Effects of Concerted Hydrogen Bonding of Distal Histidine on Active Site Structures of Horseradish Peroxidase. Resonance Raman Studies with Asn70 Mutants

Heme-copper and Heme O2-derived synthetic (bioinorganic) chemistry toward an understanding of cytochrome c oxidase dioxygen chemistry

Cytochrome c oxidase (CcO), also widely known as mitochondrial electron-transport-chain complex IV, is a multi-subunit transmembrane protein responsible for catalyzing the last step of the electron transport chain, dioxygen reduction to water, which is essential to the establishment and maintenance of the membrane proton gradient that drives ATP synthesis. Although many intermediates in the CcO catalytic cycle have been spectroscopically and/or computationally authenticated, the specifics regarding the IP intermediate, hypothesized to be a heme-Cu (hydro)peroxo species whose O-O bond homolysis is supported by a hydrogen-bonding network of water molecules, are largely obscured by the fast kinetics of the A (FeIII-O2•-/CuI/Tyr) → PM (FeIV=O/CuII-OH/Tyr•) step. In this review, we have focused on the recent advancements in the design, development, and characterization of synthetic heme-peroxo‑copper model complexes, which can circumvent the abovementioned limitation, for the investigation of the formation of IP and its O-O cleavage chemistry. Novel findings regarding (a) proton and electron transfer (PT/ET) processes, together with their contributions to exogenous phenol induced O-O cleavage, (b) the stereo-electronic tunability of the secondary coordination sphere (especially hydrogen-bonding) on the geometric and spin state alteration of the heme-peroxo‑copper unit, and (c) a plausible mechanism for the Tyr-His cofactor biogenesis, are discussed in great detail. Additionally, since the ferric-superoxide and the ferryl-oxo (Compound II) species are critically involved in the CcO catalytic cycle, this review also highlights a few fundamental aspects of these heme-only (i.e., without copper) species, including the structural and reactivity influences of electron-donating trans-axial ligands and Lewis acid-promoted H-bonding.

Lignin peroxidase-catalyzed direct oxidation of trace organic pollutants through a long-range electron transfer mechanism: Using propranolol as an example

In this work, lignin peroxidase (LiP) was extracted for the in vitro degradation of a persistent compound (propranolol, PPN). The results showed that 94.2% of PPN was degraded at 30 U L^-1 LiP activity and 10 mg L^-1 PPN. The PPN degradation rate increased from 33.5% to 94.2% when the veratryl alcohol (VA) concentration varied from 0 to 180 µM, but decreased to 73.1% with further VA addition. This phenomenon confirmed that VA was indispensable, however, it also acted as a competitive inhibitor of PPN oxidation. Computational analysis revealed that the Trp171…iron porphyrin (TRP-FeP) path was responsible for specific substrate (e.g., VA) transformation, and another long-range electron transfer (LRET) path through His-Asp…FeP (HSP-FeP) was discovered for non-specific substrate (e.g., PPN) degradation. These two electron-transfer routes shared one catalytic center, and VA protected the enzyme from H₂O₂-dependent inactivation. The HSP-FeP path transformed PPN through single electron transfer or H abstraction mechanisms. In addition, hydroxyl radicals generated in the LiP/H₂O₂ system were involved in the hydroxylation of the PPN intermediates. Possible degradation pathways were deduced using these degradation mechanisms and mass-spectrometry analysis. The multipath degradation mechanism endowed LiP with a remarkable capacity for removing various recalcitrant pollutants in environmental remediation.

A new inhibition mechanism in the multifunctional catalytic hemoglobin dehaloperoxidase as revealed by the DHP A(V59W) mutant: A spectroscopic and crystallographic study

As multifunctional catalytic hemoglobins, dehaloperoxidase isoenzymes A and B (DHP A and B) are among the most versatile hemoproteins in terms of activities displayed. The ability of DHP to bind over twenty different substrates in the distal pocket might appear to resemble the promiscuousness of monooxygenase enzymes, yet there are identifiable substrate-specific interactions that can steer the type of oxidation (O-atom vs. electron transfer) that occurs inside the DHP distal pocket. Here, we have investigated the DHP A(V59W) mutant in order to probe the limits of conformational flexibility in the distal pocket as it relates to the genesis of this substrate-dependent activity differentiation. The X-ray crystal structure of the metaquo DHP A(V59W) mutant (PDB 3K3U) and the V59W mutant in complex with fluoride [denoted as DHP A(V59W-F)] (PDB 7MNH) show significant mobility of the tryptophan in the distal pocket, with two parallel conformations having W59-N[Formula: see text] H-bonded to a heme-bound ligand (H2O or F[Formula: see text], and another conformation [observed only in DHP A(V59W-F)] that brings W59 sufficiently close to the heme as to preclude axial ligand binding. UV-vis and resonance Raman spectroscopic studies show that DHP A(V59W) is 5-coordinate high spin (5cHS) at pH 5 and 6-coordinate high spin (6cHS) at pH 7, whereas DHP A(V59W-F) is 6cHS from pH 5 to 7. Enzyme assays confirm robust peroxidase activity at pH 5, but complete loss of activity at pH 7. We find no evidence that tryptophan plays a role in the oxidation mechanism ([Formula: see text]. radical formation). Instead, the data reveal a new mechanism of DHP inhibition, namely a shift towards a non-reactive form by OH[Formula: see text] ligation to the heme-Fe that is strongly stabilized (presumably through H-bonding interactions) by the presence of W59 in the distal cavity.

