Membrane Dynamics of the Amphiphilic Siderophore, Acinetoferrin

Lipocalin-2 is an essential component of the innate immune response to Acinetobacter baumannii infection

Acinetobacter baumannii is an opportunistic pathogen and an emerging global health threat. Within healthcare settings, major presentations of A. baumannii include bloodstream infections and ventilator-associated pneumonia. The increased prevalence of ventilated patients during the COVID-19 pandemic has led to a rise in secondary bacterial pneumonia caused by multidrug resistant (MDR) A. baumannii. Additionally, due to its MDR status and the lack of antimicrobial drugs in the development pipeline, the World Health Organization has designated carbapenem-resistant A. baumannii to be its priority critical pathogen for the development of novel therapeutics. To better inform the design of new treatment options, a comprehensive understanding of how the host contains A. baumannii infection is required. Here, we investigate the innate immune response to A. baumannii by assessing the impact of infection on host gene expression using NanoString technology. The transcriptional profile observed in the A. baumannii infected host is characteristic of Gram-negative bacteremia and reveals expression patterns consistent with the induction of nutritional immunity, a process by which the host exploits the availability of essential nutrient metals to curtail bacterial proliferation. The gene encoding for lipocalin-2 (Lcn2), a siderophore sequestering protein, was the most highly upregulated during A. baumannii bacteremia, of the targets assessed, and corresponds to robust LCN2 expression in tissues. Lcn2^-/- mice exhibited distinct organ-specific gene expression changes including increased transcription of genes involved in metal sequestration, such as S100A8 and S100A9, suggesting a potential compensatory mechanism to perturbed metal homeostasis. In vitro, LCN2 inhibits the iron-dependent growth of A. baumannii and induces iron-regulated gene expression. To elucidate the role of LCN2 in infection, WT and Lcn2^-/- mice were infected with A. baumannii using both bacteremia and pneumonia models. LCN2 was not required to control bacterial growth during bacteremia but was protective against mortality. In contrast, during pneumonia Lcn2^-/- mice had increased bacterial burdens in all organs evaluated, suggesting that LCN2 plays an important role in inhibiting the survival and dissemination of A. baumannii. The control of A. baumannii infection by LCN2 is likely multifactorial, and our results suggest that impairment of iron acquisition by the pathogen is a contributing factor. Modulation of LCN2 expression or modifying the structure of LCN2 to expand upon its ability to sequester siderophores may thus represent feasible avenues for therapeutic development against this pathogen.

A lack of therapeutic options has prompted the World Health Organization to designate multidrug-resistant Acinetobacter baumannii as its priority critical pathogen for research into new treatment strategies. The mechanisms employed by A. baumannii to cause disease and the host tactics exercised to constrain infection are not fully understood. Here, we further characterize the innate immune response to A. baumannii infection. We identify nutritional immunity, a process where the availability of nutrient metals is exploited to restrain bacterial growth, as being induced during infection. The gene encoding for lipocalin-2 (Lcn2), a protein that can impede iron uptake by bacteria, is highly upregulated in infected mice, and corresponds to robust LCN2 detection in the tissues. We find that LCN2 is crucial to reducing mortality from A. baumannii bacteremia and inhibits dissemination of the pathogen during pneumonia. In wild-type and Lcn2-deficient mice, broader transcriptional profiling reveals expression patterns consistent with the known response to Gram-negative bacteremia. Although the role of LCN2 in infection is likely multifactorial, we find its antimicrobial effects are at least partly exerted by impairing iron acquisition by A. baumannii. Facets of nutritional immunity, such as LCN2, may be exploited as novel therapeutics in combating A. baumannii infection.

