Effects of a thiolate axial ligand on the pi-->pi* electronic states of oxoferryl porphyrins: a study of the optical and resonance Raman spectra of compounds I and II of chloroperoxidase

Bacterial nitric oxide reductase (NorBC) models employing click chemistry

Bacterial NO Reductase (NorBC or cNOR) is a membrane-bound enzyme found in denitrifying bacteria that catalyzes the two-electron reduction of NO to N₂O and water. The mechanism by which NorBC operates is highly debated, due to the fact that this enzyme is difficult to work with, and no intermediates of the NO reduction reaction could have been identified so far. The unique active site of NorBC consists of a heme b₃/non-heme Fe_B diiron center. Synthetic model complexes provide the opportunity to obtain insight into possible mechanistic alternatives for this enzyme. In this paper, we present three new synthetic model systems for NorBC, consisting of a tetraphenylporphyrin-derivative clicked to modified BMPA-based ligands (BMPA = bis(methylpyridyl)amine) that model the non-heme site in the enzyme. These complexes have been characterized by EPR, IR and UV-Vis spectroscopy. The reactivity with NO was then investigated, and it was found that the complex with the BMPA-carboxylate ligand as the non-heme component has a very low affinity for NO at the non-heme iron site. If the carboxylate functional group is replaced with a phenolate or pyridine group, reactivity is restored and formation of a diiron dinitrosyl complex was observed. Upon one-electron reduction of the nitrosylated complexes, following the semireduced pathway for NO reduction, formation of dinitrosyl iron complexes (DNICs) was observed in all three cases, but no N₂O could be detected.

Role of an orphan substrate-binding protein MhuP in transient heme transfer in Mycobacterium tuberculosis

The redox property of iron makes it an essential cofactor for numerous enzymes involved in various metabolic processes. In vertebrates, iron is attached to either heme molecules or with other circulatory proteins, making its accessibility restricted for bacterial pathogens residing inside the host. Due to this importance, there is always an ongoing battle between the host system and pathogens, known as nutritional immunity. To capture the bound iron from the human hosts, intracellular pathogens like Mycobacterium tuberculosis secrete siderophore molecules which are ultimately uptaken by versatile transport machinery such as ATP-binding cassette (ABC) transporters. Earlier reports have suggested the presence of a heme uptake protein MhuP (ORF id: Rv0265c) in M. tuberculosis, which transiently transfers the bound iron to the protein DppA for further heme transport by utilizing its cognate transport machinery (DppBCD). In the present study, we report the crystal structure of MhuP. The binding experiments of heme with MhuP suggest its specific nature. Molecular docking studies confirm the binding of the protein MhuP with heme as well as to the protein DppA. Thus, the results indicate the binding of heme to MhuP and its probable transient transport via the DppABCD transport system in M. tuberculosis.

A Comparative Review on the Catalytic Mechanism of Nonheme Iron Hydroxylases and Halogenases

Enzymatic halogenation and haloperoxidation are unusual processes in biology; however, a range of halogenases and haloperoxidases exist that are able to transfer an aliphatic or aromatic C–H bond into C–Cl/C–Br. Haloperoxidases utilize hydrogen peroxide, and in a reaction with halides (Cl−/Br−), they react to form hypohalides (OCl−/OBr−) that subsequently react with substrate by halide transfer. There are three types of haloperoxidases, namely the iron-heme, nonheme vanadium, and flavin-dependent haloperoxidases that are reviewed here. In addition, there are the nonheme iron halogenases that show structural and functional similarity to the nonheme iron hydroxylases and form an iron(IV)-oxo active species from a reaction of molecular oxygen with α-ketoglutarate on an iron(II) center. They subsequently transfer a halide (Cl−/Br−) to an aliphatic C–H bond. We review the mechanism and function of nonheme iron halogenases and hydroxylases and show recent computational modelling studies of our group on the hectochlorin biosynthesis enzyme and prolyl-4-hydroxylase as examples of nonheme iron halogenases and hydroxylases. These studies have established the catalytic mechanism of these enzymes and show the importance of substrate and oxidant positioning on the stereo-, chemo- and regioselectivity of the reaction that takes place.

Spectroscopic studies of the cytochrome P450 reaction mechanisms

The cytochrome P450 monooxygenases (P450s) are thiolate heme proteins that can, often under physiological conditions, catalyze many distinct oxidative transformations on a wide variety of molecules, including relatively simple alkanes or fatty acids, as well as more complex compounds such as steroids and exogenous pollutants. They perform such impressive chemistry utilizing a sophisticated catalytic cycle that involves a series of consecutive chemical transformations of heme prosthetic group. Each of these steps provides a unique spectral signature that reflects changes in oxidation or spin states, deformation of the porphyrin ring or alteration of dioxygen moieties. For a long time, the focus of cytochrome P450 research was to understand the underlying reaction mechanism of each enzymatic step, with the biggest challenge being identification and characterization of the powerful oxidizing intermediates. Spectroscopic methods, such as electronic absorption (UV-Vis), electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), electron nuclear double resonance (ENDOR), Mössbauer, X-ray absorption (XAS), and resonance Raman (rR), have been useful tools in providing multifaceted and detailed mechanistic insights into the biophysics and biochemistry of these fascinating enzymes. The combination of spectroscopic techniques with novel approaches, such as cryoreduction and Nanodisc technology, allowed for generation, trapping and characterizing long sought transient intermediates, a task that has been difficult to achieve using other methods. Results obtained from the UV-Vis, rR and EPR spectroscopies are the main focus of this review, while the remaining spectroscopic techniques are briefly summarized. This article is part of a Special Issue entitled: Cytochrome P450 biodiversity and biotechnology, edited by Erika Plettner, Gianfranco Gilardi, Luet Wong, Vlada Urlacher, Jared Goldstone.

