Efficient Coupled Oxidation of Heme by an Active Site Variant of Horse Heart Myoglobin

Heme binding to the SARS-CoV-2 spike glycoprotein

The target for humoral immunity, SARS-CoV-2 spike glycoprotein, has become the focus of vaccine research and development. Previous work demonstrated that the N-terminal domain (NTD) of SARS-CoV-2 spike binds biliverdin-a product of heme catabolism-causing a strong allosteric effect on the activity of a subset of neutralizing antibodies. Herein, we show that the spike glycoprotein is also able to bind heme (KD = 0.5 ± 0.2 μM). Molecular modeling indicated that the heme group fits well within the same pocket on the SARS-CoV-2 spike NTD. Lined by aromatic and hydrophobic residues (W104, V126, I129, F192, F194, I203, and L226), the pocket provides a suitable environment to stabilize the hydrophobic heme. Mutagenesis of N121 has a substantive effect on heme binding (KD = 3000 ± 220 μM), confirming the pocket as a major heme binding location of the viral glycoprotein. Coupled oxidation experiments in the presence of ascorbate indicated that the SARS-CoV-2 glycoprotein can catalyze the slow conversion of heme to biliverdin. The heme trapping and oxidation activities of the spike may allow the virus to reduce levels of free heme during infection to facilitate evasion of the adaptive and innate immunity.

Engineered metalloregulation of azide binding affinity and reduction potential of horse heart myoglobin

Metal ion binding to a previously reported variant of horse heart myoglobin (Lys45Glu/Lys63Glu) with a metal ion binding site on the surface of the protein that is adjacent to the haem binding site has been shown to influence ligand binding and electrochemical properties of the protein. For example, the K(d) (μM) for binding of azide to this variant decreases from 277 ± 9 to 32 ± 3 following addition of a saturating concentration of Mn(2+) (the value for the wild-type protein under the same conditions is 26 ± 1). Similarly, the midpoint reduction potential E(m) (mV vs. standard hydrogen electrode) increases from 9 to 40 in the presence of a saturating concentration of Mn(2+) (the value for the wild-type protein under the same conditions is 45 ± 2). These results demonstrate the potential value of engineered metal ion binding sites as a means of regulating the functional properties of even simple haem proteins.

Heme uptake and metabolism in bacteria

All but a few bacterial species have an absolute need for heme, and most are able to synthesize it via a pathway that is highly conserved among all life domains. Because heme is a rich source for iron, many pathogenic bacteria have also evolved processes for sequestering heme from their hosts. The heme biosynthesis pathways are well understood at the genetic and structural biology levels. In comparison, much less is known about the heme acquisition, trafficking, and degradation processes in bacteria. Gram-positive and Gram-negative bacteria have evolved similar strategies but different tactics for importing and degrading heme, likely as a consequence of their different cellular architectures. The differences are manifested in distinct structures for molecules that perform similar functions. Consequently, the aim of this chapter is to provide an overview of the structural biology of proteins and protein-protein interactions that enable Gram-positive and Gram-negative bacteria to sequester heme from the extracellular milieu, import it to the cytosol, and degrade it to mine iron.

Optical properties of Yeast Cytochrome c monolayer on gold: an in situ spectroscopic ellipsometry investigation

The adsorption of Yeast Cytochrome c (YCC) on well defined, flat gold substrates has been studied by Spectroscopic Ellipsometry (SE) in the 245-1000 nm wavelength range. The investigation has been performed in aqueous ambient at room temperature, focusing on monolayer-thick films. In situ δΨ and δΔ difference spectra have shown reproducibly well-defined features related to molecular optical absorptions typical of the so-called heme group. The data have been reproduced quantitatively by a simple isotropic optical model, accounting for the molecular absorption spectrum and film-substrate interface effects. The simulations allowed a reliable estimate of the film thickness and the determination of the position and the shape of the so-called Soret absorption peak that, within the experimental uncertainty, is the same found for molecules in liquid. These findings suggest that YCC preserves its native structure upon adsorption. The same optical model was able to reproduce also ex situ results on rinsed and dried samples, dominated by the spectral features associated to the polypeptide chain that tend to overwhelm the heme absorption features.

Direct electrochemistry and electrocatalysis of myoglobin-based nanocomposite membrane electrode

The direct electron transfer of myoglobin (Mb) was achieved based on the immobilization of Mb/Silver nanoparticles (AgNPs) on glassy carbon electrode by multi-wall carbon nanotubes (MWNTs)-chitosan(Chit) film. The immobilized Mb displayed a pair of well-defined and reversible redox peaks with a formal potential (E(θ')) of -24 mV (vs. Ag/AgCl) in 0.1 M pH 7.0 phosphate buffer solution. The apparent heterogeneous electron transfer rate constants (k(s)) of Mb confined to Chit-MWNTs film was evaluated as 5.47 s(-1) according to Laviron's equation. The surface concentration (Γ(*)) of the electroactive Mb in the Chit-MWNTs film was estimated to be (4.16±0.35)×10(-9) mol cm(-2). Meanwhile, the catalytic ability of Mb toward the reduction of H(2)O(2) was studied. Its apparent Michaelis-Menten constant for H(2)O(2) was 0.024 mM, showing a good affinity. The linear range for H(2)O(2) determination was from 2.5×10(-5) M to 2.0×10(-4) M with a detection limit of 1.02×10(-6) M (S/N=3). Moreover, the biosensor displays rapid response to H(2)O(2) and good stability and reproducibility.

