High resolution structure of an alternate form of the ferric ion binding protein from Haemophilus influenzae

Conformational multiplicity of bacterial ferric binding protein revealed by small angle x-ray scattering and molecular dynamics calculations

This study combines molecular dynamics (MD) simulations with small angle x-ray scattering (SAXS) measurements to investigate the range of conformations that can be adopted by a pH/ionic strength (IS) sensitive protein and to quantify its distinct populations in solution. To explore how the conformational distribution of proteins may be modified in the environmental niches of biological media, we focus on the periplasmic ferric binding protein A (FbpA) from Haemophilus influenzae involved in the mechanism by which bacteria capture iron from higher organisms. We examine iron-binding/release mechanisms of FbpA in varying conditions simulating its biological environment. While we show that these changes fall within the detectable range for SAXS as evidenced by differences observed in the theoretical scattering patterns calculated from the crystal structure models of apo and holo forms, detection of conformational changes due to the point mutation D52A and changes in ionic strength (IS) from SAXS scattering profiles have been challenging. Here, to reach conclusions, statistical analyses with SAXS profiles and results from different techniques were combined in a complementary fashion. The SAXS data complemented by size exclusion chromatography point to multiple and/or alternative conformations at physiological IS, whereas they are well-explained by single crystallographic structures in low IS buffers. By fitting the SAXS data with unique conformations sampled by a series of MD simulations under conditions mimicking the buffers, we quantify the populations of the occupied substates. We also find that the D52A mutant that we predicted by coarse-grained computational modeling to allosterically control the iron binding site in FbpA, responds to the environmental changes in our experiments with conformational selection scenarios that differ from those of the wild type.

Structural and functional insights into iron acquisition from lactoferrin and transferrin in Gram-negative bacterial pathogens

Iron is an essential element for various lifeforms but is largely insoluble due to the oxygenation of Earth’s atmosphere and oceans during the Proterozoic era. Metazoans evolved iron transport glycoproteins, like transferrin (Tf) and lactoferrin (Lf), to keep iron in a non-toxic, usable form, while maintaining a low free iron concentration in the body that is unable to sustain bacterial growth. To survive on the mucosal surfaces of the human respiratory tract where it exclusively resides, the Gram-negative bacterial pathogen Moraxella catarrhalis utilizes surface receptors for acquiring iron directly from human Tf and Lf. The receptors are comprised of a surface lipoprotein to capture iron-loaded Tf or Lf and deliver it to a TonB-dependent transporter (TBDT) for removal of iron and transport across the outer membrane. The subsequent transport of iron into the cell is normally mediated by a periplasmic iron-binding protein and inner membrane transport complex, which has yet to be determined for Moraxella catarrhalis. We identified two potential periplasm to cytoplasm transport systems and performed structural and functional studies with the periplasmic binding proteins (FbpA and AfeA) to evaluate their role. Growth studies with strains deleted in the fbpA or afeA gene demonstrated that FbpA, but not AfeA, was required for growth on human Tf or Lf. The crystal structure of FbpA with bound iron in the open conformation was obtained, identifying three tyrosine ligands that were required for growth on Tf or Lf. Computational modeling of the YfeA homologue, AfeA, revealed conserved residues involved in metal binding.

Proteomic profiling of the outer membrane fraction of the obligate intracellular bacterial pathogen Ehrlichia ruminantium

