Hemes and hemoproteins. 5: Kinetics of the peroxidatic activity of microperoxidase-8: model for the peroxidase enzymes

Novel Identification of a Sulfur-Centered, Radical-Derived 5,5-Dimethyl-1-pyrroline N-Oxide Nitrone Adduct Formed from the Oxidation of DTT by LC/ELISA, LC/Electrospray Ionization-MS, and LC/Tandem MS

The detection of highly reactive free radicals generated in biological systems by an ESR spin-trapping technique is always difficult and limited due to the short lifetimes of ESR active spin-trapping radical adducts and poor structural information provided by ESR spectra. In this investigation, we have for the first time employed anti-5,5-dimethyl-1-pyrroline N-oxide (DMPO) polyclonal antiserum that specifically recognizes stable, ESR silent end products of DMPO radical adducts and combined HPLC with ELISA, electrospray ionization mass spectrometry (ESI-MS), and tandem mass spectrometry (MS/MS) to separate and characterize DMPO nitrone adducts derived from free radical metabolites. When mircoperoxidase-11 (MP-11) reacted with DTT in the presence of DMPO with or without H2O2, we detected radical-derived DMPO nitrone adducts by ELISA. Similar results were obtained when MP-11 was replaced by hemin. To identify the DMPO nitrone adducts formed in both reaction systems, LC separation was carried out, and the fractions eluted from the LC column were collected and analyzed by ELISA. In both reaction mixtures, we found that only one peak with the same retention time showed a strong positive ELISA signal, suggesting that this peak was from radical-derived DMPO nitrone adducts and that both systems produced the same free radical metabolites. Using online LC/ESI-MS, LC/MS/MS, and (1)H NMR, we demonstrated that the DMPO nitrone adducts formed are from the DMPO adducts of the sulfur-centered radical of DTT. The successful application of LC/ELISA, LC/MS, and LC/MS/MS in this study makes it possible to separate and identify the stable DMPO nitrone adducts derived from free radical metabolites generated in biological systems.

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.

Kinetic Stability of the Peroxidase Activity of Unfolded Cytochrome c: Heme Degradation and Catalyst Inactivation by Hydrogen Peroxide

Unfolding converts Paracoccus versutus cytochrome c-550 into a potent peroxidase (Diederix, R. E. M.; Ubbink, M.; Canters, G. W. ChemBioChem 2002, 3, 110-112). The catalytic activity is accompanied by peroxide-driven inactivation that is prevented, in part, by reducing substrate. Here, the kinetics of inactivation are described, and evidence is presented for the occurrence of a labile intermediate on the catalytic peroxidase pathway of unfolded cytochrome c-550. This intermediate represents a branching point, whereby the protein proceeds along either the productive pathway or self-inactivates. Reducing substrate suppresses inactivation by decreasing the steady-state concentration of the labile intermediate. Inactivation is accompanied by heme degradation. Its chemical reactivity, UV-vis, and EPR properties identify the first intermediate as hydroxyheme-cytochrome c-550, i.e. with heme hydroxylated at one of the heme meso positions. The occurrence of this species argues for the peroxo-iron species in the peroxidase mechanism as the labile intermediate leading to inactivated cytochrome c-550.

Hydrogen peroxide biosensor based on microperoxidase-11 entrapped in lipid membrane

A highly catalytic activity microperoxidase-11 (MP-11) biosensor for H(2)O(2) was developed to immobilizing the heme peptide in didodecyldimethylammonium bromide (DDAB) lipid membrane. The enzyme electrode thus obtained responded to H(2)O(2) without electron mediator or promoter, at a potential of +0.10 V versus Agmid R:AgCl. A linear calibration curve is obtained over the range from 2.0 x 10(-5) to 2.4 x 10(-3) M. The biosensor responds to hydrogen peroxide in 15 s and has a detection limit of 8 x 10(-7) M (S/N=3) Providing a natural environment with lipid membrane for protein immobilization and maintenance of protein functions is a suitable option for the design of biosensors.

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.

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.

Peroxidase activity as a tool for studying the folding of c-type cytochromes

The peroxidase activity of c-type cytochromes increases substantially by unfolding. This phenomenon was used to study the equilibrium unfolding of ferricytochrome c. The peroxidase activity is already enhanced at low denaturant concentrations. The lowest free energy folding intermediate is easily detected by this method, while it is invisible using fluorescence or optical spectroscopy. The free energy difference between this folding intermediate and the native state depends on the strength of the sixth ligand of the heme-iron and the increase in peroxidase activity upon unfolding is shown to be a sensitive indicator of the strength of this ligand. Under fully denaturing conditions, the peroxidase activity is inhibited by protein-based ligands. It is shown that at least three different ligand groups can be responsible for this inhibition, and that at neutral or alkaline pH, the predominant ligand is not histidine. The use of peroxidase activity assays as a method to study the unfolding of cytochrome c is evaluated.