WITHDRAWN: Direct electrochemistry and electrocatalysis of myoglobin on redox-active self-assembling monolayers derived from nitroaniline modified electrode

The adsorption processes and electrochemical behavior of 4-nitroaniline (4-NA) and 2-nitroaniline (2-NA) adsorbed onto glassy carbon electrodes (GCE) have been investigated in aqueous 0.1M nitric acid (HNO(3)) electrolyte solutions using cyclic voltammetry (CV). Nitroaniline adsorbs onto GCE surfaces and upon potential cycling past -0.55 V is transformed into the arylhydroxylamine (ArHA), which exhibits a well-behaved pH dependent redox couple centered at 0.32 V (pH 1.5). This modified electrode can be readily used as an immobilization matrix to entrap proteins and enzymes. In our studies, myoglobin (Mb) was chosen as a model protein for investigation. A pair of well-defined reversible redox peaks for Mb(Fe(III)-Fe(II)) was obtained at the Mb/arylhydroxylamine modified glassy carbon electrode (Mb/HAGCE) by direct electron transfer between the protein and the GCE. The formal potential (E(0')), the surface coverage (Gamma) and the electron transfer rate constant (k(s)) were calculated as -0.317 V, 4.15+/-0.5 x 10(-11)mol/cm(2) and 51+/-5s(-1), respectively. Dramatically enhanced biocatalytic activity was exemplified at the Mb/HAGCE for the reduction of hydrogen peroxide (H(2)O(2)), trichloroacetic acid (TCA) and oxygen (O(2)). The Mb/ArHA film was also characterized by UV-vis spectra, scanning electron microscope (SEM) indicating excellent stability and good biocompatibility for protein in the film. The applicability of the method to the determination of H(2)O(2) ( approximately 3%) in a commercial antiseptic solution and soft-contact lenses cleaning solutions were demonstrated. This new Mb/HAGCE exhibited rapid electrochemical response (with in 2s) with good stability in physiological condition.

Detection of heme oxygenase activity in a library of four-helix bundle proteins: towards the de novo synthesis of functional heme proteins

Design and chemical synthesis of de novo heme proteins with enzymatic activity on cellulose membranes is described. 352 antiparallel four-helix bundle proteins with a single histidine for heme ligation were assembled from three different sets of short amphipathic helices on membrane-bound peptide templates. The templates were coupled by linkers to cellulose membranes of microplate format, which could be cleaved for control of intermediate and final products. The incorporation of heme and the heme oxygenase activity of the 352 proteins were monitored by measuring UV-visible spectra directly on the cellulose. The kinetics of the heme oxygenase reaction was studied by monitoring the decrease of the Soret band and the transient absorbance of verdoheme being an intermediate product in the formation of biliverdin. Four of the proteins covering a broad range of the enzymatic rate constants were selected and synthesized in solution for further characterization. Detailed studies by redox potentiometry, analytical ultracentrifugation, and electron paramagnetic resonance spectroscopy yielded information about the aggregation state of the proteins, the spin state and the putative coordination environment of the iron. The amount of five-coordinated high-spin iron and a positive reduction potential were found to promote the oxygenase activity of the proteins.

Myoglobin/arylhydroxylamine film modified electrode: Direct electrochemistry and electrochemical catalysis

The adsorption processes and electrochemical behavior of 4-nitroaniline (4-NA) adsorbed onto glassy carbon electrodes (GCE) have been investigated in aqueous 0.1M nitric acid (HNO(3)) electrolyte solutions using cyclic voltammetry (CV). 4-NA adsorbs onto GCE surfaces, and upon potential cycling past -0.2V, is transformed into the arylhydroxylamine (ArHA) derivative which exhibits a well-behaved pH dependent redox couple centered at 0.32V at pH 1.5. It is noted as arylhydroxylamine modified glassy carbon electrodes (HAGCE). This modified electrode can be readily used as an immobilization matrix to entrap proteins and enzymes. In our studies, myoglobin (Mb) was used as a model protein for investigation. A pair of well-defined reversible redox peaks of Mb (Fe(III)-Fe(II)) was obtained at the Mb/arylhydroxylamine modified glassy carbon electrode (Mb/HAGC) by direct electron transfer between the protein and the GCE. The formal potential ( [Formula: see text] ), the apparent coverage (Gamma(*)) and the electron-transfer rate constant (k(s)) were calculated as -0.317V, 8.26x10(-12)mol/cm(2) and 51+/-5s(-1), respectively. Dramatically enhanced biocatalytic activity was exemplified at the Mb/HAGC electrode by the reduction of hydrogen peroxide (H(2)O(2)), trichloroacetic acid (TCA) and oxygen (O(2)). The Mb/arylhydroxylamine film was also characterized by UV-visible spectroscopy (UV-vis), scanning electron microscope (SEM) indicating excellent stability and good biocompatibility of the protein in the arylhydroxylamine modified electrode. This new Mb/HAGC electrode exhibited rapid electrochemical response (2s) for H(2)O(2) and had good stability in physiological condition, showing the potential applicability of the films in the preparation of third generation biosensors or bioreactors based on direct electrochemistry of the proteins.