Electronic structure and reactivity of heme bound insulin

Insulin resistance as well as insulin deficiency are said to be principal to the development of type 2 diabetes mellitus (T2Dm). Heme has also been suggested to play an important role in the disease etiology since many of the heme deficiency symptoms constitute the common pathological features of T2Dm. Besides, iron overload, higher heme iron intake and transfusion requiring diseases are associated with a higher risk of T2Dm development. In this study the interaction between these two key components i.e. heme and insulin has been studied spectroscopically under different conditions which include the effect of excess peptide as well as increasing pH. The resultant heme-insulin complexes in their reduced state are found to produce very little partially reduced oxygen species (PROS) on getting oxidized by molecular oxygen. The interaction between insulin and previously reported T2Dm relevant heme-amylin complex were also examined using absorption and resonance Raman spectroscopy. The corresponding data suggest that insulin sequesters heme from heme-amylin to form the much less cytotoxic heme-insulin.

Single-Molecule Studies of Allosteric Inhibition of Individual Enzyme on a DNA Origami Reactor

Unraveling the conformational changes of enzymes together with inhibition kinetics during an enzymatic reaction has great potential in screening therapeutic candidates; however, it remains challenging due to the transient nature of each intermediate step. We report our study on the noncompetitive inhibition of horseradish peroxidase with single-turnover resolution using single-molecule fluorescence microscopy. By introducing DNA origami as an addressable nanoreactor, we observe the coexistence of nascent-formed fluorescent product on both catalytic and docking sites. We further propose a single-molecule kinetic model to reveal the interplay between product generation and noncompetitive inhibition and find three distinct inhibitor releasing pathways. Moreover, the kinetic isotope effect experiment indicates a strong correlation between catalytic and docking sites, suggesting an allosteric conformational change in noncompetitive inhibition. A memory effect is also observed. This work provides an in-depth understanding of the correlation between enzyme behavior and enzymatic conformational fluctuation, substrate conversion, and product releasing pathway and kinetics.

Lewis Acid Assisted Nitrate Reduction with Biomimetic Molybdenum Oxotransferase Complex

The reduction of nitrate (NO₃^-) to nitrite (NO₂^-) is of significant biological and environmental importance. While Mo^IV(O) and Mo^VI(O)₂ complexes that mimic the active site structure of nitrate reducing enzymes are prevalent, few of these model complexes can reduce nitrate to nitrite through oxygen atom transfer (OAT) chemistry. We present a novel strategy to induce nitrate reduction chemistry of a previously known catalyst Mo^IV(O)(SN)₂ (2), where SN = bis(4- tert-butylphenyl)-2-pyridylmethanethiolate, that is otherwise incapable of achieving OAT with nitrate. Addition of nitrate with the Lewis acid Sc(OTf)₃ (OTf = trifluoromethanesulfonate) to 2 results in an immediate and clean conversion of 2 to Mo^VI(O)₂(SN)₂ (1). The Lewis acid additive further reacts with the OAT product, nitrite, to form N₂O and O₂. This work highlights the ability of Sc³⁺ additives to expand the reactivity scope of an existing Mo^IV(O) complex together with which Sc³⁺ can convert nitrate to stable gaseous molecules.

A Water Dimer Shift Activates a Proton Pumping Pathway in the PR → F Transition of ba3 Cytochrome c Oxidase

Broken-symmetry density functional calculations have been performed on the [Fe_a3⁴⁺,Cu_B²⁺] state of the dinuclear center (DNC) for the P_R → F part of the catalytic cycle of ba₃ cytochrome c oxidase (CcO) from Thermus thermophilus (Tt), using the OLYP-D3-BJ functional. The calculations show that the movement of the H₂O molecules in the DNC affects the pK_a values of the residue side chains of Tyr237 and His376⁺, which are crucial for proton transfer/pumping in ba₃ CcO from Tt. The calculated lowest energy structure of the DNC in the [Fe_a3⁴⁺,Cu_B²⁺] state (state F) is of the form Fe_a3⁴⁺═O^2-···Cu_B²⁺, in which the H₂O ligand that resulted from protonation of the OH^- ligand in the P_R state is dissociated from the Cu_B²⁺ site. The calculated Fe_a3⁴⁺═O^2- distance in F (1.68 Å) is 0.03 Å longer than that in P_R (1.65 Å), which can explain the different Fe_a3⁴⁺═O^2- stretching modes in P (804 cm^-1) and F (785 cm^-1) identified by resonance Raman experiments. In this F state, the Cu_B²⁺···O^2- (ferryl-oxygen) distance is only around 2.4 Å. Hence, the subsequent O_H state [Fe_a3³⁺-OH^--Cu_B²⁺] with a μ-hydroxo bridge can be easily formed, as shown by our calculations.

The crystal structure and catalytic mechanism of hydroxynitrile lyase from passion fruit, Passiflora edulis

Hydroxynitrile lyases (HNLs) are enzymes used in the synthesis of chiral cyanohydrins. The HNL from Passiflora edulis (PeHNL) is R-selective and is the smallest HNL known to date. The crystal structures of PeHNL and its C-terminal peptide depleted derivative were determined by molecular replacement method using the template structure of a heat stable protein, SP1, from Populus tremula at 2.8 and 1.8 Å resolution, respectively. PeHNL belongs to dimeric α+β barrel superfamily consisting of a central β-barrel in the middle of a dimer. The structure of PeHNL complexed with (R)-mandelonitrile ((R)-MAN) was also determined. The hydroxyl group of (R)-MAN forms hydrogen bonds with His8 and Tyr30 in the active site, whereas the nitrile group is oriented toward the carboxyl group of Glu54, unlike other HNLs, where it interacts with basic residues typically. The results of mutational analysis indicate that the catalytic dyad of His8-Asn101 is critical for the enzymatic reaction. The length of the hydrogen bond between His-Nδ1 and Asn101-Oδ1 is short in the PeHNL-(R)-MAN complex (~ 2.6 Å), which would increase the basicity of His8 to abstract a proton from the hydroxyl group of (R)-MAN. The cyanide ion released from the nitrile group abstracts a proton from the protonated His8 to generate a hydrogen cyanide. Thus, the His8 in the active site of PeHNL acts both as a general acid and a general base in the reaction.

