Nitrosyliron(III) hemoglobin: autoreduction and spectroscopy

Spectroscopic Characterization of a Diferric Mycobacterial Hemerythrin-Like Protein with Unprecedented Reactivity toward Nitric Oxide

Hemerythrin-like proteins (HLPs) are broadly distributed across taxonomic groups and appear to play highly diverse functional roles in prokaryotes. Mycobacterial HLPs contribute to the survival of these pathogenic bacteria in mammalian macrophages, but their modes of action remain unclear. A recent crystallographic characterization of Mycobacterium kansasii HLP (Mka-HLP) revealed the unexpected presence of a tyrosine sidechain (Tyr54) near the coordination sphere of one of the two iron centers. Here, we show that Tyr54 is a true ligand to the Fe2(III) ion which, in conjunction with the presence of a μ-oxo group bridging the two iron(III), brings unique reactivity toward nitric oxide (NO). Monitoring the titration of Mka-HLP with NO by Fourier-transform infrared and electron paramagnetic resonance spectroscopies shows that both diferric and diferrous forms of Mka-HLP accumulate an uncoupled high-spin and low-spin {FeNO}⁷ pair. We assign the reactivity of the diferric protein to an initial radical reaction between NO and the μ-oxo bridge to form nitrite and a mixed-valent diiron center that can react further with NO. Amperometric measurements of NO consumption by Mka-HLP confirm that this reactivity can proceed at low micromolar concentrations of NO, before additional NO consumption, supporting a NO scavenging role for mycobacterial HLPs.

Protein nitration induced by Hemin/NO: A complementary mechanism through the catalytic functions of hemin and NO-scavenging

Hemin and heme-peroxidases have been considered essential catalysts for the nitrite/hydrogen peroxide (H₂O₂)-mediated protein nitration in vitro, understood as one of the main pathways for protein modification in biological systems. However, the role of nitric oxide (^●NO) in the heme/hemin-induced protein nitration has not been studied in-depth. This is despite its reductive nitrosylating effects following binding to hemin and the possible involvement of the reactive nitrogen species in the nitration of various functional proteins. Here, the ^●NO-binding affinity of hemin has been studied along with the influence of ^●NO on the internalization of hemin into MDA-MB-231 cells and the accompanying changes in the profile of intracellular nitrated proteins. Moreover, to further understand the mechanism involved, bovine serum albumin (BSA) nitration was studied after treatment with hemin and ^●NO, with an investigation of the effects of pH of the reaction medium, generation of H₂O₂, and the oxidation of the tyrosine residues as the primary sites for the nitration. We demonstrated that hemin nitrosylation enhanced its cellular uptake and induced the one-electron oxidation and nitration of different intracellular proteins along with its ^●NO-scavenging efficiency. Moreover, the hemin/NO-mediated BSA nitration was proved to be dependent on the concentration of ^●NO and the pH of the reaction medium, with a vital role being played by the scavenging effects of protein for the free hemin molecules. Collectively, our results reaffirm the involvement of hemin and ^●NO in the nitration mechanism, where the nitrosylation products can induce protein nitration while promoting the effects of the components of the nitrite/H₂O₂-mediated pathway.

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

Covalent attachment of the heme to Synechococcus hemoglobin alters its reactivity toward nitric oxide

The cyanobacterium Synechococcus sp. PCC 7002 produces a monomeric hemoglobin (GlbN) implicated in the detoxification of reactive nitrogen and oxygen species. GlbN contains a b heme, which can be modified under certain reducing conditions. The modified protein (GlbN-A) has one heme-histidine C-N linkage similar to the C-S linkage of cytochrome c. No clear functional role has been assigned to this modification. Here, optical absorbance and NMR spectroscopies were used to compare the reactivity of GlbN and GlbN-A toward nitric oxide (NO). Both forms of the protein are capable of NO dioxygenase activity and both undergo heme bleaching after multiple NO challenges. GlbN and GlbN-A bind NO in the ferric state and form diamagnetic complexes (Fe^III-NO) that resist reductive nitrosylation to the paramagnetic Fe^II-NO forms. Dithionite reduction of Fe^III-NO GlbN and GlbN-A, however, resulted in distinct outcomes. Whereas GlbN-A rapidly formed the expected Fe^II-NO complex, NO binding to Fe^II GlbN caused immediate heme loss and, remarkably, was followed by slow heme rebinding and HNO (nitrosyl hydride) production. Additionally, combining Fe^III GlbN, ¹⁵N-labeled nitrite, and excess dithionite resulted in the formation of Fe^II-H¹⁵NO GlbN. Dithionite-mediated HNO production was also observed for the related GlbN from Synechocystis sp. PCC 6803. Although ferrous GlbN-A appeared capable of trapping preformed HNO, the histidine-heme post-translational modification extinguished the NO reduction chemistry associated with GlbN. Overall, the results suggest a role for the covalent modification in Fe^II GlbNs: protection from NO-mediated heme loss and prevention of HNO formation.

Ferric Heme-Nitrosyl Complexes: Kinetically Robust or Unstable Intermediates?

