From a total synthesis of cepabactin and its 3:1 ferric complex to the isolation of a 1:1:1 mixed complex between iron (III), cepabactin and pyochelin

On the coordination chemistry of a bacterial siderophore cepabactin from a theoretical perspective

Iron is one of the essential metals required by almost all living organisms. However, nature has certain constraints in distributing this element among tissues. Since polymeric oxide-bridged Fe (III) is the prominent source of Fe (III) ions, the insolubility of Fe (III) ions in aqueous systems reduces the direct uptake by cells. Secondly, the free-Fe entities which generate ^.OH radicals pave the way to the destruction of the cells. Hence, a protective coordination environment via sophisticated chemical systems is required for the acquisition of Fe, its successive transport, storage, and effective utilization in various tissues. Siderophores are polydentate ligands used by bacterial cells for Fe acquisition, with a relatively high affinity for Fe (III) ions. Secreted from the bacterial cells into the external aqueous medium, they sequester Fe to give a soluble complex which re-enters the organism at a specific receptor. Once it gets inside the cell, the Fe is released from the complex and utilized for essential biochemical reactions. The medicinal applications of these natural ligands, developing progressively in various research groups, necessitate the theoretical aspects of their coordination chemistry. This research paper deals with the coordination chemistry of one of the siderophores, cepabactin (Cep). The chemical computations confirm that the Fe^III(Cep)₃ complex is octahedral and high spin. The oxygen atoms of Cep, which are hard and negatively charged, thus act as electron donors in the Fe^III(Cep)₃ complex formation. This in turn makes the siderophores relatively less attractive towards Fe (II) ions.

Emerging Target-Directed Approaches for the Treatment and Diagnosis of Microbial Infections

With the rising levels of drug resistance, developing efficient antimicrobial therapies has become a priority. A promising strategy is the conjugation of antibiotics with relevant moieties that can potentiate their activity by target-directing. The conjugation of siderophores with antibiotics allows them to act as Trojan horses by hijacking the microorganisms' highly developed iron transport systems and using them to carry the antibiotic into the cell. Through the analysis of relevant examples of the past decade, this Perspective aims to reveal the potential of siderophore-antibiotic Trojan horses for the treatment of infections and the role of siderophores in diagnostic techniques. Other conjugated molecules will be the subject of discussion, namely those involving vitamin B12, carbohydrates, and amino acids, as well as conjugated compounds targeting protein degradation and β-lactamase activated prodrugs.

Burkholderia pseudomallei known siderophores and hemin uptake are dispensable for lethal murine melioidosis

Burkholderia pseudomallei is a mostly saprophytic bacterium, but can infect humans where it causes the difficult-to-manage disease melioidosis. Even with proper diagnosis and prompt therapeutic interventions mortality rates still range from >20% in Northern Australia to over 40% in Thailand. Surprisingly little is yet known about how B. pseudomallei infects, invades and survives within its hosts, and virtually nothing is known about the contribution of critical nutrients such as iron to the bacterium's pathogenesis. It was previously assumed that B. pseudomallei used iron-acquisition systems commonly found in other bacteria, for example siderophores. However, our previous discovery of a clinical isolate carrying a large chromosomal deletion missing the entire malleobactin gene cluster encoding the bacterium's major high-affinity siderophore while still being fully virulent in a murine melioidosis model suggested that other iron-acquisition systems might make contributions to virulence. Here, we deleted the major siderophore malleobactin (mba) and pyochelin (pch) gene clusters in strain 1710b and revealed a residual siderophore activity which was unrelated to other known Burkholderia siderophores such as cepabactin and cepaciachelin, and not due to increased secretion of chelators such as citrate. Deletion of the two hemin uptake loci, hmu and hem, showed that Hmu is required for utilization of hemin and hemoglobin and that Hem cannot complement a Hmu deficiency. Prolonged incubation of a hmu hem mutant in hemoglobin-containing minimal medium yielded variants able to utilize hemoglobin and hemin suggesting alternate pathways for utilization of these two host iron sources. Lactoferrin utilization was dependent on malleobactin, but not pyochelin synthesis and/or uptake. A mba pch hmu hem quadruple mutant could use ferritin as an iron source and upon intranasal infection was lethal in an acute murine melioidosis model. These data suggest that B. pseudomallei may employ a novel ferritin-iron acquisition pathway as a means to sustain in vivo growth.