Discovery and genetic identification of amphiphilic coprogen siderophores from Trichoderm hypoxylon

Siderophores are small molecular iron chelators and participate in the multiple cellular processes in fungi. In this study, we discovered and identified five amphiphilic coprogen siderophores including three new natural products according to LC-MS-guided separation strategy from Trichoderm hypoxylon. The structures of three new coprogens were elucidated by NMR spectroscopy, and high-resolution (HR)-ESI-MS analysis. Genetic deletions of dfcA and dfcB abolished the production of compounds 1-5 that implied their involvement in the biosynthesis of coprogens. Interestingly, cultivations of ΔdfcA and ΔdfcB mutants with pathogenic fungi Fusarium oxysporum and Mucor corcinelloides showed the weaker inhibitions in comparison to wild type that demonstrated coprogen's role in combating the pathogenic fungi. Our study not only enriched the diversities of siderophores but also provided an approach for finding the rare amphiphilic coprogen siderophores in fungi. Furthermore, this work provided a basis for investigation on the biosynthesis of fungal amphiphilic siderophores and their ecological roles in nature. KEY POINTS: • A series of amphiphilic coprogens were found. • The gene cluster of amphiphilic coprogens and ecological roles were elucidated.

Iron-meditated fungal starvation by lupine rhizosphere-associated and extremotolerant Streptomyces sp. S29 desferrioxamine production

Siderophores are iron-chelating compounds that aid iron uptake, one of the key strategies for microorganisms to carve out ecological niches in microbially diverse environments. Desferrioxamines are the principal siderophores produced by Streptomyces spp. Their biosynthesis has been well studied and as a consequence, the chemical potential of the pathway continues to expand. With all of this in mind, our study aimed to explore extremotolerant and lupine rhizosphere-derived Streptomyces sp. S29 for its potential antifungal capabilities. Cocultivation of isolate S29 was carried out with Aspergillus niger and Botrytis cinerea, both costly fungal phytopathogens in the wine industry, to simulate their interaction within the rhizosphere. The results indicate that not only is Streptomyces sp. S29 extraordinary at producing hydroxamate siderophores but uses siderophore production as a means to 'starve' the fungi of iron. High resolution LC-MS/MS followed by GNPS molecular networking was used to observe the datasets for desferrioxamines and guided structure elucidation of new desferrioxamine analogues. Comparing the new chemistry, using tools like molecular networking and MS2LDA, with the known biosynthesis, we show that the chemical potential of the desferrioxamine pathway has further room for exploration.

Acinetobacter baumannii can use multiple siderophores for iron acquisition, but only acinetobactin is required for virulence

Acinetobacter baumannii is an emerging pathogen that poses a global health threat due to a lack of therapeutic options for treating drug-resistant strains. In addition to acquiring resistance to last-resort antibiotics, the success of A. baumannii is partially due to its ability to effectively compete with the host for essential metals. Iron is fundamental in shaping host-pathogen interactions, where the host restricts availability of this nutrient in an effort to curtail bacterial proliferation. To circumvent restriction, pathogens possess numerous mechanisms to obtain iron, including through the use of iron-scavenging siderophores. A. baumannii elaborates up to ten distinct siderophores, encoded from three different loci: acinetobactin and pre-acinetobactin (collectively, acinetobactin), baumannoferrins A and B, and fimsbactins A-F. The expression of multiple siderophores is common amongst bacterial pathogens and often linked to virulence, yet the collective contribution of these siderophores to A. baumannii survival and pathogenesis has not been investigated. Here we begin dissecting functional redundancy in the siderophore-based iron acquisition pathways of A. baumannii. Excess iron inhibits overall siderophore production by the bacterium, and the siderophore-associated loci are uniformly upregulated during iron restriction in vitro and in vivo. Further, disrupting all of the siderophore biosynthetic pathways is necessary to drastically reduce total siderophore production by A. baumannii, together suggesting a high degree of functional redundancy between the metabolites. By contrast, inactivation of acinetobactin biosynthesis alone impairs growth on human serum, transferrin, and lactoferrin, and severely attenuates survival of A. baumannii in a murine bacteremia model. These results suggest that whilst A. baumannii synthesizes multiple iron chelators, acinetobactin is critical to supporting growth of the pathogen on host iron sources. Given the acinetobactin locus is highly conserved and required for virulence of A. baumannii, designing therapeutics targeting the biosynthesis and/or transport of this siderophore may represent an effective means of combating this pathogen.

