Substrate binding favors enhanced NO binding to P450cam

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

Experimental and Computational Insight into the Mechanism of NO Binding to Ferric Microperoxidase. The Likely Role of Tautomerization to Account for the pH Dependence

According to the current paradigm, the metal-hydroxo bond in a six-coordinate porphyrin complex is believed to be significantly less reactive in ligand substitution than the analogous metal-aqua bond, due to a much higher strength of the former bond. Here, we report kinetic studies for nitric oxide (NO) binding to a heme-protein model, acetylated microperoxidase-11 (AcMP-11), that challenge this paradigm. In the studied pH range 7.4-12.6, ferric AcMP-11 exists in three acid-base forms, assigned in the literature as [(AcMP-11)FeIII(H2O)(HisH)] (1), [(AcMP-11)FeIII(OH)(HisH)] (2), and [(AcMP-11)FeIII(OH)(His-)] (3). From the pH dependence of the second-order rate constant for NO binding (kon), we determined individual rate constants characterizing forms 1-3, revealing only a ca. 10-fold decrease in the NO binding rate on going from 1 (kon(1) = 3.8 × 106 M-1 s-1) to 2 (kon(2) = 4.0 × 105 M-1 s-1) and the inertness of 3. These findings lead to the abandonment of the dissociatively activated mechanism, in which the reaction rate can be directly correlated with the Fe-OH bond energy, as the mechanistic explanation for the process with regard to 2. The reactivity of 2 is accounted for through the existence of a tautomeric equilibrium between the major [(AcMP-11)FeIII(OH)(HisH)] (2a) and minor [(AcMP-11)FeIII(H2O)(His-)] (2b) species, of which the second one is assigned as the NO binding target due to its labile Fe-OH2 bond. The proposed mechanism is further substantiated by quantum-chemical calculations, which confirmed both the significant labilization of the Fe-OH2 bond in the [(AcMP-11)FeIII(H2O)(His-)] tautomer and the feasibility of the tautomer formation, especially after introducing empirical corrections to the computed relative acidities of the H2O and HisH ligands based on the experimental pKa values. It is shown that the "effective lability" of the axial ligand (OH-/H2O) in 2 may be comparable to the lability of the H2O ligand in 1.

High-Pressure Mechanistic Insight into Bioinorganic NO Chemistry

Pressure is one of the most important parameters controlling the kinetics of chemical reactions. The ability to combine high-pressure techniques with time-resolved spectroscopy has provided a powerful tool in the study of reaction mechanisms. This review is focused on the supporting role of high-pressure kinetic and spectroscopic methods in the exploration of nitric oxide bioinorganic chemistry. Nitric oxide and other reactive nitrogen species (RNS) are important biological mediators involved in both physiological and pathological processes. Understanding molecular mechanisms of their interactions with redox-active metal/non-metal centers in biological targets, such as cofactors, prosthetic groups, and proteins, is crucial for the improved therapy of various diseases. The present review is an attempt to demonstrate how the application of high-pressure kinetic and spectroscopic methods can add additional information, thus enabling the mechanistic interpretation of various NO bioinorganic reactions.

Inorganic reaction mechanisms. A personal journey

This review covers highlights of the work performed in the van Eldik group on inorganic reaction mechanisms over the past two decades in the form of a personal journey. Topics that are covered include, from NO to HNO chemistry, peroxide activation in model porphyrin and enzymatic systems, the wonder-world of Ru^III(edta) chemistry, redox chemistry of Ru(iii) complexes, Ru(ii) polypyridyl complexes and their application, relevant physicochemical properties and reaction mechanisms in ionic liquids, and mechanistic insight from computational chemistry. In each of these sections, typical examples of mechanistic studies are presented in reference to related work reported in the literature.

One small step for cytochrome P450 in its catalytic cycle, one giant leap for enzymology

The intermediates operating in the cytochrome P450 catalytic cycle have been investigated for more than half a century, fascinating many enzymologists. Each intermediate has its unique role to carry out diverse oxidations. Natural time course of the catalytic cycle is quite fast, hence, not all of the reactive intermediates could be isolated during physiological catalysis. Different high-valent iron intermediates have been proposed as primary oxidants: the candidates are compound 0 (Cpd 0, [FeOOH][Formula: see text]P450) and compound I (Cpd I, Fe(IV)[Formula: see text]O por[Formula: see text]P450). Among them, the role of Cpd I in hydroxylation is fairly well understood due the discovery of the peroxide shunt. This review endeavors to put the outstanding research efforts conducted to isolate and characterize the intermediates together. In addition to spectral features of each intermediate in the catalytic cycle, the oxidizing powers of Cpd 0 and Cpd I will be discussed along with most recent scientific findings.

Nitrosyl- versus nitroxyl-cobalamin?

