Heme uptake across the outer membrane as revealed by crystal structures of the receptor-hemophore complex

Gaseous ligand binding to Porphyromonas gingivalis HmuY hemophore-like protein in complex with heme

Porphyromonas gingivalis is the main pathogenic player in the development of periodontitis. To acquire heme, being an essential source of iron and protoporphyrin IX, P. gingivalis utilizes TonB-dependent outer membrane heme receptor (HmuR) and heme-binding hemophore-like protein (HmuY) as the main system for heme uptake from host hemoproteins. In this work, we present an extensive spectroscopic characterization of the binding of exogenous gaseous ligands to the holo-form of the HmuY (HmuY-heme) to unravel the mechanistic basis of heme release. Our data are consistent with a scenario where heme release from HmuY-heme is a multistep process that requires the initial rupture of one of the two heme‑iron coordination bonds with endogenous histidines.

Structural Basis and Mechanism of a Unique Haemophore in the Haem-Iron Acquisition by Riemerella anatipestifer

Several bacterial pathogens employ haemophores to scavenge haem from host haemoprotein to obtain an iron source. However, no homologues of well-characterized haemophores are found in Riemerella anatipestifer, a bacterium belonging to the order Flavobacteriales that encodes haem uptake systems. Herein, a unique haemophore RhuH is characterized in this bacterium. R. anatipestifer used RhuH to grow when duck hemoglobin serves as the sole iron resource. RhuH is secreted as a component of outer membrane vesicles. Recombinant RhuH exhibited a high binding affinity for haem (Kd of 3.44 × 10-11 m) and can extract haem from duck hemoglobin. X-ray crystallography elucidated the 3D structure of RhuH at 2.85 Å resolution, showing a dimeric conformation with each monomer exhibiting a unique structure. Structure modeling of RhuH-haem, coupled with mutagenesis, haemin utilization, and binding affinity assays, show that haem is captured in the β-barrel-like region, displaying the classic iron coordination. The RhuH homologues are predominantly distributed in Weeksellaceae and Flavobacteriaceae. Finally, the homologues of RhuH in Riemerella columbina, Flavobacterium columnare, and Flavobacterium soli are used as a proof of concept, demonstrating that these homologues exhibit conserved structures and functions.

Exploring heme and iron acquisition strategies of Porphyromonas gingivalis-current facts and hypotheses

Iron and heme are crucial for pathogenic bacteria living in the human host but are not available in free form due to their binding by iron- and heme-sequestering proteins. Porphyromonas gingivalis causes dysbiosis in the oral microbiome and is considered a keystone pathogen in the onset and progression of periodontal diseases. Its ability to infect and multiply in host cells and its presence in distant tissues and fluids highlights its pathogenic versatility and explains the relationship between periodontal diseases and systemic or neurodegenerative diseases. Porphyromonas gingivalis has evolved specialized mechanisms that allow it to thrive in the host under adverse nutrient-limited conditions. This review presents the updated summary of the mechanisms of iron and heme acquisition by P. gingivalis, with a central role played by gingipains and the unique Hmu system. The potential role of other iron and heme acquisition systems, such as Hus and Iht, indicates the importance of the partially conserved heme biosynthesis pathway, involving homologs of the HemN, HemG, and HemH proteins. In light of increasing antibiotic resistance, difficulties with diagnosis, and drug administration, targeting the mechanisms of heme and iron acquisition of P. gingivalis represents a promising target for developing diagnostic tests, preventive or therapeutic strategies.

Dynamic interplay between a TonB-dependent heme transporter and a TonB protein in a membrane environment

The envelope of Gram-negative bacteria is composed of two membranes separated by the periplasmic space. This organization imposes geometrical and distance constraints that are key for the mechanism of action of multicomponent systems spanning the envelope. However, consideration of all three compartments by experimental approaches is still elusive. Here, we have used the state-of-the-art molecular dynamics simulation in an Escherichia coli envelope model to obtain a dynamic view of molecular interactions between the outer membrane heme transporter HasR and the inner membrane TonB-like protein HasB. Their interaction allows the transfer of the inner membrane proton-motive force derived energy to the transporter for heme internalization. The simulations that incorporate both membranes show the key role of periplasmic domains of both proteins and their dynamics in complex formation and stability. They revealed a previously unidentified network of HasR-HasB protein-protein interactions in the periplasm. Experimental validation (mutations, in vivo phenotypic and biophysical assays) provides support for the simulation-predicted interactions. Based on structural and sequence conservation, the network of interaction revealed in this study is expected to occur in other nutrient import systems.

IMPORTANCE

Gram-negative bacteria import scarce nutrients such as metals and vitamins by an energized mechanism involving a multicomponent protein system that spans the cell envelope. It consists of an outer membrane TonB-dependent transporter (TBDT) and a TonB complex in the inner membrane that provides the proton motive force energy for the nutrient entry. Despite the intense research efforts focused on this system (a) from structural and fundamental microbiology perspectives and (b) for the interest in the development of new antibacterial strategies, the molecular mechanism of the system is not at all well understood. The lack of understanding comes from incomplete structural data and the experimental difficulties of studying an inherently flexible multicomponent complex that resides within the heterogeneous environment of the double membrane bacterial cell envelope. To address these challenges and obtain a comprehensive view of the molecular interactions at atomic level, here, we have used the combined power of advanced molecular simulations and complementary microbiology and biochemical experiments. Our results represent a significant step forward in understanding the structural and molecular bases of this vital mechanism.

Heme homeostasis and its regulation by hemoproteins in bacteria

Heme is an important cofactor and a regulatory molecule involved in various physiological processes in virtually all living cellular organisms, and it can also serve as the primary iron source for many bacteria, particularly pathogens. However, excess heme is cytotoxic to cells. In order to meet physiological needs while preventing deleterious effects, bacteria have evolved sophisticated cellular mechanisms to maintain heme homeostasis. Recent advances in technologies have shaped our understanding of the molecular mechanisms that govern the biological processes crucial to heme homeostasis, including synthesis, acquisition, utilization, degradation, trafficking, and efflux, as well as their regulation. Central to these mechanisms is the regulation of the heme, by the heme, and for the heme. In this review, we present state-of-the-art findings covering the biochemical, physiological, and structural characterization of important, newly identified hemoproteins/systems involved in heme homeostasis.