ENZYMES

EC 4.1.2.10 DATABASE: Structural data are available in PDB database under the accession numbers 5XZQ, 5XZT, and 5Y02.

Vibrational Probes of Molybdenum Cofactor-Protein Interactions in Xanthine Dehydrogenase

The pyranopterin dithiolene (PDT) ligand is an integral component of the molybdenum cofactor (Moco) found in all molybdoenzymes with the sole exception of nitrogenase. However, the roles of the PDT in catalysis are still unknown. The PDT is believed to be bound to the proteins by an extensive hydrogen-bonding network, and it has been suggested that these interactions may function to fine-tune Moco for electron- and atom-transfer reactivity in catalysis. Here, we use resonance Raman (rR) spectroscopy to probe Moco-protein interactions using heavy-atom congeners of lumazine, molecules that bind tightly to both wild-type xanthine dehydrogenase (wt-XDH) and its Q102G and Q197A variants following enzymatic hydroxylation to the corresponding violapterin product molecules. The resulting enzyme-product complexes possess intense near-IR absorption, allowing high-quality rR spectra to be collected on wt-XDH and the Q102G and Q197A variants. Small negative frequency shifts relative to wt-XDH are observed for the low-frequency Moco vibrations. These results are interpreted in the context of weak hydrogen-bonding and/or electrostatic interactions between Q102 and the -NH₂ terminus of the PDT, and between Q197 and the terminal oxo of the Mo≡O group. The Q102A, Q102G, Q197A, and Q197E variants do not appreciably affect the kinetic parameters k_red and k_red/K_D, indicating that a primary role for these glutamine residues is to stabilize and coordinate Moco in the active site of XO family enzymes but to not directly affect the catalytic throughput. Raman frequency shifts between wt-XDH and its Q102G variant suggest that the changes in the electron density at the Mo ion that accompany Mo oxidation during electron-transfer regeneration of the catalytically competent active site are manifest in distortions at the distant PDT amino terminus. This implies a primary role for the PDT as a conduit for facilitating enzymatic electron-transfer reactivity in xanthine oxidase family enzymes.

Probing nitrite coordination in horseradish peroxidase by resonance Raman spectroscopy: Detection of two binding sites

Nitrite is a powerful oxidant that affects the activity of peroxidases towards various substrates and leads to heme macrocycle modifications in members of the peroxidase family, such as the horseradish peroxidase (HRP). We have applied resonance Raman spectroscopy to investigate the structural properties of the species formed in the reaction of NO₂^- with the ferric form of HRP. Our data demonstrate that the heme nitrovinyl group is partially formed at near neutral pH, without coordination of NO₂^- to the heme Fe. Nitrite coordinates to the heme Fe at acidic pH in the nitro binding mode, characterized by the detection of the ν(Fe-NO₂) at 563cm^-1, δ(FeNO₂) at 822cm^-1 and ν_sym(NO₂) at 1272cm^-1. The sensitivity of the vibrations of the heme Fe-nitro complex to H/D exchange indicates H-bonding interaction of the heme-bound ligand with the distal environment that determines the NO₂^- binding mode. A model describing the different modes of NO₂^- binding in HRP is presented.

A bio-mimetic zinc/tau protein as an artificial catalase

In this study, the catalase-like activity of monomeric tau protein was reported in the presence of of zinc (Zn(II)) ions at low pH value. Monomeric tau protein contains two SH groups that are a target of disulfide bond formation. However these SH groups are able to interact with Zn(II) ion at pH 7.2 which creates a thiol bond as a mimetic model of chloroperoxidase active site which performs catalase like activity at low pH. Zn(II)/tau protein complex decomposed H₂O₂ with a high rate (V_m) as well as an efficient turn oven number (k_cat) at pH 3. This remarkable catalase like activity is may be attributed to the conformational reorientation of protein at low pH. Circular dichroism (CD) studies did not demonstrate any secondary structural changes of tau protein after addition of Zn(II) ions at pH 7.2. In addition, tau protein shows identical CD bands at pH 7.2 and 3. Moreover, fluorescence quenching of tau by Zn(II) at pH 7.2 was initiated by complex formation rather than by dynamic collision. A significant red shift (6nm) was observed in the emission maximum of the fluorescence spectra when the protein was dissolved at pH 3 compared to pH 7.2. This conformational change can provide information regarding the rearrangements of the protein structure and exposure of Cys-Zn(II) group to the solvent which induces easy access of active site to H2O2 molecules and corresponding enhanced catalytic activity of Zn(II)/tau protein complex. This study introduces tau protein as a bio-inspired high performing scaffold for transition metal encapsulation and introducing an engineered apoprotein-induced biomimetic enzyme.

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.

