Benzohydroxamic Acid−Peroxidase Complexes: Spectroscopic Characterization of a Novel Heme Spin Species

Identification of Fe(iii)–OH species as a catalytic intermediate in plant peroxidases at high H2O2 concentration

The structure for compound III formed after exposure of plant heme peroxidases to excess H2O2 seems to be a hydroxylated form, providing new evidence for understanding the structural basis of the substrate-induced suicidal behavior of these enzymes.

Reaction intermediate rotation during the decarboxylation of coproheme to heme b in C. diphtheriae

Monoderm bacteria utilize coproheme decarboxylases (ChdCs) to generate heme b by a stepwise decarboxylation of two propionate groups of iron coproporphyrin III (coproheme), forming two vinyl groups. This work focuses on actinobacterial ChdC from Corynebacterium diphtheriae (CdChdC) to elucidate the hydrogen peroxide-mediated decarboxylation of coproheme via monovinyl monopropionyl deuteroheme (MMD) to heme b, with the principal aim being to understand the reorientation mechanism of MMD during turnover. Wild-type CdChdC and variants, namely H118A, H118F, and A207E, were studied by resonance Raman and ultraviolet-visible spectroscopy, mass spectrometry, and molecular dynamics simulations. As actinobacterial ChdCs use a histidine (H118) as a distal base, we studied the H118A and H118F variants to elucidate the effect of 1) the elimination of the proton acceptor and 2) steric constraints within the active site. The A207E variant mimics the proximal H-bonding network found in chlorite dismutases. This mutation potentially increases the rigidity of the proximal site and might impair the rotation of the reaction intermediate MMD. We found that both wild-type CdChdC and the variant H118A convert coproheme mainly to heme b upon titration with H2O2. Interestingly, the variant A207E mostly accumulates MMD along with small amounts of heme b, whereas H118F is unable to produce heme b and accumulates only MMD. Together with molecular dynamics simulations, the spectroscopic data provide insight into the reaction mechanism and the mode of reorientation of MMD, i.e., a rotation in the active site versus a release and rebinding.

Unique Electronic Structures of the Highly Ruffled Hemes in Heme-Degrading Enzymes of Staphylococcus aureus, IsdG and IsdI, by Resonance Raman and Electron Paramagnetic Resonance Spectroscopies

Staphylococcus aureus uses IsdG and IsdI to convert heme into a mixture of staphylobilin isomers, 15-oxo-β-bilirubin and 5-oxo-δ-bilirubin, formaldehyde, and iron. The highly ruffled heme found in the heme-IsdI and IsdG complexes has been proposed to be responsible for the unique heme degradation products. We employed resonance Raman (RR) and electron paramagnetic resonance (EPR) spectroscopies to examine the coordination and electronic structures of heme bound to IsdG and IsdI. Heme complexed to IsdG and IsdI is coordinated by a neutral histidine. The trans ligand is hydroxide in the ferric alkaline form of both proteins. In the ferric neutral form at pH 6.0, heme is six-coordinated with water as the sixth ligand for IsdG and is in the mixture of the five-coordinated and six-coordinated species for IsdI. In the ferrous CO-bound form, CO is strongly hydrogen bonded with a distal residue. The marker lines, ν₂ and ν₃, appear at frequencies that are distinct from other proteins having planar hemes. The EPR spectra for the ferric hydroxide and cyanide states might be explained by assuming the thermal mixing of the d-electron configurations, (d_xy)²(d_xz,d_yz)³ and (d_xz,d_yz)⁴(d_xy)¹. The fraction for the latter becomes larger for the ferric cyanide form. In the ferric neutral state at pH 6.0, the quantum mechanical mixing of the high and intermediate spin configurations might explain the peculiar frequencies of ν₂ and ν₃ in the RR spectra. The heme ruffling imposed by IsdG and IsdI gives rise to unique electronic structures of heme, which are expected to modulate the first and subsequent steps of the heme oxygenation.