The Commentary is in answer to the comment of a reader that objected against the use of the term ‘nitroxylcobalamin’ in two recent reports in JBC from our group. We use this opportunity to explain to the reader where this terminology originated from.

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

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

Thermodynamic characterization of five key kinetic parameters that define neuronal nitric oxide synthase catalysis

NO synthase (NOS) enzymes convert L-arginine to NO in two sequential reactions whose rates (k(cat1) and k(cat2)) are both limited by the rate of ferric heme reduction (k(r)). An enzyme ferric heme-NO complex forms as an immediate product complex and then undergoes either dissociation (at a rate that we denote as k(d)) to release NO in a productive manner, or reduction (k(r)) to form a ferrous heme-NO complex that must react with O2 (at a rate that we denote as k(ox)) in a NO dioxygenase reaction that regenerates the ferric enzyme. The interplay of these five kinetic parameters (k(cat1), k(cat2), k(r), k(d) and k(ox)) determines NOS specific activity, O2 concentration response, and pulsatile versus steady-state NO generation. In the present study, we utilized stopped-flow spectroscopy and single catalytic turnover methods to characterize the individual temperature dependencies of the five kinetic parameters of rat neuronal NOS. We then incorporated the measured kinetic values into computer simulations of the neuronal NOS reaction using a global kinetic model to comprehensively model its temperature-dependent catalytic behaviours. The results obtained provide new mechanistic insights and also reveal that the different temperature dependencies of the five kinetic parameters significantly alter neuronal NOS catalytic behaviours and NO release efficiency as a function of temperature.

Cytochrome P450–catalyzed L-tryptophan nitration in thaxtomin phytotoxin biosynthesis

Thaxtomin phytotoxins produced by plant-pathogenic Streptomyces species contain a nitro group that is essential for phytotoxicity. The N,N’-dimethyldiketopiperazine core of thaxtomins is assembled from L-phenylalanine and L-4-nitrotryptophan by a nonribosomal peptide synthetase and nitric oxide synthase-generated NO is incorporated into the nitro group, but the biosynthesis of the non-proteinogenic amino acid L-4-nitrotryptophan is unclear. Here we report that TxtE, a unique cytochrome P450, catalyzes L-tryptophan nitration using NO and O₂.

Oxidation of methyl and ethyl nitrosamines by cytochrome P450 2E1 and 2B1

Cytochrome P450 (P450) 2E1 is the major enzyme that oxidizes N-nitrosodimethylamine [N,N-dimethylnitrosamine (DMN)], a carcinogen and also a representative of some nitrosamines formed endogenously. Oxidation of DMN by rat or human P450 2E1 to HCHO showed a high apparent intrinsic kinetic deuterium isotope effect (KIE), ≥8. The KIE was not attenuated in noncompetitive intermolecular experiments with rat liver microsomes {(D)V = 12.5; (D)(V/K) = 10.9 [nomenclature of Northrop, D. B. (1982) Methods Enzymol. 87, 607-625]} but was with purified human P450 2E1 [(D)V = 3.3; (D)(V/K) = 3.7], indicating that C-H bond breaking is partially rate-limiting with human P450 2E1. With N-nitrosodiethylamine [N,N-diethylnitrosamine (DEN)], the intrinsic KIE was slightly lower and was not expressed [e.g., (D)(V/K) = 1.2] in noncompetitive intermolecular experiments. The same general pattern of KIEs was also seen in the (D)(V/K) results with DMN and DEN for the minor products resulting from the denitrosation reactions (CH(3)NH(2), CH(3)CH(2)NH(2), and NO(2)(-)). Experiments with deuterated N-nitroso-N-methyl-N-ethylamine demonstrated that the lower KIEs associated with ethyl versus methyl oxidation could be distinguished within a single molecule. P450 2E1 oxidized DMN and DEN to aldehydes and then to the carboxylic acids. No kinetic lags were observed in acid formation; pulse-chase experiments with carrier aldehydes showed only limited equilibration with P450 2E1-bound aldehydes, indicative of processive reactions, as reported for P450 2A6 [Chowdhury, G., et al. (2010) J. Biol. Chem. 285, 8031-8044]. These same features (no lag phase for HCO(2)H formation and a lack of equilibration in pulse-chase assays) were also seen with (rat) P450 2B1, which has a lower catalytic efficiency for DMN oxidation and a larger active site. Thus, the processivity of dialkyl nitrosamine oxidation appears to be shared by a number of P450s.

