Resonance Raman evidence for oxygen exchange between the FeIV = O heme and bulk water during enzymic catalysis of horseradish peroxidase and its relation with the heme-linked ionization

Colorimetric determination of cholesterol based on the peroxidase-like activity of Cu-Salt-Fe nanozyme with multiple active sites

A strategy to enhance the peroxidase-like activity of the hemin composite is presented. The Cu-Salt-Fe with enhanced activity was synthesized by one-step heat treatment and applied to the colorimetric determination of free cholesterol in human serum. Phosphate can act in complexing Cu²⁺ to form a carrier Cu-Floc with a large specific surface area. The coordination effect of Cu-Floc with hemin was used to disperse and load hemin to form Cu-Floc-Hemin from which Cu-Salt-Fe was prepared. The Cu-Salt-Fe exhibits a synergistic catalytic effect of Cu-Salt, Fe²⁺, Fe³⁺, or Fe-Nx active sites in amplification of H₂O₂ oxidation. As expected, Cu-Salt-Fe triggered H₂O₂-mediated oxidation of 3,3',5,5'-tetramethylbenzidine (TMB), resulting in H₂O₂-dependent absorbance changes at 652 nm. A cholesterol oxidase (ChOx)/Cu-Salt-Fe-TMB colorimetric sensing system exhibited excellent cholesterol determination performance in the range 5-1200 μM with a detection limit of 2.73 μM. Cholesterol recoveries from the three-level spiked serum ranged from 92.2 to 98.9% (RSD ≤ 5.4). This colorimetric sensing system not only provided a strategy for the determination of endogenous substances with H₂O₂ as an intermediate, but also provided a new design idea (carrier selection, activity enhancement method) for the development of other artificial enzymes with the same catalytic core.

Spin state dependent peroxidase activity of heme bound amyloid β peptides relevant to Alzheimer's disease

The colocalization of heme rich deposits in the senile plaque of Aβ in the cerebral cortex of the Alzheimer's disease (AD) brain along with altered heme homeostasis and heme deficiency symptoms in AD patients has invoked the association of heme in AD pathology. Heme bound Aβ complexes, depending on the concentration of the complex or peptide to heme ratio, exhibit an equilibrium between a high-spin mono-His bound peroxidase-type active site and a low-spin bis-His bound cytochrome b type active site. The high-spin heme-Aβ complex shows higher peroxidase activity than free heme, where compound I is the reactive oxidant. It is also capable of oxidizing neurotransmitters like serotonin in the presence of peroxide, owing to the formation of compound I. The low-spin bis-His heme-Aβ complex on the other hand shows enhanced peroxidase activity relative to high-spin heme-Aβ. It reacts with H2O2 to produce two stable intermediates, compound 0 and compound I, which are characterized by absorption, EPR and resonance Raman spectroscopy. The stability of compound I of low-spin heme-Aβ is accountable for its enhanced peroxidase activity and oxidation of the neurotransmitter serotonin. The effect of the second sphere Tyr10 residue of Aβ on the formation and stability of the intermediates of low-spin heme-Aβ has also been investigated. The higher stability of compound I for low-spin heme-Aβ is likely due to H-bonding interactions involving Tyr10 in the distal pocket.

Asymmetric Sulfoxidation of Thioether Catalyzed by Soybean Pod Shell Peroxidase to Form Enantiopure Sulfoxide in Water-in-Oil Microemulsions: A Kinetic Model

Esomeprazole with chiral sulfoxides structure is used to treat gastric ulcer disease. Soybean pod shell peroxidase (SPSP) is a peroxidase extracted from soybean pods shells which are one of the most abundant natural resources in the world. In the production of chiral sulfoxides catalyzed by SPSP, it is very important to establish the reaction kinetic model and explore the reaction mechanism for the development of the process, however, there is no report on the establishment of the model. Asymmetric sulfoxidation reactions catalyzed by SPSP in water-in-oil microemulsions were carried out, and the King-Altman approach was used to establish a kinetic model. A yield of 91% and e.e. value of 96% for esomeprazole were obtained at the activity of SPSP of 3200 U ml-1 and 50 °C for 5 h. The mechanism with a two-electron reduction of SPSP-I is accompanied with a single-electron transfer to SPSP-I and nonenzymatic reactions, indicating that three concomitant sub-mechanisms contribute to the asymmetric oxidation involving five enzymatic and two nonenzymatic reactions, which can represent the asymmetric sulfoxidation of organic sulfides to form enantiopure sulfoxides. With 5.44% of the average relative deviation, a kinetic model fitting experimental data was developed. The enzymatic reactions may follow ping-pong mechanism with substrate inhibition of H2 O2 and product inhibition of esomeprazole, while nonenzymatic reactions follow a power law. Those results indicate that SPSP with a lower cost and higher thermal stability may be used as an effective substitute for horseradish peroxidase.

