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

Multiresolution molecular dynamics simulations reveal the interplay between conformational variability and functional interactions in membrane-bound cytochrome P450 2B4

Cytochrome P450 2B4 (CYP 2B4) is one of the best-characterized CYPs and serves as a key model system for understanding the mechanisms of microsomal class II CYPs, which metabolize most known drugs. The highly flexible nature of CYP 2B4 is apparent from crystal structures that show the active site with either a wide open or a closed heme binding cavity. Here, we investigated the conformational ensemble of the full-length CYP 2B4 in a phospholipid bilayer, using multiresolution molecular dynamics (MD) simulations. Coarse-grained MD simulations revealed two predominant orientations of CYP 2B4's globular domain with respect to the bilayer. Their refinement by atomistic resolution MD showed adaptation of the enzyme's interaction with the lipid bilayer, leading to open configurations that facilitate ligand access to the heme binding cavity. CAVER analysis of enzyme tunnels, AquaDuct analysis of water routes, and Random Acceleration Molecular Dynamics simulations of ligand dissociation support the conformation-dependent passage of molecules between the active site and the protein surroundings. Furthermore, simulation of the re-entry of the inhibitor bifonazole into the open conformation of CYP 2B4 resulted in binding at a transient hydrophobic pocket within the active site cavity that may play a role in substrate binding or allosteric regulation. Together, these results show how the open conformation of CYP 2B4 facilitates the binding of substrates from and release of products to the membrane, whereas the closed conformation prolongs the residence time of substrates or inhibitors and selectively allows the passage of smaller reactants via the solvent and water channels.

Substrate Conformational Switch Enables the Stereoselective Dimerization in P450 NascB: Insights from Molecular Dynamics Simulations and Quantum Mechanical/Molecular Mechanical Calculations

P450 NascB catalyzes the coupling of cyclo-(l-tryptophan-l-proline) (1) to generate (-)-naseseazine C (2) through intramolecular C-N bond formation and intermolecular C-C coupling. A thorough understanding of its catalytic mechanism is crucial for the engineering or design of P450-catalyzed C-N dimerization reactions. By employing MD simulations, QM/MM calculations, and enhanced sampling, we assessed various mechanisms from recent works. Our study demonstrates that the most favorable pathway entails the transfer of a hydrogen atom from N7-H to Cpd I. Subsequently, there is a conformational change in the substrate radical, shifting it from the Re-face to the Si-face of N7 in Substrate 1. The Si-face conformation of Substrate 1 is stabilized by the protein environment and the π-π stacking interaction between the indole ring and heme porphyrin. The subsequent intermolecular C3-C6' bond formation between Substrate 1 radical and Substrate 2 occurs via a radical attack mechanism. The conformational switch of the Substrate 1 radical not only lowers the barrier of the intermolecular C3-C6' bond formation but also yields the correct stereoselectivity observed in experiments. In addition, we evaluated the reactivity of the ferric-superoxide species, showing it is not reactive enough to initiate the hydrogen atom abstraction from the indole NH group of the substrate. Our simulation provides a comprehensive mechanistic insight into how the P450 enzyme precisely controls both the intramolecular C-N cyclization and intermolecular C-C coupling. The current findings align with the available experimental data, emphasizing the pivotal role of substrate dynamics in governing P450 catalysis.

Depicting the proton relay network in human aromatase: New insights into the role of the alcohol-acid pair

Human aromatase is the cytochrome P450 catalyzing the conversion of androgens into estrogens in a three steps reaction essential to maintain steroid hormones balance. Here we report the capture and spectroscopic characterization of its compound I (Cpd I), the main reactive species in cytochromes P450. The typical spectroscopic transitions indicating the formation of Cpd I are detected within 0.8 s when mixing aromatase with meta-chloroperoxybenzoic acid. The estrogen product is obtained from the same reaction mixture, demonstrating the involvement of Cpd I in aromatization reaction. Site-directed mutagenesis is applied to the acid-alcohol pair D309 and T310 and to R192, predicted to be part of the proton relay network. Mutants D309N and R192Q do not lead to Cpd I with an associated loss of activity, confirming that these residues are involved in proton delivery for Cpd I generation. Cpd I is captured for T310A mutant and shows 2.9- and 4.4-fold faster rates of formation and decay, respectively, compared to wild-type (WT). However, its activity is lower than the WT and a larger amount of H2 O2 is produced during catalysis, indicating that T310 has an essential role in proton gating for generation of Cpd 0 and Cpd I and for their stabilization. The data provide new evidences on the role of threonine belonging to the conserved "acid-alcohol" pair and known to be crucial for oxygen activation in cytochromes P450.