Exploitation of two siderophore receptors, BauA and BfnH, for protection against Acinetobacter baumannii infection

Iron uptake system is expressed in early stages of Acinetobacter baumannii infections under iron-restricted conditions. This study is aimed at the evaluation of immuno-protectivity of BfnH in comparison with BauA in both mature and selected fragmental proteins. The study was designed in single and combined forms of antigens. BfnH is presented in 3472 strains of A. baumannii with more than 97% identity. The preliminary immune-informatics analysis of this protein indicated a region from the β-barrel domain including exposed loops 2-5, with antigenic score comparable to that of BfnH. There was a significant rise in the specific IgG response in all test groups. The bacterial challenge with a lethal dose of A. baumannii demonstrated partial protection of whole proteins which coincides with a significant reduction in the bacterial population colonized in the main organs and an increase in the survival level. Passive immunization of the mice brought about 50% survival in the mice groups immunized with BfnH and with a combination of BfnH and BauA. The protectivity of siderophore receptors suggests their potential immunogenic role that could be considered as a component of multivalent subunit vaccine candidates against A. baumannii.

Cyanobacterial Siderophores-Physiology, Structure, Biosynthesis, and Applications

Siderophores are low-molecular-weight metal chelators that function in microbial iron uptake. As iron limits primary productivity in many environments, siderophores are of great ecological importance. Additionally, their metal binding properties have attracted interest for uses in medicine and bioremediation. Here, we review the current state of knowledge concerning the siderophores produced by cyanobacteria. We give an overview of all cyanobacterial species with known siderophore production, finding siderophores produced in all but the most basal clades, and in a wide variety of environments. We explore what is known about the structure, biosynthesis, and cycling of the cyanobacterial siderophores that have been characterized: Synechobactin, schizokinen and anachelin. We also highlight alternative siderophore functionality and technological potential, finding allelopathic effects on competing phytoplankton and likely roles in limiting heavy-metal toxicity. Methodological improvements in siderophore characterization and detection are briefly described. Since most known cyanobacterial siderophores have not been structurally characterized, the application of mass spectrometry techniques will likely reveal a breadth of variation within these important molecules.

Fimsbactin and Acinetobactin Compete for the Periplasmic Siderophore Binding Protein BauB in Pathogenic Acinetobacter baumannii

Environmental and pathogenic microbes produce siderophores as small iron-binding molecules to scavenge iron from natural environments. It is common for microbes to produce multiple siderophores to gain a competitive edge in mixed microbial environments. Strains of human pathogenic Acinetobacter baumannii produce up to three siderophores: acinetobactin, baumannoferrin, and fimsbactin. Production of acinetobactin and baumannoferrin is highly conserved among clinical isolates while fimsbactin production appears to be less common. Fimsbactin is structurally related to acinetobactin through the presence of catecholate and phenolate oxazoline metal-binding motifs, and both are derived from nonribosomal peptide assembly lines with similar catalytic domain orientations and identities. Here we report on the chemical, biochemical, and microbiological investigation of fimsbactin and acinetobactin alone and in combination. We show that fimsbactin forms a 1:1 complex with iron(III) that is thermodynamically more stable than the 2:1 acinetobactin ferric complex. Alone, both acinetobactin and fimsbactin stimulate A. baumannii growth, but in combination the two siderophores appear to compete and collectively inhibit bacterial growth. We show that fimsbactin directly competes with acinetobactin for binding the periplasmic siderophore-binding protein BauB suggesting a possible biochemical mechanism for the phenomenon where the buildup of apo-siderophores in the periplasm leads to iron starvation. We propose an updated model for siderophore utilization and competition in A. baumannii that frames the molecular, biochemical, and cellular interplay of multiple iron acquisition systems in a multidrug resistant Gram-negative human pathogen.

