Heme Transfer to the Bacterial Cell Envelope Occurs via a Secreted Hemophore in the Gram-positive Pathogen Bacillus anthracis

To initiate and sustain an infection in mammals, bacterial pathogens must acquire host iron. However, the host''s compartmentalization of large amounts of iron in heme, which is bound primarily by hemoglobin in red blood cells, acts as a barrier to bacterial iron assimilation. Bacillus anthracis, the causative agent of the disease anthrax, secretes two NEAT (near iron transporter) proteins, IsdX1 and IsdX2, which scavenge heme from host hemoglobin and promote growth under low iron conditions. The mechanism of heme transfer from these hemophores to the bacterial cell is not known. We present evidence that the heme-bound form of IsdX1 rapidly and directionally transfers heme to IsdC, a NEAT protein covalently attached to the cell wall, as well as to IsdX2. In both cases, the transfer of heme is mediated by a physical association between the donor and recipient. Unlike Staphylococcus aureus, whose NEAT proteins acquire heme from hemoglobin directly at the bacterial surface, B. anthracis secretes IsdX1 to capture heme in the extracellular milieu and relies on NEAT-NEAT interactions to deliver the bound heme to the envelope via IsdC. Understanding the mechanism of NEAT-mediated iron transport into pathogenic Gram-positive bacteria may provide an avenue for the development of therapeutics to combat infection.     

Red blood cells upregulate cytoprotective proteins and the labile iron pool in dividing human T cells despite a reduction in oxidative stress

We have recently reported that red blood cells (RBC) promote T cell growth and survival by inhibiting activation-induced T cell death. In the present study, we have examined parameters of oxidative stress and intracellular iron in activated T cells and correlated these data with the expression of ferritin, heme oxygenase-1 (HO-1), and the transferrin receptor CD71. T cells growing in the presence of RBC had reduced levels of reactive oxygen species (ROS) and oxidatively modified proteins, suggesting that RBC efficiently counteracted ROS production on the activated T cells. Flow cytometry and immunodetection demonstrated that T cells dividing in the presence of RBC had increased levels of intracellular ferritin rich in L-subunits and HO-1 along with a downmodulation in CD71 expression. Finally, using the fluorescent iron indicator calcein and flow cytometry analysis, we were able to show that a relative amount of the labile iron pool (LIP) was upregulated in T cells growing in the presence of RBC. These findings are consistent with a typical response to iron overload. However, neither heme compounds nor ferric iron reproduced the levels of expansion and survival of T cells induced by intact RBC. Altogether, these data suggest that RBC inhibit apoptosis of activated T cells by a combination of ROS scavenging and upregulation of cytoprotective proteins such as ferritin and HO-1, which may counteract a possible toxic effect of the increased intracellular free iron.     

The interplay between heme iron and protein sulfhydryls in the reaction of dimeric Scapharca inaequivalvis hemoglobin with nitric oxide

The homodimeric hemoglobin from the mollusk Scapharca inaequivalvis possesses a single reactive cysteine residue per monomer, Cys92, which is located in the subunit interface in the vicinity of the heme group. The interplay between the heme iron and Cys92 towards the reaction with NO has been investigated by the combined use of electrospray mass spectrometry, FTIR and UV-Visible spectroscopy. When the ferrous liganded or unliganded protein reacts with free NO in solution Cys92 is not modified, but undergoes nitrosation when the hemoglobin is exposed to the nitric oxide releaser S-nitrosocysteine. When the ferric protein reacts with free NO under anaerobic conditions the heme iron is reduced and Cys92 is nitrosated. At variance with other hemeproteins investigated to date, in Scapharca HbI the heme-iron NO driven reduction is not accompanied by the formation of a ferric iron nitrosyl intermediate in detectable amounts. The results are consistent with the hypothesis that the nitrosating agent is the NO(+) species, which is generated during the NO driven reduction of the ferric heme iron. The possible reaction mechanism is discussed in comparison with recent findings on human hemoglobin and myoglobin.     

