Geometric and electronic structure/function correlations in non-heme iron enzymes

Mechanism of aromatic hydroxylation by an activated FeIV=O core in tetrahydrobiopterin-dependent hydroxylases

The chemical pathways leading to the hydroxylated aromatic amino acids in phenylalanine and tryptophan hydroxylases have been investigated by means of hybrid density functional theory. In the catalytic core of these non-heme iron enzymes, dioxygen reacts with the pterin cofactor and is likely to be activated by forming an iron(IV)=O complex. The capability of this species to act as a hydroxylating intermediate has been explored. Depending on the protonation state of the ligands of the metal, two different mechanisms are found to be energetically possible for the hydroxylation of phenylalanine and tryptophan by the high-valent iron-oxo species. With a hydroxo ligand the two-electron oxidation of the aromatic ring passes through a radical, while an arenium cation is involved when a water replaces the hydroxide. After the attack of the activated oxygen on the substrate, it is also found that a 1,2-hydride shift (known as an NIH shift) generates a keto intermediate, which can decay to the true product through an intermolecular keto-enol tautomerization. The benzylic hydroxylation of 4-methylphenylalanine by the Fe(IV)=O species has also been investigated according to the rebound mechanism. The computed energetics lead to the conclusion that Fe(IV)=O is capable not only of aromatic hydroxylation, but also of benzylic hydroxylation.

DFT calculations of isomer shifts and quadrupole splitting parameters in synthetic iron-oxo complexes: applications to methane monooxygenase and ribonucleotide reductase

To predict isomer shifts and quadrupole splitting parameters of Fe atoms in the protein active sites of methane monooxygenase and ribonucleotide reductase, a correlation between experimental isomer shifts ranging 0.1-1.5 mm s(-)(1) for Fe atoms in a training set with the corresponding density functional theory (DFT) calculated electron densities at the Fe nuclei in those complexes is established. The geometries of the species in the training set, consisting of synthetic polar monomeric and dimeric iron complexes, are taken from the Cambridge structural database. A comparison of calculated Mössbauer parameters for Fe atoms from complexes in the training set with their corresponding experimental values shows very good agreement (standard deviation of 0.11 mm/s, correlation coefficient of -0.94). However, for the Fe atoms in the active sites of the structurally characterized proteins of methane monooxygenase and ribonucleotide reductase, the calculated Mössbauer parameters deviate more from their experimentally measured values. The high correlation that exists between calculated and observed quadrupole splitting and isomer shift parameters for the synthetic complexes leads us to conclude that the main source of the error arising for the protein active sites is due to the differing degrees of atomic-level resolution for the protein structural data, compared to the synthetic complexes in the training set. Much lower X-ray resolutions associated with the former introduce uncertainty in the accuracy of several bond lengths. This is ultimately reflected in the calculated isomer shifts and quadrupole splitting parameters of the Fe sites in the proteins. For the proteins, the closest correspondence between predicted and observed Mössbauer isomer shifts follows the order MMOH(red), RNR(red), MMOH(ox), and RNR(ox), with average deviations from experiment of 0.17, 0.17, 0.17-0.20, and 0.32 mm/s, but this requires DFT geometry optimization of the iron-oxo dimer complexes.

Regioselective hydroxylation of the xylyl linker in a diiron(III) complex having a carboxylate-rich ligand with H2O2

Reaction of a diiron(III) complex having a xylta4- ligand (N,N,N',N'-m-xylylenediamine tetraacetate) with H2O2 resulted in regioselective hydroxylation of the m-xylyl linker. The reaction mimics the self-hydroxylation of a phenylalanine side chain found for ribonucleotide reductase (R2-W48F/D84E).

An unusual dinuclear ruthenium(III) complex with a conjugated bridging ligand derived from cleavage of a 1,4-dihydro-1,2,4,5-tetrazine ring. Synthesis, structure, and UV-vis-NIR spectroelectrochemical characterization of a five-membered redox chain incorporating two mixed-valence states

Reaction of [Ru(acac)(2)(CH(3)CN)(2)] with 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,4-dihydro-1,2,4,5-tetrazine (H(2)L) results in formation of an unexpected dinuclear complex [(acac)(2)Ru(III)(L(1))Ru(III)(acac)(2)] (1) in which the bridging ligand [L(1)](2)(-) contains an (-)HN[bond]C[double bond]N[bond]N[double bond]C[bond]NH(-) unit arising from two-electron reduction of the 1,4-dihydro-1,2,4,5-tetrazine component of H(2)L. The crystal structure of complex 1 confirms the oxidation assignment of the metal ions as Ru(III) and clearly shows the consequent arrangement of double and single bonds in the bridging ligand, which acts as a bis-bidentate chelate having two pyrazolyl/amido chelating sites. Cyclic voltammetry of the complex shows the presence of four reversible one-electron redox couples, assigned as two Ru(III)/Ru(IV) couples (oxidations with respect to the starting material) and two Ru(II)/Ru(III) couples (reductions with respect to the starting material). The separation between the two Ru(III)/Ru(IV) couples (Delta E(1/2) = 700 mV) is much larger than that between the two Ru(II)/Ru(III) couples (Delta E(1/2) = 350 mV) across the same bridging pathway, because of the better ability of the dianionic bridging ligand to delocalize an added hole (in the oxidized mixed-valence state) than an added electron (in the reduced mixed-valence state), implying some ligand-centered character for the oxidations. UV-vis-NIR spectroelectrochemical measurements were performed in all five oxidation states; the Ru(II)-Ru(III) mixed-valence state of [1](-) has a strong IVCT transition at 2360 nm whose parameters give an electronic coupling constant of V(ab) approximately 1100 cm(-1), characteristic of a strongly interacting but localized (class II) mixed-valence state. In the Ru(III)-Ru(IV) mixed-valence state [1](+), no low-energy IVCT could be detected despite the strong electronic interaction, possibly because it is in the visible region and obscured by LMCT bands.

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.

Oxygen activation by nonheme iron(II) complexes: alpha-keto carboxylate versus carboxylate

Mononuclear iron(II) alpha-keto carboxylate and carboxylate compounds of the sterically hindered tridentate face-capping ligand Tp(Ph2) (Tp(Ph2) = hydrotris(3,5-diphenylpyrazol-1-yl)borate) were prepared as models for the active sites of nonheme iron oxygenases. The structures of an aliphatic alpha-keto carboxylate complex, [Fe(II)(Tp(Ph2))(O(2)CC(O)CH(3))], and the carboxylate complexes [Fe(II)(Tp(Ph2))(OBz)] and [Fe(II)(Tp(Ph2))(OAc)(3,5-Ph(2)pzH)] were determined by single-crystal X-ray diffraction, all of which have five-coordinate iron centers. Both the alpha-keto carboxylate and the carboxylate compounds react with dioxygen resulting in the hydroxylation of a single ortho phenyl position of the Tp(Ph2) ligand. The oxygenation products were characterized spectroscopically, and the structure of the octahedral iron(III) phenolate product [Fe(III)(Tp(Ph2))(OAc)(3,5-Ph(2)pzH)] was established by X-ray diffraction. The reaction of the alpha-keto carboxylate model compounds with oxygen to produce the phenolate product occurs with concomitant oxidative decarboxylation of the alpha-keto acid. Isotope labeling studies show that (18)O(2) ends up in the Tp(Ph2) phenolate oxygen and the carboxylate derived from the alpha-keto acid. The isotope incorporation mirrors the dioxygenase nature of the enzymatic systems. Parallel studies on the carboxylate complexes demonstrate that the oxygen in the hydroxylated ligand is also derived from molecular oxygen. The oxygenation of the benzoylformate complex is demonstrated to be first order in metal complex and dioxygen, with activation parameters DeltaH++ = 25 +/- 2 kJ mol(-1) and DeltaS++ = -179 +/- 6 J mol(-1) K(-1). The rate of appearance of the iron(III) phenolate product is sensitive to the nature of the substituent on the benzoylformate ligand, exhibiting a Hammett rho value of +1.3 indicative of a nucleophilic mechanism. The proposed reaction mechanism involves dioxygen binding to produce an iron(III) superoxide species, nucleophilic attack of the superoxide at the alpha-keto functionality, and oxidative decarboxylation of the adduct to afford the oxidizing species that attacks the Tp(Ph2) phenyl ring. Interestingly, the alpha-keto carboxylate complexes react 2 orders of magnitude faster than the carboxylate complexes, thus emphasizing the key role that the alpha-keto functionality plays in oxygen activation by alpha-keto acid-dependent iron enzymes.