Acinetobactin Isomerization Enables Adaptive Iron Acquisition in Acinetobacter baumannii through pH-Triggered Siderophore Swapping

Pathogenic strains of Acinetobacter baumannii excrete multiple siderophores that enhance iron scavenging from host sources. The oxazoline siderophore pre-acinetobactin undergoes an unusual non-enzymatic isomerization, producing the isoxazolidinone acinetobactin. In this study, we explored the kinetics, mechanism, and biological consequence of this siderophore swapping. Pre-acinetobactin is excreted to the extracellular space where the isomerization to acinetobactin occurs with a pH-rate profile consistent with 5-exo-tet cyclization at C5' with clean stereochemical inversion. Pre-acinetobactin persists at pH <6, and acinetobactin is rapidly formed at pH >7, matching each siderophore's pH preference for iron(III) chelation and A. baumannii growth promotion. Acinetobactin isomerization provides two siderophores for the price of one, enabling A. baumannii to sequester iron over a broad pH range likely to be encountered during the course of an infection.

Characterization and Fungal Inhibition Activity of Siderophore from Wheat Rhizosphere Associated Acinetobacter calcoaceticus Strain HIRFA32

Acinetobacter calcoaceticus HIRFA32 from wheat rhizosphere produced catecholate type of siderophore with optimum siderophore (ca. 92 % siderophore units) in succinic acid medium without FeSO4 at 28 °C and 24 h of incubation. HPLC purified siderophore appeared as pale yellow crystals with molecular weight [M(+1)] m/z 347.18 estimated by LCMS. The structure elucidated by (1)H NMR, (13)C NMR, HMQC, HMBC, NOESY and decoupling studies, revealed that siderophore composed of 2,3-dihydroxybenzoic acid with hydroxyhistamine and threonine as amino acid subunits. In vitro study demonstrated siderophore mediated mycelium growth inhibition (ca. 46.87 ± 0.5 %) of Fusarium oxysporum. This study accounts to first report on biosynthesis of acinetobactin-like siderophore by the rhizospheric strain of A. calcoaceticus and its significance in inhibition of F. oxysporum.

Microbial tailoring of acyl peptidic siderophores

Marine bacteria produce an abundance of suites of acylated siderophores characterized by a unique, species-dependent headgroup that binds iron(III) and one of a series of fatty acid appendages. Marinobacter sp. DS40M6 produces a suite of seven acylated marinobactins, with fatty acids ranging from saturated and unsaturated C12–C18 fatty acids. In the present study, we report that in the late log phase of growth, the fatty acids are hydrolyzed by an amide hydrolase producing the peptidic marinobactin headgroup. Halomonas aquamarina str. DS40M3, another marine bacterium isolated originally from the same sample of open ocean water as Marinobacter sp. DS40M6, produces the acyl aquachelins, also as a suite composed of a peptidic headgroup distinct from that of the marinobactins. In contrast to the acyl marinobactins, hydrolysis of the suite of acyl aquachelins is not detected, even when H. aquamarina str. DS40M3 is grown into the stationary phase. The Marinobacter cell-free extract containing the acyl amide hydrolase is active toward exogenous acyl-peptidic siderophores (e.g., aquachelin C, loihichelin C, as well as octanoyl homoserine lactone used in quorum sensing). Further, when H. aquamarina str. DS40M3 is cultured together with Marinobacter sp. DS40M6, the fatty acids of both suites of siderophores are hydrolyzed, and the aquachelin headgroup is also produced. The present study demonstrates that coculturing bacteria leads to metabolically tailored metabolites compared to growth in a single pure culture, which is interesting given the importance of siderophore-mediated iron acquisition for bacterial growth and that Marinobacter sp. DS40M6 and H. aquamarina str. DS40M3 were isolated from the same sample of seawater.

