Reaction of Aplysia limacina metmyoglobin with hydrogen peroxide

Myoglobin (Mb) from gastropod mollusc Aplysia limacina shows only 20% sequence homology to the 'prototype' sperm whale Mb but exhibits a typical Mb fold and can reversibly bind oxygen. An intriguing feature of aplysia Mb is that it lacks the distal histidine and displays a ligand stabilisation based on an arginine. Here we report the reaction of aplysia metMb with hydrogen peroxide studied by optical and electron paramagnetic resonance (EPR) spectroscopies. Two electron oxidation of the protein by H2O2 results in formation of two intermediates typical for this class of reactions, the oxoferryl haem state and a globin-bound free radical. An unusual characteristic of the aplysia Mb reaction is formation, prior to haem oxidation, of an optically distinct compound with an EPR spectrum typical of the low spin Fe3+ haem state. This compound is interpreted as the complex between H2O2 and the ferric haem state (Compound), formed prior to cleavage of the dioxygen bond. We conclude that H2O2 is singly deprotonated in Compound which can thus be notated as [Fe3+--OOH]. A new low spin ferric haem state has been observed over the period of Compound decay, and hypotheses have been formulated as to its identity and role. The location of the protein bound radical observed in aplysia Mb is discussed in light of the fact that the protein does not have any tyrosine residues, the most common site of free radical formation in the haem protein/peroxide systems. All intermediates of the reaction are kinetically characterised.     

Generation of High-Oxidation States of Myoglobin in the Nanosecond Time-Scale by Laser Photoionization

Abstract— The 248 nm laser flash photolysis of myoglobin in various redox states (oxy, met and ferryl) in neutral aqueous solution yielded hydrated electrons with concurrent changes in the visible absorption spectrum of the heme. The results could be ascribed to the photoionization of both the peptide and the heme group, in approximately equal yields. The ionization of met- and ferrylmyoglobin was biphotonic, but that of oxymyoglobin was a mixture of mono- and biphotonic processes. Using appropriate electron and radical scavengers, the changes in the heme absorption could be investigated at times ≥100 ns and were shown to be associated with a +1 increase of the formal oxidation state of the heme. Using this method, the formal iron(V) state of native myoglobin could be spectroscopically characterized for the first time. Its absorption, blue-shifted and less intense relative to the ferryl state, is reminiscent of that of the compound I of peroxidases, which contains a ferryl-oxo (iron[IV]) group and a porphyrin radical cation. On this basis, the same structure is proposed for the formal iron(V) state of native myoglobin. The transition from oxy- to metmyoglo-bin took -5 μs, which may reflect the kinetics of exchange of oxygen with water as ligand. The transitions from the met to the ferryl state, and from ferryl to iron(V) states were faster (∽250 ns), consistent with processes that involve proton or electron movements but no changes in the iron coordination state.     

A molecular dynamics investigation of the resting, hydrogen peroxide-bound and compound II forms of cytochrome C peroxidase and Artromyces ramosus peroxidase

Molecular Dynamics (MD) simulations have been used to study structural and dynamic properties of the resting, hydrogen peroxide adduct and compound II forms of cytochrome C peroxidase (CCP) and Artromyces ramosus peroxidase (ARP). MD simulations of CCP show that: (i) hydrogen peroxide might form an outer sphere complex within the active site of the enzyme before the coordination to the iron centre takes place; (ii) Trp51 and His52 residues play a crucial role in the recognition and binding of hydrogen peroxide, while Arg48 is not directly involved; (iii) distal histidine (His52) allows an easy proton 1,2 shift within the H₂O₂ molecule, while Arg48 is not expected to play a role as crucial as His52 in promoting the heterolytic O–O bond breaking; (iv) the large mobility (about 2 Å) of the side chain of Arg48 in the compound II form allows the formation of a hydrogen bond (H-bond) with the ferryl oxygen, which contributes to the stabilisation of such an intermediate. The active site of the ARP enzyme is characterised by structural and dynamic features slightly different from the CCP active site. In particular, (i) the outer sphere complex with hydrogen peroxide occurring in CCP is not observed in ARP because of the substitution of Trp51 of CCP with the more hydrophobic residue Phe55 of ARP; (ii) His56 and the carbonyl group of Arg52 are determinant in controlling the hydrogen peroxide binding and its orientation in the active site. In ARP, both H₂O₂ and His56 have orientation different than in CCP, but still suited for an easy 1,2 proton shift. (iii) Arg52 in ARP is on average more distant from the heme-iron than in CCP, but its relative orientation is suited to promote an easy cleavage of H₂O₂. (iv) In compound II form of ARP, the Arg52 side chain is too far from the oxy-ferryl group to form a hydrogen bond and therefore ARP looses a stabilising factor, which is present in the corresponding form of CCP.     

