Pyochelin enantiomers and their outer-membrane siderophore transporters in fluorescent pseudomonads: structural bases for unique enantiospecific recognition

Functional biosynthetic stereodivergence in a gene cluster via a dihydrosydnone N-oxide

Chirality plays a critical role in the biochemistry of life and often only one enantiomeric series is observed (homochirality). Only a few natural products have been obtained as racemates, e.g. the signalling molecule valdiazen produced by Burkholderia cenocepacia H111. In this study, we investigated the ham biosynthetic gene cluster and discovered that both the enantiomerically pure (R)-fragin and the racemic valdiazen result from the same pathway. This stereodivergence is based on the unusual heterocyclic intermediate dihydrosydnone N-oxide, as evident from gene knockout, stable isotope feeding experiments, and mass spectrometry experiments. Both non-enzymatic racemisation via keto-enol tautomerisation and enzyme-mediated dynamic kinetic resolution were found to be crucial to this stereodivergent pathway. This novel mechanism underpins the production of configurationally and biologically distinct metabolites from a single gene cluster. Our findings highlight the intricate design of an intertwined biosynthetic pathway and provide a deeper understanding of microbial secondary metabolism related to microbial communication.

Siderophores as tools and treatments

In the search for iron, an essential element in many biochemical processes, microorganisms biosynthesise dedicated chelators, known as siderophores, to sequester iron from their environment and actively transport the siderophore complex into the cell. This process has been implicated in bacterial pathogenesis and exploited through siderophore-antibiotic conjugates as a method for selective antibiotic delivery. Here we review this Trojan-horse approach including design considerations and potential in diagnostics and infection imaging.

Antimicrobial Potency of Nor-Pyochelin Analogues and Their Cation Complexes against Multidrug-Resistant Pathogens

The opportunistic pathogen Pseudomonas aeruginosa develops increasing resistance toward even the most potent antibiotics. Like other bacteria, the pathogen produces a number of virulence factors including metallophores, which constitute an important group. Pseudomonads produce the iron-chelating metallophore (siderophore) pyochelin, which, in addition to its iron-scavenging ability, is an effector for the transcriptional regulator PchR in its FeIII-bound form (ferripyochelin). In the present study, docking studies predicted a major ferripyochelin binding site in PchR, which prompted the exploration of nor-pyochelin analogues to produce tight binding to PchR, and thereby upregulation of the pyochelin metabolism. In addition, we investigated the effects of using the analogues to bind the antimicrobial cations GaIII and InIII. Selected analogues of nor-pyochelin were synthesized, and their GaIII- and InIII-based complexes were assessed for antimicrobial activity. The results indicate that the GaIII complexes inhibit the pathogens under iron-limited conditions, while the InIII-based systems are more effective in iron-rich media. Several of the GaIII complexes were shown to be highly effective against a multidrug-resistant P. aeruginosa clinical isolate, with minimum inhibitory concentrations (MICs) of ≤1 μg/mL. Similarly, two of the InIII-based systems were particularly effective against the isolate, with an MIC of 8 μg/mL. These results show high promise in comparison with other, traditionally potent antibiotics, as the compounds generally indicated low cytotoxicity toward mammalian cells. Preliminary mechanistic investigations using pseudomonal transposon mutants suggested that the inhibitory effects of the InIII-based systems could be due to acute iron deficiency as a result of InIII-bound bacterioferritin.

Plants, Microorganisms and Their Metabolites in Supporting Asbestos Detoxification-A Biological Perspective in Asbestos Treatment

Many countries banned asbestos due to its toxicity, but considering its colossal use, especially in the 1960s and 1970s, disposing of waste containing asbestos is the current problem. Today, many asbestos disposal technologies are known, but they usually involve colossal investment and operating expenses, and the end- and by-products of these methods negatively impact the environment. This paper identifies a unique modern direction in detoxifying asbestos minerals, which involves using microorganisms and plants and their metabolites. The work comprehensively focuses on the interactions between asbestos and plants, bacteria and fungi, including lichens and, for the first time, yeast. Biological treatment is a prospect for in situ land reclamation and under industrial conditions, which can be a viable alternative to landfilling and an environmentally friendly substitute or supplement to thermal, mechanical, and chemical methods, often characterized by high cost intensity. Plant and microbial metabolism products are part of the green chemistry trend, a central strategic pillar of global industrial and environmental development.