Theoretical investigations of the hydrolysis pathway of verdoheme to biliverdin

Conversion of iron(II) verdoheme to iron biliverdin in the presence of OH(-) was investigated using B3LYP method. Both 3-21G and 6-31G* basis sets were employed for geometry optimization calculation as well as energy stabilization estimation. Calculation at 6-31G* level was found necessary for a correct spin state estimation of the iron complexes. Two possible pathways for the conversion of iron verdoheme to iron biliverdin were considered. In one path the iron was six-coordinate while in the other it was considered to be five-coordinate. In the six-coordinated pathway, the ground state of bis imidazole iron verdoheme is singlet while that for open chain iron biliverdin it is triplet state with 4.86 kcal/mol more stable than the singlet state. The potential energy surface suggests that a spin inversion take place during the course of reaction after TS. The ring opening process in the six-coordinated pathway is in overall -2.26 kcal/mol exothermic with a kinetic barrier of 9.76 kcal/mol. In the five-coordinated pathway the reactant and product are in the ground triplet state. In this path, hydroxyl ion attacks the iron center to produce a complex, which is only 1.59 kcal/mol more stable than when OH(-) directly attacks the macrocycle. The activation barrier for the conversion of iron hydroxy species to the iron biliverdin complex by a rebound mechanism is estimated to be 32.68 kcal/mol. Large barrier for rebound mechanism, small barrier of 4.18 kcal/mol for ring opening process of the hydroxylated macrocycle, and relatively same stabilities for complexes resulted by the attack of nucleophile to the iron and macrocycle indicate that five-coordinated pathway with direct attack of nucleophile to the 5-oxo position of macrocycle might be the path for the conversion of verdoheme to biliverdin.

Electron Distribution in Iron Octaethyloxophlorin Complexes. Importance of the Fe(III) Oxophlorin Trianion Form in the Bis-pyridine and Bis-imidazole Complexes

The apportionment of electrons between iron and the porphyrinic macrocycle in complexes of octaethyloxophlorin (H3OEPO) has been a vexing problem. In particular, for (Py)2Fe(OEPO), which is an important intermediate in heme degradation, three resonance structures involving Fe(III), Fe(II), or Fe(I), respectively, have been considered. To clarify this matter, the electronic and geometric structures of (Py)2Fe(III)(OEPO), (Im)2Fe(III)(OEPO).2THF, and (Im)2Fe(III)(OEPO).1.6CHCl3 have been examined by single-crystal X-ray diffraction, measurement of magnetic moments as a function of temperature, and EPR and NMR spectral studies. The results clearly show that both complexes exist in the Fe(III)/oxophlorin trianion form rather than the Fe(II)/oxophlorin radical form previously established for (2,6-xylylNC)(2)Fe(II)(OEPO.). In the solid state from 10 to 300 K, (Py)2Fe(III)(OEPO) exists in the high-spin (S = 5/2) state with the axial ligands in parallel planes, a planar porphyrin, and long axial Fe-N distances. However, in solution it exists predominantly in a low-spin (S = 1/2) form. In contrast, the structures of (Im)2Fe(III)(OEPO).2THF and (Im)2Fe(III)(OEPO).1.6CHCl3 consist of porphyrins with a severe ruffled distortion, axial ligands in nearly perpendicular planes, and relatively short axial Fe-N distances. The crystallographic, magnetic, EPR, and NMR results all indicate that (Im)2Fe(III)(OEPO) exists in the low-spin Fe(III) form in both the solid state and in solution.

Direct Electrochemistry of Myoglobin in Titanate Nanotubes Film

Titanate nanotubes (TNT) were proven to be efficient support matrixes for the immobilization of myoglobin (Mb). A comparative study was performed using the corresponding analogue, nanocrystalline anatase TiO2 (TNP). UV-visible absorption and FT-IR spectra show that Mb was not obviously denatured in TNT film in contrast to the significant denaturation of Mb in TNP film. Cyclic voltammetry and square wave voltammetry measurements were carried out using the Mb-TNT or Mb-TNP cast film-covered basal plane pyrolytic graphite electrode. The Mb-TNT film gave a well-defined, nearly reversible redox couple with the apparent formal peak potential (Ep) of -0.239 V (vs Ag/AgCl) in pH 5.5 buffer, whereas a relatively smaller, quais-reversible redox pair with Ep of -0.263 V was observed for the Mb-TNP film. The amounts of electroactive Mb in TNT film and TNP film were 15 and 10%, respectively. Moreover, the Mb-TNT film exerted facile direct electron transfer with the apparent heterogeneous electron-transfer rate constant (kET) of 86+/-7 s-1, almost 4 times the 22+/-5 s-1 value for the Mb-TNP membrane and higher than other Mb-entrapped films reported. Additionally, the Mb-TNT film demonstrates good electrocatalytic reduction of hydrogen peroxide with a detection limit of 0.6 microM, much lower than the 3.0 microM value for the Mb-TNP electrode and other Mb-related film-modified electrodes reported so far. The Mb-TNT film exhibits higher peroxidase-like activity with the apparent Michaelis-Menton constant (KM) of 140 microM, significantly lower than the 1300 microM value for the Mb-TNP film. The functional hydroxyl group and the surface charge as well as tubular morphology of TNT are important factors to stabilize the bound protein.