Consecutive Marcus Electron and Proton Transfer in Heme Peroxidase Compound II-Catalysed Oxidation Revealed by Arrhenius Plots

Electron and proton transfer reactions in enzymes are enigmatic and have attracted a great deal of theoretical, experimental, and practical attention. The oxidoreductases provide model systems for testing theoretical predictions, applying experimental techniques to gain insight into catalytic mechanisms, and creating industrially important bio(electro)conversion processes. Most previous and ongoing research on enzymatic electron transfer has exploited a theoretically and practically sound but limited approach that uses a series of structurally similar ("homologous") substrates, measures reaction rate constants and Gibbs free energies of reactions, and analyses trends predicted by electron transfer theory. This approach, proposed half a century ago, is based on a hitherto unproved hypothesis that pre-exponential factors of rate constants are similar for homologous substrates. Here, we propose a novel approach to investigating electron and proton transfer catalysed by oxidoreductases. We demonstrate the validity of this new approach for elucidating the kinetics of oxidation of "non-homologous" substrates catalysed by compound II of Coprinopsis cinerea and Armoracia rusticana peroxidases. This study - using the Marcus theory - demonstrates that reactions are not only limited by electron transfer, but a proton is transferred after the electron transfer event and thus both events control the reaction rate of peroxidase-catalysed oxidation of substrates.

Electron Transfer Control of Reductase versus Monooxygenase: Catalytic C-H Bond Hydroxylation and Alkene Epoxidation by Molecular Oxygen

Catalytic oxidation of organic substrates, using a green oxidant like O₂, has been a long-term goal of the scientific community. In nature, these oxidations are performed by metalloenzymes that generate highly oxidizing species from O₂, which, in turn, can oxidize very stable organic substrates, e.g., mono-/dioxygenases. The same oxidants are produced during O₂ reduction/respiration in the mitochondria but are reduced by electron transfer, i.e., reductases. Iron porphyrin mimics of the active site of cytochrome P450 (Cyt P450) are created atop a self-assembled monolayer covered electrode. The rate of electron transfer from the electrode to the iron porphyrin site is attenuated to derive monooxygenase reactivity from these constructs that otherwise show O₂ reductase activity. Catalytic hydroxylation of strong C-H bonds to alcohol and epoxidation of alkenes, using molecular O₂ (with ¹⁸O₂ incorporation), is demonstrated with turnover numbers >10⁴. Uniquely, one of the two iron porphyrin catalysts used shows preferential oxidation of 2° C-H bonds of cycloalkanes to alcohols over 3° C-H bonds without overoxidation to ketones. Mechanistic investigations with labeled substrates indicate that a compound I (Fe^IV=O bound to a porphyrin cation radical) analogue, formed during O₂ reduction, is the primary oxidant. The selectivity is determined by the shape of the distal pocket of the catalyst, which, in turn, is determined by the substituents on the periphery of the porphyrin macrocycle.

The Nature and Reactivity of Ferryl Heme in Compounds I and II

Aerobic organisms have evolved to activate oxygen from the atmosphere, which allows them to catalyze the oxidation of different kinds of substrates. This activation of oxygen is achieved by a metal center (usually iron or copper) buried within a metalloprotein. In the case of iron-containing heme enzymes, the activation of oxygen is achieved by formation of transient iron-oxo (ferryl) intermediates; these intermediates are called Compound I and Compound II. The Compound I and II intermediates were first discovered in the 1930s in horseradish peroxidase, and it is now known that these same species are used across the family of heme enzymes, which include all of the peroxidases, the heme catalases, the P450s, cytochrome c oxidase, and NO synthase. Many years have passed since the first observations, but establishing the chemical nature of these transient ferryl species remains a fundamental question that is relevant to the reactivity, and therefore the usefulness, of these species in biology. This Account summarizes experiments that were conceived and conducted at Leicester and presents our ideas on the chemical nature, stability, and reactivity of these ferryl heme species. We begin by briefly summarizing the early milestones in the field, from the 1940s and 1950s. We present comparisons between the nature and reactivity of the ferryl species in horseradish peroxidase, cytochrome c peroxidase, and ascorbate peroxidase; and we consider different modes of electron delivery to ferryl heme, from different substrates in different peroxidases. We address the question of whether the ferryl heme is best formulated as an (unprotonated) Fe^IV═O or as a (protonated) Fe^IV-OH species. A range of spectroscopic approaches (EXAFS, resonance Raman, Mossbauer, and EPR) have been used over many decades to examine this question, and in the last ten years, X-ray crystallography has also been employed. We describe how information from all of these studies has blended together to create an overall picture, and how the recent application of neutron crystallography has directly identified protonation states and has helped to clarify the precise nature of the ferryl heme in cytochrome c peroxidase and ascorbate peroxidase. We draw comparisons between the Compound I and Compound II species that we have observed in peroxidases with those found in other heme systems, notably the P450s, highlighting possible commonality across these heme ferryl systems. The identification of proton locations from neutron structures of these ferryl species opens the door for understanding the proton translocations that need to occur during O-O bond cleavage.

Spectroscopic Investigations of Catalase Compound II: Characterization of an Iron(IV) Hydroxide Intermediate in a Non-thiolate-Ligated Heme Enzyme

We report on the protonation state of Helicobacter pylori catalase compound II. UV/visible, Mössbauer, and X-ray absorption spectroscopies have been used to examine the intermediate from pH 5 to 14. We have determined that HPC-II exists in an iron(IV) hydroxide state up to pH 11. Above this pH, the iron(IV) hydroxide complex transitions to a new species (pKa = 13.1) with Mössbauer parameters that are indicative of an iron(IV)-oxo intermediate. Recently, we discussed a role for an elevated compound II pKa in diminishing the compound I reduction potential. This has the effect of shifting the thermodynamic landscape toward the two-electron chemistry that is critical for catalase function. In catalase, a diminished potential would increase the selectivity for peroxide disproportionation over off-pathway one-electron chemistry, reducing the buildup of the inactive compound II state and reducing the need for energetically expensive electron donor molecules.

