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

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.

Cardiolipin activates cytochrome c peroxidase activity since it facilitates H(2)O(2) access to heme

In this work, the effect of liposomes consisting of tetraoleyl cardiolipin and dioleyl phosphatidylcholine (1 : 1, mol/mol) on the rate of three more reactions of Cyt c heme with H2O2 was studied: (i) Cyt c (Fe2+) oxidation to Cyt c (Fe3+), (ii) Fe...S(Met80) bond breaking, and (iii) heme porphyrin ring decomposition. It was revealed that the rates of all those reactions increased greatly in the presence of liposomes containing cardiolipin and not of those consisting of only phosphatidylcholine, and approximately to the same extent as peroxidase activity. These data suggest that cardiolipin activates specifically Cyt c peroxidase activity not only because it promotes Fe...S(Met80) bond breaking but also facilitates H2O2 penetration to the reaction center.

Peroxidase Activity of the Hemeoctapeptide N-Acetylmicroperoxidase-8

The pH dependence of the peroxidase activity (guaiacol assay) of the ferric hemeoctapeptide N-acetylmicroperoxidase-8 (N-AcMP8) was studied under conditions where formation of the Compound I analogue of the peroxidase enzymes is rate limiting. The pH profile of the reaction rate is consistent with a mechanism where both H2O2 and HO2- can displace H2O coordinated trans to neutral His but where the hydroxo complex of the hemepeptide (OH- trans to His) is kinetically inert. At pH > 11, where the proximal His ligand of Fe(III) ionizes to form a histidinate, the hydroxo complex (OH- trans to His-) becomes kinetically labile. A comparison of DeltaH(double dagger) and DeltaS(double dagger) values for the reaction of H2O2 and HO2-, obtained from the temperature dependence of the rate constants, with values for CN- and cysteine reported previously, shows that the activation parameters depend on the identity of the incoming ligand. This suggests that ligand substitution at Fe(III) in N-AcMP8 proceeds through an interchange mechanism.

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 influence of pH on antioxidant properties and the mechanism of antioxidant action of hydroxyflavones

The effect of the pH on antioxidant properties of a series of hydroxyflavones was investigated. The pKa of the individual hydroxyl moieties in the hydroxyflavones was compared to computer-calculated deprotonation energies. This resulted in a quantitative structure activity relationship (QSAR), which enables the estimation of pKa values of individual hydroxyl moieties, also in hydroxyflavones for which these pKa values are not available. Comparison of the pKa values to the pH-dependent antioxidant profiles, determined by the TEAC assay, reveals that for various hydroxyflavones the pH-dependent behavior is related to hydroxyl moiety deprotonation, resulting in an increase of the antioxidant potential upon formation of the deprotonated forms. Comparison of these experimental results to computer calculated O-H bond dissociation energies (BDE) and ionization potentials (IP) of the nondeprotonated and the deprotonated forms of the various hydroxyflavones indicates that especially the parameter reflecting the ease of electron donation, i.e., the IP, and not the BDE, is greatly influenced by the deprotonation. Based on these results it is concluded that upon deprotonation the TEAC value increases (radical scavenging capacity increases) because electron-, not H*-, donation becomes easier. Taking into account that the mechanism of radical scavenging antioxidant activity of the neutral form of the hydroxyflavones is generally considered to be hydrogen atom donation, this implies than not only the ease of radical scavenging, but also the mechanism of antioxidant action changes upon hydroxyflavone deprotonation.

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.

Miniaturized hemoproteins

The present paper highlights and reviews current research in the field of hemoprotein models. Hemoproteins have been extensively studied in order to understand structure-function relationships, and to design new molecules with desired functions. A wide number of synthetic analogues have been developed, using quite different approaches. They differ in molecular structures, ranging from simple meso-substituted tetraaryl-metalloporphyrins and peptide-porphyrin conjugates. In this paper we summarize the state of the art on peptide based hemoprotein models. We also report here the approach used by us to develop a new class of molecules, named mimochromes. They can be regarded as miniaturized hemoproteins, because mimochromes are low molecular weight compounds with some structural and functional properties common to those of the parent high molecular weight protein. The basic structure of mimochromes is a deuteroporphyrin ring covalently linked to two helical peptide chains. Two molecules of this series have been fully characterized. All the information derived from their structural analysis has been applied to the design of new analogues with additional functions.

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.