Spectroscopic and Kinetic Evidence for the Crucial Role of Compound 0 in the P450cam -Catalyzed Hydroxylation of Camphor by Hydrogen Peroxide

The hydroperoxo iron(III) intermediate P450cam Fe(III) -OOH, being the true Compound 0 (Cpd 0) involved in the natural catalytic cycle of P450cam , could be transiently observed in the peroxo-shunt oxidation of the substrate-free enzyme by hydrogen peroxide under mild basic conditions and low temperature. The prolonged lifetime of Cpd 0 enabled us to kinetically examine the formation and reactivity of P450cam Fe(III) -OOH species as a function of varying reaction conditions, such as pH, and concentration of H2 O2 , camphor, and potassium ions. The mechanism of hydrogen peroxide binding to the substrate-free form of P450cam differs completely from that observed for other heme proteins possessing the distal histidine as a general acid-base catalyst and is mainly governed by the ability of H2 O2 to undergo deprotonation at the hydroxo ligand coordinated to the iron(III) center under conditions of pH≥p${K{{{\rm P450}\hfill \atop {\rm a}\hfill}}}$. Notably, no spectroscopic evidence for the formation of either Cpd I or Cpd II as products of heterolytic or homolytic OO bond cleavage, respectively, in Cpd 0 could be observed under the selected reaction conditions. The kinetic data obtained from the reactivity studies involving (1R)-camphor, provide, for the first time, experimental evidence for the catalytic activity of the P450Fe(III) -OOH intermediate in the oxidation of the natural substrate of P450cam .

Synthesis and characterization of M(ii) (M = Mn, Fe and Co) azafulvene complexes and their X3− derivatives

Structural and electronic flexibility in a tripodal ligand platform featuring a secondary coordination sphere.

Probing the conformational mobility of the active site of a heme peroxidase

We have previously demonstrated (Badyal et al., J. Biol. Chem., 2006, 281, 24512) that removal of the active site tryptophan (Trp41) in ascorbate peroxidase increases the conformational mobility of the distal histidine residue (His42) and that His42 coordinates to the iron in the oxidised W41A enzyme to give a 6-coordinate, low-spin peroxidase. In this work, we probe the conformational flexibility of the active site in more detail. We examine whether other residues (Cys, Tyr, Met) can also ligate to the heme at position 42; we find that introduction of other ligating amino acids created a cavity in the heme pocket, but that formation of 6-coordinate heme is not observed. In addition, we examine the role of Asn-71, which hydrogen bonds to His42 and tethers the distal histidine in the active site pocket; we find that removal of this hydrogen bond increases the proportion of low-spin heme. We suggest that, in addition to its well-known role in facilitating the reaction with peroxide, His42 also plays a role in defining the shape and folding of the active site pocket.

Reduction potential of yeast cytochrome c peroxidase and three distal histidine mutants: dependence on pH

The pH dependence of the Fe(III) reduction potential, E(0)', for yeast cytochrome c peroxidase (yCcP) and three distal pocket mutants, CcP(H52L), CcP(H52Q), and CcP(R48L/W51L/H52L), has been determined between pH 4 and 8. E(0)' values at pH 7.0 for the yCcP, CcP(H52L), CcP(H52Q), and CcP(R48L/W51L/H52L) are -189, -170, -224, and -146mV, respectively. A heme-linked ionization in the reduced enzyme affects the reduction potential for yCcP and all three mutants. Apparent pK(A) values for the heme-linked ionization are 7.5±0.2, 6.5±0.3, 6.4±0.2, and 7.0±0.3 for yCcP and the H52L, H52Q, and R48L/W51L/H52L mutants, respectively. A cooperative, two-proton ionization causing a spectroscopically-detectable transition was observed in the ferrous states of yCcP, CcP(H52L) and CcP(H52Q), with apparent pK(A) values of 7.7±0.2, 7.4±0.1 and 7.8±0.1, respectively. These data indicate that: (1) the distal histidine in CcP is not the site of proton binding upon reduction of the ferric CcP, (2) the distal histidine is not one of the two groups involved in the cooperative, two-proton ionization observed in ferrous CcP, and (3) the proton-binding site is not involved in the cooperative, two-proton ionization observed in the reduced enzyme.

Role of the secondary coordination sphere in metal-mediated dioxygen activation

Alfred Werner proposed nearly 100 years ago that the secondary coordination sphere has a role in determining the physical properties of transition-metal complexes. We now know that the secondary coordination sphere impacts nearly all aspects of transition-metal chemistry, including the reactivity and selectivity in metal-mediated processes. These features are highlighted in the binding and activation of dioxygen by transition-metal complexes. There are clear connections between control of the secondary coordination sphere and the ability of metal complexes to (1) reversibly bind dioxygen or (2) bind and activate dioxygen to form highly reactive metal-oxo complexes. In this Forum Article, several biological and synthetic examples are presented and discussed in terms of structure-function relationships. Particular emphasis is given to systems with defined noncovalent interactions, such as intramolecular H-bonds involving dioxygen-derived ligands. To further illustrate these effects, the homolytic cleavage of C-H bonds by metal-oxo complexes with basic oxo ligands is described.

Redox properties of heme peroxidases

Peroxidases are heme enzymes found in bacteria, fungi, plants and animals, which exploit the reduction of hydrogen peroxide to catalyze a number of oxidative reactions, involving a wide variety of organic and inorganic substrates. The catalytic cycle of heme peroxidases is based on three consecutive redox steps, involving two high-valent intermediates (Compound I and Compound II), which perform the oxidation of the substrates. Therefore, the thermodynamics and the kinetics of the catalytic cycle are influenced by the reduction potentials of three redox couples, namely Compound I/Fe3+, Compound I/Compound II and Compound II/Fe3+. In particular, the oxidative power of heme peroxidases is controlled by the (high) reduction potential of the latter two couples. Moreover, the rapid H2O2-mediated two-electron oxidation of peroxidases to Compound I requires a stable ferric state in physiological conditions, which depends on the reduction potential of the Fe3+/Fe2+ couple. The understanding of the molecular determinants of the reduction potentials of the above redox couples is crucial for the comprehension of the molecular determinants of the catalytic properties of heme peroxidases. This review provides an overview of the data available on the redox properties of Fe3+/Fe2+, Compound I/Fe3+, Compound I/Compound II and Compound II/Fe3+ couples in native and mutated heme peroxidases. The influence of the electron donor properties of the axial histidine and of the polarity of the heme environment is analyzed and the correlation between the redox properties of the heme group with the catalytic activity of this important class of metallo-enzymes is discussed.

