Dioxygen-activating iron center in putidamonooxin. Electron spin resonance investigation of the nitrosylated putidamonooxin

Reactivity of Thiolate and Hydrosulfide with a Mononuclear {FeNO}7 Complex Featuring a Very High N-O Stretching Frequency

Synthesis, characterization, electronic structure, and redox reactions of a mononuclear {FeNO}7 complex with a very high N-O stretching frequency in solution are presented. Nitrosylation of [(LKP)Fe(DMF)]2+ (1) (LKP = tris((1-methyl-4,5-diphenyl-1H-imidazol-2-yl)methyl)amine) produced a five-coordinate {FeNO}7 complex, [(LKP)Fe(NO)]2+ (2). While complex 2 could accommodate an additional water molecule to generate a six-coordinate {FeNO}7 complex, [(LKP)Fe(NO)(H2O)]2+ (3), the coordinated H2O in 3 dissociates to generate 2 in solution. The molecular structure of 2 features a nearly linear Fe-N-O unit with an Fe-N distance of 1.744(4) Å, N-O distance of 1.162(5) Å, and Collapse

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Affiliation(s)

Soumik Karmakar School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
Suman Patra School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
Koushik Pramanik School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
Amit Adhikary Department of Chemistry, Technology Campus, University of Calcutta, JD Block, Sector III, Salt Lake, Kolkata 700098, India
Abhishek Dey School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
Amit Majumdar School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India

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Rate-Determining Attack on Substrate Precedes Rieske Cluster Oxidation during Cis-Dihydroxylation by Benzoate Dioxygenase

Rieske dearomatizing dioxygenases utilize a Rieske iron-sulfur cluster and a mononuclear Fe(II) located 15 Å across a subunit boundary to catalyze O2-dependent formation of cis-dihydrodiol products from aromatic substrates. During catalysis, O2 binds to the Fe(II) while the substrate binds nearby. Single-turnover reactions have shown that one electron from each metal center is required for catalysis. This finding suggested that the reactive intermediate is Fe(III)-(H)peroxo or HO-Fe(V)═O formed by O-O bond scission. Surprisingly, several kinetic phases were observed during the single-turnover Rieske cluster oxidation. Here, the Rieske cluster oxidation and product formation steps of a single turnover of benzoate 1,2-dioxygenase are investigated using benzoate and three fluorinated analogues. It is shown that the rate constant for product formation correlates with the reciprocal relaxation time of only the fastest kinetic phase (RRT-1) for each substrate, suggesting that the slower phases are not mechanistically relevant. RRT-1 is strongly dependent on substrate type, suggesting a role for substrate in electron transfer from the Rieske cluster to the mononuclear iron site. This insight, together with the substrate and O2 concentration dependencies of RRT-1, indicates that a reactive species is formed after substrate and O2 binding but before electron transfer from the Rieske cluster. Computational studies show that RRT-1 is correlated with the electron density at the substrate carbon closest to the Fe(II), consistent with initial electrophilic attack by an Fe(III)-superoxo intermediate. The resulting Fe(III)-peroxo-aryl radical species would then readily accept an electron from the Rieske cluster to complete the cis-dihydroxylation reaction.

Photolability of NO in designed metal nitrosyls with carboxamido-N donors: a theoretical attempt to unravel the mechanism

During the past few years, photoactive metal nitrosyls (NO complexes of metals) have drawn attention as potential drugs for delivery of nitric oxide (NO) to biological targets under the control of light. Major success in this area has been achieved with designed metal nitrosyls derived from ligands that contain carboxamide group(s). A number of iron, manganese and ruthenium {MNO}(6) nitrosyls of such kind exhibit excellent NO photolability under low-power visible and near-IR light. The results of theoretical studies on these NO-donors have provided insight into (a) the electronic transitions that lead to photorelease of NO and (b) the structural features of the ligands that dictate the sensitivity of the nitrosyls to light of specific wavelengths. In addition, the results have afforded clear understanding of the electronic configurations of the various nitrosyls. This article highlights these results in a coherent manner. Good matches between the predicted and observed spectral features and NO photolability strongly suggest that theoretical studies should be an integral part of the smart design of such NO-donors in the future research.

