Microperoxidase/H2O2-catalyzed aromatic hydroxylation proceeds by a cytochrome-P-450-type oxygen-transfer reaction mechanism

Electrocatalysis by Heme Enzymes—Applications in Biosensing

Heme proteins take part in a number of fundamental biological processes, including oxygen transport and storage, electron transfer, catalysis and signal transduction. The redox chemistry of the heme iron and the biochemical diversity of heme proteins have led to the development of a plethora of biotechnological applications. This work focuses on biosensing devices based on heme proteins, in which they are electronically coupled to an electrode and their activity is determined through the measurement of catalytic currents in the presence of substrate, i.e., the target analyte of the biosensor. After an overview of the main concepts of amperometric biosensors, we address transduction schemes, protein immobilization strategies, and the performance of devices that explore reactions of heme biocatalysts, including peroxidase, cytochrome P450, catalase, nitrite reductase, cytochrome c oxidase, cytochrome c and derived microperoxidases, hemoglobin, and myoglobin. We further discuss how structural information about immobilized heme proteins can lead to rational design of biosensing devices, ensuring insights into their efficiency and long-term stability.

Delineating Heme-Mediated versus Direct Protein Oxidation in Peroxidase-Activated Cytochrome c by Top-Down Mass Spectrometry

Oxidation of key residues in cytochrome c (cyt c) by chloramine T (CT) converts the protein from an electron transporter to a peroxidase. This peroxidase-activated state represents an important model system for exploring the early steps of apoptosis. CT-induced transformations include oxidation of the distal heme ligand Met80 (MetO, +16 Da) and carbonylation (LysCHO, -1 Da) in the range of Lys53/55/72/73. Remarkably, the 15 remaining Lys residues in cyt c are not susceptible to carbonylation. The cause of this unusual selectivity is unknown. Here we applied top-down mass spectrometry (MS) to examine whether CT-induced oxidation is catalyzed by heme. To this end, we compared the behavior of cyt c with (holo-cyt c) and without heme (apo_SS-cyt c). CT caused MetO formation at Met80 for both holo- and apo_SS-cyt c, implying that this transformation can proceed independently of heme. The aldehyde-specific label Girard's reagent T (GRT) reacted with oxidized holo-cyt c, consistent with the presence of several LysCHO. In contrast, oxidized apo-cyt c did not react with GRT, revealing that LysCHO forms only in the presence of heme. The heme dependence of LysCHO formation was further confirmed using microperoxidase-11 (MP11). CT exposure of apo_SS-cyt c in the presence of MP11 caused extensive nonselective LysCHO formation. Our results imply that the selectivity of LysCHO formation at Lys53/55/72/73 in holo-cyt c is caused by the spatial proximity of these sites to the reactive (distal) heme face. Overall, this work highlights the utility of top-down MS for unravelling complex oxidative modifications.

Electrochemical Biosensors Employing Natural and Artificial Heme Peroxidases on Semiconductors

Heme peroxidases are widely used as biological recognition elements in electrochemical biosensors for hydrogen peroxide and phenolic compounds. Various nature-derived and fully synthetic heme peroxidase mimics have been designed and their potential for replacing the natural enzymes in biosensors has been investigated. The use of semiconducting materials as transducers can thereby offer new opportunities with respect to catalyst immobilization, reaction stimulation, or read-out. This review focuses on approaches for the construction of electrochemical biosensors employing natural heme peroxidases as well as various mimics immobilized on semiconducting electrode surfaces. It will outline important advances made so far as well as the novel applications resulting thereof.

Enhancing microperoxidase activity and selectivity: immobilization in metal-organic frameworks

Microperoxidases 8 (MP8) and 11 (MP11) are heme-containing peptides obtained by the proteolytic digestion of Cytochrome c. They act as mini-enzymes that combine both peroxidase-like and Cytochrome P450-like activities that may be useful in the synthesis of fine chemicals or in the degradation of environmental pollutants. However, their use is limited by their instability in solution due to (i) the bleaching of the heme in the presence of an excess of H2O2, (ii) the decoordination of the distal histidine ligand of the iron under acidic conditions and, (iii) their tendency to aggregate in aqueous alkaline solutions, even at low concentrations. Additionally, both MP8 and MP11 show relatively low selectivity, due to the lack of control of the substrates by a specific catalytic pocket on the distal face of the heme. Both stability and selectivity issues can be effectively addressed by immobilization of microperoxidases in solid matrices, which can also lead to their possible recycling from the reaction medium. Considering their relatively small size, the pore inclusion of MPs into Metal-Organic Frameworks appeared to be more adequate compared to other immobilization methods that have been widely investigated for decades. The present minireview describes the catalytic activities of MP8 and MP11, their limitations, and various results describing their immobilization into MOFs which led to MP11- or MP8@MOF hybrid materials that display good activity in the oxidation of dyes and phenol derivatives, with remarkable recyclability due to the stabilization of the MPs inside the MOF cavities. An example of selective oxidation of dyes according to their charge by MP8@MOF hybrid materials is also highlighted.

Solvent Composition Drives the Rebinding Kinetics of Nitric Oxide to Microperoxidase

The rebinding kinetics of NO after photodissociation from microperoxidase (Mp-9) is studied in different solvent environments. In mixed glycerol/water (G/W) mixtures the dissociating ligand rebinds with a yield close to 1 due to the cavities formed by the solvent whereas in pure water the ligand can diffuse into the solvent after photodissociation. In the G/W mixture, only geminate rebinding on the sub-picosecond and 5 ps time scales was found and the rebinding fraction is unity which compares well with available experiments. Contrary to that, simulations in pure water find two time scales – ~10 ps and ~200 ps - indicating that both, geminate rebinding and rebinding after diffusion of NO in the surrounding water contribute. The rebinding fraction is around 0.63 within 1 ns which is in stark contrast with experiment. Including ions (Na and Cl) at 0.15 M concentration in water leads to rebinding kinetics tending to that in the glycerol/water mixture and yields agreement with experiments. The effect of temperature is also probed and found to be non-negligible. The present simulations suggest that NO rebinding in Mp is primarily driven by thermal fluctuations which is consistent with recent resonance Raman spectroscopy experiments and simulations on MbNO.