Ground State and Excited State Tuning in Ferric Dipyrrin Complexes Promoted by Ancillary Ligand Exchange

Three ferric dipyrromethene complexes featuring different ancillary ligands were synthesized by one electron oxidation of ferrous precursors. Four-coordinate iron complexes of the type (^ArL)FeX₂ [^ArL = 1,9-(2,4,6-Ph₃C₆H₂)₂-5-mesityldipyrromethene] with X = Cl or ^tBuO were prepared and found to be high-spin (S = ⁵/₂), as determined by superconducting quantum interference device magnetometry, electron paramagnetic resonance, and ⁵⁷Fe Mössbauer spectroscopy. The ancillary ligand substitution was found to affect both ground state and excited properties of the ferric complexes examined. While each ferric complex displays reversible reduction and oxidation events, each alkoxide for chloride substitution results in a nearly 600 mV cathodic shift of the Fe^III/II couple. The oxidation event remains largely unaffected by the ancillary ligand substitution and is likely dipyrrin-centered. While the alkoxide substituted ferric species largely retain the color of their ferrous precursors, characteristic of dipyrrin-based ligand-to-ligand charge transfer (LLCT), the dichloride ferric complex loses the prominent dipyrrin chromophore, taking on a deep green color. Time-dependent density functional theory analyses indicate the weaker-field chloride ligands allow substantial configuration mixing of ligand-to-metal charge transfer into the LLCT bands, giving rise to the color changes observed. Furthermore, the higher degree of covalency between the alkoxide ferric centers is manifest in the observed reactivity. Delocalization of spin density onto the tert-butoxide ligand in (^ArL)FeCl(O^tBu) is evidenced by hydrogen atom abstraction to yield (^ArL)FeCl and HO^tBu in the presence of substrates containing weak C-H bonds, whereas the chloride (^ArL)FeCl₂ analogue does not react under these conditions.

A new look at the role of thiolate ligation in cytochrome P450

Protonated ferryl (or iron(IV)hydroxide) intermediates have been characterized in several thiolate-ligated heme proteins that are known to catalyze C-H bond activation. The basicity of the ferryl intermediates in these species has been proposed to play a critical role in facilitating this chemistry, allowing hydrogen abstraction at reduction potentials below those that would otherwise lead to oxidative degradation of the enzyme. In this contribution, we discuss the events that led to the assignment and characterization of the unusual iron(IV)hydroxide species, highlighting experiments that provided a quantitative measure of the ferryl basicity, the iron(IV)hydroxide pKa. We then turn to the importance of the iron(IV)hydroxide state, presenting a new way of looking at the role of thiolate ligation in these systems.

Beyond ferryl-mediated hydroxylation: 40 years of the rebound mechanism and C-H activation

Since our initial report in 1976, the oxygen rebound mechanism has become the consensus mechanistic feature for an expanding variety of enzymatic C-H functionalization reactions and small molecule biomimetic catalysts. For both the biotransformations and models, an initial hydrogen atom abstraction from the substrate (R-H) by high-valent iron-oxo species (Feⁿ=O) generates a substrate radical and a reduced iron hydroxide, [Fe^n-1-OH ·R]. This caged radical pair then evolves on a complicated energy landscape through a number of reaction pathways, such as oxygen rebound to form R-OH, rebound to a non-oxygen atom affording R-X, electron transfer of the incipient radical to yield a carbocation, R⁺, desaturation to form olefins, and radical cage escape. These various flavors of the rebound process, often in competition with each other, give rise to the wide range of C-H functionalization reactions performed by iron-containing oxygenases. In this review, we first recount the history of radical rebound mechanisms, their general features, and key intermediates involved. We will discuss in detail the factors that affect the behavior of the initial caged radical pair and the lifetimes of the incipient substrate radicals. Several representative examples of enzymatic C-H transformations are selected to illustrate how the behaviors of the radical pair [Fe^n-1-OH ·R] determine the eventual reaction outcome. Finally, we discuss the powerful potential of "radical rebound" processes as a general paradigm for developing novel C-H functionalization reactions with synthetic, biomimetic catalysts. We envision that new chemistry will continue to arise by bridging enzymatic "radical rebound" with synthetic organic chemistry.

Heme-thiolate ferryl of aromatic peroxygenase is basic and reactive

A kinetic and spectroscopic characterization of the ferryl intermediate (APO-II) from APO, the heme-thiolate peroxygenase from Agrocybe aegerita, is described. APO-II was generated by reaction of the ferric enzyme with metachloroperoxybenzoic acid in the presence of nitroxyl radicals and detected with the use of rapid-mixing stopped-flow UV-visible (UV-vis) spectroscopy. The nitroxyl radicals served as selective reductants of APO-I, reacting only slowly with APO-II. APO-II displayed a split Soret UV-vis spectrum (370 nm and 428 nm) characteristic of thiolate ligation. Rapid-mixing, pH-jump spectrophotometry revealed a basic pKa of 10.0 for the Fe(IV)-O-H of APO-II, indicating that APO-II is protonated under typical turnover conditions. Kinetic characterization showed that APO-II is unusually reactive toward a panel of benzylic C-H and phenolic substrates, with second-order rate constants for C-H and O-H bond scission in the range of 10-10(7) M(-1)⋅s(-1). Our results demonstrate the important role of the axial cysteine ligand in increasing the proton affinity of the ferryl oxygen of APO intermediates, thus providing additional driving force for C-H and O-H bond scission.