Models and mechanisms of O-O bond activation by cytochrome P450. A critical assessment of the potential role of multiple active intermediates in oxidative catalysis

Cytochrome P450 enzymes promote a number of oxidative biotransformations including the hydroxylation of unactivated hydrocarbons. Whereas the long-standing consensus view of the P450 mechanism implicates a high-valent iron-oxene species as the predominant oxidant in the radicalar hydrogen abstraction/oxygen rebound pathway, more recent studies on isotope partitioning, product rearrangements with 'radical clocks', and the impact of threonine mutagenesis in P450s on hydroxylation rates support the notion of the nucleophilic and/or electrophilic (hydro)peroxo-iron intermediate(s) to be operative in P450 catalysis in addition to the electrophilic oxenoid-iron entity; this may contribute to the remarkable versatility of P450s in substrate modification. Precedent to this mechanistic concept is given by studies with natural and synthetic P450 biomimics. While the concept of an alternative electrophilic oxidant necessitates C-H hydroxylation to be brought about by a cationic insertion process, recent calculations employing density functional theory favour a 'two-state reactivity' scenario, implicating the usual ferryl-dependent oxygen rebound pathway to proceed via two spin states (doublet and quartet); state crossing is thought to be associated with either an insertion or a radicalar mechanism. Hence, challenge to future strategies should be to fold the disparate and sometimes contradictory data into a harmonized overall picture.

Iron nitrosyls of a pentadentate ligand containing a single carboxamide group: syntheses, structures, electronic properties, and photolability of NO

Three iron complexes of a pentadentate ligand N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide (PaPy(3)H, H is the dissociable amide proton) have been synthesized. All three species, namely, two nitrosyls [(PaPy(3))Fe(NO)](ClO(4))(2) (2) and [(PaPy(3))Fe(NO)](ClO(4)) (3) and one nitro complex [(PaPy(3))Fe(NO(2))](ClO(4)) (4), have been structurally characterized. These complexes provide the opportunity to compare the structural and spectral properties of a set of isostructural [Fe-NO](6,7) complexes (2 and 3, respectively) and an analogous genuine Fe(III) complex with an "innocent" sixth ligand ([(PaPy(3))Fe(NO(2))](ClO(4)), 4). The most striking difference in the structural features of 2 and 3 is the Fe-N-O angle (Fe-N-O = 173.1(2) degrees in the case of 2 and 141.29(15) degrees in the case of 3). The clean (1)H NMR spectrum of 2 in CD(3)CN reveals its S = 0 ground state and confirms its [Fe-NO](6) configuration. The binding of NO at the non-heme iron center in 2 is completely reversible and the bound NO is photolabile. Mössbauer data, electron paramagnetic resonance signal at g approximately 2.00, and variable temperature magnetic susceptibility measurements indicate the S = (1)/(2) spin state of the [Fe-NO](7) complex 3. Analysis of the spectroscopic data suggests Fe(II)-NO(+) and Fe(II)-NO(*) formulations for 2 and 3, respectively. The bound NO in 3 does not show any photolability. However, in MeCN solution, it reacts rapidly with dioxygen to afford the nitro complex 4, which has also been synthesized independently from [(PaPy(3))Fe(MeCN)](2+) and NO(2)(-). Nucleophilic attack of hydroxide ion to the N atom of the NO ligand in 2 in MeCN in the dark gives rise to 4 in high yield.

Spectroscopic and computational study of a non-heme iron [Fe-NO]7 system: exploring the geometric and electronic structures of the nitrosyl adduct of iron superoxide dismutase

