When challenged by low-iron conditions several Gram-negative pathogens secrete a hemophore (HasA) to scavenge hemin from its host and deliver it to a receptor (HasR) on their outer membrane for internalization. Here we report results from studies aimed at probing the structural and dynamic processes at play in the loading of the apo-hemophore secreted by P. aeruginosa (apo-HasAp) with hemin. The structure of apo-HasAp shows a large conformational change in the loop harboring axial ligand His32 relative to the structure of holo-HasAp, whereas the loop bearing the other axial ligand, Tyr75, remains intact. To investigate the role played by the axial ligand-bearing loops in the process of hemin capture we investigated the H32A mutant, which was found to exist as a monomer in its apo-form and as a mixture of monomers and dimers in its holoform. We obtained an X-ray structure of dimeric H32A holo-HasAp, which revealed that the two subunits are linked by cofacial interactions of two hemin molecules and that the conformation of the Ala32 loop in the dimer is identical to that exhibited by the His32 loop in wild type apoHasAp. Additional data suggest that the conformation of the Ala32 loop in the dimer is mainly a consequence of dimerization. Hence, to investigate the effect of hemin loading on the topology of the His32 loop we also obtained the crystal structure of monomeric H32A holo-HasAp coordinated by imidazole (H32A-imidazole) and investigated the monomeric H32A HasAp and H32A-imidazole species in solution by NMR spectroscopy. The structure of H32A-imidazole * To whom correspondence should be addressed: mrivera@ku.edu.Coordinates and structure factors have been deposited to the Protein Databank with the accession codes 3MOK (Apo-HasAp), 3MOL (HasAp H32A dimer) and 3MOM (HasAp H32A imidazole complex). SUPPORTING INFORMATION AVAILABLEA view of apo-HasAp showing the location of Na + and PO 4 3 − ions, simulation system of hemin bound apo-HasAp with and without KCl ions or solution, apo-HasAp His32 loop region showing the stabilizing aromatic side chains, resonance Raman and EPR spectra of H32A holo-HasAp, electronic absorption spectra and binding curve of imidazole binding to H32A holo-HasAp, 15 N, 1 H-HSQC-TROSY spectra of H32A HasAp and H32A-imidazole, tables of backbone NMR assignments for H32A HasAp and H32A-imidazole, and complete references 30, 38 and 39 . This material is available free of charge via the Internet at

Pseudomonas aeruginosa secretes a 205 residue long hemophore (full-length HasAp) that is subsequently cleaved at the C'-terminal domain to produce mainly a 184 residue long truncated HasAp that scavenges heme [Letoffé, S., Redeker, V., and Wandersman, C. (1998) Mol. Microbiol. 28, 1223-1234. HasAp has been characterized by X-ray crystallography and in solution by NMR spectroscopy. The X-ray crystal structure of truncated HasAp revealed a polypeptide αβ fold and a ferriheme coordinated axially by His32 and Tyr75, with the side chain of His83 poised to accept a hydrogen bond from the Tyr75 phenolic acid group. NMR investigations conducted with full-length HasAp showed that the carboxyl terminal tail (21 residues) is disordered and conformationally flexible. NMR spectroscopic investigations aimed at studying a complex between apo-HasAp and human met-hemoglobin were stymied by the rapid heme capture by the hemophore. In an effort to circumvent this problem NMR spectroscopy was used to monitor the titration of 15 Nlabeled holo-HasAp with hemoglobin. These studies allowed identification of a specific area on the surface of truncated HasAp, encompassing the axial ligand His32 loop that serves as a transient site of interaction with hemoglobin. These findings are discussed in the context of a putative encounter complex between apo-HasAp and hemoglobin that leads to efficient hemoglobin-heme capture by the hemophore. Similar experiments conducted with full-length 15 N-labeled HasAp and hemoglobin revealed a transient interaction site in full-length HasAp similar to that observed in the truncated hemophore. The spectral perturbations observed while investigating these interactions, however, are weaker than those observed for the interactions between hemoglobin and truncated HasAp, † This work was supported by grants from the National Institute of Health, GM-50503 (M.R.), NSF-MCB-0818488 (M.R.) and NSF-MCB-0811888 (P.M.L.). ‡ Coordinates and crystallographic structure factors for HasAp have been deposited in the protein data bank under accession code 3ELL.Backbone resonance assignments for truncated and full-length holo-HasAp have been deposited in the BMRB under access code 15962 and 15963, respectively. NIH Public Access Author ManuscriptBiochemistry. Author manuscript; available in PMC 2010 January 13. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscriptsuggesting that the disordered tail in the full-length HasAp must be proteolyzed in the extracellular milieu to make HasAp a more efficient hemophore.The preferred aerobic metabolism of Pseudomonas aeruginosa requires respiratory enzymes that need iron or iron-containing cofactors for their function. The extremely low concentrations of free iron in mammalian hosts trigger a stress response in the opportunistic P. aeruginosa (and in many other pathogens) that involves the deployment of several iron-acquisition systems (1-4). The systems involved in the capture of iron typically fall in two categories, (i) excretion of low-molecular weight i...

