Compound ES of Dehaloperoxidase Decays via Two Alternative Pathways Depending on the Conformation of the Distal Histidine

Computational Insights into the Catalysis of the pH Dependence of Bromite Decomposition Catalyzed by Chlorite Dismutase from Dechloromonas aromatica (DaCld)

The heme-containing chlorite dismutases catalyze the rapid and efficient decomposition of chlorite (ClO2-) to yield Cl- and O2, and the catalytic efficiency of chlorite dismutase from Dechloromonas aromatica (DaCld) in catalyzing the decomposition of bromite (BrO2-) was dependent on pH, which was supposed to be caused by the conversion of active Cpd I to the inactive Cpd II by proton-coupled electron transfer (PCET) from the pocket Tyr118 to the propionate side chain of heme at high pH. However, the direct evidence of PCET and how the pH affects the efficiency of DaCld, as well as whether Cpd II is really inactive, are still poorly understood. Here, on the basis of the high-resolution crystal structures, the computational models in both acidic (pH 5.0) and alkaline (pH 9.0) environments were constructed, and a series of quantum mechanical/molecular mechanical calculations were performed. On the basis of our calculation results, the O-Br bond cleavage of BrO2- always follows the homolytic mode to generate Cpd II rather than Cpd I. It is different from the O-O cleavage of O2/H2O2 or peracetic acid catalyzed by the other heme-containing enzymes. Thus, in the subsequent O-O rebound reaction, it is the Fe(IV)═O in Cpd II that combines with the O-Br radical. Because the porphyrin ring in Cpd II does not bear an unpaired electron, the previously suggested PCET from Tyr118 to the propionate side chain of heme was not theoretically recognized in an alkaline environment. In addition, the O-O rebound step in an alkaline solution corresponds to an energy barrier that is larger than that in an acidic environment, which can well explain the pH dependence of the activity of DaCld. In addition, the protonation state of the propionic acid side chains of heme and the surrounding hydrogen bond networks were calculated to have a significant impact on the barriers of the O-O rebound step, which is mainly achieved by affecting the reactivity of the Fe(IV)═O group in Cpd II. In an acidic environment, the relatively weaker coordination of the O2 atom to Fe leads to its higher reactivity toward the O-O rebound reaction. These observations may provide useful information for understanding the catalysis of chlorite dismutases.

Comparison of the Backbone Dynamics of Dehaloperoxidase-Hemoglobin Isoenzymes

Dehaloperoxidase (DHP) is a multifunctional hemeprotein with a functional switch generally regulated by the chemical class of the substrate. Its two isoforms, DHP-A and DHP-B, differ by only five amino acids and have an almost identical protein fold. However, the catalytic efficiency of DHP-B for oxidation by a peroxidase mechanism ranges from 2- to 6-fold greater than that of DHP-A depending on the conditions. X-ray crystallography has shown that many substrates and ligands have nearly identical binding in the two isoenzymes, suggesting that the difference in catalytic efficiency could be due to differences in the conformational dynamics. We compared the backbone dynamics of the DHP isoenzymes at pH 7 through heteronuclear relaxation dynamics at 11.75, 16.45, and 19.97 T in combination with four 300 ns MD simulations. While the overall dynamics of the isoenzymes are similar, there are specific local differences in functional regions of each protein. In DHP-A, Phe35 undergoes a slow chemical exchange between two conformational states likely coupled to a swinging motion of Tyr34. Moreover, Asn37 undergoes fast chemical exchange in DHP-A. Given that Phe35 and Asn37 are adjacent to Tyr34 and Tyr38, it is possible that their dynamics modulate the formation and migration of the active tyrosyl radicals in DHP-A at pH 7. Another significant difference is that both distal and proximal histidines have a 15-18% smaller S2 value in DHP-B, thus their greater flexibility could account for the higher catalytic activity. The distal histidine grants substrate access to the distal pocket. The greater flexibility of the proximal histidine could also accelerate H2O2 activation at the heme Fe by increased coupling of an amino acid charge relay to stabilize the ferryl Fe(IV) oxidation state in a Poulos-Kraut "push-pull"-type peroxidase mechanism.