Tyrosine residues as redox cofactors in human hemoglobin: implications for engineering nontoxic blood substitutes

Respiratory proteins such as myoglobin and hemoglobin can, under oxidative conditions, form ferryl heme iron and protein-based free radicals. Ferryl myoglobin can safely be returned to the ferric oxidation state by electron donation from exogenous reductants via a mechanism that involves two distinct pathways. In addition to direct transfer between the electron donor and ferryl heme edge, there is a second pathway that involves "through-protein" electron transfer via a tyrosine residue (tyrosine 103, sperm whale myoglobin). Here we show that the heterogeneous subunits of human hemoglobin, the alpha and beta chains, display significantly different kinetics for ferryl reduction by exogenous reductants. By using selected hemoglobin mutants, we show that the alpha chain possesses two electron transfer pathways, similar to myoglobin. Furthermore, tyrosine 42 is shown to be a critical component of the high affinity, through-protein electron transfer pathway. We also show that the beta chain of hemoglobin, lacking the homologous tyrosine, does not possess this through-protein electron transfer pathway. However, such a pathway can be engineered into the protein by mutation of a specific phenylalanine residue to a tyrosine. High affinity through-protein electron transfer pathways, whether native or engineered, enhance the kinetics of ferryl removal by reductants, particularly at low reductant concentrations. Ferryl iron has been suggested to be a major cause of the oxidative toxicity of hemoglobin-based blood substitutes. Engineering hemoglobin with enhanced rates of ferryl removal, as we show here, is therefore likely to result in molecules better suited for in vivo oxygen delivery.     

Switching Patients with Non-Dialysis Chronic Kidney Disease from Oral Iron to Intravenous Ferric Carboxymaltose: Effects on Erythropoiesis-Stimulating Agent Requirements,Costs, Hemoglobin and Iron Status

Background

Patients with non-dialysis-dependent chronic kidney disease (ND-CKD) often receive an erythropoiesis-stimulating agent (ESA) and oral iron treatment. This study evaluated whether a switch from oral iron to intravenous ferric carboxymaltose can reduce ESA requirements and improve iron status and hemoglobin in patients with ND-CKD.

Methods

This prospective, single arm and single-center study included adult patients with ND-CKD (creatinine clearance ≤40 mL/min), hemoglobin 11–12 g/dL and iron deficiency (ferritin <100 μg/L or transferrin saturation <20%), who were regularly treated with oral iron and ESA during 6 months prior to inclusion. Study patients received an intravenous ferric carboxymaltose dose of 1,000 mg iron, followed by a 6-months ESA/ ferric carboxymaltose maintenance regimen (target: hemoglobin 12 g/dL, transferrin saturation >20%). Outcome measures were ESA dose requirements during the observation period after initial ferric carboxymaltose treatment (primary endpoint); number of hospitalizations and transfusions, renal function before and after ferric carboxymaltose administration, number of adverse reactions (secondary endpoints). Hemoglobin, mean corpuscular volume, ferritin and transferrin saturation were measured monthly from baseline until end of study. Creatinine clearance, proteinuria, C-reactive protein, aspartate aminotransferase, alanine aminotransferase and alkaline phosphatase bimonthly from baseline until end of study.

Results

Thirty patients were enrolled (age 70.1±11.4 years; mean±SD). Mean ESA consumption was significantly reduced by 83.2±10.9% (from 41,839±3,668 IU/patient to 6,879±4,271 IU/patient; p<0.01). Hemoglobin increased by 0.7±0.3 g/dL, ferritin by 196.0±38.7 μg/L and transferrin saturation by 5.3±2.9% (month 6 vs. baseline; all p<0.01). No ferric carboxymaltose-related adverse events were reported and no patient withdrew or required transfusions during the study.

Conclusion

Among patients with ND-CKD and stable normal or borderline hemoglobin, switching from oral iron to intravenous ferric carboxymaltose was associated with significant improvements in hematological and iron parameters and a significant reduction in ESA dose requirements in this single-center pilot study.