Reaction of ferric Caldariomyces fumago chloroperoxidase with meta-chloroperoxybenzoic acid: sequential formation of compound I, compound II and regeneration of the ferric state using one reactant

The mechanism of the reaction between ferric Caldariomyces fumago chloroperoxidase (CCPO) and meta-chloroperoxybenzoic acid (mCPBA) has been examined. It has previously been established that an Fe(IV) -oxo porphyrin radical species known as Compound I (Cpd I) is formed by two-electron oxidation of the native ferric enzyme by a variety of oxidants including organic peracids and hydroperoxides. Cpd I can return to the ferric state either by direct oxygen insertion into an organic substrate (e.g. a P450 oxygenase-like reaction), or by two consecutive one-electron additions, the first resulting in an intermediate Fe(IV) -oxo species known as Compound II (Cpd II). There has been much debate over the role of Cpd II and the details of its structure. In the present study, both CCPO Fe(IV) -oxo intermediates are formed, but unlike most CCPO reactions, Cpd I and Cpd II are formed using the same reactant, mCPBA. Thus, the peracid is used as an oxo donor to produce Cpd I and then as a reductant to reduce Cpd I to Cpd II, and finally, Cpd II to the ferric state. The observation of saturation kinetics with respect to mCPBA concentration for each step is consistent with the formation of CCPO-mCPBA complexes in each phase of the reaction. The original reaction mechanism for ferric CCPO with mCPBA was hypothesized to involve a scrambling mechanism with a unique Fe -OOO-C(O)R intermediate formed with no observed Cpd II intermediate. The data reported herein clearly demonstrate the formation of Cpd II in returning the oxidized enzyme back to its native ferric state.

Catalytic intermediates of inducible nitric-oxide synthase stabilized by the W188H mutation

Nitric-oxide synthase (NOS) catalyzes nitric oxide (NO) synthesis via a two-step process: L-arginine (L-Arg) → N-hydroxy-L-arginine → citrulline + NO. In the active site the heme is coordinated by a thiolate ligand, which accepts a H-bond from a nearby tryptophan residue, Trp-188. Mutation of Trp-188 to histidine in murine inducible NOS was shown to retard NO synthesis and allow for transient accumulation of a new intermediate with a Soret maximum at 420 nm during the L-Arg hydroxylation reaction (Tejero, J., Biswas, A., Wang, Z. Q., Page, R. C., Haque, M. M., Hemann, C., Zweier, J. L., Misra, S., and Stuehr, D. J. (2008) J. Biol. Chem. 283, 33498-33507). However, crystallographic data showed that the mutation did not perturb the overall structure of the enzyme. To understand how the proximal mutation affects the oxygen chemistry, we carried out biophysical studies of the W188H mutant. Our stopped-flow data showed that the 420-nm intermediate was not only populated during the L-Arg reaction but also during the N-hydroxy-L-arginine reaction. Spectroscopic data and structural analysis demonstrated that the 420-nm intermediate is a hydroxide-bound ferric heme species that is stabilized by an out-of-plane distortion of the heme macrocycle and a cation radical centered on the tetrahydrobiopterin cofactor. The current data add important new insights into the previously proposed catalytic mechanism of NOS (Li, D., Kabir, M., Stuehr, D. J., Rousseau, D. L., and Yeh, S. R. (2007) J. Am. Chem. Soc. 129, 6943-6951).

Detection and kinetic characterization of a highly reactive heme-thiolate peroxygenase compound I

The extracellular heme-thiolate peroxygenase from Agrocybe aegerita (AaeAPO) has been shown to hydroxylate alkanes and numerous other substrates using hydrogen peroxide as the terminal oxidant. We describe the kinetics of formation and decomposition of AaeAPO compound I upon its reaction with mCPBA. The UV-vis spectral features of AaeAPO-I (361, 694 nm) are similar to those of chloroperoxidase-I and the recently described cytochrome P450-I. The second-order rate constant for AaeAPO-I formation was 1.0 (±0.4) × 10(7) M(-1) s(-1) at pH 5.0, 4 °C. The relatively slow decomposition rate, 1.4 (±0.03) s(-1), allowed the measurement of its reactivity toward a panel of substrates. The observed rate constants, k2', spanned 5 orders of magnitude and correlated linearly with bond dissociation enthalpies (BDEs) of strong C-H bond substrates with a log k2' vs BDE slope of ∼0.4. However, the hydroxylation rate was insensitive to a C-H BDE below 90 kcal/mol, similar to the behavior of the tert-butoxyl radical. The shape and slope of the Brønsted-Evans-Polanyi plot indicate a symmetrical transition state for the stronger C-H bonds and suggest entropy control of the rate in an early transition state for weaker C-H bonds. The AaeAPO-II Fe(IV)O-H BDE was estimated to be ∼103 kcal/mol. All results support the formation of a highly reactive AaeAPO oxoiron(IV) porphyrin radical cation intermediate that is the active oxygen species in these hydroxylation reactions.

Spectroscopic features of cytochrome P450 reaction intermediates

Cytochromes P450 constitute a broad class of heme monooxygenase enzymes with more than 11,500 isozymes which have been identified in organisms from all biological kingdoms [1]. These enzymes are responsible for catalyzing dozens chemical oxidative transformations such as hydroxylation, epoxidation, N-demethylation, etc., with very broad range of substrates [2,3]. Historically these enzymes received their name from 'pigment 450' due to the unusual position of the Soret band in UV-vis absorption spectra of the reduced CO-saturated state [4,5]. Despite detailed biochemical characterization of many isozymes, as well as later discoveries of other 'P450-like heme enzymes' such as nitric oxide synthase and chloroperoxidase, the phenomenological term 'cytochrome P450' is still commonly used as indicating an essential spectroscopic feature of the functionally active protein which is now known to be due to the presence of a thiolate ligand to the heme iron [6]. Heme proteins with an imidazole ligand such as myoglobin and hemoglobin as well as an inactive form of P450 are characterized by Soret maxima at 420nm [7]. This historical perspective highlights the importance of spectroscopic methods for biochemical studies in general, and especially for heme enzymes, where the presence of the heme iron and porphyrin macrocycle provides rich variety of specific spectroscopic markers available for monitoring chemical transformations and transitions between active intermediates of catalytic cycle.