The divergent pH dependence of substrate turnover in dehaloperoxidases A and B

The pH-dependent peroxidase activity in both dehaloperoxidases A and B was studied by a kinetic assay, stopped flow spectroscopy, resonance Raman spectroscopy, and high-performance liquid chromatography at pH 5.0, 6.0, and 7.0. At pH 7.0, both isozymes follow the peroxidase ping-pong kinetic model derived from the three-step reaction scheme using the steady-state approximation. However, deviation from standard saturation behavior is observed at pH < 6.0 and [TCP] > 0.7 mM, owing to multiple processes: a) self-inhibition of TCP by internal binding; b) oxidation of the product by a pH- and concentration-dependent secondary reaction; and c) formation of an inactive species known as compound RH in the absence of oxidizable substrate. Although DHP-A and DHP-B differ by only 5 amino acids, they show a complete trend reversal in their observed peroxidase kinetics and product yields. Although at pH 7.0 DHP-B had higher TCP oxidation activity than DHP-A as reported previously, as pH was lowered, DHP-A appeared to have a higher peroxidase activity than DHP-B. This is an unprecedented result. However, the fact that there are multiple processes contributing to both kinetics and yield of TCP oxidation complicates interpretation of these data. Deactivation via compound RH and self-inhibition are pH dependent reactions that compete with substrate oxidation. Compound RH formation was observed to be rapid at low pH. A complete set of control experiments were conducted to differentiate the various contributions to the observed enzyme kinetics.

Oxidation of bisphenol A (BPA) and related compounds by the multifunctional catalytic globin dehaloperoxidase

Dehaloperoxidase (DHP) from the marine polychaete Amphitrite ornata is a multifunctional enzyme that possesses peroxidase, peroxygenase, oxidase and oxygenase activities. Herein, we investigated the reactivity of DHP B with bisphenol A (BPA) and related compounds (bisphenol E, bisphenol F, tetrachlorobisphenol A, 2,2'-biphenol, 3,3'-biphenol, 4,4'-biphenol, and 3,3'-dibromo-4,4'-biphenol). As a previously unknown substrate for DHP B, BPA (as a representative substrate) is an endocrine disruptor widely used in polycarbonate and epoxy resins, thus resulting in human exposure. Reactivity studies with these substrates were investigated using high performance liquid chromatography (HPLC), and their corresponding oxidation products were determined by mass spectrometry (GC-MS/ LC-MS). BPA undergoes oxidation in the presence of DHP B and hydrogen peroxide yielding two cleavage products (4-isopropenylphenol and 4-(2-hydroxypropan-2-yl)phenol), and oligomers with varying degrees of oxidation. ¹⁸O-labeling studies confirmed that the O-atom incorporated into the products was derived exclusively from water, consistent with substrate oxidation via a peroxidase-based mechanism. The X-ray crystal structures of DHP bound with bisphenol E (1.48 Å), bisphenol F (1.75 Å), 2,2'-biphenol (1.90 Å) and 3,3'-biphenol (1.30 Å) showed substrate binding sites are in the distal pocket of the heme cofactor, similar to other previously studied DHP substrates. Stopped-flow UV-visible spectroscopy was utilized to investigate the mechanistic details and enzyme oxidation states during substrate turnover, and a reaction mechanism is proposed. The data presented here strongly suggest that DHP B can catalyze the oxidation of bisphenols and biphenols, thus providing evidence of how infaunal invertebrates can contribute to the biotransformation of these marine pollutants.

Bridging the functional gap between reactivity and inhibition in dehaloperoxidase B from Amphitrite ornata: Mechanistic and structural studies with 2,4- and 2,6-dihalophenols

The multifunctional catalytic globin dehaloperoxidase (DHP) from the marine worm Amphitrite ornata was shown to catalyze the H₂O₂-dependent oxidation of 2,4- and 2,6-dihalophenols (DXP; X = F, Cl, Br). Product identification by LC-MS revealed multiple monomeric products with varying degrees of oxidation and/or dehalogenation, as well as oligomers with n up to 6. Mechanistic and ¹⁸O-labeling studies demonstrated sequential dihalophenol oxidation via peroxidase and peroxygenase activities. Binding studies established that 2,4-DXP (X = Cl, Br) have the highest affinities of any known DHP substrate. X-ray crystallography identified different binding positions for 2,4- and 2,6-DXP substrates in the hydrophobic distal pocket of DHP. Correlation between the number of halogens and the substrate binding orientation revealed a halogen-dependent binding motif for mono- (4-halophenol), di- (2,4- and 2,6-dihalophenol) and trihalophenols (2,4,6-trihalopenol). Taken together, the findings here on dihalophenol reactivity with DHP advance our understanding of how these compounds bridge the inhibitory and oxidative functions of their mono- and trihalophenol counterparts, respectively, and provide further insight into the protein structure-function paradigm relevant to multifunctional catalytic globins in comparison to their monofunctional analogs.