Chaperone-assisted cryo-EM structure of P. aeruginosa PhuR reveals molecular basis for heme binding

Pathogenic bacteria, such as Pseudomonas aeruginosa, depend on scavenging heme for the acquisition of iron, an essential nutrient. The TonB-dependent transporter (TBDT) PhuR is the major heme uptake protein in P. aeruginosa clinical isolates. However, a comprehensive understanding of heme recognition and TBDT transport mechanisms, especially PhuR, remains limited. In this study, we employed single-particle cryogenic electron microscopy (cryo-EM) and a phage display-generated synthetic antibody (sAB) as a fiducial marker to enable the determination of a high-resolution (2.5 Å) structure of PhuR with a bound heme. Notably, the structure reveals iron coordination by Y529 on a conserved extracellular loop, shedding light on the role of tyrosine in heme binding. Biochemical assays and negative-stain EM demonstrated that the sAB specifically targets the heme-bound state of PhuR. These findings provide insights into PhuR's heme binding and offer a template for developing conformation-specific sABs against outer membrane proteins (OMPs) for structure-function investigations.

Prevalence of Slam-dependent hemophilins in Gram-negative bacteria

Iron acquisition systems are crucial for pathogen growth and survival in iron-limiting host environments. To overcome nutritional immunity, bacterial pathogens evolved to use diverse mechanisms to acquire iron. Here, we examine a heme acquisition system that utilizes hemophores called hemophilins which are also referred to as HphAs in several Gram-negative bacteria. In this study, we report three new HphA structures from Stenotrophomonas maltophilia, Vibrio harveyi, and Haemophilus parainfluenzae. Structural determination of HphAs revealed an N-terminal clamp-like domain that binds heme and a C-terminal eight-stranded β-barrel domain that shares the same architecture as the Slam-dependent Neisserial surface lipoproteins. The genetic organization of HphAs consists of genes encoding a Slam homolog and a TonB-dependent receptor (TBDR). We investigated the Slam-HphA system in the native organism or the reconstituted system in Escherichia coli cells and found that the efficient secretion of HphA depends on Slam. The TBDR also played an important role in heme uptake and conferred specificity for its cognate HphA. Furthermore, bioinformatic analysis of HphA homologs revealed that HphAs are conserved in the alpha, beta, and gammaproteobacteria. Together, these results show that the Slam-dependent HphA-type hemophores are prevalent in Gram-negative bacteria and further expand the role of Slams in transporting soluble proteins.

IMPORTANCE

This paper describes the structure and function of a family of Slam (Type IX secretion System) secreted hemophores that bacteria use to uptake heme (iron) while establishing an infection. Using structure-based bioinformatics analysis to define the diversity and prevalence of this heme acquisition pathway, we discovered that a large portion of gammaproteobacterial harbors this system. As organisms, including Acinetobacter baumannii, utilize this system to facilitate survival during host invasion, the identification of this heme acquisition system in bacteria species is valuable information and may represent a target for antimicrobials.

Chaperone-assisted cryo-EM structure of P. aeruginosa PhuR reveals molecular basis for heme binding

Pathogenic bacteria, such as Pseudomonas aeruginosa, depend on scavenging heme for the acquisition of iron, an essential nutrient. The TonB-dependent transporter (TBDT) PhuR is the major heme uptake protein in P. aeruginosa clinical isolates. However, a comprehensive understanding of heme recognition and TBDT transport mechanisms, especially PhuR, remains limited. In this study, we employed single-particle cryogenic electron microscopy (cryo-EM) and a phage display-generated synthetic antibody (sAB) as a fiducial marker to enable the determination of a high-resolution (2.5 Å) structure of PhuR with a bound heme. Notably, the structure reveals iron coordination by Y529 on a conserved extracellular loop, shedding light on the role of tyrosine in heme binding. Biochemical assays and negative-stain EM demonstrated that the sAB specifically targets the heme-bound state of PhuR. These findings provide insights into PhuR's heme binding and offer a template for developing conformation-specific sABs against outer membrane proteins (OMPs) for structure-function investigations.

Protein-Porphyrin Complex Photosensitizers for Anticancer and Antimicrobial Photodynamic Therapies

Photodynamic therapy (PDT) efficiently induces apoptosis through visible-light irradiation of photosensitizers (PSs) within tumors and microbial cells. Porphyrin analogues serve as widely utilized photosensitizing agents with their therapeutic abilities being governed by molecular structures and central metal ions. However, these macrocyclic compounds tend to agglutinate and form stacks in aqueous environments, resulting in a loss of photochemical activity. To overcome this limitation, encapsulation within liposomes and polymer micelles enables the dispersion of porphyrins as monomolecular entities in aqueous solutions, preventing undesirable deactivation. Recently, the use of reconstituted hemoproteins containing various metal-porphyrins and protein cages incorporating porphyrins has garnered significant interest as a new generation of biocompatible PSs. In this concept paper, we provide a comprehensive review of recent developments and trends of protein-porphyrin complex PSs for applications in anticancer and antimicrobial PDTs.

TonB-Dependent Transport Across the Bacterial Outer Membrane

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

First report of whole genome sequence of septicemic Pasteurella multocida serovar B:2 'Soron' strain isolated from swine

Pig pasteurellosis, caused by Pasteurella multocida, is an acute infection that also has economic implications for pig farmers. We report the complete genome sequence of a P. multocida, serovar B:2 'Soron' strain isolated from the blood of a pig that had died of pasteurellosis in India. The isolate was not found to be haemorrhagic septicaemia (HS) specific B:2 by the PCR assay. The genome of 'Soron' strain is a single circular chromosome of 2,272,124 base pairs in length and contains 2014 predicted coding regions, 4 ribosomal RNA operons, and 52 tRNAs. It has 1812 protein-coding genes that were also found in reference sequence PmP52Vac. Phylogenetic analysis revealed that Pm_P52VAc and P. multocida 'Soron' serovar B:2 were clustered in different clades. Pasteurella multocida 'Soron' serovar B:2 was found to cluster with the same ancestor of Pm70, which is of avian origin. The genome was found to contain regions that encode proteins which may confer resistance to various antibiotics including cephalosporin, which is used to treat pasteurellosis. The isolate was also found to harbour a phage region. This strain represents a novel multi-locus sequence type (MLST) that has not been previously identified, as all of the alleles used for MLST were found, but did not match any of the alleles in the database with 100% nucleotide identity. The most closely related ST was ST221. This is the first whole-genome sequence from P. multocida serovar B:2 of pig origin.

Structures and coordination chemistry of transporters involved in manganese and iron homeostasis

A repertoire of transporters plays a crucial role in maintaining homeostasis of biologically essential transition metals, manganese, and iron, thus ensuring cell viability. Elucidating the structure and function of many of these transporters has provided substantial understanding into how these proteins help maintain the optimal cellular concentrations of these metals. In particular, recent high-resolution structures of several transporters bound to different metals enable an examination of how the coordination chemistry of metal ion-protein complexes can help us understand metal selectivity and specificity. In this review, we first provide a comprehensive list of both specific and broad-based transporters that contribute to cellular homeostasis of manganese (Mn2+) and iron (Fe2+ and Fe3+) in bacteria, plants, fungi, and animals. Furthermore, we explore the metal-binding sites of the available high-resolution metal-bound transporter structures (Nramps, ABC transporters, P-type ATPase) and provide a detailed analysis of their coordination spheres (ligands, bond lengths, bond angles, and overall geometry and coordination number). Combining this information with the measured binding affinity of the transporters towards different metals sheds light into the molecular basis of substrate selectivity and transport. Moreover, comparison of the transporters with some metal scavenging and storage proteins, which bind metal with high affinity, reveal how the coordination geometry and affinity trends reflect the biological role of individual proteins involved in the homeostasis of these essential transition metals.