A complete volume profile for the reversible binding of camphor to cytochrome P450(cam)

The effect of pressure on the kinetics and thermodynamics of the reversible binding of camphor to cytochrome P450(cam) was studied as a function of the K(+) concentration. The determination of the reaction and activation volumes enabled the construction of the first complete volume profile for the reversible binding of camphor to P450(cam). Although the volume profiles constructed for the reactions conducted at low and high K(+) concentrations are rather similar, and both show a drastic volume increase on going from the reactant to the transition state and a relatively small volume change on going from the transition to the product state, the position of the transition state is largely affected by the K(+) concentration in solution. Similarly, the activation volume determined for the dissociation of camphor is influenced by the presence of K(+), which reflects changes in the ease of water entering the active site of camphor-bound P450(cam) that depends on the K(+) concentration. Careful analysis of the components that contribute to the observed volume changes allowed the estimation of the total number of water molecules expelled to the bulk solvent during the binding of camphor to P450(cam) and the subsequent spin transition. The results are discussed in reference to other studies reported in the literature that deal with the kinetics and thermodynamics of the binding of camphor to P450(cam) under various reaction conditions.

A "sliding scale rule" for selectivity among NO, CO, and O₂ by heme protein sensors

Selectivity among NO, CO, and O₂ is crucial for the physiological function of most heme proteins. Although there is a million-fold variation in equilibrium dissociation constants (K(D)), the ratios for NO:CO:O₂ binding stay roughly the same, 1:~10(3):~10(6), when the proximal ligand is a histidine and the distal site is apolar. For these proteins, there is a "sliding scale rule" for plots of log(K(D)) versus ligand type that allows predictions of K(D) values if one or two are missing. The predicted K(D) for binding of O₂to Ns H-NOX coincides with the value determined experimentally at high pressures. Active site hydrogen bond donors break the rule and selectively increase O₂ affinity with little effect on CO and NO binding. Strong field proximal ligands such as thiolate, tyrosinate, and imidazolate exert a "leveling" effect on ligand binding affinity. The reported picomolar K(D) for binding of NO to sGC deviates even more dramatically from the sliding scale rule, showing a NO:CO K(D) ratio of 1:~10(8). This deviation is explained by a complex, multistep process, in which an initial low-affinity hexacoordinate NO complex with a measured K(D) of ≈54 nM, matching that predicted from the sliding scale rule, is formed initially and then is converted to a high-affinity pentacoordinate complex. This multistep six-coordinate to five-coordinate mechanism appears to be common to all NO sensors that exclude O₂ binding to capture a lower level of cellular NO and prevent its consumption by dioxygenation.

Mechanistic studies on the reactions of cyanide with a water-soluble Fe(III) porphyrin and their effect on the binding of NO

The reaction of the water-soluble Fe(III)(TMPS) porphyrin with CN(-) in basic solution leads to the stepwise formation of Fe(III)(TMPS)(CN)(H(2)O) and Fe(III)(TMPS)(CN)(2). The kinetics of the reaction of CN(-) with Fe(III)(TMPS)(CN)(H(2)O) was studied as a function of temperature and pressure. The positive value of the activation volume for the formation of Fe(III)(TMPS)(CN)(2) is consistent with the operation of a dissociatively activated mechanism and confirms the six-coordinate nature of the monocyano complex. A good agreement between the rate constants at pH 8 and 9 for the formation of the dicyano complex implies the presence of water in the axial position trans to coordinated cyanide in the monocyano complex and eliminates the existence of Fe(III)(TMPS)(CN)(OH) under the selected reaction conditions. Both Fe(III)(TMPS)(CN)(H(2)O) and Fe(III)(TMPS)(CN)(2) bind nitric oxide (NO) to form the same nitrosyl complex, namely, Fe(II)(TMPS)(CN)(NO(+)). Kinetic studies indicate that nitrosylation of Fe(III)(TMPS)(CN)(2) follows a limiting dissociative mechanism that is supported by the independence of the observed rate constant on [NO] at an appropriately high excess of NO, and the positive values of both the activation parameters ΔS(‡) and ΔV(‡) found for the reaction under such conditions. The relatively small first-order rate constant for NO binding, namely, (1.54 ± 0.01) × 10(-2) s(-1), correlates with the rate constant for CN(-) release from the Fe(III)(TMPS)(CN)(2) complex, namely, (1.3 ± 0.2) × 10(-2) s(-1) at 20 °C, and supports the proposed nitrosylation mechanism.

Interactions of cytochrome P450s with their ligands

Cytochrome P450s (CYPs) are heme-containing monooxygenases that contribute to an enormous range of enzymatic function including biosynthetic and detoxification roles. This review summarizes recent studies concerning interactions of CYPs with ligands including substrates, inhibitors, and diatomic heme-ligating molecules. These studies highlight the complexity in the relationship between the heme spin state and active site occupancy, the roles of water in directing protein-ligand and ligand-heme interactions, and the details of interactions between heme and gaseous diatomic CYP ligands. Both kinetic and thermodynamic aspects of ligand binding are considered.