Active Sites of O2-Evolving Chlorite Dismutases Probed by Halides and Hydroxides and New Iron-Ligand Vibrational Correlations

O₂-evolving chlorite dismutases (Clds) fall into two subfamilies, which efficiently convert ClO₂^- to O₂ and Cl^-. The Cld from Dechloromonas aromatica (DaCld) represents the chlorite-decomposing homopentameric enzymes found in perchlorate- and chlorate-respiring bacteria. The Cld from the Gram-negative human pathogen Klebsiella pneumoniae (KpCld) is representative of the second subfamily, comprising homodimeric enzymes having truncated N-termini. Here steric and nonbonding properties of the DaCld and KpCld active sites have been probed via kinetic, thermodynamic, and spectroscopic behaviors of their fluorides, chlorides, and hydroxides. Cooperative binding of Cl^- to KpCld drives formation of a hexacoordinate, high-spin aqua heme, whereas DaCld remains pentacoordinate and high-spin under analogous conditions. Fluoride coordinates to the heme iron in KpCld and DaCld, exhibiting ν(Fe^III-F) bands at 385 and 390 cm^-1, respectively. Correlation of these frequencies with their CT1 energies reveals strong H-bond donation to the F^- ligand, indicating that atoms directly coordinated to heme iron are accessible to distal H-bond donation. New vibrational frequency correlations between either ν(Fe^III-F) or ν(Fe^III-OH) and ν(Fe^II-His) of Clds and other heme proteins are reported. These correlations orthogonalize proximal and distal effects on the bonding between iron and exogenous π-donor ligands. The axial Fe-X vibrations and the relationships between them illuminate both similarities and differences in the H-bonding and electrostatic properties of the distal and proximal heme environments in pentameric and dimeric Clds. Moreover, they provide general insight into the structural basis of reactivity toward substrates in heme-dependent enzymes and their mechanistic intermediates, especially those containing the ferryl moiety.

Autoxidation of Reduced Horse Heart Cytochrome c Catalyzed by Cardiolipin-Containing Membranes

Visible circular dichroism, absorption, and fluorescence spectroscopy were used to probe the binding of horse heart ferrocytochrome c to anionic cardiolipin (CL) head groups on the surface of 1,1',2,2'-tetraoleoyl cardiolipin (TOCL)/1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) (20%:80%) liposomes in an aerobic environment. We found that ferrocytochrome c undergoes a conformational transition upon binding that leads to complete oxidation of the protein at intermediate and high CL concentrations. At low lipid concentrations, the protein maintains a structure that is only slightly different from its native one, whereas an ensemble of misligated predominantly hexacoordinated low-spin states become increasingly populated at high lipid concentrations. A minor fraction of conformations with either high- or quantum-mixed-spin states were detected at a CL to protein ratio of 200 (the largest one investigated). The population of the non-native state is less pronounced than that found for cytochrome c-CL interactions initiated with oxidized cytochrome c. Under anaerobic conditions, the protein maintains its reduced state but still undergoes some conformational change upon binding to CL head groups on the liposome surface. Our data suggest that CL-containing liposomes function as catalysts by reducing the activation barrier for a Fe²⁺ → O₂ electron transfer. Adding NaCl to the existing cytochrome-liposome mixtures under aerobic conditions inhibits protein autoxidation of ferrocytochrome c and stabilizes the reduced state of the membrane-bound protein.

Insertion of heme b into the structure of the Cys34-carbamidomethylated human lipocalin α(1)-microglobulin: formation of a [(heme)(2) (α(1)-Microglobulin)](3) complex

α(1)-Microglobulin (α(1)m) is a 26 kDa plasma and tissue protein belonging to the lipocalin protein family. Previous investigations indicate that the protein interacts with heme and suggest that it has a function in heme metabolism. However, detailed characterizations of the α(1)m-heme interactions are lacking. Here, we report for the first time the preparation and analysis of a stable α(1)m-heme complex upon carbamidomethylation of the reactive Cys34 by using recombinantly expressed human α(1)m. Analytical size-exclusion chromatography coupled with a diode-array absorbance spectrophotometry demonstrates that at first an α(1)m-heme monomer is formed. Subsequently, a second heme triggers oligomerization that leads to trimerization. The resulting (α(1)m[heme](2))(3) complex was characterized by resonance Raman and EPR spectroscopy, which support the presence of two ferrihemes, thus indicating an unusual spin-state admixed ground state with S=(3)/(2), (5)/(2).