Preclinical characterisation of gallium-68 labeled ferrichrome siderophore stereoisomers for PET imaging applications

BACKGROUND

Siderophores are small iron-binding molecules produced by microorganisms to facilitate iron acquisition from the environment. Radiolabelled siderophores offer a promising solution for infection imaging, as they can specifically target the pathophysiological mechanisms of pathogens. Gallium-68 can replace the iron in siderophores, enabling molecular imaging with positron emission tomography (PET). Stereospecific interactions play a crucial role in the recognition of receptors, transporters, and iron utilisation. Furthermore, these interactions have an impact on the host environment, affecting pharmacokinetics and biodistribution. This study examines the influence of siderophore stereoisomerism on imaging properties, with a focus on ferrirubin (FR) and ferrirhodin (FRH), two cis-trans isomeric siderophores of the ferrichrome type.

RESULTS

Tested siderophores were labelled with gallium-68 with high radiochemical purity. The resulting complexes differed in their in vitro characteristics. [68Ga]Ga-FRH showed less hydrophilic properties and higher protein binding values than [68Ga]Ga-FR. The stability studies confirmed the high radiochemical stability of both [68Ga]Ga-siderophores in all examined media. Both siderophores were found to be taken up by S. aureus, K. pneumoniae and P. aeruginosa with similar efficacy. The biodistribution tested in normal mice showed rapid renal clearance with low blood pool retention and fast clearance from examined organs for [68Ga]Ga-FR, whereas [68Ga]Ga-FRH showed moderate retention in blood, resulting in slower pharmacokinetics. PET/CT imaging of mice injected with [68Ga]Ga-FR and [68Ga]Ga-FRH confirmed findings from ex vivo biodistribution studies. In a mouse model of S. aureus myositis, both radiolabeled siderophores showed radiotracer accumulation at the site of infection.

CONCLUSIONS

The 68Ga-complexes of stereoisomers ferrirubin and ferrirhodin revealed different pharmacokinetic profiles. In vitro uptake was not affected by isomerism. Both compounds had uptake with the same bacterial culture with similar efficacy. PET/CT imaging showed that the [68Ga]Ga-complexes accumulate at the site of S. aureus infection, highlighting the potential of [68Ga]Ga-FR as a promising tool for infection imaging. In contrast, retention of the radioactivity in the blood was observed for [68Ga]Ga-FRH. In conclusion, the stereoisomerism of potential radiotracers should be considered, as even minor structural differences can influence their pharmacokinetics and, consequently, the results of PET imaging.

TonB-Dependent Transport Across the Bacterial Outer Membrane

TonB-dependent transporters (TBDTs) are present in all gram-negative bacteria and mediate energy-dependent uptake of molecules that are too scarce or large to be taken up efficiently by outer membrane (OM) diffusion channels. This process requires energy that is derived from the proton motive force and delivered to TBDTs by the TonB-ExbBD motor complex in the inner membrane. Together with the need to preserve the OM permeability barrier, this has led to an extremely complex and fascinating transport mechanism for which the fundamentals, despite decades of research, are still unclear. In this review, we describe our current understanding of the transport mechanism of TBDTs, their potential role in the delivery of novel antibiotics, and the important contributions made by TBDT-associated (lipo)proteins.