Research progress on iron uptake pathways and mechanisms of foodborne microorganisms and their application in the food sector

Iron is one of the essential nutrients for almost all microorganisms. Under iron-limited conditions, bacteria can secrete siderophores to the outside world to absorb iron for survival. This process requires the coordinated action of energy-transducing proteins, transporters, and receptors. The spoilage factors of some spoilage bacteria and the pathogenic mechanism of pathogenic bacteria are also closely related to siderophores. Meanwhile, some siderophores have also gradually evolved toward beneficial aspects. First, a variety of siderophores are classified into three aspects. In addition, representative iron uptake systems of Gram-negative and Gram-positive bacteria are described in detail to understand the common and specific pathways of iron uptake by various bacteria. In particular, the causes of siderophore-induced bacterial pathogenicity and the methods and mechanisms of inhibiting bacterial iron absorption under the involvement of siderophores are presented. Then, the application of siderophores in the food sector is mainly discussed, such as improving the food quality of dairy products and meat, inhibiting the attack of pathogenic bacteria on food, improving the plant growth environment, and promoting plant growth. Finally, this review highlights the unresolved fate of siderophores in the iron uptake system and emphasizes further development of siderophore-based substitutes for traditional drugs, new antibiotic-resistance drugs, and vaccines in the food and health sectors.

Tetraphenylporphyrin Enters the Ring: First Example of a Complex Between Highly Bulky Porphyrins and a Protein

Tetraphenylporphyrin (TPP) is a symmetrically substituted synthetic porphyrin whose properties can be readily modified, providing it with significant advantages over naturally occurring porphyrins. Herein, we report the first example of a stable complex between a native biomolecule, the haemoprotein HasA, and TPP as well as its derivatives. The X-ray crystal structures of nine different HasA-TPP complexes were solved at high resolutions. HasA capturing TPP derivatives was also demonstrated to inhibit growth of the opportunistic pathogen Pseudomonas aeruginosa . Mutant variants of HasA binding FeTPP were shown to possess a different mode of coordination, permitting the cyclopropanation of styrene.

The diversity of heme sensor systems - heme-responsive transcriptional regulation mediated by transient heme protein interactions

Heme is a versatile molecule that is vital for nearly all cellular life by serving as prosthetic group for various enzymes or as nutritional iron source for diverse microbial species. However, elevated levels of heme molecule are toxic to cells. The complexity of this stimulus has shaped the evolution of diverse heme sensor systems, which are involved in heme-dependent transcriptional regulation in eukaryotes and prokaryotes. The functions of these systems are manifold - ranging from the specific control of heme detoxification or uptake systems to the global integration of heme and iron homeostasis. This review focuses on heme sensor systems, regulating heme homeostasis by transient heme protein interaction. We provide an overview of known heme-binding motifs in prokaryotic and eukaryotic transcription factors. Besides the central ligands, the surrounding amino acid environment was shown to play a pivotal role in heme binding. The diversity of heme-regulatory systems therefore illustrates that prediction based on pure sequence information is hardly possible and requires careful experimental validation. Comprehensive understanding of heme-regulated processes is not only important for our understanding of cellular physiology, but also provides a basis for the development of novel antibacterial drugs and metabolic engineering strategies.

A Heme-Acquisition Protein Reconstructed with a Cobalt 5-Oxaporphyrinium Cation and Its Growth-Inhibition Activity Toward Multidrug-Resistant Pseudomonas aeruginosa

Using artificial hemes for the reconstruction of natural heme proteins represents a fascinating approach to enhance the bioactivity of the latter. Here, we report the synthesis of various metal 5-oxaporphyrinium cations as cofactors, and a cobalt 5-oxaporphyrinium cation was successfully incorporated into the heme-acquisition protein (HasA) secreted by Pseudomonas aeruginosa. We hypothesize that the oxaporphyrinium cation strongly bound to the HasA-specific outer membrane receptor (HasR) due to its cationic charge, which prevents the subsequent acquisition of heme. In fact, the reconstructed HasA inhibited the growth of Pseudomonas aeruginosa and even of multidrug-resistant P. aeruginosa.

Extracellular haem utilization by the opportunistic pathogen Pseudomonas aeruginosa and its role in virulence and pathogenesis

Iron is an essential micronutrient for all bacteria but presents a significant challenge given its limited bioavailability. Furthermore, iron's toxicity combined with the need to maintain iron levels within a narrow physiological range requires integrated systems to sense, regulate and transport a variety of iron complexes. Most bacteria encode systems to chelate and transport ferric iron (Fe³⁺) via siderophore receptor mediated uptake or via cytoplasmic energy dependent transport systems. Pathogenic bacteria have further lowered the barrier to iron acquisition by employing systems to utilize haem as a source of iron. Haem, a lipophilic and toxic molecule, presents a significant challenge for transport into the cell. As such pathogenic bacteria have evolved sophisticated cell surface signaling (CSS) and transport systems to sense and obtain haem from the host. Once internalized haem is cleaved by both oxidative and non-oxidative mechanisms to release iron. Herein we summarize our current understanding of the mechanism of haem sensing, uptake and utilization in Pseudomonas aeruginosa, its role in pathogenesis and virulence, and the potential of these systems as antimicrobial targets.

A Slam-dependent hemophore contributes to heme acquisition in the bacterial pathogen Acinetobacter baumannii

Nutrient acquisition systems are often crucial for pathogen growth and survival during infection, and represent attractive therapeutic targets. Here, we study the protein machinery required for heme uptake in the opportunistic pathogen Acinetobacter baumannii. We show that the hemO locus, which includes a gene encoding the heme-degrading enzyme, is required for high-affinity heme acquisition from hemoglobin and serum albumin. The hemO locus includes a gene coding for a heme scavenger (HphA), which is secreted by a Slam protein. Furthermore, heme uptake is dependent on a TonB-dependent receptor (HphR), which is important for survival and/or dissemination into the vasculature in a mouse model of pulmonary infection. Our results indicate that A. baumannii uses a two-component receptor system for the acquisition of heme from host heme reservoirs.