Eukaryotic extracellular catalase-peroxidase from Magnaporthe grisea - Biophysical/chemical characterization of the first representative from a novel phytopathogenic KatG group

All phytopathogenic fungi have two catalase-peroxidase paralogues located either intracellularly (KatG1) or extracellularly (KatG2). Here, for the first time a secreted bifunctional, homodimeric catalase-peroxidase (KatG2 from the rice blast fungus Magnaporthe grisea) has been produced heterologously with almost 100% heme occupancy and comprehensively investigated by using a broad set of methods including UV-Vis, ECD and resonance Raman spectroscopy (RR), thin-layer spectroelectrochemistry, mass spectrometry, steady-state & presteady-state spectroscopy. RR spectroscopy reveals that MagKatG2 shows a unique mixed-spin state, non-planar heme b, and a proximal histidine with pronounced imidazolate character. At pH 7.0 and 25 °C, the standard reduction potential E°' of the Fe(III)/Fe(II) couple for the high-spin native protein was found to fall in the range typical for the KatG family. Binding of cyanide was relatively slow at pH 7.0 and 25 °C and with a K(d) value significantly higher than for the intracellular counterpart. Demonstrated by mass spectrometry MagKatG2 has the typical Trp118-Tyr251-Met277 adduct that is essential for its predominantly catalase activity at the unique acidic pH optimum. In addition, MagKatG2 acts as a versatile peroxidase using both one- and two-electron donors. Based on these data, structure-function relationships of extracellular eukaryotic KatGs are discussed with respect to intracellular KatGs and possible role(s) in host-pathogen interaction.

Ligand changes in ferric species of the giant extracellular hemoglobin ofGlossoscolex paulistusas function of pH: correlations between redox, spectroscopic and oligomeric properties and general implications with different hemoproteins

The present review is focused on the relationship between oligomeric and heme properties of HbGp, emphasizing the characteristics that can be generalized to other hemoproteins. This study represents the state-of-the-art with respect to the approaches for investigating giant extracellular hemoglobins as well as the correlation between oligomeric assembly alterations and their consequent changes in the first coordination sphere. A wide introduction focused on the properties of this hemoglobin is developed. Indeed, this hemoprotein is considered an interesting prototype of blood substitute and biosensor due to its peculiar properties, such as resistance to autoxidation and oligomeric stability. Previous studies by our group employing UV-vis, EPR and CD spectroscopies have been revised in a complete approach, in agreement with recent and relevant data from the literature. In fact, a consistent and inter-related spectroscopic study is described propitiating a wide assignment of "fingerprint" peaks found in the techniques evaluated in this paper. This review furnishes physicochemical information regarding the identification of ferric heme species of hemoproteins and metallic complexes through their spectroscopic bands. This effort at the attribution of UV-vis, EPR and CD peaks is not restricted to HbGp, and includes a comparative analysis of several hemoproteins involving relevant implications regarding several types of iron-porphyrin systems.

Linear Free-Energy Relationships in the Protonation Equilibria and Acid–Base Catalysed Reaction of 4-Substituted Benzohydroxamic Acids

As part of our investigations on the structure-reactivity relationships in protonation and hydrolysis of hydroxamic acids, we have now studied a series of 4-substituted benzohydroxamic acids (4-X-C6H4.CO.NH(OH), with X = H, Me, OMe, Cl, Br, F and NO2) in aqueous sulfuric acid. Good correlation was observed pointing out the validity of the Hammett equation. The protonation constant, ionization constant and rate constant for acid- and base-catalysed hydrolysis reactions seems are to be related in a linear free-energy (LEER) fashion.

Kinetics and Mechanism of the Mineral Acid Catalysed Reactions of Hydroxamic Acids

There is currently a great deal of interest in the chemistry of hydroxamic acids. In recent years we have been studying the synthesis, structure and nucleophilicities of hydroxamic acids. This paper reports a kinetic study of reactivity of some hydroxamic acids RC(O)·N(OH)R′; R=C6H5·CH=CH, R′=4-CH3·C6H4 [p-tolylcinnamo hydroxamic acid]; R=C6H5, R′=4-CH3·C6H4 [p-tolylbenzo hydroxamic acid]; R=C6H5, R′=2-CH3·C6H4 [o-tolylbenzo hydroxamic acid] in aqueous mineral acids (HCl, HClO4 and H2SO4). The rate data of hydrolysis reaction revealed that the reactivity/stability sequence of the compounds is generally p-TBHA>o-TBHA>p-TCHA. An excess acidity analysis reveals that the reaction proceeds by nucleophilic attack of water molecule on the protonated substrate.