Pseudomonas aeruginosa and its multiple strategies to access iron

Pseudomonas aeruginosa is a ubiquitous bacterium found in many natural and man-made environments. It is also a pathogen for plants, animals, and humans. As for almost all living organisms, iron is an essential nutrient for the growth of P. aeruginosa. The bacterium has evolved complex systems to access iron and maintain its homeostasis to survive in diverse natural and dynamic host environments. To access ferric iron, P. aeruginosa is able to produce two siderophores (pyoverdine and pyochelin), as well as use a variety of siderophores produced by other bacteria (mycobactins, enterobactin, ferrioxamine, ferrichrome, vibriobactin, aerobactin, rhizobactin and schizokinen). Furthermore, it can also use citrate, in addition to catecholamine neuromediators and plant-derived mono catechols, as siderophores. The P. aeruginosa genome also encodes three heme-uptake pathways (heme being an iron source) and one ferrous iron acquisition pathway. This review aims to summarize current knowledge concerning the molecular mechanisms involved in all the iron and heme acquisition strategies used by P. aeruginosa.

Genomics-driven discovery of chiral triscatechol siderophores with enantiomeric Fe(iii) coordination

Ferric complexes of triscatechol siderophores may assume one of two enantiomeric configurations at the iron site. Chirality is known to be important in the iron uptake process, however an understanding of the molecular features directing stereospecific coordination remains ambiguous. Synthesis of the full suite of (DHB^L/DLys^L/DSer)₃ macrolactone diastereomers, which includes the siderophore cyclic trichrysobactin (CTC), enables the effects that the chirality of Lys and Ser residues exert on the configuration of the Fe(iii) complex to be defined. Computationally optimized geometries indicate that the Λ/Δ configurational preferences are set by steric interactions between the Lys sidechains and the peptide backbone. The ability of each (DHB^L/DLys^L/DSer)₃ diastereomer to form a stable Fe(iii) complex prompted a genomic search for biosynthetic gene clusters (BGCs) encoding the synthesis of these diastereomers in microbes. The genome of the plant pathogen Dickeya chrysanthemi EC16 was sequenced and the genes responsible for the biosynthesis of CTC were identified. A related but distinct BGC was identified in the genome of the opportunistic pathogen Yersinia frederiksenii ATCC 33641; isolation of the siderophore from Y. frederiksenii ATCC 33641, named frederiksenibactin (FSB), revealed the triscatechol oligoester, linear-(DHB^LLys^LSer)₃. Circular dichroism (CD) spectroscopy establishes that Fe(iii)-CTC and Fe(iii)-FSB are formed in opposite enantiomeric configuration, consistent with the results of the ferric complexes of the cyclic (DHB^L/DLys^L/DSer)₃ diastereomers.

Iron coordination to pyochelin siderophore influences dynamics of FptA receptor from Pseudomonas aeruginosa: a molecular dynamics simulation study

FptA is a TonB-dependent transporter that permits the high affinity binding and transport of Fe(III)-pyochelin complex across the outer membrane of Pseudomonas aeruginosa. Molecular dynamics simulations were employed to FptA receptor and its complexes with pyochelin, and co-crystallized Fe(III)-pyochelin-ethanediol and Fe(III)-pyochelin-water embedded in dilauroyl phosphatidyl choline bilayer for the evaluation of their structural and dynamical properties. The evaluation of properties of the receptor bound to pyochelin molecule and Fe(III)-pyochelin complexes helped to figure out the iron coordination effect on the receptor properties. Moreover, comparison of these four simulation systems revealed further information on the dynamical changes occurred in extracellular loops, in particular loop-7 corresponding to the missing amino acid residues including the close-by loop-8 that was largely affected by the metal coordination to pyochelin. The binding of iron to pyochelin molecule affected the overall structure of the receptor therefore, evaluation fo the gyration radii and hydrogen bonding were evaluated as well as analysis of the pore size were also carried out to understand the effect of metal coordination on the dynamics of the helices which form a kind of translocation channel to transport the siderophore across the FptA protein into the periplasmic space. The properties of each component of the molecular systems were therefore observed to be perturbed by the incorporation of iron to the pyochelin molecule thus demonstrating that the bacteria use its receptor to abstract and transport iron from extracellular environment for its survival and that was made possible to understand at the molecular level through successful implementation of molecular dynamics simulations.