Design and engineering of artificial oxygen-activating metalloenzymes

Many efforts are being made in the design and engineering of metalloenzymes with catalytic properties fulfilling the needs of practical applications. Progress in this field has recently been accelerated by advances in computational, molecular and structural biology. This review article focuses on the recent examples of oxygen-activating metalloenzymes, developed through the strategies of de novo design, miniaturization processes and protein redesign. Considerable progress in these diverse design approaches has produced many metal-containing biocatalysts able to adopt the functions of native enzymes or even novel functions beyond those found in Nature.

Various strategies for obtaining oxidative artificial hemoproteins with a catalytic oxidative activity: from "Hemoabzymes" to "Hemozymes"?

The design of artificial hemoproteins that could lead to new biocatalysts for selective oxidation reactions using clean oxidants such as O 2 or H 2 O 2 under ecocompatible conditions constitutes a really promising challenge for a wide range of industrial applications. In vivo, such reactions are performed by heme-thiolate proteins, cytochromes P450, that catalyze the oxidation of drugs by dioxygen in the presence of electrons delivered from NADPH by cytochrome P450 reductase. Several strategies were used to design new artificial hemoproteins to mimic these enzymes, that associate synthetic metalloporphyrin derivatives to a protein that is supposed to induce a selectivity in the catalyzed reaction. A first generation of artificial hemoproteins or "hemoabzymes" was obtained by the non-covalent association of synthetic hemes such as N-methyl-mesoporphyrin IX, Fe(III) -α3β-tetra-o-carboxyphenylporphyrin or microperoxidase 8 with monoclonal antibodies raised against these cofactors. The obtained antibody-metalloporphyrin complexes displayed a peroxidase activity and some of them catalyzed the regio-selective nitration of phenols by H 2 O 2/ NO 2 and the stereo-selective oxidation of sulphides by H 2 O 2. A second generation of artificial hemoproteins or "hemozymes", was obtained by the non-covalent association of non-relevant proteins with metalloporphyrin derivatives. Several strategies were used, the most successful of which, named "host-guest" strategy involved the non-covalent incorporation of metalloporphyrin derivatives into easily affordable proteins. The artificial hemoproteins obtained were found to be able to perform efficiently the stereoselective oxidation of organic compounds such as sulphides and alkenes by H 2 O 2 and KHSO 5.

From “hemoabzymes” to “hemozymes”: towards new biocatalysts for selective oxidations

Two generations of artificial hemoproteins have been obtained: “hemoabzymes”, by non-covalent association of synthetic hemes with monoclonal antibodies raised against these cofactors and “hemozymes”, by non-covalent association of non-relevant proteins with metalloporphyrin derivatives. A review of the different strategies employed as well as their structural and catalytic properties is presented here.

Solution Coordination Chemistry of Protohaemin IX Peptide Derivatives

As part of an investigation aimed at developing artificial models for mimicking haemoprotein activity, four C-aminoacyl protohaemin IX derivatives with one or two -L-Leu-L- HisOMe (complexes 3 and 1) or -β-Ala-L- HisOMe (complexes 4 and 2) peptide fragments have been studied by means of electronic and ESR spectroscopy. The bis-His complexes 1 and 2 are predominantly low-spin ( S = 1/2) species, while the mono-His complexes 3 and 4 are predominantly high-spin ( S = 5/2) species. Products of the treatment of starting complexes 1–4 with trichloroacetic acid, acetic acid, methylimidazole, sodium cyanide and sodium hydroxide have been analysed by electronic spectroscopy. Weak-field ligands lead to predominantly high-spin hexacoordinated complexes, while strong-field ligands result in predominantly low-spin ones.

Modification of norfloxacin by a Microbacterium sp. strain isolated from a wastewater treatment plant

Antimicrobial residues found in municipal wastewater may increase selective pressure on microorganisms for development of resistance, but studies with mixed microbial cultures derived from wastewater have suggested that some bacteria are able to inactivate fluoroquinolones. Medium containing N-phenylpiperazine and inoculated with wastewater was used to enrich fluoroquinolone-modifying bacteria. One bacterial strain isolated from an enrichment culture was identified by 16S rRNA gene sequence analysis as a Microbacterium sp. similar to a plant growth-promoting bacterium, Microbacterium azadirachtae (99.70%), and a nematode pathogen, "M. nematophilum" (99.02%). During growth in medium with norfloxacin, this strain produced four metabolites, which were identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and nuclear magnetic resonance (NMR) analyses as 8-hydroxynorfloxacin, 6-defluoro-6-hydroxynorfloxacin, desethylene norfloxacin, and N-acetylnorfloxacin. The production of the first three metabolites was enhanced by ascorbic acid and nitrate, but it was inhibited by phosphate, amino acids, mannitol, formate, and thiourea. In contrast, N-acetylnorfloxacin was most abundant in cultures supplemented with amino acids. This is the first report of defluorination and hydroxylation of a fluoroquinolone by an isolated bacterial strain. The results suggest that some bacteria may degrade fluoroquinolones in wastewater to metabolites with less antibacterial activity that could be subject to further degradation by other microorganisms.

Insights into porphyrin chemistry provided by the microperoxidases, the haempeptides derived from cytochrome c

The water-soluble haem-containing peptides obtained by proteolytic digestion of cytochrome c, the microperoxidases, have been used to explore aspects of the chemistry of iron porphyrins, and as mimics for some reactions catalysed by the haemoproteins, including the reactions catalysed by the peroxidases and the cytochromes P450. The preparation of the microperoxidases, their physical and chemical properties including their electronic structure, the kinetics and thermodynamics of their reactions with ligands, electrochemical studies and examples of their uses as haemoproteins mimics, is reviewed.

Proton assisted oxygen-oxygen bond splitting in cytochrome p450

Proton assisted O-O bond splitting of cytochromes' P450 hydroperoxo Compound 0 has been investigated by density functional theory, showing a barrier for the slightly endothermic formation of the iron-oxo Compound I. The barrier and the endothermicity increase with decreasing acidity of the distal proton source. Protonation of the proximal iron heme ligand favors the O-O bond scission and provides an important regulatory component in the catalytic cycle. The Compound 0 --> I conversion is slightly exothermic for the peroxidase and catalase models. Implications of the energetic relationship between the two reactive intermediates are discussed in terms of possible oxidative pathways.