Spectroscopic characterization of the [Fe(His)(4)(Cys)] site in 2Fe-superoxide reductase from Desulfovibrio vulgaris

The electronic and vibrational properties of the [Fe(His)(4)(Cys)] site (Center II) responsible for catalysis of superoxide reduction in the two-iron superoxide reductase (2Fe-SOR) from Desulfovibrio vulgaris have been investigated using the combination of EPR, resonance Raman, UV/visible/near-IR absorption, CD, and VTMCD spectroscopies. Deconvolution of the spectral contributions of Center II from those of the [Fe(Cys)(4)] site (Center I) has been achieved by parallel investigations of the C13S variant, which does not contain Center I. The resonance Raman spectrum of ferric Center II has been assigned based on isotope shifts for (34)S and (15)N globally labeled proteins. As for the [Fe(His)(4)(Cys)] active site in 1Fe-SOR from Pyrococcus furiosus, the spectroscopic properties of ferric and ferrous Center II in D. vulgaris 2Fe-SOR are indicative of distorted octahedral and square-pyramidal coordination geometries, respectively. Differences in the properties of the ferric [Fe(His)(4)(Cys)] sites in 1Fe- and 2Fe-SORs are apparent in the rhombicity of the S=5/2 ground state ( E/ D=0.06 and 0.28 in 1Fe- and 2Fe-SORs, respectively), the energy of the CysS(-)(p(pi))-->Fe(3+)(d(pi)) CT transition (15150+/-150 cm(-1) and 15600+/-150 cm(-1) in 1Fe- and 2Fe-SORs, respectively) and in changes in the Fe-S stretching region of the resonance Raman spectrum indicative of a weaker Fe-S(Cys) bond in 2Fe-SORs. These differences are interpreted in terms of small structural perturbations in the Fe coordination sphere with changes in the Fe-S(Cys) bond strength resulting from differences in the peptide N-H.S(Cys) hydrogen bonding within a tetrapeptide bidentate "chelate". Observation of the characteristic intervalence charge transfer transition of a cyano-bridged [Fe(III)-NC-Fe(II)(CN)(5)] unit in the near-IR VTMCD spectra of ferricyanide-oxidized samples of both P. furiosus 1Fe-SOR and D. vulgaris 2Fe-SOR has confirmed the existence of novel ferrocyanide adducts of the ferric [Fe(His)(4)(Cys)] sites in both 1Fe- and 2Fe-SORs.

Chain cleavage and sulfoxidation of thiastearoyl-ACP upon reaction with stearoyl-ACP desaturase

The fatty acid analogues 9- and 10-thiastearate were converted to acyl-ACP derivatives by in vitro enzymatic synthesis and reacted with the reconstituted soluble stearoyl-ACP Delta9 desaturase complex. Electrospray ionization mass spectral analysis of the acyl chains purified from the reaction mixtures showed that 10-thiastearoyl-ACP was converted to the 10-sulfoxide as the sole product. In the presence of (18)O(2), the sulfoxide oxygen was found to be derived exclusively from O(2). This result confirms the ability of the soluble stearoyl-ACP desaturase to catalyze O atom transfer in the presence of the appropriate substrate analogue. Inhibition studies showed that 10-thiastearoyl-ACP was a mixed-type inhibitor of 18:0-ACP, with an apparent K(I) of approximately 10 microM. Comparable reactions of the stearoyl-ACP desaturase complex with 9-thiastearoyl-ACP gave the 9-sulfoxide as approximately 5% of the total products, with the O atom again exclusively derived from O(2). The remaining 95% of the total products arose from an acyl chain cleavage reaction between S-9 and C-10. Matrix-assisted laser desorption ionization time-of-flight mass spectral analysis showed that 9-thiastearoyl-ACP had a mass of 9443 amu while the acyl chain cleavage product had a mass of 9322 amu, corresponding to the calculated mass of 8-mercaptooctanoyl-ACP. Recovery of the acyl chain from the ACP product gave the disulfide of 8-mercaptooctanoate (mass of 349.1 amu), arising from the dimerization of 8-mercaptooctanoate during product workup. Gas chromatography-mass spectral analysis also showed the accumulation of nonanal in sealed reaction vials, accounting for the other product of the acyl chain cleavage reaction. The reactivity at both the 9 and 10 positions of the thia-substituted acyl-ACPs is consistent with the proximity of both positions to the diiron center oxidant in the enzyme-substrate complex. Moreover, the differential reactivity of the 9- and 10-thiastearoyl-ACPs also suggests position-dependent consequences of the reaction within the Delta9D active site. Mechanisms accounting for both sulfoxidation and acyl cleavage reactions by the stearoyl-ACP Delta9 desaturase are proposed.

The first direct characterization of a high-valent iron intermediate in the reaction of an alpha-ketoglutarate-dependent dioxygenase: a high-spin FeIV complex in taurine/alpha-ketoglutarate dioxygenase (TauD) from Escherichia coli

The Fe(II)- and alpha-ketoglutarate(alphaKG)-dependent dioxygenases have roles in synthesis of collagen and sensing of oxygen in mammals, in acquisition of nutrients and synthesis of antibiotics in microbes, and in repair of alkylated DNA in both. A consensus mechanism for these enzymes, involving (i) addition of O(2) to a five-coordinate, (His)(2)(Asp)-facially coordinated Fe(II) center to which alphaKG is also bound via its C-1 carboxylate and ketone oxygen; (ii) attack of the uncoordinated oxygen of the bound O(2) on the ketone carbonyl of alphaKG to form a bicyclic Fe(IV)-peroxyhemiketal complex; (iii) decarboxylation of this complex concomitantly with formation of an oxo-ferryl (Fe(IV)=O(2)(-)) intermediate; and (iv) hydroxylation of the substrate by the Fe(IV)=O(2)(-) complex via a substrate radical intermediate, has repeatedly been proposed, but none of the postulated intermediates occurring after addition of O(2) has ever been detected. In this work, an oxidized Fe intermediate in the reaction of one of these enzymes, taurine/alpha-ketoglutarate dioxygenase (TauD) from Escherichia coli, has been directly demonstrated by rapid kinetic and spectroscopic methods. Characterization of the intermediate and its one-electron-reduced form (obtained by low-temperature gamma-radiolysis of the trapped intermediate) by Mössbauer and electron paramagnetic resonance spectroscopies establishes that it is a high-spin, formally Fe(IV) complex. Its Mössbauer isomer shift is, however, significantly greater than those of other known Fe(IV) complexes, suggesting that the iron ligands in the TauD intermediate confer significant Fe(III) character to the high-valent site by strong electron donation. The properties of the complex and previous results on related alphaKG-dependent dioxygenases and other non-heme-Fe(II)-dependent, O(2)-activating enzymes suggest that the TauD intermediate is most probably either the Fe(IV)-peroxyhemiketal complex or the taurine-hydroxylating Fe(IV)=O(2)(-) species. The detection of this intermediate sets the stage for a more detailed dissection of the TauD reaction mechanism than has previously been reported for any other member of this important enzyme family.