We have determined a convenient method for the bulk synthesis of high-purity ferric heme-nitrosyl complexes ({FeNO}⁶ in the Enemark-Feltham notation); this method is based on the chemical or electrochemical oxidation of corresponding {FeNO}⁷ precursors. We used this method to obtain the five- and six-coordinate complexes [Fe(TPP)(NO)]⁺ (TPP^2- = tetraphenylporphyrin dianion) and [Fe(TPP)(NO)(MI)]⁺ (MI = 1-methylimidazole) and demonstrate that these complexes are stable in solution in the absence of excess NO gas. This is in stark contrast to the often-cited instability of such {FeNO}⁶ model complexes in the literature, which is likely due to the common presence of halide impurities (although other impurities could certainly also play a role). This is avoided in our approach for the synthesis of {FeNO}⁶ complexes via oxidation of pure {FeNO}⁷ precursors. On the basis of these results, {FeNO}⁶ complexes in proteins do not show an increased stability toward NO loss compared to model complexes. We also prepared the halide-coordinated complexes [Fe(TPP)(NO)(X)] (X = Cl^-, Br^-), which correspond to the elusive, key reactive intermediate in the so-called autoreduction reaction, which is frequently used to prepare {FeNO}⁷ complexes from ferric precursors. All of the complexes were characterized using X-ray crystallography, UV-vis, IR, and nuclear resonance vibrational spectroscopy (NRVS). On the basis of the vibrational data, further insight into the electronic structure of these {FeNO}⁶ complexes, in particular with respect to the role of the axial ligand trans to NO, is obtained.

The production of nitrous oxide by the heme/nonheme diiron center of engineered myoglobins (Fe(B)Mbs) proceeds through a trans-iron-nitrosyl dimer

Denitrifying NO reductases are transmembrane protein complexes that are evolutionarily related to heme/copper terminal oxidases. They utilize a heme/nonheme diiron center to reduce two NO molecules to N₂O. Engineering a nonheme Fe_B site within the heme distal pocket of sperm whale myoglobin has offered well-defined diiron clusters for the investigation of the mechanism of NO reduction in these unique active sites. In this study, we use FTIR spectroscopy to monitor the production of N₂O in solution and to show that the presence of a distal Fe_B^II is not sufficient to produce the expected product. However, the addition of a glutamate side chain peripheral to the diiron site allows for 50% of a productive single-turnover reaction. Unproductive reactions are characterized by resonance Raman spectroscopy as dinitrosyl complexes, where one NO molecule is bound to the heme iron to form a five-coordinate low-spin {FeNO}⁷ species with ν(FeNO)_heme and ν(NO)_heme at 522 and 1660 cm^–1, and a second NO molecule is bound to the nonheme Fe_B site with a ν(NO)_FeB at 1755 cm^–1. Stopped-flow UV–vis absorption coupled with rapid-freeze-quench resonance Raman spectroscopy provide a detailed map of the reaction coordinates leading to the unproductive iron-nitrosyl dimer. Unexpectedly, NO binding to Fe_B is kinetically favored and occurs prior to the binding of a second NO to the heme iron, leading to a (six-coordinate low-spin heme-nitrosyl/Fe_B-nitrosyl) transient dinitrosyl complex with characteristic ν(FeNO)_heme at 570 ± 2 cm^–1 and ν(NO)_FeB at 1755 cm^–1. Without the addition of a peripheral glutamate, the dinitrosyl complex is converted to a dead-end product after the dissociation of the proximal histidine of the heme iron, but the added peripheral glutamate side chain in Fe_BMb2 lowers the rate of dissociation of the promixal histidine which in turn allows the (six-coordinate low-spin heme-nitrosyl/Fe_B-nitrosyl) transient dinitrosyl complex to decay with production of N₂O at a rate of 0.7 s^–1 at 4 °C. Taken together, our results support the proposed trans mechanism of NO reduction in NORs.

Gas-phase spectroscopy of ferric heme-NO complexes

Weakly bound complexes between ferric heme cations and NO were synthesised in the gas phase from ion-molecule reactions, and their absorption measured based on photodissociation yields. The Soret band, which serves as an important marker band for heme-protein spectroscopy, is maximal at 357±5 nm and significantly blue-shifted compared to ferric heme nitrosyl proteins (maxima between 408 and 422 nm). This is in stark contrast to the Q-band absorption where the protein microenvironment is nearly innocent in perturbing the electronic structure of the porphyrin macrocycle. Photodissociation is primarily through loss of NO. In contrast to the Q-band region, two-photon absorption was seen in the Soret band despite NO loss only requiring ∼1 eV. A model based on intersystem crossing to a long-lived triplet state where a barrier has to be surmounted is suggested. Finally, we summarise the measured absorption maxima of heme and its complexes with amino acids and NO.

Quantitative detection of nitroxyl upon trapping with glutathione and labeling with a specific fluorogenic reagent

Donors of nitroxyl (HNO) have shown promise for treatment of stroke, heart failure, alcoholism and cancer. However, comparing the pharmacological capacities of various donors is difficult without first quantifying the amount of HNO released from each donor. Detection and quantitation of HNO has been complicated by the rapid self-consumption of HNO through irreversible dimerization, poor selectivity of trapping agents against other nitrogen oxides, and/or low sensitivity towards HNO. Here, an assay is described for the trapping of HNO by glutathione (GSH) followed by labeling of GSH with the fluorogenic agent, naphthalene-2,3-dicarboxaldehyde (NDA), and subsequent quantitation by fluorescence difference. The newly developed assay was used to validate the pH-dependence of HNO release from isopropylamine NONOate (IPA/NO), which is a dual donor of HNO and NO at physiological pH. Furthermore, varied assay conditions were utilized to suggest the ratios of the products of the reaction of GSH with HNO. At intracellular concentrations of GSH, the disulfide (GSSG) was the major product, but significant concentrations of glutathione sulfinamide (GS(O)NH₂) were also detected. This suggests that GS(O)NH₂, which is a selective biomarker of HNO, may be produced in concentrations that are amenable to in vivo analysis.

The specificity of nitroxyl chemistry is unique among nitrogen oxides in biological systems

The importance of nitric oxide in mammalian physiology has been known for nearly 30 years. Similar attention for other nitrogen oxides such as nitroxyl (HNO) has been more recent. While there has been speculation as to the biosynthesis of HNO, its pharmacological benefits have been demonstrated in several pathophysiological settings such as cardiovascular disorders, cancer, and alcoholism. The chemical biology of HNO has been identified as related to, but unique from, that of its redox congener nitric oxide. A summary of these findings as well as a discussion of possible endogenous sources of HNO is presented in this review.