Trial Registration

ClinicalTrials.gov NCT02232906     

A new sensitive assay reveals that hemoglobin is oxidatively modified in vivo

Free radical formation in heme proteins is recognised as a factor in mediating the toxicity of peroxides in oxidative stress. As well as initiating free radical damage, heme proteins damage themselves. Under extreme conditions, where oxidative stress and low pH coincide (e.g., myoglobin in the kidney following rhabdomyolysis and hemoglobin in the CSF subsequent to subarachnoid hemorrhage), peroxide can induce covalent heme to protein cross-linking. In this paper we show that, even at neutral pH, the heme in hemoglobin is covalently modified by oxidation. The product, which we term OxHm, is a "green heme" iron chlorin with a distinct optical spectrum. OxHm formation can be quantitatively prevented by reductants of ferryl iron, e.g., ascorbate. We have developed a simple, robust, and reproducible HPLC assay to study the extent of OxHm formation in the red cell in vivo. We show that hemoglobin is oxidatively damaged even in normal blood; approximately 1 in 2,000 heme groups exist as OxHm in the steady state. We used a simple model (physical exercise) to demonstrate that OxHm increases significantly during acute oxidative stress. The exercise-induced increase is short-lived, suggesting the existence of an active mechanism for repairing or removing the damaged heme proteins.     

Alteration of sperm whale myoglobin heme axial ligation by site-directed mutagenesis

Three mutant proteins of sperm whale myoglobin (Mb) that exhibit altered axial ligations were constructed by site-directed mutagenesis of a synthetic gene for sperm whale myoglobin. Substitution of distal pocket residues, histidine E7 and valine E11, with tyrosine and glutamic acid generated His(E7)Tyr Mb and Val(E11)Glu Mb. The normal axial ligand residue, histidine F8, was also replaced with tyrosine, resulting in His(F8)Tyr Mb. These proteins are analogous in their substitutions to the naturally occurring hemoglobin M mutants (HbM). Tyrosine coordination to the ferric heme iron of His(E7)Tyr Mb and His(F8)Tyr Mb is suggested by optical absorption and EPR spectra and is verified by similarities to resonance Raman spectral bands assigned for iron-tyrosine proteins. His(E7)Tyr Mb is high-spin, six-coordinate with the ferric heme iron coordinated to the distal tyrosine and the proximal histidine, resembling Hb M Saskatoon [His(beta E7)Tyr], while the ferrous iron of this Mb mutant is high-spin, five-coordinate with ligation provided by the proximal histidine. His(F8)Tyr Mb is high-spin, five-coordinate in both the oxidized and reduced states, with the ferric heme iron liganded to the proximal tyrosine, resembling Hb M Iwate [His(alpha F8)Tyr] and Hb M Hyde Park [His(beta F8)Tyr]. Val(E11)Glu Mb is high-spin, six-coordinate with the ferric heme iron liganded to the F8 histidine. Glutamate coordination to the ferric iron of this mutant is strongly suggested by the optical and EPR spectral features, which are consistent with those observed for Hb M Milwaukee [Val(beta E11)Glu]. The ferrous iron of Val(E11)Glu Mb exhibits a five-coordinate structure with the F8 histidine-iron bond intact.(ABSTRACT TRUNCATED AT 250 WORDS)     

Optical and magnetic resonance studies of formate binding to horse liver catalase and sperm whale myoglobin.

The binding of formate ion, a substrate for the peroxidatic reaction of catalase, has been investigated by magnetic resonance techniques. Comparative studies of formate binding to ferric myoglobin have also been performed. The nuclear magnetic relaxation (NMR) rate of formate and water protons is enhanced by the presence of ferric horse liver catalase. The enhancement is not changed significantly by the addition of cyanide, indicating that water and formate are still bound in the presence of cyanide. Formate proton to heme iron distances determined by magnetic resonance techniques indicate that formate does not directly bind to the heme iron of catalase or myoglobin but to the globin, and NMR relaxation occurs as a result of outersphere mechanisms. Evidence that water forms an innersphere complex with the iron atom of the catalase heme is presented. In similar experiments with ferric myoglobin, the addition of cyanide caused a large decrease in the enhancement of the proton relaxation rate of both formate and water, indicating the displacement of water and formate from the heme and the vicinity of the heme, respectively. Broad, high-spin, ferric ion electron paramagnetic resonance absorptions of catalase and myoglobin at room temperature obtained in the presence and absence of formate show that formate does not alter appreciably the heme environment of catalase or myoglobin or the spin state of the heme iron. Studies on the binding of formate to catalase as monitored by changes in the heme absorption spectrum in the visible region show one-to-one stoichiometry with heme concentration. However, the small changes observed in the visible region of the optical spectrum on addition of formate ion are attributed to a secondary effect of formate on the heme environment, rather than direct binding of formate to the heme moiety.     