Crystal structure of the pristine peroxidase ferryl center and its relevance to proton-coupled electron transfer

The reaction of peroxides with peroxidases oxidizes the heme iron from Fe(III) to Fe(IV)=O and a porphyrin or aromatic side chain to a cationic radical. X-ray-generated hydrated electrons rapidly reduce Fe(IV), thereby requiring very short exposures using many crystals, and, even then, some reduction cannot be avoided. The new generation of X-ray free electron lasers capable of generating intense X-rays on the tenths of femtosecond time scale enables structure determination with no reduction or X-ray damage. Here, we report the 1.5-Å crystal structure of cytochrome c peroxidase (CCP) compound I (CmpI) using data obtained with the Stanford Linear Coherent Light Source (LCLS). This structure is consistent with previous structures. Of particular importance is the active site water structure that can mediate the proton transfer reactions required for both CmpI formation and reduction of Fe(IV)=O to Fe(III)-OH. The structures indicate that a water molecule is ideally positioned to shuttle protons between an iron-linked oxygen and the active site catalytic His. We therefore have carried out both computational and kinetic studies to probe the reduction of Fe(IV)=O. Kinetic solvent isotope experiments show that the transfer of a single proton is critical in the peroxidase rate-limiting step, which is very likely the proton-coupled reduction of Fe(IV)=O to Fe(III)-OH. We also find that the pKa of the catalytic His substantially increases in CmpI, indicating that this active site His is the source of the proton required in the reduction of Fe(IV)=O to Fe(IV)-OH.

Setting an upper limit on the myoglobin iron(IV)hydroxide pK(a): insight into axial ligand tuning in heme protein catalysis

To provide insight into the iron(IV)hydroxide pK(a) of histidine ligated heme proteins, we have probed the active site of myoglobin compound II over the pH range of 3.9-9.5, using EXAFS, Mössbauer, and resonance Raman spectroscopies. We find no indication of ferryl protonation over this pH range, allowing us to set an upper limit of 2.7 on the iron(IV)hydroxide pK(a) in myoglobin. Together with the recent determination of an iron(IV)hydroxide pK(a) ∼ 12 in the thiolate-ligated heme enzyme cytochrome P450, this result provides insight into Nature's ability to tune catalytic function through its choice of axial ligand.

Peroxidase-type reactions suggest a heterolytic/nucleophilic O-O joining mechanism in the heme-dependent chlorite dismutase

Heme-containing chlorite dismutases (Clds) catalyze a highly unusual O-O bond-forming reaction. The O-O cleaving reactions of hydrogen peroxide and peracetic acid (PAA) with the Cld from Dechloromonas aromatica (DaCld) were studied to better understand the Cl-O cleavage of the natural substrate and subsequent O-O bond formation. While reactions with H2O2 result in slow destruction of the heme, at acidic pH heterolytic cleavage of the O-O bond of PAA cleanly yields the ferryl porphyrin cation radical (compound I). At alkaline pH, the reaction proceeds more rapidly, and the first observed intermediate is a ferryl heme. Freeze-quench EPR confirmed that the latter has an uncoupled protein-based radical, indicating that compound I is the first intermediate formed at all pH values and that radical migration is faster at alkaline pH. These results suggest by analogy that two-electron Cl-O bond cleavage to yield a ferryl-porphyrin cation radical is the most likely initial step in O-O bond formation from chlorite.

Direct observation of intermediates formed during steady-state electrocatalytic O2 reduction by iron porphyrins

Heme/porphyrin-based electrocatalysts (both synthetic and natural) have been known to catalyze electrochemical O2, H(+), and CO2 reduction for more than five decades. So far, no direct spectroscopic investigations of intermediates formed on the electrodes during these processes have been reported; and this has limited detailed understanding of the mechanism of these catalysts, which is key to their development. Rotating disk electrochemistry coupled to resonance Raman spectroscopy is reported for iron porphyrin electrocatalysts that reduce O2 in buffered aqueous solutions. Unlike conventional single-turnover intermediate trapping experiments, these experiments probe the system while it is under steady state. A combination of oxidation and spin-state marker bands and metal ligand vibrations (identified using isotopically enriched substrates) allow in situ identification of O2-derived intermediates formed on the electrode surface. This approach, combining dynamic electrochemistry with resonance Raman spectroscopy, may be routinely used to investigate a plethora of metalloporphyrin complexes and heme enzymes used as electrocatalysts for small-molecule activation.

Effects of imidazole deprotonation on vibrational spectra of high-spin iron(II) porphyrinates

The effects of the deprotonation of coordinated imidazole on the vibrational dynamics of five-coordinate high-spin iron(II) porphyrinates have been investigated using nuclear resonance vibrational spectroscopy. Two complexes have been studied in detail with both powder and oriented single-crystal measurements. Changes in the vibrational spectra are clearly related to structural differences in the molecular structures that occur when imidazole is deprotonated. Most modes involving the simultaneous motion of iron and imidazolate are unresolved, but the one mode that is resolved is found at higher frequency in the imidazolates. These out-of-plane results are in accord with earlier resonance Raman studies of heme proteins. We also show the imidazole vs imidazolate differences in the in-plane vibrations that are not accessible to resonance Raman studies. The in-plane vibrations are at lower frequency in the imidazolate derivatives; the doming mode shifts are inconclusive. The stiffness, an experimentally determined force constant that averages the vibrational details to quantify the nearest-neighbor interactions, confirms that deprotonation inverts the relative strengths of axial and equatorial coordination.