What factors influence the rate constant of substrate epoxidation by compound I of cytochrome P450 and analogous iron(IV)-oxo oxidants?

The cytochromes P450 are a versatile range of mono-oxygenase enzymes that catalyze a variety of different chemical reactions, of which the key reactions include aliphatic hydroxylation and C=C double bond epoxidation. To establish the fundamental factors that govern substrate epoxidation by these enzymes we have done a systematic density functional theory study on substrate epoxidation by the active species of P450 enzymes, namely the iron(IV)-oxo porphyrin cation radical oxidant or Compound I. We show here, for the first time, that the rate constant of substrate epoxidation, and hence the activation energy, correlates with the ionization potential of the substrate as well as with intrinsic electronic properties of the active oxidant such as the polarizability volume. To explain these findings we present an electron-transfer model for the reaction mechanism that explains the factors that determine the barrier heights and developed a valence bond (VB) curve crossing mechanism to rationalize the observed trends. In addition, we have found a correlation for substrate epoxidation reactions catalyzed by a range of heme and nonheme iron(IV)-oxo oxidants with the strength of the O-H bond in the iron-hydroxo complex, i.e. BDE(OH), which is supported by the VB model. Finally, the fundamental factors that determine the regioselectivity change between substrate hydroxylation and epoxidation are discussed. It is shown that the regioselectivity of aliphatic hydroxylation versus double bond epoxidation is not influenced by the choice of the oxidant but is purely substrate dependent.

Quantitative production of compound I from a cytochrome P450 enzyme at low temperatures. Kinetics, activation parameters, and kinetic isotope effects for oxidation of benzyl alcohol

Cytochrome P450 enzymes are commonly thought to oxidize substrates via an iron(IV)-oxo porphyrin radical cation transient termed Compound I, but kinetic studies of P450 Compounds I are essentially nonexistent. We report production of Compound I from cytochrome P450 119 (CYP119) in high conversion from the corresponding Compound II species at low temperatures in buffer mixtures containing 50% glycerol by photolysis with 365 nm light from a pulsed lamp. Compound I was studied as a reagent in oxidations of benzyl alcohol and its benzylic mono- and dideuterio isotopomers. Pseudo-first-order rate constants obtained at -50 degrees C with concentrations of substrates between 1.0 and 6.0 mM displayed saturation kinetics that gave binding constants for the substrate in the Compound I species (K(bind)) and first-order rate constants for the oxidation reactions (k(ox)). Representative results are K(bind) = 214 M(-1) and k(ox) = 0.48 s(-1) for oxidation of benzyl alcohol. For the dideuterated substrate C(6)H(5)CD(2)OH, kinetics were studied between -50 and -25 degrees C, and a van't Hoff plot for complexation and an Arrhenius plot for the oxidation reaction were constructed. The H/D kinetic isotope effects (KIEs) at -50 degrees C were resolved into a large primary KIE (P = 11.9) and a small, inverse secondary KIE (S = 0.96). Comparison of values extrapolated to 22 degrees C of both the rate constant for oxidation of C(6)H(5)CD(2)OH and the KIE for the nondeuterated and dideuterated substrates to values obtained previously in laser flash photolysis experiments suggested that tunneling could be a significant component of the total rate constant at -50 degrees C.

Kinetics of oxidation of benzphetamine by compounds I of cytochrome P450 2B4 and its mutants

Cytochromes P450 are ubiquitous heme-containing enzymes that catalyze a wide range of reactions in nature including many oxidation reactions. The active oxidant species in P450 enzymes are widely thought to be iron(IV)-oxo porphyrin radical cations, termed Compound I species, but these intermediates have not been observed under turnover conditions. We prepared Compounds I of the mammalian hepatic P450 enzyme CYP2B4 and three mutants (E301Q, T302A, and F429H) by laser flash photolysis of the Compound II species that, in turn, were prepared by reaction of the resting enzymes with peroxynitrite. The PN treatment resulted in a small amount of nitration of the P450 as determined by mass spectrometry but no change in reactivity of the P450 in a test reaction. CYP2B4 Compound I oxidized benzphetamine to norbenzphetamine in high yield in bulk studies. In direct kinetic studies of benzphetamine oxidations, Compounds I displayed saturation kinetics with similar binding equilibrium constants (K(bind)) for each. The first-order oxidation rate constants (k(ox)) were comparable for Compounds I of CYP2B4, the E301Q mutant, and the T302A mutant, whereas the k(ox) for Compound I of the F429H mutant was reduced by a factor of 2. CYP119 Compound I, studied for comparison purposes, reacted with benzphetamine with a binding constant that was nearly an order of magnitude smaller than that of CYP2B4 but a rate constant that was similar. Substrate binding constants for P450 Compound I are important for controlling overall rates of oxidation reactions, and the intrinsic reactivities of Compounds I from various P450 enzymes are comparable.