Integrated metabolomics approach facilitates discovery of an unpredicted natural product suite from Streptomyces coelicolor M145

Natural products exhibit a broad range of biological properties and have been a crucial source of therapeutic agents and novel scaffolds. Although bacterial secondary metabolomes are widely explored, they remain incompletely cataloged by current isolation and characterization strategies. To identify metabolites residing in unexplored chemical space, we have developed an integrated discovery approach that combines bacterial growth perturbation, accurate mass spectrometry, comparative mass spectra data analysis, and fragmentation spectra clustering for the identification of low-abundant, novel compounds from complex biological matrices. In this investigation, we analyzed the secreted metabolome of the extensively studied Actinomycete, Streptomyces coelicolor M145, and discovered a low-abundant suite of 15 trihydroxamate, amphiphilic siderophores. Compounds in this class have primarily been observed in marine microorganisms making their detection in the soil-dwelling S. coelicolor M145 significant. At least 10 of these ferrioxamine-based molecules are not known to be produced by any organism, and none have previously been detected from S. coelicolor M145. In addition, we confirmed the production of ferrioxamine D1, a relatively hydrophilic family member that has not been shown to be biosynthesized by this organism. The identified molecules are part of only a small list of secondary metabolites that have been discovered since sequencing of S. coelicolor M145 revealed that it possessed numerous putative secondary metabolite-producing gene clusters with no known metabolites. Thus, the identified siderophores represent the unexplored metabolic potential of both well-studied and new organisms that could be uncovered with our sensitive and robust approach.

A suite of citrate-derived siderophores from a marine Vibrio species isolated following the Deepwater Horizon oil spill

Nearly all microbes require iron for growth. The low concentration of iron found in the ocean makes iron acquisition a particularly difficult task. In response to these low iron conditions, many bacteria produce low-molecular-weight iron-binding molecules called siderophores to aid in iron uptake. We report herein the isolation and structural characterization of a suite of amphiphilic siderophores called the ochrobactins-OH, which are produced by a Vibrio species isolated from the Gulf of Mexico after the 2010 Deepwater Horizon oil spill. The citrate-based ochrobactins-OH are derivatives of aerobactin, replacing the acetyl groups with fatty acid appendages ranging in size from C8 to C12, and are distinctly different from the ochrobactins in that the fatty acid appendages are hydroxylated rather than unsaturated. The discovery of the marine amphiphilic ochrobactin-OH suite of siderophores increases the geographic and phylogenetic diversity of siderophore-producing bacteria.

Mechanisms of peroxynitrite interactions with heme proteins

Oxygenated heme proteins are known to react rapidly with nitric oxide (NO) to produce peroxynitrite (PN) at the heme site. This process could lead either to attenuation of the effects of NO or to nitrosative protein damage. PN is a powerful nitrating and oxidizing agent that has been implicated in a variety of cell injuries. Accordingly, it is important to delineate the nature and variety of reaction mechanisms of PN interactions with heme proteins. In this Forum, we survey the range of reactions of PN with heme proteins, with particular attention to myoglobin and cytochrome c. While these two proteins are textbook paradigms for oxygen binding and electron transfer, respectively, both have recently been shown to have other important functions that involve NO and PN. We have recently described direct evidence that ferrylmyolgobin (ferrylMb) and nitrogen dioxide (NO(2)) are both produced during the reaction of PN and metmyolgobin (metMb) (Su, J.; Groves, J. T. J. Am. Chem. Soc. 2009, 131, 12979-12988). Kinetic evidence indicates that these products evolve from the initial formation of a caged radical intermediate [Fe(IV) horizontal lineO.NO(2)]. This caged pair reacts mainly via internal return with a rate constant k(r) to form metMb and nitrate in an oxygen-rebound scenario. Detectable amounts of ferrylMb are observed by stopped-flow spectrophotometry, appearing at a rate consistent with the rate, k(obs), of heme-mediated PN decomposition. Freely diffusing NO(2), which is liberated concomitantly from the radical pair (k(e)), preferentially nitrates myoglobin Tyr103 and added fluorescein. For cytochrome c, Raman spectroscopy has revealed that a substantial fraction of cytochrome c converts to a beta-sheet structure, at the expense of turns and helices at low pH (Balakrishnan, G.; Hu, Y.; Oyerinde, O. F.; Su, J.; Groves, J. T.; Spiro, T. G. J. Am. Chem. Soc., 2007, 129, 504-505). It is proposed that a short beta-sheet segment, comprising residues 37-39 and 58-61, extends itself into the large 37-61 loop when the latter is destabilized by protonation of H26, which forms an anchoring hydrogen bond to loop residue P44. This conformation change ruptures the Met80-Fe bond, as revealed by changes in ligation-sensitive Raman bands. It also induces peroxidase activity with the same temperature profile. This process is suggested to model the apoptotic peroxidation of cardiolipin by cytochrome c.