The quaternary hemoglobin conformation regulates the formation of the nitrite-induced bioactive intermediate and the dissociation of nitric oxide from this intermediate

Deoxyhemoglobin reduces nitrite to nitric oxide (NO). In order to study the effect of the hemoglobin quaternary conformation on the nitrite reaction, we compared T-state deoxyhemoglobin with R-state deoxyhemoglobin produced by reacting hemoglobin with carboxypeptidase-A prior to deoxygenation. The nitrite reaction with deoxyhemoglobin was followed by chemiluminescence, electron paramagnetic resonance and visible spectroscopy. The initial steps in this reaction involve the binding of nitrite to deoxyhemoglobin followed by the formation of an electron delocalized metastable intermediate that retains potential NO bioactivity. This reaction is shown by visible spectroscopy to occur 5.6 times faster in the R-state than in the T-state. However, the dissociation of NO from the delocalized intermediate is shown to be facilitated by the T-quaternary conformation with a 9.6 fold increase in the rate constant. The preferred NO-release in the T-state, which has a higher affinity for the membrane, can result in the NO diffusing out of the RBC and being released to the vasculature at low partial pressures of oxygen.

Heme-based sensing by the mammalian circadian protein CLOCK

Heme is emerging as a key player in the synchrony of circadian-coupled transcriptional regulation. Current evidence suggests that levels of circadian-linked transcription are regulated in response to both the availability of intracellular heme and heme-based sensing of carbon monoxide (CO) and possibly nitric oxide (NO). The protein CLOCK is central to the regulation and maintenance of circadian rhythms in mammals. CLOCK comprises two PAS domains, each with a heme binding site. Our studies focus on the functionality of the murine CLOCK PAS-A domain (residues 103-265). We show that CLOCK PAS-A binds iron(III) protoporhyrin IX to form a complex with 1:1 stoichiometry. Optical absorbance and resonance Raman studies reveal that the heme of ferric CLOCK PAS-A is a six-coordinate, low-spin complex whose resonance Raman signature is insensitive to pH over the range of protein stability. Ferrous CLOCK PAS-A is a mixture of five-coordinate, high-spin and six-coordinate, low-spin complexes. Ferrous CLOCK PAS-A forms complexes with CO and NO. Ferric CLOCK PAS-A undergoes reductive nitrosylation in the presence of NO to generate a CLOCK PAS-A-NO, which is a five-coordinate {FeNO}(7) complex. Formation of the highly stable {FeNO}(7) heme complex from either ferrous or ferric heme makes possible the binding of NO at very low concentration, a characteristic of NO sensors. Comparison of the spectroscopic properties and CO-binding kinetics of CLOCK PAS-A with other CO sensor proteins reveals that CLOCK PAS-A exhibits chemical properties consistent with a heme-based gas sensor protein.

Reductive coupling of nitrogen monoxide (*NO) facilitated by heme/copper complexes

The interactions of nitrogen monoxide (*NO; nitric oxide) with transition metal centers continue to be of great interest, in part due to their importance in biochemical processes. Here, we describe *NO((g)) reductive coupling chemistry of possible relevance to that process (i.e., nitric oxide reductase (NOR) biochemistry), which occurs at the heme/Cu active site of cytochrome c oxidases (CcOs). In this report, heme/Cu/*NO((g)) activity is studied using 1:1 ratios of heme and copper complex components, (F(8))Fe (F(8) = tetrakis(2,6-difluorophenyl)porphyrinate(2-)) and [(tmpa)Cu(I)(MeCN)](+) (TMPA = tris(2-pyridylmethyl)amine). The starting point for heme chemistry is the mononitrosyl complex (F(8))Fe(NO) (lambda(max) = 399 (Soret), 541 nm in acetone). Variable-temperature (1)H and (2)H NMR spectra reveal a broad peak at delta = 6.05 ppm (pyrrole) at room temperature (RT), which gives rise to asymmetrically split pyrrole peaks at 9.12 and 8.54 ppm at -80 degrees C. A new heme dinitrosyl species, (F(8))Fe(NO)(2), obtained by bubbling (F(8))Fe(NO) with *NO((g)) at -80 degrees C, could be reversibly formed, as monitored by UV-vis (lambda(max) = 426 (Soret), 538 nm in acetone), EPR (silent), and NMR spectroscopies; that is, the mono-NO complex was regenerated upon warming to RT. (F(8))Fe(NO)(2) reacts with [(tmpa)Cu(I)(MeCN)](+) and 2 equiv of acid to give [(F(8))Fe(III)](+), [(tmpa)Cu(II)(solvent)](2+), and N(2)O((g)), fitting the stoichiometric *NO((g)) reductive coupling reaction: 2*NO((g)) + Fe(II) + Cu(I) + 2H(+) --> N(2)O((g)) + Fe(III) + Cu(II) + H(2)O, equivalent to one enzyme turnover. Control reaction chemistry shows that both iron and copper centers are required for the NOR-type chemistry observed and that, if acid is not present, half the *NO is trapped as a (F(8))Fe(NO) complex, while the remaining nitrogen monoxide undergoes copper complex promoted disproportionation chemistry. As part of this study, [(F(8))Fe(III)]SbF(6) was synthesized and characterized by X-ray crystallography, along with EPR (77 K: g = 5.84 and 6.12 in CH(2)Cl(2) and THF, respectively) and variable-temperature NMR spectroscopies. These structural and physical properties suggest that at RT this complex consists of an admixture of high and intermediate spin states.