EPR and ENDOR studies of Fe(II) hemoproteins reduced and oxidized at 77 K

gamma-irradiation of frozen solutions of Fe(II) hemoproteins at 77 K generates both electron paramagnetic resonance (EPR) active singly reduced and oxidized heme centers trapped in the conformation of the Fe(II) precursors. The reduction products of pentacoordinate (S = 2) Fe(II) globins, peroxidases and cytochrome P450cam show EPR and electron-nuclear double resonance (ENDOR) spectra characteristic of (3d 7) Fe(I) species. In addition, cryoreduced Fe(II) alpha-chains of hemoglobin and myoglobin exhibit an S = 3/2 spin state produced by antiferromagnetic coupling between a porphyrin anion radical and pentacoordinate (S = 2) Fe(II). The spectra of cryoreduced forms of Fe(II) hemoglobin alpha-chains and deoxymyoglobin reveal that the Fe(II) precursors adopt multiple conformational substates. Reduction of hexacoordinate Fe(II) cytochrome c and cytochrome b5 as well as carboxy complexes of deoxyglobins produces only Fe(II) porphyrin pi-anion radical species. The low-valent hemoprotein intermediates produced by cryoreduction convert to the Fe(II) states at T > 200 K. Cryogenerated Fe(III) cytochrome c and cytochrome b5 have spectra similar to these for the resting Fe(III) states, whereas the spectra of the products of cryooxidation of pentacoordinate Fe(II) globins and peroxidases are different. Cryooxidation of CO-Fe(II) globins generates Fe(III) hemes with quantum-mechanically admixed S = 3/2, 5/2 ground states. The trapped Fe(III) species relax to the equilibrium ferric states upon annealing at T > 190 K. Both cryooxidized and reduced centers provide very sensitive EPR/ENDOR structure probes of the EPR-silent Fe(II) state.

Modification of the active site of Mycobacterium tuberculosis KatG after disruption of the Met-Tyr-Trp cross-linked adduct

Mycobacterium tuberculosis catalase-peroxidase (Mtb KatG) is a bifunctional enzyme that possesses both catalase and peroxidase activities and is responsible for the activation of the antituberculosis drug isoniazid. Mtb KatG contains an unusual adduct in its distal heme-pocket that consists of the covalently linked Trp107, Tyr229, and Met255. The KatG(Y229F) mutant lacks this adduct and has decreased steady-state catalase activity and enhanced peroxidase activity. In order to test a potential structural role of the adduct that supports catalase activity, we have used resonance Raman spectroscopy to probe the local heme environment of KatG(Y229F). In comparison to wild-type KatG, resting KatG(Y229F) contains a significant amount of 6-coordinate, low-spin heme and a more planar heme. Resonance Raman spectroscopy of the ferrous-CO complex of KatG(Y229F) suggest a non-linear Fe-CO binding geometry that is less tilted than in wild-type KatG. These data provide evidence that the Met-Tyr-Trp adduct imparts structural stability to the active site of KatG that seems to be important for sustaining catalase activity.

Phosphate group effects upon the equilibrium of iron(III) meso-tetrakis (4-N-methylpyridiniumyl) porphyrin in aqueous solution

Iron(III) meso-tetrakis (4-N-methylpyridiniumyl) porphyrin (FeTMPyP) undergoes a complex equilibrium in aqueous solution as a function of pH. Use of phosphate buffers, a common practice in biomedical applications of porphyrins, suggests the complexation of phosphate anion at the sixth coordination position to the iron, which contributes to the complexity of the equilibrium in the pH range from 1 to 4. In the absence of phosphate the equilibrium is simplified in a similar way as in the presence of high salt concentrations. Combined use of optical absorption, (1)H NMR and infrared spectroscopies, together with the literature data, suggest the formation of hexacoordinated monoaqueous-phosphate FeTMPyP complex in a limited acidic pH range. Discussion of the behavior of cationic FeTMPyP as compared to anionic iron(III) meso-tetrakis (4-sulfonatophenyl) porphyrin (FeTPPS(4)) is presented in regard to equilibrium of different species to explain the observed complex equilibrium.