Complementarity of neutron, XFEL and synchrotron crystallography for defining the structures of metalloenzymes at room temperature

Room-temperature macromolecular crystallography allows protein structures to be determined under close-to-physiological conditions, permits dynamic freedom in protein motions and enables time-resolved studies. In the case of metalloenzymes that are highly sensitive to radiation damage, such room-temperature experiments can present challenges, including increased rates of X-ray reduction of metal centres and site-specific radiation-damage artefacts, as well as in devising appropriate sample-delivery and data-collection methods. It can also be problematic to compare structures measured using different crystal sizes and light sources. In this study, structures of a multifunctional globin, dehaloperoxidase B (DHP-B), obtained using several methods of room-temperature crystallographic structure determination are described and compared. Here, data were measured from large single crystals and multiple microcrystals using neutrons, X-ray free-electron laser pulses, monochromatic synchrotron radiation and polychromatic (Laue) radiation light sources. These approaches span a range of 18 orders of magnitude in measurement time per diffraction pattern and four orders of magnitude in crystal volume. The first room-temperature neutron structures of DHP-B are also presented, allowing the explicit identification of the hydrogen positions. The neutron data proved to be complementary to the serial femtosecond crystallography data, with both methods providing structures free of the effects of X-ray radiation damage when compared with standard cryo-crystallography. Comparison of these room-temperature methods demonstrated the large differences in sample requirements, data-collection time and the potential for radiation damage between them. With regard to the structure and function of DHP-B, despite the results being partly limited by differences in the underlying structures, new information was gained on the protonation states of active-site residues which may guide future studies of DHP-B.

Rigidifying a De Novo Enzyme Increases Activity and Induces a Negative Activation Heat Capacity

Conformational sampling profoundly impacts the overall activity and temperature dependence of enzymes. Peroxidases have emerged as versatile platforms for high-value biocatalysis owing to their broad palette of potential biotransformations. Here, we explore the role of conformational sampling in mediating activity in the de novo peroxidase C45. We demonstrate that 2,2,2-triflouoroethanol (TFE) affects the equilibrium of enzyme conformational states, tending toward a more globally rigid structure. This is correlated with increases in both stability and activity. Notably, these effects are concomitant with the emergence of curvature in the temperature-activity profile, trading off activity gains at ambient temperature with losses at high temperatures. We apply macromolecular rate theory (MMRT) to understand enzyme temperature dependence data. These data point to an increase in protein rigidity associated with a difference in the distribution of protein dynamics between the ground and transition states. We compare the thermodynamics of the de novo enzyme activity to those of a natural peroxidase, horseradish peroxidase. We find that the native enzyme resembles the rigidified de novo enzyme in terms of the thermodynamics of enzyme catalysis and the putative distribution of protein dynamics between the ground and transition states. The addition of TFE apparently causes C45 to behave more like the natural enzyme. Our data suggest robust, generic strategies for improving biocatalytic activity by manipulating protein rigidity; for functional de novo protein catalysts in particular, this can provide more enzyme-like catalysts without further rational engineering, computational redesign, or directed evolution.

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.

Carbon-fluorine bond cleavage mediated by metalloenzymes

Fluorochemicals are a widely distributed class of compounds and have been utilized across a wide range of industries for decades. Given the environmental toxicity and adverse health threats of some fluorochemicals, the development of new methods for their decomposition is significant to public health. However, the carbon-fluorine (C-F) bond is among the most chemically robust bonds; consequently, the degradation of fluorinated hydrocarbons is exceptionally difficult. Here, metalloenzymes that catalyze the cleavage of this chemically challenging bond are reviewed. These enzymes include histidine-ligated heme-dependent dehaloperoxidase and tyrosine hydroxylase, thiolate-ligated heme-dependent cytochrome P450, and four nonheme oxygenases, namely, tetrahydrobiopterin-dependent aromatic amino acid hydroxylase, 2-oxoglutarate-dependent hydroxylase, Rieske dioxygenase, and thiol dioxygenase. While much of the literature regarding the aforementioned enzymes highlights their ability to catalyze C-H bond activation and functionalization, in many cases, the C-F bond cleavage has been shown to occur on fluorinated substrates. A copper-dependent laccase-mediated system representing an unnatural radical defluorination approach is also described. Detailed discussions on the structure-function relationships and catalytic mechanisms provide insights into biocatalytic defluorination, which may inspire drug design considerations and environmental remediation of halogenated contaminants.