Solution 1H NMR study of the accommodation of the side chain of n-butyl-etiohemin-I incorporated into the active site of cyano-metmyoglobin

In order to identify the most readily deformable portion of the heme pocket in myoglobin, equine myoglobin was reconstituted with a meso-n-butyl substituent on centrosymmetric etiohemin-I. Solution 1H NMR data for the low-spin iron(III) cyanide complex of oxidized myoglobin that include 2D nuclear Overhauser enhancement spectroscopy contacts, paramagnetic relaxation, and dipolar shifts resulting from magnetic anisotropy show that the heme binds uniquely to the iron in a manner that arranges the methyl and ethyl substituents on a given pyrrole in a clockwise manner when viewed from the proximal side, and with the n-butyl group seated at the canonical alpha-meso position of native protohemin-IX. The butyl group is oriented sharply toward the proximal side and its protein contacts demonstrate that it is oriented largely into the "xenon hole" in myoglobin. The location of the n-butyl group on the proximal side near the vacancies places it within the region found to be most flexible in molecular dynamics simulation. A small, counterclockwise rotation of the pyrrole N-Fe-N vector of n-butyl-etiohemin-I relative to that for native protohemin, indicated by both the prosthetic group methyl contact shift pattern and the prosthetic group contacts to heme pocket residues, is proposed to allow the xenon hole to accommodate better the n-butyl group. In contrast to previous work, which showed that a bulky polar substituent on etiohemin-I preferentially seats at the canonical gamma-meso position, the nonpolar n-butyl group selects the alpha-meso position.

Ligand binding properties of myoglobin reconstituted with iron porphycene: unusual O2 binding selectivity against CO binding

Sperm whale myoglobin, an oxygen storage hemoprotein, was successfully reconstituted with the iron porphycene having two propionates, 2,7-diethyl-3,6,12,17-tetramethyl-13,16-bis(carboxyethyl)porphycenatoiron. The physicochemical properties and ligand bindings of the reconstituted myoglobin were investigated. The ferric reconstituted myoglobin shows the remarkable stability against acid denaturation and only a low-spin characteristic in its EPR spectrum. The Fe(III)/Fe(II) redox potential (-190 mV vs NHE) determined by the spectroelectrochemical measurements was much lower than that of the wild-type. These results can be attributed to the strong coordination of His93 to the porphycene iron, which is induced by the nature of the porphycene ring symmetry. The O2 affinity of the ferrous reconstituted myoglobin is 2600-fold higher than that of the wild-type, mainly due to the decrease in the O2 dissociation rate, whereas the CO affinity is not so significantly enhanced. As a result, the O2 affinity of the reconstituted myoglobin exceeds its CO affinity (M' = K(CO)/K(O2) < 1). The ligand binding studies on H64A mutants support the fact that the slow O2 dissociation of the reconstituted myoglobin is primarily caused by the stabilization of the Fe-O2 sigma-bonding. The IR spectra for the carbon monoxide (CO) complex of the reconstituted myoglobin suggest several structural and/or electrostatic conformations of the Fe-C-O bond, but this is not directly correlated with the CO dissociation rate. The high O2 affinity and the unique characteristics of the myoglobin with the iron porphycene indicate that reconstitution with a synthesized heme is a useful method not only to understand the physiological function of myoglobin but also to create a tailor-made function on the protein.

Oxidative Verdoheme Formation and Stabilization by Axial Isocyanide Ligation

The effect of isocyanides as axial ligands on the formation and stability of verdoheme by oxidation has been examined. The reaction of [Fe(III)(OEPO)]2 with t-butyl isocyanide under dioxygen-free conditions results in the formation of (t-BuNC)2Fe(II)(OEPO*) with an electron paramagnetic resonance at g=2.009 with a peak-to-peak separation of 23.5 G at 4 K. (OEPO is the trianion of octaethyloxophlorin and OEPO* is the radical dianion obtained from OEPO by one-electron oxidation.) Exposure of chloroform solutions of either (2,6-xylylNC)2Fe(II)(OEPO*) or (t-BuNC)2Fe(II)(OEPO*) to dioxygen followed by the addition of ammonium hexafluorophosphate results in their transformation into the diamagnetic verdohemes, [(2,6-xylylNC)2Fe(II)(OEOP)](PF6) and [(t-BuNC)2Fe(II)(OEOP)](PF6), yields 68 and 70%, respectively. (OEOP is the anion of octaethyl-5-oxaporphyrin.) The oxidation reactions of (2,6-xylylNC)2Fe(II)(OEPO*) and (t-BuNC)2Fe(II)(OEPO*) have also been monitored by 1H NMR spectroscopy. No resonances due to paramagnetic products could be detected, the reactions appear to result only in the formation of the diamagnetic verdohemes, and the products are not susceptible to further oxidation.

The Effects of Axial Ligands on Electron Distribution and Spin States in Iron Complexes of Octaethyloxophlorin, Intermediates in Heme Degradation

The results presented here show that the nature of the axial ligand can alter the distribution of electrons between the metal and the porphyrin in complexes where there is an oxygen atom replacing one of the meso protons. The complexes (1-MeIm)(2)Fe(III)(OEPO) and (2,6-xylylNC)(2)Fe(II)(OEPO(*)) (where OEPO is the trianionic octaethyloxophlorin ligand and OEPO(*) is the dianionic octaethyloxophlorin radical) were prepared by addition of an excess of the appropriate axial ligand to a slurry of [Fe(III)(OEPO)](2) in chloroform under anaerobic conditions. The magnetic moment of (2,6-xylylNC)(2)Fe(II)(OEPO(*)) is temperature invariant and consistent with a simple S = (1)/(2) ground state. This complex with an EPR resonance at g = 2.004 may be considered as a model for the free-radical like EPR signal seen when the meso-hydroxylated heme/heme oxygenase complex is treated with carbon monoxide. In contrast, the magnetic moment of (1-MeIm)(2)Fe(III)(OEPO) drops with temperature and indicates a spin-state change from an S = (5)/(2) or an admixed S = (3)/(2),(5)/(2) state at high temperatures (near room temperature) to an S = (1)/(2) state at temperatures below 100 K. X-ray diffraction studies show that each complex crystallizes in centrosymmetric form with the expected six-coordinate geometry. The structure of (1-MeIm)(2)Fe(III)(OEPO) has been determined at 90, 129, and 296 K and shows a gradual and selective lengthening of the Fe-N(axial bond). This behavior is consistent with population of a higher spin state at elevated temperatures.