Like many non-heme iron enzymes, reduced iron superoxide dismutase (Fe(2+)SOD) reacts with nitric oxide (NO) to yield an [Fe-NO]7 system. Electron paramagnetic resonance (EPR) data obtained for this Fe-NO adduct of FeSOD (NO-FeSOD) exhibit two rhombic S = 3/2 signals of comparable population; E/D = 0.128 (42%) and 0.154 (58%). While similar results were previously reported for NO-FeSOD [Niederhoffer, E. C.; Fee, J. A.; Papaefthymiou, V.; Münck, E. Magnetic Resonance Studies Involving Iron Superoxide Dismutase from Escherichia coli. Isotope and Nuclear Chemistry Division Annual Report; Los Alamos National Laboratory: Los Alamos, NM, 1987], detailed geometric and electronic structure descriptions of these [Fe-NO]7 systems had not yet been developed. Therefore, in addition to EPR spectroscopy, we have used electronic absorption, magnetic circular dichroism (MCD), variable-temperature, variable-field MCD, and resonance Raman spectroscopies to determine ground-state spin Hamiltonian parameters, electronic transition energies, oscillator strengths, and transition polarizations for NO-FeSOD. These spectroscopic parameters have been used in conjunction with density functional theory (DFT) and semiempirical INDO/S-CI calculations to generate an experimentally calibrated active site model for NO-FeSOD. Our studies indicate that NO binds to the active site of Fe(2+)SOD to form a six-coordinate [Fe-NO]7 system with an Fe-N-O angle of approximately 145 degrees. DFT computations performed on this model of NO-FeSOD reveal that the NO ligand is formally reduced by the ferrous center to yield NO(-) and an Fe(3+) center that are strongly antiferromagnetically coupled. DFT calculations reveal that NO binding to Fe(2+)SOD also lowers the pK of the coordinated water ligand by at least 3.3 pH units, suggesting that this process is associated with increased acidity and probable ionization of the axial solvent ligand. To explore the origin of the two [Fe-NO]7 systems observed by EPR spectroscopy, additional calculations have been performed on slightly perturbed NO-FeSOD models. Significantly, semiempirical INDO/S-CI computations reveal that the rhombicity of NO-FeSOD is altered by changes in the Fe-N-O angle or rotation about the Fe-N(O) bond, suggesting that the two species observed by EPR spectroscopy merely differ slightly with respect to the orientation of the NO ligand. Indeed, our EPR data obtained on NO-FeSOD variants indicate that the relative population of the S = 3/2 signals can be altered by perturbations in the second sphere of the protein active site. These results provide compelling evidence that the second coordination sphere is able to modulate the geometric and electronic structures of NO-FeSOD.

Nitric oxide binding at the mononuclear active site of reduced Pyrococcus furiosus superoxide reductase

Nitric oxide (NO) has been used as a substrate analog to explore the structural and electronic determinants of enzymatic superoxide reduction at the mononuclear iron active site of Pyrococcus furiosus superoxide reductase (SOR) through the use of EPR, resonance Raman, Fourier transform IR, UV-visible absorption, and variable-temperature variable-field magnetic CD spectroscopies. The NO adduct of reduced SOR is shown to have a near-axial S = 32 ground state with ED = 0.06 and D = 12 +/- 2 cm(-1) (where D and E are the axial and rhombic zero-field splitting parameters, respectively) and the UV-visible absorption and magnetic CD spectra are dominated by an out-of-plane NO(-)(pi*)-to-Fe(3+)(dpi) charge-transfer transition, polarized along the zero-field splitting axis. Resonance Raman studies indicate that the NO adduct is six-coordinate with NO ligated in a bent conformation trans to the cysteinyl S, as evidenced by the identification of nu(N-O) at 1,721 cm(-1), nu(Fe-NO) at 475 cm(-1), and nu(Fe-S(Cys), at 291 cm(-1), via (34)S and (15)NO isotope shifts. The electronic and vibrational properties of the S = 32 (FeNO)(7) unit are rationalized in terms of a limiting formulation involving a high-spin (S = 52) Fe(3+) center antiferromagnetically coupled to a (S = 1) NO(-) anion, with a highly covalent Fe(3+)-NO(-) interaction. The results support a catalytic mechanism for SOR, with the first step involving oxidative addition of superoxide to form a ferric-peroxo intermediate, and indicate the important roles that the Fe spin state and the trans cysteinate ligand play in effecting superoxide reduction and peroxide release.