Axial Heme Coordination by the Tyr-His Motif in the Extracellular Hemophore HasAp Is Critical for the Release of Heme to the HasR Receptor of Pseudomonas aeruginosa

Pseudomonas aeruginosa senses extracellular heme via an extra cytoplasmic function σ factor that is activated upon interaction of the hemophore holo-HasAp with the HasR receptor. Herein, we show Y75H holo-HasAp interacts with HasR but is unable to release heme for signaling and uptake. To understand this inhibition, we undertook a spectroscopic characterization of Y75H holo-HasAp by resonance Raman (RR), electron paramagnetic resonance (EPR), and X-ray crystallography. The RR spectra are consistent with a mixed six-coordinate high-spin (6cHS), six-coordinate low-spin (6cLS) heme configuration and an H₂¹⁸O exchangeable Fe^III-O stretching frequency with ¹⁶O/¹⁸O and H/D isotope shifts that support a two-body Fe-OH₂ oscillator with (iron-hydroxy)-like character as both hydrogen atoms are engaged in short hydrogen bond interactions with protein side chains. Further support comes from the EPR spectrum of Y75H holo-HasAp that shows a LS rhombic signal with ligand-field splitting values intermediate between those of His-hydroxy and bis-His ferric hemes. The crystal structure of Y75H holo-HasAp confirmed the coordinated solvent molecule hydrogen bonded through H75 and H83. The long-range conformational rearrangement of HasAp upon heme binding can still take place in Y75H holo-HasAp, because the intercalation of a hydroxy ligand between the heme iron and H75 allows the variant to reproduce the heme binding pocket observed in wild-type holo-HasAp. However, in the absence of a covalent linkage to the Y75 loop combined with the malleability provided by the bracketing H75 and H83 hydrogen bonds, either the hydroxy sixth ligand remains bound after complexation of Y75H holo-HasAp with HasR or rearrangement and coordination of H85 prevent heme transfer.

Siderophores: Importance in bacterial pathogenesis and applications in medicine and industry

Iron is an essential element for all microorganisms. Siderophores are low-weight, high-affinity iron chelating molecules produced in response to iron deficiency by Gram-positive and Gram-negative bacteria which also known as essential virulence factors of bacteria. Several studies have indicated that defective production and/or function of these molecules as well as iron acquisition systems in pathogens are associated with a reduction in pathogenicity of bacteria. Because of their potential role in various biological pathways, siderophores have been received special attention as secondary metabolites. Siderophores can detect iron levels in a variety of environments with a biosensor function. In medicine, siderophores are used to deliver antibiotics (Trojan horse strategy) to resistant bacteria and to treat diseases such as cancer and malaria. In this review, we discuss the iron acquisition pathways in Gram-positive and -negative bacteria, importance of siderophore production in pathogenesis of bacteria, classification of siderophores, and main applications of siderophores in medicine and industry.

Transporters Involved in the Biogenesis and Functionalization of the Mycobacterial Cell Envelope

The biology of mycobacteria is dominated by a complex cell envelope of unique composition and structure and of exceptionally low permeability. This cell envelope is the basis of many of the pathogenic features of mycobacteria and the site of susceptibility and resistance to many antibiotics and host defense mechanisms. This review is focused on the transporters that assemble and functionalize this complex structure. It highlights both the progress and the limits of our understanding of how (lipo)polysaccharides, (glyco)lipids, and other bacterial secretion products are translocated across the different layers of the cell envelope to their final extra-cytoplasmic location. It further describes some of the unique strategies evolved by mycobacteria to import nutrients and other products through this highly impermeable barrier.

Ferric iron reductases and their contribution to unicellular ferrous iron uptake

Iron is a necessary element for nearly all forms of life, and the ability to acquire this trace nutrient has been identified as a key virulence factor for the establishment of infection by unicellular pathogens. In the presence of O2, iron typically exists in the ferric (Fe3+) oxidation state, which is highly unstable in aqueous conditions, necessitating its sequestration into cofactors and/or host proteins to remain soluble. To counter this insolubility, and to compete with host sequestration mechanisms, many unicellular pathogens will secrete low molecular weight, high-affinity Fe3+ chelators known as siderophores. Once acquired, unicellular pathogens must liberate the siderophore-bound Fe3+ in order to assimilate this nutrient into metabolic pathways. While these organisms may hydrolyze the siderophore backbone to release the chelated Fe3+, this approach is energetically costly. Instead, iron may be liberated from the Fe3+-siderophore complex through reduction to Fe2+, which produces a lower-affinity form of iron that is highly soluble. This reduction is performed by a class of enzymes known as ferric reductases. Ferric reductases are broadly-distributed electron-transport proteins that are expressed by numerous infectious organisms and are connected to the virulence of unicellular pathogens. Despite this importance, ferric reductases remain poorly understood. This review provides an overview of our current understanding of unicellular ferric reductases (both soluble and membrane-bound), with an emphasis on the important but underappreciated connection between ferric-reductase mediated Fe3+ reduction and the transport of Fe2+ via ferrous iron transporters.

Bacterial iron detoxification at the molecular level

Iron is an essential micronutrient, and, in the case of bacteria, its availability is commonly a growth-limiting factor. However, correct functioning of cells requires that the labile pool of chelatable "free" iron be tightly regulated. Correct metalation of proteins requiring iron as a cofactor demands that such a readily accessible source of iron exist, but overaccumulation results in an oxidative burden that, if unchecked, would lead to cell death. The toxicity of iron stems from its potential to catalyze formation of reactive oxygen species that, in addition to causing damage to biological molecules, can also lead to the formation of reactive nitrogen species. To avoid iron-mediated oxidative stress, bacteria utilize iron-dependent global regulators to sense the iron status of the cell and regulate the expression of proteins involved in the acquisition, storage, and efflux of iron accordingly. Here, we survey the current understanding of the structure and mechanism of the important members of each of these classes of protein. Diversity in the details of iron homeostasis mechanisms reflect the differing nutritional stresses resulting from the wide variety of ecological niches that bacteria inhabit. However, in this review, we seek to highlight the similarities of iron homeostasis between different bacteria, while acknowledging important variations. In this way, we hope to illustrate how bacteria have evolved common approaches to overcome the dual problems of the insolubility and potential toxicity of iron.