Spectroscopic characterization of soybean lipoxygenase-1 mutants: the role of second coordination sphere residues in the regulation of enzyme activity

Lipoxygenases are non-heme iron enzymes, which catalyze the stereo- and regiospecific hydroperoxidation of unsaturated fatty acids. Spectroscopic studies on soybean lipoxygenase have shown that the ferrous form of the enzyme is a mixture of five- and six-coordinate species (40 and 60%, respectively). Addition of substrate leads to a purely six-coordinate form. A series of mutations in the second coordination sphere (Q697E, Q697N, Q495A, and Q495E) were generated, and the structures of the mutants were solved by crystallography [Tomchick et al. (2001) Biochemistry 40, 7509-7517]. While this study clearly showed the contribution of H-bond interactions between the first and the second coordination spheres in catalysis, no correlation with the coordination environment of the Fe(II) was observed. A recent study using density-functional theory [Lehnert and Solomon (2002) J. Biol. Inorg. Chem. 8, 294-305] indicated that coordination flexibility, involving the Asn694 ligand, is regulated via H-bond interactions. In this paper, we investigate the solution structures of the second coordination sphere mutants using CD and MCD spectroscopy since these techniques are more sensitive indicators of the first coordination sphere ligation of Fe(II) systems. Our data demonstrate that the iron coordination environment directly relates to activity, with the mutations that have the ability to form a five-coordinate/six-coordinate mixture being more active. We propose that the H-bond between the weak Asn694 ligand and the Gln697 plays a key role in the modulation of the coordination flexibility of Asn694, and thus, is crucial for the regulation of enzyme reactivity.

Spectroscopic study of [Fe2O2(5-Et3-TPA)2]3+: nature of the Fe2O2 diamond core and its possible relevance to high-valent binuclear non-heme enzyme intermediates

The spectroscopic properties and electronic structure of an Fe(2)(III,IV) bis-mu-oxo complex, [Fe(2)O(2)(5-Et(3)-TPA)(2)](ClO(4))(3) where 5-Et(3)-TPA = tris(5-ethyl-2-pyridylmethyl)amine, are explored to determine the molecular origins of the unique electronic and geometric features of the Fe(2)O(2) diamond core. Low-temperature magnetic circular dichroism (MCD) allows the two features in the broad absorption envelope (4000-30000 cm(-)(1)) to be resolved into 13 transitions. Their C/D ratios and transition polarizations from variable temperature-variable field MCD saturation behavior indicate that these divide into three types of electronic transitions; t(2) --> t(2) involving excitations between metal-based orbitals with pi Fe-O overlap (4000-10000 cm(-)(1)), t(2)/t(2) --> e involving excitations to metal-based orbitals with sigma Fe-O overlap (12500-17000 cm(-)(1)) and LMCT (17000-30000 cm(-)(1)) and allows transition assignments and calibration of density functional calculations. Resonance Raman profiles show the C(2)(h)() geometric distortion of the Fe(2)O(2) core results in different stretching force constants for adjacent Fe-O bonds (k(str)(Fe-O(long)) = 1.66 and k(str)(Fe-O(short)) = 2.72 mdyn/A) and a small ( approximately 20%) difference in bond strength between adjacent Fe-O bonds. The three singly occupied pi-metal-based orbitals form strong superexchange pathways which lead to the valence delocalization and the S = (3)/(2) ground state. These orbitals are key to the observed reactivity of this complex as they overlap with the substrate C-H bonding orbital in the best trajectory for hydrogen atom abstraction. The electronic structure implications of these results for the high-valent enzyme intermediates X and Q are discussed.

Site-directed mutagenesis of an extradiol dioxygenase involved in tetralin biodegradation identifies residues important for activity or substrate specificity

The sequence of the extradiol dioxygenase ThnC, involved in tetralin biodegradation, was aligned with other extradiol dioxygenases involved in biodegradation of polycyclic compounds, and a three-dimensional model of ThnC, based on the structure of the previously crystallized 2,3-dihydroxybiphenyl dioxygenase from Burkholderia fungorum LB400, was built. In order to assess the functional importance of some non-active-site residues whose relevance could not be established by structural information, a number of positions surrounding the substrate-binding site were mutated in ThnC. Ten mutant proteins were purified and their activity towards 1,2-dihydroxytetralin, 1,2-dihydroxynaphthalene and 2,3-dihydroxybiphenyl was characterized. N213H, Q198H, G206M, A282R and A282G mutants increased k(cat)/K(m) at least twofold using 1,2-dihydroxytetralin as the substrate, thus showing that activity of ThnC is not maximized for this substrate. N213H and Q198H mutants increased k(cat)/K(m) using any of the substrates tested, thus showing the relevance for activity of these two histidines, which are highly conserved in dihydroxybiphenyl dioxygenases, but not present in dihydroxynaphthalene dioxygenases. Different substitutions in position 282 had different effects on general activity or substrate specificity, thus showing the functional importance of the most C-terminal beta-sheet of the protein. A251M and G206M mutants showed increased activity specifically for a particular substrate. N213H, G206M, A282R, A282G and Y177I substitutions resulted in enzymes more tolerant to acidic pH, the most striking effect being observed in mutant Y177I, which showed maximal activity at pH 5.5. In addition, Q198D and V175D mutants, which had altered K(m), also showed altered sensitivity to substrate inhibition, thus indicating that inhibition is exerted through the same binding site. This mutational analysis, therefore, identified conserved residues important for activity or substrate specificity, and also shed some light on the mechanism of substrate inhibition exhibited by extradiol dioxygenases.

EPR and ENDOR evidence for a 1-His, hydroxo-bridged mixed-valent diiron site in Desulfovibrio vulgaris rubrerythrin

Key features differentiating the coordination environment of the two irons in the mixed-valent (Fe(2+),Fe(3+)) diiron site of Desulfovibrio vulgaris rubrerythrin (Rbr(mv)) were determined by continuous wave (CW) and pulsed ENDOR spectroscopy at 35GHz. (14)N ENDOR evidence indicates that a nitrogen is bound only to the Fe(2+) ion of the mixed-valent site. Assuming that this nitrogen is from His131Ndelta, the same one that furnishes an iron ligand in the crystal structure of the diferric site, the ENDOR data allow us to specify the Fe(2+) and Fe(3+) positions within the molecular reference frame. In addition, the (1,2)H ENDOR on Rbr(mv) indicates the presence of a solvent-derived aqua/hydroxo ligand bound either terminally or in a bridging mode to Fe(3+) in the mixed-valent site. The relatively large g anisotropy of Rbr(mv) and weak antiferromagnetic coupling, J approximately -8 cm(-)(1) (in the 2JS(1)*S(2) formalism), between the irons is more consistent with a bridging than terminal hydroxo ligand. gamma-Irradiation was used to cryoreduce Rbr at 77 K, thereby producing a mixed-valent diiron site [(Rbr(ox))(mv)] that retains the structure of the diferric site. The EPR spectrum of (Rbr(ox))(mv) was nearly identical to that of the as-isolated or chemically reduced samples. This near identity implies that the structure of the mixed-valent Rbr diiron site is essentially identical to that of the diferric site, except for protonation of the oxo bridge, which apparently occurred via a proton jump from hydrogen-bonded solvent at 77 K. The EPR spectrum of (Rbr(ox))(mv) thus supports the (14)N ENDOR-assigned His131 ligation to Fe(2+) and assignment of the solvent-derived ligand observed in the (1,2)H ENDOR to a hydroxo bridge between the irons of the mixed-valent diiron site.