The outer membrane proteins (OMPs) of Gram-negative bacteria play a crucial role in virulence and pathogenesis. Identification of these proteins represents an important goal for bacterial proteomics, because it aids in vaccine development. Here, we have developed such an approach for Ehrlichia ruminantium, the obligate intracellular bacterium that causes heartwater. A preliminary whole proteome analysis of elementary bodies, the extracellular infectious form of the bacterium, had been performed previously, but information is limited about OMPs in this organism and about their role in the protective immune response. Identification of OMPs is also essential for understanding Ehrlichia's OM architecture, and how the bacterium interacts with the host cell environment. First, we developed an OMP extraction method using the ionic detergent sarkosyl, which enriched the OM fraction. Second, proteins were separated via one-dimensional electrophoresis, and digested peptides were analyzed via nano-liquid chromatographic separation coupled with mass spectrometry (LC-MALDI-TOF/TOF). Of 46 unique proteins identified in the OM fraction, 18 (39%) were OMPs, including 8 proteins involved in cell structure and biogenesis, 4 in transport/virulence, 1 porin, and 5 proteins of unknown function. These experimental data were compared to the predicted subcellular localization of the entire E. ruminantium proteome, using three different algorithms. This work represents the most complete proteome characterization of the OM fraction in Ehrlichia spp. The study indicates that suitable subcellular fractionation experiments combined with straightforward computational analysis approaches are powerful for determining the predominant subcellular localization of the experimentally observed proteins. We identified proteins potentially involved in E. ruminantium pathogenesis, which are good novel targets for candidate vaccines. Thus, combining bioinformatics and proteomics, we discovered new OMPs for E. ruminantium that are valuable data for those investigating new vaccines against this organism. In summary, we provide both pioneering data and novel insights into the pathogenesis of this obligate intracellular bacterium.

Bordetella pertussis FbpA binds both unchelated iron and iron siderophore complexes

Bordetella pertussis is the causative agent of whooping cough. This pathogenic bacterium can obtain the essential nutrient iron using its native alcaligin siderophore and by utilizing xeno-siderophores such as desferrioxamine B, ferrichrome, and enterobactin. Previous genome-wide expression profiling identified an iron repressible B. pertussis gene encoding a periplasmic protein (FbpA_Bp). A previously reported crystal structure shows significant similarity between FbpA_Bp and previously characterized bacterial iron binding proteins, and established its iron-binding ability. Bordetella growth studies determined that FbpA_Bp was required for utilization of not only unchelated iron, but also utilization of iron bound to both native and xeno-siderophores. In this in vitro solution study, we quantified the binding of unchelated ferric iron to FbpA_Bp in the presence of various anions and importantly, we demonstrated that FbpA_Bp binds all the ferric siderophores tested (native and xeno) with μM affinity. In silico modeling augmented solution data. FbpA_Bp was incapable of iron removal from ferric xeno-siderophores in vitro. However, when FbpA_Bp was reacted with native ferric-alcaligin, it elicited a pronounced change in the iron coordination environment, which may signify an early step in FbpA_Bp-mediated iron removal from the native siderophore. To our knowledge, this is the first time the periplasmic component of an iron uptake system has been shown to bind iron directly as Fe³⁺ and indirectly as a ferric siderophore complex.

Binding of oxo-Cu2 clusters to ferric ion-binding protein A from Neisseria gonorrhoeae: a structural insight

The ferric ion-binding protein A (FbpA), a member of transferrin superfamily, is a periplasmic iron transporter employed by many Gram-negative pathogens. Our experiments indicated copper(ii) could bind with Neisseria gonorrhoeae FbpA (NgFbpA), and the binding constant reached up to (8.7 ± 0.2) × 10(8) M(-1)via UV-vis titration. The crystal structure of recombinant Cu-NgFbpA at 2.1 Å revealed that the oxo-Cu2 clusters (dinuclear centres) assembled in the iron binding cleft and were bound to the two adjacent tyrosine residues (Y195 and Y196) of the protein, two Cu ions coordinated with two tyrosines, Y195 and Y196, respectively, which was different from the binding model of Fe ion with FbpA, in which Y195 and Y196 coordinated together with one Fe ion. While this was similar to the binding of Zr and Hf ion clusters, Y195 and Y196 coordinated with two metal ions and the μ-oxo-bridges linking the metal ions. Structural superimposition demonstrated that oxo-Cu2-NgFbpA still keeping an open conformation, similar to the apo-form of NgFbpA. The structure presented additional information towards an understanding of the function of FbpA, and provided a detailed binding model for FbpA protein with the possible metal ions in a biological system.