Characterization of the peroxidase mechanism upon reaction of prostacyclin synthase with peracetic acid. Identification of a tyrosyl radical intermediate

Prostacyclin synthase (PGIS) is a membrane-bound class III cytochrome P450 that catalyzes an isomerization of prostaglandin H(2), an endoperoxide, to prostacyclin. We report here the characterization of the PGIS intermediates in reactions with other peroxides, peracetic acid (PA), and iodosylbenzene. Rapid-scan stopped-flow experiments revealed an intermediate with an absorption spectrum similar to that of compound ES (Cpd ES), which is an oxo-ferryl (Fe(IV)O) plus a protein-derived radical. Cpd ES, formed upon reaction with PA, has an X-band (9 GHz) EPR signal of g = 2.0047 and a half-saturation power, P(1/2), of 0.73 mW. High-field (130 GHz) EPR reveals the presence of two species of tyrosyl radicals in Cpd ES with their g-tensor components (g(x), g(y), g(z)) of 2.00970, 2.00433, 2.00211 and 2.00700, 2.00433, 2.00211 at a 1:2 ratio, indicating that one is involved in hydrogen bonding and the other is not. The line width of the g = 2 signal becomes narrower, while its P(1/2) value becomes smaller as the reaction proceeds, indicating migration of the unpaired electron to an alternative site. The rate of electron migration ( approximately 0.2 s(-1)) is similar to that of heme bleaching, suggesting the migration is associated with the enzymatic inactivation. Moreover, a g = 6 signal that is presumably a high-spin ferric species emerges after the appearance of the amino acid radical and subsequently decays at a rate comparable to that of enzymatic inactivation. This loss of the g = 6 species thus likely indicates another pathway leading to enzymatic inactivation. The inactivation, however, was prevented by the exogenous reductant guaiacol. The studies of PGIS with PA described herein provide a mechanistic model of a peroxidase reaction catalyzed by the class III cytochromes P450.

Spectra and kinetic studies of the compound I derivative of cytochrome P450 119

The Compound I derivative of cytochrome P450 119 (CYP119) was produced by laser flash photolysis of the corresponding Compound II derivative, which was first prepared by reaction of the resting enzyme with peroxynitrite. The UV-vis spectrum of the Compound I species contained an asymmetric Soret band that could be resolved into overlapping transitions centered at approximately 367 and approximately 416 nm and a Q band with lambda(max) approximately 650 nm. Reactions of the Compound I derivative with organic substrates gave epoxidized (alkene oxidation) and hydroxylated (C-H oxidation) products, as demonstrated by product studies and oxygen-18 labeling studies. The kinetics of oxidations by CYP119 Compound I were measured directly; the reactions included hydroxylations of benzyl alcohol, ethylbenzene, Tris buffer, lauric acid, and methyl laurate and epoxidations of styrene and 10-undecenoic acid. Apparent second-order rate constants, equal to the product of the equilibrium binding constant (K(bind)) and the first-order oxidation rate constant (k(ox)), were obtained for all of the substrates. The oxidations of lauric acid and methyl laurate displayed saturation kinetic behavior, which permitted the determination of both K(bind) and k(ox) for these substrates. The unactivated C-H positions of lauric acid reacted with a rate constant of k(ox) = 0.8 s(-1) at room temperature. The CYP119 Compound I derivative is more reactive than model Compound I species [iron(IV)-oxo porphyrin radical cations] and similar in reactivity to the Compound I derivative of the heme-thiolate enzyme chloroperoxidase. Kinetic isotope effects (kH/kD) for oxidations of benzyl alcohol and ethylbenzene were small, reflecting the increased reactivity of the Compound I derivative in comparison to models. Nonetheless, CYP119 Compound I apparently is much less reactive than the oxidizing species formed in the P450 cam reaction cycle. Studies of competition kinetics employing CYP119 activated by hydrogen peroxide indicated that the same oxidizing transient is formed in the photochemical reaction and in the hydrogen peroxide shunt reaction.

Caldariomyces fumago chloroperoxidase catalyzes the oxidative dehalogenation of chlorophenols by a mechanism involving two one-electron steps

We have employed rapid scan stopped-flow spectroscopy to examine whether the mechanism of oxidative dehalogenation catalyzed by C. fumago chloroperoxidase (CCPO) involves two consecutive one-electron steps or a single two-electron oxidation. First, we optimized the formation of CCPO compound I (CCPO-I) [Fe(IV)=O/porphyrin radical] and CCPO compound II (CCPO-II) [Fe(IV)=O] for use in double mixing rapid scan stopped-flow experiments. Reaction of CCPO-I with 2,4,6-trichlorophenol (TCP) quickly yielded CCPO-II. Reaction of CCPO-II, a one-electron oxidant, with TCP rapidly regenerated the ferric resting state of the enzyme. The rates of the reaction of both CCPO-I and -II with TCP are first-order with respect to [TCP]. In the absence of organic substrate, CCPO-I is slowly reduced to CCPO-II and then the ferric state. The ability of both CCPO-I and -II to carry out the oxidative dehalogenation reaction is consistent with a mechanism involving two consecutive one-electron oxidations. In contrast, reaction of CCPO-I with thioanisole generated the ferric enzyme with no evidence of CCPO-II, consistent with a single two-electron oxidation by insertion of an oxygen atom. The relative stability of CCPO-I and -II has allowed us to differentiate between one- and two-electron substrate oxidations using rapid scan stopped-flow techniques.

Kinetics of two-electron oxidations by the compound I derivative of chloroperoxidase, a model for cytochrome P450 oxidants

[structure: see text] Rate constants for two-electron oxidation reactions of Compound I from chloroperoxidase (CPO) with a variety of substrates were measured by stopped-flow kinetic techniques. The thiolate ligand of CPO Compound I activates the iron-oxo species with the result that oxidation reactions are 2 to 3 orders of magnitude faster than oxidations by model iron(IV)-oxo porphyrin radical cations containing weaker binding counterions.