Substrate-induced conformational changes in Escherichia coli taurine/alpha-ketoglutarate dioxygenase and insight into the oligomeric structure

The enzymes in the alpha-ketoglutarate (alphaKG) dependent dioxygenase superfamily represent the largest class of non-heme iron oxidases and have important medical, ecological, and biotechnological roles. One such enzyme, taurine/alpha-ketoglutarate dioxygenase (TauD), catalyzes the conversion of 2-aminoethanesulfonate (taurine) to sulfite and aminoacetaldehyde while decomposing alphaKG to succinate and CO(2). This alphaKG dependent dioxygenase is expressed in Escherichia coli under sulfur starvation conditions and allows the cell to utilize taurine, and other similar sulfonates in the environment, as an alternative sulfur source. In this work, we report the structures of the apo and holo forms of TauD to 1.9 A resolution (R(cryst) = 21.2%, R(free) = 24.9%) and 2.5 A resolution (R(cryst) = 22.5%, R(free) = 27.8%), respectively. The models reported herein provide significant new insight into the substrate orientations at the active site and the conformational changes that are induced upon taurine binding. Furthermore, analysis of our crystallographic data coupled with reanalysis of the crystallographic model (resolution = 3.0 A, R(cryst) = 28.1, R(free) = 32.0) presented by Elkins et al. (Biochemistry (2002) 41, 5185-5192) reveals an alternative oligomeric arrangement for the enzyme that is consistent with the conserved primary and secondary structure elements of other alphaKG dependent dioxygenases.

Spectroscopic and kinetic studies of PKU-inducing mutants of phenylalanine hydroxylase: Arg158Gln and Glu280Lys

Phenylalanine hydroxylase (PAH) is a tetrahydrobiopterin-dependent, nonheme iron enzyme that catalyzes the hydroxylation of L-Phe to L-Tyr in the rate-limiting step of phenylalanine catabolism. This reaction is tightly coupled in the wild-type enzyme to oxidation of the tetrahydropterin cofactor. Dysfunction of PAH activity in humans leads to the disease phenylketonuria (PKU). We have investigated two PKU-inducing mutants, Arg158Gln and Glu280Lys, using kinetic methods, magnetic circular dichrosim (MCD) spectroscopy, and X-ray absorption spectroscopy (XAS). Analysis of the products produced by the mutant enzymes shows that although both oxidize pterin at more than twice the rate of wild-type enzyme, these reactions are only approximately 20% coupled to production of L-Tyr. Previous MCD and XAS studies had demonstrated that the resting Fe(II) site is six-coordinate in the wild-type enzyme and converts to a five-coordinate site when both L-Phe and reduced pterin are present in the active site. Although the Arg158Gln mutant forms the five-coordinate site when both cosubstrates are bound, the Fe(II) site of the Glu280Lys mutant remains six-coordinate. These results provide insight into the PAH reaction and disease mechanism at a molecular level, indicating that the first step of the mechanism is formation of a peroxy-pterin species, which subsequently reacts with the Fe(II) site if the pterin is properly oriented for formation of an Fe-OO-pterin bridge and an open coordination position is available on the Fe(II).

Fe(II) and Fe(III) mononuclear complexes with a pentadentate ligand built on the 1,3-diaminopropane unit. Structures and spectroscopic and electrochemical properties. Reaction with H2O2

Two new iron complexes, [L(5)(3)Fe(II)Cl]PF(6) (1.PF(6)) and [(L(5)(3)H(+))Fe(III)Cl(3)]PF(6) (2.PF(6)), were synthesized (L(5)(3) = N-methyl-N,N',N'-tris(2-pyridylmethyl)propane-1,3-diamine), and their molecular structures were determined by X-ray crystallography. Their behavior in solution was studied by UV-vis spectroscopy and electrochemistry. Upon addition of a base to an acetonitrile solution of 2, the new unsymmetrical dinuclear complex [L(5)(3)Fe(III)OFe(III)Cl(3)](+) was detected. Treating 1 with hydrogen peroxide has allowed us to detect the low spin [L(5)(3)Fe(III)OOH](2+). Its spectroscopic properties (UV-vis, EPR and resonance Raman) are similar to those reported for related FeOOH complexes obtained with amine/pyridine ligands. Using stopped-flow absorption spectroscopy, the formation and degradation of [L(5)(3)Fe(III)OOH](2+) has been monitored, and a mechanism is proposed to reproduce the kinetic data.

An FeIV=O complex of a tetradentate tripodal nonheme ligand

The reaction of [Fe(II)(tris(2-pyridylmethyl)amine, TPA)(NCCH(3))(2)](2+) with 1 equiv. peracetic acid in CH(3)CN at -40 degrees C results in the nearly quantitative formation of a pale green intermediate with lambda(max) at 724 nm ( epsilon approximately 300 M(-1).cm(-1)) formulated as [Fe(IV)(O)(TPA)](2+) by a combination of spectroscopic techniques. Its electrospray mass spectrum shows a prominent feature at mz 461, corresponding to the [Fe(IV)(O)(TPA)(ClO(4))](+) ion. The Mössbauer spectra recorded in zero field reveal a doublet with DeltaE(Q) = 0.92(2) mms and delta = 0.01(2) mms; analysis of spectra obtained in strong magnetic fields yields parameters characteristic of S = 1 Fe(IV)O complexes. The presence of an Fe(IV)O unit is also indicated in its Fe K-edge x-ray absorption spectrum by an intense 1-s --> 3-d transition and the requirement for an ON scatterer at 1.67 A to fit the extended x-ray absorption fine structure region. The [Fe(IV)(O)(TPA)](2+) intermediate is stable at -40 degrees C for several days but decays quantitatively on warming to [Fe(2)(mu-O)(mu-OAc)(TPA)(2)](3+). Addition of thioanisole or cyclooctene at -40 degrees C results in the formation of thioanisole oxide (100% yield) or cyclooctene oxide (30% yield), respectively; thus [Fe(IV)(O)(TPA)](2+) is an effective oxygen-atom transfer agent. It is proposed that the Fe(IV)O species derives from OO bond heterolysis of an unobserved Fe(II)(TPA)-acyl peroxide complex. The characterization of [Fe(IV)(O)(TPA)](2+) as having a reactive terminal Fe(IV)O unit in a nonheme ligand environment lends credence to the proposed participation of analogous species in the oxygen activation mechanisms of many mononuclear nonheme iron enzymes.