Low-Lying Electromeric States in Chloro-Ligated Iron(IV)-Oxo Porphyrin as a Model for Compound I, Studied with Second-Order Perturbation Theory Based on Density Matrix Renormalization Group

Employing second-order perturbation theory based on the density matrix renormalization group (DMRG-CASPT2), this work aims at providing a quantitative description of the spin state energetics of a chloro-ligated iron(IV)-oxo porphyrin as a model for the cytochromes P450 active species, also known as compound I (Cpd I). We explored DMRG-CASPT2 to its full extent with an extensive active space (up to 31 active orbitals) as well as a large number of renormalized states m (up to 10000). Different flavors of DMRG-CASPT2, using either the costly exact 4-particle reduced density matrix (4-RDM) or the cheaper cumulant approximated 4-RDM (cu(4)), were analyzed. All flavors essentially converge to similar relative energies between different spin states. Including a correction for the protein environment, we found a quartet Fe^IVO ground state and, more importantly, a thermally accessible doublet Fe^VO excited state that might directly contribute to the reactivity of this iron-oxo species. Our results also showed that cheaper approaches, such as CASPT2 based on a smaller active space or the cumulant approximation DMRG-cu(4)-CASPT2, are capable of accurately describing the spin state energetics of this species.

CYP1A1-Mediated Intramolecular Rearrangement of Aminoazepane in GDC-0339

GDC-0339 is a novel small molecule pan-Pim kinase inhibitor that was discovered as a potential treatment of multiple myeloma. During the in vitro and in vivo metabolite profiling of GDC-0339, a metabolite was detected that had the same elemental composition as the parent but was distinct with respect to its chromatographic separation and mass spectrometric fragmentation pattern. High resolution tandem mass spectrometry data indicated the metabolite was modified at the aminoazepane moiety. The structure was solved by nuclear magnetic resonance analysis of the isolated metabolite and further confirmed by comparing it to a synthetic standard. These results indicated that the metabolite was formed by an intramolecular amine replacement reaction with the primary amine forming a new attachment to pyrazole without any change in stereochemistry. In vitro experiments showed cytochrome P450s catalyzed the reaction and demonstrated high isoform selectivity by CYP1A1. Results from kinetic experiments showed that the CYP1A1-mediated rearrangement of GDC-0339 was an efficient reaction with apparent turnover number (kcat) and Michaelis constant (Km) of 8.4 minutes-1 and 0.6 μM, respectively. The binding of GDC-0339 to the cytochrome P450 active site was examined by characterizing the direct inhibition of CYP1A1-mediated phenacetin O-deethylation, and GDC-0339 was a potent competitive inhibitor with Ki of 0.9 μM. This high affinity binding was unexpected given a narrow active site for CYP1A1 and GDC-0339 does not conform structurally to known CYP1A1 substrates, which are mostly polyaromatic planar molecules. Further, we explored some of the structural requirements for the rearrangement reaction and identified several analogs to GDC-0339 that undergo this biotransformation.

Reaction Intermediates and Molecular Mechanism of Peroxynitrite Activation by NO Synthases

The activation of the peroxynitrite anion (PN) by hemoproteins, which leads to its detoxification or, on the contrary to the enhancement of its cytotoxic activity, is a reaction of physiological importance that is still poorly understood. It has been known for some years that the reaction of hemoproteins, notably cytochrome P450, with PN leads to the buildup of an intermediate species with a Soret band at ∼435 nm (I435). The nature of this intermediate is, however, debated. On the one hand, I435 has been presented as a compound II species that can be photoactivated to compound I. A competing alternative involves the assignment of I435 to a ferric-nitrosyl species. Similar to cytochromes P450, the buildup of I435 occurs in nitric oxide synthases (NOSs) upon their reaction with excess PN. Interestingly, the NOS isoforms vary in their capacity to detoxify/activate PN, although they all show the buildup of I435. To better understand PN activation/detoxification by heme proteins, a definitive assignment of I435 is needed. Here we used a combination of fine kinetic analysis under specific conditions (pH, PN concentrations, and PN/NOSs ratios) to probe the formation of I435. These studies revealed that I435 is not formed upon homolytic cleavage of the O-O bond of PN, but instead arises from side reactions associated with excess PN. Characterization of I435 by resonance Raman spectroscopy allowed its identification as a ferric iron-nitrosyl complex. Our study indicates that the model used so far to depict PN interactions with hemo-thiolate proteins, i.e., leading to the formation and accumulation of compound II, needs to be reconsidered.