Dynamic hydrogen-bonding network in the distal pocket of the nitrosyl complex of Pseudomonas aeruginosa cd1 nitrite reductase

cd(1) nitrite reductase (NIR) is a key enzyme in the denitrification process that reduces nitrite to nitric oxide (NO). It contains a specialized d(1)-heme cofactor, found only in this class of enzymes, where the substrate, nitrite, binds and is converted to NO. For a long time, it was believed that NO must be released from the ferric d(1)-heme to avoid enzyme inhibition by the formation of ferrous-nitroso complex, which was considered as a dead-end product. However, recently an enhanced rate of NO dissociation from the ferrous form, not observed in standard b-type hemes, has been reported and attributed to the unique d(1)-heme structure (Rinaldo, S.; Arcovito, A.; Brunori, M.; Cutruzzolà, F. J. Biol. Chem. 2007, 282, 14761-14767). Here, we report on a detailed study of the spatial and electronic structure of the ferrous d(1)-heme NO complex from Pseudomonas aeruginosa cd(1) NIR and two mutants Y10F and H369A/H327A in solution, searching for the unique properties that are responsible for the relatively fast release. There are three residues at the "distal" side of the heme (Tyr(10), His(327), and His(369)), and in this work we focus on the identification and characterization of possible H-bonds they can form with the NO, thereby affecting the stability of the complex. For this purpose, we have used high field pulse electron-nuclear double resonance (ENDOR) combined with density functional theory (DFT) calculations. The DFT calculations were essential for assigning and interpreting the ENDOR spectra in terms of geometric structure. We have shown that the NO in the nitrosyl d(1)-heme complex of cd(1) NIR forms H-bonds with Tyr(10) and His(369), whereas the second conserved histidine, His(327), appears to be less involved in NO H-bonding. This is in contrast to the crystal structure that shows that Tyr(10) is removed from the NO. We have also observed a larger solvent accessibility to the distal pocket in the mutants as compared to the wild-type. Moreover, it was shown that the H-bonding network within the active site is dynamic and that a change in the protonation state of one of the residues does affect the strength and position of the H-bonds formed by the others. In the Y10F mutant, His(369) is closer to the NO, whereas mutation of both distal histidines displaces Tyr(10), removing its H-bond. The implications of the H-bonding network found in terms of the complex stability and catalysis are discussed.

Reaction of Mycobacterium tuberculosis cytochrome P450 enzymes with nitric oxide

During the initial growth infection stage of Mycobacterium tuberculosis (Mtb), (*)NO produced by host macrophages inhibits heme-containing terminal cytochrome oxidases, inactivates iron/sulfur proteins, and promotes entry into latency. Here we evaluate the potential of (*)NO as an inhibitor of Mtb cytochrome P450 enzymes, as represented by CYP130, CYP51, and the two previously uncharacterized enzymes CYP125 and CYP142. Using UV-visible absorption, resonance Raman, and stopped-flow spectroscopy, we investigated the reactions of (*)NO with these heme proteins in their ferric resting form. (*)NO coordinates tightly to CYP125 and CYP142 (submicromolar) and with a lower affinity (micromolar) to CYP130 and CYP51. Anaerobic reduction of the ferric-NO species with sodium dithionite led to the formation of two spectrally distinct classes of five-coordinate ferrous-NO complexes. Exposure of these species to O(2) revealed that the ferrous-NO forms of CYP125 and CYP142 are labile and convert back to the ferric state within a few minutes, whereas ferrous CYP130 and CYP51 bind (*)NO almost irreversibly. This work clearly indicates that, at physiological concentrations (approximately 1 microM), (*)NO would impair the activity of CYP130 and CYP51, whereas CYP125 and CYP142 are more resistant. Selective P450 inhibition may contribute to the inhibitory effects of (*)NO on Mtb growth.

Application of high pressure laser flash photolysis in studies on selected hemoprotein reactions

This article focuses on the application of high pressure laser flash photolysis for studies on selected hemoprotein reactions with the objective to establish details of the underlying reaction mechanisms. In this context, particular attention is given to the reactions of small molecules such as dioxygen, carbon monoxide, and nitric oxide with selected hemoproteins (hemoglobin, myoglobin, neuroglobin and cytochrome P450(cam)), as well as to photo-induced electron transfer reactions occurring in hemoproteins (particularly in various types of cytochromes). Mechanistic conclusions based on the interpretation of the obtained activation volumes are discussed in this account.