Heme-thiolate haloperoxidases: versatile biocatalysts with biotechnological and environmental significance

Heme-thiolate haloperoxidases are undoubtedly the most versatile biocatalysts of the hemeprotein family and share catalytic properties with at least three further classes of heme-containing oxidoreductases, namely, classic plant and fungal peroxidases, cytochrome P450 monooxygenases, and catalases. For a long time, only one enzyme of this type--the chloroperoxidase (CPO) of the ascomycete Caldariomyces fumago--has been known. The enzyme is commercially available as a fine chemical and catalyzes the unspecific chlorination, bromination, and iodation (but no fluorination) of a variety of electrophilic organic substrates via hypohalous acid as actual halogenating agent. In the absence of halide, CPO resembles cytochrome P450s and epoxidizes and hydroxylates activated substrates such as organic sulfides and olefins; aromatic rings, however, are not susceptible to CPO-catalyzed oxygen-transfer. Recently, a second fungal haloperoxidase of the heme-thiolate type has been discovered in the agaric mushroom Agrocybe aegerita. The UV-Vis adsorption spectrum of the isolated enzyme shows little similarity to that of CPO but is almost identical to a resting-state P450. The Agrocybe aegerita peroxidase (AaP) has strong brominating as well as weak chlorinating and iodating activities, and catalyzes both benzylic and aromatic hydroxylations (e.g., of toluene and naphthalene). AaP and related fungal peroxidases could become promising biocatalysts in biotechnological applications because they seemingly fill the gap between CPO and P450 enzymes and act as "self-sufficient" peroxygenases. From the environmental point of view, the existence of a halogenating mushroom enzyme is interesting because it could be linked to the multitude of halogenated compounds known from these organisms.

A new microperoxidase from Marinobacter hydrocarbonoclasticus

The preparation and characterization of a new microperoxidase obtained from proteinase K-treated cytochrome c(552) from Marinobacter hydrocarbonoclasticus (previously known as Pseudomonas nautica) are presented. This microperoxidase (MMP-5) has novel structural properties relative to previously reported microperoxidases, as the two intervening amino acid (X) residues within the consensual CXXCH c-type heme binding motif are missing, yielding a heme-pentapeptide with increased solubility in aqueous solvents and a 1-2 order of magnitude higher stability of the monomeric state relative to canonical microperoxidases. The electronic spectra in the near-UV and visible regions have been studied as a function of MMP-5 concentration and pH. The spectroscopic properties of MMP-5 are typical of microperoxidases with high-spin hexa- or pentacoordinate heme species dominant in the 1-8 pH range and low-spin states prevailing at higher pH values. In the presence of hydrogen peroxide, MMP-5 displays peroxidatic activities towards several compounds.

The coordination of imidazole and substituted pyridines by the hemeoctapeptide N-acetyl-ferromicroperoxidase-8 (FeIINAcMP8)

The N-terminus acetylated ferric hemeoctapeptide from cytochrome c, N-acetylmicroperoxidase-8 (Fe(III)-NAcMP8) can be reduced by dithionite in aqueous solution to produce Fe(II)-NAcMP8. The UV-Vis spectrum has a broad Soret band and relatively poorly defined Q bands which is consistent with a mixture of a five-coordinate high spin species with His as the axial ligand and a six-coordinate, predominantly high spin species with His/H(2)O as axial ligands. There are two spectroscopically observable pK(a)s at 8.7+/-0.1 and 10.9+/-0.2 which are attributed to ionization of a heme propionic acid group and coordinated H(2)O, respectively; a pK(a) > or = 14 is due to ionization of the proximal His ligand. Equilibrium constants were determined by UV-Vis spectrophotometry at 25.0+/-0.2 degrees C and 0.5 M ionic strength (NaClO(4)) for the coordination of imidazole and a number of substituted pyridines, and complement available data for the ferric hemepeptide, allowing a comparison to be made of the affinity of an iron porphyrin with Fe in the +2 and +3 oxidation states towards these ligands. Imidazole is coordinated more strongly by the ferric porphyrin (log K=4.08) than by the ferrous porphyrin (log K=3.40). The equilibrium constants for coordination of pyridines by the ferric and ferrous porphyrins increase and decrease, respectively, with increasing ligand basicity. Values determined by cyclic voltammetry show the same dependence on the identity of the ligand. In the ferric porphyrin, the stability of the complex increases with the basicity of the ligand and hence its ability to donate electron density onto the metal. In the case of the more electron rich ferrous porphyrin, greater stability occurs with pyridine ligands that have an electron withdrawing group and hence can accept electron density from the metal. This is consistent with the midpoint reduction potentials E(1/2) of the pyridine complexes determined by cyclic voltammetry; E(1/2) is linearly dependent on, and becomes more negative with an increase in, ligand basicity. Log K for coordination of pyridines by the ferrous hemepeptide correlates well with the energy of the ligand frontier orbital with pi symmetry, suggesting that pi-bonding effects are significant in determining the strength of binding of pyridines by a ferrous porphyrin.

Spectroscopic investigation of isonitrile complexes of ferric and ferrous microperoxidase 8

Microperoxidase 8 (MP8) is able to react with alkyl- and aryl-isonitriles (RNC) both in its reduced and oxidized states, to form MP8Fe(II)- and MP8Fe(III)-CNR complexes. The coordination and spin states of these complexes have been fully characterized by UV-visible and resonance Raman spectroscopies. Both MP8Fe(II)- and MP8Fe(III)-CNR complexes are hexacoordinate low-spin complexes, which bear a single RNC ligand on the distal face of the heme and keep the His 18 ligand on its proximal face, trans to the RNC ligand. A comparison of these characteristics with those of the Fe-CNR complexes of other hemoproteins suggests that both MP8Fe(II)- and MP8Fe(III)-CNR complexes present a Fe-C-N linear arrangement. This may be due to the lack of any interactions of the RNC ligand with the octapeptide of MP8 that is mainly located over the opposite face of the heme. Finally the formation of hexacoordinate low-spin MP8Fe(II)- and MP8Fe(III)-CNR complexes constitutes a new example of the reactivity of MP8 with a new class of weak sigma-donating and strong pi-accepting ligands, which adds to its already very rich coordination chemistry.