Benzoate 1,2-dioxygenase from Pseudomonas putida: single turnover kinetics and regulation of a two-component Rieske dioxygenase

The benzoate 1,2-dioxygenase system (BZDOS) from Pseudomonas putida mt-2 catalyzes the NADH-dependent oxidation of benzoate to 1-carboxy-1,2-cis-dihydroxycyclohexa-3,5-diene. Both the oxygenase (BZDO) and reductase (BZDR) components of BZDOS have been purified and characterized kinetically and by optical, EPR, and Mössbauer spectroscopies. BZDO has an (alpha beta)(3) subunit structure in which each alpha subunit contains a Rieske [2Fe-2S] cluster and a mononuclear iron site. Two different purification protocols were developed for BZDO allowing the mononuclear iron to be stabilized in either the Fe(III) or the Fe(II) state for spectroscopic characterization. Using single turnover reactions, it is shown that fully reduced BZDO alone is capable of yielding the cis-diol product in high yield at rates that exceed the BZDOS turnover number. At the conclusion of turnover, quantification of each oxidation state of the metal sites by EPR and Mössbauer spectroscopies shows that the Rieske cluster and mononuclear iron are each oxidized in amounts equal to the product yield, suggesting that the two electrons required for catalysis derive from the two metal centers. These results are in agreement with our previous study of naphthalene 1,2-dioxygenase [Wolfe, M. D., Parales, J. V., Gibson, D. T., and Lipscomb, J. D. (2001) J. Biol. Chem. 276, 1945-1953], which belongs to a different Rieske dioxygenase subclass, suggesting that it is a universal characteristic of Rieske dioxygenases that oxygen activation and substrate oxidation are catalyzed by the oxygenase component alone. The EPR spectrum of the Fe(III) center after a single turnover is distinct from either of those of substrate-free or substrate-bound enzyme. The complex with this spectrum is not formed by addition of cis-diol product to the resting Fe(III) form of the enzyme but is observed when the Fe(II) form is oxidized in the presence of product. Together, these results suggest that product exchange occurs only when the mononuclear iron is reduced. Stopped-flow and rapid scan analyses monitoring the oxidation of the Rieske cluster during the single turnover reaction show that it occurs in three phases that are kinetically competent for catalysis. The rate of each phase was found to be dependent on the type of substrate present, suggesting that the substrate influences the rate of electron transfer between the metal clusters. The participation of substrate in the oxygen activation reaction suggests a new aspect of the mechanism of this process by the Rieske dioxygenase class.

A comprehensive phylogenetic analysis of Rieske and Rieske-type iron-sulfur proteins

The Rieske iron-sulfur center consists of a [2Fe-2S] cluster liganded to a protein via two histidine and two cysteine residues present in conserved sequences called Rieske motifs. Two protein families possessing Rieske centers have been defined. The Rieske proteins occur as subunits in the cytochrome bc1 and cytochrome b6f complexes of prokaryotes and eukaryotes or form components of archaeal electron transport systems. The Rieske-type proteins encompass a group of bacterial oxygenases and ferredoxins. Recent studies have uncovered several new proteins containing Rieske centers, including archaeal Rieske proteins, bacterial oxygenases, bacterial ferredoxins, and, intriguingly, eukaryotic Rieske oxygenases. Since all these proteins contain a Rieske motif, they probably form a superfamily with one common ancestor. Phylogenetic analyses have, however, been generally limited to similar sequences, providing little information about relationships within the whole group of these proteins. The aim of this work is, therefore, to construct a dendrogram including representatives from all Rieske and Rieske-type protein classes in order to gain insight into their evolutionary relationships and to further define the phylogenetic niches occupied by the recently discovered proteins mentioned above.