Reaction mechanisms of 15-hydroperoxyeicosatetraenoic acid catalyzed by human prostacyclin and thromboxane synthases

Prostacyclin synthase (PGIS) and thromboxane synthase (TXAS) are atypical cytochrome P450s. They do not require NADPH or dioxygen for isomerization of prostaglandin H(2) (PGH(2)) to produce prostacyclin (PGI(2)) and thromboxane A(2) (TXA(2)). PGI(2) and TXA(2) have opposing actions on platelet aggregation and blood vessel tone. In this report, we use a lipid hydroperoxide, 15-hydroperoxyeicosatetraenoic acid (15-HPETE), to explore the active site characteristics of PGIS and TXAS. The two enzymes transformed 15-HPETE not only into 13-hydroxy-14,15-epoxy-5,8,11-eicosatrienoic acid (13-OH-14,15-EET), like many microsomal P450s, but also to 15-ketoeicosatetraenoic acid (15-KETE) and 15-hydroxyeicosatetraenoic acid (15-HETE). 13-OH-14,15-EET and 15-KETE result from homolytic cleavage of the O-O bond, whereas 15-HETE results from heterolytic cleavage, a common peroxidase pathway. About 80% of 15-HPETE was homolytically cleaved by PGIS and 60% was homolytically cleaved by TXAS. The V(max) of homolytic cleavage is 3.5-fold faster than heterolytic cleavage for PGIS-catalyzed reactions (1100 min(-1)vs. 320 min(-1)) and 1.4-fold faster for TXAS (170 min(-1)vs. 120 min(-1)). Similar K(M) values for homolytic and heterolytic cleavages were found for PGIS ( approximately 60 microM 15-HPETE) and TXAS ( approximately 80 microM 15-HPETE), making PGIS a more efficient catalyst for the 15-HPETE reaction.

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

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

Structure and quantum chemical characterization of chloroperoxidase compound 0, a common reaction intermediate of diverse heme enzymes

We have determined the crystal structure of the chloroperoxidase (CPO) hydroperoxo reaction intermediate (CPO compound 0) at 1.75-A resolution. The intermediate was generated through controlled photoreduction of the CPO oxygen complex during x-ray data collection, which was monitored by recording of the crystal absorption spectra. Initially, the peroxo-anion species was formed and then protonated to yield compound 0. Quantum chemical calculations indicate that the peroxo-anion species is not stable and collapses instantaneously to compound 0. Compound 0 is present in the ferric low-spin doublet ground state and is characterized by a long O O bond length of 1.5 A and a Fe O bond distance of 1.8 A, which is also observed in the crystal structure.

Evidence for Basic Ferryls in Cytochromes P450

Using a combination of Mössbauer spectroscopy and density functional calculations, we have determined that the ferryl forms of P450(BM3) and P450cam are protonated at physiological pH. Density functional calculations were performed on large active-site models of these enzymes to determine the theoretical Mössbauer parameters for the ferryl and protonated ferryl (Fe(IV)OH) species. These calculations revealed a significant enlargement of the quadrupole splitting parameter upon protonation of the ferryl unit. The calculated quadrupole splittings for the protonated and unprotonated ferryl forms of P450(BM3) are DeltaE(Q) = 2.17 mm/s and DeltaE(Q) = 1.05 mm/s, respectively. For P450cam, they are DeltaE(Q) = 1.84 mm/s and DeltaE(Q) = 0.66 mm/s, respectively. The experimentally determined quadrupole splittings (P450(BM3), DeltaE(Q) = 2.16 mm/s; P450cam, DeltaE(Q) = 2.06 mm/s) are in good agreement with the values calculated for the protonated forms of the enzymes. Our results suggest that basic ferryls are a natural consequence of thiolate-ligated hemes.

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

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

In silico design of a mutant of cytochrome P450 containing selenocysteine

A mutant of P450(cam), in which the cysteine ligand was replaced by selenocysteine, was designed theoretically using hybrid QM/MM (quantum mechanical/molecular mechanical) calculations. The calculations of the active species, Se-CpdI (selenocysteine-Compound I), of the mutant enzyme indicate that Se-Cpd I will be formed faster than the wild-type species and be consumed more slowly in C-H hydroxylation. As such, our calculations suggest that Se-Cpd I can be observed unlike the elusive species of the wild-type enzyme (Denisov, I. G.; Makris, T. M.; Sligar, S. G.; Schlichting, I. Chem. Rev. 2005, 105, 2253-2277). Spectral features of Se-Cpd I were calculated and may assist such eventual characterization. The observation of Se-Cpd I will resolve the major puzzle in the catalytic cycle of a key enzyme in nature.

Evidence for Two Ferryl Species in Chloroperoxidase Compound II

Using a combination of density functional calculations and Mössbauer spectroscopy, we have examined chloroperoxidase compound II (CPO-II). The Mössbauer spectrum of CPO-II suggests the presence of two distinct ferryl species in an approximately 70:30 ratio. Density functional calculations and cryogenic reduction and annealing experiments allow us to assign the major species as an Fe(IV)OH intermediate. The Mössbauer parameters of the minor component are indicative of an authentic iron(IV)oxo species, but we have found the 70:30 ratio to be pH invariant. The unchanging ratio of component concentrations is in agreement with CPO-II's visible absorption spectrum, which shows no change over the enzyme's range of pH stability.