Characterization of a novel microperoxidase from Marinobacter hydrocarbonoclasticus by electrospray ionization tandem mass spectrometry

Microperoxidases are small heme-peptides obtained by proteolytic digestion of cytochrome c, exhibiting peroxidase activity. They consist of a short- or medium-length polypeptide chain, covalently linked to an iron protoporphyrin IX moiety via two thioether bonds involving Cys residues at the c-porphyrin A and B pyrrole rings. These small molecules are interesting for a wide range of possible applications. We have structurally characterized, by means of electrospray ionization (ESI) mass and tandem mass spectrometric experiments, a novel microperoxidase called MMP-5 (Marinobacter MicroPeroxidase-5), obtained by proteolytic digestion of cytochrome c552, a monoheminic electron-transfer protein isolated from Marinobacter hydrocarbonoclasticus. This microperoxidase, which still maintains the functional peptide moieties for peroxidase activity, is devoid of the two amino acids intercalating the Cys residues linked to the c-porphyrin, thus increasing its water solubility. Once submitted to the ESI source potential, MMP-5 showed an interesting tendency for the reduction of the iron protoporphyrin substructure. This behaviour was clearly evidenced by the mass shift exhibited by the reduced form.

Models and mechanisms of O-O bond activation by cytochrome P450. A critical assessment of the potential role of multiple active intermediates in oxidative catalysis

Cytochrome P450 enzymes promote a number of oxidative biotransformations including the hydroxylation of unactivated hydrocarbons. Whereas the long-standing consensus view of the P450 mechanism implicates a high-valent iron-oxene species as the predominant oxidant in the radicalar hydrogen abstraction/oxygen rebound pathway, more recent studies on isotope partitioning, product rearrangements with 'radical clocks', and the impact of threonine mutagenesis in P450s on hydroxylation rates support the notion of the nucleophilic and/or electrophilic (hydro)peroxo-iron intermediate(s) to be operative in P450 catalysis in addition to the electrophilic oxenoid-iron entity; this may contribute to the remarkable versatility of P450s in substrate modification. Precedent to this mechanistic concept is given by studies with natural and synthetic P450 biomimics. While the concept of an alternative electrophilic oxidant necessitates C-H hydroxylation to be brought about by a cationic insertion process, recent calculations employing density functional theory favour a 'two-state reactivity' scenario, implicating the usual ferryl-dependent oxygen rebound pathway to proceed via two spin states (doublet and quartet); state crossing is thought to be associated with either an insertion or a radicalar mechanism. Hence, challenge to future strategies should be to fold the disparate and sometimes contradictory data into a harmonized overall picture.

Microperoxidase 8 adsorbed on a roughened silver electrode as a monomeric high-spin penta-coordinated species: characterization by SERR spectroscopy and electrochemistry

Microperoxidase 8 (MP8), a heme octapeptide obtained by hydrolytic digestion of cytochrome c, was adsorbed at the surface of a roughened silver electrode in order to provide a new supported biomimetic system for hemoproteins. A combination of two techniques was used to study its redox and coordination properties: electrochemistry and surface-enhanced resonance Raman (SERR) spectroscopy. This allowed us to show that MP8 could be adsorbed as a monolayer at the surface of the roughened silver electrode, where it could undergo a reversible electron transfer. Under those conditions, a redox potential of -0.4 V vs. SCE (-0.16 V vs. NHE) was measured for MP8, which was almost identical to that reported for N-acetyl-MP8 in aqueous solution. In addition, whereas MP8 appeared to aggregate in solution, and led to a mixture of high-spin penta-coordinated (5cHS) and low-spin hexa-coordinated (6cLS) iron(III) or iron(II) species, it was recovered almost exclusively as a monomeric high-spin penta-coordinated species at the surface of the electrode, both in the reduced and in the oxidized states. This then allowed a free coordination site on the iron, on the distal face of MP8 accessible to ligands. Accordingly, experiments performed in the presence of potassium cyanide demonstrated that MP8 adsorbed on a silver electrode could be ligated by a sixth CN(-) ligand. Thus there is the possibility of binding several kinds of ligands such as O(2) or H(2)O(2), which will open the way to biocatalysis of oxidation reactions at the surface of an electrode, or ligands such as drugs which will lead to the design of new biosensors for molecules of biological interest.

Oxidation of pyocyanin, a cytotoxic product from Pseudomonas aeruginosa, by microperoxidase 11 and hydrogen peroxide

Pyocyanin (1-hydroxy-N-methylphenazine) is a cytotoxic pigment secreted by the bacterial species Pseudomonas aeruginosa, which frequently infects the lungs of immunosuppressed patients as well as those with cystic fibrosis. Pyocyanin toxicity results presumably from the ability of the compound to undergo reduction by NAD(P)H and subsequent generation of superoxide and H2O2 directly in the lungs. We report that in the presence of peroxidase mimics, microperoxidase 11, or hemin, pyocyanin undergoes oxidation by H2O2, as evidenced by loss of the pigment's characteristic absorption spectrum and by EPR detection of a free radical metabolite. The oxidation of pyocyanin is irreversible, suggesting an extensive modification of the pigment's phenazine chromophore. Oxidation of pyocyanin was observed also when exogenous H2O2 was replaced by a H2O2-generating system consisting of NADH and the pigment itself. That the oxidation involves the phenolate group of pyocyanin was verified by the observation that a related pigment, phenazine methosulfate, which is devoid of this group, does not undergo oxidation by microperoxidase 11/H2O2. In contrast to intact pyocyanin, oxidized pyocyanin was less efficient in NADH oxidation and stimulation of interleukin-8 release by human alveolar epithelial A549 cells in vitro, suggesting that oxidation of pyocyanin leads to its inactivation. This study demonstrates that pyocyanin may play a dual role in biological systems, first as an oxidant and ROS generator, and second as a substrate for peroxidases, contributing to H2O2 removal. This latter property may cause pyocyanin degradation and inactivation, which may be of considerable biomedical interest.