Iron Acquisition Systems of Gram-negative Bacterial Pathogens Define TonB-Dependent Pathways to Novel Antibiotics

Iron is an indispensable metabolic cofactor in both pro- and eukaryotes, which engenders a natural competition for the metal between bacterial pathogens and their human or animal hosts. Bacteria secrete siderophores that extract Fe3+ from tissues, fluids, cells, and proteins; the ligand gated porins of the Gram-negative bacterial outer membrane actively acquire the resulting ferric siderophores, as well as other iron-containing molecules like heme. Conversely, eukaryotic hosts combat bacterial iron scavenging by sequestering Fe3+ in binding proteins and ferritin. The variety of iron uptake systems in Gram-negative bacterial pathogens illustrates a range of chemical and biochemical mechanisms that facilitate microbial pathogenesis. This document attempts to summarize and understand these processes, to guide discovery of immunological or chemical interventions that may thwart infectious disease.

Mining Public Mass Spectrometry Data to Characterize the Diversity and Ubiquity of P. aeruginosa Specialized Metabolites

Pseudomonas aeruginosa is a ubiquitous environmental bacterium that causes chronic infections of burn wounds and in the lungs of cystic fibrosis (CF) patients. Vital to its infection is a myriad of specialized metabolites that serve a variety of biological roles including quorum sensing, metal chelation and inhibition of other competing bacteria. This study employed newly available algorithms for searching individual tandem mass (MS/MS) spectra against the publicly available Global Natural Product Social Molecular Networking (GNPS) database to identify the chemical diversity of these compounds and their presence in environmental, laboratory and clinical samples. For initial characterization, the metabolomes of eight clinical isolates of P. aeruginosa were analyzed using liquid chromatography-tandem mass spectrometry (LC-MS/MS) and uploaded to GNPS for spectral searching. Quinolones, rhamnolipids, phenazines and siderophores were identified and characterized; including the discovery of modified forms of the iron chelator pyochelin. Quinolones were highly diverse with the three base forms Pseudomonas quinolone signal 2-heptyl-3-hydroxy-4(1H)-quinolone (PQS), 4-heptyl-4(1H)-quinolone (HHQ) and 2-heptyl-4-quinolone-N-oxide (HQNO) having extensive variation in the length of their acyl chain from as small as 3 carbons to as large as 17. Rhamnolipids were limited to either one or two sugars with a limited set of fatty acyl chains, but the base lipid form without the rhamnose was also detected. These specialized metabolites were identified from diverse sources including ant-fungal mutualist dens, soil, plants, human teeth, feces, various lung mucus samples and cultured laboratory isolates. Their prevalence in fecal samples was particularly notable as P. aeruginosa is not known as a common colonizer of the human gut. The chemical diversity of the compounds identified, particularly the quinolones, demonstrates a broad spectrum of chemical properties within these the metabolite groups with likely significant impacts on their biological functions. Mining public data with GNPS enables a new approach to characterize the chemical diversity of biological organisms, which includes enabling the discovery of new chemistry from pathogenic bacteria.

The structure of the bacterial iron-catecholate transporter Fiu suggests that it imports substrates via a two-step mechanism

The ferric iron uptake (Fiu) transporter from Escherichia coli functions in the transport of iron–catecholate complexes across the bacterial outer membrane, providing the bacterium with iron, which is essential for growth. Recently it has become clear that Fiu also represents a liability for E. coli because its activity allows import of antimicrobial compounds that mimic catecholate. This inadvertent import suggests the potential utility of antimicrobial catechol siderophore mimetics in managing bacterial infections. However, to fully exploit these compounds, a detailed understanding of the mechanism of transport through Fiu and related transporters is required. To address this question, we determined the crystal structure of Fiu at 2.1–2.9 Å and analyzed its function in E. coli. Through analysis of the Fiuo crystal structure, in combination with in silico docking and mutagenesis, we provide insight into how Fiu and related transporters bind catecholate in a surface-exposed cavity. Moreover, through determination of the structure of Fiu in multiple crystal states, we revealed the presence of a large, selectively gated cavity in the interior of this transporter. This chamber is large enough to accommodate the Fiu substrate and may allow import of substrates via a two-step mechanism. This would avoid channel formation through the transporter and inadvertent import of toxic molecules. As Fiu and its homologs are the targets of substrate-mimicking antibiotics, these results may assist in the development of these compounds.