Quantification of intermediates formed during the reduction of nitrite by deoxyhemoglobin

Nitric oxide (NO) plays a crucial role in human physiology by regulating vascular tone and blood flow. The short life-span of NO in blood requires a mechanism to retain NO bioactivity in the circulation. Recent studies have suggested a mechanism involving the reduction of nitrite back to NO by deoxyhemoglobin in RBCs. A role for RBCs in transporting NO must, however, bypass the scavenging of NO in RBCs by hemoglobin. To understand how the nitrite reaction can deliver bioactive NO to the vasculature, we have studied the intermediates formed during the reaction. A reliable measure of the total concentration of heme-associated nitrite/NO intermediates formed was provided by combining filtration to measure free nitrite by chemiluminescence and electron paramagnetic resonance to measure the final product Hb(II)NO. By modifying the chemiluminescence method used to detect NO, we have been able to identify two intermediates: 1) a heme-associated nitrite complex that is released as NO in acid solution in the presence of ascorbate and 2) an intermediate that releases NO at neutral pH in the presence of ferricyanide when reacted with an Fe(III) ligand like azide. This species designated as "Hb(II)NO(+) (<--)(-->)Hb(III)NO" has properties of both isomeric forms resulting in a slower NO dissociation rate and much higher stability than Hb(III)NO, but provides a potential source for bioactive NO, which can be released from the RBC. This detailed analysis of the nitrite reaction with deoxyHb provides important insights into the mechanism for nitrite induced vasodilation by RBCs.

The conformational manifold of ferricytochrome c explored by visible and far-UV electronic circular dichroism spectroscopy

The oxidized state of cytochrome c is a subject of continuous interest, owing to the multitude of conformations which the protein can adopt in solution and on surfaces of artificial and cell membranes. The structural diversity corresponds to a variety of functions in electron transfer, peroxidase and apoptosis processes. In spite of numerous studies, a comprehensive analysis and comparison of native and non-native states of ferricytochrome c has thus far not been achieved. This results in part from the fact that the influence of solvent conditions (i.e., ionic strength, anion concentration, temperature dependence of pH values) on structure, function and equilibrium thermodynamics has not yet been thoroughly assessed. The current study is a first step in this direction, in that it provides the necessary experimental data to compare different non-native states adopted at high temperature and alkaline pH. To this end, we employed visible electronic circular dichroism (ECD) and absorption spectroscopy to probe structural changes of the heme environment in bovine and horse heart ferricytochrome c as a function of temperature between 278 and 363 K at different neutral and alkaline pH values. A careful selection of buffers enabled us to monitor the partial unfolding of the native state at room temperature while avoiding a change to an alkaline state at high temperatures. We found compelling evidence for the existence of a thermodynamic intermediate of the thermal unfolding/folding process, termed III h, which is structurally different from the alkaline states, IV 1 and IV 2, contrary to current belief. At neutral or slightly acidic pH, III h is populated in a temperature region between 320 and 345 K. The unfolded state of the protein becomes populated at higher temperatures. The ECD spectra of the B-bands of bovine and horse heart cytochrome c (pH 7.0) exhibit a pronounced couplet that is maintained below 343 K, before protein unfolding replaces it by a rather strong positive Cotton band. A preliminary vibronic analysis of the B-band profile reveals that the couplet reflects a B-band splitting of 350 cm (-1), which is mostly of electronic origin, due to the internal electric field in the heme cavity. Our results suggest that the conformational transition from the native state, III, into a thermally activated intermediate state, III h, does not substantially affect the internal electric field and causes only moderate rearrangements of the heme pocket, which involves changes, rather than a rupture, of the Fe (3+)-M80 linkage. In the unfolded state, as well as in the alkaline states IV and V, the band splitting is practically eliminated, but the positive Cotton effect observed for the B-band suggests that the proximal environment, encompassing H18 and the two cysteine residues 14 and 17, is most likely still intact and covalently bound to the heme chromophore. Both alkaline states IV and V were found to melt via intermediate states. Unfolded states probed at neutral and alkaline pH can be discriminated, owing to the different intensities of the Cotton bands of the respective B-band transitions. Differences between the ECD intensities of the B-bands of the different unfolded states and alkaline states most likely reflect different degrees of openness of the corresponding heme crevice.

The transcription regulator RcoM-2 from Burkholderia xenovorans is a cysteine-ligated hemoprotein that undergoes a redox-mediated ligand switch

Spectroscopic characterization of the newly discovered heme-PAS domain sensor protein BxRcoM-2 reveals that this protein undergoes redox-dependent ligand switching and CO- and NO-induced ligand displacement. The aerobic bacterium Burkholderia xenovorans expresses two homologous heme-containing proteins that promote CO-dependent transcription in vivo. These regulators of CO metabolism, BxRcoM-1 and BxRcoM-2, are gas-responsive heme-PAS domain proteins like mammalian neuronal PAS domain protein 2 (NPAS2) and the direct oxygen sensor from Escherichia coli ( EcDos). BxRcoM-2 was studied using electronic absorption, MCD, resonance Raman, and EPR spectroscopies. In the Fe(III) oxidation state, the heme is low-spin and six-coordinate with a cysteine(thiolate) as one of the two ligands. The sixth ligand is a histidine (His (74)), which is present in all states of the protein that were studied. Reduction to the Fe(II) oxidation state results in replacement of the cysteine(thiolate) with a neutral thioether ligand, Met (104). CO and NO bind to the Fe(II) BxRcoM-2 heme opposite the histidine ligand. Thus, BxRcoM-2 employs coordination state changes similar to those known for CO-sensing CooA, with redox-dependent loss of a cysteine(thiolate) ligand and displacement of a relatively weakly bound axial ligand by the effector gas molecule. Like EcDos, the weakly bound axial ligand that is displaced is methionine.