Molecular structures and electron distributions of higher-valent iron and manganese porphyrins: Density functional theory calculations and some preliminary open-shell coupled-cluster results

Density functional theory (DFT) calculations have been carried out for a variety of iron and manganese porphyrin complexes with metal oxidation states of +3 and above. In general, DFT gives very good descriptions of molecular structure and electron distributions, but appears less reliable in predictions of the relative energetics of different spin states of transition metal compounds. For instance, the fact that our calculations favor a quartet state for the five-coordinate complex, ( P ) FeCl , over a sextet state may be regarded as a shortcoming of currently available exchange-correlation functionals. This problem is corrected at the considerably more demanding spin-restricted open-shell coupled-cluster level of theory. For both +3 and +4 metal oxidation states, the manganese 3d subshells appear to be significantly more spatially contracted than the 3d subshells of analogous iron compounds. This appears to be a key factor underlying the anomalously low MnO stretching frequencies of Mn(IV) -oxo porphyrins, compared to the FeO stretching frequencies of Fe(IV) -oxo porphryins. Similarly, a dramatic ruffling-induced redistribution of unpaired spin density in the case of an A2 u-type iron(IV)-oxo porphyrin cation radical, recently reported by Deeth, has been ascribed to a metal( dxy)–porphyrin( a2 u) orbital interaction that becomes symmetry-allowed on ruffling of the porphyrin ring. If the axial ligand in a compound I species is strongly basic such as imidazolate (e.g. horseradish peroxidase) or mercaptide (chloroperoxidase and cytochrome P450), it too can be strongly noninnocent and carry a significant unpaired electron spin population. Similarly, theoretical modeling of a synthetic 'perferryl' complex, reported by Morishima and co-workers, has revealed a noninnocent axial methoxide ligand.

Insight into the mechanism of an iron dioxygenase by resolution of steps following the FeIV=HO species

Iron oxygenases generate elusive transient oxygen species to catalyze substrate oxygenation in a wide range of metabolic processes. Here we resolve the reaction sequence and structures of such intermediates for the archetypal non-heme Fe(II) and alpha-ketoglutarate-dependent dioxygenase TauD. Time-resolved Raman spectra of the initial species with (16)O(18)O oxygen unequivocally establish the Fe(IV) horizontal lineO structure. (1)H/(2)H substitution reveals direct interaction between the oxo group and the C1 proton of substrate taurine. Two new transient species were resolved following Fe(IV) horizontal lineO; one is assigned to the nu(FeO) mode of an Fe(III) horizontal line O(H) species, and a second is likely to arise from the vibration of a metal-coordinated oxygenated product, such as Fe(II) horizontal line O horizontal line C(1) or Fe(II) horizontal line OOCR. These results provide direct insight into the mechanism of substrate oxygenation and suggest an alternative to the hydroxyl radical rebinding paradigm.

Evidence for an ionic intermediate in the transformation of fatty acid hydroperoxide by a catalase-related allene oxide synthase from the Cyanobacterium Acaryochloris marina

Allene oxides are reactive epoxides biosynthesized from fatty acid hydroperoxides by specialized cytochrome P450s or by catalase-related hemoproteins. Here we cloned, expressed, and characterized a gene encoding a lipoxygenase-catalase/peroxidase fusion protein from Acaryochloris marina. We identified novel allene oxide synthase (AOS) activity and a by-product that provides evidence of the reaction mechanism. The fatty acids 18.4omega3 and 18.3omega3 are oxygenated to the 12R-hydroperoxide by the lipoxygenase domain and converted to the corresponding 12R,13-epoxy allene oxide by the catalase-related domain. Linoleic acid is oxygenated to its 9R-hydroperoxide and then, surprisingly, converted approximately 70% to an epoxyalcohol identified spectroscopically and by chemical synthesis as 9R,10S-epoxy-13S-hydroxyoctadeca-11E-enoic acid and only approximately 30% to the 9R,10-epoxy allene oxide. Experiments using oxygen-18-labeled 9R-hydroperoxide substrate and enzyme incubations conducted in H2(18)O indicated that approximately 72% of the oxygen in the epoxyalcohol 13S-hydroxyl arises from water, a finding that points to an ionic intermediate (epoxy allylic carbocation) during catalysis. AOS and epoxyalcohol synthase activities are mechanistically related, with a reacting intermediate undergoing a net hydrogen abstraction or hydroxylation, respectively. The existence of epoxy allylic carbocations in fatty acid transformations is widely implicated although for AOS reactions, without direct experimental support. Our findings place together in strong association the reactions of allene oxide synthesis and an ionic reaction intermediate in the AOS-catalyzed transformation.

