Reversible formation of high-valent-iron-oxo porphyrin intermediates in heme-based catalysis: revisiting the kinetic model for horseradish peroxidase

New mixed amino acids complexes of iron(III) and zinc(II) with isonitrosoacetophenone: Synthesis, spectral characterization, DFT study and anticancer activity

New mixed Fe(III) and Zn(II) complexes with isonitrosoacetophenone (HINAP) and l-amino acids (such as l-histidine, l-phenylalanine and l-proline) have been synthesized and characterized by elemental analyses, UV-Vis, IR and ESR spectroscopy and thermal analyses. The values of molar conductance of the complexes in DMSO solution at 10^-3 M concentration indicate their non-electrolyte nature. IR spectroscopy has revealed the coordination of deprotonated ligands to metal through nitrogen and oxygen atoms in an N₂O₂ arrangement. Density functional theory (DFT) calculations were performed using the hybrid functional of Truhlar and Zhao (M06) with basis set of double zeta quality LANL2DZ to evaluate the cis and trans coordination modes and to ascertain dipole moment, HOMO-LUMO energy gap, chemical hardness, softness and electrophilicity. The magnetic moments and ESR measurements suggest that there is an admixture of S = 5/2 and S = 1/2 spins in Fe(III) complexes. UV-Visible spectra indicate a distorted octahedral geometry around the metal ions. Thermal analyses show the presence of hydrated and coordinated water. The antimicrobial activity was investigated against (G⁺ and G^-) bacteria (Staphylococcus aureus, bacillus subtilis, Escherichia coli, Pseudomonas aeruginosa) and fungi (Candida albicans). The iron and zinc complexes were found to be more active against Gram-positive than Gram negative bacteria. They also show considerable growth inhibition against the fungi tested. In vitro antitumor activity assayed against cancer cell lines of the HEP2 type (cancer cells of the larynx) exhibited significant toxicity of the ligands and their mixed complexes.

On the role of water in peroxidase catalysis: a theoretical investigation of HRP compound I formation

We have investigated the dynamics of water molecules in the distal pocket of horseradish peroxidase to elucidate the role that they may play in the formation of the principal active species of the enzymatic cycle (compound I, Por(o+)-Fe(IV)=O) upon reaction of the resting Fe(III) state with hydrogen peroxide. The equilibrium molecular dynamics simulations show that, in accord with experimental evidence, the active site access channel is hydrated with an average of two to three water molecules within 5 A from the bound hydrogen peroxide. Although the channel is always hydrated, the specific conformations in which a water molecule bridges H(2)O(2) and the distal histidine, which were found (Derat; et al. J. Am. Chem. Soc. 2007, 129, 6346.) to display a low-energy barrier for the initial acid-base step of the reaction, occur with low probability but are relevant within the time scale of catalysis. Metadynamics simulations, which were used to reconstruct the free-energy landscape of water motion in the access channel, revealed that preferred interaction sites within the channel are separated by small energy barriers (<1.5 kcal/mol). Most importantly, water-bridged conformations lie on a shoulder just 1 kcal/mol above one local minimum and thus are easily accessible. Such an energy landscape appears as a requisite for the effectiveness of compound I formation, whereby the H-bonding pattern involving reactants and catalytic residues (including the intervening water molecule) has to rearrange to deliver the proton to the distal OH moiety of the hydrogen peroxide and thereby lead to heterolytic O-O cleavage. Our study provides an example of a system for which the "reactive configurations" (i.e., structures characterized by a low barrier for the chemical transformation) correspond to a minor population of the system and show how equilibrium molecular dynamics and free-energy calculations may conveniently be used to ascertain that such reactive conformations are indeed accessible to the system. Once again, the MD and QM/MM combination shows that a single water molecule acts as a biocatalyst in the cycle of HRP.