Pentacoordinate and hexacoordinate ferric hemes in acid medium: EPR, UV–Vis and CD studies of the giant extracellular hemoglobin of Glossoscolex paulistus

The equilibrium complexity involving different axially coordinated hemes is peculiar to hemoglobins. The pH dependence of the spontaneous exchange of ligands in the extracellular hemoglobin from Glossoscolex paulistus was studied using UV-Vis, EPR, and CD spectroscopies. This protein has a complex oligomeric assembly with molecular weight of 3.1 MDa that presents an important cooperative effect. A complex coexistence of different species was observed in almost all pH values, except pH 7.0, where just aquomet species is present. Four new species were formed and coexist with the aquomethemoglobin upon acidification: (i) a "pure" low-spin hemichrome (Type II), also called hemichrome B, with an usual spin state (d(xy))(2)(d(xz),d(yz))(3); (ii) a strong g(max) hemichrome (Type I), also showing an usual spin state (d(xy))(2)(d(xz),d(yz))(3); (iii) a hemichrome with unusual spin state (d(xz),d(yz))(4)(d(xy))(1) (Type III); (iv) and a high-spin pentacoordinate species. CD measurements suggest that the mechanism of species formation could be related with an initial process of acid denaturation. However, it is worth mentioning that based on EPR the aquomet species remains even at acidic pH, indicating that the transitions are not complete. The "pure" low-spin hemichrome presents a parallel orientation of the imidazole ring planes but the strong g(max) hemichrome is a HALS (highly anisotropic low-spin) species indicating a reciprocally perpendicular orientation of the imidazole ring planes. The hemichromes and pentacoordinate formation mechanisms are discussed in detail.

Fifteen years of Raman spectroscopy of engineered heme containing peroxidases: what have we learned?

Spectroscopic techniques have been fundamental to the comprehension of peroxidase function under physiological conditions. This Account examines the contribution to our understanding of heme peroxidases provided by electronic and resonance Raman spectroscopies in conjunction with site-directed mutagenesis. The results obtained over 15 years with several heme peroxidases and selected mutants have provided important insights into the influence exerted by the protein in the vicinity of the active site via key amino acids on the functionality and stability of the enzymes. Moreover, resonance Raman spectroscopy has revealed that a common feature of heme peroxidases is the presence of an extensive network of H-bonds coupling the distal and proximal sides, which has a profound influence on the heme ligation, affecting both the fifth and the sixth coordination sites.

New Insight into the Peroxidase−Hydroxamic Acid Interaction Revealed by the Combination of Spectroscopic and Crystallographic Studies

Aromatic hydroxamic acids, such as salicylhydroxamic (SHA) and benzohydroxamic (BHA) acids, are commonly used as probes for studying the active sites of peroxidases. In this paper, we have extended the study of the complexes of Arthromyces ramosus peroxidase (ARP/CIP) with BHA and SHA by analyzing their Raman spectra in solution and in single crystals. The experiments were carried out under various conditions to identify the best experimental conditions, and hence, avoid artifacts deriving from the preparation of the samples or collection of the spectra. The analysis of the data takes also into account the characteristic of the electronic absorption spectra in solution and the crystal structures of the complexes. The results showed small differences between the solution and the crystal phases even though the coordination state can be dramatically affected by the physical or chemical conditions. The greater sensitivity of the spectroscopic technique enabled us to establish the existence of multiple species upon complexation of the protein with the hydroxamic acids that could not be detected by ordinary X-ray crystallography. Furthermore, SHA titration experiments and singular value decomposition analysis of the absorption spectra indicated the presence of two binding sites in the protein, one with a high affinity (K(d) = 1.7 mM), which should correspond to the SHA bound protein as determined by X-ray, and the other with a very low affinity (K(d) > 80 mM) probably located in a non-heme site. This suggests that the heterogeneous titration line shape involves ligand binding to a non-heme site in competition with the canonical heme site. In contrast, the titration profile obtained with the BHA ligand is monophasic, in agreement with all the peroxidases so far studied.