Facile Ring Opening of Iron(III) and Iron(II) Complexes of meso-Amino-octaethylporphyrin by Dioxygen

Pyridine solutions of ClFe(III)(meso-NH(2)-OEP) undergo oxidative ring opening when exposed to dioxygen. The high-spin iron(III) complex, ClFe(III)(meso-NH(2)-OEP), has been isolated and characterized by X-ray crystallography. In the solid state, it has a five-coordinate structure typical for high-spin (S = 5/2) iron(III) complex. In chloroform-d solution, ClFe(III)(meso-NH(2)-OEP) displays an (1)H NMR spectrum characteristic of a high-spin, five-coordinate complex and is unreactive toward dioxygen. However, in pyridine-d(5) solution a temperature-dependent equilibrium exists between the high-spin (S = 5/2), six-coordinate complex, [(py)ClFe(III)(meso-NH(2)-OEP)], and the six-coordinate, low spin (S = 1/2 with the less common (d(xz)d(yz))(4)(d(xy))(1) ground state)) complex, [(py)(2)Fe(III)(meso-NH(2)-OEP)](+). Such pyridine solutions are air-sensitive, and the remarkable degradation has been monitored by (1)H NMR spectroscopy. These studies reveal a stepwise conversion of ClFe(III)(meso-NH(2)-OEP) into an open-chain tetrapyrrole complex in which the original amino group and the attached meso carbon atom have been converted into a nitrile group. Additional oxidation at an adjacent meso carbon occurs to produce a ligand that binds iron by three pyrrole nitrogen atoms and the oxygen atom introduced at a meso carbon. This open-chain tetrapyrrole complex itself is sensitive to attack by dioxygen and is converted into a tripyrrole complex that is stable to further oxidation and has been isolated. The process of oxidation of the Fe(III) complex, ClFe(III)(meso-NH(2)-OEP), is compared with that of the iron(II) complex, (py)(2)Fe(II)(meso-NH(2)-OEP); both converge to form identical products.

Formation and Isolation of an Iron-Tripyrrole Complex from Heme Degradation

Addition of dioxygen to a solution of (py)2Fe(OEPO) (1) (where OEPO is the trianion of octaethyloxophlorin) in pyridine-d5 at 23 degrees C results in a series of oxidative changes that have been followed by 1H NMR spectroscopy. After 4 h, the resonances of 1 have vanished, while several sets of new resonances have developed. The most intense of these are identified as belonging to (py)2FeIII(OEB) (2) by comparison with the previously obtained spectrum of this species, which contains a ring-opened tetrapyrrole. After the sample was left standing for 1 day, further changes occur which produce a sample that is stable, because 1H NMR spectroscopic examination reveals no further changes on continued exposure to dioxygen or air. The product, (py)2FeIII(HETP) (3), where HETP is the dianion of a hexaethyltripyrrole, has been converted into (n-Bu4N)[ClFeIII(HETP)] by treatment with tetra(n-butyl)ammonium chloride. Red (n-Bu4N)[ClFeIII(HETP)] has been isolated and characterized by single-crystal X-ray diffraction.

Coupled oxidation vs heme oxygenation: insights from axial ligand mutants of mitochondrial cytochrome b5

Mutation of His-39, one of the axial ligands in rat outer mitochondrial membrane cytochrome b(5) (OM cyt b(5)), to Val produces a mutant (H39V) capable of carrying out the oxidation of heme to biliverdin when incubated with hydrazine and O(2). The reaction proceeds via the formation of an oxyferrous complex (Fe(II)(-)O(2)) that is reduced by hydrazine to a ferric hydroperoxide (Fe(III)(-)OOH) species. The latter adds a hydroxyl group to the porphyrin to form meso-hydroxyheme. The observation that catalase does not inhibit the oxidation of the heme in the H39V mutant is consistent with the formation of a coordinated hydroperoxide (Fe(III)(-)OOH), which in heme oxygenase is the precursor of meso-hydroxyheme. By comparison, mutation of His-63, the other axial ligand in OM cyt b(5), to Val results in a mutant (H63V) capable of oxidizing heme to verdoheme in the absence of catalase. However, the oxidation of heme by H63V is completely inhibited by catalase. Furthermore, whereas the incubation of Fe(III)(-)H63V with H(2)O(2) leads to the nonspecific degradation of heme, the incubation of Fe(II)(-)H63V with H(2)O(2) results in the formation of meso-hydroxyheme, which upon exposure to O(2) is rapidly converted to verdoheme. These findings revealed that although meso-hydroxyheme is formed during the degradation of heme by the enzyme heme oxygenase or by the process of coupled oxidation of model hemes and hemoproteins not involved in heme catabolism, the corresponding mechanisms by which meso-hydroxyheme is generated are different. In the coupled oxidation process O(2) is reduced to noncoordinated H(2)O(2), which reacts with Fe(II)-heme to form meso-hydroxyheme. In the heme oxygenation reaction a coordinated O(2) molecule (Fe(II)(-)O(2)) is reduced to a coordinated peroxide molecule (Fe(III)(-)OOH), which oxidizes heme to meso-hydroxyheme.