Reactivity study on microperoxidase-8

The catalytic activity of the microperoxidase-8/H(2)O(2) system toward tyramine and 3-(4-hydroxyphenyl)propionic acid has been determined in acetate buffer, pH 5.0. Operating with a strong excess of hydrogen peroxide, the rate-determining step of the reaction was substrate oxidation. Owing to the fast microperoxidase-8 degradation, only the very initial phase of the reactions were analyzed. The reaction rates follow a substrate saturation behavior, with turnover numbers [ k(cat)=26+/-1 s(-1) for 3-(4-hydroxyphenyl)propionic acid and k(cat)=22+/-1 s(-1) for tyramine] that were similar for the two substrates. In contrast, the K(M) values indicated a reduced affinity for the catalyst active species by the positively charged phenol, probably due to repulsive interaction with the protonated N-terminal microperoxidase-8 amino group. The reactivity of the catalyst active species was studied upon incubation of microperoxidase-8 with a small excess hydrogen peroxide, followed by reaction with the phenolic substrates. The kinetic analysis showed that more than two active species are accumulated. The species responsible for the faster reactions was present in solution as a minor fraction. The active intermediate which accumulated in a larger amount (intermediate III) has a reduced substrate oxidation activity. Comparison of this activity with the kinetic constants obtained under turnover experiments shows that intermediate III is not involved in the microperoxidase-8 catalytic cycle. The active species of the catalytic process are intermediates I and II, which in the absence of substrate rapidly convert to intermediate III.

Effects of the environment on microperoxidase-11 and on its catalytic activity in oxidation of organic sulfides to sulfoxides

Microperoxidase-11 (MP-11, also known as heme undecapeptide of cytochrome c) was immobilized by encapsulation into sol-gel silica glass and by physisorption, chemisorption, and covalent attachment to silica gel. We then compared these species with one another and with dissolved microperoxidase-11 as catalysts for the sulfoxidation of methyl phenyl sulfide by hydrogen peroxide. MP-11 is prone to oligomerization in solution, both via axial ligation and via intermolecular interactions. When the ligation oligomerization is suppressed upon immobilization, heme becomes more accessible, and the sulfoxide yield increases 4-6 times, from 15% up to 95%. When the ligation oligomerization of dissolved MP-11 is suppressed by protonation and acetylation of amino groups and by addition of methanol, sodium dodecyl sulfate (SDS), or trifluoroethanol, the sulfoxide yield increases 3-5 times (up to 76%). The oligomerization via intermolecular interactions is important for preserving enantioselectivity in immobilized and dissolved MP-11. For MP-11 in amine-rich and especially alcohol-rich environments, the enantioselectivity is vanishingly low, presumably because amino and hydroxyl groups cause a conformation change in the catalyst. In other environments, the MP-11 species are aggregated via intermolecular interactions in micellar (SDS) solution and on the surface of the silica gel, or via axial ligation in aqueous buffer at pH 6.0. Under these conditions, the enantioselectivity is enhanced; the enantiomeric excess (ee) becomes as high as 46%. An understanding of the effects of the aggregation state and consequent properties on the catalytic activity of MP-11 allowed us to control the yield and enantioselectivity of sulfoxidation reaction.

Microperoxidase 8 catalysed nitrogen oxides formation from oxidation of N-hydroxyguanidines by hydrogen peroxide

Nitric oxide (NO) is a potent intra- and intercellular messenger involved in the control of vascular tone, neuronal signalling and host response to infection. In mammals, NO is synthesized by oxidation of l-arginine catalysed by hemeproteins called NO-synthases with intermediate formation of Nomega-hydroxy-l-arginine (NOHA). NOHA and some hydroxyguanidines have been shown to be able to deliver nitrogen oxides including NO in the presence of various oxidative systems. In this study, NOHA and a model compound, N-(4-chlorophenyl)-N'-hydroxyguanidine, were tested for their ability to generate NO in the presence of a haemprotein model, microperoxidase 8 (MP8), and hydrogen peroxide. Nitrite and nitrate production along with selective formation of 4-chlorophenylcyanamide was observed from incubations of N-(4-chlorophenyl)-N'-hydroxyguanidine in the presence of MP8 and hydrogen peroxide. In the case of NOHA, the corresponding cyanamide, Ndelta-cyano-L-ornithine, was too unstable under the conditions used and l-citrulline was the only product identified. A NO-specific conversion of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide to 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl and formation of MP8-Fe-NO complexes were observed by EPR spectroscopy and low-temperature UV/visible spectroscopy, respectively. These results clearly demonstrate the formation of nitrogen oxides including NO from the oxidation of exogenous hydroxyguanidines by hydrogen peroxide in the presence of a minienzyme such as MP8. The importance of the bioactivation of endogenous (NOHA) or exogenous N-hydroxyguanidines by peroxidases of physiological interest remains to be established in vivo.

Hemoabzymes: towards new biocatalysts for selective oxidations

Catalytic antibodies with a metalloporphyrin cofactor or <<hemoabzymes>>, used as models for hemoproteins like peroxidases and cytochrome P450, represent a promising route to catalysts tailored for selective oxidation reactions. A brief overview of the literature shows that until now, the first strategy for obtaining such artificial hemoproteins has been to produce antiporphyrin antibodies, raised against various free-base, N-substituted Sn-, Pd- or Fe-porphyrins. Five of them exhibited, in the presence of the corresponding Fe-porphyrin cofactor, a significant peroxidase activity, with k(cat)/K(m) values of 3.7 x 10(3) - 2.9 x 10(5) M(-1) min(-1). This value remained, however, low when compared to that of peroxidases. This strategy has also led to a few models of cytochrome P450. The best of them, raised against a water-soluble tin(IV) porphyrin containing an axial alpha-naphtoxy ligand, was reported to catalyze the stereoselective oxidation of aromatic sulfides by iodosyl benzene using a Ru(II)-porphyrin cofactor. The relatively low efficiency of the porphyrin-antibody complexes is probably due, at least in part, to the fact that no proximal ligand of Fe has been induced in those antibodies. We then proposed to use, as a hapten, microperoxidase 8 (MP8), a heme octapeptide in which the imidazole side chain of histidine 18 acts as a proximal ligand of the iron atom. This led to the production of seven antibodies recognizing MP8, the best of them, 3A3, binding it with an apparent binding constant of 10(-7) M. The corresponding 3A3-MP8 complex was found to have a good peroxidase activity characterized by a k(cat)/K(m) value of 2 x 10(6) M(-1) min(-1), which constitutes the best one ever reported for an antibody-porphyrin complex. Active site topology studies suggest that the binding of MP8 occurs through interactions of its carboxylate substituents with amino acids of the antibody and that the protein brings a partial steric hindrance of the distal face of the heme of MP8. Consequently, the use of the 3A3-MP8 complexes for the selective oxidation of substrates, such as sulfides, alkanes and alkenes will be undertaken in the future.