Optical band splitting and electronic perturbations of the heme chromophore in cytochrome C at room temperature probed by visible electronic circular dichroism spectroscopy

We have measured the electronic circular dichroism (ECD) of the ferri- and ferro-states of several natural cytochrome c derivatives (horse heart, chicken, bovine, and yeast) and the Y67F mutant of yeast in the region between 300 and 750 nm. Thus, we recorded the ECD of the B- and Q-band region as well as the charge-transfer band at approximately 695 nm. The B-band region of the ferri-state displays a nearly symmetric couplet at the B0-position that overlaps with a couplet 790 cm-1 higher in energy, which we assigned to a vibronic side-band transition. For the ferro-state, the couplet is greatly reduced, but still detectable. The B-band region is dominated by a positive Cotton effect at energies lower than B0 that is attributed to a magnetically allowed iron-->heme charge-transfer transition as earlier observed for nitrosyl myoglobin and hemoglobin. The Q-band region of the ferri-state is poorly resolved, but displays a pronounced positive signal at higher wavenumbers. This must result from a magnetically allowed transition, possibly from the methionine ligand to the dxy-hole of Fe3+. For the ferro-state, the spectra resolve the vibronic structure of the Qv-band. A more detailed spectral analysis reveals that the positively biased spectrum can be understood as a superposition of asymmetric couplets of split Q0 and Qv-states. Substantial qualitative and quantitative differences between the respective B-state and Q-state ECD spectra of yeast and horse heart cytochrome c can clearly be attributed to the reduced band splitting in the former, which results from a less heterogeneous internal electric field. Finally, we investigated the charge-transfer band at 695 nm in the ferri-state spectrum and found that it is composed of at least three bands, which are assignable to different taxonomic substates. The respective subbands differ somewhat with respect to their Kuhn anisotropy ratio and their intensity ratios are different for horse and yeast cytochrome c. Our data therefore suggests different substate populations for these proteins, which is most likely assignable to a structural heterogeneity of the distal Fe-M80 coordination of the heme chromophore.

S-Nitrosohemoglobin: A mechanism for its formation in conjunction with nitrite reduction by deoxyhemoglobin

The formation of S-nitrosohemoglobin (SNOHb) in red cells has been a major point of contention among researchers in this field. We have delineated a new mechanism for the formation of SNOHb coupled to nitrite reduction by deoxygenated hemoglobin chains at low oxygen pressures. The establishment of this mechanism required the development of a chemiluminescence assay utilizing Cu(II) and ascorbic acid to directly measure nitrosothiols without any interference from nitrite or heme-NO. The formation of SNOHb was shown to involve a dominant nitrite-reduction intermediate with electron delocalized between the heme iron and the bound NO. The possible mechanisms for the formation of SNOHb from this intermediate in the absence of oxygen are discussed including the role for an expansion of the electron delocalized intermediate to include the beta-93 cysteine residue. This extended delocalization was supported by a direct reaction with unbound NO, simultaneously producing SNOHb and Hb(II)NO, when NO reacts with metHb. The SNOHb found in red cells in vivo can, thus, be explained as originating from nitrite reduction that takes place at reduced oxygen pressures.

Regulation of the monomer-dimer equilibrium in inducible nitric-oxide synthase by nitric oxide

The oxygenase domain of inducible nitric-oxide synthase exists as a functional tight homodimer in the presence of the substrate L-arginine and the cofactor tetrahydrobiopterin (H4B). In the absence of H4B, the enzyme is a mixture of monomer and loose dimer. We show that exposure of H4B-free enzyme to NO induces dissociation of the loose dimer into monomers in a reaction that follows single exponential decay kinetics with a lifetime of approximately 300 min. It is followed by a faster autoreduction reaction of the heme iron with a lifetime of approximately 30 min and the concurrent breakage of the proximal iron-thiolate bond, forming a five-coordinate NO-bound ferrous species. Mass spectrometry revealed that the NO-induced monomerization is associated with intramolecular disulfide bond formation between Cys104 and Cys109, located in the zinc-binding motif. The regulatory effect of NO as a dimer inhibitor is discussed in the context of the structure/function relationships of this enzyme.

Kinetics and mechanistic studies of the reactions of metleghemoglobin, ferrylleghemoglobin, and nitrosylleghemoglobin with reactive nitrogen species

It is now established that nitrogen monoxide is produced not only in animals but also in plants. However, much less is known about the pathways of generation and the functions of NO. in planta. One of the possible targets of NO. is leghemoglobin (Lb), the hemoprotein found in high concentrations in the root nodules of legumes that establish a symbiosis with nitrogen-fixing bacteria. In analogy to hemoglobin and myoglobin, we have shown that different forms of Lb react not only with NO. but also with so-called reactive nitrogen species derived from it, among others peroxynitrite and nitrite. Because of the wider active-site pocket, the rate constants measured in this work for NO. and for nitrite binding to metLb are 1 order of magnitude larger than the corresponding values for binding of these species to metmyoglobin and methemoglobin. Moreover, we showed that reactive nitrogen species are able to react with two forms of Lb that are produced in vivo but that cannot bind oxygen: ferrylLb is reduced by NO. and nitrite, and nitrosylLb is oxidized by peroxynitrite. The second-order rate constants of these reactions are on the order of 10(2), 10(6), and 10(5) M-1 s-1, respectively. In all cases, the final reaction product is metLb, a further Lb form that has been detected in vivo. Since a specific reductase is active in nodules, which reduces metLb, reactive nitrogen species could contribute to the recycling of these inactive forms to regenerate deoxyLb, the oxygen-binding form of Lb.