The role of copper and protons in heme-copper oxidases: kinetic study of an engineered heme-copper center in myoglobin

To probe the role of copper and protons in heme-copper oxidase (HCO), we have performed kinetic studies on an engineered heme-copper center in sperm whale myoglobin (Leu-29 --> HisPhe-43 --> His, called Cu(B)Mb) that closely mimics the heme-copper center in HCO. In the absence of metal ions, the engineered Cu(B) center in Cu(B)Mb decreases the O(2) binding affinity of the heme. However, addition of Ag(I), a redox-inactive mimic of Cu(I), increases the O(2)-binding affinity. More importantly, copper ion in the Cu(B) center is essential for O(2) reduction, as no O(2) reduction can be observed in copper-free, Zn(II), or Ag(I) derivatives of Cu(B)Mb. Instead of producing a ferryl-heme as in HCO, the Cu(B)Mb generates verdoheme because the engineered Cu(B)Mb may lack a hydrogen bonding network that delivers protons to promote the heterolytic OO cleavage necessary for the formation of ferryl-heme. Reaction of oxidized Cu(B)Mb with H(2)O(2), a species equivalent in oxidation state to 2e(-), reduced O(2) but, possessing the extra protons, resulted in ferryl-heme formation, as in HCO. The results showed that the Cu(B) center plays a critical role in O(2) binding and reduction, and that proton delivery during the O(2) reduction is important to avoid heme degradation and to promote the HCO reaction.

Oxophlorin and metallooxophlorin radicals--DFT studies

A flexible oxophlorin macrocycle, which allows the location of labile hydrogen atoms alternatively at the pyrrole nitrogen, oxygen, or meso-carbon atoms, has been studied by density functional theory (DFT). DFT calculations were carried out on oxophlorin 1, 5-hydroxyporphyrin 2, and two isomers of oxophlorin 3 and 4 (the proton added at the tetrahedral C(15) or the C(10) meso-carbon, respectively). The oxophlorin-hydroxyporphyrin structural changes are appropriately reflected by the significant changes of the meso carbon-oxygen bond lengths, which are in the limits of typical C=O and C-O distances. The rearrangement that creates the iso-oxophlorin macrocycle 3 (4) results in near tetrahedral geometry around the C(15) (C(10)) carbon atom, with the C(14)-C(15) and C(15)-C(16) (C(9)-C(10) and C(10)-C(11)) bond lengths corresponding to a single C-C bond. 5-Hydroxyporphyrin 2 is aromatic and has a bond pattern resembling that of regular porphyrin. In 1, 3, and 4, a localization of single and double bonds was seen, which agrees with the nonaromatic nature of oxophlorin, or isooxophlorin. The relative stability decreases in the order: 2 (0) > 3 (4.85) > 1 (5.11) > 4 (10.04) > 3-cis (12.89) (the number in parentheses is the relative energy, in kcal mol-1). The energy difference between the NH-cis and NH-trans tautomers, which is 8.04 kcal mol-1 for 3, results from a destabilizing NH-NH cis-interaction. DFT calculations were performed on the oxophlorin dianion radical (OP.)2- and a series of metallooxophlorin radicals ([(OP.)LiI]-, [(OP.)ZnII], [(OP.)GaIII]+, and (OP.)GaIIIF, in order to assess their electronic structures. Typically, the largest atomic spin density was found at the C(10) (C(20)) and C(15) meso positions, with the spin density at C(15) being twice as large as that at C(10). The spin density at the C(5) atom is negligible. A large spin density was found at the O(5) oxygen atom. The amount of spin density at the meso positions decreased as the cationic charge increased. When considering the absolute values of the spin densities, the opposite trend was observed at the pyrrolic carbon atoms. The spin density at the nucleus (Fermi contact terms) has also been analyzed. The spin distributions of iron oxophlorins determined by NMR were attributed to an oxophlorin radical electronic structure. The calculated spin density maps accounted for the essential NMR spectroscopic features of important intermediates in the heme degradation process--iron oxophlorin complexes. The DFT calculations reproduced the following spectroscopic patterns: a) |delta H(15)|>|delta H(10)|>>|delta (beta-H)|, b) a sign alteration of the contact shifts for identical substituents located on the same pyrrole ring.