Red-excitation resonance Raman analysis of the nu(Fe=O) mode of ferryl-oxo hemoproteins

The Raman excitation profile of the nuFe O mode of horseradish peroxidase compound II exhibits a maximum at 580 nm. This maximum is located within an absorption band with a shoulder assignable to an oxygen-to-iron charge transfer band on the longer wavelength side of the alpha-band. Resonance Raman bands of the nuFe O mode of various ferryl-oxo type hemoproteins measured at 590 nm excitation indicate that many hemoproteins in the ferryl-oxo state have an oxygen-to-iron charge transfer band in the visible region. Since this red-excited resonance Raman technique causes much less photochemical damage in the proteins relative to blue-excited resonance Raman spectroscopy, it produces a higher signal-to-noise ratio and thus represents a powerful tool for investigations of ferryl-oxo intermediates of hemoproteins.

Spectroscopic description of an unusual protonated ferryl species in the catalase from Proteus mirabilis and density functional theory calculations on related models. Consequences for the ferryl protonation state in catalase, peroxidase and chloroperoxidase

The catalase from Proteus mirabilis peroxide-resistant bacteria is one of the most efficient heme-containing catalases. It forms a relatively stable compound II. We were able to prepare samples of compound II from P. mirabilis catalase enriched in (57)Fe and to study them by spectroscopic methods. Two different forms of compound II, namely, low-pH compound II (LpH II) and high-pH compound II (HpH II), have been characterized by Mössbauer, extended X-ray absorption fine structure (EXAFS) and UV-vis absorption spectroscopies. The proportions of the two forms are pH-dependent and the pH conversion between HpH II and LpH II is irreversible. Considering (1) the Mössbauer parameters evaluated for four related models by density functional theory methods, (2) the existence of two different Fe-O(ferryl) bond lengths (1.80 and 1.66 A) compatible with our EXAFS data and (3) the pH dependence of the alpha band to beta band intensity ratio in the absorption spectra, we attribute the LpH II compound to a protonated ferryl Fe(IV)-OH complex (Fe-O approximately 1.80 A), whereas the HpH II compound corresponds to the classic ferryl Fe(IV)=O complex (Fe=O approximately 1.66 A). The large quadrupole splitting value of LpH II (measured 2.29 mm s(-1) vs. computed 2.15 mm s(-1)) compared with that of HpH II (measured 1.47 mm s(-1) vs. computed 1.46 mm s(-1)) reflects the protonation of the ferryl group. The relevancy and involvement of such (Fe(IV)=O/Fe(IV)-OH) species in the reactivity of catalase, peroxidase and chloroperoxidase are discussed.

Resonance Raman spectroscopy of chloroperoxidase compound II provides direct evidence for the existence of an iron(IV)-hydroxide

We report direct evidence for the existence of an iron(IV)-hydroxide. Resonance Raman measurements on chloroperoxidase compound II (CPO-II) reveal an isotope ((18)O and (2)H)-sensitive band at nu(Fe-O) = 565 cm(-1). Preparation of CPO-II in H(2)O using H(2)(18)O(2) results in a red-shift of 22 cm(-1), while preparation of CPO-II in (2)H(2)O using H(2)O(2) results in a red-shift of 13 cm(-1). These values are in good agreement with the isotopic shifts predicted (23 and 12 cm(-1), respectively) for an Fe-OH harmonic oscillator. The measured Fe-O stretching frequency is also in good agreement with the 1.82-A Fe-O bond reported for CPO-II. A Badger's rule analysis of this distance provides an Fe-O stretching frequency of nu(Badger) = 563 cm(-1). We also present X-band electron nuclear double resonance (ENDOR) data for cryoreduced CPO-II. Cryogenic reduction (77 K) of the EPR-silent Fe(IV)OH center in CPO-II results in an EPR-active Fe(III)OH species with a strongly coupled (13.4 MHz) exchangeable proton. Based on comparisons with alkaline myoglobin, we assign this resonance to the hydroxide proton of cryoreduced CPO-II.

Resonance Raman spectroscopy of oxoiron(IV) porphyrin π-cation radical and oxoiron(IV) hemes in peroxidase intermediates

The catalytic cycle intermediates of heme peroxidases, known as compounds I and II, have been of long standing interest as models for intermediates of heme proteins, such as the terminal oxidases and cytochrome P450 enzymes, and for non-heme iron enzymes as well. Reports of resonance Raman signals for compound I intermediates of the oxo-iron(IV) porphyrin pi-cation radical type have been sometimes contradictory due to complications arising from photolability, causing compound I signals to appear similar to those of compound II or other forms. However, studies of synthetic systems indicated that protein based compound I intermediates of the oxoiron(IV) porphyrin pi-cation radical type should exhibit vibrational signatures that are different from the non-radical forms. The compound I intermediates of horseradish peroxidase (HRP), and chloroperoxidase (CPO) from Caldariomyces fumago do in fact exhibit unique and characteristic vibrational spectra. The nature of the putative oxoiron(IV) bond in peroxidase intermediates has been under discussion in the recent literature, with suggestions that the Fe(IV)O unit might be better described as Fe(IV)-OH. The generally low Fe(IV)O stretching frequencies observed for proteins have been difficult to mimic in synthetic ferryl porphyrins via electron donation from trans axial ligands alone. Resonance Raman studies of iron-oxygen vibrations within protein species that are sensitive to pH, deuteration, and solvent oxygen exchange, indicate that hydrogen bonding to the oxoiron(IV) group within the protein environment contributes to substantial lowering of Fe(IV)O frequencies relative to those of synthetic model compounds.