Mycobacterium tuberculosis can exist in the actively growing state of the overt disease or in a latent quiescent state that can be induced, among other things, by anaerobiosis. Eradication of the latent state is particularly difficult with the available drugs and requires prolonged treatment. DevS is a member of the DevS-DevR two-component regulatory system that is thought to mediate the cellular response to anaerobiosis. Here we report the cloning, expression, and initial characterization of a truncated version of DevS (DevS642) containing only the N-terminal GAF sensor domain (GAF-A) and of the full-length protein DevS. The DevS truncated construct quantitatively binds heme in a 1:1 stoichiometry, and the complex of the protein with ferrous heme reversibly binds O2, NO, and CO. UV-vis and resonance Raman spectroscopy of the wild-type protein and the H149A mutant confirm that His149 is the proximal ligand to the heme iron atom. While the heme-CO complex is present as two conformers in the GAF-A domain, a single set of [Fe-C-O] vibrations is observed with the full-length protein, suggesting that interactions between domains within DevS influence the distal pocket environment of the heme in the GAF-A domain.

In Bacillus subtilis, NsrR is required for the upregulation of ResDE-dependent genes in the presence of nitric oxide (NO). NsrR was shown to bind to the promoters of these genes and inhibit their transcription in vitro. NO relieves this inhibition by an unknown mechanism. Here we use spectroscopic techniques (UV-vis, resonance Raman, and EPR) to show that anaerobically isolated NsrR from B. subtilis contains a [4Fe-4S] 2+ cluster which reacts with NO to form dinitrosyl iron complexes. This method of NO sensing is analogous to that of the FNR protein of Escherichia coli. The Fe-S cluster of NsrR is also reactive toward other exogenous ligands such as cyanide, dithiothreitol, and O 2 . These results, together with the presence of only three cysteine residues in NsrR, suggest that the 4Fe-4S cluster contains a non-cysteinyl labile ligand to one of the iron atoms, leading to high reactivity. Size exclusion chromatography and crosslinking experiments show that NsrR adopts a dimeric structure in its [4Fe-4S] 2+ holo form as well as in the apo form. These findings provide a first steppingstone to investigate the mechanism of NO sensing in NsrR.The ability to adapt to anaerobic conditions is vital for a great diversity of microorganisms. It is particularly true of pathogenic organisms which may encounter oxygen limitation within their hosts. Such bacteria also commonly face other stresses such as nitric oxide (NO) produced by phagocytic cells as part of the immune response of the host (1). Thus, an understanding of how bacteria sense and respond to these conditions is of considerable importance.An example of the bacterial response to oxygen limitation comes from Bacillus subtilis which, in the absence of oxygen, can grow by nitrate respiration (2). The ResDE two-component regulatory system is required for the induction of genes involved in nitrate respiration (3). These genes include fnr, (the gene encoding anaerobic transcription factor) (4), nasDEF (nitrite reductase genes) (5), and hmp (flavohemoglobin gene) (6). However, anaerobic conditions alone are not sufficient for the full induction of the ResDE-controlled genes, in particular hmp and nasDEF, and the presence of NO is required to attain the full induction of these genes (7). The effect of NO is abrogated in an nsrR mutant strain, indicating a role for NsrR in NOmediated control of the ResDE regulon (8).NsrR belongs to the Rrf2 family of transcription regulators and is widely found in various bacteria (9). Sequence analyses predict that NsrR contains a helix-turn-helix which likely binds to the promoter region of its target genes (9). In vivo studies of nsrR mutants and the effect of * To whom correspondence should be addressed. Tel: 503-748-167, Fax: 503-7481464, Email: ploccoz@ebs.ogi Materials & Methods Purification of N-terminal and C-terminal His 6 -tagged NsrR proteins (NH-NsrR & CH-NsrR)The various strains and plasmids used in this study are listed in Table S1 (see Supporting Information). NH-NsrR expression plasmid, pMMN648, was previously cons...