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.

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.

The peroxidase activity of cytochrome c-550 from Paracoccus versutus

Next to their natural electron transport capacities, c-type cytochromes possess low peroxidase and cytochrome P-450 activities in the presence of hydrogen peroxide. These catalytic properties, in combination with their structural robustness and covalently bound cofactor make cytochromes c potentially useful peroxidase mimics. This study reports on the peroxidase activity of cytochrome c-550 from Paracoccus versutus and the loss of this activity in presence of H2O2. The rate-determining step in the peroxidase reaction of cytochrome c-550 is the formation of a reactive intermediate, following binding of peroxide to the haem iron. The reaction rate is very low compared to horseradish peroxidase (approximately one millionth), because of the poor accessibility of the haem iron for H2O2, and the lack of a base catalyst such as the distal His of the peroxidases. This is corroborated by the linear dependence of the reaction rate on the peroxide concentration up to at least 1 M H2O2. Steady-state conversion of a reducing substrate, guaiacol, is preceded by an activation phase, which is ascribed to the build-up of amino-acid radicals on the protein. The inactivation kinetics in the absence of reducing substrate are mono-exponential and shown to be concurrent with haem degradation up to 25 mM H2O2 (pH 8.0). At still higher peroxide concentrations, inactivation kinetics are biphasic, as a result of a remarkable protective effect of H2O2, involving the formation of superoxide and ferrocytochrome c-550.

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.

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.

Covalently modified microperoxidases as heme-peptide models for peroxidases

Microperoxidase-8 (MP8) and microperoxidase-9 (MP9) have been covalently modified by attachment of proline-containing residues to the amino terminal peptide chain in order to obtain new peroxidase model systems. The catalytic activities of these derivatives in the oxidation of p-cresol by hydrogen peroxide have been compared to that of MP8. The presence of steric hindrance above the heme reduces the formation rate of the catalytically active species, while the reactivity is increased when the amino group of a proline residue is close to the iron. The modification of the catalyst affects the rate of degradation processes undergone by the heme group during catalysis. A bulky aromatic group on the distal side decreases the stability of the complex because it reduces the mobility of a phenoxy radical species formed during catalysis, while the presence of proline residues increases the number of turnovers of the heme catalysts before degradation. The complex Pro2-MP8 obtained by addition of two proline residues to MP8 exhibits the best catalytic performance in terms of activity and chemical stability.

Mechanism of inhibition of electron transfer by amino acid replacement K362M in a proton channel of Rhodobacter sphaeroides cytochrome c oxidase

The three-dimensional structure of cytochrome coxidase (COX) reveals two potential input proton channels connecting the redox core of the enzyme with the negatively charged (N-) aqueous phase. These are denoted as the K-channel (for the highly conserved lysine residue, K362 in Rhodobacter sphaeroides COX) and the D-channel (for the highly conserved aspartate gating the channel at the N-side, D132 in R. sphaeroides). In this paper, it is shown that the K362M mutant form of COX from R. sphaeroides, although unable to turnover with dioxygen as electron acceptor, can utilize hydrogen peroxide as an electron acceptor, with either cytochrome c or ferrocyanide as electron donors, with turnover that is close to that of the wild-type enzyme. The peroxidase activity is similar to that of the wild-type oxidase and is coupled to the generation of a membrane potential and to proton pumping. In contrast, no peroxidase activity is revealed in the D-channel mutants of COX, D132N, and E286Q. Reduction by dithionite of heme a3 in the fully oxidized oxidase is severely inhibited in the K362M mutant, but not in the D132N mutant. Apparently, mutations in the D-channel arrest COX turnover by inhibiting proton uptake associated with the proton-pumping peroxidase phase of the COX catalytic cycle. In contrast, the K-channel appears to be dispensable for the peroxidase phase of the catalytic cycle, but is required for the initial reduction of the heme-copper binuclear center in the first half of the catalytic cycle.