On the status of ferryl protonation

We examine the issue of ferryl protonation in heme proteins. An analysis of the results obtained from X-ray crystallography, resonance Raman spectroscopy, and extended X-ray absorption spectroscopy (EXAFS) is presented. Fe-O bond distances obtained from all three techniques are compared using Badger's rule. The long Fe-O bond lengths found in the ferryl crystal structures of myoglobin, cytochrome c peroxidase, horseradish peroxidase, and catalase deviate substantially from the values predict by Badger's rule, while the oxo-like distances obtained from EXAFS measurements are in good agreement with the empirical formula. Density functional calculations, which suggest that Mössbauer spectroscopy can be used to determine ferryl protonation states, are presented. Our calculations indicate that the quadrupole splitting (DeltaE(Q)) changes significantly upon ferryl protonation. New resonance Raman data for horse-heart myoglobin compound II (Mb-II, pH 4.5) are also presented. An Fe-O stretching frequency of 790cm(-1) (shifting to 754cm(-1) with (18)O substitution) was obtained. This frequency provides a Badger distance of r(Fe-O)=1.66A. This distance is in agreement with the 1.69A Fe-O bond distance obtained from EXAFS measurements but is significantly shorter than the 1.93A bond found in the crystal structure of Mb-II (pH 5.2). In light of the available evidence, we conclude that the ferryl forms of myoglobin (pKa4), horseradish peroxidase (pKa4), cytochrome c peroxidase (pKa4), and catalase (pKa7) are not basic. They are authentic Fe(IV)oxos with Fe-O bonds on the order of 1.65A.

The mechanism of Compound I formation revisited

The most recently proposed mechanisms for the formation of the Compound I intermediates of the peroxidases and catalases have been based on the crystallographic elucidation of the enzyme structures. It has been assumed that these mechanisms are compatible with an earlier proposal of the formation of a reversible enzyme-substrate intermediate called Compound 0, which was based on data that pre-dated the availability of the enzyme structures. However, it is argued here that this is not the case and some modifications of the existing mechanism are proposed which reconcile the structural, kinetic and energetic data for the reactions. This paper focuses attention on horseradish peroxidase isoenzyme C and particularly on the acid-base properties of the imidazole side chain of distal histidine 42. This imidazole group has an exceptionally low pK(a) value in the resting enzyme, which is higher in Compound I and higher still in Compound II. The pK(a) value must also be greatly increased following Compound 0 formation so that the imidazole can become an effective proton acceptor. An explanation is offered in a dielectric insertion (DI) model, in which the peroxide substrate, or fragments thereof, screens the influence of the positively charged heme iron on the pK(a) value of the imidazole group. It is proposed that Compound 0 is converted to a second intermediate, Compound 0*, by intramolecular proton transfer along a pre-existing hydrogen bond, a process which reduces the energy requirements of charge separation in the deprotonation of hydrogen peroxide.

Oxidation of 2,4-dichlorophenol catalyzed by horseradish peroxidase: characterization of the reaction mechanism by UV-visible spectroscopy and mass spectrometry

The hydrogen peroxide-oxidation of 2,4-dichlorophenol catalyzed by horseradish peroxidase has been studied by means of UV-visible spectroscopy and mass spectrometry in order to clarify the reaction mechanism. The dimerization of 2,4-dichlorophenol to 2,4-dichloro-6-(2,4-dichlorophenoxy)-phenol and its subsequent oxidation to 2-chloro-6-(2,4-dichlorophenoxy)-1,4-benzoquinone together with chloride release were observed. The reaction rate was found to be pH-dependent and to be influenced by the pK(a) value of 2,4-dichlorophenol. The dissociation constants of the 2,4-dichlorophenol/horseradish peroxidase (HRP) adduct at pH 5.5 and 8.5 were also determined: their values indicate the unusual stability of the adduct at pH 5.5 with respect to several adducts of HRP with substituted phenols.

Direct detection of Fe(IV)[double bond]O intermediates in the cytochrome aa3 oxidase from Paracoccus denitrificans/H2O2 reaction

We report the first evidence for the formation of the "607- and 580-nm forms" in the cytochrome oxidase aa3/H2O2 reaction without the involvement of tyrosine 280. The pKa of the 607-580-nm transition is 7.5. The 607-nm form is also formed in the mixed valence cytochrome oxidase/O2 reaction in the absence of tyrosine 280. Steady-state resonance Raman characterization of the reaction products of both the wild-type and Y280H cytochrome aa3 from Paracoccus denitrificans indicate the formation of six-coordinate low spin species, and do not support, in contrast to previous reports, the formation of a porphyrin pi-cation radical. We observe three oxygen isotope-sensitive Raman bands in the oxidized wild-type aa3/H2O2 reaction at 804, 790, and 358 cm-1. The former two are assigned to the Fe(IV)[double bond]O stretching mode of the 607- and 580-nm forms, respectively. The 14 cm-1 frequency difference between the oxoferryl species is attributed to variations in the basicity of the proximal to heme a3 His-411, induced by the oxoferryl conformations of the heme a3-CuB pocket during the 607-580-nm transition. We suggest that the 804-790 cm-1 oxoferryl transition triggers distal conformational changes that are subsequently communicated to the proximal His-411 heme a3 site. The 358 cm-1 mode has been found for the first time to accumulate with the 804 cm-1 mode in the peroxide reaction. These results indicate that the mechanism of oxygen reduction must be reexamined.