Chemistry and biology of siderophores

Siderophores are compounds produced by bacteria, fungi and graminaceous plants for scavenging iron from the environment. They are low-molecular-weight compounds (500-1500 daltons) possessing a high affinity for iron(III) (Kf > 1030), the biosynthesis of which is regulated by iron levels and the function of which is to supply iron to the cell. This article briefly describes the classification and chemical properties of siderophores, before outlining research on siderophore biosynthesis and transport. Clinically important siderophores and the therapeutic potential of siderophore design are described. Appendix 1 provides a comprehensive list of siderophore structures.

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.

Design and synthesis of a siderophore conjugate as a potent PSMA inhibitor and potential diagnostic agent for prostate cancer

A siderophore conjugate was designed as a potential PSMA inhibitor and diagnostic agent for prostate cancer. A semi-rigid spacer was incorporated to avoid competitive participation of iron binding by the enzyme inhibitor relative to the siderophore component. Biological test results showed that, even with the extended scaffold, this compound is a potent PSMA inhibitor with an IC50 of 4 nM. This siderophore conjugate may be useful for detection of prostate-derived cancer cells by magnetic resonance imaging (MRI).

Picomolar inhibitors as transition-state probes of 5'-methylthioadenosine nucleosidases

Transition states can be predicted from an enzyme's affinity to related transition-state analogues. 5'-Methylthioadenosine nucleosidases (MTANs) are involved in bacterial quorum sensing pathways and thus are targets for antibacterial drug design. The transition-state characteristics of six MTANs are compared by analyzing dissociation constants (K(d)) with a small array of representative transition-state analogues. These inhibitors mimic early or late dissociative transition states with K(d) values in the picomolar range. Our results indicate that the K(d) ratio for mimics of early and late transition states are useful in distinguishing between these states. By this criterion, the transition states of Neisseria meningitides and Helicobacter pylori MTANs are early dissociative, whereas Escherichia coli, Staphylococcus aureus, Streptococcus pneumoniae, and Klebsiella pneumoniae MTANs have late dissociative characters. This conclusion is confirmed independently by the characteristic [1'- (3)H] and [1'- (14)C] kinetic isotope effects (KIEs) of these enzymes. Large [1'- (3)H] and unity [1'- (14)C] KIEs are observed for late dissociative transition states, whereas early dissociative states showed close-to-unity [1'- (3)H] and significant [1'- (14)C] KIEs. K d values of various MTANs for individual transition-state analogues provide tentative information about transition-state structures due to varying catalytic efficiencies of enzymes. Comparing K d ratios for mimics of early and late transition states removes limitations inherent to the enzyme and provides a better predictive tool in discriminating between possible transition-state structures.

