Biscatecholate–Monohydroxamate Mixed Ligand Siderophore–Carbacephalosporin Conjugates are Selective Sideromycin Antibiotics that Target Acinetobacter baumannii

Chemical syntheses and biological evaluation of biscatecholate–monohydroxamate mixed ligand sideromycins utilizing the carbacephalosporin β-lactam antibiotic loracarbef and the fluoroquinolone antibiotic ciprofloxacin are described. The mixed ligand β-lactam sideromycin (<b>1b</b>) had remarkably selective and extremely potent antibacterial activity against the Gram-negative pathogen Acinetobacter baumannii ATCC 17961 (MIC = 0.0078 μM). The antibacterial activity of the β-lactam sideromycin was inversely related to the iron­(III) concentration in the testing media and was antagonized by the presence of the competing parent siderophore. These data suggested that active transport of the mixed ligand β-lactam sideromycin across the outer cell membrane of A. baumannii via siderophore-uptake pathways was responsible for the selective and potent antibacterial activity

Rigid Oxazole Acinetobactin Analog Blocks Siderophore Cycling in <i>Acinetobacter baumannii</i>

The emergence of multidrug resistant (MDR) Gram-negative bacterial pathogens has raised global concern. Nontraditional therapeutic strategies, including antivirulence approaches, are gaining traction as a means of applying less selective pressure for resistance <i>in vivo</i>. Here, we show that rigidifying the structure of the siderophore preacinetobactin from MDR <i>Acinetobacter baumannii</i> via oxidation of the phenolate-oxazoline moiety to a phenolate-oxazole results in a potent inhibitor of siderophore transport and imparts a bacteriostatic effect at low micromolar concentrations under infection-like conditions

Iron(III)-Templated Macrolactonization of Trihydroxamate Siderophores

A method was developed to synthesize macrocyclic trihydroxamate siderophores using optimized Yamaguchi macrolactonization conditions. The natural ability of siderophores to bind iron(III) was exploited to template the reactions and allowed for rapid reaction rates, high product conversions, and the formation of large macrolactone rings up to 35 atoms. An X-ray structure of a 33-membered macrolactone siderophore–Fe(III) complex is presented

Immobilized FhuD2 Siderophore-Binding Protein Enables Purification of Salmycin Sideromycins from <i>Streptomyces violaceus</i> DSM 8286

Siderophores are a structurally diverse class of natural products common to most bacteria and fungi as iron­(III)-chelating ligands. Siderophores, including trihydroxamate ferrioxamines, are used clinically to treat iron overload diseases and show promising activity against many other iron-related human diseases. Here, we present a new method for the isolation of ferrioxamine siderophores from complex mixtures using affinity chromatography based on resin-immobilized FhuD2, a siderophore-binding protein (SBP) from <i>Staphylococcus aureus</i>. The SBP-resin enabled purification of charge positive, charge negative, and neutral ferrioxamine siderophores. Treatment of culture supernatants from <i>Streptomyces violaceus</i> DSM 8286 with SBP-resin provided an analytically pure sample of the salmycins, a mixture of structurally complex glycosylated sideromycins (siderophore–antibiotic conjugates) with potent antibacterial activity toward human pathogenic <i>Staphylococcus aureus</i> (minimum inhibitory concentration (MIC) = 7 nM). Siderophore affinity chromatography could enable the rapid discovery of new siderophore and sideromycin natural products from complex mixtures to aid drug discovery and metabolite identification efforts in a broad range of therapeutic areas

Mechanistic Basis for ATP-Dependent Inhibition of Glutamine Synthetase by Tabtoxinine-β-lactam

Tabtoxinine-β-lactam (TβL), also known as wildfire toxin, is a time- and ATP-dependent inhibitor of glutamine synthetase produced by plant pathogenic strains of <i>Pseudomonas syringae</i>. Here we demonstrate that recombinant glutamine synthetase from <i>Escherichia coli</i> phosphorylates the C3-hydroxyl group of the TβL 3-(<i>S</i>)-hydroxy-β-lactam (3-HβL) warhead. Phosphorylation of TβL generates a stable, noncovalent enzyme–ADP–inhibitor complex that resembles the glutamine synthetase tetrahedral transition state. The TβL β-lactam ring remains intact during enzyme inhibition, making TβL mechanistically distinct from traditional β-lactam antibiotics such as penicillin. Our findings could enable the design of new 3-HβL transition state inhibitors targeting enzymes in the ATP-dependent carboxylate-amine ligase superfamily with broad therapeutic potential in many disease areas