Spectroscopic study on structure of horseradish peroxidase in water and dimethyl sulfoxide mixture

The structure of horseradish peroxidase (HRP) in phosphate buffered saline (PBS)/dimethyl sulfoxide (DMSO) mixed solvents at different compositions is investigated by IR, electronic absorption, and fluorescence spectroscopies. The fluorescence spectra and the amide I spectra of ferric HRP [HRP(Fe3+)] show that overall structural changes are relatively small up to 60% DMSO. Although the amide I band of HRP(Fe3+) shows a gradual change in the secondary structure and a decrease in the contents of a helices, its fluorescence spectra indicate that the distance between the heme and Trp173 is almost constant. In contrast, the changes in the positions of the Soret bands for resting HRP(Fe3+) and catalytic intermediates (compounds I and II) and the IR spectra at the C-O stretching vibration mode of carbonyl ferrous HRP [HRP(Fe2+)-CO] show that the microenvironment in the distal heme pocket is altered, even with low DMSO contents. The large reduction of the catalytic activity of HRP even at low DMSO contents can be attributed to the structural transition in the distal heme pocket. In PBS/DMSO mixtures containing more than 70 vol % DMSO, HRP undergoes large structural changes, including a large loss of the secondary structure and a dissociation of the heme from the apoprotein. The presence of the components of the amide I band that can be assigned to strongly hydrogen bonding amide C=O groups at 1616 and 1684 cm(-1) suggests that the denatured HRP may aggregate through strong hydrogen bonds.

Oxygen ligand exchange at metal sites - implications for the O2 evolving mechanism of photosystem II

The mechanism for photosynthetic O2 evolution by photosystem II is currently a topic of intense debate. Important questions remain as to what is the nature of the binding sites for the substrate water and how does the O-O bond form. Recent measurements of the 18O exchange between the solvent water and the photogenerated O2 as a function of the S-state cycle have provided some surprising insights to these questions (W. Hillier, T. Wydrzynski, Biochemistry 39 (2000) 4399-4405). The results show that one substrate water molecule is bound at the beginning of the catalytic sequence, in the S0 state, while the second substrate water molecule binds in the S3 state or possibly earlier. It may be that the second substrate water molecule only enters the catalytic sequence following the formation of the S3 state. Most importantly, comparison of the observed exchange rates with oxygen ligand exchange in various metal complexes reveal that the two substrate water molecules are most likely bound to separate Mn(III) ions, which do not undergo metal-centered oxidations through to the S3 state. The implication of this analysis is that in the S1 state, all four Mn ions are in the +3 oxidation state. This minireview summarizes the arguments for this proposal.

Resonance Raman spectroscopy of soybean peroxidase

Resonance Raman spectra (600-1700 cm-1) for the heme enzyme soybean peroxidase (Rz = 2.5) were obtained using Soret band excitation at 406.7 nm. The vibrational frequencies and depolarization data indicate a strong similarity between the active sites of soybean and horseradish peroxidase. This similarity suggests that the active site in the resting form of soybean peroxidase contains a ferric iron, is a high-spin 5-coordinate heme binding His as a fifth axial ligand.

Kinetic studies on the oxidation of phenols by the horseradish peroxidase compound II

Oxidation of substituted phenols by horseradish peroxidase compound II were studied using stopped-flow technique. Dissociation constants (K(D)) of HRP-II-phenol complexes were deduced from the kinetic data. Magnitudes of K(D) fall in a relatively narrow range of 3-11 mM. These are comparable to 3-10 mM reported for the binding of substituted phenols to native HRP, suggesting that the mode of binding of phenols to native HRP and HRP compound II may be similar. pH dependence of the apparent second order rate constants (k(app)) of the reactions of all the phenols were determined. The k(app) values of reactions other than the reaction of tyrosine, were observed to increase in the acidic region but decreased in the alkaline region. The increase was attributed to the deprotonation of distal carboxylic acid residue on enzyme with pK(a) values of 4.2-5.2. For tyrosine, however, the apparent second-order rate constant was observed to further increase non linearly on increasing the pH in the alkaline region. Results were interpreted quantitatively on the basis that protonated form of the enzyme reacted with the protonated form of the phenol with different individual rate constants.

Binding of iodide to Arthromyces ramosus peroxidase investigated with X-ray crystallographic analysis, 1H and 127I NMR spectroscopy, and steady-state kinetics

The site and characteristics of iodide binding to Arthromyces ramosus peroxidase were examined by x-ray crystallographic analysis, 1H and 127I NMR, and kinetic studies. X-ray analysis of an A. ramosus peroxidase crystal soaked in a KI solution at pH 5.5 showed that a single iodide ion is located at the entrance of the access channel to the distal side of the heme and lies between the two peptide segments, Phe90-Pro91-Ala92 and Ser151-Leu152-Ile153, 12.8 A from the heme iron. The distances between the iodide ion and heme peripheral methyl groups were all more than 10 A. The findings agree with the results obtained with 1H NMR in which the chemical shift and intensity of the methyl groups in the hyperfine shift region of A. ramosus peroxidase were hardly affected by the addition of iodide, unlike the case of horseradish peroxidase. Moreover, 127I NMR and steady-state kinetics showed that the binding of iodide depends on protonation of an amino acid residue with a pKa of about 5.3, which presumably is the distal histidine (His56), 7.8 A away from the iodide ion. The mechanism of electron transfer from the iodide ion to the heme iron is discussed on the basis of these findings.