Interconversion of two oxidized forms of taurine/alpha-ketoglutarate dioxygenase, a non-heme iron hydroxylase: evidence for bicarbonate binding

Taurinealpha-ketoglutarate (alphaKG) dioxygenase, or TauD, is a mononuclear non-heme iron hydroxylase that couples the oxidative decarboxylation of alphaKG to the decomposition of taurine, forming sulfite and aminoacetaldehyde. Prior studies revealed that taurine-free TauD catalyzes an O(2)- and alphaKG-dependent self-hydroxylation reaction involving Tyr-73, yielding an Fe(III)-catecholate chromophore with a lambda(max) of 550 nm. Here, a chromophore (lambda(max) 720 nm) is described and shown to arise from O(2)-dependent self-hydroxylation of TauD in the absence of alphaKG, but requiring the product succinate. A similar chromophore rapidly develops with the alternative oxidant H(2)O(2). Resonance Raman spectra indicate that the approximately 700-nm chromophore also arises from an Fe(III)-catecholate species, and site-directed mutagenesis studies again demonstrate Tyr-73 involvement. The approximately 700-nm and 550-nm species are shown to interconvert by the addition or removal of bicarbonate, consistent with the alphaKG-derived CO(2) remaining tightly bound to the oxidized metal site as bicarbonate. The relevance of the metal-bound bicarbonate in TauD to reactions of other members of this enzyme family is discussed.

A density functional investigation of the extradiol cleavage mechanism in non-heme iron catechol dioxygenases

The mechanism for extradiol cleavage in non-heme iron catechol dioxygenase was modelled theoretically via density functional theory. Based on the Fe(II)-His,His,Glu motif observed in enzymes, an active site model complex, [Fe(acetate)(imidazole)(2)(catecholate)(O(2))](-), was optimized for states with six, four and two unpaired electrons (U6, U4 and U2, respectively). The transfer of the terminal atom of the coordinated dioxygen leading to "ferryl" Fe=O intermediates spontaneously generates an extradiol epoxide. The computed barriers range from 19 kcal mol(-1) on the U6 surface to approximately 25 kcal mol(-1) on the U4 surface, with overall reaction energies of +11.6, 6.3 and 7.1 kcal mol(-1) for U6, U4 and U2, respectively. The calculations for a protonated process reveal the terminal oxygen of O(2) to be the thermodynamically favoured site but subsequent oxygen transfer to the catechol has a barrier of approximately 30-40 kcal mol(-1), depending on the spin state. Instead, protonating the acetate group gives a slightly higher energy species but a subsequent barrier on the U4 surface of only 7 kcal mol(-1) relative to the hydroperoxide complex. The overall exoergicity increases to 13 kcal mol(-1). The favoured proton-assisted pathway does not involve significant radical character and has features reminiscent of a Criegee rearrangement which involves the participation of the aromatic ring pi-orbitals in the formation of the new carbon-oxygen bond. The subsequent collapse of the epoxide, attack by the coordinated hydroxide and final product formation proceeds with an overall exoergicity of approximately 75 kcal mol(-1) on the U4 surface.

Non-heme iron enzymes: contrasts to heme catalysis

Non-heme iron enzymes catalyze a wide range of O(2) reactions, paralleling those of heme systems. Non-heme iron active sites are, however, much more difficult to study because they do not exhibit the intense spectral features characteristic of the porphyrin ligand. A spectroscopic methodology was developed that provides significant mechanistic insight into the reactivity of non-heme ferrous active sites. These studies reveal a general mechanistic strategy used by these enzymes and differences in substrate and cofactor interactions dependent on their requirement for activation by iron. Contributions to O(2) activation have been elucidated for non-heme relative to heme ligand sets, and major differences in reactivity are defined with respect to the heterolytic and homolytic cleavage of O-O bonds.

Synthesis, X-ray crystal structure, and redox and electronic properties of iron(III)-polyimidazole complexes relevant to the metal sites of iron proteins

A new tripod N(3) ligand (L), containing three imidazole rings, was synthesized in good yield. At variance with usual aromatic ligands with N(2) or N(3) donor sets such as pyridine or pyrazole derivatives, L stabilizes the Fe(III) oxidation state. The corresponding iron(III) complexes [Fe(L)Cl(3)] (1) and [Fe(L)(2)](ClO(4))(3) (2) were prepared and characterized by X-ray structural analysis and spectroscopic methods. The coordination environment around all the Fe(III) centers has a distorted octahedral geometry. [Fe(L)Cl(3)] (1) belongs to the monoclinic system, space group P2(1)/n, a = 9.7406(5) A, b = 17.207(2) A, c = 14.615(2) A, beta = 104.448(9)(o) Z = 4, V = 2372.1(4) A(3); R = 0.044, R(w) = 0.055. [Fe(L)(2)](ClO(4))(3) (2) belongs to the monoclinic system, space group P2(1)/c, a = 16.1057(15) A, b = 11.1079(12) A, c = 26.283(2) A, beta = 102.062(10)(o), Z = 4, V = 4598.2(8) A(3); R = 0.0465, R(w) = 0.0902. The Fe-N((i)PrIm) bond lengths are systematically longer than the Fe-N(MeIm) ones. Compound 2 is a highly anisotropic low-spin Fe(III) complex displaying a rather unusual EPR spectrum with a sharp signal at g = 3.5 and a broad one at g approximately 1.6. The fitting of this EPR spectrum is discussed.

The selectivity and inhibition of AlkB

AlkB is one of four proteins involved in the adaptive response to DNA alkylation damage in Escherichia coli and is highly conserved from bacteria to humans. Recent analyses have verified the prediction that AlkB is a member of the Fe(II) and 2-oxoglutarate (2OG)-dependent oxygenase family of enzymes. AlkB mediates repair of methylated DNA by direct demethylation of 1-methyladenine and 3-methylcytosine lesions. Other members of the Fe(II) and 2OG-dependent oxygenase family, including those involved in the hypoxic response, are targets for therapeutic intervention. Assays measuring 2OG turnover were used to investigate the selectivity of AlkB. 1-Methyladenosine, 1-methyl-2'-deoxyadenosine, 3-methylcytidine, and 3-methyl-2'-deoxycytidine all stimulated 2OG turnover by AlkB but were not demethylated indicating an uncoupling of 2OG and prime substrate oxidation and that oligomeric DNA is required for hydroxylation and subsequent demethylation. In contrast the equivalent unmethylated nucleosides did not stimulate 2OG turnover indicating that the presence of a methyl group in the substrate is important in initiating oxidation of 2OG. Stimulation of 2OG turnover by 1-methyladenosine was highly dependent on the presence of a reducing agent, ascorbate or dithiothreitol. Following the observation that AlkB is inhibited by high concentrations of 2OG, analogues of 2OG, including 2-mercaptoglutarate, were found to specifically inhibit AlkB. The flavonoid quercetin inhibits both AlkB and the 2OG oxygenase factor-inhibiting hypoxia-inducible factor (FIH) in vitro. FIH inhibition by quercetin occurs in the presence of excess iron indicating a specific interaction, while the inhibition of AlkB by quercetin is, predominantly, due to nonspecific iron chelation.