Marine amphiphilic siderophores: marinobactin structure, uptake, and microbial partitioning

Marinobactins A-E are a suite of amphiphilic siderophores which have a common peptidic head group that coordinates Fe(III), and a fatty acid which varies in length and saturation. As a result of the amphiphilic properties of these siderophores it is difficult to study siderophore-mediated uptake of iron, because the amphiphilic siderophores partition indiscriminately in microbial and other membranes. An alternative method to distinguish amphiphilic siderophore partitioning versus siderophore-mediated active uptake for Fe(III)-marinobactin E has been developed. In addition, a new member of the marinobactin family of siderophores is also reported, marinobactin F, which has a C(18) fatty acid with one double bond and which is substantially more hydrophobic that marinobactins A-E.

Towards understanding performance differences between approximate density functionals for spin states of iron complexes

Density functional theory has been widely used to investigate the structural and electronic properties of heme-containing proteins such as cytochrome P450. Nevertheless, recent studies have shown that approximate exchange-correlation energy density functionals can incorrectly predict the stability order of spin states in, for instance, iron-containing pyridine and imidazole systems. This raises questions about the validity of earlier theoretical studies. In this work, we systematically investigate a few typical inorganic and organic iron-containing complexes and try to understand the performance difference of various density functionals. Two oxidation states of iron, Fe(II) and Fe(III), with different spin states and both adiabatic and vertical structures are considered. A different description of the outmost molecular orbital is found to play the crucial role. Local density and generalized gradient based functionals bias the lower spin state and produce a more localized frontier orbital that is higher in energy than the hybrid functionals. Energy component analysis has been performed, together with comparison of numerous structural and electronic properties. Implications of the present work to the theoretical study of heme-containing biological molecules and other spin-related systems are discussed.

Structure and membrane affinity of new amphiphilic siderophores produced by Ochrobactrum sp. SP18

The coastal alpha-proteobacterium Ochrobactrum sp. SP18 produces a suite of three citrate-derived, cell-associated amphiphilic siderophores, ochrobactins A-C. The ochrobactins are composed of a citric acid backbone amide-linked to two lysine residues. Each epsilon-amine of lysine is hydroxylated and acylated forming two hydroxamic acid moieties. One of the acylated appendages of each ochrobactin is (E)-2-decenoic acid. The other acylated appendages for ochrobactins A-C are (E)-2-octenoic acid, octanoic acid and (E)-2-decenoic acid, respectively. The ferric ochrobactin complexes are photoreactive in UV light, producing an oxidized ligand with loss of 46 mass units that can still coordinate Fe(III). The relative partitioning of the apo-ochrobactins, Fe(III) ochrobactins and Fe(III) photoproducts into 1,2-dimyristoyl-sn-glycero-3-phosphocholine vesicles is presented. The ochrobactins are the first example of aerobactin-based siderophores with two fatty acid appendages produced in a suite with varying acyl appendage lengths.

How pathogenic bacteria evade mammalian sabotage in the battle for iron

Many bacteria, including numerous human pathogens, synthesize small molecules known as siderophores to scavenge iron. Enterobactin, a siderophore produced by enteric bacteria, is surprisingly ineffective as an iron-scavenging agent for bacteria growing in animals because of its hydrophobicity and its sequestration by the mammalian protein siderocalin, a component of the innate immune system. However, pathogenic strains of Escherichia coli and Salmonella use enzymes encoded by the iroA gene cluster to tailor enterobactin by glycosylation and linearization. The resulting modified forms of enterobactin, known as salmochelins, can evade siderocalin and are less hydrophobic than enterobactin, restoring this siderophore's iron-scavenging ability in mammals.

Marine siderophores and microbial iron mobilization

Iron is essential for the growth of nearly all microorganisms yet iron is only sparingly soluble near the neutral pH, aerobic conditions in which many microorganisms grow. The pH of ocean water is even higher, thereby further lowering the concentration of dissolved ferric ion. To compound the problem of availability, the total iron concentration is surprisingly low in surface ocean water, yet nevertheless, marine microorganisms still require iron for growth. Like terrestrial bacterial, bacteria isolated from open ocean water often produce siderophores, which are low molecular weight chelating ligands that facilitate the microbial acquisition of iron. The present review summarizes the structures of siderophores produced by marine bacteria and the emerging characteristics that distinguish marine siderophores.