Microperoxidases catalytically degrade reactive oxygen species and may be anti-cataract agents

microPx-11, a ferriheme undecapeptide proteolytic degradation product of cytochrome C is shown to be a peroxidase with broad specificity degrading H2O2 and tertiary butyl hydroperoxide. It is also capable of effectively eliminating superoxide and hydroxyl radical. The peroxidase loses activity in the presence of peroxide unless it is stabilized by ascorbate (Asc) or solutions such as aqueous humor or medium 199. While thiol but not disulfides inactivates the microPx-11, it is not inhibited in the presence of the rat lens which has a high GSH content. microPx-11 at concentrations 10 to 50 fold greater than are required to achieve good protective activity exhibits no toxicity based on cell viability, ATP levels and lens transparency after long-term incubations of alpha TN4-1 cells or cultured rat lens. The peroxidase is capable of protecting cultured rat lenses from photochemical stress where H2O2, O2.- and OH. are generated based on transparency, choline transport, epithelial cell viability and protein integrity as indicated by SDS-PAGE of the rat lens protein. In the absence of the peroxidase, extensive epithelial cell death and other degradative changes are observed. The DNA of alpha TN4-1 cells can also be protected from H2O2 induced single strand breaks by the microPx-11. The overall results suggest that a number of cytochrome C proteolytic degradation products are peroxidases which may be effective anti-cataract agents protecting the lens from oxidative stress.

Microperoxidase/H2O2-mediated alkoxylating dehalogenation of halophenol derivatives in alcoholic media

The results of this study report the H2O2-driven microperoxidase-8 (MP8)-catalyzed dehalogenation of halophenols such as 4-fluorophenol, 4-chlorophenol, 4-bromophenol, and 2-fluorophenol in alcoholic solvents. In methanol, the conversion of the para-halophenols and 2-fluorophenol to, respectively, 4-methoxyphenol and 2-methoxyphenol, as the major dehalogenated products is observed. In ethanol, 4-ethoxyphenol is the principal dehalogenated product formed from 4-fluorophenol. Two mechanisms are suggested for this MP8-dependent alkoxylating dehalogenation reaction. In one of these mechanisms the oxene resonant form of compound I of MP8 is suggested to react with methanol forming a cofactor-peroxide-alkyl intermediate. This intermediate reacts with the reactive pi-electrons of the substrate, leading to the formation of the alkoxyphenols and the release of the fluorine substituent as fluoride anion.

The influence of the peptide chain on the kinetics and stability of microperoxidases

Microperoxidases with increasing lengths of the peptide attached to the heme moiety have been isolated after proteolytic digestion of horse-heart cytochrome c (microperoxidases 6, 8, and 11) and of cytochrome c550 from Thiobacillus versutus (microperoxidase 17). The different microperoxidases catalyze the H2O2-dependent para-hydroxylation of aniline relatively efficiently but are rapidly inactivated under turnover conditions. The horse-heart cytochrome-c-derived microperoxidases have identical values for Vmax but show a decrease of the K(m) for aniline and a higher stability when the attached peptide is longer. The kinetic constants obtained for microperoxidase 17, differ markedly from the microperoxidases derived from horse-heart cytochrome c. Possible factors underlying the observed differences are discussed.

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

The mechanism of aromatic hydroxylation of aniline and phenol derivatives in a H2O2-driven microperoxidase-8(MP8)-catalyzed reaction was investigated. It was shown that the reaction was not inhibited by the addition of scavengers of superoxide anion or hydroxyl radicals, which demonstrates that the reaction mechanism differs from that of the aromatic hydroxylation catalyzed by a horseradish peroxidase/ dihydroxyfumarate system. Additional experiments with 18O-labelled H2 18O2 demonstrated that the oxygen incorporated into aniline to give 4-aminophenol originates from H2O2. Furthermore, it was found that the addition of ascorbic acid efficiently blocks all peroxidase-type reactions that can be catalyzed by the MP8/H2O2 system, but does not inhibit the aromatic hydroxylation of aniline and phenol derivatives. Together, these observations exclude reaction mechanisms for the aromatic hydroxylation that proceed through peroxidase-type mechanisms in which the oxygen incorporated into the substrate originates from O2 or H2O. The mechanism instead seems to proceed by an initial attack of the high-valent iron-oxo intermediate of MP8 on the pi-electrons of the aromatic ring of the substrate leading to product formation by a cytochrome-P-450-type of sigma-O-addition or oxygen-rebound mechanism. This implies that MP8, which has a histidyl and not a cysteinate fifth axial ligand, is able to react by a cytochrome-P-450-like oxygen-transfer reaction mechanism.