Unravelling the intrinsic features of NO binding to iron(II)- and iron(III)-hemes

Electrospray ionization of appropriate precursors is used to deliver [Fe (III)-heme] (+) and [Fe (II)-hemeH] (+) ions as naked species in the gas phase where their ion chemistry has been examined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. In the naked, four-coordinate [Fe (II)-hemeH] (+) and [Fe (III)-heme] (+) ions, the intrinsic reactivity of iron(II)- and iron(III)-hemes is revealed free from any influence due to axial ligand, counterion, or solvent effects. Ligand (L) addition and ligand transfer equilibria with a series of selected neutrals are attained when [Fe (II)-hemeH] (+), corresponding to protonated Fe (II)-heme, is allowed to react in the FT-ICR cell. A Heme Cation Basicity (HCB) ladder for the various ligands toward [Fe (II)-hemeH] (+), corresponding to -Delta G degrees for the process [Fe (II)-hemeH] (+) + L --> [Fe (II)-hemeH(L)] (+) and named HCB (II), can thus be established. The so-obtained HCB (II) values are compared with the corresponding HCB (III) values for [Fe (III)-heme] (+). In spite of pronounced differences displayed by various ligands, NO shows a quite similar HCB of about 67 kJ mol (-1) at 300 K toward both ions, estimated to correspond to a binding energy of 124 kJ mol (-1). Density Functional Theory (DFT) computations confirm the experimental results, yielding very similar values of NO binding energies to [Fe (II)-hemeH] (+) and [Fe (III)-heme] (+), equal to 140 and 144 kJ mol (-1), respectively. The kinetic study of the NO association reaction supports the equilibrium HCB data and reveals that the two species share very close rate constant values both for the forward and for the reverse reaction. These gas phase results diverge markedly from the kinetics and thermodynamic behavior of NO binding to iron(II)- and iron(III)-heme proteins and model complexes in solution. The requisite of either a very labile or a vacant coordination site on iron for a facile addition of NO to occur, suggested to explain the bias for typically five-coordinate iron(II) species in solution, is fully supported by the present work.

Influence of an Extremely Negatively Charged Porphyrin on the Reversible Binding Kinetics of NO to Fe(III) and the Subsequent Reductive Nitrosylation

The polyanionic, water-soluble, and non-micro-oxo dimer-forming iron porphyrin (hexadecasodium iron 54,104,154,204-tetra-t-butyl-52,56,102,106,152,156,202,206-octakis[2,2-bis(carboxylato)ethyl]-5,10,15,20-tetraphenylporphyrin), (P16-)FeIII, with 16 negatively charged meso substituents on the porphyrin was synthesized and fully characterized by UV-vis and 1H NMR spectroscopy. A single pKa1 value of 9.90 +/- 0.01 was determined for the deprotonation of coordinated water in the six-coordinate (P16-)FeIII(H2O)2 and as attributed to the formation of the five-coordinate monohydroxo-ligated form, (P16-)FeIII(OH). The porphyrin complex reversibly binds NO in aqueous solution to yield the nitric oxide adduct, (P16-)FeII(NO+)(L), where L = H2O or OH-. The kinetics for the reversible binding of NO were studied as a function of pH, temperature, and pressure using the stopped-flow technique. The data for the binding of NO to the diaqua complex are consistent with the operation of a dissociative mechanism on the basis of the significantly positive values of DeltaS and DeltaV, whereas the monohydroxo complex favors an associatively activated mechanism as determined from the corresponding negative activation parameters. The rate constant, kon = 3.1 x 104 M-1 s-1 at 25 degrees C, determined for the NO binding to (P16-)FeIII(OH) at higher pH, is significantly lower than the corresponding value measured for (P16-)FeIII(H2O)2 at lower pH, namely, kon = 11.3 x 105 M-1 s-1 at 25 degrees C. This decrease in the reactivity is analogous to that reported for other diaqua- and monohydroxo-ligated ferric porphyrin complexes, and is accounted for in terms of a mechanistic changeover observed for (P16-)FeIII(H2O)2 and (P16-)FeIII(OH). The formed nitrosyl complex, (P16-)FeII(NO+)(H2O), undergoes subsequent reductive nitrosylation to produce (P16-)FeII(NO), which is catalyzed by nitrite produced during the reaction. Concentration-, pH-, temperature-, and pressure-dependent kinetic data are reported for this reaction. Data for the reversible binding of NO and the subsequent reductive nitrosylation reaction are discussed in reference to that available for other iron(III) porphyrins in terms of the influence of the porphyrin periphery.

Thermodynamic and Kinetic Studies on the Binding of Nitric Oxide to a New Enzyme Mimic of Cytochrome P450

A new model for the P450 enzyme carrying a SO(3)(-) ligand coordinated to iron(III) (complex 2) reversibly binds NO to yield the nitrosyl adduct. The rate constant for NO binding to 2 in toluene is of the same order of magnitude as that found for the nitrosylation of the native, substrate-bound form of P450(cam) (E.S-P450(cam)). Large and negative activation entropy and activation volume values for the binding of NO to complex 2 support a mechanism that is dominated by bond formation with concomitant iron spin change from S = (5)/(2) to S = 0, as proposed for the reaction between NO and E.S-P450(cam). In contrast, the dissociation of NO from 2(NO) was found to be several orders of magnitude faster than the corresponding reaction for the E.S-P450(cam)/NO system. In a coordinating solvent such as methanol, the alcohol coordinates to iron(III) of 2 at the distal position, generating a six-coordinate, high-spin species 5. The reaction of NO with 5 in methanol was found to be much slower in comparison to the nitrosylation reaction of 2 in toluene. This behavior can be explained in terms of a mechanism in which methanol must be displaced during Fe-NO bond formation. The thermodynamic and kinetic data for NO binding to the new model complexes of P450 (2 and 5) are discussed in reference to earlier results obtained for closely related nitrosylation reactions of cytochrome P450(cam) (in the presence and in the absence of the substrate) and a thiolate-ligated iron(III) model complex.