A flavodiiron protein and high molecular weight rubredoxin from Moorella thermoacetica with nitric oxide reductase activity

A five-gene "oxidative stress protection" cluster has recently been described from the strictly anaerobic, acetogenic bacterium, Moorella thermoacetica [Das, A., et al. (2001) J. Bacteriol. 183, 1560-1567]. Within this cluster are two cotranscribed genes, fprA (for A-type flavoprotein) and hrb (for high molecular weight rubredoxin) whose encoded proteins have no known functions. Here we show that FprA and Hrb are expressed in M. thermoacetica under normal anaerobic growth conditions and report characterizations of the recombinant FprA and Hrb. FprA contains flavin mononucleotide (FMN) and a non-heme diiron site. Mössbauer spectroscopy shows that the irons of the diferric site are antiferromagnetically coupled, implying a single-atom, presumably solvent, bridge between the irons. Hrb contains FMN and a rubredoxin-like [Fe(SCys)4] site. NADH does not directly reduce either the FMN or the diiron site in FprA, whereas Hrb functions as an efficient NADH:FprA oxidoreductase. Substitution of zinc for iron in Hrb completely abolished this activity. The observation that homologues of FprA from other organisms show O2 and/or anaerobic NO consumption activity prompted an examination of these activities for M. thermoacetica FprA. The Hrb/FprA combination does indeed have both NADH:O2 and NADH:NO oxidoreductase activities. The NO reductase activity, however, was significantly more efficient due to a lower Km for NO (4 M) and to progressive and irreversible inactivation of FprA during O2 reductase turnover but retention of activity during NO reductase turnover. Substitution of zinc for iron in FprA completely abolished these reductase activities. The stoichiometry of 1 mol of NADH oxidized:2 mol of NO consumed implies reduction to N2O. Fits of an appropriate rate law to the kinetics data are consistent with a mechanism in which 2NO's react at each FprA active site in the committed step. Expression of FprA in an Escherichia coli strain deficient in NO reductase restored the anaerobic growth phenotype of cultures exposed to otherwise toxic levels of exogenous NO. The accumulated results indicate that Hrb/FprA is fully capable of functioning in nitrosative stress protection in M. thermoacetica.

Reaction of (mu-oxo)diiron(III) core with CO2 in N-methylimidazole: formation of mono(mu-carboxylato)(mu-oxo)diiron(III) complexes with N-methylimidazole as ligands

Several iron(III) complexes with N-methylimidazole (N-MeIm) as the ligand have been synthesized by using N-MeIm as the solvent. Under anaerobic conditions, [Fe(N-MeIm)(6)](ClO(4))(3) (1) reacts with stoichiometric amounts of water in N-MeIm to afford the (mu-oxo)diiron(III) complex, [Fe(2)(mu-O)(N-MeIm)(10)](ClO(4))(4) (3). Exposure of a solution of 3 in N-MeIm to stoichiometric and excess CO(2) gives rise to the (mu-oxo)(mu-carboxylato)diiron(III) species [Fe(2)(mu-O)(mu-HCO(2))(N-MeIm)(8)](ClO(4))(3) (4) and the methyl carbonate complex [Fe(2)(mu-O)(mu-CH(3)OCO(2))(N-MeIm)(8)](ClO(4))(3) (5), respectively. Formation of the formato-bridged complex 4 upon fixation of CO(2) by 3 in N-MeIm is unprecedentated. Methyl transfer from N-MeIm to a bicarbonato-bridged (mu-oxo)diiron(III) intermediate appears to give rise to 5. Complex 3 is a good starting material for the synthesis of (mu-oxo)mono(mu-carboxylato)diiron(III) species [Fe(2)(mu-O)(mu-RCO(2))(N-MeIm)(8)](ClO(4))(3) (where R = H (4), CH(3) (6), or C(6)H(5) (7)); addition of the respective carboxylate ligand in stoichiometric amount to a solution of 3 in N-MeIm affords these complexes in high yields. Attempts to add a third bridge to complexes 4, 6, and 7 to form the (mu-oxo)bis(mu-carboxylato)diiron(III) species result in the isolation of the previously known triiron(III) mu-eta(3)-oxo clusters [[Fe(mu-RCO(2))(2)(N-MeIm)](3)O](ClO(4)) (8). The structures of 3, 4, 6, and 7 allow one, for the first time, to inspect the various features of the [Fe(2)(mu-O)(mu-RCO(2))](3+) moiety with no strain from the ligand framework.

Studies on the regulatory properties of the pterin cofactor and dopamine bound at the active site of human phenylalanine hydroxylase

The catalytic activity of phenylalanine hydroxylase (PAH, phenylalanine 4-monooxygenase EC 1.14.16.1) is regulated by three main mechanisms, i.e. substrate (l-phenylalanine, L-Phe) activation, pterin cofactor inhibition and phosphorylation of a single serine (Ser16) residue. To address the molecular basis for the inhibition by the natural cofactor (6R)-l-erythro-5,6,7,8-tetrahydrobiopterin, its effects on the recombinant tetrameric human enzyme (wt-hPAH) was studied using three different conformational probes, i.e. the limited proteolysis by trypsin, the reversible global conformational transition (hysteresis) triggered by L-Phe binding, as measured in real time by surface plasmon resonance analysis, and the rate of phosphorylation of Ser16 by cAMP-dependent protein kinase. Comparison of the inhibitory properties of the natural cofactor with the available three-dimensional crystal structure information on the ligand-free, the binary and the ternary complexes, have provided important clues concerning the molecular mechanism for the negative modulatory effects. In the binary complex, the binding of the cofactor at the active site results in the formation of stabilizing hydrogen bonds between the dihydroxypropyl side-chain and the carbonyl oxygen of Ser23 in the autoregulatory sequence. L-Phe binding triggers local as well as global conformational changes of the protomer resulting in a displacement of the cofactor bound at the active site by 2.6 A (mean distance) in the direction of the iron and Glu286 which causes a loss of the stabilizing hydrogen bonds present in the binary complex and thereby a complete reversal of the pterin cofactor as a negative effector. The negative modulatory properties of the inhibitor dopamine, bound by bidentate coordination to the active site iron, is explained by a similar molecular mechanism including its reversal by substrate binding. Although the pterin cofactor and the substrate bind at distinctly different sites, the local conformational changes imposed by their binding at the active site have a mutual effect on their respective binding affinities.

Biomimetic aryl hydroxylation derived from alkyl hydroperoxide at a nonheme iron center. Evidence for an Fe(IV)=O oxidant

