Iron Chelation Properties of an Extracellular Siderophore Exochelin MS

Synthesis of Modular Desferrioxamine Analogues and Evaluation of Zwitterionic Derivatives for Zirconium Complexation

The natural siderophore desferrioxamine B (DFOB) has been used for targeted PET imaging with 89 Zr before. However, Zr-DFOB has a limited stability and a number of derivatives have been developed with improved chelation properties for zirconium. We describe the synthesis of pseudopeptidic analogues of DFOB with azido side chains. These are termed AZA-DFO (hexadentate) and AZA-DFO* (octadentate) and are assembled via a modular synthesis from Orn-β-Ala and Lys-β-Ala. Nine different chelators have been conjugated to zwitterionic moieties by copper-catalyzed azide-alkyne cycloaddition (CuAAC). The resulting water-soluble chelators form Zr complexes under mild conditions (room temperature for 90 min). Transchelation assays with 1000-fold excess of EDTA and 300-fold excess of DFOB revealed that a short spacing of hydroxamates in (Orn-β-Ala)3-4 leads to improved complex stability compared to a longer spacing in (Lys-β-Ala)3-4 . We found that the alignment of amide groups in the pseudopeptide backbone and the presence of zwitterionic sidechains did not compromise the stability of the Zr-complexes with our chelators. We believe that the octadentate derivative AZA-DFO* is particularly valuable for the preparation of new Zr-chelators for targeted imaging which combine tunable pharmacokinetic properties with high complex stability and fast Zr-complexation kinetics.

The Mycobactin Biosynthesis Pathway: A Prospective Therapeutic Target in the Battle against Tuberculosis

The alarming rise in drug-resistant clinical cases of tuberculosis (TB) has necessitated the rapid development of newer chemotherapeutic agents with novel mechanisms of action. The mycobactin biosynthesis pathway, conserved only among the mycolata family of actinobacteria, a group of intracellularly surviving bacterial pathogens that includes Mycobacterium tuberculosis, generates a salicyl-capped peptide mycobactin under iron-stress conditions in host macrophages to support the iron demands of the pathogen. This in vivo essentiality makes this less explored mycobactin biosynthesis pathway a promising endogenous target for novel lead-compounds discovery. In this Perspective, we have provided an up-to-date account of drug discovery efforts targeting selected enzymes (MbtI, MbtA, MbtM, and PPTase) from the mbt gene cluster (mbtA-mbtN). Furthermore, a succinct discussion on non-specific mycobactin biosynthesis inhibitors and the Trojan horse approach adopted to impair iron metabolism in mycobacteria has also been included in this Perspective.

Iron Supplementation Therapy, A Friend and Foe of Mycobacterial Infections?

Iron is an essential element that is required for oxygen transfer, redox, and metabolic activities in mammals and bacteria. Mycobacteria, some of the most prevalent infectious agents in the world, require iron as growth factor. Mycobacterial-infected hosts set up a series of defense mechanisms, including systemic iron restriction and cellular iron distribution, whereas mycobacteria have developed sophisticated strategies to acquire iron from their hosts and to protect themselves from iron’s harmful effects. Therefore, it is assumed that host iron and iron-binding proteins, and natural or synthetic chelators would be keys targets to inhibit mycobacterial proliferation and may have a therapeutic potential. Beyond this hypothesis, recent evidence indicates a host protective effect of iron against mycobacterial infections likely through promoting remodeled immune response. In this review, we discuss experimental procedures and clinical observations that highlight the role of the immune response against mycobacteria under various iron availability conditions. In addition, we discuss the clinical relevance of our knowledge regarding host susceptibility to mycobacteria in the context of iron availability and suggest future directions for research on the relationship between host iron and the immune response and the use of iron as a therapeutic agent.

Tuning the Anti(myco)bacterial Activity of 3-Hydroxy-4-pyridinone Chelators through Fluorophores

Controlling the sources of Fe available to pathogens is one of the possible strategies that can be successfully used by novel antibacterial drugs. We focused our interest on the design of chelators to address Mycobacterium avium infections. Taking into account the molecular structure of mycobacterial siderophores and considering that new chelators must be able to compete for Fe(III), we selected ligands of the 3-hydroxy-4-pyridinone class to achieve our purpose. After choosing the type of chelating unit it was also our objective to design chelators that could be monitored inside the cell and for that reason we designed chelators that could be functionalized with fluorophores. We didn’t realize at the time that the incorporation a fluorophore, to allow spectroscopic detection, would be so relevant for the antimycobacterial effect or to determine the affinity of the chelators towards biological membranes. From a biophysical perspective, this is a fascinating illustration of the fact that functionalization of a molecule with a particular label may lead to a change in its membrane permeation properties and result in a dramatic change in biological activity. For that reason we believe it is interesting to give a critical account of our entire work in this area and justify the statement “to label means to change”. New perspectives regarding combined therapeutic approaches and the use of rhodamine B conjugates to target closely related problems such as bacterial resistance and biofilm production are also discussed.