Nitrosyl heme production compared in endotoxemic and hemorrhagic shock

We compared nitric oxide production and nitrosyl hemoglobin steady state concentrations during the early phases of endotoxemic and hemorrhagic shock of equivalent severity. Sprague-Dawley rats were randomly assigned to (1) sham-operated control, (2) hemorrhage, and (3) intravenous endotoxin. Electron paramagnetic resonance spectroscopy was used to measure NO in the vasculature (binding to hemoglobin) and in the liver (binding to cytochrome P450). Despite similar changes in cardiorespiratory variables and identical microvascular pO(2), nitrosyl hemoglobin concentrations were significantly higher in endotoxemic rats than in rats in hemorrhagic shock, suggesting increased rates of NO production. A substantial venous minus arterial concentration gradient was observed for nitrosyl hemoglobin. This increased in line with the plasma total nitrite + nitrate concentration. Nitrosyl hemoglobin formation is likely to occur predominantly in the venous pool, suggesting that removal of NO from hemoglobin in the presence of oxygen may be faster than previously thought. In the liver, an increase in intracellular heme-NO complexes was detected in endotoxemic rats compared with rats in hemorrhagic shock; this was associated with increased reduction of the mitochondrial respiratory chain and is suggestive of NO inhibition of mitochondrial respiration.

Five-coordinate Fe(III)NO and Fe(II)CO porphyrinates: where are the electrons and why does it matter?

Recent years have seen dramatic growth in our understanding of the biological roles of nitric oxide (NO). Yet, the fundamental underpinnings of its reactivities with transition metal centers in proteins and enzymes, the stabilities of their structures, and the relationships between structure and reactivity remains, to a significant extent, elusive. This is especially true for the so-called ferric heme nitrosyls ([FeNO](6) in the Enemark-Feltham scheme). The Fe-CO and C-O bond strengths in the isoelectronic ferrous carbonyl complexes are widely recognized to be inversely correlated and sensitive to structural, environmental, and electronic factors. On the other hand, the Fe-NO and N-O bonds in [FeNO](6) heme complexes exhibit seemingly inconsistent behavior in response to varying structure and environment. This report contains resonance Raman and density functional theory results that suggest a new model for FeNO bonding in five-coordinate [FeNO](6) complexes. On the basis of resonance Raman and FTIR data, a direct correlation between the nu(Fe)(-)(NO) and nu(N)(-)(O) frequencies of [Fe(OEP)NO](ClO(4)) and [Fe(OEP)NO](ClO(4)).CHCl(3) (two crystal forms of the same complex) has been established. Density functional theory calculations show that the relationship between Fe-NO and N-O bond strengths is responsive to FeNO electron density in three molecular orbitals. The highest energy orbital of the three is sigma-antibonding with respect to the entire FeNO unit. The other two comprise a lower-energy, degenerate, or nearly degenerate pair that is pi-bonding with respect to Fe-NO and pi-antibonding with respect to N-O. The relative sensitivities of the electron density distributions in these orbitals are shown to be consistent with all published indicators of Fe-N-O bond strengths and angles, including the examples reported here.

Critical redox and allosteric aspects of nitric oxide interactions with hemoglobin

Nitric oxide (NO) is an important signaling molecule. Relatively long-lived NO adducts at the heme and SH groups of hemoglobin (Hb) could enable NO to carry out long-range signaling functions. In spite of significant advances, there remain as yet unresolved issues regarding the possible role of Hb in moderating NO-signaling events that affect blood pressure regulation. In this review, we summarize recent reports concerning the redox and allosteric aspects of NO/Hb interactions that have advanced our understanding of the physiological significance of NO binding to heme groups (forming NO-Hb) and of reactions promoting formation of S-nitrosated Hb (SNO-Hb). Allosteric mechanisms modify the bioactivity of NO/Hb complexes by altering the lifetime of NO-Hb and the properties of SNO-Hb. Redox reactions are significant because of the complex chemistry possible for NO and its oxidation products. Reactions at ferrous and ferric heme sites have differing consequences and affinities for interactions with NO. Moreover, redox changes at heme groups affect reactivity of SH groups and vice versa. In spite of low levels of NO-Hb and SNO-Hb found in vivo, recent findings do not rule out participation of NO-Hb or SNO-Hb in NO-dependent signaling reactions.

Electrochemical assay for determining nitrosyl derivatives of human hemoglobin: nitrosylhemoglobin and S-nitrosylhemoglobin

Nitric oxide (NO) is an important biological regulator. It can bind to heme iron and form NO+, involved in the synthesis of S-nitrosothiols (-SNOs). NO reacts with human hemoglobin (Hb) to produce the derivatives: S-nitrosylhemoglobin (-SNOHb) and nitrosylhemoglobin (HbNO). At neutral pH values, free NO does not react directly with the -SH groups of Hb. The reductive nitrosylation of Fe(III) heme upon reaction with NO has long been studied, but it is not yet completely known. To quantify the reaction of NO with Hb, we developed a new, sensitive (nanomolar concentration range) electrochemical assay to selectively measure HbNO and -SNOHb. The assay also allows the monitoring of free NO during the reaction with human Fe(III)Hb and Fe(II)HbO(2).

Reactivity Studies of the Fe(III) and Fe(II)NO Forms of Human Neuroglobin Reveal a Potential Role against Oxidative Stress

Neuroglobin, recently discovered in the brain and in the retina of vertebrates, belongs to the class of hexacoordinate globins, in which the distal histidine coordinates the iron center in both the Fe(II) and Fe(III) forms. As for most other hexacoordinate globins, the physiological function of neuroglobin is still unclear, but seems to be related to neuronal survival following acute hypoxia. In this study, we have addressed the question whether human neuroglobin could act as a scavenger of toxic species, such as nitrogen monoxide, peroxynitrite, and hydrogen peroxide, which are generated at high levels in the brain during hypoxia; we have also investigated the kinetics of the reactions of its Fe(III) (metNGB) and Fe(II)NO forms with several reagents. Binding of cyanide or NO* to metNGB follows bi-exponential kinetics, showing the existence of two different protein conformations. In the presence of excess NO*, metNGB is converted into NGBFe(II)NO by reductive nitrosylation, in analogy to the reactions of NO* with metmyoglobin and methemoglobin. The Fe(II)NO form of neuroglobin is oxidized to metNGB by peroxynitrite and dioxygen, two reactions that also take place in hemoglobin, albeit at lower rates. In contrast to myoglobin and hemoglobin, metNGB unexpectedly does not generate the cytotoxic ferryl form of the protein upon addition of either peroxynitrite or hydrogen peroxide. Taken together, our data indicate that human neuroglobin may be an efficient scavenger of reactive oxidizing species and thus may play a role in the cellular defense against oxidative stress.