Role of the Proximal Cysteine Hydrogen Bonding Interaction in Cytochrome P450 2B4 Studied by Cryoreduction, Electron Paramagnetic Resonance, and Electron-Nuclear Double Resonance Spectroscopy

Crystallographic studies have shown that the F429H mutation of cytochrome P450 2B4 introduces an H-bond between His429 and the proximal thiolate ligand, Cys436, without altering the protein fold but sharply decreases the enzymatic activity and stabilizes the oxyferrous P450 2B4 complex. To characterize the influence of this hydrogen bond on the states of the catalytic cycle, we have used radiolytic cryoreduction combined with electron paramagnetic resonance (EPR) and (electron-nuclear double resonance (ENDOR) spectroscopy to study and compare their characteristics for wild-type (WT) P450 2B4 and the F429H mutant. (i) The addition of an H-bond to the axial Cys436 thiolate significantly changes the EPR signals of both low-spin and high-spin heme-iron(III) and the hyperfine couplings of the heme-pyrrole (14)N but has relatively little effect on the (1)H ENDOR spectra of the water ligand in the six-coordinate low-spin ferriheme state. These changes indicate that the H-bond introduced between His and the proximal cysteine decreases the extent of S → Fe electron donation and weakens the Fe(III)-S bond. (ii) The added H-bond changes the primary product of cryoreduction of the Fe(II) enzyme, which is trapped in the conformation of the parent Fe(II) state. In the wild-type enzyme, the added electron localizes on the porphyrin, generating an S = (3)/2 state with the anion radical exchange-coupled to the Fe(II). In the mutant, it localizes on the iron, generating an S = (1)/2 Fe(I) state. (iii) The additional H-bond has little effect on g values and (1)H-(14)N hyperfine couplings of the cryogenerated, ferric hydroperoxo intermediate but noticeably slows its decay during cryoannealing. (iv) In both the WT and the mutant enzyme, this decay shows a significant solvent kinetic isotope effect, indicating that the decay reflects a proton-assisted conversion to Compound I (Cpd I). (v) We confirm that Cpd I formed during the annealing of the cryogenerated hydroperoxy intermediate and that it is the active hydroxylating species in both WT P450 2B4 and the F429H mutant. (vi) Our data also indicate that the added H-bond of the mutation diminishes the reactivity of Cpd I.

F429 Regulation of Tunnels in Cytochrome P450 2B4: A Top Down Study of Multiple Molecular Dynamics Simulations

The root causes of the outcomes of the single-site mutation in enzymes remain by and large not well understood. This is the case of the F429H mutant of the cytochrome P450 (CYP) 2B4 enzyme where the substitution, on the proximal surface of the active site, of a conserved phenylalanine 429 residue with histidine seems to hamper the formation of the active species, Compound I (porphyrin cation radical-Fe^(IV) = O, Cpd I) from the ferric hydroperoxo (Fe^(III)OOH^-, Cpd 0) precursor. Here we report a study based on extensive molecular dynamic (MD) simulations of 4 CYP-2B4 point mutations compared to the WT enzyme, having the goal of better clarifying the importance of the proximal Phe429 residue on CYP 2B4 catalytic properties. To consolidate the huge amount of data coming from five simulations and extract the most distinct structural features of the five species studied we made an extensive use of cluster analysis. The results show that all studied single polymorphisms of F429, with different side chain properties: i) drastically alter the reservoir of conformations accessible by the protein, perturbing global dynamics ii) expose the thiolate group of residue Cys436 to the solvent, altering the electronic properties of Cpd0 and iii) affect the various ingress and egress channels connecting the distal sites with the bulk environment, altering the reversibility of these channels. In particular, it was observed that the wild type enzyme exhibits unique structural features as compared to all mutant species in terms of weak interactions (hydrogen bonds) that generate a completely different dynamical behavior of the complete system. Albeit not conclusive, the current computational investigation sheds some light on the subtle and critical effects that proximal single-site mutations can exert on the functional mechanisms of human microsomal CYPs which should go rather far beyond local structure characterization.