Accessibility of oxygen with respect to the heme pocket in horseradish peroxidase

Oxygen and other molecules of similar size take part in a variety of protein reactions. Therefore, it is critical to understand how these small molecules penetrate the protein matrix. The protein system studied in this case is horseradish peroxidase (HRP). We have converted the native HRP into a phosphorescent analog by replacing the heme prosthetic group by Pd-mesoporphyrin. Oxygen readily quenches the phosphorescence of Pd porphyrins, and this can be used to characterize oxygen diffusion through the protein matrix. Our measurements indicate that solvent viscosity and pH modulate the accessibility of the heme pocket relative to small molecules. The binding of the substrate benzohydroxamic acid (BHA) to the protein drastically impedes oxygen access to the heme pocket. These results indicate that, first, the penetration of small molecules through the protein matrix is a function of protein dynamics, and second, there are specific pathways for the diffusion of these molecules. The effect of substrate and pH on protein dynamics has been investigated with the use of molecular dynamics calculations. We demonstrate that the model of a "fluctuating entry point," as suggested by Zwanzig (J Chem Phys 1992;97:3587-3589), properly describes the diffusion of oxygen through the protein matrix.

Heme structural perturbation of PEG-modified horseradish peroxidase C in aromatic organic solvents probed by optical absorption and resonance Raman dispersion spectroscopy

The heme structure perturbation of poly(ethylene glycol)-modified horseradish peroxidase (HRP-PEG) dissolved in benzene and toluene has been probed by resonance Raman dispersion spectroscopy. Analysis of the depolarization ratio dispersion of several Raman bands revealed an increase of rhombic B(1g) distortion with respect to native HRP in water. This finding strongly supports the notion that a solvent molecule has moved into the heme pocket where it stays in close proximity to one of the heme's pyrrole rings. The interactions between the solvent molecule, the heme, and the heme cavity slightly stabilize the hexacoordinate high spin state without eliminating the pentacoordinate quantum mixed spin state that is dominant in the resting enzyme. On the contrary, the model substrate benzohydroxamic acid strongly favors the hexacoordinate quantum mixed spin state and induces a B(2g)-type distortion owing to its position close to one of the heme methine bridges. These results strongly suggest that substrate binding must have an influence on the heme geometry of HRP and that the heme structure of the enzyme-substrate complex (as opposed to the resting state) must be the key to understanding the chemical reactivity of HRP.

Conformational differences in Mycobacterium tuberculosis catalase-peroxidase KatG and its S315T mutant revealed by resonance Raman spectroscopy

KatG from Mycobacterium tuberculosis is a heme-containing catalase-peroxidase, which belongs to the class I peroxidases and is important for activation of the prodrug isoniazid (INH), a front-line antituberculosis drug. In many clinical isolates, resistance to INH has been linked to mutations on the katG gene, and the most prevalent mutation, S315T, suggests that modification of the heme pocket has occurred. Electronic absorption and resonance Raman spectra of ferric wild-type (WT) KatG and its INH-resistant mutant KatG(S315T) at different pH values and their complexes with INH and benzohydroxamic acid (BHA) are reported. At neutral pH, a quantum mechanically mixed spin state (QS) is revealed, which coexists with five-coordinate and six-coordinate high-spin hemes in WT KatG. The QS heme is the major species in KatG(S315T). Addition of either INH or BHA to KatG induces only minor changes in the resonance Raman spectra, indicating that both compounds do not directly interact with the heme iron. New vibrational modes are observed at 430, 473, and 521 cm(-1), and these modes are indicative of a change in conformation in the KatG heme pocket. The intensity of these modes and the relative population of the QS heme are stable in KatG(S315T) but not in the WT enzyme. This indicates that there are differences in heme pocket stability between WT KatG and KatG(S315T). We will discuss the stabilization of the QS heme and propose a model for the inhibition of INH oxidation by KatG(S315T).

Effect of protein dynamics upon reactions that occur in the heme pocket of horseradish peroxidase

Free base and Pd porphyrin derivatives of horseradish peroxidase show long-lived excited states that are quenched by the presence of the peroxidase inhibitor, benzhydroxamic acid. The relaxation times of the excited-state luminescence and the rates of the quenching reaction for these derivatives of peroxidase were monitored as a function of pH, temperature, and viscosity with the view of examining how protein dynamics affect the quenching reaction. As solvent viscosity increases, the rate decreases, but at the limit of very high viscosity (i.e., high glycerol or sugar glass) the quenching still occurs. A model is presented that is consistent with the known structure of the enzyme-inhibitor complex. It is considered that the inhibitor is held at an established position but that solvent-dependent and independent motions allow a limited diffusion of the two reactants. Since there is a steep dependence upon distance and orientation, the diffusion toward the favorable position for reaction enhances the reaction rate. The solvent viscosity dependent and independent effects were separated and analyzed. The importance of internal reaction dynamics is demonstrated in the observation that rigidity of solvent imposed by incorporating the protein into glass at room temperature allows the reaction to occur, while the reaction is inhibited at low temperature. The results emphasize that protein dynamics plays a role in determining reaction rates.