Activation of heme-regulated eukaryotic initiation factor 2alpha kinase by nitric oxide is induced by the formation of a five-coordinate NO-heme complex: optical absorption, electron spin resonance, and resonance raman spectral studies

Heme-regulated eukaryotic initiation factor 2alpha kinase (HRI) regulates the synthesis of hemoglobin in reticulocytes in response to heme availability. HRI contains a tightly bound heme at the N-terminal domain. Earlier reports show that nitric oxide (NO) regulates HRI catalysis. However, the mechanism of this process remains unclear. In the present study, we utilize in vitro kinase assays, optical absorption, electron spin resonance (ESR), and resonance Raman spectra of purified full-length HRI for the first time to elucidate the regulation mechanism of NO. HRI was activated via heme upon NO binding, and the Fe(II)-HRI(NO) complex displayed 5-fold greater eukaryotic initiation factor 2alpha kinase activity than the Fe(III)-HRI complex. The Fe(III)-HRI complex exhibited a Soret peak at 418 nm and a rhombic ESR signal with g values of 2.49, 2.28, and 1.87, suggesting coordination with Cys as an axial ligand. Interestingly, optical absorption, ESR, and resonance Raman spectra of the Fe(II)-NO complex were characteristic of five-coordinate NO-heme. Spectral findings on the coordination structure of full-length HRI were distinct from those obtained for the isolated N-terminal heme-binding domain. Specifically, six-coordinate NO-Fe(II)-His was observed but not Cys-Fe(III) coordination. It is suggested that significant conformational change(s) in the protein induced by NO binding to the heme lead to HRI activation. We discuss the role of NO and heme in catalysis by HRI, focusing on heme-based sensor proteins.

Kinetics of the reactions of nitrogen monoxide and nitrite with ferryl hemoglobin

Hemoglobin released in the circulation from ruptured red blood cells can be oxidized by hydrogen peroxide or peroxynitrite to generate the highly oxidizing iron(IV)oxo species HbFe(IV)z=O. Nitrogen monoxide, produced in large amounts by activated inducible nitric oxide synthase, can have indirect cytotoxic effects, mainly through the generation of peroxynitrite from its very fast reaction with superoxide. In the present work we have determined the rate constant for the reaction of HbFe(IV)z=O with NO(*), 2.4 x 10(7) M(-1)s(-1) at pH 7.0 and 20 degrees C. The reaction proceeds via the intermediate HbFe(III)ONO, which then dissociates to metHb and nitrite. As these products are not oxidizing and because of its large rate, the reaction of HbFe(IV)z=O with NO(*) may be important to remove the high valent form of hemoglobin, which has been proposed to be at least in part responsible for oxidative lesions. In addition, we have determined that the rate constant for the reaction of HbFe(IV)z=O with nitrite is significantly lower (7.5 x 10(2) M(-1)s(-1) at pH 7.0 and 20 degrees C), but increases with decreasing pH (1.8 x 10(3) M(-1)s(-1) at pH 6.4 and 20 degrees C). Thus, under acidic conditions as found in ischemic tissues, this reaction may also have a physiological relevance.

Interaction of nitric oxide with human heme oxygenase-1

NO and CO may complement each other as signaling molecules in some physiological situations. We have examined the binding of NO to human heme oxygenase-1 (hHO-1), an enzyme that oxidizes heme to biliverdin, CO, and free iron, to determine whether inhibition of hHO-1 by NO can contribute to the signaling interplay of NO and CO. An Fe(3+)-NO hHO-1-heme complex is formed with NO or the NO donors NOC9 or 2-(N,N-diethylamino)-diazenolate-2-oxide.sodium salt. Resonance Raman spectroscopy shows that ferric hHO-1-heme forms a 6-coordinated, low spin complex with NO. The nu(N-O) vibration of this complex detected by Fourier transform IR is only 4 cm(-1) lower than that of the corresponding metmyoglobin (met-Mb) complex but is broader, suggesting a greater degree of ligand conformational freedom. The Fe(3+)-NO complex of hHO-1 is much more stable than that of met-Mb. Stopped-flow studies indicate that k(on) for formation of the hHO-1-heme Fe(3+)-NO complex is approximately 50-times faster, and k(off) 10 times slower, than for met-Mb, resulting in K(d) = 1.4 microm for NO. NO thus binds 500-fold more tightly to ferric hHO-1-heme than to met-Mb. The hHO-1 mutations E29A, G139A, D140A, S142A, G143A, G143F, and K179A/R183A do not significantly diminish the tight binding of NO, indicating that NO binding is not highly sensitive to mutations of residues that normally stabilize the distal water ligand. As expected from the K(d) value, the enzyme is reversibly inhibited upon exposure to pathologically, and possibly physiologically, relevant concentrations of NO. Inhibition of hHO-1 by NO may contribute to the pleiotropic responses to NO and CO.