Mechanism of and exquisite selectivity for O-O bond formation by the heme-dependent chlorite dismutase

Chlorite dismutase (Cld) is a heme b-dependent, O-O bond forming enzyme that transforms toxic chlorite (ClO(2)(-)) into innocuous chloride and molecular oxygen. The mechanism and specificity of the reaction with chlorite and alternate oxidants were investigated. Chlorite is the sole source of dioxygen as determined by oxygen-18 labeling studies. Based on ion chromatography and mass spectrometry results, Cld is highly specific for the dismutation of chlorite to chloride and dioxygen with no other side products. Cld does not use chlorite as an oxidant for oxygen atom transfer and halogenation reactions (using cosubstrates guaiacol, thioanisole, and monochlorodimedone, respectively). When peracetic acid or H(2)O(2) was used as an alternative oxidant, oxidation and oxygen atom transfer but not halogenation reactions occurred. Monitoring the reaction of Cld with peracetic acid by rapid-mixing UV-visible spectroscopy, the formation of the high valent compound I intermediate, [(Por(*+))Fe(IV) = O], was observed [k(1) = (1.28 +/- 0.04) x 10(6) M(-1) s(-1)]. Compound I readily decayed to form compound II in a manner that is independent of peracetic acid concentration (k(2) = 170 +/- 20 s(-1)). Both compound I and a compound II-associated tryptophanyl radical that resembles cytochrome c peroxidase (Ccp) compound I were observed by EPR under freeze-quench conditions. The data collectively suggest an O-O bond-forming mechanism involving generation of a compound I intermediate via oxygen atom transfer from chlorite, and subsequent recombination of the resulting hypochlorite and compound I.

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.

A comparative reactivity study of microperoxidases based on hemin, mesohemin and deuterohemin

Three microperoxidases--hemin-6(7)-gly-gly-his methyl ester (HGGH), mesohemin-6(7)-gly-gly-his methyl ester (MGGH) and deuterohemin-6(7)-gly-gly-his methyl ester (DGGH)--have been prepared as models for heme-containing peroxidases by condensation of glycyl-glycyl-L-histidine methyl ester with the propionic side chains of hemin, mesohemin and deuterohemin, respectively. The three microperoxidases differ in two substituents, R, of the protoporphyrin IX framework (HGGH: R=vinyl, MGGH: R=ethyl, DGGH: R=H). X-band and high field EPR spectra show that the microperoxidases exhibit spectroscopic properties similar to those of metmyoglobin, i.e. a high spin ferric S=5/2 signal at g(perpendicular)=6 and g parallel)=2 and an estimated D value of 7.5+/-1cm(-1). The catalytic activities of the microperoxidases towards K4[Fe(CN)6], L-tyrosine methyl ester and 2,2'-azino(bis(3-ethylbenzothiazoline-6-sulfonic acid)) (ABTS) have been investigated. It was found that all three microperoxidases exhibit peroxidase activity and that the reactions follow the generally accepted peroxidase reaction scheme [Biochem. J. 145 (1975) 93-103] with the exception that the initial formation of a Compound I analogue is the rate-limiting step for the whole process. The general activity trend was found to be MGGH approximately DGGH>HGGH. For each microperoxidase, DFT calculations (B3LYP) were made on the reactions of compounds 0, I and II with H+, e- and H+ + e-, respectively, in order to probe the possible relationship between the nature of the 2- and 4-substituents of the hemin and the observed reactivity. The computational modeling indicates that the relative energy differences are very small; solvation and electrostatic effects may be factors that decide the relative activities of the microperoxidases.

Monooxygenation of an aromatic ring by F43W/H64D/V68I myoglobin mutant and hydrogen peroxide. Myoglobin mutants as a model for P450 hydroxylation chemistry

Myoglobin (Mb) is used as a model system for other heme proteins and the reactions they catalyze. The latest novel function to be proposed for myoglobin is a P450 type hydroxylation activity of aromatic carbons (Watanabe, Y., and Ueno, T. (2003) Bull. Chem. Soc. Jpn. 76, 1309-1322). Because Mb does not contain a specific substrate binding site for aromatic compounds near the heme, an engineered tryptophan in the heme pocket was used to model P450 hydroxylation of aromatic compounds. The monooxygenation product was not previously isolated because of rapid subsequent oxidation steps (Hara, I., Ueno, T., Ozaki, S., Itoh, S., Lee, K., Ueyama, N., and Watanabe, Y. (2001) J. Biol. Chem. 276, 36067-36070). In this work, a Mb variant (F43W/H64D/V68I) is used to characterize the monooxygenated intermediate. A modified (+16 Da) species forms upon the addition of 1 eq of H2O2. This product was digested with chymotrypsin, and the modified peptide fragments were isolated and characterized as 6-hydroxytryptophan using matrix-assisted laser desorption ionization time-of-flight tandem mass spectroscopy and 1H NMR. This engineered Mb variant represents the first enzyme to preferentially hydroxylate the indole side chain of Trp at the C6 position. Finally, heme extraction was used to demonstrate that both the formation of the 6-hydroxytryptophan intermediate (+16 Da) and subsequent oxidation to form the +30 Da final product are catalyzed by the heme cofactor, most probably via the compound I intermediate. These results provide insight into the mechanism of hydroxylation of aromatic carbons by heme proteins, demonstrating that non-thiolate-ligated heme enzymes can perform this function. This establishes Mb compound I as a model for P450 type aromatic hydroxylation chemistry.

