Among multidrug-resistant bacteria, methicillin-resistant Staphylococcus aureus is emerging as one of the most threatening pathogens. S. aureus exploits different mechanisms for its iron supply, but the preferred one is acquisition of organic iron through the expression of hemoglobin (Hb) receptors. One of these, IsdB, belonging to the Isd (Iron-Regulated Surface Determinant) system, was shown to be essential for bacterial growth and virulence. Therefore, interaction of IsdB with Hb represents a promising target for the rational design of a new class of antibacterial molecules. However, despite recent investigations, many structural and mechanistic details of complex formation and heme extraction process are still elusive. By combining site-directed mutagenesis, absorption spectroscopy, surface plasmon resonance and molecular dynamics simulations, we tackled most of the so far unanswered questions: (i) the exact complex stoichiometry, (ii) the microscopic kinetic rates of complex formation, (iii) the IsdB selectivity for binding to, and extracting heme from, α and β subunits of Hb, iv) the role of specific amino acid residues and structural regions in driving complex formation and heme transfer, and (v) the structural/dynamic effect played by the hemophore on Hb. The ability of bacterial pathogens to establish infections relies on the adaptation of bacterial metabolism to the environment found within the host, which is often severely nutrient-restricted and a site for competition with commensal microorganisms and other pathogens 1,2. A well-characterized mechanism of adaptation to the nutritional environment of the host is represented by the expression of redundant iron-acquisition systems, both in Gram + and Grambacteria, to provide iron supply to support invasion and proliferation 3. Iron is an essential nutrient for both the pathogen and the host, but also a toxic element due to its involvement in Fenton chemistry and Haber-Weiss reactions that generate reactive oxygen species (ROS) 4. For this reason, vertebrates have put in place strategies to maintain very low concentrations of free iron in body fluids (down to 10 −18-10 −24 M) 4,5 achieving two goals, the first being limiting toxicity and the second creating an iron-restricted environment for pathogens 6,7. In S. aureus, iron scavenging is achieved by at least three different mechanisms: i) acquisition of inorganic iron through production of siderophores, ii) expression of hemoglobin (Hb) receptors, and iii) acquisition of inorganic iron through transporters 4. Heme is the preferred iron source during the initial phase of infection 8

Nutritional immunity is a form of innate immunity widespread in both vertebrates and invertebrates. The term refers to a rich repertoire of mechanisms set up by the host to inhibit bacterial proliferation by sequestering trace minerals (mainly iron, but also zinc and manganese). This strategy, selected by evolution, represents an effective front-line defense against pathogens and has thus inspired the exploitation of iron restriction in the development of innovative antimicrobials or enhancers of antimicrobial therapy. This review focuses on the mechanisms of nutritional immunity, the strategies adopted by opportunistic human pathogen Staphylococcus aureus to circumvent it, and the impact of deletion mutants on the fitness, infectivity, and persistence inside the host. This information finally converges in an overview of the current development of inhibitors targeting the different stages of iron uptake, an as-yet unexploited target in the field of antistaphylococcal drug discovery.

In bacteria and plants, serine acetyltransferase (CysE) and O-acetylserine sulfhydrylase-A sulfhydrylase (CysK) collaborate to synthesize L-Cys from L-Ser. CysE and CysK bind one another with high affinity to form the cysteine synthase complex (CSC). We demonstrate that bacterial CysE is activated when bound to CysK. CysE activation results from the release of substrate inhibition, with the K i for L-Ser increasing from 4 mM for free CysE to 16 mM for the CSC. Feedback inhibition of CysE by L-Cys is also relieved in the bacterial CSC. These findings suggest that the CysE active site is allosterically altered by CysK to alleviate substrate and feedback inhibition in the context of the CSC. Author contributions BC, SB, and AM conceived and supervised the study; RB, ODB, GP, and NF performed experiments; CSH provided the expression vectors; BC, RB, and ODB analyzed the data; BC prepared the original draft; BC, SB, CSH, and AM reviewed and edited the manuscript. Supporting informationAdditional Supporting Information may be found online in the supporting information tab for this article. HHS Public Access Author Manuscript Author ManuscriptAuthor Manuscript Author ManuscriptPlants and bacteria share a common two-reaction pathway for the synthesis of L-cysteine (LCys) from L-serine (L-Ser; Fig. 1). Serine acetyltransferase (CysE) catalyzes an acyl transfer from acetyl-CoA to L-Ser using a random-order kinetic mechanism [1]. The second reaction is catalyzed by O-acetylserine sulfhydrylase-A (CysK), a pyridoxal 5′-phosphate (PLP)-dependent enzyme that displaces the acetoxy group from O-acetylserine with bisulfide to yield L-Cys [2][3][4][5][6][7][8]. Many bacteria also encode O-acetylserine sulfhydrylase-B (CysM) [9,10] that is thought to play an important role in L-Cys biosynthesis under stress conditions [11].Kredich et al. [2,12] first discovered that CysE and CysK from Salmonella Typhimurium bind to one another with high affinity, and they called this assembly the cysteine synthase complex (CSC; Fig. 1). The CysE-CysK interaction is highly conserved across species, and the plant enzymes also form a high-affinity CSC. Although there is no experimentally solved structure available for the CSC, biochemical and spectroscopic approaches revealed that the C-terminal tail of CysE inserts into the CysK active site to anchor the interaction. CysE proteins that lack C-terminal residues are unable to bind CysK [13][14][15], and CSC formation is disrupted by millimolar O-acetylserine, which competes with CysE for binding to the CysK active site [12,16,17]. These findings are supported by crystal structures of CysE Cterminal peptides bound in the active site of CysK. These structures show that the C-terminal Ile residue of CysE engages in the same specific interactions with the active site as Oacetylserine substrate [18,19]. The stoichiometry of CysE to CysK has been determined to be 3:2 for CSCs from S. Typhimurium and Haemophilus influenzae. Because CysK forms homodimers and CysE exists as a dimer of trimers [20,21],...

Significance During infection, the human pathogen Staphylococcus aureus expresses a surface-exposed receptor, Iron surface determinant B (IsdB), that captures free human hemoglobin (Hb) and removes heme to retrieve iron, an essential nutrient for bacterial proliferation inside the host. Using single-particle cryo-electron microscopy, we solved the structure of two complexes between Hb and IsdB that represent snapshots of the initial interaction, where heme is still bound to Hb, and the final complex after completion of heme extraction. The structural and dynamic details unlocked through these structures will boost the design of inhibitors of IsdB:Hb interaction that might work as innovative antimicrobials.

Infections caused by Staphylococcus aureus depend on its ability to acquire nutrients. One essential nutrient is iron, which is obtained from the heme of the human host hemoglobin (Hb) through a protein machinery called Iron-regulated Surface Determinant (Isd). IsdB is the protein in charge of heme extraction from Hb, which is the first step of the chain of events leading to iron transfer to the bacterium cell interior. In order to elucidate the molecular events leading from the formation of the initial IsdB:Hb complex to heme extraction, we have performed a time-resolved X-ray solution scattering (TR-XSS) investigation combined with a rapid mixing triggering approach. We succeeded in defining the stoichiometry of IsdB:Hb binding and in describing the kinetics of the subsequent structural changes. The presented approach is potentially applicable to unveil the complex kinetic pathways generated by protein-protein interaction in different biological systems.