Improving B1 field homogeneity in dielectric tube resonators for EPR spectroscopy via controlled shaping of the dielectric insert

We propose an approach for improving the homogeneity of microwave magnetic field amplitude in a dielectric tube resonator for electron paramagnetic resonance spectroscopy at X-band. The improvement is achieved by "shaping" (controllable variation of the outer diameter of a dielectric insert along its axial direction). Various shaping scenarios based on the principle of discrete solenoids and electromagnetic calculations have been considered. The dielectric insert having the most promising shape was manufactured from a bismuth germanate single crystal. The shaped insert increases the area at B₁ > 0.9 B_1max from 5.06 to 7.36 mm. Higher sensitivity and lower likelihood of quantitative errors have been achieved in pulse EPR experiments for "long" samples (whose length was comparable to that of the dielectric insert) in a shaped dielectric insert.

The multifunctional globin dehaloperoxidase strikes again: Simultaneous peroxidase and peroxygenase mechanisms in the oxidation of EPA pollutants

The multifunctional catalytic hemoglobin dehaloperoxidase (DHP) from the terebellid polychaete Amphitrite ornata was found to catalyze the H₂O₂-dependent oxidation of EPA Priority Pollutants (4-Me-o-cresol, 4-Cl-m-cresol and pentachlorophenol) and EPA Toxic Substances Control Act compounds (o-, m-, p-cresol and 4-Cl-o-cresol). Biochemical assays (HPLC/LC-MS) indicated formation of multiple oxidation products, including the corresponding catechol, 2-methylbenzoquinone (2-MeBq), and oligomers with varying degrees of oxidation and/or dehalogenation. Using 4-Br-o-cresol as a representative substrate, labeling studies with ¹⁸O confirmed that the O-atom incorporated into the catechol was derived exclusively from H₂O₂, whereas the O-atom incorporated into 2-MeBq was from H₂O, consistent with this single substrate being oxidized by both peroxygenase and peroxidase mechanisms, respectively. Stopped-flow UV-visible spectroscopic studies strongly implicate a role for Compound I in the peroxygenase mechanism leading to catechol formation, and for Compounds I and ES in the peroxidase mechanism that yields the 2-MeBq product. The X-ray crystal structures of DHP bound with 4-F-o-cresol (1.42 Å; PDB 6ONG), 4-Cl-o-cresol (1.50 Å; PDB 6ONK), 4-Br-o-cresol (1.70 Å; PDB 6ONX), 4-NO₂-o-cresol (1.80 Å; PDB 6ONZ), o-cresol (1.60 Å; PDB 6OO1), p-cresol (2.10 Å; PDB 6OO6), 4-Me-o-cresol (1.35 Å; PDB 6ONR) and pentachlorophenol (1.80 Å; PDB 6OO8) revealed substrate binding sites in the distal pocket in close proximity to the heme cofactor, consistent with both oxidation mechanisms. The findings establish cresols as a new class of substrate for DHP, demonstrate that multiple oxidation mechanisms may exist for a given substrate, and provide further evidence that different substituents can serve as functional switches between the different activities performed by dehaloperoxidase. More broadly, the results demonstrate the complexities of marine pollution where both microbial and non-microbial systems may play significant roles in the biotransformations of EPA-classified pollutants, and further reinforces that heterocyclic compounds of anthropogenic origin should be considered as environmental stressors of infaunal organisms.

Tyrosyl radical in haemoglobin and haptoglobin-haemoglobin complex: how does haptoglobin make haemoglobin less toxic?