Proposed docking interface between peptidoglycan and the target recognition domain of zoocin A

A docking model is proposed for the target recognition domain of the lytic exoenzyme zoocin A with the peptidoglycan on the outer cell surface of sensitive bacterial strains. Solubilized fragments from such peptidoglycans perturb specific backbone and side chain amide resonances in the recombinant form of the domain designated rTRD as detected in two-dimensional (1)H-(15)N correlation NMR spectra. The affected residues comprise a shallow surface cleft on the protein surface near W115, N53, N117, and Q105 among others, which interacts with the peptide portion of the peptidoglycan. Calculations with AutoDock Vina provide models of the docking interface. There is approximate homology between the rTDR-peptidoglycan docking site and the antigen binding site of Fab antibodies with the immunoglobin fold. EDTA was also found to bind to rTRD, but at a site distinct from the proposed peptidoglycan docking site.

FbpA--a bacterial transferrin with more to offer

BACKGROUND

Gram negative bacteria require iron for growth and virulence. It has been shown that certain pathogenic bacteria such as Neisseria gonorrhoeae possess a periplasmic protein called ferric binding protein (FbpA), which is a node in the transport of iron from the cell exterior to the cytosol.

SCOPE OF REVIEW

The relevant literature is reviewed which establishes the molecular mechanism of FbpA mediated iron transport across the periplasm to the inner membrane.

MAJOR CONCLUSIONS

Here we establish that FbpA may be considered a bacterial transferrin on structural and functional grounds. Data are presented which suggest a continuum whereby FbpA may be considered as a naked iron carrier, as well as a Fe-chelate carrier, and finally a member of the larger family of periplasmic binding proteins.

GENERAL SIGNIFICANCE

An investigation of the molecular mechanisms of action of FbpA as a member of the transferrin super family enhances our understanding of bacterial mechanisms for acquisition of the essential nutrient iron, as well as the modes of action of human transferrin, and may provide approaches to the control of pathogenic diseases. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Transcriptional profiling of the iron starvation response in Bordetella pertussis provides new insights into siderophore utilization and virulence gene expression

Serological studies of patients with pertussis and the identification of antigenic Bordetella pertussis proteins support the hypothesis that B. pertussis perceives an iron starvation cue and expresses multiple iron source utilization systems in its natural human host environment. Furthermore, previous studies using a murine respiratory tract infection model showed that several of these B. pertussis iron systems are required for colonization and persistence and are differentially expressed over the course of infection. The present study examined genome-wide changes in B. pertussis gene transcript abundance in response to iron starvation in vitro. In addition to known iron source utilization genes, we identified a previously uncharacterized iron-repressed cytoplasmic membrane transporter system, fbpABC, that is required for the utilization of multiple structurally distinct siderophores including alcaligin, enterobactin, ferrichrome, and desferrioxamine B. Expression of type III secretion system genes was also found to be upregulated during iron starvation in both B. pertussis strain Tohama I and Bordetella bronchiseptica strain RB50. In a survey of type III secretion system protein production by an assortment of B. pertussis laboratory-adapted and low-passage clinical isolate strains, iron limitation increased the production and secretion of the type III secretion system-specific translocation apparatus tip protein Bsp22 in all Bvg-proficient strains. These results indicate that iron starvation in the infected host is an important environmental cue influencing not only Bordetella iron transport gene expression but also the expression of other important virulence-associated genes.

Conserved interaction between transferrin and transferrin-binding proteins from porcine pathogens

Gram-negative porcine pathogens from the Pasteurellaceae family possess a surface receptor complex capable of acquiring iron from porcine transferrin (pTf). This receptor consists of transferrin-binding protein A (TbpA), a transmembrane iron transporter, and TbpB, a surface-exposed lipoprotein. Questions remain as to how the receptor complex engages pTf in such a way that iron is positioned for release, and whether divergent strains present distinct recognition sites on Tf. In this study, the TbpB-pTf interface was mapped using a combination of mass shift analysis and molecular docking simulations, localizing binding uniquely to the pTf C lobe for multiple divergent strains of Actinobacillus plueropneumoniae and suis. The interface was further characterized and validated with site-directed mutagenesis. Although targeting a common lobe, variants differ in preference for the two sublobes comprising the iron coordination site. Sublobes C1 and C2 participate in high affinity binding, but sublobe C1 contributes in a minor fashion to the overall affinity. Further, the TbpB-pTf complex does not release iron independent of other mediators, based on competitive iron binding studies. Together, our findings support a model whereby TbpB efficiently captures and presents iron-loaded pTf to other elements of the uptake pathway, even under low iron conditions.