Siderophore production by streptomycetes-stability and alteration of ferrihydroxamates in heavy metal-contaminated soil

Heavy metal-contaminated soil derived from a former uranium mining site in Ronneburg, Germany, was used for sterile mesocosms inoculated with the extremely metal-resistant Streptomyces mirabilis P16B-1 or the sensitive control strain Streptomyces lividans TK24. The production and fate of bacterial hydroxamate siderophores in soil was analyzed, and the presence of ferrioxamines E, B, D, and G was shown. While total ferrioxamine concentrations decreased in water-treated controls after 30 days of incubation, the sustained production by the bacteria was seen. For the individual molecules, alteration between neutral and cationic forms and linearization of hydroxamates was observed for the first time. Mesocosms inoculated with biomass of either strain showed changes of siderophore contents compared with the non-treated control indicating for auto-alteration and consumption, respectively, depending on the vital bacteria present. Heat stability and structural consistency of siderophores obtained from sterile culture filtrate were shown. In addition, low recovery (32 %) from soil was shown, indicating adsorption to soil particles or soil organic matter. Fate and behavior of hydroxamate siderophores in metal-contaminated soils may affect soil properties as well as conditions for its inhabiting (micro)organisms.

Role of porins in iron uptake by Mycobacterium smegmatis

Many bacteria rely on siderophores to extract iron from the environment. However, acquisition of iron-loaded siderophores is dependent on high-affinity uptake systems that are not produced under high-iron conditions. The fact that bacteria are able to maintain iron homeostasis in the absence of siderophores indicates that alternative iron acquisition systems exist. It has been speculated that such low-affinity uptake of iron in Gram-negative bacteria includes diffusion of iron ions or chelates across the outer membrane through porins. The outer membrane of the saprophytic Mycobacterium smegmatis contains the Msp family of porins, which enable the diffusion of small and hydrophilic solutes, such as monosaccharides, amino acids, and phosphate. However, it is unknown how cations cross the outer membrane of mycobacteria. Here, we show that the Msp porins of M. smegmatis are involved in the acquisition of soluble iron under high-iron conditions. Uptake of ferric ions by a triple porin mutant was reduced compared to wild-type (wt) M. smegmatis. An intracellular iron reporter indicated that derepression of iron-responsive genes occurs at higher iron concentrations in the porin mutant. This was consistent with the finding that the porin mutant produced more siderophores under low-iron conditions than wt M. smegmatis. In contrast, uptake of the exochelin MS, the main siderophore of M. smegmatis, was not affected by the lack of porins, indicating that a specific outer membrane siderophore receptor exists. These results provide, to our knowledge, the first experimental evidence that general porins are indeed the outer membrane conduit of low-affinity iron acquisition systems in bacteria.

Fluorescent 3-hydroxy-4-pyridinone hexadentate iron chelators: intracellular distribution and the relevance to antimycobacterial properties

We report the synthesis and characterization of a fluorescent iron chelator (4), shown to be effective in inhibiting the growth of Mycobacterium avium in macrophages, together with the synthesis and characterization of two unsuccessful analogues selected to facilitate identification of the molecular properties responsible for the antimicrobial activity. Partition of the chelators in liposomes was investigated and the compounds were assessed with respect to uptake by macrophages, responsiveness to iron overload/iron deprivation and intracellular distribution by flow cytometry and confocal microscopy. The synthesis of the hexadentate chelators is based on a tetrahedral structure to which three bidentate 3-hydroxy-4-pyridinone chelating units are linked via amide bonds. The structure is synthetically versatile, allowing further addition of functional groups such as fluorophores. Here, we analyse the non-functionalized hexadentate unit (3) and the corresponding rhodamine B (4) and fluorescein (5) labelled chelators. The iron(III) stability constant was determined for 3 and the values log beta = 34.4 and pFe(3+) = 29.8 indicate an affinity for iron of the same order of magnitude as that of mycobacteria siderophores. Fluorescence properties in the presence of liposomes show that 4 strongly interacts with the lipid phase, whereas 5 does not. Such different behaviour may explain their distinct intracellular localization as revealed by confocal microscopy. The flow cytometry and confocal microscopy studies indicate that 4 is readily engulfed by macrophages and targeted to cytosol and vesicles of the endolysosomal continuum, whereas 5 is differentially distributed and only partially colocalizes with 4 after prolonged incubation. Differential distribution of the compounds is likely to account for their different efficacy against mycobacteria.