The diheme enzyme MauG catalyzes the posttranslational modification of the precursor protein of methylamine dehydrogenase (preMADH) to complete biosynthesis of its protein-derived tryptophan tryptophylquinone (TTQ) cofactor. Catalysis proceeds through a high valent bis-Fe(IV) redox state and requires long-range electron transfer (ET), as the distance between the modified residues of pre-MADH and the nearest heme iron of MauG is 19.4 Å. Trp199 of MauG resides at the MauG-preMADH interface, positioned midway between the residues that are modified and the nearest heme. W199F and W199K mutations did not affect the spectroscopic and redox properties of MauG, or its ability to stabilize the bis-Fe(IV) state. Crystal structures of complexes of W199F/K MauG with pre-MADH showed no significant perturbation of the MauG-preMADH structure or protein interface. However, neither MauG variant was able to synthesize TTQ from preMADH. In contrast, an ET reaction from diferrous MauG to quinone MADH, which does not require the bis-Fe(IV) intermediate, was minimally affected by the W199F/K mutations. W199F/K MauGs were able to oxidize quinol MADH to form TTQ, the putative final two-electron oxidation of the biosynthetic process, but with k cat ∕K m values approximately 10% that of wild-type MauG. The differential effects of the W199F/K mutations on these three different reactions are explained by a critical role for Trp199 in mediating multistep hopping from preMADH to bis-Fe(IV) MauG during the long-range ET that is required for TTQ biosynthesis.cytochrome | electron hopping | peroxidase | protein oxidation | protein radical L ong-range electron transfer (ET) through proteins is required for biological processes including respiration, photosynthesis, and metabolism. Mechanisms by which ET occurs over large distances to specific sites within a protein have been extensively studied (1-4). For interprotein ET, kinetic mechanisms are more complex, as the overall redox reaction requires additional steps such as protein-protein association and reorientation of the protein complex to optimize the system for ET (5, 6). "Long-range catalysis" is a related process in which the redox center that provides the oxidizing or reducing power is physically distinct from the site of chemical reaction of the substrate, so that long-range ET is required for catalysis. Thus far two enzymes have been postulated to employ long-range catalysis. Ribonucleotide reductase (RNR) catalyzes the formation of deoxyribonucleotides from ribonucleotides by long-range ET via multiple tyrosyl residues (7,8). DNA photolyase is a flavoprotein that catalyzes DNA repair of pyrimidine-pyrimidine dimers via multiple tryptophan residues (9). In these enzymes it is believed that the long-range ET proceeds by hopping (10) through residues that can stabilize a radical state, rather than via a single long-range electron tunneling event.

oxidase, where no oxygen was released into the medium from nonenzymatic peroxide decomposition), product formation and peroxide consumption were tightly coupled, and the rate of product formation was identical to that measured under aerobic conditions. Peroxide reactivity was eliminated by a mutation at the Cu H center, which should not be involved in the peroxide shunt. Our data lend support to recent proposals that Cu(II)-superoxide is the active species.