Non-specific interference of cobalt with siderophore-dependent iron uptake pathways

Presence of Co²⁺affects the production of the siderophore Pyochelin inPseudomonas aeruginosa. This repression is not Fur-dependent but due to competition of Pyochelin–Co²⁺with Pyochein–Fe³⁺for PchR (transcriptional activator).

Preacinetobactin not acinetobactin is essential for iron uptake by the BauA transporter of the pathogen Acinetobacter baumannii

New strategies are urgently required to develop antibiotics. The siderophore uptake system has attracted considerable attention, but rational design of siderophore antibiotic conjugates requires knowledge of recognition by the cognate outer-membrane transporter. Acinetobacter baumannii is a serious pathogen, which utilizes (pre)acinetobactin to scavenge iron from the host. We report the structure of the (pre)acinetobactin transporter BauA bound to the siderophore, identifying the structural determinants of recognition. Detailed biophysical analysis confirms that BauA recognises preacinetobactin. We show that acinetobactin is not recognised by the protein, thus preacinetobactin is essential for iron uptake. The structure shows and NMR confirms that under physiological conditions, a molecule of acinetobactin will bind to two free coordination sites on the iron preacinetobactin complex. The ability to recognise a heterotrimeric iron-preacinetobactin-acinetobactin complex may rationalize contradictory reports in the literature. These results open new avenues for the design of novel antibiotic conjugates (trojan horse) antibiotics.

Advances in the Chemical Biology of Desferrioxamine B

Desferrioxamine B (DFOB) was discovered in the late 1950s as a hydroxamic acid metabolite of the soil bacterium Streptomyces pilosus. The exquisite affinity of DFOB for Fe(III) identified its potential for removing excess iron from patients with transfusion-dependent hemoglobin disorders. Many studies have used semisynthetic chemistry to produce DFOB adducts with new properties and broad-ranging functions. More recent approaches in chemical biology have revealed some nuances of DFOB biosynthesis and discovered new DFOB-derived drugs and radiometal imaging agents. The current and potential applications of DFOB continue to inspire a rich body of chemical biology research focused on this bacterial metabolite.

Synthesis and antibiotic activity of oxazolidinone-catechol conjugates against Pseudomonas aeruginosa

Pseudomonas aeruginosa is a Gram-negative pathogenic bacterium responsible for severe infections in which resistance to most of the approved families of antibiotics is increasing. Oxazolidinone antibiotics are active against many Gram-positive bacteria, but are only weakly active against Gram-negative pathogens. We describe the synthesis of conjugates between a catechol moiety and oxazolidinone antibiotics. These conjugates were significantly more active against P. aeruginosa (218-1024 μM) than linezolid (MIC > 1024 μM), the reference molecule from the oxazolidinone family. Antibiotic activity was slightly higher in medium depleted of iron, suggesting the involvement of a bacterial iron uptake system in this biological activity. The bacterial iron uptake pathway involved in the transport is still to be addressed, but the present data excluded a contribution of the enterobactin transporter PfeA.

Systematic analysis of protein-detergent complexes applying dynamic light scattering to optimize solutions for crystallization trials

Detergents are widely used for the isolation and solubilization of membrane proteins to support crystallization and structure determination. Detergents are amphiphilic molecules that form micelles once the characteristic critical micelle concentration (CMC) is achieved and can solubilize membrane proteins by the formation of micelles around them. The results are presented of a study of micelle formation observed by in situ dynamic light-scattering (DLS) analyses performed on selected detergent solutions using a newly designed advanced hardware device. DLS was initially applied in situ to detergent samples with a total volume of approximately 2 µl. When measured with DLS, pure detergents show a monodisperse radial distribution in water at concentrations exceeding the CMC. A series of all-trans n-alkyl-β-D-maltopyranosides, from n-hexyl to n-tetradecyl, were used in the investigations. The results obtained verify that the application of DLS in situ is capable of distinguishing differences in the hydrodynamic radii of micelles formed by detergents differing in length by only a single CH2 group in their aliphatic tails. Subsequently, DLS was applied to investigate the distribution of hydrodynamic radii of membrane proteins and selected water-insoluble proteins in presence of detergent micelles. The results confirm that stable protein-detergent complexes were prepared for (i) bacteriorhodopsin and (ii) FetA in complex with a ligand as examples of transmembrane proteins. A fusion of maltose-binding protein and the Duck hepatitis B virus X protein was added to this investigation as an example of a non-membrane-associated protein with low water solubility. The increased solubility of this protein in the presence of detergent could be monitored, as well as the progress of proteolytic cleavage to separate the fusion partners. This study demonstrates the potential of in situ DLS to optimize solutions of protein-detergent complexes for crystallization applications.