Investigation of the proton-assisted pathway to formation of the catalytically active, ferryl species of P450s by molecular dynamics studies of P450eryF

The recently determined crystal structure of cytochrome P450eryF (6-deoxyerythronolide B hydroxylase; CYP107A1) in its ferric heme substrate-bound form has been used to address one of the most fundamental unresolved aspects of the mechanism of oxidation common to this ubiquitous family of metabolizing heme proteins, the pathway from the twice reduced dioxygen species to the putative catalytically active ferryl oxygen species. Both of these species are too transient to have been characterized experimentally, and the transformation from one to the other has been only partially characterized. The observed requirement of two protons and the formation of water in this transformation suggests a proton-assisted dioxygen bond cleavage as a plausible pathway. However, this pathway is difficult to establish by experiment alone, and the source of the protons in the largely hydrophobic binding pocket of the P450s remains unclear. In this work we have performed molecular dynamics simulations of the twice reduced dioxygen substrate-bound form of this isozyme in order to (i) determine the plausibility of the proposed pathway to compound I formation, a proton-assisted cleavage of the dioxygen bond, and (ii) investigate the possible source of these protons. The analysis of the molecular dynamics trajectories of this species does indeed provide further evidence for this pathway and points to a source of protons. Specifically, two dynamically stable hydrogen bonds to the distal oxygen atom of the dioxygen ligand, one by the substrate and the other by a bound water, are found, consistent with the proposed proton-assisted cleavage of the bond and formation of water. In addition, an extensive dynamically stable hydrogen bond network is formed that connects the distal oxygen to Glu 360, a well-conserved residue in a channel accessible to solvent that could be the ultimate source of protons. The simulations were done for both a protonated and unprotonated Glu and led to a proposed mechanism of proton transfer by it to the distal oxygen atom. In order to validate the procedures used for the simulation of this transient twice-reduced species, we have used these same procedures to perform molecular dynamics simulations of two other forms of P450eryF, the ferric and ferryl substrate-bound species, and compared the results with experiment. The results for the ferric substrate-bound species were assessed by comparisons to the experimentally determined X-ray structure and fluctuations, and good agreement was found. The simulations performed for the ferryl substrate-bound species led to the correct prediction of the observed regio- and stereospecific hydroxylation of its natural substrate, 6-deoxyerythronolide B (6-DEB) at the 6S position. The results of these two additional studies lend credibility to the important mechanistic inferences from the simulations of the transient twice reduced dioxygen species: further evidence for a proton-assisted pathway from it to the catalytically active ferryl species and a possible source of the protons.     

Regulating the Coordination State of a Heme Protein by a Designed Distal Hydrogen-Bonding Network

Heme coordination state determines the functional diversity of heme proteins. Using myoglobin as a model protein, we designed a distal hydrogen-bonding network by introducing both distal glutamic acid (Glu29) and histidine (His43) residues and regulated the heme into a bis-His coordination state with native ligands His64 and His93. This resembles the heme site in natural bis-His coordinated heme proteins such as cytoglobin and neuroglobin. A single mutation of L29E or F43H was found to form a distinct hydrogen-bonding network involving distal water molecules, instead of the bis-His heme coordination, which highlights the importance of the combination of multiple hydrogen-bonding interactions to regulate the heme coordination state. Kinetic studies further revealed that direct coordination of distal His64 to the heme iron negatively regulates fluoride binding and hydrogen peroxide activation by competing with the exogenous ligands. The new approach developed in this study can be generally applicable for fine-tuning the structure and function of heme proteins.     