Characterizing the Intermediates Compound I and II in the Cytochrome P450 Catalytic Cycle with Nonlinear X-ray Spectroscopy: A Simulation Study

Cytochrome P450 enzymes are an important family of biocatalysts that oxidize chemically inert CH bonds. There are many unresolved questions regarding the catalytic reaction intermediates, in particular P450 Compound I (Cpd-I) and II (Cpd-II). By using simple molecular models, we simulate various X-ray spectroscopy signals, including X-ray absorption near-edge structure (XANES), resonant inelastic X-ray scattering (RIXS), and stimulated X-ray Raman spectroscopy (SXRS) of the low- and high-spin states of Cpd-I and II. Characteristic peak patterns are presented and connected to the corresponding electronic structures. These X-ray spectroscopy techniques are complementary to more conventional infrared and optical spectroscopy and they help to elucidate the evolving electronic structures of transient species along the reaction path.

Monooxygenase, peroxidase and peroxygenase properties and reaction mechanisms of cytochrome P450 enzymes

This review examines the monooxygenase, peroxidase and peroxygenase properties and reaction mechanisms of cytochrome P450 (CYP) enzymes in bacterial, archaeal and mammalian systems. CYP enzymes catalyze monooxygenation reactions by inserting one oxygen atom from O2 into an enormous number and variety of substrates. The catalytic versatility of CYP stems from its ability to functionalize unactivated carbon-hydrogen (C-H) bonds of substrates through monooxygenation. The oxidative prowess of CYP in catalyzing monooxygenation reactions is attributed primarily to a porphyrin π radical ferryl intermediate known as Compound I (CpdI) (Por•+FeIV=O), or its ferryl radical resonance form (FeIV-O•). CYP-mediated hydroxylations occur via a consensus H atom abstraction/oxygen rebound mechanism involving an initial abstraction by CpdI of a H atom from the substrate, generating a highly-reactive protonated Compound II (CpdII) intermediate (FeIV-OH) and a carbon-centered alkyl radical that rebounds onto the ferryl hydroxyl moiety to yield the hydroxylated substrate. CYP enzymes utilize hydroperoxides, peracids, perborate, percarbonate, periodate, chlorite, iodosobenzene and N-oxides as surrogate oxygen atom donors to oxygenate substrates via the shunt pathway in the absence of NAD(P)H/O2 and reduction-oxidation (redox) auxiliary proteins. It has been difficult to isolate the historically elusive CpdI intermediate in the native NAD(P)H/O2-supported monooxygenase pathway and to determine its precise electronic structure and kinetic and physicochemical properties because of its high reactivity, unstable nature (t½~2 ms) and short life cycle, prompting suggestions for participation in monooxygenation reactions of alternative CYP iron-oxygen intermediates such as the ferric-peroxo anion species (FeIII-OO-), ferric-hydroperoxo species (FeIII-OOH) and FeIII-(H2O2) complex.

Reactive intermediates in cytochrome p450 catalysis

Recently, we reported the spectroscopic and kinetic characterizations of cytochrome P450 compound I in CYP119A1, effectively closing the catalytic cycle of cytochrome P450-mediated hydroxylations. In this minireview, we focus on the developments that made this breakthrough possible. We examine the importance of enzyme purification in the quest for reactive intermediates and report the preparation of compound I in a second P450 (P450ST). In an effort to bring clarity to the field, we also examine the validity of controversial reports claiming the production of P450 compound I through the use of peroxynitrite and laser flash photolysis.

Multireference and multiconfiguration ab initio methods in heme-related systems: what have we learned so far?

This work reviews the recent applications of ab initio multireference/multiconfiguration (MR/MC) electronic structure methods to heme-related systems, involving tetra-, penta-, and hexa-coordinate species, as well as the high-valent iron-oxo species. The current accuracy of these methods in the various systems is discussed, with special attention to potential sources of systematic errors. Thus, the review summarizes and tries to rationalize the key elements of MR/MC calculations, namely, the choice of the employed active space, especially the so-called double-shell effect that has already been recognized to be important in transition-metal-containing systems, and the impact of these elements on the spin-state energetics of heme species, as well as on the bonding mechanism of small molecules to the heme. It is shown that expansion of the MC wave function into one based on localized orbitals provides a compact and insightful view on some otherwise complex electronic structures. The effects of protein environment on the MR/MC results are summarized for the few available quantum mechanical/molecular mechanical (QM/MM) studies. Comparisons with corresponding DFT results are also made wherever available. Potential future directions are proposed.