A cell biological view of the siderophore pyochelin iron uptake pathway in Pseudomonas aeruginosa

Pyochelin (PCH) is a siderophore produced and secreted by Pseudomonas aeruginosa for iron capture. Using (55) Fe uptake and binding assays, we showed that PCH-Fe uptake in P. aeruginosa involves, in addition to the highly studied outer membrane transporter FptA, the inner membrane permease FptX, which recognizes PCH-(55) Fe with an affinity of 0.6 ± 0.2 nM and transports the ferri-siderophore complex from the periplasm into the cytoplasm: fptX deletion inhibited (55) Fe accumulation in the bacterial cytoplasm. Chromosomal replacement was used to generate P. aeruginosa strains producing fluorescent fusions with FptX, PchR (an AraC regulator), PchA (the first enzyme involved in the PCH biosynthesis) and PchE (a non-ribosomic peptide-synthetase involved in a further step). Fluorescence imaging and cellular fractionation showed a uniform repartition of FptX in the inner membrane. PchA and PchE were found in the cytoplasm, associated to the inner membrane all over the bacteria and also concentrated at the bacterial poles. PchE clustering at the bacterial poles was dependent on PchA expression, but on the opposite PchA clustering and membrane association was PchE-independent. PchA and PchE cellular organization suggests the existence of a siderosome for PCH biosynthesis as previously proposed for pyoverdine biosynthesis (another siderophore produced by P. aeruginosa).

Siderophore-dependent iron uptake systems as gates for antibiotic Trojan horse strategies against Pseudomonas aeruginosa

Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen responsible for nosocomial infections. The prevalence of antibiotic-resistant P. aeruginosa strains is increasing, necessitating the urgent development of new strategies to improve the control of this pathogen. Its bacterial envelope constitutes of an outer and an inner membrane enclosing the periplasm. This structure plays a key role in the resistance of the pathogen, by decreasing the penetration and the biological impact of many antibiotics. However, this barrier may also be seen as the "Achilles heel" of the bacterium as some of its functions provide opportunities for breaching bacterial defenses. Siderophore-dependent iron uptake systems act as gates in the bacterial envelope and could be used in a "Trojan horse" strategy, in which the conjugation of an antibiotic to a siderophore could significantly increase the biological activity of the antibiotic, by enhancing its transport into the bacterium. In this review, we provide an overview of the various siderophore-antibiotic conjugates that have been developed for use against P. aeruginosa and show that an accurate knowledge of the structural and functional features of the proteins involved in this transmembrane transport is required for the design and synthesis of effective siderophore-antibiotic Trojan horse conjugates.

Synthesis and biological properties of thiazole-analogues of pyochelin, a siderophore of Pseudomonas aeruginosa

Pyochelin is a siderophore common to all strains of Pseudomonas aeruginosa utilized by this Gram-negative bacterium to acquire iron(III). FptA is the outer membrane transporter responsible of ferric-pyochelin uptake in P. aeruginosa. We describe in this Letter the synthesis and the biological properties ((55)Fe uptake, binding to FptA) of several thiazole analogues of pyochelin. Among them we report in this Letter the two first pyochelin analogues able to bind FptA without promoting any iron uptake in P. aeruginosa.