Oxidizing O,O,O-trimethyl phosphorothioate with hydrogen peroxide or ozone

Oxidization of O,O,O-trimethyl phosphorothioate to the corresponding oxo (P=O) compound is accomplished under mild conditions with hydrogen peroxide or ozone. Urea hydroperoxide was found to be a serviceable solid-state source of hydrogen peroxide; it was a better oxidizing agent than 30% hydrogen peroxide and resulted in better yield. It was also proved that ozone was the most effective oxidizer, with formation of purer products in higher yield (90.1%) than with hydrogen peroxide.     

QM/MM studies of the electronic structure of the compound I intermediate in cytochrome c peroxidase and ascorbate peroxidase

Cytochrome c peroxidase (CcP) and ascorbate peroxidase (APX) both involve reactive haem oxoferryl intermediates known as 'compound I' species. These two enzymes also have a very similar structure, especially in the vicinity of the haem group. Despite this similarity, the electronic structure of compound I in the two enzymes is known to be very different. Compound I intermediates have three unpaired electrons, two of which are always situated on the Fe-O core, whilst the third is located in a porphyrin orbital in APX and many other compound I species. In CcP, however, this third unpaired electron is positioned on a tryptophan residue lying close to the haem ring. The same residue is present in the same position in APX, yet it is not oxidized in that case. We report QM/MM calculations, using accurate B3LYP density functional theory for the QM region, on the active intermediate for both enzymes. We reproduce the observed difference in electronic structure, and show that it arises as a result of subtle electrostatic effects which affect the ionization potential of both the tryptophan and porphyrin groups. The computed structures of both enzymes do not involve deprotonation of the tryptophan group, or protonation of the oxoferryl oxygen.     

Fix L, a haemoglobin that acts as an oxygen sensor: signalling mechanism and structural basis of its homology with PAS domains

Fix L, which contains a haemoglobin domain homologous to the PAS family and a histidine kinase domain, forms, with Fix J, a two-component signalling complex that regulates expression of nitrogenase genes in Rhizobium. Spin transitions of its haem iron trigger stereochemical changes in and around the haem that, together with steric effects, control the activity of the kinase. Homology with the PAS family is based on a common core of about 20 structurally equivalent sites from which polar residues are excluded.     

The H93G myoglobin cavity mutant as a versatile template for modeling heme proteins: ferrous, ferric, and ferryl mixed-ligand complexes with imidazole in the cavity

One of the difficulties in preparing accurate ambient-temperature model complexes for heme proteins, particularly in the ferric state, has been the generation of mixed-ligand adducts: complexes with different ligands on either side of the heme. The difference in the accessibility of the two sides of the heme in the H93G cavity mutant of myoglobin (Mb) provides a potential general solution to this problem. To demonstrate the versatility of H93G Mb for the preparation of heme protein models, numerous mixed-ligand adducts of ferrous, ferric, and ferryl imidazole-ligated H93G (H93G(Im) Mb) have been prepared. The complexes have been characterized by electronic absorption and magnetic circular dichroism (MCD) spectroscopy in comparison to analogous derivatives of wild type Mb. The starting ferric H93G(Im) Mb state spectroscopically resembles wild-type ferric Mb as expected for a complex containing a single imidazole in the proximal cavity and water bound on the distal side. Addition of a sixth ligand to ferric H93G(Im) Mb, whether charge neutral (imidazole) or anionic (cyanide and azide), results in formation of six-coordinate low-spin complexes with MCD characteristics similar to those of parallel derivatives of wild-type ferric Mb. Reduction of ferric H93G(Im) Mb and subsequent exposure to either CO, NO, or O2 produces ferrous complexes (deoxy, CO, NO, and O2) that consistently exhibit MCD spectra similar to the analogous ferrous species of wild-type ferrous Mb. Most interestingly, reaction of ferric H93G(Im) Mb with H2O2 results in the formation of a stable high-valent oxoferryl complex with MCD characteristics that are essentially identical to those of oxoferryl wild-type Mb. The generation of such a wide array of mixed-ligand heme complexes demonstrates the efficacy of the H93G Mb cavity mutant as a template for the preparation of heme protein model complexes.     