The role of vicinal tyrosine residues in the function of Haemophilus influenzae ferric-binding protein A

The periplasmic FbpA (ferric-binding protein A) from Haemophilus influenzae plays a critical role in acquiring iron from host transferrin, shuttling iron from the outer-membrane receptor complex to the inner-membrane transport complex responsible for transporting iron into the cytoplasm. In the present study, we report on the properties of a series of site-directed mutants of two adjacent tyrosine residues involved in iron co-ordination, and demonstrate that, in contrast with mutation of equivalent residues in the N-lobe of human transferrin, the mutant FbpAs retain significant iron-binding affinity regardless of the nature of the replacement amino acid. The Y195A and Y196A FbpAs are not only capable of binding iron, but are proficient in mediating periplasm-to-cytoplasm iron transport in a reconstituted FbpABC pathway in a specialized Escherichia coli reporter strain. This indicates that their inability to mediate iron acquisition from transferrin is due to their inability to compete for iron with receptor-bound transferrin. Wild-type iron-loaded FbpA could be crystalized in a closed or open state depending upon the crystallization conditions. The synergistic phosphate anion was not present in the iron-loaded open form, suggesting that initial anchoring of iron was mediated by the adjacent tyrosine residues and that alternate pathways for iron and anion binding and release may be considered. Collectively, these results demonstrate that the presence of a twin-tyrosine motif common to many periplasmic iron-binding proteins is critical for initially capturing the ferric ion released by the outer-membrane receptor complex.

Role of citrate and phosphate anions in the mechanism of iron(III) sequestration by ferric binding protein: kinetic studies of the formation of the holoprotein of wild-type FbpA and its engineered mutants

Ferric binding protein A (FbpA) plays a central role in the iron acquisition processes of pathogenic Neisseria gonorrheae, Neisseria meningitidis, and Haemophilus influenzae. FbpA functions as an iron shuttle within the periplasmic space of these Gram-negative human pathogens. Iron is picked up by FbpA at the periplasmic aspect of the outer membrane with concomitant acquisition of a synergistic anion. Here we report the kinetics and mechanisms involved with loading of iron(III) into iron-free FbpA using iron(III) citrate as an iron source in the presence of excess citrate or phosphate (physiologically available anions) at pH 6.5. In the presence of excess phosphate, iron(III) citrate loads into apo-FbpA in three kinetically distinguishable steps, while in the presence of excess citrate, only two steps are discernible. A stable intermediate containing iron(III) citrate-bound FbpA is observed in each case. The observation of an additional kinetic step and moderate increase in apparent rate constants suggests an active role for phosphate in the iron insertion process. To further elucidate a mechanism for iron loading, we report on the sequestration kinetics of iron(III) citrate in the presence of phosphate with binding site mutant apo-FbpAs, H9E, E57D, E57Q, Q58A, Y195F, and Y196H. Tyrosine mutations drastically alter the kinetics and hamper iron sequestration ability. H9E, E57D, and E57Q have near native iron sequestration behavior; however, iron binding rates are altered, enabling assignment of sequential side chain interactions. Additionally, this investigation elaborates on the function of FbpA as a carrier for iron chelates as well as "naked" or free iron as originally proposed.

Kinetics and mechanism of exogenous anion exchange in FeFbpA-NTA: significance of periplasmic anion lability and anion binding activity of ferric binding protein A