Many nonheme iron-dependent enzymes activate dioxygen to catalyze hydroxylations of arene substrates. Key features of this chemistry have been developed from complexes of a family of tetradentate tripodal ligands obtained by modification of tris(2-pyridylmethyl)amine (TPA) with single alpha-arene substituents. These included the following: -C(6)H(5) (i.e., 6-PhTPA), L(1); -o-C(6)H(4)D, o-d(1)-L(1); -C(6)D(5), d(5)-L(1); -m-C(6)H(4)NO(2), L(2); -m-C(6)H(4)CF(3), L(3); -m-C(6)H(4)Cl, L(4); -m-C(6)H(4)CH(3), L(5); -m-C(6)H(4)OCH(3), L(6); -p-C(6)H(4)OCH(3), L(7). Additionally, the corresponding ligand with one alpha-phenyl and two alpha-methyl substituents (6,6-Me(2)-6-PhTPA, L(8)) was also synthesized. Complexes of the formulas [(L(1))Fe(II)(NCCH(3))(2)](ClO(4))(2), [(L(n)())Fe(II)(OTf)(2)] (n = 1-7, OTf = (-)O(3)SCF(3)), and [(L(8))Fe(II)(OTf)(2)](2) were obtained and characterized by (1)H NMR and UV-visible spectroscopies and by X-ray diffraction in the cases of [(L(1))Fe(II)(NCCH(3))(2)](ClO(4))(2), [(L(6))Fe(II)(OTf)(2)], and [(L(8))Fe(II)(OTf)(2)](2). The complexes react with tert-butyl hydroperoxide ((t)()BuOOH) in CH(3)CN solutions to give iron(III) complexes of ortho-hydroxylated ligands. The product complex derived from L(1) was identified as the solvated monomeric complex [(L(1)O(-))Fe(III)](2+) in equilibrium with its oxo-bridged dimer [(L(1)O(-))(2)Fe(III)(2)(mu(2)-O)](2+), which was characterized by X-ray crystallography as the BPh(4)(-) salt. The L(8) product was also an oxo-bridged dimer, [(L(8)O(-))(2)Fe(III)(2)(mu(2)-O)](2+). Transient intermediates were observed at low temperature by UV-visible spectroscopy, and these were characterized as iron(III) alkylperoxo complexes by resonance Raman and EPR spectroscopies for L(1) and L(8). [(L(1))Fe(II)(OTf)(2)] gave rise to a mixture of high-spin (S = 5/2) and low-spin (S = 1/2) Fe(III)-OOR isomers in acetonitrile, whereas both [(L(1))Fe(OTf)(2)] in CH(2)Cl(2) and [(L(8))Fe(OTf)(2)](2) in acetonitrile afforded only high-spin intermediates. The L(1) and L(8) intermediates both decomposed to form respective phenolate complexes, but their reaction times differed by 3 orders of magnitude. In the case of L(1), (18)O isotope labeling indicated that the phenolate oxygen is derived from the terminal peroxide oxygen via a species that can undergo partial exchange with exogenous water. The iron(III) alkylperoxo intermediate is proposed to undergo homolytic O-O bond cleavage to yield an oxoiron(IV) species as an unobserved reactive intermediate in the hydroxylation of the pendant alpha-aryl substituents. The putative homolytic chemistry was confirmed by using 2-methyl-1-phenyl-2-propyl hydroperoxide (MPPH) as a probe, and the products obtained in the presence and in the absence of air were consistent with formation of alkoxy radical (RO(*)). Moreover, when one ortho position was labeled with deuterium, no selectivity was observed between hydroxylation of the deuterated and normal isotopomeric ortho sites, but a significant 1,2-deuterium shift ("NIH shift") occurred. These results provide strong mechanistic evidence for a metal-centered electrophilic oxidant, presumably an oxoiron(IV) complex, in these arene hydroxylations and support participation of such a species in the mechanisms of the nonheme iron- and pterin-dependent aryl amino acid hydroxylases.

(4-Hydroxyphenyl)pyruvate dioxygenase from Streptomyces avermitilis: the basis for ordered substrate addition

(4-hydroxyphenyl)pyruvate dioxygenase (HPPD) catalyzes the second step in the pathway for the catabolism of tyrosine, the conversion of (4-hydroxyphenyl)pyruvate (HPP) to homogentisate (HG). This reaction involves decarboxylation, substituent migration, and aromatic oxygenation. HPPD is a member of the alpha-keto acid dependent oxygenases that require Fe(II) and an alpha-keto acid substrate to oxygenate an organic molecule. We have examined the binding of ligands to HPPD from Streptomyces avermitilis. Our data show that HPP binds to the apoenzyme and that the apo-HPPD.HPP complex does not bind Fe(II) to generate active holoenzyme. The binding of HPP, phenylpyruvate (PPA), and pyruvate to the holoenzyme produces a weak ligand charge-transfer band at approximately 500 nm that is indicative of bidentate binding of the 1-carboxylate and 2-keto pyruvate oxygen atoms to the active site metal ion. For HPPD from this organism the 4-hydroxyl group of (4-hydroxyphenyl)pyruvate is a requirement for catalysis; no turnover is observed in the presence of phenylpyruvate. The rate constant for the dissociation of Fe(II) from the holoenzyme is 0.0006 s(-)(1) and indicates that this phenomenon is not significantly relevant in steady-state turnover. The addition of HPP and molecular oxygen to the holoenzyme is formally random. The basis of the ordered bi bi steady-state kinetic mechanism previously observed by Rundgren (Rundgren, M. (1977) J. Biol. Chem. 252, 5094-9) is the 3600-fold increase in oxygen reactivity when holo-HPPD is in complex with HPP. This complex reacts with molecular oxygen with a second-order rate constant of 1.4 x 10(5) M(-)(1) s(-)(1) inducing the formation of an intermediate that decays at the catalytically relevant rate of 7.8 s(-)(1).

Crystallographic and spectroscopic characterization of a nonheme Fe(IV)-O complex

Following the heme paradigm, it is often proposed that dioxygen activation by nonheme monoiron enzymes involves an iron(IV)=oxo intermediate that is responsible for the substrate oxidation step. Such a transient species has now been obtained from a synthetic complex with a nonheme macrocyclic ligand and characterized spectroscopically. Its high-resolution crystal structure reveals an iron-oxygen bond length of 1.646(3) angstroms, demonstrating that a terminal iron(IV)=oxo unit can exist in a nonporphyrin ligand environment and lending credence to proposed mechanisms of nonheme iron catalysis.

Spectroscopic and computational studies of a Ni(+)-CO model complex: implications for the acetyl-CoA synthase catalytic mechanism

The four-coordinate Ni(+) complex [PhTt(t)(Bu)]Ni(I)CO, where PhTt(t)()(Bu) = phenyltris((tert-buthylthio)methyl)borate (a tridentate thioether donor ligand), serves as a possible model for key Ni-CO reaction intermediates in the acetyl-CoA synthase (ACS) catalytic cycle. Resonance Raman, electronic absorption, magnetic circular dichroism (MCD), variable-temperature variable-field MCD, and electron paramagnetic resonance spectroscopies were utilized in conjunction with density functional theory and semiemperical INDO/S-CI calculations to investigate the ground and excited states of [PhTt(t)()(Bu)]Ni(I)CO. These studies reveal extensive Ni(+) --> CO pi-back-bonding interactions, as evidenced by a low C-O stretching frequency (1995 cm(-)(1)), a calculated C-O stretching force constant of 15.5 mdyn/A (as compared to k(CO)(free CO) = 18.7 mdyn/A), and strong Ni(+) --> CO charge-transfer absorption intensities. Calculations reveal that this high degree of pi-back-bonding is due to the fact that the Ni(+) 3d orbitals are in close energetic proximity to the CO pi acceptor orbitals. In the ACS "paramagnetic catalytic cycle", the high degree of pi-back-bonding in the putative Ni(+)-CO intermediate (the NiFeC species) is not expected to preclude methyl transfer from CH(3)-CoFeSP.

Spectroscopic studies of the effect of ligand donor strength on the Fe-NO bond intradiol dioxygenases

The geometric and electronic structure of NO bound to reduced protocatechuate 3,4-dioxygenase and its substrate (3,4-dihydroxybenzoate, PCA) complex have been examined by X-ray absorption (XAS), UV-vis absorption (Abs), magnetic circular dichroism (MCD), and variable temperature variable field (VTVH) MCD spectroscopies. The results are compared to those previously published on model complexes described as [FeNO]7 systems in which an S = 5/2 ferric center is antiferromagnetically coupled to an S = 1 NO-. XAS pre-edge analysis indicates that the Fe-NO units in FeIIIPCD[NO-] and FeIIIPCD[PCA,NO-] lack the greatly increased pre-edge intensity representative of most [FeNO]7 model sites. Furthermore, from extended X-ray absorption fine structure (EXAFS) analysis, the FeIIIPCD[NO-] and FeIIIPCD[PCA,NO-] active sites are shown to have an Fe-NO distance of at least 1.91 A, approximately 0.2 A greater than those found in the model complexes. The weakened Fe-NO bond is consistent with the overall lengthening of the bond lengths and the fact that VTVH MCD data show that NO(-)-->FeIII CT transitions are no longer polarized along the z-axis of the zero-field splitting tensor. The weaker Fe-NO bond derives from the strong donor interaction of the endogenous phenolate and substrate catecholate ligands, which is observed from the increased intensity in the CT region relative to that of [FeNO]7 model complexes, and from the shift in XAS edge position to lower energy. As NO is an analogue of O2, the effect of endogenous ligand donor strength on the Fe-NO bond has important implications with respect to O2 activation by non-heme iron enzymes.