Substrate and solvent isotope effects on the fate of the active oxygen species in substrate-modulated reactions of putidamonooxin

Using 4-methoxybenzoate monooxygenase from Pseudomonas putida, the substrate deuterium isotope effect on product formation and the solvent isotope effect on the stoichiometry of oxygen uptake, NADH oxidation, product and/or H2O2 (D2O2) formation for tight couplers, partial uncouplers, and uncouplers as substrates were measured. These studies revealed for the true, intrinsic substrate deuterium isotope effect on the oxygenation reaction a k1H/k2H ratio of < 2.0, derived from the inter- and intramolecular substrate isotope effects. This value favours a concerted oxygenation mechanism of the substrate. Deuterium substitution in a tightly coupling substrate initiated a partial uncoupling of oxygen reduction and substrate oxygenation, with release of H2O2 corresponding to 20% of the overall oxygen uptake. This H2O2 (D2O2) formation (oxidase reaction) almost completely disappeared when the oxygenase function was increased by deuterium substitution in the solvent. The electron transfer from NADH to oxygen, however, was not affected by deuterium substitution in the substrate and/or the solvent. With 4-trifluoromethylbenzoate as uncoupling substrate and D2O as solvent, a reduction (peroxidase reaction) of the active oxygen complex was initiated in consequence of its extended lifetime. These additional two electron-transfer reactions to the active oxygen complex were accompanied by a decrease of both NADH oxidation and oxygen uptake rates. These findings lead to the following conclusions: (a) under tightly coupling conditions the rate-limiting step must be the formation time and lifetime of an active transient intermediate within the ternary complex iron/peroxo/substrate, rather than an oxygenative attack on a suitable C-H bond or electron transfer from NADH to oxygen. Water is released after the monooxygenation reaction; (b) under uncoupling conditions there is competition in the detoxification of the active oxygen complex between its protonation (deuteronation), with formation of H2O2 (D2O2) and its further reduction to water. The additional two electron-transfer reactions onto the active oxygen complex then become rate limiting for the oxygen uptake rate.

Nitric oxide and iron proteins

Nitric oxide interactions with iron are the most important biological reactions in which NO participates. Reversible binding to ferrous haem iron is responsible for the observed activation of guanylate cyclase and inhibition of cytochrome oxidase. Unlike carbon monoxide or oxygen, NO can also bind reversibly to ferric iron. The latter reaction is responsible for the inhibition of catalase by NO. NO reacts with the oxygen adduct of ferrous haem proteins (e.g. oxyhaemoglobin) to generate nitrate and ferric haem; this reaction is responsible for the majority of NO metabolism in the vasculature. NO can also interact with iron-sulphur enzymes (e.g. aconitase, NADH dehydrogenase). This review describes the underlying kinetics, thermodynamics, mechanisms and biological role of the interactions of NO with iron species (protein and non-protein bound). The possible significance of iron reactions with reactive NO metabolites, in particular peroxynitrite and nitroxyl anion, is also discussed.

Benzene-induced uncoupling of naphthalene dioxygenase activity and enzyme inactivation by production of hydrogen peroxide

Naphthalene dioxygenase (NDO) is a multicomponent enzyme system that oxidizes naphthalene to (+)-cis-(1R,2S)-1,2-dihydroxy-1, 2-dihydronaphthalene with consumption of O2 and two electrons from NAD(P)H. In the presence of benzene, NADH oxidation and O2 utilization were partially uncoupled from substrate oxidation. Approximately 40 to 50% of the consumed O2 was detected as hydrogen peroxide. The rate of benzene-dependent O2 consumption decreased with time, but it was partially increased by the addition of catalase in the course of the O2 consumption by NDO. Detailed experiments showed that the total amount of O2 consumed and the rate of benzene-induced O2 consumption increased in the presence of hydrogen peroxide-scavenging agents, and further addition of the terminal oxygenase component (ISPNAP) of NDO. Kinetic studies showed that ISPNAP was irreversibly inactivated in the reaction that contained benzene, but the inactivation was relieved to a high degree in the presence of catalase and partially relieved in the presence of 0.1 mM ferrous ion. Benzene- and naphthalene-reacted ISPNAP gave almost identical visible absorption spectra. In addition, hydrogen peroxide added at a range of 0.1 to 0.6 mM to the reaction mixtures inactivated the reduced ISPNAP containing mononuclear iron. These results show that hydrogen peroxide released during the uncoupling reaction acts both as an inhibitor of benzene-dependent O2 consumption and as an inactivator of ISPNAP. It is proposed that the irreversible inactivation of ISPNAP occurs by a Fenton-type reaction which forms a strong oxidizing agent, hydroxyl radicals (. OH), from the reaction of hydrogen peroxide with ferrous mononuclear iron at the active site. Furthermore, when [14C]benzene was used as the substrate, cis-benzene 1,2-dihydrodiol formed by NDO was detected. This result shows that NDO also couples a trace amount of benzene to both O2 consumption and NADH oxidation.