Coordination structure of the ferric heme iron in engineered distal histidine myoglobin mutants.

Recombinant human myoglobin mutants with the distal His residue (E7, His64) replaced by Leu, Val, or Gln residues were prepared by site-directed mutagenesis and expression in Escherichia coli. Electronic and coordination structures of the ferric heme iron in the recombinant myoglobin proteins were examined by optical absorption, EPR, 1H NMR, magnetic circular dichroism, and x-ray spectroscopy. Mutations, His-->Val and His-->Leu, remove the heme-bound water molecule resulting in a five-coordinate heme iron at neutral pH, while the heme-bound water molecule appears to be retained in the engineered myoglobin with His-->Gln substitution as in the wild-type protein. The distal Val and distal Leu ferric myoglobin mutants at neutral pH exhibited EPR spectra with g perpendicular values smaller than 6, which could be interpreted as an admixture of intermediate (S = 3/2) and high (S = 5/2) spin states. At alkaline pH, the distal Gln mutant is in the same so-called "hydroxy low spin" form as the wild-type protein, while the distal Leu and distal Val mutants are in high spin states. The ligand binding properties of these recombinant myoglobin proteins were studied by measurements of azide equilibrium and cyanide binding. The distal Leu and distal Val mutants exhibited diminished azide affinity and extremely slow cyanide binding, while the distal Gln mutant showed azide affinity and cyanide association rate constants similar to those of the wild-type protein.     

Heme oxygenase-2 knockout neurons are less vulnerable to hemoglobin toxicity

When cortical neurons are exposed to hemoglobin, they undergo oxidative stress that ultimately results in iron-dependent cell death. Heme oxygenase (HO)-2 is constitutively expressed in neurons and catalyzes heme breakdown. Its role in the cellular response to hemoglobin is unclear. We tested the hypothesis that HO-2 attenuates hemoglobin neurotoxicity by comparing reactive oxygen species (ROS) formation and cell death in wild-type and HO-2 knockout cortical cultures. Consistent with prior observations, hemoglobin increased ROS generation, detected by fluorescence intensity after dihydrorhodamine 123 or dichlorofluorescin-diacetate loading, in wild-type neurons. This fluorescence was significantly attenuated in cultures prepared from HO-2 knockout mice, and cell death as determined by propidium iodide staining was decreased. In other experiments, hemoglobin exposure was continued for 19 h; cell death as quantified by LDH release was decreased in knockout cultures, and was further diminished by treatment with the HO inhibitor tin protoporphyrin IX. In contrast, HO-2 knockout neurons were more vulnerable than wild-type neurons to inorganic iron. HO-1, ferritin, and superoxide dismutase expression in HO-2 -/- cultures did not differ significantly from that observed in HO-2 +/+ cultures; cellular glutathione levels were slightly higher in knockout cultures. These results suggest that heme breakdown by heme oxygenase accelerates the oxidative neurotoxicity of hemoglobin, and may contribute to neuronal injury after CNS hemorrhage.     

Mitofusin 2 deficiency leads to oxidative stress that contributes to insulin resistance in rat skeletal muscle cells

Mitofusin 2 (Mfn2) is a dynamin-like protein anchored in the outer mitochondrial membrane that plays a crucial role in ensuring optimal mitochondrial morphological homeostasis. It has been shown that reduced expression of Mfn2 is associated with insulin resistance, but the mechanism is still unclear. We investigated whether Mfn2 deficiency leads to impaired insulin sensitivity via elevated oxidative stress. L6 skeletal muscle cells were treated with palmitate and Mfn2 expression was repressed by transfection with antisense Mfn2. Levels of antioxidant enzymes, reactive oxygen species (ROS), the phosphorylation of c-Jun N-terminal Kinase (JNK) and nuclear factor-κB (NF-κB) and the mitochondrial membrane potential (Δψ_m) were measured. The results showed palmitate-induced insulin resistance of skeletal muscle cells was accompanied by Mfn2 repression. Meanwhile, the cells had decreased Δψ_m and activity of antioxidant enzymes which could increase production of ROS, phosphorylation of JNK and NF-κB. When Mfn2 was up-regulated in palmitate-treated cells, oxidative stress and insulin resistance was alleviated. Furthermore, knock-down of Mfn2 in control cells enhanced oxidative stress. Mfn2 deficiency led to increased superoxide concentration and activation of JNK as well as NF-κB associated with insulin signaling. In conclusion, Mfn2 is a potent repressor for oxidative stress and regulation of Mfn2 expression may prove to be a potential method to circumvent insulin resistance.     