The bacterial transferrin ferric binding protein A (FbpA) requires an exogenous anion to facilitate iron sequestration, and subsequently to shuttle the metal across the periplasm to the cytoplasmic membrane. In the diverse conditions of the periplasm, numerous anions are known to be present. Prior in vitro experiments have demonstrated the ability of multiple anions to fulfill the synergistic iron-binding requirement, and the identity of the bound anion has been shown to modulate important physicochemical properties of iron-bound FbpA (FeFbpA). Here we address the kinetics and mechanism of anion exchange for the FeFbpA-nitrilotriacetate (NTA) assembly with several biologically relevant anions (citrate, oxalate, phosphate, and pyrophosphate), with nonphysiologic NTA serving as a representative synergistic anion/chelator. The kinetic data are consistent with an anion-exchange process that occurs in multiple steps, dependent on the identity of both the entering anion and the leaving anion. The exchange mechanism may proceed either as a direct substitution or through an intermediate FeFbpA-X* assembly based on anion (X) identity. Our kinetic results further develop an understanding of exogenous anion lability in the periplasm, as well as address the final step of the iron-free FbpA (apo-FbpA)/Fe(3+) sequestration mechanism. Our results highlight the kinetic significance of the FbpA anion binding site, demonstrating a correlation between apo-FbpA/anion affinity and the FeFbpA rate of anion exchange, further supporting the requirement of an exogenous anion to complete tight sequestration of iron by FbpA, and developing a mechanism for anion exchange within FeFbpA that is dependent on the identity of both the entering anion and the leaving anion.

Structural biology of bacterial iron uptake

To fulfill their nutritional requirement for iron, bacteria utilize various iron sources which include the host proteins transferrin and lactoferrin, heme, and low molecular weight iron chelators termed siderophores. The iron sources are transported into the Gram-negative bacterial cell via specific uptake pathways which include an outer membrane receptor, a periplasmic binding protein (PBP), and an inner membrane ATP-binding cassette (ABC) transporter. Over the past two decades, structures for the proteins involved in bacterial iron uptake have not only been solved, but their functions have begun to be understood at the molecular level. However, the elucidation of the three dimensional structures of all components of the iron uptake pathways is currently limited. Despite the low sequence homology between different bacterial species, the available three-dimensional structures of homologous proteins are strikingly similar. Examination of the current three-dimensional structures of the outer membrane receptors, PBPs, and ABC transporters provides an overview of the structural biology of iron uptake in bacteria.

High-affinity binding by the periplasmic iron-binding protein from Haemophilus influenzae is required for acquiring iron from transferrin

The periplasmic iron-binding protein, FbpA (ferric-ion-binding protein A), performs an essential role in iron acquisition from transferrin in Haemophilus influenzae. A series of site-directed mutants in the metal-binding amino acids of FbpA were prepared to determine their relative contribution to iron binding and transport. Structural studies demonstrated that the mutant proteins crystallized in an open conformation with the iron atom associated with the C-terminal domain. The iron-binding properties of the mutant proteins were assessed by several assays, including a novel competitive iron-binding assay. The relative ability of the proteins to compete for iron was pH dependent, with a rank order at pH 6.5 of wild-type, Q58L, H9Q>H9A, E57A>Y195A, Y196A. The genes encoding the mutant FbpA were introduced into H. influenzae and the resulting strains varied in the level of ferric citrate required to support growth on iron-limited medium, suggesting a rank order for metal-binding affinities under physiological conditions comparable with the competitive binding assay at pH 6.5 (wild-type=Q58L>H9Q>H9A, E57A>Y195A, Y196A). Growth dependence on human transferrin was only obtained with cells expressing wild-type, Q58L or H9Q FbpAs, proteins with stability constants derived from the competition assay >2.0x10(18) M(-1). These results suggest that a relatively high affinity of iron binding by FbpA is required for removal of iron from transferrin and its transport across the outer membrane.

The role of the synergistic phosphate anion in iron transport by the periplasmic iron-binding protein from Haemophilus influenzae