Rapid Freeze-Quench ENDOR Study of Chloroperoxidase Compound I: The Site of the Radical

The classical heme-monooxygenase active intermediate, compound I (Cpd-I), incorporates a heme which is oxidized by two equivalents above the resting ferric state, one equivalent associated with a ferryl center, [Fe=O]2+ (FeS = 1), and the other with an active-site radical (RS = 1/2). Theoretical calculations on models of a Cpd-I with a thiolato axial ligand have presented divergent views about its electronic structure. In one picture, the radical is on the porphyrin; in the other, it is on the sulfur. In this report, ENDOR spectroscopy answers the question, does Cpd-I of the enzyme chloroperoxidase contain a porphyrin pi-cation radical or an iron-bound cysteinyl radical: the radical is predominantly on the porphyrin, with spin density on sulfur having an upper bound, rhoS </= rhoSmax approximately 0.23. We further suggest that the same answer applies to Cpd-I of cytochromes P450.

Electrochemical generation of a high-valent state of cytochrome P450

Cyclic voltammetry performed at rapid scan rates on cytochrome P450 from Pseudomonas putida (P450CAM) in didodecyldimethylammonium bromide (DDAB) films on graphite electrodes revealed a couple (E) at 830mV (vs Ag/AgCl). E was not significantly observed at scan rates less than 30V/s at room temperature, suggesting that the oxidized species is unstable. The lifetime of E could be prolonged at 4 degrees C, which allowed reversible access to E at scan rates as low as 1V/s. E was found to be sensitive to imidazole in solution and to variations in pH, suggesting that the redox reaction is occurring at the metal center (i.e., Fe(IV/III)). Electrolysis reactions with different P450 substrates revealed that the electrochemically generated high-valent species is able to convert thioanisole to methyl phenyl sulfoxide.

Structures of the high-valent metal-ion haem–oxygen intermediates in peroxidases, oxygenases and catalases

Peroxidases, oxygenases and catalases have similar high-valent metal-ion intermediates in their respective reaction cycles. In this review, haem-based examples will be discussed. The intermediates of the haem-containing enzymes have been extensively studied for many years by different spectroscopic methods like UV-Vis, EPR (electron paramagnetic resonance), resonance Raman, Mössbauer and MCD (magnetic circular dichroism). The first crystal structure of one of these high-valent intermediates was on cytochrome c peroxidase in 1987. Since then, structures have appeared for catalases in 1996, 2002, 2003, putatively for cytochrome P450 in 2000, for myoglobin in 2002, for horseradish peroxidase in 2002 and for cytochrome c peroxidase again in 1994 and 2003. This review will focus on the most recent structural investigations for the different intermediates of these proteins. The structures of these intermediates will also be viewed in light of quantum mechanical (QM) calculations on haem models. In particular quantum refinement, which is a combination of QM calculations and crystallography, will be discussed. Only small structural changes accompany the generation of these intermediates. The crystal structures show that the compound I state, with a so called pi-cation radical on the haem group, has a relatively short iron-oxygen bond (1.67-1.76A) in agreement with a double-bond character, while the compound II state or the compound I state with a radical on an amino acid residue have a relatively long iron-oxygen bond (1.86-1.92A) in agreement with a single-bond character where the oxygen-atom is protonated.

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.

The status of high-valent metal oxo complexes in the P450 cytochromes

The oxidative prowess of the P450 cytochromes in physiological reactions is attributed to the production of a high-valent iron-oxo complex, or Compound I intermediate, in the reaction cycle. Despite many years of study, however, the full electronic description of this fleeting intermediate still remains an active area of study. In this manuscript, the current status of the isolation and characterization of the P450 oxo-Fe(IV) is examined and compared to analogous states in related heme enzymes. In addition, the utilization of cofactor exchange to stabilize high-valent oxo-states in the P450 is addressed. Structural and spectroscopic studies on manganese reconstituted P450, and its corresponding oxo-complex, are presented.

On the status of ferryl protonation

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

Characterization of heme environment and mechanism of peroxide bond cleavage in human prostacyclin synthase

Prostacyclin is a potent mediator of vasodilation and anti-platelet aggregation. It is synthesized from prostaglandin H(2) by prostacyclin synthase (PGIS), a member of Family 8 in the cytochrome P450 superfamily. Unlike most P450s, which require exogenous reducing equivalents and an oxygen molecule for mono-oxygenation, PGIS catalyzes an isomerization with an initial step of endoperoxide bond cleavage of prostaglandin H(2) (PGH(2)). The low abundance of PGIS in natural tissues necessitates heterologous expression for studies of structure/function relationships and reaction mechanism. We report here a high-yield prokaryotic system for expression of enzymatically active human PGIS. The PGIS cDNA is modified by replacing the hydrophobic amino-terminal sequence with the more hydrophilic amino-terminal sequence from P450 2C5 and by adding a four-histidine tag at the carboxyl terminus. The resulting recombinant PGIS associates with host cell membranes and was purified to electrophoretic homogeneity by nickel affinity, hydroxyapatite and CM Sepharose column chromatography. The recombinant PGIS, with a heme:protein ratio of 0.9:1, catalyzes prostacyclin formation at a K(m) of 13.3 muM PGH(2) and a V(max) of 980 per min. The dithionite-reduced PGIS binds CO with an on-rate of 5.6 x 10(5) M(-1) s(-1) and an off-rate of 15 s(-1). The ferrous-CO complex of PGIS is very short-lived and decays at a rate of 0.7 s(-1). Spectral binding assays showed that imidazole binds weakly to PGIS (K(d) approximately 0.5 mM,) but clotrimazole, a bulky and rigid imidazole derivative, binds strongly (K(d) approximately 1 microM). The transient nature of the CO complex and the weak imidazole binding seem to support an earlier proposal that PGIS active site has a limited space, but the tight binding of clotrimazole argues against this view. It appears that the heme distal pocket of PGIS is fairly adaptable to ligands of various structures. UV-visible absorption, magnetic circular dichroism and electron paramagnetic resonance spectra indicate that PGIS has a typical low-spin heme with a hydrophobic active site. PGIS catalyzes homolytic scission of the peroxide bond of a test substrate, 10-hydroperoxyoctadeca-8,12-dienoic acid, accompanied by formation of a heme intermediate with a Compound II-like optical spectrum.