X-ray crystal structure of the fluoride derivative of Aplysia limacina ferric myoglobin at 2.0 A resolution. Stabilization of the fluoride ion by hydrogen bonding to Arg66 (E10)

The X-ray crystal structure of the fluoride derivative of Aplysia limacina ferric myoglobin has been solved and refined at 2.0 A resolution; the crystallographic R-factor is 13.6%. The fluoride ion binds to the sixth co-ordination position of the heme iron, 2.2 A from the metal. Binding of the negatively charged ligand on the distal side of the heme pocket of this myoglobin, which lacks the distal His, is associated with a network of hydrogen bonds that includes the fluoride ion, the residue Arg66 (E10), the heme propionate III, three ordered water molecules and backbone or side-chain atoms from the CD region. A comparison of fluoride and oxygen dissociation rate constants of A. limacina myoglobin, sperm whale (Physeter catodon) myoglobin and Glycera dibranchiata monomeric hemoglobin, suggests that the conformational readjustment of Arg66 (E10) in A. limacina myoglobin may represent the molecular basis for ligand stabilization, in the absence of a hydrogen-bond donor residue at the distal E7 position.     

Intra- and intermolecular oxidation of oxymyoglobin and oxyhemoglobin induced by hydroxyl and carbonate radicals

The mechanism of the reactions of myoglobin and hemoglobin with *OH and CO3*- in the presence of oxygen was studied using pulse and gamma-radiolysis. Unlike *NO2, which adds to the porphyrin iron, *OH and CO3*- form globin radicals. These secondary radicals oxidize the Fe(II) center through both intra- and intermolecular processes. The intermolecular pathway was further demonstrated when BSA radicals derived from *OH or CO3*- oxidized oxyhemoglobin and oxymyoglobin to their respective ferric states. The oxidation yields obtained by pulse radiolysis were lower compared to gamma-radiolysis, where the contribution of radical-radical reactions is negligible. Full oxidation yields by *OH-derived globin radicals could be achieved only at relatively high concentrations of the heme protein mainly via an intermolecular pathway. It is suggested that CO3*- reaction with the protein yields Tyr and/or Trp-derived phenoxyl radicals, which solely oxidize the porphyrin iron under gamma-radiolysis conditions. The *OH particularly adds to aromatic residues, which can undergo elimination of H2O forming the phenoxyl radical, and/or react rapidly with O2 yielding peroxyl radicals. The peroxyl radical can oxidize a neighboring porphyrin iron and/or give rise to superoxide, which neither oxidize nor reduce the porphyrin iron. The potential physiological implications of this chemistry are that hemoglobin and myoglobin, being present at relatively high concentrations, can detoxify highly oxidizing radicals yielding the respective ferric states, which are not toxic.     

Axial coordination of ferric Aplysia myoglobin

Resonance Raman spectra of ferric Aplysia myoglobin in the ligand-free and the azide-bound forms have been studied over a wide pH range to determine the coordination states of the heme iron atom. In the hydroxide form at high pH (approximately 9) the iron is six-coordinate and is in a high/low spin equilibrium. As the pH is lowered below the acid/alkaline transition (pKa = 7.5), the heme becomes five-coordinate. When the pH is lowered even further no other changes in the resonance Raman spectrum are detected; thus, the heme remains five-coordinate down to pH 4, the lowest value studied. For ferric azide-bound Aplysia myoglobin, the iron is six-coordinate in a high/low spin equilibrium at all pH values (4.8-9). These data indicate (i) that the unusual reactivity toward azide previously observed at neutral pH is indeed related to the absence of a coordinated water molecule, and (ii) that causes other than the heme coordination are responsible for the spectral differences and the ligand-binding kinetics differences observed below pH 6.     