Burkholderia pseudomallei is the etiologic agent of melioidosis, a multifaceted deadly and difficult to treat disease of equatorial regions of the world. Disease manifestations range from acute infections to long term chronic infections. The factors by which this bacterium causes disease are not yet well understood. Studies thus far focused on elucidation of the roles of traditional virulence factors such as secreted proteins and exopolysaccharides, but virtually nothing is known about the roles of nutrient acquisition systems in B. pseudomallei's survival in its mammalian hosts. One nutrient that is essential for bacterial metabolism and pathogenicity is iron. As free iron is not readily available in nature, bacteria developed numerous mechanisms for iron acquisition from abiotic and biotic sources. These mechanisms include siderophores and hemin/hemoglobin utilization systems, and it is therefore not too surprising that mutants defective in these systems are often impaired in virulence. In this study we show that defined B. pseudomallei mutants defective in siderophore and hemin/hemoglobin utilization systems remain fully lethal in a murine melioidosis model and present evidence for in vitro ferritin-iron acquisition which may be one or perhaps the main means by which this pathogen sustains in vivo growth.

Mammalian siderophores, siderophore-binding lipocalins, and the labile iron pool

Bacteria use tight-binding, ferric-specific chelators called siderophores to acquire iron from the environment and from the host during infection; animals use proteins such as transferrin and ferritin to transport and store iron. Recently, candidate compounds that could serve endogenously as mammalian siderophore equivalents have been identified and characterized through associations with siderocalin, the only mammalian siderophore-binding protein currently known. Siderocalin, an antibacterial protein, acts by sequestering iron away from infecting bacteria as siderophore complexes. Candidate endogenous siderophores include compounds that only effectively transport iron as ternary complexes with siderocalin, explaining pleiotropic activities in normal cellular processes and specific disease states.

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.

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

Pyochelin (Pch) and enantiopyochelin (EPch) are enantiomeric siderophores, with three chiral centers, produced under iron limitation conditions by Pseudomonas aeruginosa and Pseudomonas fluorescens , respectively. After iron chelation in the extracellular medium, Pch-Fe and EPch-Fe are recognized and transported by their specific outer-membrane transporters: FptA in P. aeruginosa and FetA in P. fluorescens . Structural analysis of FetA-EPch-Fe and FptA-Pch-Fe, combined with mutagenesis and docking studies revealed the structural basis of the stereospecific recognition of these enantiomers by their respective transporters. Whereas FetA and FptA have a low sequence identity but high structural homology, the Pch and EPch binding pockets do not share any structural homology, but display similar physicochemical properties. The stereospecific recognition of both enantiomers by their corresponding transporters is imposed by the configuration of the siderophore's C4'' and C2'' chiral centers. This recognition involves specific hydrogen bonds between the Arg91 guanidinium group and EPch-Fe for FetA and between the Leu117-Leu116 main chain and Pch-Fe for FptA. FetA and FptA are the first membrane receptors to be structurally described with opposite binding enantioselectivities for their ligands, giving insights into the structural basis of their enantiospecificity.

New roles for bacterial siderophores in metal transport and tolerance

Siderophores are chelators with extremely strong affinity for ferric iron and are best known for their capacity to feed microorganisms with this metal. Despite their preference for iron, they can also chelate numerous other metals with variable affinities. There is also increasing evidence that metals other than iron can activate the production of siderophores by bacteria, thereby implicating siderophores in the homeostasis of metals other than iron and especially heavy metal tolerance. This article considers this new concept that siderophores play a role in protecting bacteria against metal toxicity and discusses the possible contribution of these chelators to the transport of biological relevant metals in addition to iron.

Synthesis and iron sequestration equilibria of novel exocyclic 3-hydroxy-2-pyridinone donor group siderophore mimics