A Sec14-like phosphatidylinositol transfer protein paralog defines a novel class of heme-binding proteins

Yeast Sfh5 is an unusual member of the Sec14-like phosphatidylinositol transfer protein (PITP) family. Whereas PITPs are defined by their abilities to transfer phosphatidylinositol between membranes in vitro, and to stimulate phosphoinositide signaling in vivo, Sfh5 does not exhibit these activities. Rather, Sfh5 is a redox-active penta-coordinate high spin Fe^III hemoprotein with an unusual heme-binding arrangement that involves a co-axial tyrosine/histidine coordination strategy and a complex electronic structure connecting the open shell iron d-orbitals with three aromatic ring systems. That Sfh5 is not a PITP is supported by demonstrations that heme is not a readily exchangeable ligand, and that phosphatidylinositol-exchange activity is resuscitated in heme binding-deficient Sfh5 mutants. The collective data identify Sfh5 as the prototype of a new class of fungal hemoproteins, and emphasize the versatility of the Sec14-fold as scaffold for translating the binding of chemically distinct ligands to the control of diverse sets of cellular activities.

A Glance on the Role of Bacterial Siderophore from the Perspectives of Medical and Biotechnological Approaches

Iron, which is described as the most basic component found in nature, is hard to be assimilated by microorganisms. It has become increasingly complicated to obtain iron from nature as iron (II) in the presence of oxygen oxidized to press (III) oxide and hydroxide, becoming unsolvable at neutral pH. Microorganisms appeared to produce organic molecules known as siderophores in order to overcome this condition. Siderophore's essential function is to connect with iron (II) and make it dissolvable and enable cell absorption. These siderophores, apart from iron particles, have the ability to chelate various other metal particles that have collocated away to focus the use of siderophores on wound care items. There is a severe clash between the host and the bacterial pathogens during infection. By producing siderophores, small ferric iron-binding molecules, microorganisms obtain iron. In response, host immune cells produce lipocalin 2 to prevent bacterial reuptake of siderophores loaded with iron. Some bacteria are thought to produce lipocalin 2-resistant siderophores to counter this risk. The aim of this article is to discuss the recently described roles and applications of bacterial siderophore.

Targeted Killing of Pseudomonas aeruginosa by Pyocin G Occurs via the Hemin Transporter Hur

Pseudomonas aeruginosa is a priority pathogen for the development of new antibiotics, particularly because multi-drug-resistant strains of this bacterium cause serious nosocomial infections and are the leading cause of death in cystic fibrosis patients. Pyocins, bacteriocins of P. aeruginosa, are potent and diverse protein antibiotics that are deployed during bacterial competition. Pyocins are produced by more than 90% of P. aeruginosa strains and may have utility as last resort antibiotics against this bacterium. In this study, we explore the antimicrobial activity of a newly discovered pyocin called pyocin G (PyoG). We demonstrate that PyoG has broad killing activity against a collection of clinical P. aeruginosa isolates and is active in a Galleria mellonella infection model. We go on to identify cell envelope proteins that are necessary for the import of PyoG and its killing activity. PyoG recognizes bacterial cells by binding to Hur, an outer-membrane TonB-dependent transporter. Both pyocin and Hur interact with TonB1, which in complex with ExbB-ExbD links the proton motive force generated across the inner membrane with energy-dependent pyocin translocation across the outer membrane. Inner-membrane translocation of PyoG is dependent on the conserved inner-membrane AAA+ ATPase/protease, FtsH. We also report a functional exploration of the PyoG receptor. We demonstrate that Hur can bind to hemin in vitro and that this interaction is blocked by PyoG, confirming the role of Hur in hemin acquisition.

Contributions of the heme coordinating ligands of the Pseudomonas aeruginosa outer membrane receptor HasR to extracellular heme sensing and transport

Pseudomonas aeruginosa exhibits a high requirement for iron, which it can acquire via several mechanisms, including the acquisition and utilization of heme. The P. aeruginosa genome encodes two heme uptake systems, the heme assimilation system (Has) and the Pseudomonas heme utilization (Phu) system. Extracellular heme is sensed via the Has system, which encodes an extracytoplasmic function (ECF) σ factor system. Previous studies have shown that the transfer of heme from the extracellular hemophore HasAp to the outer membrane receptor HasR is required for activation of the σ factor HasI and upregulation of has operon expression. Here, employing site-directed mutagenesis, allelic exchange, quantitative PCR analyses, immunoblotting, and ¹³C-heme uptake experiments, we delineated the differential contributions of the extracellular FRAP/PNPNL loop residue His-624 in HasR and of His-221 in its N-terminal plug domain required for heme capture to heme transport and signaling, respectively. Specifically, we show that substitution of the N-terminal plug His-221 disrupts both signaling and transport, leading to dysregulation of both the Has and Phu uptake systems. Our results are consistent with a model wherein heme release from HasAp to the N-terminal plug of HasR is required to initiate signaling, whereas His-624 is required for simultaneously closing off the heme transport channel from the extracellular medium and triggering heme transport. Our results provide critical insight into heme release, signaling, and transport in P. aeruginosa and suggest a functional link between the ECF σ factor and Phu heme uptake system.

Apohemoglobin-haptoglobin complex attenuates the pathobiology of circulating acellular hemoglobin and heme

Haptoglobin (Hp) is the plasma protein that binds and clears cell-free hemoglobin (Hb), whereas apohemoglobin (apoHb, i.e., Hb devoid of heme) can bind heme. Therefore, the apoHb-Hp protein complex should facilitate holoHb-apoHb αβ-dimer exchange and apoHb-heme intercalation. Thus, we hypothesized that apoHb-Hp could facilitate both Hb and heme clearance, which, if not alleviated, could have severe microcirculatory consequences. In this study, we characterized apoHb-Hp and Hb/heme ligand interactions and assessed their in vivo consequences. Hb exchange and heme binding with the apoHb-Hp complex was studied with transfer assays using size-exclusion high-performance liquid chromatography coupled with UV-visible spectrophotometry. Exchange/transfer experiments were conducted in guinea pigs dosed with Hb or heme-albumin followed by a challenge with equimolar amounts of apoHb-Hp. Finally, systemic and microcirculatory parameters were studied in hamsters instrumented with a dorsal window chamber via intravital microscopy. In vitro and in vivo Hb exchange and heme transfer experiments demonstrated proof-of-concept Hb/heme ligand transfer to apoHb-Hp. Dosing with the apoHb-Hp complex reversed Hb- and heme-induced systemic hypertension and microvascular vasoconstriction, reduced microvascular blood flow, and diminished functional capillary density. Therefore, this study highlights the apoHb-Hp complex as a novel therapeutic strategy to attenuate the adverse systemic and microvascular responses to intravascular Hb and heme exposure.NEW & NOTEWORTHY This study highlights the apoHb-Hp complex as a novel therapeutic strategy to attenuate the adverse systemic and microvascular responses to intravascular Hb and heme exposure. In vitro and in vivo Hb exchange and heme transfer experiments demonstrated proof-of-concept Hb/heme ligand transfer to apoHb-Hp. The apoHb-Hp complex reverses Hb- and heme-induced systemic hypertension and microvascular vasoconstriction, preserves microvascular blood flow, and functional capillary density. In summary, the unique properties of the apoHb-Hp complex prevent adverse systemic and microvascular responses to Hb and heme-albumin exposure and introduce a novel therapeutic approach to facilitate simultaneous removal of extracellular Hb and heme.