The rate-limiting step in P450 hydroxylation of hydrocarbons a direct comparison of the "somersault" versus the "consensus" mechanism involving compound I

Model theoretical quantum mechanical (QM) calculations are described for the P-450 hydroxylation of methane, isobutane, and camphor that compare the concerted somersault H-abstraction mechanism with the oxidation step involving Cpd I. Special emphasis has been placed on maintaining a balanced basis set in the oxidation step. QM calculations, employing the 6-311+G(d,p) basis set on the Fe atom and all of the key surrounding atoms involved in the C-H abstraction step, reaffirm a mechanism involving rearrangement of the iron hydroperoxide group (FeO-OH --> FeO...HO(*)) in concert with hydrogen abstraction from the C-H bond of the substrate by the incipient bound hydroxyl radical HO(*). The barrier for the somersault rearrangement of model Cpd 0 (FeO-OH) is calculated to be 21.4 kcal/mol in the absence of substrate. The overall activation energy for the oxidation of camphor involving the somersault motion of the FeO-OH group of P450 model porphyrin iron(III) hydroperoxide [Por(SH)Fe(III)-OOH(-)] --> [Por(SH)Fe(III)-O....HO(-)] in concert with hydrogen abstraction is DeltaE(++) = 12.4 kcal/mol. The corresponding abstraction of the hydrogen atom from the C-H bond of camphor by Cpd I has an activation barrier of 17.6 kcal/mol. Arguments are presented that the somersault rearrangement is induced by steric compression at the active site. Kinetic isotope effect data are discussed that provides compelling evidence for a rate-limiting step involving C-H bond cleavage.

Low temperature photo-oxidation of chloroperoxidase Compound II

Oxidation of the heme-thiolate enzyme chloroperoxidase (CPO) from Caldariomyces fumago with peroxynitrite (PN) gave the Compound II intermediate, which was photo-oxidized with 365 nm light to give a reactive oxidizing species. Cryo-solvents at pH ≈ 6 were employed, and reactions were conducted at temperatures as low as -50° C. The activity of CPO as evaluated by the chlorodimedone assay was unaltered by treatment with PN or by production of the oxidizing transient and subsequent reaction with styrene. EPR spectra at 77K gave the amount of ferric protein at each stage in the reaction sequence. The PN oxidation step gave a 6:1 mixture of Compound II and ferric CPO, the photolysis step gave an approximate 1:1 mixture of active oxidant and ferric CPO, and the final mixture after reaction with excess styrene contained ferric CPO in 80% yield. In single turnover reactions at -50°C, styrene was oxidized to styrene oxide in high yield. Kinetic studies of styrene oxidation at -50°C displayed saturation kinetics with an equilibrium constant for formation of the complex of K(bind)=3.8 x 10(4)M(-1) and an oxidation rate constant of k(ox)=0.30s(-1). UV-Visible spectra of mixtures formed in the photo-oxidation sequence at ca. -50° C did not contain the signature Q-band absorbance at 690 nm ascribed to CPO Compound I prepared by chemical oxidation of the enzyme, indicating that different species were formed in the chemical oxidation and the photo-oxidation sequence.

Stabilization and spectroscopic characterization of the dioxygen complex of wild-type cytochrome P4502B4 (CYP2B4) and its distal side E301Q, T302A and proximal side F429H mutants at subzero temperatures

Mammalian cytochrome P450 2B4 (CYP2B4) is a phenobarbital-inducible rabbit hepatic monooxygenase that catalyzes the N-demethylation of benzphetamine and metabolism of numerous other compounds. To probe the interactions of the heme environment and bound benzphetamine with the dioxygen (O₂) complex of CYP2B4, homogeneous O₂ complexes of the wild-type enzyme and three mutants at sites of conserved amino acids, two on the heme distal side (T302A and E301Q) and one on the proximal side (F429H), have been prepared and stabilized at ~-50°C in mixed solvents (60-70% v/v glycerol). We report that the magnetic circular dichroism and electronic absorption spectra of wild-type oxyferrous CYP2B4, in the presence and absence of substrate, are quite similar to those of the dioxygen complex of bacterial cytochrome P450-CAM (CYP101). However, the oxyferrous complexes of the T302A and E301Q CYP2B4 mutants have significantly perturbed electronic structure (~4 nm and ~3 nm red-shifted Soret features, respectively) compared to that of the wild-type oxyferrous complex. On the other hand, the heme proximal side mutant, CYP2B4 F429H, undergoes relatively facile conversion to a partially (~50%) denatured (P420) form upon reduction. The structural changes in the heme pocket environments of the CYP2B4 mutants that lead to the spectroscopic distinctions reported herein can be related to the differences in oxidation activities of wild-type CYP2B4 and its E301Q, T302A and F429H mutants.