Surface plasmon resonance analysis of antifungal azoles binding to CYP3A4 with kinetic resolution of multiple binding orientations

The heme-containing cytochrome P450s (CYPs) are a major enzymatic determinant of drug clearance and drug-drug interactions. The CYP3A4 isoform is inhibited by antifungal imidazoles or triazoles, which form low-spin heme iron complexes via formation of a nitrogen-ferric iron coordinate bond. However, CYP3A4 also slowly oxidizes the antifungal itraconazole (ITZ) at a site that is approximately 25 A from the triazole nitrogens, suggesting that large antifungal azoles can adopt multiple orientations within the CYP3A4 active site. Here, we report a surface plasmon resonance (SPR) analysis with kinetic resolution of two binding modes of ITZ, and the related drug ketoconazole (KTZ). SPR reveals a very slow off-rate for one binding orientation. Multiphasic binding kinetics are observed, and one of the two binding components resolved by curve fitting exhibits "equilibrium overshoot". Preloading of CYP3A4 with the heme ligand imidazole abolishes this component of the antifungal azole binding trajectories, and it eliminates the conspicuously slow off-rate. The fractional populations of CYP3A4 complexes corresponding to different drug orientations can be manipulated by altering the duration of the pulse of drug exposure. UV-vis difference absorbance titrations yield low-spin spectra and K(D) values that are consistent with the high-affinity complex resolved by SPR. These results demonstrate that ITZ and KTZ bind in multiple orientations, including a catalytically productive mode and a slowly dissociating inhibitory mode. Most importantly, they provide the first example of a SPR-based method for the kinetic characterization of binding of a drug to any human CYP, including mechanistic insight not available from other methods.

The ferrous-dioxygen intermediate in human cytochrome P450 3A4. Substrate dependence of formation and decay kinetics

The oxy-ferrous complex is the first of three branching intermediates in the catalytic cycle of cytochrome P450, in which the total efficiency of substrate turnover is curtailed by the side reaction of autoxidation. For human membrane-bound cytochromes P450, the oxy complex is believed to be the primary source of cytotoxic superoxide and peroxide, although information on the properties and stability of this intermediate is lacking. Here we document stopped-flow spectroscopic studies of the formation and decay of the oxy-ferrous complex in the most abundant human cytochrome P450 (CYP3A4) as a function of temperature in the substrate-free and substrate-bound form. CYP3A4 solubilized in purified monomeric form in nanoscale POPC bilayers is functionally and kinetically homogeneous. In substrate-free CYP3A4, the oxy complex is extremely unstable with a half-life of approximately 30 ms at 5 degrees C. Saturation with testosterone or bromocriptine stabilizes the oxy-ferrous intermediate. Comparison of the autoxidation rates with the available data on CYP3A4 turnover kinetics suggests that the oxy complex may be an important route for uncoupling.

Transition State Characterization for the Reversible Binding of Dihydrogen to Bis(2,2'-bipyridine)rhodium(I) from Temperature- and Pressure-Dependent Experimental and Theoretical Studies

Thermodynamic and kinetic parameters for the oxidative addition of H2 to [Rh(I)(bpy)2]+ (bpy = 2,2'-bipyridine) to form [Rh(III)(H)2(bpy)2]+ were determined from either the UV-vis spectrum of equilibrium mixtures of [Rh(I)(bpy)2]+ and [Rh(III)(H)2(bpy)2]+ or from the observed rates of dihydride formation following visible-light irradiation of solutions containing [Rh(III)(H)2(bpy)2]+ as a function of H2 concentration, temperature, and pressure in acetone and methanol. The activation enthalpy and entropy in methanol are 10.0 kcal mol(-1) and -18 cal mol(-1) K(-1), respectively. The reaction enthalpy and entropy are -10.3 kcal mol(-1) and -19 cal mol(-1) K(-1), respectively. Similar values were obtained in acetone. Surprisingly, the volumes of activation for dihydride formation (-15 and -16 cm(3) mol(-1) in methanol and acetone, respectively) are very close to the overall reaction volumes (-15 cm(3) mol(-1) in both solvents). Thus, the volumes of activation for the reverse reaction, elimination of dihydrogen from the dihydrido complex, are approximately zero. B3LYP hybrid DFT calculations of the transition-state complex in methanol and similar MP2 calculations in the gas phase suggest that the dihydrogen has a short H-H bond (0.823 and 0.810 Angstroms, respectively) and forms only a weak Rh-H bond (1.866 and 1.915 Angstroms, respectively). Equal partial molar volumes of the dihydrogenrhodium(I) transition state and dihydridorhodium(III) can account for the experimental volume profile found for the overall process.