Aspartate 205 in the catalytic domain of naphthalene dioxygenase is essential for activity

The naphthalene dioxygenase enzyme system carries out the first step in the aerobic degradation of naphthalene by Pseudomonas sp. strain NCIB 9816-4. The crystal structure of naphthalene dioxygenase (B. Kauppi, K. Lee, E. Carredano, R. E. Parales, D. T. Gibson, H. Eklund, and S. Ramaswamy, Structure 6:571-586, 1998) indicates that aspartate 205 may provide the most direct route of electron transfer between the Rieske [2Fe-2S] center of one alpha subunit and mononuclear iron in the adjacent alpha subunit. In this study, we constructed four site-directed mutations that changed aspartate 205 to alanine, glutamate, asparagine, or glutamine to test whether this residue is essential for naphthalene dioxygenase activity. The mutant proteins were very inefficient in oxidizing naphthalene to cis-naphthalene dihydrodiol, and oxygen uptake in the presence of naphthalene was below detectable levels. The purified mutant protein with glutamine in place of aspartate 205 had identical spectral properties to wild-type naphthalene dioxygenase and was reduced by NADH in the presence of catalytic amounts of ferredoxinNAP and reductaseNAP. Benzene, an effective uncoupler of oxygen consumption in purified naphthalene dioxygenase, did not elicit oxygen uptake by the mutant protein. These results indicate that electron transfer from NADH to the Rieske center in the mutant oxygenase is intact, a finding consistent with the proposal that aspartate 205 is a necessary residue in the major pathway of electron transfer to mononuclear iron at the active site.

A Carboxylate-Bridged Non-Heme Diiron Dinitrosyl Complex

The reaction of nitric oxide with the carboxylate-bridged diiron(II) complex [Fe(2)(Et-HPTB)(O(2)CPh)](BF(4))(2) (1a) afforded the dinitrosyl adduct, [Fe(2)(NO)(2)(Et-HPTB)(O(2)CPh)](BF(4))(2) (1b), where Et-HPTB = N,N,N',N'-tetrakis(N-ethyl-2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane, in 69% yield. Compound 1b further reacts with dioxygen to form the bis(nitrato) complex, [Fe(2)(Et-HPTB)(NO(3))(2)(OH)](BF(4))(2) (1c). The structure of 1b was determined by X-ray crystallography (triclinic, P&onemacr;, a = 13.5765(8) Å, b = 15.4088(10) Å, c = 16.2145(10) Å, alpha = 73.656(1) degrees, beta = 73.546(1) degrees, gamma = 73.499(1) degrees, V = 3043.8(7) Å(3), T = -80 degrees C, Z = 2, and R = 0.085 and R(w) = 0.095 for 5644 independent reflections with I > 3sigma(I)). The two nitrosyl units are equivalent with an average Fe-N-O angle of 167.4 +/- 0.8 degrees. Spectroscopic characterization of solid 1b revealed an NO stretch at 1785 cm(-)(1) in the infrared and Mössbauer parameters of delta = 0.67 mm s(-)(1) and DeltaE(Q) = 1.44 mm s(-)(1) at 4.2 K. These data are comparable to those for other {FeNO}(7) systems. An S = (3)/(2) spin state was assigned from magnetic susceptibility studies to the two individual {FeNO} centers, each of which has a nitrosyl ligand antiferromagnetically coupled to iron. A least-squares fit of the chi vs temperature plots to a theoretical model yielded an exchange coupling constant J of -23 cm(-)(1), where H = -2JS(1).S(2), indicating that the two S = (3)/(2) centers are antiferromagnetically coupled to one another. An extended Hückel calculation on a model complex, [Fe(2)(NO)(2)(NH(3))(6)(O(2)CH)(OH)](2+), revealed that the magnitudes of Fe-N-O angles are dictated by pi-bonding interactions between the Fe d(xz)() and NO pi orbitals.