Structural analysis of an open active site conformation of nonheme iron halogenase CytC3

CytC3, a member of the recently discovered class of nonheme Fe(II) and α-ketoglutarate (αKG)-dependent halogenases, catalyzes the double chlorination of l-2-aminobutyric acid (Aba) to produce a known Streptomyces antibiotic, γ,γ-dichloroaminobutyrate. Unlike the majority of the Fe(II)-αKG-dependent enzymes that catalyze hydroxylation reactions, halogenases catalyze a transfer of halides. To examine the important enzymatic features that discriminate between chlorination and hydroxylation, the crystal structures of CytC3 both with and without αKG/Fe(II) have been solved to 2.2 Å resolution. These structures capture CytC3 in an open active site conformation, in which no chloride is bound to iron. Comparison of the open conformation of CytC3 with the closed conformation of another nonheme iron halogenase, SyrB2, suggests two important criteria for creating an enzyme-bound Fe—Cl catalyst: (1) the presence of a hydrogen-bonding network between the chloride and surrounding residues, and (2) the presence of a hydrophobic pocket in which the chloride resides.

Pyrazolate-bridging dinucleating ligands containing hydrogen-bond donors: synthesis and structure of their cobalt analogues

Convergent preparative routes to new urea-pyrazolate dinucleating ligands are described. Metal complexes of these ligands have hydrogen bond donors that are proximal to the metal centers that interact with other coordinated species. This is exemplified by Co(II) dimers with Co(II)-mu-Cl-Co(II) motifs, in which the chloro ligand is involved in four intramolecular hydrogen bonds. These noncovalent interactions appear to influence the Co(II)-Cl bonds, which are unusually long, having lengths greater than 2.5 A.

Resonance Raman Study of Ferric Heme Adducts of Dehaloperoxidase from Amphitrite ornata

The study of axial ligation by anionic ligands to ferric heme iron by resonance Raman spectroscopy provides a basis for comparison of the intrinsic electron donor ability of the proximal histidine in horse heart myoglobin (HHMb), dehaloperoxidase (DHP), and horseradish peroxidase (HRP). DHP is a dimeric hemoglobin (Hb) originally isolated from the terebellid polychaete Amphitrite ornata. The monomers are structurally related to Mb and yet DHP has a peroxidase function. The core size marker modes, v2 and v3, were observed using Soret excitation, and DHP-X was compared to HHMb-X for the ligand series X = F, Cl, Br, SCN, OH, N3, and CN. Special attention was paid to the hydroxide adduct, which is also formed during the catalytic cycle of peroxidases. The Fe-OH stretching frequency was observed and confirmed by deuteration and is higher in DHP than in HHMb. The population of high-spin states of the heme iron in DHP was determined to be intermediate between HHMb and HRP. The data provide the first direct measurement of the effect of axial ligation on the heme iron in DHP. The Raman data support a modified charge relay in DHP, in which a strongly hydrogen-bonded backbone carbonyl (>C=O) polarizes the proximal histidine. The charge relay mechanism by backbone carbonyl >C=O-His-Fe is the analogue of the Asp-His-Fe of peroxidases and Glu-His-Fe of flavohemoglobins.

Horseradish and soybean peroxidases: comparable tools for alternative niches?

Horseradish and soybean peroxidases (HRP and SBP, respectively) are useful biotechnological tools. HRP is often termed the classical plant heme peroxidase and although it has been studied for decades, our understanding has deepened since its cloning and subsequent expression, enabling numerous mutational and protein engineering studies. SBP, however, has been neglected until recently, despite offering a real alternative to HRP: SBP actually outperforms HRP in terms of stability and is now used in numerous biotechnological applications, including biosensors. Review of both is timely. This article summarizes and discusses the main insights into the structure and mechanism of HRP, with special emphasis on HRP mutagenesis, and outlines its use in a variety of applications. It also reviews the current knowledge and applications to date of SBP, particularly biosensors. The final paragraphs speculate on the future of plant heme-based peroxidases, with probable trends outlined and explored.

X-ray Absorption Spectroscopy and Density Functional Theory Studies of [(H3buea)FeIII-X]n- (X = S2-, O2-, OH-): Comparison of Bonding and Hydrogen Bonding in Oxo and Sulfido Complexes

Iron L-edge, iron K-edge, and sulfur K-edge X-ray absorption spectroscopy was performed on a series of compounds [Fe(III)H(3)buea(X)](n-) (X = S(2-), O(2-), OH(-)). The experimentally determined electronic structures were used to correlate to density functional theory calculations. Calculations supported by the data were then used to compare the metal-ligand bonding and to evaluate the effects of H-bonding in Fe(III)(-)O vs Fe(III)(-)S complexes. It was found that the Fe(III)(-)O bond, while less covalent, is stronger than the Fe(III)(-)S bond. This dominantly reflects the larger ionic contribution to the Fe(III)(-)O bond. The H-bonding energy (for three H-bonds) was estimated to be -25 kcal/mol for the oxo as compared to -12 kcal/mol for the sulfide ligand. This difference is attributed to the larger charge density on the oxo ligand resulting from the lower covalency of the Fe-O bond. These results were extended to consider an Fe(IV)(-)O complex with the same ligand environment. It was found that hydrogen bonding to Fe(IV)(-)O is less energetically favorable than that to Fe(III)(-)O, which reflects the highly covalent nature of the Fe(IV)(-)O bond.