The synthesis of a novel class of exocyclic bis- and tris-3,2-hydroxypyridinone (HOPO) chelators built on N(2) and N(3) aza-macrocyclic scaffolds and the thermodynamic solution characterization of their complexes with Fe(III) are described. The chelators for this study were prepared by reaction of either piperazine or N,N',N''-1,4,7-triazacyclononane with a novel electrophilic HOPO iminium salt in good yields. Subsequent removal of the benzyl protecting groups using HBr/acetic acid gave bis-HOPO chelators N(2)(etLH)(2) and N(2)(prLH)(2), and tris-HOPO chelator N(3)(etLH)(3) in excellent yields. Solution thermodynamic characterization of their complexes with Fe(III) was accomplished using spectrophotometric, potentiometric, and electrospray ionization-mass spectrometry (ESI-MS) methods. The pK(a)'s of N(2)(etLH)(2), N(2)(prLH)(2), and N(3)(etLH)(3), were determined spectrophotometrically and potentiometrically. The Fe(III) complex stability constants for the tetradentate N(2)(etLH)(2) and N(2)(prLH)(2), and hexadentate N(3)(etLH)(3), were measured by spectrophotometric and potentiometric titration, and by competition with ethylenediaminetetraacetic acid (EDTA). N(3)(etLH)(3) forms a 1:1 complex with Fe(III) with log β(110) = 27.34 ± 0.04. N(2)(prLH)(2) forms a 3:2 L:Fe complex with Fe(III) where log β(230) = 60.46 ± 0.04 and log β(110) = 20.39 ± 0.02. While N(2)(etLH)(2) also forms a 3:2 L:Fe complex with Fe(III), solubility problems precluded determining log β(230); log β(110) was found to be 20.45 ± 0.04. The pFe values of 26.5 for N(3)(etLH)(3) and 24.78 for N(2)(prLH)(2) are comparable to other siderophore molecules used in the treatment of iron overload, suggesting that these hydroxypyridinone ligands may be useful in the development of new chelation therapy agents.

Iron acquisition with the natural siderophore enantiomers pyochelin and enantio-pyochelin in Pseudomonas species

The bacterial siderophore pyochelin is composed of salicylate and two cysteine-derived heterocycles, the second of which is modified by reduction and N-methylation during biosynthesis. In Pseudomonas aeruginosa, the first cysteine residue is converted to its D-isoform during thiazoline ring formation, whereas the second cysteine remains in its L-configuration. Stereochemistry is opposite in the Pseudomonas fluorescens siderophore enantio-pyochelin, in which the first ring originates from L-cysteine and the second ring from D-cysteine. Both siderophores promote growth of the producer organism during iron limitation and induce the expression of their biosynthesis genes by activating the transcriptional AraC-type regulator PchR. However, neither siderophore is functional as an iron carrier or as a transcriptional inducer in the other species, demonstrating that both processes are highly stereospecific. Stereospecificity of pyochelin/enantio-pyochelin-mediated iron uptake is ensured at two levels: (i) by the outer membrane siderophore receptors and (ii) by the cytosolic PchR regulators.

The Pseudomonas aeruginosa pyochelin-iron uptake pathway and its metal specificity

Pyochelin (Pch) is one of the two major siderophores produced and secreted by Pseudomonas aeruginosa PAO1 to assimilate iron. It chelates iron in the extracellular medium and transports it into the cell via a specific outer membrane transporter, FptA. We used the fluorescent properties of Pch to show that this siderophore chelates, in addition to Fe(3+) albeit with substantially lower affinities, Ag(+), Al(3+), Cd(2+), Co(2+), Cr(2+), Cu(2+), Eu(3+), Ga(3+), Hg(2+), Mn(2+), Ni(2+), Pb(2+), Sn(2+), Tb(3+), Tl(+), and Zn(2+). Surprisingly, the Pch complexes with all these metals bound to FptA with affinities in the range of 10 nM to 4.8 microM (the affinity of Pch-Fe is 10 nM) and were able to inhibit, with various efficiencies, Pch-(55)Fe uptake in vivo. We used inductively coupled plasma atomic emission spectrometry to follow metal uptake by P. aeruginosa. Energy-dependent metal uptake, in the presence of Pch, was efficient only for Fe(3+). Co(2+), Ga(3+), and Ni(2+) were also transported, but the uptake rates were 23- to 35-fold lower than that for Fe(3+). No uptake was seen for all the other metals. Thus, cell surface FptA has broad metal specificity at the binding stage but is much more selective for the metal uptake process. This uptake pathway does not appear to efficiently assimilate any metal other than Fe(3+).