In silico determination and validation of baumannii acinetobactin utilization a structure and ligand binding site

Acinetobacter baumannii is a deadly nosocomial pathogen. Iron is an essential element for the pathogen. Under iron-restricted conditions, the bacterium expresses iron-regulated outer membrane proteins (IROMPs). Baumannii acinetobactin utilization (BauA) is the most important member of IROMPs in A. baumannii. Determination of its tertiary structure could help deduction of its functions and its interactions with ligands. The present study unveils BauA 3D structure via in silico approaches. Apart from ab initio, other rational methods such as homology modeling and threading were invoked to achieve the purpose. For homology modeling, BLAST was run on the sequence in order to find the best template. The template was then served to model the 3D structure. All the models built were evaluated qualitatively. The best model predicted by LOMETS was selected for analyses. Refinement of 3D structure as well as determination of its clefts and ligand binding sites was carried out on the structure. In contrast to the typical trimeric arrangement found in porins, BauA is monomeric. The barrel is formed by 22 antiparallel transmembrane β -strands. There are short periplasmic turns and longer surface-located loops. An N-terminal domain referred to either as the cork, the plug, or the hatch domain occludes the β -barrel.

Fate of ferrisiderophores after import across bacterial outer membranes: different iron release strategies are observed in the cytoplasm or periplasm depending on the siderophore pathways

Siderophore production and utilization is one of the major strategies deployed by bacteria to get access to iron, a key nutrient for bacterial growth. The biological function of siderophores is to solubilize iron in the bacterial environment and to shuttle it back to the cytoplasm of the microorganisms. This uptake process for Gram-negative species involves TonB-dependent transporters for translocation across the outer membranes. In Escherichia coli and many other Gram-negative bacteria, ABC transporters associated with periplasmic binding proteins import ferrisiderophores across cytoplasmic membranes. Recent data reveal that in some siderophore pathways, this step can also be carried out by proton-motive force-dependent permeases, for example the ferrichrome and ferripyochelin pathways in Pseudomonas aeruginosa. Iron is then released from the siderophores in the bacterial cytoplasm by different enzymatic mechanisms depending on the nature of the siderophore. Another strategy has been reported for the pyoverdine pathway in P. aeruginosa: iron is released from the siderophore in the periplasm and only siderophore-free iron is transported into the cytoplasm by an ABC transporter having two atypical periplasmic binding proteins. This review presents recent findings concerning both ferrisiderophore and siderophore-free iron transport across bacterial cytoplasmic membranes and considers current knowledge about the mechanisms involved in iron release from siderophores.

Inner-membrane transporters for the siderophores pyochelin in Pseudomonas aeruginosa and enantio-pyochelin in Pseudomonas fluorescens display different enantioselectivities

Iron uptake and transcriptional regulation by the enantiomeric siderophores pyochelin (Pch) and enantio-pyochelin (EPch) of Pseudomonas aeruginosa and Pseudomonas fluorescens, respectively, are stereospecific processes. The iron-loaded forms of Pch (ferriPch) and of EPch (ferriEPch) are recognized stereospecifically (i) at the outer membrane by the siderophore receptors FptA in P. aeruginosa and FetA in P. fluorescens and (ii) in the cytoplasm by the two AraC-type regulators PchR, which are activated by their cognate siderophore. Here, stereospecific siderophore recognition is shown to occur at the inner membrane also. In P. aeruginosa, translocation of ferriPch across the inner membrane is carried out by the single-subunit siderophore transporter FptX. In contrast, the uptake of ferriEPch into the cytoplasm of P. fluorescens was found to involve a classical periplasmic binding protein-dependent ABC transporter (FetCDE), which is encoded by the fetABCDEF operon. Expression of a translational fetA-gfp fusion was repressed by ferric ions, and activated by the cognate siderophore bound to PchR, thus resembling the analogous regulation of the P. aeruginosa ferriPch transport operon fptABCX. The inner-membrane transporters FetCDE and FptX were expressed in combination with either of the two siderophore receptors FetA and FptA in a siderophore-negative P. aeruginosa mutant deleted for the fptABCX operon. Growth tests conducted under iron limitation with ferriPch or ferriEPch as the iron source revealed that FptX was able to transport ferriPch as well as ferriEPch, whereas FetCDE specifically transported ferriEPch. Thus, stereospecific siderophore recognition occurs at the inner membrane by the FetCDE transporter.

