification of five new hydroquinone-based metabolites, choerosponols A-E (1-5), bearing unsaturated hydrocarbon chains. The structures of the natural products were determined using a combination of 1D and 2D NMR, HRESIMS, ECD spectroscopy, and Mosher ester analyses. The purified compounds were evaluated for their antiproliferative and cytotoxic activities, revealing that 1, which contains a benzofuran moiety, exhibited over 50-fold selective antiproliferative activity against Ewing sarcoma and medulloblastoma cells with growth inhibitory (GI50) values of 0.19 and 0.07 µM, respectively. The effects of 1 were evaluated in a larger panel of cancer cell lines, and these data were used in turn to interrogate the Project Achilles cancer dependency database, leading to the identification of the MCT1 transporter as a functional target of 1. These data highlight the utility of publicly available cancer dependency databases such as Project Achilles to facilitate the identification of the mechanisms of action of compounds with selective activities among cancer cell lines, which can be a major challenge in natural products drug discovery.

CRISPR-Cas9 Genome-Wide Knockout Screen Identifies Mechanism of Selective Activity of Dehydrofalcarinol in Mesenchymal Stem-like Triple-Negative Breast Cancer Cells.

There are no targeted therapies available for triple-negative breast cancers (TNBCs) in part because they represent a heterogeneous group of tumors with diverse oncogenic drivers. Our goal is to identify targeted therapies for subtypes of these cancers using a mechanism-blind screen of natural product extract libraries. An extract from Desmanthodium guatemalense was 4-fold more potent for cytotoxicity against MDA-MB-231 cells, which represent the mesenchymal stem-like (MSL) subtype, as compared to cells of other TNBC subtypes. Bioassay-guided fractionation led to the isolation of six polyacetylenes, and subsequent investigations of plant sources known to produce polyacetylenes yielded six additional structurally related compounds. A subset of these compounds retained selective cytotoxic effects in MSL subtype cells. Studies suggest that these selective effects do not appear to be due to PPARγ agonist activities that have previously been reported for polyacetylenes. A CRISPR-Cas9-mediated gene knockout screen was employed to identify the mechanism of selective cytotoxic activity of the most potent and selective compound, dehydrofalcarinol (1a). This genomic screen identified HSD17B11, the gene encoding the enzyme 17ß-hydroxysteroid dehydrogenase type 11, as a mediator of the selective cytotoxic effects of 1a in MDA-MB-231 cells that express high levels of this protein. The Project Achilles cancer dependency database further identified a subset of Ewing sarcoma cell lines as highly dependent on HSD17B11 expression, and it was found these were also highly sensitive to 1a. This report demonstrates the value of CRISPR-Cas9 genome-wide screens to identify the mechanisms underlying the selective activities of natural products.

Chemoenzymatic synthesis of daptomycin analogs active against daptomycin-resistant strains.

Daptomycin is a last resort antibiotic for the treatment of infections caused by many Gram-positive bacterial strains, including vancomycin-resistant Enterococcus (VRE) and methicillin- and vancomycin-resistant Staphylococcus aureus (MRSA and VRSA). However, the emergence of daptomycin-resistant strains of S. aureus and Enterococcus in recent years has renewed interest in synthesizing daptomycin analogs to overcome resistance mechanisms. Within this context, three aromatic prenyltransferases have been shown to accept daptomycin as a substrate, and the resulting prenylated analog was shown to be more potent against Gram-positive strains than the parent compound. Consequently, utilizing prenyltransferases to derivatize daptomycin offered an attractive alternative to traditional synthetic approaches, especially given the molecule's structural complexity. Herein, we report exploiting the ability of prenyltransferase CdpNPT to synthesize alkyl-diversified daptomycin analogs in combination with a library of synthetic non-native alkyl-pyrophosphates. The results revealed that CdpNPT can transfer a variety of alkyl groups onto daptomycin's tryptophan residue using the corresponding alkyl-pyrophosphates, while subsequent scaled-up reactions suggested that the enzyme can alkylate the N1, C2, C5, and C6 positions of the indole ring. In vitro antibacterial activity assays using 16 daptomycin analogs revealed that some of the analogs displayed 2-80-fold improvements in potency against MRSA, VRE, and daptomycin-resistant strains of S. aureus and Enterococcus faecalis. Thus, along with the new potent analogs, these findings have established that the regio-chemistry of alkyl substitution on the tryptophan residue can modulate daptomycin's potency. With additional protein engineering to improve the regio-selectivity, the described method has the potential to become a powerful tool for diversifying complex indole-containing molecules. KEY POINTS: • CdpNPT displays impressive donor promiscuity with daptomycin as the acceptor. • CdpNPT catalyzes N1-, C2-, C5-, and C6-alkylation on daptomycin's tryptophan residue. • Differential alkylation of daptomycin's tryptophan residue modulates its activity.

Unique amalgamation of primary and secondary structural elements transform peptaibols into potent bioactive cell-penetrating peptides.

Mass-spectrometry-based metabolomics and molecular phylogeny data were used to identify a metabolically prolific strain of Tolypocladium that was obtained from a deep-water Great Lakes sediment sample. An investigation of the isolate's secondary metabolome resulted in the purification of a 22-mer peptaibol, gichigamin A (1). This peptidic natural product exhibited an amino acid sequence including several ß-alanines that occurred in a repeating ααß motif, causing the compound to adopt a unique right-handed 311 helical structure. The unusual secondary structure of 1 was confirmed by spectroscopic approaches including solution NMR, electronic circular dichroism (ECD), and single-crystal X-ray diffraction analyses. Artificial and cell-based membrane permeability assays provided evidence that the unusual combination of structural features in gichigamins conferred on them an ability to penetrate the outer membranes of mammalian cells. Compound 1 exhibited potent in vitro cytotoxicity (GI50 0.55 ± 0.04 µM) and in vivo antitumor effects in a MIA PaCa-2 xenograft mouse model. While the primary mechanism of cytotoxicity for 1 was consistent with ion leakage, we found that it was also able to directly depolarize mitochondria. Semisynthetic modification of 1 provided several analogs, including a C-terminus-linked coumarin derivative (22) that exhibited appreciably increased potency (GI50 5.4 ± 0.1 nM), but lacked ion leakage capabilities associated with a majority of naturally occurring peptaibols such as alamethicin. Compound 22 was found to enter intact cells and induced cell death in a process that was preceded by mitochondrial depolarization.

An Electrophilic Natural Product Provides a Safe and Robust Odor Neutralization Approach To Counteract Malodorous Organosulfur Metabolites Encountered in Skunk Spray.

The anal secretions of skunks comprise several types of malodorous organosulfur compounds. The pungent metabolites are used defensively by skunks to repel threats posed by predators, and in many parts of the world, those perceived threats include humans and their pets. The extremely low thresholds for detection of the organosulfur metabolites make efforts to "de-skunk" people, animals, and clothing a process fraught with many challenges. The fungal-derived metabolite pericosine A (4) is a promiscuous yet stabile electrophilic compound that we propose is used by some fungi as a novel form of chemical defense. Our investigations have indicated that pericosine A readily reacts with skunk-spray secretions to transform them into odorless products. Mechanistic and computational studies suggested that pericosine A and its synthetic analogues react via SN2'-type mechanisms with thiols and thioacetates under aqueous conditions to generate stable thioethers. Testing revealed that pericosine A did not cause skin or eye irritation and was highly effective at deodorizing skunk anal gland secretions when formulated to include adjunctive cosmetic ingredients.