Ultrafast infrared spectroscopy reveals water-mediated coherent dynamics in an enzyme active site

Understanding the impact of fast dynamics upon the chemical processes occurring within the active sites of proteins and enzymes is a key challenge that continues to attract significant interest, though direct experimental insight in the solution phase remains sparse. Similar gaps in our knowledge exist in understanding the role played by water, either as a solvent or as a structural/dynamic component of the active site. In order to investigate further the potential biological roles of water, we have employed ultrafast multidimensional infrared spectroscopy experiments that directly probe the structural and vibrational dynamics of NO bound to the ferric haem of the catalase enzyme from Corynebacterium glutamicum in both H₂O and D₂O. Despite catalases having what is believed to be a solvent-inaccessible active site, an isotopic dependence of the spectral diffusion and vibrational lifetime parameters of the NO stretching vibration are observed, indicating that water molecules interact directly with the haem ligand. Furthermore, IR pump–probe data feature oscillations originating from the preparation of a coherent superposition of low-frequency vibrational modes in the active site of catalase that are coupled to the haem ligand stretching vibration. Comparisons with an exemplar of the closely-related peroxidase enzyme family shows that they too exhibit solvent-dependent active-site dynamics, supporting the presence of interactions between the haem ligand and water molecules in the active sites of both catalases and peroxidases that may be linked to proton transfer events leading to the formation of the ferryl intermediate Compound I. In addition, a strong, water-mediated, hydrogen bonding structure is suggested to occur in catalase that is not replicated in peroxidase; an observation that may shed light on the origins of the different functions of the two enzymes.     

Horseradish peroxidase catalyzed nitric oxide formation from hydroxyurea

Hydroxyurea represents an approved treatment for sickle cell anemia and a number of cancers. Chemiluminescence and electron paramagnetic resonance spectroscopic studies show horseradish peroxidase catalyzes the formation of nitric oxide from hydroxyurea in the presence of hydrogen peroxide. Gas chromatographic headspace analysis and infrared spectroscopy also reveal the production of nitrous oxide in this reaction, which provides evidence for nitroxyl, the one-electron reduced form of nitric oxide. These reactions also generate carbon dioxide, ammonia, nitrite, and nitrate. None of these products form within 1 h in the absence of hydrogen peroxide or horseradish peroxidase. Electron paramagnetic resonance spectroscopy and trapping studies show the intermediacy of a nitroxide radical and a C-nitroso species during this reaction. Absorption spectroscopy indicates that both compounds I and II of horseradish peroxidase act as one-electron oxidants of hydroxyurea. Nitroxyl, generated from Angeli's salt, reacts with ferric horseradish peroxidase to produce a ferrous horseradish peroxidase-nitric oxide complex. Electron paramagnetic resonance experiments with a nitric oxide specific trap reveal that horseradish peroxidase is capable of oxidizing nitroxyl to nitric oxide. A mechanistic model that includes the observed nitroxide radical and C-nitroso compound intermediates has been forwarded to explain the observed product distribution. These studies suggest that direct nitric oxide producing reactions of hydroxyurea and peroxidases may contribute to the overall pharmacological properties of this drug.     