Refolding, purification and crystallization of the FrpB outer membrane iron transporter from Neisseria meningitidis

FrpB is an integral outer membrane protein from the human pathogen Neisseria meningitidis. It is a member of the TonB-dependent transporter family and promotes the uptake of iron across the outer membrane. There is also evidence that FrpB is an antigen and hence a potential component of a vaccine against meningococcal meningitis. FrpB incorporating a polyhistidine tag was overexpressed in Escherichia coli into inclusion bodies. The protein was then solubilized in urea, refolded and purified to homogeneity. Two separate antigenic variants of FrpB were crystallized by sitting-drop vapour diffusion. Crystals of the F5-1 variant diffracted to 2.4 Å resolution and belonged to space group C2, with unit-cell parameters a = 176.5, b = 79.4, c = 75.9 Å, β = 98.3°. Crystal-packing calculations suggested the presence of a monomer in the asymmetric unit. Crystals of the F3-3 variant also diffracted to 2.4 Å resolution and belonged to space group P2(1)2(1)2(1), with unit-cell parameters a = 85.3, b = 104.6, c = 269.1 Å. Preliminary analysis suggested the presence of an FrpB trimer in the asymmetric unit.

Pyochelin, a siderophore of Pseudomonas aeruginosa: physicochemical characterization of the iron(III), copper(II) and zinc(II) complexes

Pseudomonas aeruginosa is an opportunistic pathogen, synthesizing two major siderophores, pyoverdine (Pvd) and pyochelin (Pch), to cover its needs in iron(III). If the high affinity and specificity of Pvd toward iron(III) (pFe = 27.0) was well described in the literature, the physicochemical and coordination properties of Pch toward biologically relevant metals (Fe(III), Cu(II) or Zn(II)) have been only scarcely investigated. We report a thorough physico-chemical investigation of Pch (potentiometry, spectrophotometries, ESI/MS) that highlighted its moderate but significantly higher affinity for Fe(3+) (pFe = 16.0 at p[H] 7.4) than reported previously. We also demonstrated that Pch strongly chelates divalent metals such as Zn(II) (pZn = 11.8 at p[H] 7.4) and Cu(II) (pCu = 14.9 at p[H] 7.4) and forms predominantly 1 : 2 (M(2+)/Pch) complexes. Kinetic studies revealed that the formation of the ferric Pch complexes proceeds through a Eigen-Wilkins dissociative ligand interchange mechanism involving two protonated species of Pch and the Fe(OH)(2+) species of Fe(III). Our physico-chemical parameters supports the previous biochemical studies which proposed that siderophores are not only devoted to iron(III) shuttling but most likely display other specific biological role in the subtle metals homeostasis in microorganisms. This work also represents a step toward deciphering the role of siderophores throughout evolution.

Structure, function and binding selectivity and stereoselectivity of siderophore-iron outer membrane transporters

To get access to iron, microorganisms produce and release into their environment small organic metal chelators called siderophores. In parallel, they produce siderophore-iron outer membrane transporters (also called TonB-Dependent Transporters or TBDT) embedded in the outer membrane; these proteins actively reabsorb the siderophore loaded with iron from the extracellular medium. This active uptake requires energy in the form of the proton motive force transferred from the inner membrane to the outer membrane transporter via the inner membrane TonB complex. Siderophores produced by microorganisms are structurally very diverse with molecular weights of 150 up to 2000Da. Siderophore-iron uptake from the extracellular medium by TBDTs is a highly selective and sometimes even stereoselective process, with each siderophore having a specific TBDT. Unlike the siderophores, all TBDTs have similar structures and belong to the outer membrane β-barrel protein superfamily. The way in which the siderophore-iron complex passes through the TBDT is still unclear. In some bacteria, TBDTs are also partners of signaling cascades regulating the expression of proteins involved in siderophore biosynthesis and siderophore-iron acquisition.