One of the difficulties in creating a blood substitute on the basis of human haemoglobin (Hb) is the toxic nature of Hb when it is outside the safe environment of the red blood cells. The plasma protein haptoglobin (Hp) takes care of the Hb physiologically leaked into the plasma – it binds Hb and makes it much less toxic while retaining the Hb's high oxygen transporting capacity. We used Electron Paramagnetic Resonance (EPR) spectroscopy to show that the protein bound radical induced by H₂O₂ in Hb and Hp-Hb complex is formed on the same tyrosine residue(s), but, in the complex, the radical is found in a more hydrophobic environment and decays slower than in unbound Hb, thus mitigating its oxidative capacity. The data obtained in this study might set new directions in engineering blood substitutes for transfusion that would have the oxygen transporting efficiency typical of Hb, but which would be non-toxic.

An Aromatic Dyad Motif in Dye Decolourising Peroxidases Has Implications for Free Radical Formation and Catalysis

Dye decolouring peroxidases (DyPs) are the most recent class of heme peroxidase to be discovered. On reacting with H₂ O₂ , DyPs form a high-valent iron(IV)-oxo species and a porphyrin radical (Compound I) followed by stepwise oxidation of an organic substrate. In the absence of substrate, the ferryl species decays to form transient protein-bound radicals on redox active amino acids. Identification of radical sites in DyPs has implications for their oxidative mechanism with substrate. Using a DyP from Streptomyces lividans, referred to as DtpA, which displays low reactivity towards synthetic dyes, activation with H₂ O₂ was explored. A Compound I EPR spectrum was detected, which in the absence of substrate decays to a protein-bound radical EPR signal. Using a newly developed version of the Tyrosyl Radical Spectra Simulation Algorithm, the radical EPR signal was shown to arise from a pristine tyrosyl radical and not a mixed Trp/Tyr radical that has been widely reported in DyP members exhibiting high activity with synthetic dyes. The radical site was identified as Tyr374, with kinetic studies inferring that although Tyr374 is not on the electron-transfer pathway from the dye RB19, its replacement with a Phe does severely compromise activity with other organic substrates. These findings hint at the possibility that alternative electron-transfer pathways for substrate oxidation are operative within the DyP family. In this context, a role for a highly conserved aromatic dyad motif is discussed.

Reaction of O2 with a diiron protein generates a mixed-valent Fe2+/Fe3+ center and peroxide

The gene encoding the cyanobacterial ferritin SynFtn is up-regulated in response to copper stress. Here, we show that, while SynFtn does not interact directly with copper, it is highly unusual in several ways. First, its catalytic diiron ferroxidase center is unlike those of all other characterized prokaryotic ferritins and instead resembles an animal H-chain ferritin center. Second, as demonstrated by kinetic, spectroscopic, and high-resolution X-ray crystallographic data, reaction of O₂ with the di-Fe²⁺ center results in a direct, one-electron oxidation to a mixed-valent Fe²⁺/Fe³⁺ form. Iron-O₂ chemistry of this type is currently unknown among the growing family of proteins that bind a diiron site within a four α-helical bundle in general and ferritins in particular. The mixed-valent form, which slowly oxidized to the more usual di-Fe³⁺ form, is an intermediate that is continually generated during mineralization. Peroxide, rather than superoxide, is shown to be the product of O₂ reduction, implying that ferroxidase centers function in pairs via long-range electron transfer through the protein resulting in reduction of O₂ bound at only one of the centers. We show that electron transfer is mediated by the transient formation of a radical on Tyr40, which lies ∼4 Å from the diiron center. As well as demonstrating an expansion of the iron-O₂ chemistry known to occur in nature, these data are also highly relevant to the question of whether all ferritins mineralize iron via a common mechanism, providing unequivocal proof that they do not.

Oxidation of Pyrrole by Dehaloperoxidase-Hemoglobin: Chemoenzymatic Synthesis of Pyrrolin-2-Ones