Enzymatic tailoring of enterobactin alters membrane partitioning and iron acquisition

Enterobactin (Ent), a prototypic bacterial siderophore, is modified by both the C-glucosyltransferase IroB and the macrolactone hydrolase IroE in pathogenic bacteria that contain the iroA cluster. To investigate the possible effects of glucosylation and macrolactone hydrolysis on the physical properties of Ent, the membrane affinities and iron acquisition rates of Ent and Ent-derived siderophores were measured. The data obtained indicate that Ent has a high membrane affinity (K(x) = 1.5 x 10(4)) similar to that of ferric acinetoferrin, an amphiphile containing two eight-carbon hydrophobic chains. Glucosylation and macrolactone hydrolysis decrease the membrane affinity of Ent by 5-25-fold. Furthermore, in the presence of phospholipid vesicles, the iron acquisition rate is significantly increased by glucosylation and macrolactone hydrolysis, due to the resultant decrease in membrane sequestration of the siderophore. These results suggest that IroB and IroE enhance the ability of Ent-producing pathogens to acquire iron in membrane-rich microenvironments.

Effect of phosphatidylcholine vesicle size on chirality induction and chiral discrimination

The effect of the size of phosphatidylcholine (PC) vesicles on the induction of chirality and chiral discrimination was examined. Three kinds of vesicles formed with l-dimyristoyl, l-dipalmitoyl, or egg yolk PCs induced circular dichroisms (CDs) with the sign and intensity of the Cotton effect different from those of monomeric PCs. The CD intensity of the vesicles increased with a decrease in the vesicle size. Furthermore, the helicity of heterohelicene derivatives in a rapid equilibrium between right-handed (P) and left-handed (M) enantiomers was biased toward the M enantiomer side in l-PC vesicles, implying chiral discrimination by the vesicles. The extent of the bias toward the M enantiomer increased with an increase in vesicle size. Both the chirality induction and chiral discrimination were enhanced in a low-fluidity gel phase in comparison with those in a high-fluidity liquid-crystalline phase for every kind of vesicle of every size examined.

Synthesis and structural modeling of the amphiphilic siderophore rhizobactin-1021 and its analogs

We describe two convenient syntheses of rhizobactin-1021 (Rz), a citrate-based siderophore amphiphile produced by the nitrogen-fixing root symbiont Rhizobium meliloti-1021, and several analogs. Our approach features a singly amidated, tert-butyl-protected citrate intermediate that easily affords a variety of Rz analogs in the late stages of the synthesis. Structural modeling and the monolayer behavior of Rz and its metal complexes are consistent with a structural reorganization upon Rz-mediated iron chelation.

Mycobactin-mediated iron acquisition within macrophages

Restricting the availability of iron is an important strategy for defense against bacterial infection. Mycobacterium tuberculosis survives within the phagosomes of macrophages; consequently, iron acquisition is particularly difficult for M. tuberculosis, because the phagosomal membrane is an additional barrier for its iron access. However, little is known about the iron transport and acquisition pathways adapted by this microbe in vivo. Extracellular iron sources are usually mobilized by hydrophilic siderophores. Here, we describe direct evidence that mycobactins, the lipophilic siderophores of mycobacteria, efficiently extract intracellular macrophage iron. The metal-free siderophore is diffusely associated with the macrophage membrane, ready for iron chelation. Notably, the mycobactin-metal complex accumulates with high selectivity in macrophage lipid droplets, intracellular domains for lipid storage and sorting. In our experiments, these mycobactin-targeted lipid droplets were found in direct contact with phagosomes, poised for iron delivery. The existence of this previously undescribed iron acquisition pathway indicates that mycobacteria have taken advantage of endogenous macrophage mechanisms for iron mobilization and lipid sorting for iron acquisition during infection. The pathway could represent a new target for the control of mycobacterial infection.