Routes to S-nitroso-hemoglobin formation with heme redox and preferential reactivity in the beta subunits

Previous studies of the interactions of NO with human hemoglobin have implied the predominance of reaction channels that alternatively eliminate NO by converting it to nitrate, or tightly complex it on the alpha subunit ferrous hemes. Both channels could effectively quench NO bioactivity. More recent work has raised the idea that NO groups can efficiently transfer from the hemes to cysteine thiols within the beta subunit (cysbeta-93) to form bioactive nitrosothiols. The regulation of NO function, through its chemical position in the hemoglobin, is supported by response to oxygen and to redox agents that modulate the molecular and electronic structure of the protein. In this article, we focus on reactions in which Fe(III) hemes could provide the oxidative requirements of this NO-group transfer chemistry. We report a detailed investigation of the reductive nitrosylation of human met-Hb, in which we demonstrate the production of S-nitroso (SNO)-Hb through a heme-Fe(III)NO intermediate. The production of SNO-Hb is strongly favored (over nitrite) when NO is gradually introduced in limited total quantities; in this situation, moreover, heme nitrosylation occurs primarily within the beta subunits of the hemoglobin tetramer. SNO-Hb can similarly be produced when Fe(II)NO hemes are subjected to mild oxidation. The reaction of deoxygenated hemoglobin with limited quantities of nitrite leads to the production of beta subunit Fe(II)NO hemes, with SNO-Hb produced on subsequent oxygenation. The common theme of these reactions is the effective coupling of heme-iron and NO redox chemistries. Collectively, they establish a connectivity between hemes and thiols in Hb, through which NO is readily dislodged from storage on the heme to form bioactive SNO-Hb.

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.

Reversible sequestration of nitric oxide by hemoglobin during hemodialysis in end-stage renal disease

BACKGROUND

During hemodialysis, patients whose plasma concentrations of nitric oxide (NO) products increase reportedly experience hypotension. Therefore, whether NO bound to hemoglobin (Hb) could contribute to various clinical and laboratory changes during hemodialysis was explored in patients with end-stage renal disease (ESRD).

METHODS

Ten patients were studied during 3 hemodialysis treatments with samples of blood analyzed for RBC nitrosyl Hb (HbNO), L-arginine, asymmetric dimethylarginine (ADMA), plasma nitrite+nitrate (NOx), and buffy coat NO synthase (NOS) activities.

RESULTS

HbNO before and during hemodialysis varied considerably. Those with higher predialysis levels had lower HbNO values during dialysis, whereas HbNO levels in those with lower levels before dialysis increased. Plasma NOx did not correlate with HbNO, but change in HbNO in the first hour and change in NOx in the first 2 hours correlated with drop in diastolic and systolic blood pressures (BP), respectively. HbNO concentrations increased in patients with >35% drop in systolic BP, whereas in those with <35% drop, HbNO concentrations decreased. HbNO levels adjusted by the hematocrit showed a drop in HbNO for the <35% group and a >3-fold increase in the >35% group. HbNO levels were higher in men than in women, and levels and changes correlated with the hematocrit, skin temperatures, plasma ADMA, arginine, and buffy coat NOS.

CONCLUSIONS

In patients with >35% drop in systolic BP, NO was scavenged by Hb in the circulating RBCs, undoubtedly attenuating the degree of hypotension. These data indicate that the amount of NO that is scavenged or released by Hb in the circulating RBCS during dialysis is highly variable and reversible. Various predialysis factors relate to the concentration of HbNO before and during dialysis, which in turn influence clinical findings that occur during the interdialytic period.

Unprecedented proximal binding of nitric oxide to heme: implications for guanylate cyclase

Microbial cytochromes c' contain a 5-coordinate His-ligated heme that forms stable adducts with nitric oxide (NO) and carbon monoxide (CO), but not with dioxygen. We report the 1.95 and 1.35 A resolution crystal structures of the CO- and NO-bound forms of the reduced protein from Alcaligenes xylosoxidans. NO disrupts the His-Fe bond and binds in a novel mode to the proximal face of the heme, giving a 5-coordinate species. In contrast, CO binds 6-coordinate on the distal side. A second CO molecule, not bound to the heme, is located in the proximal pocket. Since the unusual spectroscopic properties of cytochromes c' are shared by soluble guanylate cyclase (sGC), our findings have potential implications for the activation of sGC induced by the binding of NO or CO to the heme domain.

Pyridoxalated hemoglobin polyoxyethylene: a nitric oxide scavenger with antioxidant activity for the treatment of nitric oxide-induced shock

Hemoglobins modified for therapeutic use as either hemoglobin-based oxygen carriers or scavengers of nitric oxide are currently being evaluated in clinical trials. One such product, pyridoxalated hemoglobin polyoxyethylene conjugate (PHP), is a human-derived and chemically modified hemoglobin that has yielded promising results in Phase II clinical trials, and is entering a pivotal Phase III clinical trial for the treatment of shock associated with systemic inflammatory response syndrome (SIRS). Shock associated with SIRS is a NO-induced shock. PHP, a new mechanism-based therapy, has been demonstrated in clinical trials to have the expected hemodynamic activity of raising blood pressure and reducing catecholamine use, consistent with its mechanism of action as a NO scavenger. PHP is conjugated with polyoxyethylene, which results in a surface-decorated molecule with enhanced circulation time and stability as well as in attachment of soluble red blood cell enzymes, including catalase and superoxide dismutase. PHP thus contains an antioxidant profile similar to the intact red blood cell and is therefore resistant to both initial oxidative modification by oxidants such as hydrogen peroxide and subsequent ferrylhemoglobin formation. These studies suggest both that the redox activity of modified hemoglobins can be attenuated and that modified hemoglobins containing endogenous antioxidants, such as PHP, may have reduced pro-oxidant potential. These antioxidant properties, in addition to the NO-scavenging properties, may allow the use of PHP in other indications in which excess NO, superoxide, or hydrogen peroxide is involved, including ischemia-reperfusion injury and hemorrhagic shock.