The use of oxidoreductases as biocatalysts in the syntheses of functionalized, monomeric pyrroles has been a challenge owing to, among a number of factors, undesired polypyrrole formation. Here, we have investigated the ability of dehaloperoxidase (DHP), the coelomic hemoglobin from the terebellid polychaete Amphitrite ornata, to catalyze the H2O2-dependent oxidation of pyrroles as a new class of substrate for this enzyme. Substrate oxidation was observed for all compounds employed (pyrrole, N-methylpyrrole, 2-methylpyrrole, 3-methylpyrrole and 2,5-dimethylpyrrole) under both aerobic and anaerobic conditions. Using pyrrole as a representative substrate, only a single oxidation product, 4-pyrrolin-2-one, was observed, and notably without formation of polypyrrole. Reactivity could be initiated from all three biologically relevant oxidation states for this catalytic globin: ferric, ferrous and oxyferrous. Isotope labeling studies determined that the O-atom incorporated into the 4-pyrrolin-2-one product was derived exclusively from H2O2, indicative of a peroxygenase mechanism. Consistent with this observation, single- and double-mixing stopped-flow UV-visible spectroscopic studies supported Compound I, but not Compounds ES or II, as the catalytically-relevant ferryl intermediate involved in pyrrole oxidation. Electrophilic addition of the ferryl oxygen to pyrrole is proposed as the mechanism of O-atom transfer. The results demonstrate the breadth of chemical reactivity afforded by dehaloperoxidase, and provide further evidence for establishing DHP as a multifunctional globin with practical applications as a biocatalyst.

Tyr25, Tyr58 and Trp133 ofEscherichia colibacterioferritin transfer electrons between iron in the central cavity and the ferroxidase centre

Tyr58 and Trp133 play key roles in the formation and decay of the Tyr25 radical species ofE. coliBFR.

The hydrogen-peroxide-induced radical behaviour in human cytochrome c-phospholipid complexes: implications for the enhanced pro-apoptotic activity of the G41S mutant

We have investigated whether the pro-apoptotic properties of the G41S mutant of human cytochrome c can be explained by a higher than wild-type peroxidase activity triggered by phospholipid binding. A key complex in mitochondrial apoptosis involves cytochrome c and the phospholipid cardiolipin. In this complex cytochrome c has its native axial Met(80) ligand dissociated from the haem-iron, considerably augmenting the peroxidase capability of the haem group upon H2O2 binding. By EPR spectroscopy we reveal that the magnitude of changes in the paramagnetic haem states, as well as the yield of protein-bound free radical, is dependent on the phospholipid used and is considerably greater in the G41S mutant. A high-resolution X-ray crystal structure of human cytochrome c was determined and, in combination with the radical EPR signal analysis, two tyrosine residues, Tyr(46) and Tyr(48), have been rationalized to be putative radical sites. Subsequent single and double tyrosine-to-phenylalanine mutations revealed that the EPR signal of the radical, found to be similar in all variants, including G41S and wild-type, originates not from a single tyrosine residue, but is instead a superimposition of multiple EPR signals from different radical sites. We propose a mechanism of multiple radical formations in the cytochrome c-phospholipid complexes under H2O2 treatment, consistent with the stabilization of the radical in the G41S mutant, which elicits a greater peroxidase activity from cytochrome c and thus has implications in mitochondrial apoptosis.

Peroxygenase and oxidase activities of dehaloperoxidase-hemoglobin from Amphitrite ornata

The marine globin dehaloperoxidase-hemoglobin (DHP) from Amphitrite ornata was found to catalyze the H₂O₂-dependent oxidation of monohaloindoles, a previously unknown class of substrate for DHP. Using 5-Br-indole as a representative substrate, the major monooxygenated products were found to be 5-Br-2-oxindole and 5-Br-3-oxindolenine. Isotope labeling studies confirmed that the oxygen atom incorporated was derived exclusively from H₂O₂, indicative of a previously unreported peroxygenase activity for DHP. Peroxygenase activity could be initiated from either the ferric or oxyferrous states with equivalent substrate conversion and product distribution. It was found that 5-Br-3-oxindole, a precursor of the product 5-Br-3-oxindolenine, readily reduced the ferric enzyme to the oxyferrous state, demonstrating an unusual product-driven reduction of the enzyme. As such, DHP returns to the globin-active oxyferrous form after peroxygenase activity ceases. Reactivity with 5-Br-3-oxindole in the absence of H₂O₂ also yielded 5,5′-Br₂-indigo above the expected reaction stoichiometry under aerobic conditions, and O₂-concentration studies demonstrated dioxygen consumption. Nonenzymatic and anaerobic controls both confirmed the requirements for DHP and molecular oxygen in the catalytic generation of 5,5′-Br₂-indigo, and together suggest a newly identified oxidase activity for DHP.