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.

Probing the structure and bifunctionality of catalase-peroxidase (KatG)

Catalase-peroxidases (KatGs) exhibit peroxidase and substantial catalase activities similar to monofunctional catalases. Crystal structures of four different KatGs reveal the presence of a peroxidase-conserved proximal and distal heme pocket together with features unique to KatG. To gain insight into their structure-function properties, many variants were produced and very similar results were obtained irrespective of the origin of the KatG mutated. This review focuses mainly on the electronic absorption and resonance Raman results together with the combined analysis of pre-steady and steady-state kinetics of various mutants involving both the peroxidase-conserved and the KatG-specific residues of recombinant KatG from the cyanobacterium Synechocystis. Marked differences in the structural role of conserved amino acids and hydrogen-bond networks in KatG with respect to the other plant peroxidases were found. Typically, the catalatic but not the peroxidatic activity was very sensitive to mutations that disrupted the KatG-typical extensive hydrogen-bonding network. Moreover, the integrity of this network is crucial for the formation of distinct protein radicals formed upon incubation of KatG with peroxides in the absence of one-electron donors. The correlation between the structural architecture and the bifunctional activity is discussed and compared with data obtained for KatGs from other organisms.

Detection of the His-heme Fe2+-NO species in the reduction of NO to N2O by ba3-oxidase from thermus thermophilus

Reaction pathways in the enzymatic formation and cleavage of the N-N and N-O bonds, respectively, are difficult to verify without the structure of the intermediates, but we now have such information on the heme a(3)(2+)-NO species formed in the reaction of ba(3)-oxidase with NO from resonance Raman spectroscopy. We have identified the His-heme a(3)(2+)-NO/Cu(B)(1+) species by its characteristic Fe-NO and N-O stretching frequencies at 539 and 1620 cm(-)(1), respectively. The Fe-NO and N-O frequencies in ba(3)-oxidase are 21 and 7 cm(-)(1) lower and higher, respectively, than those observed in Mb-NO. From these results and earlier Raman and FTIR measurements, we demonstrate that the protein environment of the proximal His384 that is part of the Q-proton pathway controls the strength of the Fe-His384 bond upon ligand (CO vs NO) binding. We also show by time-resolved FTIR spectroscopy that Cu(B)(1+) has a much lower affinity for NO than for CO. We suggest that the reduction of NO to N(2)O by ba(3)-oxidase proceeds by the fast binding of the first NO molecule to heme a(3) with high-affinity, and the second NO molecule binds to Cu(B) with low-affinity, producing the temporal co-presence of two NO molecules in the heme-copper center. The low-affinity of Cu(B) for NO binding also explains the NO reductase activity of the ba(3)-oxidase as opposed to other heme-copper oxidases. With the identification of the His-heme a(3)(2+)-NO/Cu(B)(1+) species, the structure of the binuclear heme a(3)-Cu(B)(1+) center in the initial step of the NO reduction mechanism is known.

Structure and Bonding in Monomeric Iron(III) Complexes with Terminal Oxo and Hydroxo Ligands

We report on the structure and bonding in the title iron(III) complexes, containing the tris[(N'-tert-butylureayl)-N-ethyl]amine ligand, with density functional theory techniques. In agreement with the experimental data, a high-spin electronic state is favored for all of the systems we considered. H bonds between the terminal oxo and hydroxo ligands and NH groups present in the organic ligand coordinated to the metal have a remarkable effect on the overall coordination geometry. In fact, the structure of model complexes without H bonds shows shorter Fe-O bond lengths. This is a consequence of the ability of the H bonds to stabilize a remarkable amount of electron density localized on the terminal oxo and hydroxo ligands. Energy analysis indicates that each H bond stabilizes the nonheme complexes by roughly 35 kJ/mol. Molecular orbital analysis indicates a reduction of two Fe-O bonding electrons on going from a complex with a terminal oxo ligand to a complex with a terminal hydroxo ligand. This reduction in the number of bonding electrons is also supported by frequency analysis.

Reaction route control by microperoxidase-9/CTAB micelle ratios

Microperoxidases (MP) as water-soluble models attract interest to studying the reaction mechanism of peroxidases because these heme peptides are able to form the same enzyme intermediates during the reaction with peroxides. In this work we have demonstrated that the association of Fe(III)MP-9 and Fe(III)MP-11 with CTAB micelles (MP-9/CTAB and MP11/CTAB) provides a microenvironment with an alkaline interface and a hydrophobic core that exhibits peroxidase behavior. This microenvironment shifts positively the redox potential of microperoxidases by approximately 100 mV. tert-Butylhydroperoxide (t-BuOOH) when added to the medium, converted Fe(III)MP-9/CTAB to MP-9/CTAB Compound II, a high valence oxidized intermediate of the heme peptide. Subsequent addition of diphenylacetaldehyde (DPAA) to MP-9/CTAB Compound II regenerated the native form of the enzyme, Fe(III)MP-9/CTAB, what characterizes the occurrence of a peroxidase cycle. Fe(III)MP-9/CTAB regenerated during the peroxidase cycle reacted with residual DPAA in the medium to form Fe(II)MP-9/CTAB, which indicates that both Fe(III)MP-9/CTAB and its oxyferryl form can use aldehydes as reducing agents. According to the determined reduction potential, Fe(III)MP-9 and Fe(III)MP-9/CTAB should be able to oxidize DPAA (reduction potential -630 mV). The reaction of MP-9/CTAB with DPAA produced benzophenone as final product, detected by infrared spectroscopy and mass spectrometry. Interestingly, a significant difference was observed in the benzophenone yield according to the micelle/MP-9 molar ratio.