A Comparative Mechanistic Study of the Reversible Binding of NO to a Water-Soluble Octa-Cationic FeIII Porphyrin Complex

The water-soluble, non-mu-oxo dimer-forming porphyrin, [5,10,15,20-tetrakis-4'-t-butylphenyl-2',6'-bis-(N-methylene-(4''-t-butylpyridinium))porphyrinato]iron(III) octabromide, (P(8+))Fe(III), with eight positively charged substituents in the ortho positions of the phenyl rings, was characterized by UV-vis and 1H NMR spectroscopy and 17O NMR water-exchange studies in aqueous solution. Spectrophotometric titrations of (P(8+))Fe(III) indicated a pKa1 value of 5.0 for coordinated water in (P(8+))Fe(III)(H2O)2. The monohydroxo-ligated (P(8+))Fe(III)(OH)(H2O) formed at 5 < pH < 12 has a weakly bound water molecule that undergoes an exchange reaction, k(ex) = 2.4 x 10(6) s(-1), significantly faster than water exchange on (P(8+))Fe(III)(H2O)2, viz. k(ex) = 5.5 x 10(4) s(-1) at 25 degrees C. The porphyrin complex reacts with nitric oxide to yield the nitrosyl adduct, (P(8+))Fe(II)(NO+)(L) (L = H2O or OH-). The diaqua-ligated (P(8+))Fe(III)(H2O)2 binds and releases NO according to a dissociatively activated mechanism, analogous to that reported earlier for other (P)Fe(III)(H2O)2 complexes. Coordination of NO to (P(8+))Fe(III)(OH)(H2O) at high pH follows an associative mode, as evidenced by negative deltaS(double dagger)(on) and deltaV(double dagger)(on) values measured for this reaction. The observed ca. 10-fold decrease in the NO binding rate on going from six-coordinate (P(8+))Fe(III)(H2O)2 (k(on) = 15.1 x 10(3) M(-1) s(-1)) to (P(8+))Fe(III)(OH)(H2O) (k(on) = 1.56 x 10(3) M(-1) s(-1) at 25 degrees C) is ascribed to the different nature of the rate-limiting step for NO binding at low and high pH, respectively. The results are compared with data reported for other water-soluble iron(III) porphyrins with positively and negatively charged meso substituents. Influence of the porphyrin periphery on the dynamics of reversible NO binding to these (P)Fe(III) complexes as a function of pH is discussed on the basis of available experimental data.

Conformational transitions and redox potential shifts of cytochrome P450 induced by immobilization

Cytochrome P450 (P450) from Pseudomonas putida was immobilized on Ag electrodes coated with self-assembled monolayers (SAMs) via electrostatic and hydrophobic interactions as well as by covalent cross-linking. The redox and conformational equilibria of the immobilized protein were studied by potential-dependent surface-enhanced resonance Raman spectroscopy. All immobilization conditions lead to the formation of the cytochrome P420 (P420) form of the enzyme. The redox potential of the electrostatically adsorbed P420 is significantly more positive than in solution and shows a steady downshift upon shortening of the length of the carboxyl-terminated SAMs, i.e., upon increasing the strength of the local electric field. Thus, two opposing effects modulate the redox potential of the adsorbed enzyme. First, the increased hydrophobicity of the heme environment brought about by immobilization on the SAM tends to upshift the redox potential by stabilizing the formally neutral ferrous form. Second, increasing electric fields tend to stabilize the positively charged ferric form, producing the opposite effect. The results provide insight into the parameters that control the structure and redox properties of heme proteins and contribute to the understanding of the apparently anomalous behavior of P450 enzymes in bioelectronic devices.