Analysis of heme structural heterogeneity in Mycobacterium tuberculosis catalase-peroxidase (KatG)

Mycobacterium tuberculosis catalase-peroxidase (KatG) is a heme enzyme considered important for virulence, which is also responsible for activation of the anti-tuberculosis pro-drug isoniazid. Here, we present an analysis of heterogeneity in KatG heme structure using optical, resonance Raman, and EPR spectroscopy. Examination of ferric KatG under a variety of conditions, including enzyme in the presence of fluoride, chloride, or isoniazid, and at different stages during purification in different buffers allowed for assignment of spectral features to both five- and six-coordinate heme. Five-coordinate heme is suggested to be representative of "native" enzyme, since this species was predominant in the enzyme examined immediately after one chromatographic protocol. Quantum mechanically mixed spin heme is the most abundant form in such partially purified enzyme. Reduction and reoxidation of six-coordinate KatG or the addition of glycerol or isoniazid restored five-coordinate heme iron, consistent with displacement of a weakly bound distal water molecule. The rate of formation of KatG Compound I is not retarded by the presence of six-coordinate heme either in wild-type KatG or in a mutant (KatG[Y155S]) associated with isoniazid resistance, which contains abundant six-coordinate heme. These results reveal a number of similarities and differences between KatG and other Class I peroxidases.

Purification and characterization of a new cationic peroxidase from fresh flowers of Cynara scolymus L

A basic heme peroxidase isoenzyme (AKPC) has been purified to homogeneity from artichoke flowers (Cynara scolymus L.). The enzyme was shown to be a monomeric glycoprotein, M(r)=42300+/-1000, (mean+/-S.D.) with an isoelectric point >9. The native enzyme exhibits a typical peroxidase ultraviolet-visible spectrum with a Soret peak at 404 nm (epsilon=137,000+/-3000 M(-1) cm(-1)) and a Reinheitzahl (Rz) value (A(404nm)/A(280nm)) of 3.8+/-0.2. The ultraviolet-visible absorption spectra of compounds I, II and III were typical of class III plant peroxidases but unlike horseradish peroxidase isoenzyme C, compound I was unstable. Resonance Raman and UV-Vis spectra of the ferric form show that between pH 5.0 and 7.0 the protein is mainly 6 coordinate high spin with a water molecule as the sixth ligand. The substrate-specificity of AKPC is characteristic of class III (guaiacol-type) peroxidases with chlorogenic and caffeic acids, that are abundant in artichoke flowers, as particularly good substrates at pH 4.5. Ferric AKPC reacts with hydrogen peroxide to yield compound I with a second-order rate constant (k(+1)) of 7.4 x 10(5) M(-1) s(-1) which is significantly slower than that reported for most other class III peroxidases. The reaction of ferric and ferrous AKPC with nitric oxide showed a potential use of this enzyme for quantitative spectrophotometric determination of NO and as a component of novel NO sensitive electrodes.

Sol-gel encapsulated horseradish peroxidase: a catalytic material for peroxidation

This study addresses the viability of sol-gel encapsulated HRP (HRP:sol-gel) as a recyclable solid-state catalytic material. Ferric, ferric-CN, ferrous, and ferrous-CO forms of HRP:sol-gel were investigated by resonance Raman and UV-visible methods. Electronic and vibrational spectroscopic changes associated with changes in spin state, oxidation state, and ligation of the heme in HRP:sol-gel were shown to correlate with those of HRP in solution, showing that the heme remains a viable ligand-binding complex. Furthermore, the high-valent HRP:sol-gel intermediates, compound I and compound II, were generated and identified by time-resolved UV-visible spectroscopy. Catalytic activity of the HRP:sol-gel material was demonstrated by enzymatic assays by using I(-), guaiacol, and ABTS as substrates. Encapsulated HRP was shown to be homogeneously distributed throughout the sol-gel host. Differences in turnover rates between guaiacol and I(-) implicate mass transport of substrate through the silicate matrix as a defining parameter in the peroxidase activity of HRP:sol-gel. HRP:sol-gel was reused as a peroxidation catalyst for multiple reaction cycles without loss of activity, indicating that such materials show promise as reusable catalytic materials.