The Staphylococcus aureus IsdH Receptor Forms a Dynamic Complex with Human Hemoglobin that Triggers Heme Release via Two Distinct Hot Spots

Iron is an essential nutrient that is actively acquired by bacterial pathogens during infections. Clinically important Staphylococcus aureus obtains iron by extracting heme from hemoglobin (Hb) using the closely related IsdB and IsdH surface receptors. In IsdH, extraction is mediated by a conserved tridomain unit that contains its second (N2) and third (N3) NEAT domains joined by a helical linker, called IsdHN2N3. Leveraging the crystal structure of the IsdHN2N3:Hb complex, we have probed the mechanism of heme capture using NMR, stopped-flow transfer kinetics measurements, and molecular dynamics (MD) simulations. NMR studies of the 220 kDa IsdHN2N3:Hb complex reveal that it is dynamic, with persistent interdomain motions enabling the linker and N3 domains in the receptor to transiently engage Hb to remove its heme. An alanine mutagenesis analysis reveals that two receptor subsites positioned ~20 Å apart trigger heme release by contacting Hb's F-helix. These subsites are located within the N3 and linker domains and appear to play distinct roles in stabilizing the heme transfer transition state. Linker domain contacts primarily function to destabilize Hb-heme interactions, thereby lowering ΔH‡, while contacts from the N3 subsite play a similar destabilizing role, but also form a bridge through which heme moves from Hb to the receptor. Interestingly, MD simulations suggest that within the transiently forming interface, both the F-helix and receptor bridge are in motion, dynamically sampling conformations that are suitable for heme transfer. Thus, IsdH triggers heme release from Hb via a flexible, low-affinity interface that forms fleetingly in solution.

Binding of HasA by its transmembrane receptor HasR follows a conformational funnel mechanism

HasR in the outer membrane of Serratia marcescens binds secreted, heme-loaded HasA and translocates the heme to the periplasm to satisfy the cell's demand for iron. The previously published crystal structure of the wild-type complex showed HasA in a very specific binding arrangement with HasR, apt to relax the grasp on the heme and assure its directed transfer to the HasR-binding site. Here, we present a new crystal structure of the heme-loaded HasA arranged with a mutant of HasR, called double mutant (DM) in the following that seemed to mimic a precursor stage of the abovementioned final arrangement before heme transfer. To test this, we performed first molecular dynamics (MD) simulations starting at the crystal structure of the complex of HasA with the DM mutant and then targeted MD simulations of the entire binding process beginning with heme-loaded HasA in solution. When the simulation starts with the former complex, the two proteins in most simulations do not dissociate. When the mutations are reverted to the wild-type sequence, dissociation and development toward the wild-type complex occur in most simulations. This indicates that the mutations create or enhance a local energy minimum. In the targeted MD simulations, the first protein contacts depend upon the chosen starting position of HasA in solution. Subsequently, heme-loaded HasA slides on the external surface of HasR on paths that converge toward the specific arrangement apt for heme transfer. The targeted simulations end when HasR starts to relax the grasp on the heme, the subsequent events being in a time regime inaccessible to the available computing power. Interestingly, none of the ten independent simulation paths visits exactly the arrangement of HasA with HasR seen in the crystal structure of the mutant. Two factors which do not exclude each other could explain these observations: the double mutation creates a non-physiologic potential energy minimum between the two proteins and /or the target potential in the simulation pushes the system along paths deviating from the low-energy paths of the native binding processes. Our results support the former view, but do not exclude the latter possibility.

A heme-binding protein produced by Haemophilus haemolyticus inhibits non-typeable Haemophilus influenzae

Commensal bacteria serve as an important line of defense against colonisation by opportunisitic pathogens, but the underlying molecular mechanisms remain poorly explored. Here, we show that strains of a commensal bacterium, Haemophilus haemolyticus, make hemophilin, a heme-binding protein that inhibits growth of the opportunistic pathogen, non-typeable Haemophilus influenzae (NTHi) in culture. We purified the NTHi-inhibitory protein from H. haemolyticus and identified the hemophilin gene using proteomics and a gene knockout. An x-ray crystal structure of recombinant hemophilin shows that the protein does not belong to any of the known heme-binding protein folds, suggesting that it evolved independently. Biochemical characterisation shows that heme can be captured in the ferrous or ferric state, and with a variety of small heme-ligands bound, suggesting that hemophilin could function under a range of physiological conditions. Hemophilin knockout bacteria show a limited capacity to utilise free heme for growth. Our data suggest that hemophilin is a hemophore and that inhibition of NTHi occurs by heme starvation, raising the possibility that competition from hemophilin-producing H. haemolyticus could antagonise NTHi colonisation in the respiratory tract.

The Role of Iron and Siderophores in Infection, and the Development of Siderophore Antibiotics

Iron is an essential nutrient for bacterial growth, replication, and metabolism. Humans store iron bound to various proteins such as hemoglobin, haptoglobin, transferrin, ferritin, and lactoferrin, limiting the availability of free iron for pathogenic bacteria. However, bacteria have developed various mechanisms to sequester or scavenge iron from the host environment. Iron can be taken up by means of active transport systems that consist of bacterial small molecule siderophores, outer membrane siderophore receptors, the TonB-ExbBD energy-transducing proteins coupling the outer and the inner membranes, and inner membrane transporters. Some bacteria also express outer membrane receptors for iron-binding proteins of the host and extract iron directly from these for uptake. Ultimately, iron is acquired and transported into the bacterial cytoplasm. The siderophores are small molecules produced and released by nearly all bacterial species and are classified according to the chemical nature of their iron-chelating group (ie, catechol, hydroxamate, α-hydroxyl-carboxylate, or mixed types). Siderophore-conjugated antibiotics that exploit such iron-transport systems are under development for the treatment of infections caused by gram-negative bacteria. Despite demonstrating high in vitro potency against pathogenic multidrug-resistant bacteria, further development of several candidates had stopped due to apparent adaptive resistance during exposure, lack of consistent in vivo efficacy, or emergence of side effects in the host. However, cefiderocol, with an optimized structure, has advanced and has been investigated in phase 1 to 3 clinical trials. This article discusses the mechanisms implicated in iron uptake and the challenges associated with the design and utilization of siderophore-mimicking antibiotics.