Spectroscopic study of substrate binding to the carbonmonoxy form of dehaloperoxidase from Amphitrite ornata

Dehaloperoxidase (DHP) is a globular heme enzyme found in the marine worm Amphitrite ornata that can catalyze the dehalogenation of halophenols to the corresponding quinones by using hydrogen peroxide as a cosubstrate. Its three-dimensional fold is surprisingly similar to that of the oxygen storage protein myoglobin (Mb). A key structural feature common to both DHP and Mb is the existence of multiple conformations of the distal histidine. In DHP, the conformational flexibility may be involved in promotion of substrate and cosubstrate entry and exit. Here we have explored the dynamics of substrate binding in DHP using Fourier transform infrared spectroscopy and flash photolysis. A number of discrete conformations at the active site were identified from the appearance of multiple CO absorbance bands in the infrared region of the spectrum. Upon photolysis at cryogenic temperatures, the CO molecules are trapped at docking sites within the protein matrix, as inferred from the appearance of several photoproduct bands characteristic of each site. Substrate binding stabilizes the protein by approximately 20 kJ/mol. The low yield of substrate-bound DHP at ambient temperature points toward a steric inhibition of substrate binding by carbon monoxide.     

INVESTIGATIONS ON THE LIGHT-EMITTING SPECIES IN THE REACTION OF METMYOGLOBIN AND METHEMOGLOBIN WITH HYDROGEN PEROXIDE

Abstract The formation of a compound I type ferryl complex in the reaction of methemoglobin (MetHb) and metmyoglobin (MetMyo) with hydrogen peroxide is accompanied by strong chemiluminescence. An approach to identify the nature of the light-emitting species was made by the use of quenchers and sensitizers reacting with singlet oxygen and compounds interfering in the formation and reactivity of other reactive oxygen species. Singlet oxygen is not the source of light emission. This could be concluded from the results obtained using the specific singlet oxygen trap 9,10-anthracenedipropionic acid (ADPA) in combination with high-performance liquid chromatography (HPLC) analysis. The singlet oxygen adduct of ADPA was not formed in the incubation systems (MetHb or MetMyo/H₂O₂). Instead, ADPA was oxidized by the ferryl ion to a different oxidation product, which was characterized by HPLC and IR spectroscopy. In the case of MetHb-related chemiluminescence, light emission does not result from a single source. Both, SH-groups and O₂^.¯ radicals are involved because blocking of thiol-groups with N-ethylmaleimide (NEM) and scavenging of O₂^.¯(by superoxide dismutase) suppressed chemiluminescence by 50% and 30%, respectively. Development of MetMyo-related chemiluminescence is not dependent on thiol groups (which are not present in the globin moiety) and also 0₂^.¯is not involved. Although generation of chemiluminescence in MetHb and MetMyo seems to follow different mechanisms, both types of light-emitting species are sensitive to antioxidants, such as uric acid and ascorbate. The detection of the respective free radicals by means of ESR demonstrates that both MetHb- and MetMyo-mediated chemiluminescence is associated with a strong one-electron oxidizing species, which seems to be identical with the light-emitting source itself. Also desferal, which was originally used to exclude the involvement of a Fenton-type reaction, was readily oxidized to the nitroxide free radical associated with a strong decrease of chemiluminescence. This quenching effect was not dependent on iron complexation because the addition of iron was ineffective. In summary, chemiluminescence is not restricted to a single chemical process but is related to different one-electron transfer reactions from globin residues to the oxo-heme center.     

Development of a new protein labeling strategy, oxidation labeling. part 1: Preliminary evaluation and synthesis of tautomycin containing a metal coordinating unit

In the current work we present our preliminary evaluation of a new protein labeling strategy, namely oxidation labeling. We found that a bis(2-picolyl) amine analogue coordinating Cu⁺ was able to oxidize histidine to oxo-histedine in a small peptide by generating reactive oxygen species upon exposure to hydrogen peroxide. The bis(2-picolyl) amine unit was then incorporated into the natural product tautomycin via an oxime linker. The compound, which showed good activity toward protein phosphatase 1γ (PP1γ), will be used in oxidation labeling studies with PP1γ.     