Kinetic and Mechanistic Studies on the Reaction of Nitric Oxide with a Water-Soluble Octa-anionic Iron(III) Porphyrin Complex

The polyanionic water-soluble and non-mu-oxo-dimer-forming iron porphyrin iron(III) 5(4),10(4),15(4),20(4)-tetra-tert-butyl-5(2),5(6),15(2),15(6)-tetrakis[2,2-bis(carboxylato)ethyl]-5,10,15,20-tetraphenylporphyrin, (P(8-))Fe(III) (1), was synthesized as an octasodium salt by applying well-established porphyrin and organic chemistry procedures to bromomethylated precursor porphyrins and characterized by standard techniques such as UV-vis and (1)H NMR spectroscopy. A single pK(a1) value of 9.26 was determined for the deprotonation of coordinated water in (P(8-))Fe(III)(H(2)O)(2) (1-H(2)()O) present in aqueous solution at pH <9. The porphyrin complex reversibly binds NO in aqueous solution to give the mononitrosyl adduct, (P(8-))Fe(II)(NO(+))(L), where L = H(2)O or OH(-). The kinetics of the binding and release of NO was studied as a function of pH, temperature, and pressure by stopped-flow and laser flash photolysis techniques. The diaqua-ligated form of the porphyrin complex binds and releases NO according to a dissociative interchange mechanism based on the positive values of the activation parameters DeltaS() and DeltaV() for the "on" and "off" reactions. The rate constant k(on) = 6.2 x 10(4) M(-1) s(-1) (24 degrees C), determined for NO binding to the monohydroxo-ligated (P(8-))Fe(III)(OH) (1-OH) present in solution at pH >9, is markedly lower than the corresponding value measured for 1-H(2)O at lower pH (k(on) = 8.2 x 10(5) M(-1) s(-1), 24 degrees C, pH 7). The observed decrease in the reactivity is contradictory to that expected for the diaqua- and monohydroxo-ligated forms of the iron(III) complex and is accounted for in terms of a mechanistic changeover observed for 1-H(2)O and 1-OH in their reactions with NO. The mechanistic interpretation offered is further substantiated by the results of water-exchange studies performed on the polyanionic porphyrin complex as a function of pH, temperature, and pressure.

pH Controls the Rate and Mechanism of Nitrosylation of Water-Soluble FeIII Porphyrin Complexes

In-depth kinetic and mechanistic studies on the reversible binding of NO to water-soluble iron(III) porphyrins as a function of pH revealed unexpected reaction kinetics for monohydroxo-ligated (P)Fe(III)(OH) species formed by deprotonation of coordinated water in diaqua-ligated (P)Fe(III)(H(2)O)(2). The observed significant decrease in the rate of NO binding to (P)Fe(OH) as compared to that of (P)Fe(H(2)O)(2) does not conform with expectations based on previous mechanistic work on NO-heme interactions, which would point to a diffusion-limited reaction for the five-coordinate Fe(III) center in (P)Fe(OH). The decrease in rate and an associatively activated mode of NO binding observed at high pH is ascribed to an increase in the activation barrier related to spin state and structural changes accompanying NO coordination to the high-spin (P)Fe(III)(OH) complex. The existence of such a barrier has previously been observed in the reactions of five-coordinate iron(II) hemes with CO and is evidenced for the first time for the process involving coordination of NO to the iron heme complex. The observed reactivity pattern, relevant in the context of studies on NO interactions with synthetic and biologically important hemes (in particular, hemoproteins), is reported here for an example of a simple water-soluble iron(III) porphyrin [meso-tetrakis(sulfonatomesityl)porphinato]-iron(III), (TMPS)Fe(III).

Mechanistic Studies on the Binding of Nitric Oxide to a Synthetic Heme−Thiolate Complex Relevant to Cytochrome P450

The synthetic heme-thiolate complex (SR) in methanol binds nitric oxide (k(on) = (2.7 +/- 0.2) x10(6) M(-)(1) s(-)(1) at 25 degrees C) to form SR(NO). The binding of NO to the SR complex in a noncoordinating solvent, such as toluene, was found to be almost 3 orders of magnitude faster than that in methanol. The activation parameters DeltaH(), DeltaS(), and DeltaV() for the formation of SR(NO) in methanol are consistent with the operation of a limiting dissociative mechanism, dominated by dissociation of methanol in SR(MeOH). In the presence of an excess of NO, the formation of SR(NO) is followed by subsequent slower reactions. The substantially negative activation entropy and activation volume values found for the second observed reaction step support an associative mechanism which involves attack of a second NO molecule on the thiolate ligand in the initially formed SR(NO) complex. The following slower reactions are strongly accelerated by a large excess of NO or by the presence of NO(2)(-) in the SR/NO reaction mixture. They can be accounted for in terms of dynamic equilibria between higher nitrogen oxides (NO(x)()) and reactive SR species, which lead to the formation of a nitrosyl-nitrite complex of SR(Fe(II)) as the final product. This finding is clearly supported by laser flash photolysis studies on the SR/NO reaction mixture, which do not reveal simple NO photolabilization from SR(Fe(III))(NO), but rather involve the generation of at least three photoinduced intermediates decaying with different rate constants to the starting material. The species formed along the proposed reaction pathways were characterized by FTIR and EPR spectroscopy. The results are discussed in terms of their relevance for the biological function of cytochrome P450 enzymes and in context of results for the reaction of NO with imidazole- and thiolate-ligated iron(III) hemoproteins.