Tyrosyl radicals in dehaloperoxidase: how nature deals with evolving an oxygen-binding globin to a biologically relevant peroxidase

Dehaloperoxidase (DHP) from Amphitrite ornata, having been shown to catalyze the hydrogen peroxide-dependent oxidation of trihalophenols to dihaloquinones, is the first oxygen binding globin that possesses a biologically relevant peroxidase activity. The catalytically competent species in DHP appears to be Compound ES, a reactive intermediate that contains both a ferryl heme and a tyrosyl radical. By simulating the EPR spectra of DHP activated by H2O2, Thompson et al. (Thompson, M. K., Franzen, S., Ghiladi, R. A., Reeder, B. J., and Svistunenko, D. A. (2010) J. Am. Chem. Soc. 132, 17501-17510) proposed that two different radicals, depending on the pH, are formed, one located on either Tyr-34 or Tyr-28 and the other on Tyr-38. To provide additional support for these simulation-based assignments and to deduce the role(s) that tyrosyl radicals play in DHP, stopped-flow UV-visible and rapid-freeze-quench EPR spectroscopic methods were employed to study radical formation in DHP when three tyrosine residues, Tyr-28, Tyr-34, and Tyr-38, were replaced either individually or in combination with phenylalanines. The results indicate that radicals form on all three tyrosines in DHP. Evidence for the formation of DHP Compound I in several tyrosine mutants was obtained. Variants that formed Compound I showed an increase in the catalytic rate for substrate oxidation but also an increase in heme bleaching, suggesting that the tyrosines are necessary for protecting the enzyme from oxidizing itself. This protective role of tyrosines is likely an evolutionary adaptation allowing DHP to avoid self-inflicted damage in the oxidative environment.

Ferric haem forms of Mycobacterium tuberculosis catalase-peroxidase probed by EPR spectroscopy: Their stability and interplay with pH

Low temperature EPR spectroscopy was used to characterise Mycobacterium tuberculosis catalase-peroxidase in its resting ferric haem state. Several high spin ferric haem forms and no low spin forms were found in the enzyme samples frozen in methanol on dry ice. The EPR spectra depended not only on the pH but also on the buffer type. As a general trend, the higher the pH, the greater the 'rhombic' fraction of the high spin ferric haem that was observed. The rhombic form was characterised by well separated two lines in the g = 6 region whereas in the 'axial' form the two lines overlap. This pH dependence of the equilibrium of axial and rhombic ferric haem forms is also seen in rapidly freeze-quenched samples. Different high spin ferric haem forms were monitored during a 3 week storage of the enzyme at 4 °C. For some forms, extremal dependences, i.e. those progressing via maxima or minima over storage time, were found. This indicates that the mechanism of the time-dependent transition from one high spin ferric haem form to another must be more complex than a simple single site oxidation.

Reactivity of deoxy- and oxyferrous dehaloperoxidase B from Amphitrite ornata: identification of compound II and its ferrous-hydroperoxide precursor

Dehaloperoxidase (DHP) from the terebellid polychaete Amphitrite ornata is a bifunctional enzyme that possesses both hemoglobin and peroxidase activities. The bifunctional nature of DHP as a globin peroxidase appears to be at odds with the traditional starting oxidation state for each individual activity. Namely, reversible oxygen binding is only mediated via a ferrous heme in globins, and peroxidase activity is initiated from ferric centers and to the exclusion of the oxyferrous oxidation state from the peroxidase cycle. Thus, to address what appears to be a paradox, herein we report the details of our investigations into the DHP catalytic cycle when initiated from the deoxy- and oxyferrous states using biochemical assays, stopped-flow UV-visible, and rapid-freeze-quench electron paramagnetic resonance spectroscopies, and anaerobic methods. We demonstrate the formation of Compound II directly from deoxyferrous DHP B upon its reaction with hydrogen peroxide and show that this occurs both in the presence and in the absence of trihalophenol. Prior to the formation of Compound II, we have identified a new species that we have preliminarily attributed to a ferrous-hydroperoxide precursor that undergoes heterolysis to generate the aforementioned ferryl intermediate. Taken together, the results demonstrate that the oxyferrous state in DHP is a peroxidase competent starting species, and an updated catalytic cycle for DHP is proposed in which the ferric oxidation state is not an obligatory starting point for the peroxidase catalytic cycle of dehaloperoxidase. The data presented herein provide a link between the peroxidase and oxygen transport activities, which furthers our understanding of how this bifunctional enzyme is able to unite its two inherent functions in one system.