NO synthase: structures and mechanisms

Production of NO from arginine and molecular oxygen is a complex chemical reaction unique to biology. Our understanding of the chemical and regulation mechanisms of the NO synthases has developed over the past two decades, uncovering some extraordinary features. This article reviews recent progress and highlights current issues and controversies. The structure of the enzyme has now been determined almost in entirety, although it is as a selection of fragments, which are difficult to assemble unambiguously. NO synthesis is driven by electron transfer through FAD and FMN cofactors, which is controlled by calmodulin binding in the constitutive mammalian enzymes. Many of the unique structural features involved have been characterised, but the mechanics of calmodulin-dependent activation are largely unresolved. Ultimately, NO is produced in the active site by the reaction of arginine with activated heme-bound oxygen in two distinct cycles. The unique role of the tetrahydrobiopterin cofactor as an electron donor in this process has now been established, but the subsequent chemical events are currently a matter of intense speculation and debate.

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 and activation parameters for oxidations of styrene by Compounds I from the cytochrome P450(BM-3) (CYP102A1) heme domain and from CYP119

Cytochrome P450 (CYP or P450) enzymes are ubiquitous in nature where they catalyze a vast array of oxidation reactions. The active oxidants in P450s have long been assumed to be iron(IV)-oxo porphyrin radical cations termed Compounds I, but P450 Compounds I have proven to be difficult to prepare. The recent development of an entry to these transients by photo-oxidation of the corresponding iron(IV)-oxo neutral porphyrin species (Compounds II) permits spectroscopic and kinetic studies. We report here application of the photo-oxidation method for production of Compound I from the heme domain of CYP102A1 (cytochrome P450(BM-3)), and product and kinetic studies of reactions of styrene with this Compound I transient and also Compound I from CYP119. The studies were performed at low temperatures in 1:1 (v:v) mixtures of glycerol and phosphate buffer. Single-turnover reactions at 0 degrees C gave styrene oxide in good yields. In kinetic studies conducted between -10 and -50 degrees C, both Compounds I displayed saturation kinetics permitting determinations of binding constants and first-order oxidation rate constants. Temperature-dependent functions for the binding constants and rate constants were determined for both Compounds I. In the temperature range studied, the Compound I transient from the CYP102A1 heme domain bound styrene more strongly than Compound I from CYP119, but the rate constants for oxidations of styrene by the latter were somewhat larger than those for the former. The temperature-dependent functions for the first-order oxidation reactions are as follows: log k = 13.2-15.2/2.303RT and log k = 13.3-14.6/2.303RT (kilocalories per mole) for Compounds I from the CYP102A1 heme domain and CYP119, respectively.

Kinetic isotope effects in hydroxylation reactions effected by cytochrome P450 compounds I implicate multiple electrophilic oxidants for P450-catalyzed oxidations

Kinetic isotope effects were measured for oxidations of (S,S)-2-(p-trifluoromethylphenyl)cyclopropylmethane containing zero, two, and three deuterium atoms on the methyl group by Compounds I from the cytochrome P450 enzymes CYP119 and CYP2B4 at 22 degrees C. The oxidations displayed saturation kinetics, which permitted solution of both binding constants (K(bind)) and first-order oxidation rate constants (k(ox)) for both enzymes with the three substrates. The binding constant for CYP2B4 Compound I was about 1 order of magnitude greater than that for CYP119 Compound I, but the oxidation rate constants were similar for the two. In oxidations of 1-d(0), k(ox) = 10.4 s(-1) for CYP119 Compound I, and k(ox) = 12.4 s(-1) for CYP2B4 Compound I. Primary kinetic isotope effects (P) and secondary kinetic isotope effects (S) were obtained from the results with the three isotopomers. The primary KIEs were large, P = 9.8 and P = 8.9 for CYP119 and CYP2B4 Compounds I, respectively, and the secondary KIEs were small and normal, S = 1.07 and S = 1.05, respectively. Large intermolecular KIEs for 1-d(0) and 1-d(3) of k(H)/k(D) = 11.2 and 9.8 found for the two Compounds I contrast with small intermolecular KIEs obtained previously for the same substrate in P450-catalyzed oxidations; these differences suggest that a second electrophilic oxidant, presumably iron-complexed hydrogen peroxide, is important in cytochrome P450 oxidations under turnover conditions.