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.

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.

Improving the catalytic performance of peroxidases in organic synthesis

Peroxidases are ubiquitous enzymes that catalyze a variety of enantioselective oxygen-transfer reactions with hydrogen peroxide (H2O2). Although they have enormous potential, their industrial application is hampered by their high price and low operational stability. Recent developments, such as the controlled addition and in situ formation of the oxidant, protein engineering and the rational design of semi-synthetic peroxidases, aim to improve the operational stability of peroxidases.

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.

Selective oxygen transfer catalysed by heme peroxidases: synthetic and mechanistic aspects

The synthetic and mechanistic aspects of the use of heme peroxidases as functional mimics of the cytochrome P450 monooxygenases in oxygen-transfer reactions have been described. The chloroperoxidase from Caldariomyces fumago (CPO) is the catalyst of choice in sulfoxidation, hydroxylation and epoxidation on account of its high activity and enantioselectivity. Other heme peroxidases were less active by orders of magnitude; protein engineering has resulted in impressive improvements but even the most active mutant was still at least an order of magnitude less active than CPO. The 'oxygen-rebound' mechanisms of oxygen transfer mediated by heme enzymes - as originally conceived - have proved to be untenable. Dual pathway mechanisms, via oxoferryl species that insert oxygen as well as iron hydroperoxide species that insert OH(+), have been proposed that accommodate all of the known experimental data.

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.

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.

Biocatalytic and biomimetic oxidations with vanadium

Approaches to the rational design of vanadium-based semi-synthetic enzymes and biomimetic models as catalysts for enantioselective oxidations are reviewed. Incorporation of vanadate ion into the active site of phytase (E.C. 3.1.3.8), which in vivo mediates the hydrolysis of phosphate esters, afforded a semi-synthetic peroxidase. It catalyzed the enantioselective oxidation of prochiral sulfides with H2O2 affording the S-sulfoxide, e.g. in 66% ee at quantitative conversion of thioanisole. Under the reaction conditions the semi-synthetic vanadium peroxidase was stable for more than 3 days with only a slight decrease in turnover frequency. Amongst the transition-metal oxoanions that are known to be potent inhibitors of phosphatases, only vanadate resulted in a semi-synthetic peroxidase when incorporated into phytase. In a biomimetic approach, vanadium complexes of chiral Schiff base complexes were encapsulated in the super cages of a hydrophobic zeolite Y. Unfortunately, these ship-in-a-bottle complexes afforded only racemic sulfoxide in the catalytic oxidation of thioanisole with H2O2.

Differential substrate behaviour of phenol and aniline derivatives during conversion by horseradish peroxidase

For the first time saturating overall k(cat) values for horseradish peroxidase (HRP) catalysed conversion of phenols and anilines are described. These k(cat) values correlate quantitatively with calculated ionisation potentials of the substrates. The correlations for the phenols are shifted to higher k(cat) values at similar ionisation potentials as compared to those for anilines. (1)H-NMR T(1) relaxation studies, using 3-methylphenol and 3-methylaniline as the model substrates, revealed smaller average distances of the phenol than of the aniline protons to the paramagnetic Fe(3+) centre in HRP. This observation, together with a possibly higher extent of deprotonation of the phenols than of the anilines upon binding to the active site of HRP, may contribute to the relatively higher HRP catalysed conversion rates of phenols than of anilines.