The acquisition of iron from transferrin by Gram-negative bacterial pathogens is dependent on a periplasmic ferric-ion-binding protein, FbpA. FbpA shuttles iron from the outer membrane to an inner membrane transport complex. A bound phosphate anion completes the iron co-ordination shell of FbpA and kinetic studies demonstrate that the anion plays a critical role in iron binding and release in vitro. The present study was initiated to directly address the hypothesis that the synergistic anion is required for transport of iron in intact cells. A series of site-directed mutants in the anion-binding amino acids of the Haemophilus influenzae FbpA (Gln-58, Asn-175 and Asn-193) were prepared to provide proteins defective in binding of the phosphate anion. Crystal structures of various mutants have revealed that alteration of the C-terminal domain ligands (Asn-175 or Asn-193) but not the N-terminal domain ligand (Gln-58) abrogated binding of the phosphate anion. The mutant proteins were introduced into H. influenzae to evaluate their ability to mediate iron transport. All of the single site-directed mutants (Q58L, N175L and N193L) were capable of mediating iron acquisition from transferrin and from limiting concentrations of ferric citrate. The results suggest that the transport of iron by FbpA is not dependent on binding of phosphate in the synergistic anion-binding site.

Genomic insights into the iron uptake mechanisms of the biomining microorganism Acidithiobacillus ferrooxidans

Commercial bioleaching of copper and the biooxidation of gold is a cost-effective and environmentally friendly process for metal recovery. A partial genome sequence of the acidophilic, bioleaching bacterium Acidithiobacillus ferrooxidans is available from two public sources. This information has been used to build preliminary models that describe how this microorganism confronts unusually high iron loads in the extremely acidic conditions (pH 2) found in natural environments and in bioleaching operations. A. ferrooxidans contains candidate genes for iron uptake, sensing, storage, and regulation of iron homeostasis. Predicted proteins exhibit significant amino acid similarity with known proteins from neutrophilic organisms, including conservation of functional motifs, permitting their identification by bioinformatics tools and allowing the recognition of common themes in iron transport across distantly related species. However, significant differences in amino acid sequence were detected in pertinent domains that suggest ways in which the periplasmic and outer membrane proteins of A. ferrooxidans maintain structural integrity and relevant protein-protein contacts at low pH. Unexpectedly, the microorganism also contains candidate genes, organized in operon-like structures that potentially encode at least 11 siderophore systems for the uptake of Fe(III), although it does not exhibit genes that could encode the biosynthesis of the siderophores themselves. The presence of multiple Fe(III) uptake systems suggests that A. ferrooxidans can inhabit aerobic environments where iron is scarce and where siderophore producers are present. It may also help to explain why it cannot tolerate high Fe(III) concentrations in bioleaching operations where it is out-competed by Leptospirillum species.

An immunoreactive 38-kilodalton protein of Ehrlichia canis shares structural homology and iron-binding capacity with the ferric ion-binding protein family

Ehrlichiae are tick-transmitted, gram-negative, obligately intracellular bacteria that live and replicate in cytoplasmic vacuoles, but little is known about iron acquisition mechanisms necessary for their survival. In this study, a genus-conserved immunoreactive ferric ion-binding protein (Fbp) of Ehrlichia canis was identified and its iron-binding capability was investigated. E. canis Fbp was homologous to a family of periplasmic Fbp's involved in iron acquisition and transport in gram-negative bacteria. E. canis Fbp had a molecular mass (38 kDa) consistent with those of Fbp's in other bacteria and exhibited substantial immunoreactivity in its native conformation. The predicted three-dimensional structure of E. canis Fbp demonstrated conservation of important Fbp family structural motifs: two domains linked with a polypeptide "hinge" region. Under iron-binding conditions, the recombinant Fbp exhibited an intense red color and an absorbance spectrum indicative of iron binding, and it bound Fe(III) but not Fe(II). Fbp was observed primarily in the cytoplasm of the reticulate forms of E. canis and Ehrlichia chaffeensis but was notably found on extracellular morula fibers in morulae containing dense-cored organisms. Although expression of Fbp is regulated through an operon of three functionally linked genes in other gram-negative bacteria, the absence of an intact fbp operon in Ehrlichia spp. suggests that genes involved in ehrlichial iron acquisition have been subject to reductive evolution.