Quantitation of the Reactive Oxygen Species Generated by the UVA Irradiation of Ascorbic Acid-Glycated Lens Proteins

The oxidation products of ascorbic acid rapidly glycate proteins and produce protein-bound, advanced glycation endproducts. These endproducts can absorb UVA light and cause the photolytic oxidation of proteins (Ortwerth, Linetsky and Olesen, Photochem. Photobiol . 62, 454–463, 1995), which is mediated by the formation of reactive oxygen species. A dialyzed preparation of calf lens proteins, which had been incubated for 4 weeks with 20 mM ascorbic acid in air, was irradiated for 1 h with 200 mW/ cm² of absorbed UVA light (λ > 338 nm), and the concentration of individual oxygen free radicals was measured. Superoxide anion attained a level of 76 μ M as determined by the superoxide dismutase (SOD)-depen-dent increase in hydrogen peroxide formation and of 52 μ M by the SOD-inhibitable reduction of cytochrome c. Hydrogen peroxide formation increased linearly to 81 μM after 1 h. Neither superoxide anion nor hydrogen peroxide, however, could account for the UVA photolysis of Trp and His seen in this system.
Singlet oxygen levels approached 1.0 mM as measured by the oxidation of histidine, which was consistent with singlet oxygen measurements by the bleaching of N,N- dimethyl-4-nitrosoaniline. High concentrations of sodium azide, a known singlet oxygen quencher, inhibited the photolytic destruction of both His and Trp. Little or no protein damage could be ascribed to hydroxyl radical based upon quenching experiments with added mannitol. Therefore, superoxide anion and H₂O₂ were generated by the UVA irradiation of ascorbate advanced glycation endproducts, however, the major reactive oxygen species formed was singlet oxygen.     

Nitrite increases the enantioselectivity of sulfoxidation catalyzed by myoglobin derivatives in the presence of hydrogen peroxide

The effect of nitrite in the sulfoxidation of organic sulfides catalyzed by myoglobin (Mb) in the presence of hydrogen peroxide has been investigated. A general improvement in enantioselectivity was found for the reaction catalyzed by horse heart metMb and a series of sperm whale metMb derivatives including the wild type protein, the active site mutants T67K Mb, T67R Mb, T67R/S92D Mb, and the T67K Mb derivative reconstituted with the modified prosthetic group protohemin-l- histidine methyl ester.     

Quantum chemical evaluation of protein control over heme ligation: CO/O2 discrimination in myoglobin

Control of O2 versus CO binding in myoglobin (Mb) is tuned by a distal histidine residue through steric and H-bonding interactions. These interactions have been evaluated via Car-Parrinello DFT calculations, whose efficiency allows full quantum mechanical treatment of the 13 closest residues surrounding the heme. The small (8 degrees ) deviation of the Fe-C-O bond angle from linearity results from the steric influence of a distal valine residue and not the distal histidine. H-bond energies were evaluated by replacing the distal histidine with the non-H-bonding residue isoleucine. Binding energies for CO and O2 decreased by 0.8 and 4.1 kcal/mol for MbCO and MbO2, in good agreement with experimental H-bond estimates. Ligand discrimination is dominated by distal histidine H-bonding, which is also found to stabilize a metastable side-on isomer of MbO2 that may play a key role in MbO2 photodynamics.     

Raman spectroscopic and electrochemical characterization of myoglobin thin film: implication of the role of histidine 64 for fast heterogeneous electron transfer.