Stereospecificity of the siderophore pyochelin outer membrane transporters in fluorescent pseudomonads

Pyochelin (Pch) and enantio-pyochelin (EPch) are enantiomer siderophores that are produced by Pseudomonas aeruginosa and Pseudomonas fluorescens, respectively, under iron limitation. Pch promotes growth of P. aeruginosa when iron is scarce, and EPch carries out the same biological function in P. fluorescens. However, the two siderophores are unable to promote growth in the heterologous species, indicating that siderophore-mediated iron uptake is highly stereospecific. In the present work, using binding and iron uptake assays, we found that FptA, the Fe-Pch outer membrane transporter of P. aeruginosa, recognized (K(d) = 2.5 +/- 1.1 nm) and transported Fe-Pch but did not interact with Fe-EPch. Likewise, FetA, the Fe-EPch receptor of P. fluorescens, was specific for Fe-EPch (K(d) = 3.7 +/- 2.1 nm) but did not bind and transport Fe-Pch. Growth promotion experiments performed under iron-limiting conditions confirmed that FptA and FetA are highly specific for Pch and EPch, respectively. When fptA and fetA along with adjacent transport genes involved in siderophore uptake were swapped between the two bacterial species, P. aeruginosa became able to utilize Fe-EPch as an iron source, and P. fluorescens was able to grow with Fe-Pch. Docking experiments using the FptA structure and binding assays showed that the stereospecificity of Pch recognition by FptA was mostly due to the configuration of the siderophore chiral centers C4'' and C2'' and was only weakly dependent on the configuration of the C4' carbon atom. Together, these findings increase our understanding of the stereospecific interaction between Pch and its outer membrane receptor FptA.

Siderophore-based iron acquisition and pathogen control

High-affinity iron acquisition is mediated by siderophore-dependent pathways in the majority of pathogenic and nonpathogenic bacteria and fungi. Considerable progress has been made in characterizing and understanding mechanisms of siderophore synthesis, secretion, iron scavenging, and siderophore-delivered iron uptake and its release. The regulation of siderophore pathways reveals multilayer networks at the transcriptional and posttranscriptional levels. Due to the key role of many siderophores during virulence, coevolution led to sophisticated strategies of siderophore neutralization by mammals and (re)utilization by bacterial pathogens. Surprisingly, hosts also developed essential siderophore-based iron delivery and cell conversion pathways, which are of interest for diagnostic and therapeutic studies. In the last decades, natural and synthetic compounds have gained attention as potential therapeutics for iron-dependent treatment of infections and further diseases. Promising results for pathogen inhibition were obtained with various siderophore-antibiotic conjugates acting as "Trojan horse" toxins and siderophore pathway inhibitors. In this article, general aspects of siderophore-mediated iron acquisition, recent findings regarding iron-related pathogen-host interactions, and current strategies for iron-dependent pathogen control will be reviewed. Further concepts including the inhibition of novel siderophore pathway targets are discussed.

Iron acquisition mechanisms of the Burkholderia cepacia complex

The Burkholderia cepacia complex (Bcc) is comprised of at least 10 closely related species of Gram-negative proteobacteria that are associated with infections in certain groups of immunocompromised individuals, particularly those with cystic fibrosis. Infections in humans tend to occur in the lungs, which present an iron-restricted environment to a prospective pathogen, and accordingly members of the Bcc appear to possess efficient mechanisms for iron capture. These bacteria specify up to four different types of siderophore (ornibactin, pyochelin, cepabactin and cepaciachelin) that employ the full repertoire of iron-binding groups present in most naturally occurring siderophores. Members of the Bcc are also capable of utilising some exogenous siderophores that they are not able to synthesise. In addition to siderophore-mediated mechanisms of iron uptake, the Bcc possess mechanisms for acquiring iron from haem and from ferritin. The Bcc therefore appear to be well-equipped for life in an iron-poor environment.

Different iron-chelating properties of pyochelin diastereoisomers revealed by LC/MS

Pyochelin is a siderophore and virulence factor common to Burkholderia cepacia and several Pseudomonas strains. It is isolated from bacterial media as a mixture of two epimers, which readily equilibrate in most solvents. Experiments based on high-performance liquid chromatography/electrospray ionization mass spectrometry are reported here, allowing the investigation of the different Fe(III)-chelating properties of pyochelin diastereomers in solution without the need for labourious isolation. It is demonstrated in this study that only one of the two pyochelin diastereomers is able to chelate Fe(III); no Fe(III) complexes of the other diastereomer could be detected. The Fe(III)-pyochelin complex exhibited a 1:1 metal-to-siderophore ratio and no evidence for other stoichiometries was found.