Novel anion-independent iron coordination by members of a third class of bacterial periplasmic ferric ion-binding proteins

The uptake of the element iron is vital for the survival of most organisms. Numerous pathogenic Gram-negative bacteria utilize a periplasm-to-cytosol ATP-binding cassette transport pathway to transport this essential atom in to the cell. In this study, we investigated the Yersinia enterocolitica (YfuA) and Serratia marcescens (SfuA) iron-binding periplasmic proteins. We have determined the 1.8-angstroms structures of iron-loaded (YfuA) and iron-free (SfuA) forms of this class of proteins. Although the sequence of these proteins varies considerably from the other members of the transferrin structural superfamily, they adopt the same three-dimensional fold. The iron-loaded YfuA structure illustrates the unique nature of this new class of proteins in that they are able to octahedrally coordinate the ferric ion in the absence of a bound anion. The iron-free SfuA structure contains a bound citrate anion in the iron-binding cleft that tethers the N- and C-terminal domains of the apo protein and stabilizes the partially open structure.

Kinetics and mechanism of iron(III) complexation by ferric binding protein: the role of phosphate

Iron transport across the periplasmic space to the cytoplasmic membrane of certain Gram-negative bacteria is mediated by a ferric binding protein (Fbp). This requires Fe(3+) loading of Fbp at the inner leaflet of the outer membrane. A synergistic anion is required for tight Fe(3+) sequestration by Fbp. Although phosphate fills this role in the protein isolated from bacterial cell lysates, nitrilotriacetate anion (NTA) can also satisfy this requirement in vitro. Here, we report the kinetics and mechanism of Fe(3+) loading of Fbp from Fe(NTA)(aq) in the presence of phosphate at pH 6.5. The reaction proceeds in four kinetically distinguishable steps to produce Fe(3+)Fbp(PO(4)) as a final product. The first three steps exhibit half-lives ranging from ca. 20 ms to 0.5 min, depending on the concentrations, and produce Fe(3+)Fbp(NTA) as an intermediate product of significant stability. The rate for the first step is accelerated with an increasing phosphate concentration, while that of the third step is retarded by phosphate. Conversion of Fe(3+)Fbp(NTA) to Fe(3+)Fbp(PO(4)) in the fourth step is a slow process (half-life approximately 2 h) and is facilitated by free phosphate. A mechanism for the Fe(3+)-loading process is proposed in which the synergistic anions, phosphate and NTA, play key roles. These data suggest that not only is a synergistic anion required for tight Fe(3+) sequestration by Fbp, but also the synergistic anion plays a critical role in the process of inserting Fe(3+) into the Fbp binding site.

The hFbpABC transporter from Haemophilus influenzae functions as a binding-protein-dependent ABC transporter with high specificity and affinity for ferric iron

Pathogenic Haemophilus influenzae, Neisseria spp. (Neisseria gonorrhoeae and N. meningitidis), Serratia marcescens, and other gram-negative bacteria utilize a periplasm-to-cytosol FbpABC iron transporter. In this study, we investigated the H. influenzae FbpABC transporter in a siderophore-deficient Escherichia coli background to assess biochemical aspects of FbpABC transporter function. Using a radiolabeled Fe3+ transport assay, we established an apparent Km=0.9 microM and Vmax=1.8 pmol/10(7)cells/min for FbpABC-mediated transport. Complementation experiments showed that hFbpABC is dependent on the FbpA binding protein for transport. The ATPase inhibitor sodium orthovanadate demonstrated dose-dependent inhibition of FbpABC transport, while the protonmotive-force-inhibitor carbonyl cyanide m-chlorophenyl hydrazone had no effect. Metal competition experiments demonstrated that the transporter has high specificity for Fe3+ and selectivity for trivalent metals, including Ga3+ and Al3+, over divalent metals. Metal sensitivity experiments showed that several divalent metals, including copper, nickel, and zinc, exhibited general toxicity towards E. coli. Significantly, gallium-induced toxicity was specific only to E. coli expressing FbpABC. A single-amino-acid mutation in the gene encoding the periplasmic binding protein, FbpA(Y196I), resulted in a greatly diminished iron binding affinity Kd=5.2 x 10(-4) M(-1), approximately 14 orders of magnitude weaker than that of the wild-type protein. Surprisingly, the mutant transporter [FbpA(Y196I)BC] exhibited substantial transport activity, approximately 35% of wild-type transport, with Km=1.2 microM and Vmax=0.5 pmol/10(7)cells/min. We conclude that the FbpABC complexes possess basic characteristics representative of the family of bacterial binding protein-dependent ABC transporters. However, the specificity and high-affinity binding characteristics suggest that the FbpABC transporters function as specialized transporters satisfying the strict chemical requirements of ferric iron (Fe3+) binding and membrane transport.