Pseudomonas aeruginosa possesses three distinct systems for sensing and using the host molecule haem

Pathogens have developed several strategies to obtain iron during infection, including the use of iron-containing molecules from the host. Haem accounts for the vast majority of the iron pool in vertebrates and thus represents an important source of iron for pathogens. Using a proteomic approach, we have identified in this work a previously uncharacterized system, which we name Hxu, that together with the known Has and Phu systems, is used by the human pathogen Pseudomonas aeruginosa to respond to haem. We show that the Has and Hxu systems are functional signal transduction pathways of the cell-surface signalling class and report the mechanism triggering the activation of these signalling systems. Both signalling cascades involve an outer membrane receptor (HasR and HxuA respectively) that upon sensing haem in the extracellular medium produces the activation of an σ^ECF factor in the cytosol. HxuA has a major role in signalling and a minor role in haem acquisition in conditions in which the HasR and PhuR receptors or other sources of iron are present. Remarkably, P. aeruginosa compensates the lack of the HasR receptor by increasing the production of HxuA, which underscores the importance of haem signalling for this pathogen.

Hijacking the Heme Acquisition System of Pseudomonas aeruginosa for the Delivery of Phthalocyanine as an Antimicrobial

To survive in the iron-devoid environment of their host, pathogenic bacteria have devised multifarious cunning tactics such as evolving intricate heme transport systems to pirate extracellular heme. Yet, the potential of heme transport systems as antimicrobial targets has not been explored. Herein we developed a strategy to deliver antimicrobials by exploiting the extracellular heme acquisition system protein A (HasA) of Pseudomonas aeruginosa. We demonstrated that, analogous to heme uptake, HasA can specifically traffic an antimicrobial, gallium phthalocyanine (GaPc), into the intracellular space of P. aeruginosa via the interaction of HasA with its outer membrane receptor HasR. HasA enables water-insoluble GaPc to be mistakenly acquired by P. aeruginosa, permitting its sterilization (>99.99%) by irradiation with near-infrared (NIR) light, irrespective of antibiotic resistance. Our findings substantiate that bacterial heme uptake via protein-protein recognition is an attractive target for antimicrobials, enabling specific and effective sterilization.

Secondary and Supersecondary Structure of Proteins in Light of the Structure of Hydrophobic Cores

The traditional classification of protein structures (with regard to their supersecondary and tertiary conformation) is based on an assessment of conformational similarities between various polypeptide chains and particularly on the presence of specific secondary structural motifs. Mutual relations between secondary folds determine the overall shape of the protein and may be used to assign proteins to specific families (such as the immunoglobulin-like family). An alternative means of conducting structural assessment focuses on the structure of the protein's hydrophobic core. In this case, the protein is treated as a quasi-micelle, which exposes hydrophilic residues on its surface while internalizing hydrophobic residues. The accordance between the actual distribution of hydrophobicity in a protein and its corresponding theoretical ("idealized") distribution can be determined quantitatively, which, in turn, enables comparative analysis of structures regarded as geometrically similar (as well as geometrically divergent structures which are nevertheless regarded as similar in the sense of the fuzzy oil drop model). In this scope, the protein may be compared to an "intelligent micelle," where local disorder is often intentional and related to biological function-unlike traditional surfactant micelles which remain highly symmetrical throughout and do not carry any encoded information.

Post-transcriptional regulation of the Pseudomonas aeruginosa heme assimilation system (Has) fine-tunes extracellular heme sensing

Pseudomonas aeruginosa is an opportunistic pathogen that utilizes heme as a primary iron source within the host. Extracellular heme is sensed via a heme assimilation system (has) that encodes an extracytoplasmic function (ECF) σ factor system. Herein, using has deletion mutants, quantitative PCR analyses, and immunoblotting, we show that the activation of the σ factor HasI requires heme release from the hemophore HasAp to the outer-membrane receptor HasR. Using RT-PCR and 5'-RACE, we observed that following transcriptional activation of the co-transcribed hasRAp, it is further processed into specific mRNAs varying in stability. We noted that the processing and variation in stability of the hasAp and hasR mRNAs in response to heme provide a mechanism for differential expression from co-transcribed genes. The multiple layers of post-transcriptional regulation of the ECF signaling cascade, including the previously reported post-transcriptional regulation of HasAp by the heme metabolites biliverdin IXβ and IXδ, allow fine-tuning of the cell-surface signaling system in response to extracellular heme levels. We hypothesize that the complex post-transcriptional regulation of the Has system provides P. aeruginosa an advantage in colonizing a variety of physiological niches in the host.

Proteomics, Bioinformatics and Structure-Function Antigen Mining For Gonorrhea Vaccines

Expanding efforts to develop preventive gonorrhea vaccines is critical because of the serious health consequences combined with the prevalence and the dire possibility of untreatable gonorrhea. Reverse vaccinology, which includes genome and proteome mining, has proven successful in the discovery of vaccine candidates against many pathogenic bacteria. Here, we describe proteomic applications including comprehensive, quantitative proteomic platforms and immunoproteomics coupled with broad-ranging bioinformatics that have been applied for antigen mining to develop gonorrhea vaccine(s). We further focus on outlining the vaccine candidate decision tree, describe the structure-function of novel proteome-derived antigens as well as ways to gain insights into their roles in the cell envelope, and underscore new lessons learned about the fascinating biology of Neisseria gonorrhoeae.

Validation and Applications of Protein-Ligand Docking Approaches Improved for Metalloligands with Multiple Vacant Sites

Decoding the interaction between coordination compounds and proteins is of fundamental importance in biology, pharmacy, and medicine. In this context, protein- ligand docking represents a particularly interesting asset to predict how small compounds could interact with biomolecules, but to date, very little information is available to adapt these methodologies to metal-containing ligands. Here, we assessed the predictive capability of a metal-compatible parameter set for the docking program GOLD for metallo ligands with multiple vacant sites and different geometries. The study first presents a benchmark of 25 well-characterized X-ray metallo ligand-protein adducts. In 100% of the cases, the docking solutions are superimposable to the X-ray determination, and in 92% the value of the root-mean-square deviation between the experimental and calculated structures is lower than 1.5 Å. After the validation step, we applied these methods to five case studies for the prediction of the binding of pharmacological active metal species to proteins: (i) the anticancer copper(II) complex [Cu^II(Br)(2-hydroxy-1-naphthaldehyde benzoyl hydrazine)(indazole)] to human serum albumin (HSA); (ii) one of the active species of antidiabetic and antitumor vanadium compounds, V^IVO²⁺ ion, to carboxypeptidase; (iii) the antiarthritic species [Au^I(PEt₃)]⁺ to HSA; (iv) the antitumor oxaliplatin to ubiquitin; (v) the antitumor ruthenium(II) compound RAPTA-PentaOH to cathepsin B. The calculations suggested that the binding modes are in good agreement with the partial information retrieved from spectroscopic and spectrometric analysis and allowed us, in certain cases, to propose additional hypotheses. This method is an important update in protein-metallo ligand docking, which could have a wide field of application, from biology and inorganic biochemistry to medicinal chemistry and pharmacology.