Magnetic circular dichroism studies of exogenous ligand and substrate binding to the non-heme ferrous active site in phthalate dioxygenase

BACKGROUND

Mononuclear non-heme iron centers are found in the active sites of a variety of enzymes that require molecular oxygen for catalysis. The mononuclear non-heme iron is believed to be the active site for catalysis, and is presumed to bind and activate molecular oxygen. The mechanism of this reaction is not understood. Phthalate dioxygenase is one such enzyme. Because it also contains a second iron site, the Rieske site, it is difficult to obtain information on the structure of the active site. We therefore used magnetic circular dichroism (MCD) spectroscopy to probe the mononuclear, non-heme Fe2+ site in this biodegradative enzyme.

RESULTS

The MCD spectrum of the resting enzyme shows features indicative of one six-coordinate Fe2+ site; substrate binding converts the site to two different five-coordinate species, opening up a coordination position for O2 binding. MCD spectra of the corresponding apoenzyme have been subtracted to account for temperature-independent contributions from the Rieske site. Azide binds both to the resting enzyme to produce a new six-coordinate species, showing that one of the ferrous ligands is exchangeable, and also to the enzyme-substrate complex to form a ternary species. The low azide binding constant for the substrate-enzyme species relative to the resting enzyme indicates steric interaction and close proximity between exogenous ligand and the substrate.

CONCLUSIONS

We have been able to provide some detailed structural insight into exogenous ligand and substrate binding to the non-heme Fe2+ site, even in the presence of the enzyme's [2Fe-2S] Rieske center. Further mechanistic studies are now required to maximize the molecular-level detail available from these spectroscopic studies.

Substrate-modulated reactions of putidamonooxin. The nature of the active oxygen species formed and its reaction mechanism

1. 4-Methoxybenzoate monooxygenase is fairly nonspecific. The enzyme system with putidamonooxin as its oxygen-activating component catalyses: (a) O-, S- and N-demethylation; (b) the oxygenation of 4-methylbenzoate and 4-methylmercaptobenzoate, with formation of 4-carboxybenzyl alcohol and 4-carboxyphenylmethyl sulfoxide, respectively, and (c) attack of the aromatic ring of 4- and 3-hydroxybenzoate and 4-aminobenzoate, yielding 3,4-dihydroxybenzoate and 4-amino-3-hydroxybenzoate, respectively. 2. Compounds which are bound by the active sites of putidamonooxin have two essential features in common: a planar aromatic ring system, and a free carboxyl group attached to it. 3. By a substrate-modulated reaction putidamonooxin can be induced to function not only as a monooxygenase but also as a peroxotransferase, i.e. it incorporates both atoms of the activated oxygen molecule into a substrate molecule. This finding supports the hypothesis that a mesomeric state of the iron.peroxo complex, [FeO2]+, is indeed the active oxygenating species of putidamonooxin. 4. The lifetime of the ternary complex consisting of enzyme.iron-peroxo-complex.substrate is significantly prolonged by uncoupling and partially uncoupling substrates, except when it is inactivated by protonation of the iron.peroxo complex by a proton transported into the active sites by a special kind of substrate (i.e. isomers of monoaminobenzoate), with the direct formation of H2O2. 5. The lifetime of the active oxygen species is determined by (a) the rate of the oxygenation reaction in the presence of tight-coupling substrates and (b) the rate of the oxygenation reaction as well as detoxification by the availability of a dissociable proton in the presence of partial uncoupling (and uncoupling) substrate analogues. 6. The rate of the oxygenation reaction depends on the lifetime of the active oxygen species, [FeO2]+, in the presence of partial uncoupling substrates. 7. The iron.peroxo complex attacks an aromatic ring system according to the empiric rules of electrophilic substitution, whereas the attack of aliphatic substituents at the aromatic ring is controlled by steric criteria.

