Ashraf Ibrahim

TLN-05220, TLN-05223, new Echinosporamicin-type antibiotics, and proposed revision of the structure of bravomicins(*).

The deposited strain of the hazimicin producer, Micromonospora echinospora ssp. challisensis NRRL 12255 has considerable biosynthetic capabilities as revealed by genome scanning. Among these is a locus containing both type I and type II PKS genes. The presumed products of this locus, TLN-05220 (1) and TLN-05223 (2), bear a core backbone composed of six fused rings starting with a 2-pyridone moiety. The structures were confirmed by conventional spectral analyses including MS, and 1D and 2D NMR experiments. Comparison of both the 1H and 13C NMR data of the newly isolated compound with those of echinosporamicin and bravomicin A led us to propose a revision of the structure of the latter to include a 2-pyridone instead of the pyran originally postulated. Both compounds (1 and 2) possessed strong antibacterial activity against a series of gram-positive pathogens including several strains of methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococci (VRE), and cytotoxic activities against several human tumor cell lines. The TLN compounds are the first of this group with reported anticancer activity.

Informatic strategies for the discovery of polyketides and nonribosomal peptides

Modern medicine underwent a major transformation when microbes were first industrially exploited for their ability to produce antibiotic agents. These natural products often belonged to two classes, known as polyketides and nonribosomal peptides. Exhaustive bioactivity screening has lead to the discovery of hundreds of these bioactive compounds, whose rediscovery confounds new exploratory efforts. Still, microbial genomes have shown us that there are many more polyketides and nonribosomal peptides remaining to be discovered. A modern challenge and opportunity exists in our ability to link genomic and metabolomic data, using novel informatic methods to find new bioactive natural products. This “genome-mining” approach, along with other recent informatic platforms for discovering polyketides and nonribosomal peptides will be the focus of this review.

Exploration of Nonribosomal Peptide Families with an Automated Informatic Search Algorithm

Microbial natural products are some of the most important pharmaceutical agents and possess unparalleled chemical diversity. Here we present an untargeted metabolomics algorithm that builds on our validated iSNAP platform to rapidly identify families of peptide natural products. By utilizing known or in silico-dereplicated seed structures, this algorithm screens tandem mass spectrometry data to elaborate extensive molecular families within crude microbial culture extracts with high confidence and statistical significance. Analysis of peptide natural product producers revealed an abundance of unreported congeners, revealing one of the largest families of natural products described to date, as well as a novel variant with greater potency. These findings demonstrate the effectiveness of the iSNAP platform as an accurate tool for rapidly profiling large families of nonribosomal peptides.

Trienylfuranol A and trienylfuranone A–B: metabolites isolated from an endophytic fungus, Hypoxylon submoniticulosum , in the raspberry Rubus idaeus

A strain of Hypoxylon submonticulosum was isolated as an endophyte from a surface-sterilized leaf of a cultivated raspberry (Rubus idaeus). The liquid culture extract displayed growth inhibition activity against Saccharomyces cerevisiae using a disc diffusion assay. The extract’s major component was identified as a new natural product, trienylfuranol A (1S,2S,4R)-1-((1′E,3′E)-hexa-1′,3′,5′-trienyl)-tetrahydro-4-methylfuran-2-ol (1), by high-resolution LC-MS and 1D and 2D NMR spectroscopy. Two additional new metabolites, trienylfuranones A (2) and B (3), were isolated as minor components of the extract and their structure elucidation revealed that they were biosynthetically related to 1. Absolute stereochemical configurations of compounds 1–3 were confirmed by NOE NMR experiments and by the preparation of Mosher esters. Complete hydrogenation of 1 yielded tetrahydrofuran 7 that was used for stereochemical characterization and assessment of antifungal activity.

Epoxynemanione A, nemanifuranones A–F, and nemanilactones A–C, from Nemania serpens, an endophytic fungus isolated from Riesling grapevines

Ten polyketide specialized metabolites, epoxynemanione A, nemanifuranones A-F, and nemanilactones A-C, were isolated from the culture filtrate of Nemania serpens (Pers.) Grey (1821), an endophytic fungus from a Riesling grapevine (Vitis vinifera) found in Canadas Niagara region. Additionally, four known metabolites 2-(hydroxymethyl)-3-methoxy-benzoic acid, phyllostine, 5-methylmellein and a nordammarane triterpenoid were isolated. A related known metabolite 2,3-dihydro-2-hydroxy-2,4-dimethyl-5-trans-propenylfuran-3-one has also been included for structural and biological comparison to the nemanifuranones. The latter was isolated from the culture filtrates of Mollisia nigrescens, an endophytic fungus from the leaves and stems of lowbush blueberry (Vaccinium angustifolium) found in the Acadian forest of Nova Scotia, Canada. Their structures were elucidated based on 1D and 2D NMR, HRESIMS measurements, X-ray crystallographic analysis of nemanifuranone A, the nordammarane triterpenoid and 2,3-dihydro-2-hydroxy-2,4-dimethyl-5-trans-propenylfuran-3-one compounds, and comparison of NOE and vicinal 1H-1H coupling constants to literature data for relative stereochemical assignments. Nemanifuranone A possesses a rare C2 hemiacetal and was active against both Gram-negative and Gram-positive bacteria.