Gauging the Relative Oxidative Powers of Compound I, Ferric-Hydroperoxide, and the Ferric-Hydrogen Peroxide Species of Cytochrome P450 Toward C−H Hydroxylation of a Radical Clock Substrate

Density functional calculations were performed in response to the controversies regarding the identity of the oxidant species in cytochrome P450. The calculations were used to gauge the relative C-H hydroxylation reactivity of three potential oxidant species of the enzyme, the high-valent oxo-iron species Compound I (Cpd I), the ferric hydroperoxide Compound 0 (Cpd 0), and the ferric-hydrogen peroxide complex Fe(H(2)O(2)). The results for the hydroxylation of a radical probe substrate, 1, show the following trends: (a) Cpd I is the most reactive species; in its presence the other two reagents will be silent. (b) In the absence of Cpd I, substrate oxidation by Cpd 0 and Fe(H(2)O(2)) will take place via a stepwise mechanism that involves initial O-O homolysis followed by H-abstraction from 1. (c) Cpd 0 will undergo mostly porphyrin hydroxylation and only approximately 15% of substrate oxidation producing mostly the rearranged alcohol, 3 (Scheme 2). (d) Fe(H(2)O(2)) will generate mostly free hydrogen peroxide (uncoupling). A small fraction will perform substrate oxidation and lead mostly to 3. Reactivity probes for these reagents are kinetic isotope effect (KIE) and the product ratio of unrearranged to rearranged alcohols, [2/3]. Thus, for substrate oxidation by Cpd 0 or Fe(H(2)O(2)) KIE will be small, approximately 2, while Cpd I will have large KIE values. Typically both Cpd 0 and Fe(H(2)O(2)) will lead to a [2/3] ratio < 1, while Cpd I will lead to ratios > 1. In addition, the product isotope effect (KIE(2)/KIE(3) not equal 1) is expected from the reactivity of Cpd I.

Freeze-quenched iron-oxo intermediates in cytochromes P450

Since the discovery of cytochromes P450 and their assignment to heme proteins a reactive iron-oxo intermediate as the hydroxylating species has been discussed. It is believed that the electronic structure of this intermediate corresponds to an iron(IV)-porphyrin-pi-cation radical system (Compound I). To trap this intermediate the reaction of P450 with oxidants (shunt pathway) has been used. The common approaches are stopped-flow experiments with UV-visible spectroscopic detection or rapid-mixing/freeze-quench studies with EPR and Mössbauer spectroscopic characterization of the trapped intermediate. Surprisingly, the two approaches seem to give conflicting results. While the stopped-flow data indicate the formation of a porphyrin-pi-cation radical, no such species is seen by EPR spectroscopy, although the Mössbauer data indicate iron(IV) for P450cam (CYP101) and P450BMP (CYP102). Instead, radicals on tyrosine and tryptophan residues are observed. These findings are reviewed and discussed with respect to intramolecular electron transfer from aromatic amino acids to a presumably transiently formed porphyrin-pi-cation radical.

X-ray absorption spectroscopy of chloroperoxidase compound I: Insight into the reactive intermediate of P450 chemistry

We report the structural characterization of a thiolate-ligated ferryl radical. Using x-ray absorption spectroscopy, we examined chloroperoxidase (CPO) compound I (CPO-I). Our results indicate that CPO-I is an authentic ferryl species with an Fe-O bond of 1.65 A. Axial-ligand interactions result in a remarkably long 2.48-A Fe-S bond. Analogous forms of cytochrome P450 and CPO have been shown to possess virtually identical coordination environments. Thus, it seems likely that our findings provide a good structural description of the elusive P450-I.

Possible involvement of singlet oxygen species as multiple oxidants in p450 catalytic reactions

Cytochrome P450 (P450) constitutes a superfamily of enzymes which activate dioxygen and carry out monooxygenation reactions of large numbers of endogenous and xenobiotic compounds. Drug metabolism is a particularly important P450 function, and, therefore, elucidating the metabolic products and pathways of drugs is essential for drug development. To explain the substrate selectivity of P450 reactions, it is necessary to understand the formation of multiple activated oxygen species to determine the type of catalyzed reactions, in addition to conducting structure analyses of P450s. Although an oxo-Fe(IV)-porphyrin-pi-cation radical is regarded as an activated oxygen species in P450 reactions, a nucleophilic Fe(III)-peroxo species has also been proposed as another oxidant. In the past decade, various studies indicated that P450-catalyzed oxygenations are complex, and that a single reaction pathway cannot explain all of the experimental results. In addition, the microsomal P450 system is known to generate reactive oxygen species (ROS). However, the contribution of ROS to P450 reactions remains unclear. We recently found that singlet oxygen (1O2) was involved in both several rat liver microsomal P450 reactions and four human CYP subfamily activities, as confirmed by the ESR spin-trapping method. In this review, we describe the studies that have been conducted on the detection and characterization of ROS in P450 reactions related to drug metabolism that involve the possibility of 1O2 in the P450 catalytic cycle. Gaining an understanding of the activated oxygen species that determine the type of drug metabolism will help us to predict the important metabolites formed.