Binding Properties of Pyochelin and Structurally Related Molecules to FptA of Pseudomonas aeruginosa

Pyochelin (Pch) is a siderophore that is produced in iron-limited conditions, by both Pseudomonas aeruginosa and Burkholderia cepacia. This iron uptake pathway could therefore be a target for the development of new antibiotics. Pch is (4'R,2''R/S,4''R)-2'-(2-hydroxyphenyl)-3''-methyl-4',5',2'',3'',4'',5''-hexahydro-[4',2'']bithiazolyl-4''-carboxylic acid, and has three chiral centres located at positions C4', C2'' and C4''. In P.aeruginosa, this siderophore chelates iron in the extracellular medium and transports it into the cells via a specific outer membrane transporter FptA. Docking experiments using the X-ray structure of FptA-Pch-Fe showed that iron-loaded or unloaded Pch diastereoisomers could bind to FptA. This was confirmed by in vivo binding assays. These binding properties and the iron uptake ability were not affected by removal of the C4' chiral centre. After removal of both the C4' and C2'' chiral centres, the molecule still bound to FptA but was unable to transport iron. The overall binding mode of this iron-complexed analogue was inverted. These findings describe the first antagonist of the Pch/FptA iron uptake pathway. Pch also complexes with iron in conjunction with other bidentate ligands such as cepabactin (Cep) or ethylene glycol. Docking experiments showed that such complexes bind to FptA via the Pch molecule. The mixed Pch-Fe-Cep complex was also recognized by FptA, having an affinity intermediate between that for Pch(2)-Fe and Cep(3)-Fe. Finally, the iron uptake properties of the different Pch-related molecules suggested a mechanism for FptA-Pch-Fe complex formation similar to that of the FpvA/Pvd uptake system. All these findings improve our understanding of specificity of the interaction between FptA and its siderophore.

Bacterial siderophores: the solution stoichiometry and coordination of the Fe(III) complexes of pyochelin and related compounds

Pyochelin, its analog 3''-nor-NH-pyochelin, and the related methyl hydroxamate, 2-(2'-hydroxyphenyl)-4,5-dihydrothiazol-4-carboxylic acid methoxymethyl amide, have been prepared together with their Fe(III) complexes. The solution stoichiometry and the coordination of the three Fe(III) complexes in methanol or buffered (pH approximately 2) 50:50 (v/v) methanol-water mixtures were determined using various spectroscopic methods: UV-vis absorption, X-ray absorption, extended X-ray absorption fine structure and electron paramagnetic resonance. All three systems showed both a 1:1 and 2:1 ligand-Fe(III) stoichiometry, but presented different coordination properties. Conditional formation constants (pH approximately 2) were determined for both the 1:1 and 2:1 complexes in all three systems. Computation of the coordination-conformational energies by semiempirical methods indicated that the coordination in the case of the 2:1 complexes of pyochelin-Fe(III) and 3''-nor-NH-pyochelin-Fe(III) was asymmetrical, with one molecule of pyochelin (or 3''-nor-NH-pyochelin) tetradentately coordinated (O1, N1, N2 and O3) to the Fe(III), and the second molecule bound bidentately (O1, N1 or N2, O3), to complete the octahedral geometry. In contrast, two molecules of the methyl hydroxamate each provided a set of tridentate ligand atoms in the formation of the 2:1 ligand-Fe(III) complex. These results are consistent with the role of pyochelin in the uptake of iron by the FptA receptor in the outer membrane of Pseudomonas aeruginosa and in several gram-negative bacteria.

Crystal Structure at High Resolution of Ferric-pyochelin and its Membrane Receptor FptA from Pseudomonas aeruginosa

Pyochelin is a siderophore and virulence factor common to Burkholderia cepacia and several Pseudomonas strains. We describe at 2.0 A resolution the crystal structure of the pyochelin outer membrane receptor FptA bound to the iron-pyochelin isolated from Pseudomonas aeruginosa. One pyochelin molecule bound to iron is found in the protein structure, providing the first three-dimensional structure at the atomic level of this siderophore. The pyochelin molecule provides a tetra-dentate coordination of iron, while the remaining bi-dentate coordination is ensured by another molecule not specifically recognized by the protein. The overall structure of the pyochelin receptor is typical of the TonB-dependent transporter superfamily, which uses the proton motive force from the cytoplasmic membrane through the TonB-ExbB-ExbD energy transducing complex to transport ferric ions across the bacterial outer membrane: a transmembrane 22 beta-stranded barrel occluded by a N-terminal domain that contains a mixed four-stranded beta-sheet. The N-terminal TonB box is disordered in two crystal forms, and loop L8 is found to point towards the iron-pyochelin complex, suggesting that the receptor is in a transport-competent conformation.