Myoglobin (Mb) thin films formed on various substrates have been characterized by using Raman spectroscopy, reflectance absorbance FT-IR, UV-vis absorption spectroscopy, and electrochemical methods. Raman spectra were obtained upon excitation within the Soret band as well as alpha-beta bands. The spin state marker bands observed from the Mb film in the 1550-1630 cm(-)(1) region (excitation at 514.5 nm) are approximately 20 cm(-)(1) higher than those of aqueous metMb having the high spin state. The 1210 cm(-)(1) band (methine bridge C-H vibration) also shifts to 1240 cm(-)(1) upon the formation of the film. These results indicate that the heme iron of myoglobin in the film is the ferric low-spin state, and the iron atom is pulled to the heme plane. A comparison of the Raman spectra of the Mb film with that of an Mb-imidazole derivative leads to the conclusion that the distal histidine is responsible for the change in the spectral characteristics. The escape of water from the sixth position upon the formation of the Mb film may result in a conformational change at the heme distal pocket: the histidine residue at the E7 helical position (H64) moves toward the central iron and is coordinated with it through the N on the imidazole ring. These structural features facilitate the fast electron transfer between the thin protein film and the electrode. Distal histidine may serve as an electron-transfer pathway as it does in cytochrome c.     

Alkylamine-ligated H93G myoglobin cavity mutant: a model system for endogenous lysine and terminal amine ligation in heme proteins such as nitrite reductase and cytochrome f

His93Gly sperm whale myoglobin (H93G Mb) has the proximal histidine ligand removed to create a cavity for exogenous ligand binding, providing a remarkably versatile template for the preparation of model heme complexes. The investigation of model heme adducts is an important way to probe the relationship between coordination structure and catalytic function in heme enzymes. In this study, we have successfully generated and spectroscopically characterized the H93G Mb cavity mutant ligated with less common alkylamine ligands (models for Lys or the amine group of N-terminal amino acids) in numerous heme iron states. All complexes have been characterized by electronic absorption and magnetic circular dichroism spectroscopy in comparison with data for parallel imidazole-ligated H93G heme iron moieties. This is the first systematic spectral study of models for alkylamine- or terminal amine-ligated heme centers in proteins. High-spin mono- and low-spin bis-amine-ligated ferrous and ferric H93G Mb adducts have been prepared together with mixed-ligand ferric heme complexes with alkylamine trans to nitrite or imidazole as heme coordination models for cytochrome c nitrite reductase or cytochrome f, respectively. Six-coordinate ferrous H93G Mb derivatives with CO, NO, and O(2) trans to the alkylamine have also been successfully formed, the latter for the first time. Finally, a novel high-valent ferryl species has been generated. The data in this study represent the first thorough investigation of the spectroscopic properties of alkylamine-ligated heme iron systems as models for naturally occurring heme proteins ligated by Lys or terminal amines.     

Engineering tyrosine-based electron flow pathways in proteins: the case of aplysia myoglobin

Tyrosine residues can act as redox cofactors that provide an electron transfer ("hole-hopping") route that enhances the rate of ferryl heme iron reduction by externally added reductants, for example, ascorbate. Aplysia fasciata myoglobin, having no naturally occurring tyrosines but 15 phenylalanines that can be selectively mutated to tyrosine residues, provides an ideal protein with which to study such through-protein electron transfer pathways and ways to manipulate them. Two surface exposed phenylalanines that are close to the heme have been mutated to tyrosines (F42Y, F98Y). In both of these, the rate of ferryl heme reduction increased by up to 3 orders of magnitude. This result cannot be explained in terms of distance or redox potential change between donor and acceptor but indicates that tyrosines, by virtue of their ability to form radicals, act as redox cofactors in a new pathway. The mechanism is discussed in terms of the Marcus theory and the specific protonation/deprotonation states of the oxoferryl iron and tyrosine. Tyrosine radicals have been observed and quantified by EPR spectroscopy in both mutants, consistent with the proposed mechanism. The location of each radical is unambiguous and allows us to validate theoretical methods that assign radical location on the basis of EPR hyperfine structure. Mutation to tyrosine decreases the lipid peroxidase activity of this myoglobin in the presence of low concentrations of reductant, and the possibility of decreasing the intrinsic toxicity of hemoglobin by introduction of these pathways is discussed.