Oxidation of thiamine on reaction with nitrogen dioxide generated by ferric myoglobin and hemoglobin in the presence of nitrite and hydrogen peroxide

It is shown that nitrogen dioxide oxidizes thiamine to thiamine disulfide, thiochrome, and oxodihydrothiochrome (ODTch). The latter is formed during oxidation of thiochrome by nitrogen dioxide. Nitrogen dioxide was produced by incubation of nitrite with horse ferric myoglobin and human hemoglobin in the presence of hydrogen peroxide. After addition of tyrosine or phenol to aqueous solutions containing oxoferryl forms of the hemoproteins, thiamine, and nitrite, the yield of thiochrome greatly increased, whereas the yield of ODTch decreased. In the presence of high concentrations of tyrosine or phenol compounds ODTch was not formed at all. The neutral form of thiamine with the closed thiazole cycle and minor tricyclic form of thiamine do not enter the heme pocket of the protein and do not interact with the oxoferryl heme complex Fe(IV=O) or porphyrin radical. The tricyclic form of thiamine is oxidized to thiochrome by tyrosyl radicals located on the surface of the hemoprotein. The thiol form of thiamine is oxidized to thiamine disulfide by both hemoprotein tyrosyl radicals and oxoferryl heme complexes. Nitrite and also tyrosine, tyramine, and phenol readily penetrate into the heme pocket of the protein and reduce the oxyferryl complex to ferric cation. These reactions yield nitrogen dioxide as well as tyrosyl and phenoxyl radicals of tyrosine molecules and phenol compounds, respectively. Tyrosyl and phenoxyl radicals of low molecular weight compounds oxidize thiamine only to thiochrome and thiamine disulfide. The effect of oxoferryl forms of myoglobin and hemoglobin, nitrogen dioxide, and phenol on thiamine oxidative transformation as well as antioxidant properties of the hydrophobic thiamine metabolites thiochrome and ODTch are discussed.     

Effects of Resistance Exercise on Iron Absorption and Balance in Iron-Deficient Rats

We have previously reported that resistance exercise improved the iron status in iron-deficient rats. The current study investigated the mechanisms underlying this exercise-related effect. Male 4-week-old rats were divided into a group sacrificed at the start (week 0) (n?=?7), a group maintained sedentary for 6 weeks (S) or a group that performed exercise for 6 weeks (E), and all rats in the latter groups were fed an iron-deficient diet (12 mg iron/kg) for 6 weeks. The rats in the E group performed climbing exercise (5 min?×?6 sets/day, 3 days/week). Compared to the week 0 rats, the rats in the S and E groups showed lower tissue iron content, and the hematocrit, hemoglobin, plasma iron, and transferrin saturation values were all low. However, the tissue iron content and blood iron status parameters, and the whole body iron content measured using the whole body homogenates of the rats, did not differ between the S group and the E group. The messenger RNA (mRNA) expression levels of hepcidin, duodenal cytochrome b, divalent metal transporter 1, and ferroportin 1 did not differ between the S group and the E group. The apparent absorption of iron was significantly lower in the E group than in the S group. Therefore, it was concluded that resistance exercise decreases iron absorption, whereas the whole body iron content is not affected, and an increase in iron recycling in the body seems to be responsible for this effect.     

The solution structure of the recombinant hemoglobin from the cyanobacterium Synechocystis sp. PCC 6803 in its hemichrome state

The product of the cyanobacterium Synechocystis sp. PCC 6803 gene slr2097 is a 123 amino acid polypeptide chain belonging to the truncated hemoglobin family. Recombinant, ferric heme-reconstituted Synechocystis sp. PCC 6803 hemoglobin displays bis-histidine coordination of the iron ion. In addition, this protein is capable of covalently attaching a reactive histidine to the heme 2-vinyl group. The structure of the protein in the low-spin ferric state with intact vinyl substituents was solved by NMR methods. It was found that the structure differs from that of known truncated hemoglobins primarily in the orientation of the E helix, which carries His46 (E10) as the distal ligand to the iron; the length and orientation of the F helix, which carries His70 (F8) as the proximal ligand to the iron; and the H-helix, which carries His117 (H16), the reactive histidine. Regions of enhanced flexibility include the short A helix, the loop connecting the E and F helices, and the last seven residues at the carboxy end. The structural data allowed for the rationalization of physical properties of the cyanobacterial protein, such as fast on-rate for small ligand binding, unstable apoprotein fold, and cross-linking ability. Comparison to the truncated hemoglobin from the green alga Chlamydomonas eugametos also suggested how the endogenous hexacoordination affected the structure.