Does P450-type catalysis proceed through a peroxo-iron intermediate? A review of studies with microperoxidase

Recent stopped-flow kinetics demonstrated the existence of an intermediate before the occurrence of the final product of the reaction of both iron-containing microperoxidase-8 (Fe(III)MP-8) and manganese-containing microperoxidase-8 (Mn(III)MP-8) with H(2)O(2). The intermediate was assigned to be (hydro)peroxo-iron. With both mini-catalysts the final state obtained after 30-40 ms showed a resemblance to PorM(IV)MP-8[double bond]O(R(+)*); (R(+)*) is a radical located at the peptide. Quantum mechanical calculations indicate that hydroperoxo-iron is inactive as a catalytic intermediate in cytochrome P450 (P450)-type catalysis. Instead, the calculations suggest that peroxo-iron acts as the catalytic intermediate in P450-type catalysis. In addition, the calculations demonstrate that, although less likely, the possibility that oxenoid-iron acts as a catalytic intermediate in P450 catalysis cannot be fully excluded. An interesting aspect of the reactions catalysed by MP-8 is the possibility that, in view of the reversibility of the reactions between (hydro)peroxo-iron and oxenoid-iron, H(2)O plays a decisive role, at least in some cytochromes P450, in the removal of halogens, avoiding the production of compounds hazardous to the organism.

The regioselectivity of glutathione adduct formation with flavonoid quinone/quinone methides is pH-dependent

In the present study, the formation of glutathionyl adducts from a series of 3',4'-dihydroxy flavonoid o-quinone/p-quinone methides was investigated with special emphasis on the regioselectivity of the glutathione addition as a function of pH. The flavonoid o-quinones were generated using horseradish peroxidase, and upon purification by HPLC, the glutathionyl adducts were identified by LC/MS as well as (1)H and (13)C NMR. The major pH effect observed for the glutathione conjugation of taxifolin and luteolin quinone is on the rate of taxifolin and luteolin conversion and, as a result, on the ratio of mono- to diglutathione adduct formation. With fisetin, 3,3',4'-trihydroxyflavone, and quercetin, decreasing the pH results in a pathway in which glutathionyl adduct formation occurs in the C ring of the flavonoid, being initiated by hydration of the quinone and H(2)O adduct formation also in the C ring of the flavonoid. With increasing pH, for fisetin and 3,3',4'-trihydroxyflavone glutathione adduct formation of the quinone occurs in the B ring at C2' as the preferential site. For quercetin, the adduct formation of its quinone/quinone methide shifts from the C ring at pH 3.5, to the A ring at pH 7.0, to the B ring at pH 9.5, indicating a significant influence of the pH and deprotonation state on the chemical electrophilic behavior of quercetin quinone/quinone methide. Together the results of the present study elucidate the mechanism of the pH-dependent electrophilic behavior of B ring catechol flavonoids, which appears more straightforward than previously foreseen.

The nature of the intermediates in the reactions of Fe(III)- and Mn(III)-microperoxidase-8 with H(2)O(2): a rapid kinetics study

Kinetic studies were performed with microperoxidase-8 (Fe(III)MP-8), the proteolytic breakdown product of horse heart cytochrome c containing an octapeptide linked to an iron protoporphyrin IX. Mn(III) was substituted for Fe(III) in Mn(III)MP-8. The mechanism of formation of the reactive metal-oxo and metal-hydroperoxo intermediates of M(III)MP-8 upon reaction of H(2)O(2) with Fe(III)MP-8 and Mn(III)MP-8 was investigated by rapid-scan stopped-flow spectroscopy and transient EPR. Two steps (k(obs1) and k(obs2)) were observed and analyzed for the reaction of hydrogen peroxide with both catalysts. The plots of k(obs1) as function of [H(2)O(2)] at pH 8.0 and pH 9.1 for Fe(III)MP-8, and at pH 10.2 and pH 10.9 for Mn(III)MP-8, exhibit saturation kinetics, which reveal the accumulation of an intermediate. Double reciprocal plots of 1/k(obs1) as function of 1/[H(2)O(2)] at different pH values reveal a competitive effect of protons in the oxidation of M(III)MP-8. This effect of protons is confirmed by the linear dependence of 1/k(obs1) on [H(+)] showing that k(obs1) increases with the pH. The UV-visible spectra of the intermediates formed at the end of the first step (k(obs1)) exhibit a spectrum characteristic of a high-valent metal-oxo intermediate for both catalysts. Transient EPR of Mn(III)MP-8 incubated with an excess of H(2)O(2), at pH 11.5, shows the detection of a free radical signal at g approximately equal to 2 and of a resonance at g approximately equal to 4 characteristic of a Mn(IV) (S = 3/2) species. On the basis of these results, the following mechanism is proposed: (i) M(III)MP-8-OH(2) is deprotonated to M(III)MP-8-OH in a rapid preequilibrium step, with a pK(a) = 9.2 +/- 0.9 for Fe(III)MP-8 and a pK(a) = 11.2 +/- 0.3 for Mn(III)MP-8; (ii) M(III)MP-8-OH reacts with H(2)O(2) to form Compound 0, M(III)MP8-OOH, with a second-order rate constant k(1) = (1.3 +/- 0.6) x 10(6) M(-1) x s(-1) for Fe(III)MP-8 and k(1) = (1.6 +/- 0.9) x 10(5) M(-1) x s(-1) for Mn(III)MP-8; (iii) this metal-hydroperoxo intermediate is subsequently converted to a high-valent metal-oxo species, M(IV)MP-8=O, with a free radical on the peptide (R(*+)). The first-order rate constants for the cleavage of the hydroperoxo group are k(2) = 165 +/- 8 s(-1) for Fe(III)MP-8 and k(2) = 145 +/- 7 s(-1) for Mn(III)MP-8; and (iv) the proposed M(IV)MP-8=O(R(*+)) intermediate slowly decays (k(obs2)) with a rate constant of k(obs2) = 13.1 +/- 1.1 s(-)(1) for Fe(III)MP-8 and k(obs2) = 5.2 +/- 1.2 s(-1) for Mn(III)MP-8. The results show that Compound 0 is formed prior to what is analyzed as a high-valent metal-oxo peptide radical intermediate.