Resonance Raman study of deoxy and ligated (O2 and CO) mesoheme IX-reconstituted myoglobin, hemoglobin and its alpha and beta subunits

In this work, we corrected the resonance Raman (RR) results presented earlier for deoxy mesoheme IX-reconstituted hemoglobin (mesoHb) alpha and beta subunits implied that mesohemes in these subunits undergo substantial structural changes upon formation of a hemoglobin tetramer (Biochemistry 29 (1990) 5087). We show that these data were probably due to the improper handling of the deoxy mesoheme subunit preparation. Additionally, we discuss the RR spectra of deoxy, oxy, and CO species of mesoheme IX-reconstituted myoglobin (mesoMb) and alpha and beta deoxy meso hemoglobin subunits, including their analogues with deuterium-substituted mesoheme IX in all methyl groups (d(12)). Based on the obtained data, we propose a complete RR band assignment for all of the investigated molecules. The most pronounced changes are observed for the gamma(7) mode (out-of-plane movement of methane carbon atoms) associated with the interaction of the ethyl groups with the globin. We also show that in mesoheme IX-reconstituted proteins, the O(2) molecule binds stronger than in the case of native species. This is manifested by the up-shift of nu(Fe-O(2)).

Bioinspired hydrogen bond motifs in ligand design: the role of noncovalent interactions in metal ion mediated activation of dioxygen

Hydrogen bonds influence secondary coordination spheres around metal ions in many proteins. To duplicate these features of molecular architecture in synthetic systems, urea-based ligands have have been developed that create rigid organic frameworks when bonded to metal ions. These frameworks position hydro-gen bond donors proximal to metal ion(s) to form specific chem-ical microenvironments. Iron(II) and manganese(II) complexes with constrained cavities activate O(2), yielding M(III) (M(III) = Fe and Mn) complexes with terminal oxo ligands. Installation of anionic sites within the cavity assists the formation of complexes with M(II/III)-OH and M(III)-O units derived directly from water. Opening the cavity promotes M(mu-O)(2)M rhombs, as illustrated by isolation of a cobalt(III) analogue, the stability of which is promoted by the hydrogen bonds surrounding the bridging oxo ligands.

Utilization of hydrogen bonds to stabilize M-O(H) units: synthesis and properties of monomeric iron and manganese complexes with terminal oxo and hydroxo ligands

Non-heme iron and manganese species with terminal oxo ligands are proposed to be key intermediates in a variety of biological and synthetic systems; however, the stabilization of these types of complexes has proven difficult because of the tendency to form oxo-bridged complexes. Described herein are the design, isolation, and properties for a series of mononuclear Fe(III) and Mn(III) complexes with terminal oxo or hydroxo ligands. Isolation of the complexes was facilitated by the tripodal ligand tris[(N'-tert-butylureaylato)-N-ethyl]aminato ([H(3)1](3-)), which creates a protective hydrogen bond cavity around the M(III)-O(H) units (M(III) = Fe and Mn). The M(III)-O(H) complexes are prepared by the activation of dioxygen and deprotonation of water. In addition, the M(III)-O(H) complexes can be synthesized using oxygen atom transfer reagents such as N-oxides and hydroxylamines. The [Fe(III)H(3)1(O)](2-) complex also can be made using sulfoxides. These findings support the proposal of a high valent M(IV)-oxo species as an intermediate during dioxygen cleavage. Isotopic labeling studies show that oxo ligands in the [M(III)H(3)1(O)](2-) complexes come directly from the cleavage of dioxygen: for [Fe(III)H(3)1(O)](2-) the nu(Fe-(16)O) = 671 cm(-1), which shifts 26 cm(-1) in [Fe(III)H(3)1((18)O)](2-) (nu(Fe-(18)O) = 645 cm(-1)); a nu(Mn-(16)O) = 700 cm(-1) was observed for [Mn(III)H(3)1((16)O)](2-), which shifts to 672 cm(-1) in the Mn-(18)O isotopomer. X-ray diffraction studies show that the Fe-O distance is 1.813(3) A in [Fe(III)H(3)1(O)](2-), while a longer bond is found in [Fe(III)H(3)1(OH)](-) (Fe-O at 1.926(2) A); a similar trend was found for the Mn(III)-O(H) complexes, where a Mn-O distance of 1.771(5) A is observed for [Mn(III)H(3)1(O)](2-) and 1.873(2) A for [Mn(III)H(3)1(OH)](-). Strong intramolecular hydrogen bonds between the urea NH groups of [H(3)1](3-) and the oxo and oxygen of the hydroxo ligand are observed in all the complexes. These findings, along with density functional theory calculations, indicate that a single sigma-bond exists between the M(III) centers and the oxo ligands, and additional interactions to the oxo ligands arise from intramolecular H-bonds, which illustrates that noncovalent interactions may replace pi-bonds in stabilizing oxometal complexes.