Resonance Raman spectra of native and mesoheme-reconstituted horseradish peroxidase and their catalytic intermediates

Resonance Raman studies of native and mesoheme-reconstituted horseradish peroxidase and their catalytic intermediates, known as Compounds I and II, have been conducted using both near UV ( approximately 350 nm) and visible (406.7 nm) excitation. Careful power studies indicate that the authentic Compound I spectra are obtainable using near UV excitation, but that use of visible excitation results in contamination of the Compound I spectrum with the spectrum of a Compound II-like photoproduct. Using H218O2, the nu(Fe=O) stretching modes for both systems are unambiguously identified, for the first time, at approximately 790 cm-1. The authentic Compound I spectra are indicative of an 2A1u-like ground state for both the native and the mesoheme-reconstituted proteins. Finally, the possible biological implications of such information are briefly discussed.

Rescue of His-42 --> Ala horseradish peroxidase by a Phe-41 --> His mutation. Engineering of a surrogate catalytic histidine

Formation of the ferryl (FeIV=O) porphyrin radical cation known as Compound I in the reaction of horseradish peroxidase (HRP) with H2O2 is catalyzed by His-42, a residue that facilitates the binding of H2O2 to the iron and subsequent rupture of the dioxygen bond. An H42A mutation was shown earlier to decrease the rate of Compound I formation by a factor of approximately 10(6) and of guaiacol oxidation by a factor of approximately 10(4). In contrast, an F41A mutation has little effect on peroxidative catalysis (Newmyer, S. L., and Ortiz de Montellano, P. R. (1995) J. Biol. Chem. 270, 19430-19438). We report here construction, expression, and characterization of the F41H/H42A double mutant. The pH profile for guaiacol oxidation by this double mutant has a broad maximum at approximately pH 6.3. Addition of H2O2 produces a Compound I species (lambdamax = 406 nm) that is reduced by 1 eq of K4Fe(CN)6 to the ferric state (lambdamax = 407 nm) without the detectable formation of Compound II. A fraction of the heme chromophore is lost in the process. The rate of Compound I formation for the F41H/H42A double mutant is 3.0 x 10(4) M-1 s-1. This is to be compared with 0.9 x 10(7) M-1 s-1 for wild-type HRP and 19 M-1 s-1 for the H42A mutant. The kcat values for guaiacol oxidation by wild-type, H42A, and F41H/H42A HRP are 300, 0.015, and 1.8 s-1. The corresponding kcat values for ABTS oxidation are 4900, 0.41, and 100 s-1, respectively. These results show that a histidine at position 41 substitutes, albeit imperfectly, for His-42 in peroxidative turnover of the enzyme. The F41H/H42A double mutant has peroxidative properties intermediate between those of the native enzyme and the H42A mutant. The F41H/H42A double mutant, however, is a considerably better thioanisole sulfoxidation and styrene epoxidation catalyst than native or H42A HRP. The surrogate catalytic residue introduced by the F41H mutation thus partially compensates for the H42A substitution used to increase access to the ferryl oxygen.

Rescue of the catalytic activity of an H42A mutant of horseradish peroxidase by exogenous imidazoles

His-42 plays a critical role in the H2O2-dependent catalytic turnover of horseradish peroxidase (HRP). This is clearly illustrated by the finding that an H42A mutation decreases the rate of Compound I formation by a factor of approximately10(6). As shown here, the addition of 2-substituted imidazoles partially rescues both the rate of formation of Compound I and the peroxidase activity of the H42A mutant. 2-Substituted imidazoles are the most effective because they do not coordinate to the iron. In contrast to native HRP, which exhibits a parabolic pH profile, and the H42A mutant, for which the activity increases linearly with increasing pH, the activity of the H42A mutant in the presence of 1,2-dimethylimidazole (pKa = 8.0) exhibits a sigmoidal pH dependence with a midpoint at pH 8.0 +/- 0.2. Similar results are obtained with 2-methylimidazole. These results establish that the free base forms of these imidazoles facilitate HRP turnover. The spectroscopic binding constants for 1, 2-dimethylimidazole and 2-methylimidazole are Kd = 2.9 +/- 1.3 and 2. 5 +/- 0.2 M, respectively. When cyanide is bound to the heme, the Kd for 1,2-dimethylimidazole is 0.17 M. This >10-fold decrease in Kd may reflect hydrogen bonding of the protonated imidazole to the iron-coordinated cyanide. The log of the rate of Compound I formation exhibits a linear dependence on the molecular volume of the imidazoles used to rescue the activity. If the rates are corrected for differences in the size of the imidazoles, the log of the rates is linearly related to the pKa of the imidazoles. This Brönsted analysis predicts that approximately 60% of a positive charge develops on the imidazole in the transition state of Compound I formation. The results confirm the acid-base role of the distal histidine, demonstrate that exogenous histidines can function as surrogates for the missing histidine in the H42A mutant, and provide a transition state model of relevance to the formation of Compound I in the native protein.