N-hydroxyguanidines as new heme ligands: UV-visible, EPR, and resonance Raman studies of the interaction of various compounds bearing a C=NOH function with microperoxidase-8

Interaction between microperoxidase-8 (MP8), a water-soluble hemeprotein model, and a wide range of N-aryl and N-alkyl N'-hydroxyguanidines and related compounds has been investigated using UV-visible, EPR, and resonance Raman spectroscopies. All the N-hydroxyguanidines studied bind to the ferric form of MP8 with formation of stable low-spin iron(III) complexes characterized by absorption maxima at 405, 535, and 560 nm. The complex obtained with N-(4-methoxyphenyl) N'-hydroxyguanidine exhibits EPR g-values at 2.55, 2.26, and 1.86. The resonance Raman (RR) spectrum of this complex is also in agreement with an hexacoordinated low-spin iron(III) structure. The dissociation constants (K(s)) of the MP8 complexes with mono- and disubstituted N-hydroxyguanidines vary between 15 and 160 microM at pH 7.4. Amidoximes also form low-spin iron(III) complexes of MP8, although with much larger dissociation constants. Under the same conditions, ketoximes, aldoximes, methoxyguanidines, and guanidines completely fail to form such complexes with MP8. The K(s) values of the MP8-N-hydroxyguanidine complexes decrease as the pH of the solution is increased, and the affinity of the N-hydroxyguanidines toward MP8 increases with the pK(a) of these ligands. Altogether these results show that compounds involving a -C(NHR)=NOH moiety act as good ligands of MP8-Fe(III) with an affinity that depends on the electron-richness of this moiety. The analysis of the EPR spectrum of the MP8-N-hydroxyguanidine complexes according to Taylor's equations shows a strong axial distortion of the iron, typical of those observed for hexacoordinated heme-Fe(III) complexes with at least one pi donor axial ligand (HO(-), RO(-), or RS(-)). These data strongly suggest that N-hydroxyguanidines bind to MP8 iron via their oxygen atom after deprotonation or weakening of their O-H bond. It thus seems that N-hydroxyguanidines could constitute a new class of strong ligands for hemeproteins and iron(III)-porphyrins.

Microperoxidase 8 catalyzed nitration of phenol by nitrogen dioxide radicals

Microperoxidase 8 (MP8) is a heme octapeptide obtained by hydrolytic digestion of horse heart cytochrome c. At pH below 9, the heme iron is axially coordinated to the imidazole side chain of His18 and to a water molecule. Replacement of this weak ligand by H2O2 allows the formation of high-valent iron-oxo species which are responsible for both peroxidase-like and cytochrome P450-like activities of MP8. This paper shows that MP8 is able to catalyze the nitration of phenol by nitrite. The reaction requires H2O2 and is inhibited by ligands having a high affinity for the iron, catalase and radical scavengers. This suggests that the nitrating species could be NO2* radicals formed by the oxidation of nitrite by high-valent iron-oxo species. This new activity of MP8 opens a new access to nitro-aromatic compounds under mild conditions and validates the use of this minienzyme to mimick heme peroxidases, especially in the reactions of NO-derived species with biomolecules under oxidative stress conditions.

Structure-activity study on the quinone/quinone methide chemistry of flavonoids

A structure-activity study on the quinone/quinone methide chemistry of a series of 3',4'-dihydroxyflavonoids was performed. Using the glutathione trapping method followed by HPLC, (1)H NMR, MALDI-TOF, and LC/MS analysis to identify the glutathionyl adducts, the chemical behavior of the quinones/quinone methides of the different flavonoids could be deduced. The nature and type of mono- and diglutathionyl adducts formed from quercetin, taxifolin, luteolin, fisetin, and 3,3',4'-trihydroxyflavone show how several structural elements influence the quinone/quinone methide chemistry of flavonoids. In line with previous findings, glutathionyl adduct formation for quercetin occurs at positions C6 and C8 of the A ring, due to the involvement of quinone methide-type intermediates. Elimination of the possibilities for efficient quinone methide formation by (i) the absence of the C3-OH group (luteolin), (ii) the absence of the C2=C3 double bond (taxifolin), or (iii) the absence of the C5-OH group (3,3',4'-trihydroxyflavone) results in glutathionyl adduct formation at the B ring due to involvement of the o-quinone isomer of the oxidized flavonoid. The extent of di- versus monoglutathionyl adduct formation was shown to depend on the ease of oxidation of the monoadduct as compared to the parent flavonoid. Finally, unexpected results obtained with fisetin provide new insight into the quinone/quinone methide chemistry of flavonoids. The regioselectivity and nature of the quinone adducts that formed appear to be dependent on pH. At pH values above the pK(a) for quinone protonation, glutathionyl adduct formation proceeds at the A or B ring following expected quinone/quinone methide isomerization patterns. However, decreasing the pH below this pK(a) results in a competing pathway in which glutathionyl adduct formation occurs in the C ring of the flavonoid, which is preceded by protonation of the quinone and accompanied by H(2)O adduct formation, also in the C ring of the flavonoid. All together, the data presented in this study confirm that quinone/quinone methide chemistry can be far from straightforward, but the study provides significant new data revealing an important pH dependence for the chemical behavior of this important class of electrophiles.