Reactivity of mono-meso-substituted iron(II) octaethylporphyrin complexes with hydrogen peroxide in the absence of dioxygen. Evidence for nucleophilic attack on the heme

Treatment of the mono-meso-substituted iron(II) octaethylporphyrin complexes, (py)2Fe(II)(meso-NO2-OEP), (py)2Fe(II)(meso-CN-OEP), (py)2Fe(II)(meso-HC(O)-OEP), (py)2Fe(II)(meso-Cl-OEP), (py)2Fe(II)(meso-OMe-OEP), (py)2Fe(II)(meso-Ph-OEP), and (py)2Fe(II)(meso-n-Bu-OEP), with hydrogen peroxide in pyridine-d5 at -30 degrees C in the strict absence of dioxygen has been monitored by 1H NMR spectroscopy. The product oxophlorin complexes are stable as long as the samples are protected from exposure to dioxygen. Hydrogen peroxide reacts cleanly with mono-meso-substituted iron(II) porphyrins in pyridine solution under an inert atmosphere to form mixtures of three possible oxygenation products, (py)2Fe(cis-meso-R-OEPO), (py)2Fe(trans-meso-R-OEPO), and (py)2Fe(OEPO). The yields of (py)2Fe(OEPO), which results from replacement of the unique meso substituent, as a function of the identity of the meso substituent decrease in the order NO2 > HC(O) approximately equal to CN approximately equal to Cl > OMe > Ph, Bu, which suggests that the species responsible for attack on the porphyrin periphery is nucleophilic in nature. A mechanism involving isoporphyrin formation through attack of hydroxide ion on a cationic iron porphyrin with an oxidized porphyrin ring is suggested. The identity of the unique meso functionality also affects the regiospecificity of substitution when the unique meso group is retained. Although random attack at the two different meso sites is expected to yield a cis/trans product ratio of 2, the observed ratios vary in the following order: cyano, 5.0; n-butyl, 4.9; chloro, 3.2; formyl, 2.6; methoxy, 1.9; phenyl 1.4.

Solution NMR determination of the seating(s) of meso-nitro-etioheme-1 in myoglobin: implications for steric constraints to meso position access in heme degradation by coupled oxidation

The highly stereoselective cleavage of hemin in myoglobin by coupled oxidation has been attributed to steric barriers that leave more space near the alpha- than the other meso-positions. The steric barriers near meso positions in myoglobin have been investigated by establishing the thermodynamics and dynamics of possible seatings in the pocket of horse myoglobin of a four-fold symmetric etioheme I modified with a bulky nitro group at a single meso position. The cyanomet complex of this reconstituted myoglobin exhibits three sets of (1)H NMR resonances that are linked dynamically and occur in approximate populations ratios of 0.82:0.10:0.08. Two dimensional (1)H NMR has been used to assign the hemin and heme pocket resonances in the major isomer in solution and to determine that the hemin is oriented with the nitro group at the canonical gamma-meso position of native hemin. The dominance of this isomer is attributed to the solvent exposure of this portion of the hemin which stabilizes the highly polar nitro group. Using a combination of magnetization transfer among methyl groups of the three isomers due to "hopping" of the hemin about its normal, the assigned resonances of an isoelectronic, bis-cyano complex of meso-nitro-etioheme I, and the known essentially constant rhombic perturbation of heme pocket sites on the hyperfine shifts of heme methyl (Kolczak, U.; Hauksson, J. B.; Davis, N. L.; Pande, U.; de Ropp, J. S.; Langry, K. C.; Smith, K. M.; LaMar, G. N. J. Am. Chem. Soc. 1999, 121, 835-843); the two minor isomers are shown to place their bulky nitro group at the canonical delta-meso (8%) and alpha-meso positions (10%). The comparable population of the isomers with nitro groups at the hydrophobic alpha- and delta-meso positions dictates that, while the static crystal structure finds more room near the alpha-meso position, the deformation at minimal energetic expense near the alpha- and delta-meso positions is comparable. These results argue that factors other than simple steric influences control the selectivity of the ring cleavage in myoglobin.

Coupled oxidation of heme covalently attached to cytochrome b562 yields a novel biliprotein

A variant of Escherichia coli cytochrome b(562) with covalently attached heme can be converted to a biliverdin-containing protein in two distinct stages by coupled oxidation and acid hydrolysis. The first stage of coupled oxidation yields a stable verdoheme-containing protein. This verdoheme protein is unusual in three respects. First, the verdoheme group is covalently bound to the protein through a c-type thioether linkage. Second, the oxidation stops at the verdoheme stage, and finally, this is the first report of verdoheme generated from a heme protein with exclusive methionine ligation to the heme iron. In addition, the oxidation process does not require denaturation of the protein. The product has been characterized by optical spectroscopy, ESI mass spectrometry, and (1)H NMR. The NMR data show that the predominant product is the result of oxidation at the alpha-meso carbon. A collective evaluation of data on the topic suggests that the electronic structure of the heme, not protein steric effects, is the main factor in controlling the regiospecificity of the oxidation site. In the second stage of conversion to a biliprotein, we demonstrate that the verdoheme ring can be opened by treatment with aqueous formic acid to give alpha-biliverdin covalently attached to the folded protein. This product, a protein-bound linear tetrapyrrole as characterized by optical spectroscopy and mass spectrometry, is an example of a phycobilin chromophore that has not been observed previously.