Fe(II)-substituted horse liver alcohol dehydrogenase, a model for non-heme iron enzymes. Various states of iron-dioxygen interaction investigated by Mössbauer and EPR spectroscopy

The catalytic Zn(II) ion of horse liver alcohol dehydrogenase (EE isozyme) was replaced by Fe(II), providing a novel iron protein with the unusual coordination of Fe(II) to two cysteines residues, one histine residue and water. The electronic structure of iron in this system was characterized by Mössbauer spectroscopy at various temperatures as well as applied magnetic fields and analysed in terms of the spin Hamiltonian formalism. The novelty we found is an unusually weak spin coupling (/J/ less than 0.1 cm-1) of a paramagnet (S = 1) with iron (S = 2). From EPR and biochemical studies we conclude that the corresponding chemical species is triplet oxygen (O2). The quantitative determination of the coupling energy was possible utilizing the competition between Zeemann interaction and spin coupling at weak magnetic fields and low temperature. Oxidation experiments followed by Mössbauer spectroscopy showed that the spin-coupled system is an outer-sphere Fe(II) . (O2)aq complex occurring as an intermediate during a Fe(II)-catalyzed dioxygen activation. We observed two additional Fe(II) species after treatment with O2 and dithionite. The spin Hamiltonian parameters of iron in the coupled system are presented. The results are compared with those of iron in other non-heme iron proteins.

Protein-protein interactions and antigenic relationships between the components of 4-methoxybenzoate monooxygenase and of benzene 1,2-dioxygenase from Pseudomonas putida

The investigations presented in this paper were performed on two enzyme systems from Pseudomonas putida: (a) 4-methoxybenzoate monooxygenase, consisting of a NADH: putidamonooxin oxidoreductase and putidamonooxin, the oxygen-activating component, and (b) benzene 1,2-dioxygenase, a three-component enzyme system with an NADH: ferredoxin oxidoreductase, functioning together with a plant-type ferredoxin as electron-transport chain, and an oxygen-activating component similar to putidamonooxin in its active sites. The influence of temperature, ionic strength, and pH on the activities of 4-methoxybenzoate monooxygenase and of NADH: putidamonooxin oxidoreductase were investigated. The studies revealed that the activity of 4-methoxybenzoate monooxygenase is determined by the behaviour of the reductase. Spectroscopic measurements showed that the interaction between the two components of 4-methoxybenzoate monooxygenase influences the optical-absorption behaviour of one or both components. As a criterion for the affinity between the two components of 4-methoxybenzoate monooxygenase, the Km value of the reductase for putidamonooxin was determined and found to be 31 +/- 11 microM. Antibodies against both components of 4-methoxybenzoate monooxygenase were obtained from rabbits. The antibodies against putidamonooxin inhibited the O-demethylation reaction (up to 80%) and also the reduction of putidamonooxin by the reductase (up to 40%). The antibodies against putidamonooxin did not interact with the oxygen-activating component of benzene 1,2-dioxygenase. The electron-transport chains of 4-methoxybenzoate monooxygenase and benzene 1,2-dioxygenase could not be replaced by one another without a complete loss of enzyme activity.

Mössbauer investigation of the cofactor iron of putidamonooxin

Mononuclear non-heme cofactor iron of putidamonooxin has been investigated in the binary oxidized 'enzyme X substrate' complex and in the ternary 'enzyme X substrate X NO' complex via Mössbauer spectroscopy. The experimental spectra were analyzed on the basis of the spin-Hamiltonian formalism. The resulting fine and hyperfine structure parameters are compared with literature values of similar compounds. From this comparison we conclude that in the binary complex (reduced and oxidized) the mononuclear non-heme cofactor iron has a coordination number higher than four. Additionally, the cofactor iron shows remarkable spectral similarities with iron in protocatechuate 3,4-dioxygenase, though the catalytic properties of the iron sites in the two proteins are different. The data obtained form the ternary 'enzyme X substrate X NO' complex indicate that the cofactor iron (a) is in the ferric intermediate spin state (S = 3/2) and (b) is pentacoordinated, which means that upon NO binding to the reduced cofactor iron at least one ligand has to be released. Comparing our data with literature values suggests that the cofactor iron in the binary as well as in the ternary NO complex is not directly bound to a sulfur atom, though biochemical arguments seem to indicate the opposite.