The extreme limitation of free iron has driven various pathogens to acquire iron from the host in the form of heme. Specifically, several Gram negative pathogens secrete a heme binding protein known as HasA to scavenge heme from the extracellular environment and to transfer it to the receptor protein HasR for import into the bacterial cell. Structures of heme-bound and apo-HasA homologues show that the heme iron(III) ligands, His32 and Tyr75, reside on loops extending from the core of the protein and that a significant conformational change must occur at the His32 loop upon heme binding. Here, we investigate the kinetics of heme acquisition by HasA from Pseudomonas aeruginosa (HasAp). The rate of heme acquisition from human met-hemoglobin (met-Hb) closely matched that of heme dissociation which suggests a passive mode of heme uptake from this source. The binding of free hemin is characterized by an initial rapid phase forming an intermediate before further conversion to the final complex. Analysis of this same reaction using an H32A variant lacking the His heme ligand shows only the rapid phase to form a heme-protein complex spectroscopically equivalent to that of the wild type intermediate. Further characterization of these reactions using EPR and resonance Raman spectroscopy of rapid freeze quench samples provided support for a model where heme is initially bound by the Tyr75 to form a high-spin heme-protein complex before slower coordination of the His32 ligand upon closing of the His loop over the heme. The slow rate of this loop closure implies that the induced-fit mechanism of heme uptake in HasAp is not based on a rapid sampling of the H32 loop between open and closed configurations, but rather, that the H32 loop motions are triggered by the formation of the high-spin heme-HasAp intermediate complex.Certain opportunistic pathogens, such as Pseudomonas aeruginosa, are able to overcome the extremely low levels of available free iron within their mammalian host by deploying several iron acquisition systems. One of these, encoded by the has (heme acquisition system) operon found in a number of gram-negative bacteria, involves the secretion of a heme-binding protein HasA1 (1). HasA binds extracellular heme with high affinity (5.3 × 10 10 M -1 ) (2) and delivers it to a specific outer membrane receptor, HasR (1,3). Once † This work was supported in part by the National Science Foundation (P.M.-L., MCB-0811888; M.R., MCB-0818488) and the National Institutes of Health (P.M.-L., GM0 74785; M.R., GM 50503).

The diheme enzyme MauG catalyzes a six-electron oxidation required for posttranslational modification of a precursor of methylamine dehydrogenase (preMADH) to complete the biosynthesis of its protein-derived tryptophan tryptophylquinone (TTQ) cofactor. Crystallographic studies had shown that Pro107, which resides in the distal pocket of the high-spin heme of MauG, changes conformation upon binding of CO or NO to the heme iron. In this study, Pro107 was converted to Cys, Val and Ser by site-directed mutagenesis. The structures of each of these MauG mutant proteins in complex with preMADH were determined, as were their physical and catalytic properties. P107C MauG was inactive and the crystal structure revealed that Cys107 had been oxidatively modified to a sulfinic acid. Mass spectrometry revealed that this modification was present prior to crystallization. P107V MauG exhibited spectroscopic and catalytic properties that were similar to wild-type MauG, but P107V MauG was more susceptible to oxidative damage. The P107S mutation caused a structural change which resulted in the five-coordinate high-spin heme being converted to a six-coordinate heme with a distal axial ligand provided by Glu113. EPR and resonance Raman spectroscopy revealed this heme remained high-spin but with much increased rhombicity as compared to the axial signal of wild-type MauG. P107S MauG was resistant to reduction by dithionite and reaction with H2O2, and unable to catalyze TTQ biosynthesis. These results show that the presence of Pro107 is critical in maintaining the proper structure of the distal heme pocket of the high-spin heme of MauG, enabling exogenous ligands to bind and directing the reactivity of the heme-activated oxygen during catalysis, thus minimizing the oxidation of other residues of MauG.