Comparison between Catalase-Peroxidase and Cytochrome c Peroxidase. The Role of the Hydrogen-Bond Networks for Protein Stability and Catalysis

A detailed resonance Raman and electronic absorption investigation has been carried out on a series of novel distal and proximal variants of recombinant catalase-peroxidase from the cyanobacterium Synechocystis PCC 6803. In particular, variants of the distal triad Pro-Asp-Asn and the proximal triad His-Asp-Trp have been studied in their ferric and ferrous states at various pH. The data suggest marked differences in the structural role of the conserved residues and hydrogen-bond networks in KatG and CCP, which might be connected to the different catalytic activity. In particular, in KatG the proximal residues have a major role in the stability of the protein architecture because the disruption of the proximal Trp-Asp hydrogen bond by mutation weakens heme binding to the protein. On the distal side, replacing the hydrogen-acceptor carboxamide group of Asn153 by an aspartate carboxylate group or an aliphatic residue alters or disrupts the hydrogen bond with the distal His. As a consequence, the basicity of His123 is altered. The effect of mutation on Asp152 is noteworthy. Replacement of the Asp152 with Ser makes the architecture of the protein very similar to that of CCP. The Asp152 residue, which has been shown to be important in the hydrogen peroxide oxidation reaction, is expected to be hydrogen bonded to the nitrogen atom of Ile248 which is part of the KatG-specific insertion LL1, as in other KatGs. This insertion is at one edge of the heme, and connects the distal side with the proximal helices E and F, the latter carrying the proximal His ligand. We found that the distal Asp-Ile hydrogen bond is important for the stability of the heme architecture and its alteration changes markedly the proximal His-Asp hydrogen-bond interaction.

LZnX complexes of tripodal ligands with intramolecular RN–H hydrogen bonding groups: structural implications of a hydrogen bonding cavity, and of X/R in the hydrogen bonding geometry/strength

Tripodal ligands N(CH2Py)3-n(CH2Py-6-NHR)n(R=H, n=1-3 L1-3, n=0 tpa; R=CH2tBu, n=1-3 L'1-3) are used to investigate the effect of different hydrogen bonding microenvironments on structural features of their LZnX complexes (X=Cl-, NO3-, OH-). The X-ray structures of [(L2)Zn(Cl)](BPh4)2.0.5(H2O.CH3CN), [(L3)Zn(Cl)](BPh4)3.CH3CN, [(L'1)Zn(Cl)](BPh4) 1', [(L'2)Zn(Cl)](BPh4)2'.CH3OH, and [(L'3)Zn(Cl)](BPh4)3' have been determined and exhibit trigonal bipyramidal geometries with intramolecular (internal) N-HCl-Zn hydrogen bonds. The structure of [(L'2)Zn(ONO2)]NO3 4'.H2O with two internal N-HO-Zn hydrogen bonds has also been determined. The axial Zn-Cl distance lengthens from 2.275 A in [(tpa)Zn(Cl)](BPh4) to 2.280-2.347 A in 1-3, 1'-3'. Notably, the average Zn-N(py) distance is also progressively lengthened from 2.069 A in [(tpa)Zn(Cl)](BPh4) to 2.159 and 2.182 A in the triply hydrogen bonding cavity of 3 and 3', respectively. Lengthening of the Zn-Cl and Zn-N(py) bonds is accompanied by a progressive shortening of the trans Zn-N bond from 2.271 A in [(tpa)Zn(Cl)](BPh4) to 2.115 A in 3 (2.113 A in 3'). As a result of the triply hydrogen bonding microenvironment the Zn-Cl and Zn-N(py) distances of 3 are at the upper end of the range observed for axial Zn-Cl bonds, whereas the axial Zn-N distance is one of shortest among N4 ligands that induce a trigonal bipyramidal geometry. Despite the rigidity of these tripodal ligands, the geometry of the intramolecular RN-HX-Zn hydrogen bonds (X=Cl-, OH-, NO3-) is strongly dependent on the nature of X, however, on average, similar for R=H, CH2tBu.

Relative importance of hydrogen bonding and coordinating groups in modulating the zinc–water acidity

The presence of second-sphere -NH(2) groups in the proximity of a zinc(ii)-bound water molecule enhances its acidity by ca. 2 pK(a) units.

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

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

Mechanism of reaction of hydrogen peroxide with horseradish peroxidase: identification of intermediates in the catalytic cycle

The mechanism of the reaction of horseradish peroxidase isoenzyme C (HRPC) with hydrogen peroxide to form the reactive enzyme intermediate compound I has been studied using electronic absorbance, rapid-scan stopped-flow, and electron paramagnetic resonance (EPR) spectroscopies at both acid and basic pH. The roles of the active site residues His42 and Arg38 in controlling heterolytic cleavage of the H(2)O(2) oxygen-oxygen bond have been probed with site-directed mutant enzymes His42 --> Leu (H42L), Arg38 --> Leu (R38L), and Arg38 --> Gly (R38G). The biphasic reaction kinetics of H42L with H(2)O(2) suggested the presence of an intermediate species and, at acid pH, a reversible second step, probably due to a neutral enzyme-H(2)O(2) complex and the ferric-peroxoanion-containing compound 0. EPR also indicated the formation of a protein radical situated more than approximately 10 A from the heme iron. The stoichiometry of the reaction of the H42L/H(2)O(2) reaction product and 2,2'-azinobis(3-ethylbenzothiazolinesulfonic acid) (ABTS) was concentration dependent and fell from a value of 2 to 1 above 0.7 mM ABTS. These data can be explained if H(2)O(2) undergoes homolytic cleavage in H42L. The apparent rate of compound I formation by H42L, while low, was pH independent in contrast to wild-type HRPC where the rate falls at acid pH, indicating the involvement of an ionizable group with pK(a) approximately 4. In R38L and R38G, the apparent pK(a) was shifted to approximately 8 but there is no evidence that homolytic cleavage of H(2)O(2) occurs. These data suggest that His42 acts initially as a proton acceptor (base catalyst) and then as a donor (acid catalyst) at neutral pH and predict the observed slower rate and lower efficiency of heterolytic cleavage observed at acid pH. Arg38 is influential in lowering the pK(a) of His42 and additionally in aligning H(2)O(2) in the active site, but it does not play a direct role in proton transfer.