In Situ Monitoring of the Degradation of Iron Porphyrins by Dioxygen with Hydrazine as Sacrificial Reductant. Detection of Paramagnetic Intermediates in the Coupled Oxidation Process by (1)H NMR Spectroscopy

The effects of using hydrazine rather than ascorbic acid on the coupled oxidation of (OEP)Fe(II)(py)(2) (OEP is the dianion of octaethylporphyrin, py is pyridine) have been investigated with the goal of directly detecting reactive intermediates during the process of heme degradation by dioxygen. The reaction products, [(OEOP)Fe(II)(py)(2)]Cl and (OEB)Fe(III)(py)(2) (OEOP is the monoanion of octaethyl-5-oxaporphyrin and OEB is the trianion of octaethylbilindione), and their yields are similar to those of the standard coupled oxidation process. The reaction has been monitored in situ in pyridine/dichloromethane mixtures by (1)H NMR spectroscopy. The recently isolated and crystallographically characterized complex, (OEPO)Fe(py)(2) (OEOP is the trianion of octaethyloxaphlorin), has been identified as a key intermediate. Addition of dioxygen to (OEP)Fe(II)(py)(2) in pyridine with hydrazine present also produces two new transient species: (OEPO)Fe(py)(N(2)D(4)) and (OEPO)Fe(N(2)D(4))(2). These complexes have also be produced independently by low-temperature titration of hydrazine into a solution of {(OEPO)Fe}(2). Thus, hydrazine acts as an axial ligand during the early stages of the coupled oxidation process. However, the two hydrazine-containing complexes eventually are converted into (OEPO)Fe(py)(2) before [(OEOP)Fe(II)(py)(2)]Cl and (OEB)Fe(III)(py)(2) are formed. The observations reported here suggest that the coupled oxidation process can be divided into two stages. The first stage involves the meso C-H bond and results in introduction of oxygen at that site with the formation of the three intermediates: (OEPO)Fe(N(2)H(4))(2), (OEPO)Fe(N(2)H(2))(py), and (OEPO)Fe(py)(2). The second stage of the coupled oxidation process involves C-C bond breaking and the conversion of the hydroxylated heme, (OEPO)Fe(py)(2), into the final products, [(OEOP)Fe(II)(py)(2)](+) and (OEB)Fe(III)(py)(2).

Evidence for differential binding of isoniazid by Mycobacterium tuberculosis KatG and the isoniazid-resistant mutant KatG(S315T)

Isoniazid is a mainstay of antibiotic therapy for the treatment of tuberculosis, but its molecular mechanism of action is unclear. Previous investigators have hypothesized that isoniazid is a prodrug that requires in vivo activation by KatG, the catalase-peroxidase of Mycobacterium tuberculosis, and that resistance to isoniazid strongly correlates with deletions or point mutations in KatG. One such mutation, KatG(S315T), is found in approximately 50% of clinical isolates exhibiting isoniazid resistance. In this work, 1H nuclear magnetic resonance T1 relaxation measurements indicate that KatG and KatG(S315T) each bind isoniazid at a position approximately 12 A from the active site heme iron. Electron paramagnetic resonance spectroscopy revealed heterogeneous populations of high-spin ferric heme in both wild-type KatG and KatG(S315T) with the ratios of each species differing between the two enzymes. Small changes in the proportions of these high-spin species upon addition of isoniazid support the finding that isoniazid binds near the heme periphery of both enzymes. Titration of wild-type KatG with isoniazid resulted in the appearance of a "type I" substrate-induced difference spectrum analogous to those seen upon substrate binding to the cytochromes P450. The difference spectrum may result from an isoniazid-induced change in a portion of the KatG heme iron from 6- to 5-coordinate. Titration of KatG(S315T) with isoniazid failed to produce a measurable difference spectrum indicating an altered active site configuration. These results suggest that KatG(S315T) confers resistance to isoniazid through subtle changes in the isoniazid binding site.

Conversion of mitochondrial cytochrome b5 into a species capable of performing the efficient coupled oxidation of heme

Histidine-63, one of the heme axial ligands in outer mitochondrial membrane cytochrome b5 (OM cyt b5) has been replaced by a methionine. The H63M variant performs the efficient and regioselective coupled oxidation of heme in order to produce >90% of the alpha-isomer of verdoheme. The variant was characterized by electronic, EPR, and NMR spectroscopic studies which indicate that the ferric form is a high-spin species whose heme is coordinated by histidine-39 in the proximal site and likely by water in the distal site. The coordination of methionine to the ferric heme was ruled out on the basis of NMR spectroscopic studies. Addition of imidazole to a solution of the ferric variant results in the formation of a species axially coordinated by imidazole and histidine-63. The reduction potential of the variant was found to be +110 mV in the absence of exogenous imidazole and -92 mV in the presence of imidazole. These values compare well with the reduction potential of myoglobin (50 mV) and wild-type OM cyt b5 (-102 mV), respectively, consistent with the axial ligation described above. The ferrous variant, on the other hand, is a low-spin species coordinated by histidine-39 and methionine-63. Carbon monoxide (CO) readily displaces Met-63 from its coordination site on the ferrous heme, whereas CO cannot completely displace Met-63 from its coordination site on verdoheme. Consequently, the mechanism of inhibition for the oxidation of verdoheme to iron-biliverdin in the H63M variant appears to be similar to that observed for the heme-heme oxygenase complex in the presence of CO.

Spectroscopic and functional studies of a novel quadruple myoglobin variant with increased peroxidase activity

A quadruple variant of horse heart myoglobin (Thr39Ile/Lys45Asp/Phe46Leu/Ile107Phe) that exhibits significantly (approximately 25-fold) greater peroxidase activity than the wild-type protein has been studied to determine its midpoint reduction potential (24(2) mV vs. SHE; pH 6.0, mu = 0.1 M, 25 degrees C) and to characterize the kinetics of its reaction with hydrogen peroxide. In addition, Fourier transform infrared (FTIR) spectra of the carbonyl and azide adducts of the protein have been obtained to gain initial insight into the effects of these substitutions on the ligand binding properties of the reduced and oxidized variant. All of the results obtained in this work are consistent with a variant heme binding pocket with increased hydrophilic character.