Porphyrin-Fe(III)-hydroperoxide and porphyrin-Fe(III)-peroxide anion as catalytic intermediates in cytochrome P450-catalyzed hydroxylation reactions: a molecular orbital study

The hydroxylation of fluorobenzene and aniline, catalyzed by the porphyrin-Fe(III)-peroxide anion with either a cysteinate- or a histidyl-type of axial ligand as well as the hydroxylation of fluorobenzene, catalyzed by porphyrin-Fe(III)-hydroperoxide with a cysteinate-type of axial ligand as catalytic intermediates, have been investigated by electronic structure calculations in local spin-density approximation. Non-repulsive potential curves are, in contrast with porphyrin-Fe(III)-hydroperoxide, obtained only in the case of porphyrin-Fe(III)-peroxide anion as catalytic intermediate. The mutual substrate-porphyrin orientation with a dihedral angle between the plane of the substrate and the porphyrin plane of 45 degrees is more favorable compared with the parallel orientation between these two planes. This orientation differs for the case of fluorobenzene hydroxylation from the corresponding one calculated by us with the ferryl-oxo-pi-cation radical complex as a catalytic intermediate. The calculated reaction profiles show also the effectiveness of the histidyl-type coordinated porphyrin-Fe(III)-peroxide involved in P450 type of hydroxylation reactions. The calculations demonstrate the predominant role of the O1-O2 moiety of the porphyrin-Fe(III)-peroxide anion in the hydroxylation process of the substrates. The results indicate that the porphyrin-Fe(III)-peroxide anion is an effective catalytic species in hydroxylation reactions. In all the studied cases irrespective of the substrate and the nature of the axial ligand, the potential curves reach minimum at approximately 130-140 pm, expressing the length of an aromatic C-O bond.

Formation of iron(II)-nitrosoalkane complexes: a new activity of microperoxidase 8

Microperoxidase 8 (MP8) is a heme octapeptide, obtained by enzymatic hydrolysis of heart cytochrome c, in which a histidine is axially coordinated to the heme iron, and acts as its fifth ligand. It exhibits two kinds of activities: a peroxidase-like activity and a cytochrome P450-like activity. We here show that MP8 is not only able to oxidize various aliphatic and aromatic hydroxylamines with the formation of MP8-Fe(II)-nitrosoalkane or -arene complexes absorbing around 414 nm, but also that these complexes can be obtained by reduction of nitroalkanes. This is the first example of fully characterized iron(II)-metabolite complexes of MP8. Such complexes constitute good models for those obtained upon oxidation of amphetamine or macrolids by cytochromes P450. In addition, this is a new catalytic activity of MP8, which validates the use of this mini-enzyme as a convenient model for hemoproteins of interest in toxicology and pharmacology such as cytochromes P450 and peroxidases.

Heme-(hydro)peroxide mediated O- and N-dealkylation. A study with microperoxidase

The mechanism of microperoxidase-8 (MP-8) mediated O- and N-dealkylation was investigated. In the absence of ascorbate (peroxidase mode), many unidentified polymeric products are formed and the extent of substrate degradation correlates (r = 0.94) with the calculated substrate ionization potential, reflecting the formation of radical intermediates. In the presence of ascorbate (P450 mode) formation of polymeric products is largely prevented but, surprisingly, dealkylation is not affected. In addition, aromatic hydroxylation and oxidative dehalogenation is observed. The results exclude a radical mechanism and indicate the involvement of a (hydro)peroxo-iron heme intermediate in P450-type of heteroatom dealkylation.

Investigation of the mechanism of action of microperoxidase-11, (MP11), a potential anti-cataract agent, with hydrogen peroxide and ascorbate

The interaction of hydrogen peroxide, ascorbate and microperoxidase-11 (MP11), a ferriheme undecapeptide derived from cytochrome c, has been investigated using spectrophotometry, oxymetry, electron paramagnetic resonance (EPR), and mass spectroscopy techniques. It is shown that in 50 m M phosphate pH 7. 0-7.4 in the absence of other reactants H(2)O(2)induces a concentration-dependent decrease in absorption at the Soret band (399 nm) of the microperoxidase, with concomitant H(2)O(2)decomposition and oxygen evolution. The reaction causes irreversible heme degradation, concomitant with loss of enzymatic activity. Ascorbate effectively protects MP11 from degradation and inhibits oxygen evolution. At ascorbate concentrations greater than that of H(2)O(2), microperoxidase degradation is almost completely prevented. Mass spectrometry showed that H(2)O(2)oxidizes the microperoxidase to a monooxygenated product, which did not form if ascorbate was included in the reaction system. There appears to be a 1:1 relationship between H(2)O(2)degradation and ascorbate oxidation. EPR experiments revealed that an ascorbate radical was formed during the reaction. These reactions may be described by a scheme where a putative 'compound I' of the microperoxidase is reduced by ascorbate back to the original redox state (ferric) of the peroxidase in two one-electron steps, concomitantly with oxidation of the ascorbate to an ascorbate radical or in one two-electron transfer step forming dehydroascorbate. In the absence of ascorbate, the 'compound I' reacts further with the peroxide causing microperoxidase degradation and partial oxygen evolution. These observations are relevant to the interaction of ferrihemes with H(2)O(2)and ascorbic acid and may be pertinent for the potential application of MP11 as an anti-cataract agent.

A mechanism for oxygen exchange between ligated oxometalloporphinates and bulk water

Oxygen exchange between high-valent metal-oxo complexes and bulk water has been monitored for nonligated model porphyrins (hemin, FeTDCPPS, MnTMPyP) and the axially ligated microperoxidase-8 (MP-8). Exchange extents up to 90% were measured for MP-8 in spite of the presence of an axial histidine ligand and accompanied by the formation of nonlabelled H(2)O(2) from H(2)(18)O(2). These results point to the existence of a mechanism for oxygen exchange between the high-valent iron-oxo complex and the solvent different from the so-called "oxo-hydroxo tautomerism." Regeneration of the primary oxidant, H(2)O(2), and oxygen exchange by axially ligated porphyrins can be explained by a mechanism involving the reversibility of compound I formation.