Mutation of residues critical for benzohydroxamic acid binding to horseradish peroxidase isoenzyme C

Aromatic substrate binding to peroxidases is mediated through hydrophobic and hydrogen bonding interactions between residues on the distal side of the heme and the substrate molecule. The effects of perturbing these interactions are investigated by an electronic absorption and resonance Raman study of benzohydroxamic acid (BHA) binding to a series of mutants of horseradish peroxidase isoenzyme C (HRPC). In particular, the Phe179 --> Ala, His42 --> Glu variants and the double mutant His42 --> Glu:Arg38 --> Leu are studied in their ferric state at pH 7 with and without BHA. A comparison of the data with those previously reported for wild-type HRPC and other distal site mutants reaffirms that in the resting state mutation of His42 leads to an increase of 6-coordinate aquo heme forms at the expense of the 5-coordinate heme state, which is the dominant species in wild-type HRPC. The His42Glu:Arg38Leu double mutant displays an enhanced proportion of the pentacoordinate heme state, similar to the single Arg38Leu mutant. The heme spin states are insensitive to mutation of the Phe179 residue. The BHA complexes of all mutants are found to have a greater amount of unbound form compared to the wild-type HRPC complex. It is apparent from the spectral changes induced on complexation with BHA that, although Phe179 provides an important hydrophobic interaction with BHA, the hydrogen bonds formed between His42 and, in particular, Arg38 and BHA assume a more critical role in the binding of BHA to the resting state.

The critical role of the proximal calcium ion in the structural properties of horseradish peroxidase

The extent to which the structural Ca(2+) ions of horseradish peroxidase (HRPC) are a determinant in defining the heme pocket architecture is investigated by electronic absorption and resonance Raman spectroscopy upon removal of one Ca(2+) ion. The Fe(III) heme states are modified upon Ca(2+) depletion, with an uncommon quantum mechanically mixed spin state becoming the dominant species. Ca(2+)-depleted HRPC forms complexes with benzohydroxamic acid and CO which display spectra very similar to those of native HRPC, indicating that any changes to the distal cavity structural properties upon Ca(2+) depletion are easily reversed. Contrary to the native protein, the Ca(2+)-depleted ferrous form displays a low-spin bis-histidyl heme state and a small proportion of high-spin heme. Furthermore, the nu(Fe-Im) stretching mode downshifts 27 cm(-1) upon Ca(2+) depletion revealing a significant structural perturbation of the proximal cavity near the histidine ligand. The specific activity of the Ca(2+)-depleted enzyme is 50% that of the native form. The effects on enzyme activity and spectral features observed upon Ca(2+) depletion are reversible upon reconstitution. Evaluation of the present and previous data firmly favors the proximal Ca(2+) ion as that which is lost upon Ca(2+) depletion and which likely plays the more critical role in regulating the heme pocket structural and catalytic properties.

Cationic ascorbate peroxidase isoenzyme II from tea: structural insights into the heme pocket of a unique hybrid peroxidase

The novel class III ascorbate peroxidase isoenzyme II from tea leaves (TcAPXII), with an unusually high specific ascorbate peroxidase activity associated with stress response, has been characterized by resonance Raman (RR), electronic absorption, and Fourier transform infrared (FT-IR) spectroscopies. Ferric and ferrous forms and the complexes with fluoride, cyanide, and CO have been studied at various pH values. The overall blue shift of the electronic absorption spectrum, the high RR frequencies of the core size marker bands, similar to those of 6-coordinate low-spin heme, and the complex RR spectrum in the low-frequency region of ferric TcAPXII indicate that this protein contains an unusual 5-coordinate quantum mechanically mixed-spin heme. The spectra of both the fluoride and the CO adducts suggest that these exogenous ligands are strongly hydrogen-bonded with a residue that appears to be unique to this peroxidase. Electronic absorption spectra also emphasize structural differences between the benzhydroxamic acid binding sites of TcAPXII and horseradish peroxidases (HRPC). It is concluded that TcAPXII is a paradigm peroxidase since it is the first example of a hybrid enzyme that combines spectroscopic signatures, structural elements, and substrate specificities previously reported only for distinct class I and class III peroxidases.