Experimental Methods for Studying Cellular Heme Signaling

The study of heme is important to our understanding of cellular bioenergetics, especially in cancer cells. The function of heme as a prosthetic group in proteins such as cytochromes is now well-documented. Less is known, however, about its role as a regulator of metabolic and energetic pathways. This is due in part to some inherent difficulties in studying heme. Due to its slightly amphiphilic nature, heme is a "sticky" molecule which can easily bind non-specifically to proteins. In addition, heme tends to dimerize, oxidize, and aggregate in purely aqueous solutions; therefore, there are constraints on buffer composition and concentrations. Despite these difficulties, our knowledge of heme's regulatory role continues to grow. This review sums up the latest methods used to study reversible heme binding. Heme-regulated proteins will also be reviewed, as well as a system for imaging the cellular localization of heme.

Biotechnology of siderophores in high-impact scientific fields

Different aspects of bacterial and fungal siderophore biotechnological applications will be discussed. Areas of application presented include, but are not limited to agriculture, medicine, pharmacology, bioremediation, biodegradation and food industry. In agriculture-related applications, siderophores could be employed to enhance plant growth due to their uptake by rhizobia. Siderophores hindered the presence of plant pathogens in biocontrol strategies. Bioremediation studies on siderophores discuss mostly the mobilization of heavy metals and radionuclides; the emulsifying effects of siderophore-producing microorganisms in oil-contaminated environments are also presented. The different applications found in literature based in medicine and pharmacological approaches range from iron overload to drug delivery systems and, more recently, vaccines. Additional research should be done in siderophore production and their metabolic relevance to have a deeper understanding for future biotechnological advances.

Expression and purification of functionally active ferrous iron transporter FeoB from Klebsiella pneumoniae

The acquisition of ferrous iron (Fe²⁺) is an important virulence factor utilized by several hospital-acquired (nosocomial) pathogens such as Klebsiella pneumoniae to establish infection within human hosts. Virtually all bacteria use the ferrous iron transport system (Feo) to acquire ferrous iron from their environments, which are often biological niches that stabilize Fe²⁺ relative to Fe³⁺. However, the details of this process remain poorly understood, likely owing to the few expression and purification systems capable of supplying sufficient quantities of the chief component of the Feo system, the integral membrane GTPase FeoB. This bottleneck has undoubtedly hampered efforts to understand this system in order to target it for therapeutic intervention. In this study, we describe the expression, solubilization, and purification of the Fe²⁺ transporter from K. pneumoniae, KpFeoB. We show that this protein may be heterologously overexpressed in Escherichia coli as the host organism. After testing several different commercially-available detergents, we have developed a solubilization and purification protocol that produces milligram quantities of KpFeoB with sufficient purity for enzymatic and biophysical analyses. Importantly, we demonstrate that KpFeoB displays robust GTP hydrolysis activity (k_cat^GTP of ∼10^-1 s^-1) in the absence of any additional stimulatory factors. Our findings suggest that K. pneumoniae may be capable of using its Feo system to drive Fe²⁺ import in an active manner.

Iron Acquisition Strategies of Bacterial Pathogens

Iron is an essential micronutrient for both microbes and humans alike. For well over half a century we have known that this element, in particular, plays a pivotal role in health and disease and, most especially, in shaping host-pathogen interactions. Intracellular iron concentrations serve as a critical signal in regulating the expression not only of high-affinity iron acquisition systems in bacteria, but also of toxins and other noted virulence factors produced by some major human pathogens. While we now are aware of many strategies that the host has devised to sequester iron from invading microbes, there are as many if not more sophisticated mechanisms by which successful pathogens overcome nutritional immunity imposed by the host. This review discusses some of the essential components of iron sequestration and scavenging mechanisms of the host, as well as representative Gram-negative and Gram-positive pathogens, and highlights recent advances in the field. Last, we address how the iron acquisition strategies of pathogenic bacteria may be exploited for the development of novel prophylactics or antimicrobials.

Gating of TonB-dependent transporters by substrate-specific forced remodelling

Membrane proteins play vital roles in inside-out and outside-in signal transduction by responding to inputs that include mechanical stimuli. Mechanical gating may be mediated by the membrane or by protein(s) but evidence for the latter is scarce. Here we use force spectroscopy, protein engineering and bacterial growth assays to investigate the effects of force on complexes formed between TonB and TonB-dependent transporters (TBDT) from Gram-negative bacteria. We confirm the feasibility of protein-only mediated mechanical gating by demonstrating that the interaction between TonB and BtuB (a TBDT) is sufficiently strong under force to create a channel through the TBDT. In addition, by comparing the dimensions of the force-induced channel in BtuB and a second TBDT (FhuA), we show that the mechanical properties of the interaction are perfectly tuned to their function by inducing formation of a channel whose dimensions are tailored to the ligand.

Extracellular Heme Uptake and the Challenge of Bacterial Cell Membranes

Iron is essential for the survival of most bacteria but presents a significant challenge given its limited bioavailability. Furthermore, the toxicity of iron combined with the need to maintain physiological iron levels within a narrow concentration range requires sophisticated systems to sense, regulate, and transport iron. Most bacteria have evolved mechanisms to chelate and transport ferric iron (Fe³⁺) via siderophore receptor systems, and pathogenic bacteria have further lowered this barrier by employing mechanisms to utilize the host's hemoproteins. Once internalized, heme is cleaved by both oxidative and nonoxidative mechanisms to release iron. Heme, itself a lipophilic and toxic molecule, presents a significant challenge for transport into the cell. As such, pathogenic bacteria have evolved sophisticated cell surface signaling and transport systems to obtain heme from the host. In this review, we summarize the structure and function of the heme-sensing and transport systems of pathogenic bacteria and the potential of these systems as antimicrobial targets.

Innovative approaches to treat Staphylococcus aureus biofilm-related infections

Many bacterial infections in humans and animals are caused by bacteria residing in biofilms, complex communities of attached organisms embedded in an extracellular matrix. One of the key properties of microorganisms residing in a biofilm is decreased susceptibility towards antimicrobial agents. This decreased susceptibility, together with conventional mechanisms leading to antimicrobial resistance, makes biofilm-related infections increasingly difficult to treat and alternative antibiofilm strategies are urgently required. In this review, we present three such strategies to combat biofilm-related infections with the important human pathogen Staphylococcus aureus: (i) targeting the bacterial communication system with quorum sensing (QS) inhibitors, (ii) a ‘Trojan Horse’ strategy to disturb iron metabolism by using gallium-based therapeutics and (iii) the use of ‘non-antibiotics’ with antibiofilm activity identified through screening of repurposing libraries.