Heme-Peptide Models for Hemoproteins. 1. Solution Chemistry of N-Acetylmicroperoxidase-8

An improved method for the preparation of the heme octapeptide acetyl-MP8, obtained by proteolysis of horse heart cytochrome c, is described. AcMP8 obeys Beer's law at pH 7.0 in aqueous solution up to a concentration of 3 x 10(-)(5) M. The self-association constant measured at 25 degrees C (log K(D) = 4.04) is an order of magnitude lower than that for MP8, reflecting the role of the N-acetyl protecting group in abolishing intermolecular coordination. However, AcMP8 does form pi-stacked dimers in aqueous solution with increasing ionic strength. A more weakly packed pi-pi dimer reaches a maximum abundance at approximately 3 M ionic strength, but a more tightly packed dimer is favored at &mgr; > 3 M. An equilibrium model based on charge neutralization by specific binding of Na(+) ions gives a total molecular charge of 3- for AcMP8 at pH 7.0 and a self-association constant log K(D) = 4.20. AcMP8 exhibits six spectroscopically active pH-dependent transitions. The Glu-21 c-terminal carboxylate binds to the heme iron at low pH (pK(a) = 2.1) but is substituted by His-18 (pK(a) = 3.12) as the pH increases. The two heme propanoic acid substituents ionize with pK(a)'s of 4.95 and 6.1. This is followed by ionization of iron-bound water with a pK(a) = 9.59, DeltaH = 48 +/- 1 kJ mol(-)(1), and DeltaS = -22 +/- 3 J K(-)(1) mol(-)(1). The electronic spectra indicate that AcMP8 is predominantly in the S = (5)/(2) state at pH 7.0, while the hydroxo complex at pH 10.5 corresponds to an equilibrium mixture of S = (5)/(2) and S = (1)/(2) states at 25 degrees C. In the final transition, His-18 ionizes to form the S = (1)/(2) histidinate complex with a pK(a) of 12.71. AcMP8 is relatively stable under alkaline conditions, dimerizing slowly at high pH (k = 2.59 +/- 0.14 M(-)(1) s(-)(1)) to form a high-spin &mgr;-oxo-bridged species. The pH-dependent behavior of AcMP8 in the presence of excess 3-cyanopyridine, however, is markedly different. At low pH, AcMP8 simultaneously binds the exogenous ligand and the Glu-21 c-terminal carboxylate with a pK(a) < 2. His-18 replaces the carboxylate ligand at higher pH (pK(a) = 2.60), and both heme propanoic acid groups ionize with a mean pK(a) = 5.10. Unlike AcMP8.OH(-), the axial histidine of the 3-CNPy complex ionizes at near neutral pH (pK(a) = 7.83), prior to being replaced by OH(-) (pK(a) = 10.13). The sixth transition in the AcMP8/3-CNPy system produces the bis(hydroxo) complex (pK(a) > 13).

High-valent intermediates in the reaction of N alpha-acetyl microperoxidase-8 with hydrogen peroxide: models for compounds 0, I and II of horseradish peroxidase

N-acetyl microperoxidase-8 (Ac-MP-8) is a water soluble, ferric heme model for the peroxidases. The reaction of Ac-MP-8 with H2O2 in 10 mM potassium phosphate over the pH range of 7-11 gives rise sequentially to relatively stable green and red species with properties that closely mimic those of HRP compounds I and II, respectively. Low-temperature stopped-flow studies of this reaction carried out in 50% v/v methanol/10 mM potassium phosphate, pH* 9.1 at -25.8 degrees C indicate that the pseudo-first-order rate constant, kobs, that describes the formation of the green intermediate exhibits saturation kinetics as a function of [H2O2] with kmaxobs = 95 s-1 and KM = 87 mM. Rapid-scan studies carried out with [H2O2] = 200 mM at -38.0 degrees C show that a compound 0 species with a characteristic band near 340 nm is formed whose conversion to the green species is rate limiting. Thus, Ac-MP-8 has high-valent forms that are models for all three known intermediates in the peroxidase cycle of horseradish peroxidase.

Oxygen activation and ligand binding by pure heme-octapeptide microperoxidase-8 (MP-8)

The rapid, efficient preparation of pure microperoxidase-8 (MP-8) is described. Ligand binding studies confirm that MP-8 is monomeric in alkaline solution. It is shown that the monomeric MP-8 activates oxygen in a similar manner to that already reported for alkaline hemin, establishing the octapeptide as a possible second generation model for the oxygen activation/insertion reactions of the cytochrome P-450.