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+).

Reactions of N-benzyloxycarbamate derivatives with stabilized carbon nucleophiles: a new synthetic approach to polyhydroxamic acids and other hydroxamate-containing mixed ligand systems

Hydroxamic acids are an important class of chelators of hard metal ions such as Fe(III), which have found applications in therapeutic, diagnostic, and separation chemistry. Hence, methods for their preparation and incorporation into various matrices are important. A new strategy for the preparation of hydroxamic acids that uses readily available N-benzyloxy carbamic acid ethyl ester, 1, has been developed. N-Alkylation of 1 occurs readily to give N-alkyl-N-benzyloxy carbamates, 2, which react with a variety of stabilized carbon nucleophiles to give functionalized protected hydroxamic acids, 3, in good to excellent yields. The O-protected hydroxamate intermediates 3 can be further alkylated with halides to access a variety of potential metal binding hosts. The usefulness of this methodology has been demonstrated by the synthesis of a novel trihydroxamic acid 6, mixed ligand systems 9 and 12, and the macrocyclic dihydroxamic acid 16.

New insights into the metal specificity of the Pseudomonas aeruginosa pyoverdine-iron uptake pathway

Pyoverdine (PvdI) is the major siderophore secreted by Pseudomonas aeruginosa PAOI in order to get access to iron. After being loaded with iron in the extracellular medium, PvdI is transported across the bacterial outer membrane by the transporter, FpvAI. We used the spectral properties of PvdI to show that in addition to Fe(3+), this siderophore also chelates, but with lower efficiencies, all the 16 metals used in our screening. Afterwards, FpvAI at the cell surface binds Ag(+), Al(3+), Cd(2+), Co(2+), Cu(2+), Fe(3+), Ga(3+), Hg(2+), Mn(2+), Ni(2+) or Zn(2+) in complex with PvdI. We used Inductively Coupled Plasma-Atomic Emission Spectrometry to monitor metal uptake in P. aeruginosa: TonB-dependent uptake, in the presence of PvdI, was only efficient for Fe(3+). Cu(2+), Ga(3+), Mn(2+) and Ni(2+) were also transported into the cell but with lower uptake rates. The presence of Al(3+), Cu(2+), Ga(3+), Mn(2+), Ni(2+) and Zn(2+) in the extracellular medium induced PvdI production in P. aeruginosa. All these data allow a better understanding of the behaviour of the PvdI uptake pathway in the presence of metals other than iron: FpvAI at the cell surface has broad metal specificity at the binding stage and it is highly selective for Fe(3+) only during the uptake process.

Density-Functional Theory Study of Iron(III) Hydrolysis in Aqueous Solution

Fe(III) hydrolysis in aqueous solution has been investigated using density-functional methods (DFT). All possible structures arising from different tautomers and multiplicities have been calculated. The solvation energy has been estimated using the UAHF-PCM method. The hydrolysis free energies have been estimated and compared with the available experimental data. The different hydrolysis species have distinct geometries and electronic structures. We have shown that improvement of theory level in calculating the electronic energy does not necessarily improve the estimated free energies in aqueous solution since the UAHF-PCM is a simple method that neglects specific interactions with the solvent. Therefore, it is important to have the correct balance between theory level used in the electronic calculation and the UAHF-PCM. The PBE/TZVP/UAHF-PCM method has been found to describe correctly the hydrolysis energies of Fe(III), deviating about 3.0 kcal mol(-1) from experimental values.

Understanding of the Mode of Action of FeIII-EDDHA as Iron Chlorosis Corrector Based on Its Photochemical and Redox Behavior

The very low reduction potential of the chelate Fe(III)-EDDHA (EDDHA = ethylenediamine N,N'-bis(2-hydroxy)phenylacetic acid) makes it unreactive in photochemically or chemically induced electron transfer processes. The lack of reactivity of this complex toward light invalidates photodegradation as an alternative mechanism for environmental elimination. However, in spite of its low reduction potential, the biological reduction of Fe(III)-EDDHA is very effective. Based on electrochemical measurements, it is proposed that Fe(III)-EDDHA itself is not the substrate of the enzyme ferric chelate reductase. Likely, at the more acidic pH in the vicinity of the roots, the ferric chelate in a closed form (FeL-) could generate a vacant coordination site that leads to an open hexacoordinate species (FeHL) where the reduction of the metal by the enzyme takes place.