Oxo-iron clusters in a bacterial iron-trafficking protein: new roles for a conserved motif

We report a set of three 1.8-1.9 A resolution X-ray crystal structures of Neisseria gonorrhoeae Fbp (ferric-ion binding protein): (i) open-cleft apo-Fbp containing bound phosphate, (ii) open-cleft mono-Fe Fbp capped by nitrilotriacetate, and (iii) open-cleft trinuclear oxo-iron Fbp, the first structure of an iron-cluster adduct of a transferrin. The nine independent molecules in the unit cells provide 'snapshots' of the versatile dynamic structural roles of the conserved dityrosyl iron-binding motif (Tyr195-Tyr196) which control the capture and, possibly, processing of iron. These findings have implications for understanding bacterial iron acquisition and dissimilation, and organic/mineral interfaces.

Crystal structure of Pasteurella haemolytica ferric ion-binding protein A reveals a novel class of bacterial iron-binding proteins

Pasteurellosis caused by the Gram-negative pathogen Pasteurella haemolytica is a serious disease leading to death in cattle. To scavenge growth-limiting iron from the host, the pathogen utilizes the periplasmic ferric ion-binding protein A (PhFbpA) as a component of an ATP-binding cassette transport pathway. We report the 1.2-A structure of the iron-free (apo) form of PhFbpA, which is a member of the transferrin structural superfamily. The protein structure adopts a closed conformation, allowing us to reliably assign putative iron-coordinating residues. Based on our analysis, PhFbpA utilizes a unique constellation of binding site residues and anions to octahedrally coordinate an iron atom. A surprising finding in the structure is the presence of two formate anions on opposite sides of the iron-binding pocket. The formate ions tether the N- and C-terminal domains of the protein and stabilize the closed structure, also providing clues as to probable candidates for synergistic anions in the iron-loaded state. PhFbpA represents a new class of bacterial iron-binding proteins.

Kinetics and mechanism of iron release from the bacterial ferric binding protein nFbp: exogenous anion influence and comparison with mammalian transferrin

Ferric binding protein, Fbp, serves an essential biological function in shuttling naked (hydrated) Fe(3+) across the periplasmic space of many Gram-negative bacteria. In this process, iron must be released at the cytoplasmic membrane to a permease. How iron is released from Fbp has yet to be resolved. Consequently, understanding the dynamics of iron release from Fbp is of both biological and chemical interest. Fbp requires an exogenous anion, e.g. phosphate when isolated from cell lysates, for tight iron sequestration. To address the role of exogenous anion identity and lability on Fe(aq)(3+) dissociation from Fbp, the kinetics of PO(4)(3-) exchange in Fe(3+) nFbp(PO(4)) ( nFbp=recombinant Fbp from Neisseria meningitidis) were investigated by dynamic (31)P NMR and the kinetics of Fe(3+) dissociation from Fe(3+) nFbp(X) (X=PO(4)(3-), citrate anion) were investigated by stopped-flow pH-jump measurements. We justify the use of non-physiological low-pH conditions because a high [H(+)] will drive the Fe(aq)(3+) dissociation reaction to completion without using competing chelators, whose presence may complicate or influence the dissociation mechanism. For perspective, these studies of nFbp (which has been referred to as a bacterial transferrin) are compared to new and previously published kinetic and thermodynamic data for mammalian transferrin. Significantly, we address the lability of the Fe(3+) coordination shell in nFbp, Fe(3+) nFbp(X) (X=PO(4)(3-), citrate), with respect to exogenous anion (X(n-)) exchange and dissociation, and ultimately complete dissociation of the protein to yield naked (hydrated) Fe(aq)(3+). These findings are a first step in understanding the process of iron donation to the bacterial permease for transport across the cytoplasmic membrane.