Computational study of the non-heme iron active site in superoxide reductase and its reaction with superoxide

The ferrous square-pyramidal [Fe(NHis)4(SCys)] site of superoxide reductases (SORs) has been shown to reduce superoxide at a nearly diffusion-controlled rate. The final products of the reaction are hydrogen peroxide and the ferric hexacoordinated SOR site, with a carboxylate group from a conserved glutamate serving as the sixth ligand trans to the cysteine sulfur. A transient intermediate absorbing at approximately 600 nm in the reaction of the ferrous pentacoordinated site with superoxide has been proposed to be a ferric-(hydro)peroxo complex (Coulter, E.; Emerson, J.; Kurtz, D. M., Jr.; Cabelli, D. J. Am. Chem. Soc. 2000, 122, 11555-11556.). In the present study, DFT and ZINDO/S-CI results are shown to support the description of the 600-nm intermediate as an end-on, low-spin ferricperoxo or--hydroperoxo complex. Side-on peroxo coordination was found to be significantly less stable than end-on because of constraints on the imidazole ligand ring orientations imposed mostly by the protein. The modeled ferric-hydroperoxo complex had a decidedly nonplanar CysC beta-S-Fe-O-O geometry that appears to be imposed by the same constraints. A single prominent visible absorption of the (hydro)peroxo model is shown to be due mainly to a CysS-->Fe(III) pi charge transfer (CT) transition with a minor portion of His-->Fe(III) pi CT character and very little peroxo-->Fe(III) CT character. On the basis of calculations of models with various mono- and diprotonated peroxo ligands, protonation of the iron-bound peroxo oxygen is a key step in the decay of the ferric(hydro)peroxo complex favoring release of hydrogen peroxide over cleavage of the O-O bond, as occurs in the heme structural analogue, cytochrome P450.

Electronic structure and reactivity of high-spin iron--alkyl- and--pterinperoxo complexes

The spectroscopic properties and electronic structure of the four-coordinate high-spin [FeIII(L3)(OOtBu)]+ complex (1; L3 = hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate; tBu = tert-butyl) are investigated and compared to the six-coordinated high-spin [Fe(6-Me3TPA)(OHx)(OOtBu)]x+ system (TPA = tris(2-pyridylmethyl)amine, x = 1 or 2) studied earlier [Lehnert, N.; Ho, R. Y. N.; Que, L., Jr.; Solomon, E. I. J. Am. Chem. Soc. 2001, 123, 12802-12816]. Complex 1 is characterized by Raman features at 889 and 830 cm-1 which are assigned to the O-O stretch (mixed with the symmetric C-C stretch) and a band at 625 cm-1 that corresponds to nu(Fe-O). The UV-vis spectrum shows a charge-transfer (CT) transition at 510 nm from the alkylperoxo pi v* (v = vertical to C-O-O plane) to a d orbital of Fe(III). A second CT is identified from MCD at 370 nm that is assigned to a transition from pi h* (h = horizontal to C-O-O plane) to an Fe(III) d orbital. For the TPA complex the pi v* CT is at 560 nm while the pi h* CT is to higher energy than 250 nm. These spectroscopic differences between four- and six-coordinate Fe(III)-OOR complexes are interpreted on the basis of their different ligand fields. In addition, the electronic structure of Fe-OOPtn complexes with the biologically relevant pterinperoxo ligand are investigated. Substitution of the tert-butyl group in 1 by pterin leads to the corresponding Fe(III)-OOPtn species (2), which shows a stronger electron donation from the peroxide to Fe(III) than 1. This is related to the lower ionization potential of pterin. Reduction of 2 by one electron leads to the Fe(II)-OOPtn complex (3), which is relevant as a model for potential intermediates in pterin-dependent hydroxylases. However, in the four-coordinate ligand field of 3, the additional electron is located in a nonbonding d orbital of iron. Hence, the pterinperoxo ligand is not activated for heterolytic cleavage of the O-O bond in this system. This is also evident from the calculated reaction energies that are endothermic by at least 20 kcal/mol.

Electrochemical Behaviour of trans-[FeH(CN)(dppe)2] Adducts

Electrochemical behaviour of various metallo cyano adducts of trans-[FeH(CN)(dppe)2], viz. the dinuclear complexes [FeH(dppe)2(μ-CN)PdCl2(PPh3)], [FeH(dppe)2(μ-CN)NiCl2(PCy3)], and trinuclear [{FeH(dppe)2(μ-CN)}2(ReOCl3)], [{FeH(dppe)2(μ-CN)}2PtCl(Ph)] and [{FeX(dppe)2(μ-CN)}2WCl3(OH)] (X = Cl or OH, dppe = 1,2-diphenylphosphinoethane, PCy3 = P(C6H11)3), as well as the benzoylisocyanide mononuclear adduct trans-[FeH(CNCOPh)- (dppe)2], is reported. All of them exhibit FeII/FeIII-based oxidations (which are reversible, except for trans-[FeH(CNCOPh)(dppe)2]). The metallocyanide bridges C≡N-Re-N≡C and C≡N-W-N≡C in [{FeH(dppe)2(μ-CN)}2(ReOCl3)] and [{FeX(dppe)2(μ-CN)}2WCl3(OH)], respectively, allow electronic communication between the iron centres, with possible generation of mixed-valence FeII/FeIII complexes whose comproportionation constant could be estimated in the former case. From the values of the measured oxidation potentials, the electrochemical PL and EL ligand parameters have been estimated for the metallocyanide ligands that were shown to behave as stronger net electron donors than organoisocyanides, although weaker than cyanide itself. Ligand-centred reduction processes were also observed to lead, in the cases of complexes trans-[FeH(CNCOPh)(dppe)2] (CNCOPh-based reduction) and [FeH(dppe)2(μ-CN)PdCl2(PPh3)] (PdII-based reduction in the CNPdCl2(PPh3)- metallocyanide ligand), to the dissociation of the adduct, with regeneration of the parent cyano complex trans-[FeH(CN)(dppe)2], thus reflecting the reductive decrease of the electrophilic (or Lewis acidic) character of the benzoyl and {PdCl2(PPh3)} groups.

Membrane-bound diiron carboxylate proteins

Four proteins have been identified recently as diiron carboxylate proteins on the basis of conservation of six amino acids (four carboxylate residues and two histidines) constituting an iron-binding motif. Unlike previously identified proteins with this motif, biochemical studies indicate that each of these proteins is membrane bound, although homology modeling rules out a transmembrane mode of binding. Therefore, the predicted structure of each protein [the alternative oxidase (AOX), the plastid terminal oxidase (PTOX), the diiron 5-demethoxyquinone hydroxylase (DMQ hydroxylase), and the aerobic Mg-protoporphyrin IX monomethylester hydroxylase (MME hydroxylase)] is that of a protein bound monotopically to one leaflet of the membrane bilayer. Three of these enzymes utilize a quinol substrate, with two oxidizing the quinol (AOX and PTOX) and one hydroxylating it (DMQ hydroxylase). MME hydroxylase is involved in synthesis of the isocyclic ring of chlorophyll. Two enzymes are involved in respiration (AOX and, indirectly, the diiron DMQ hydroxylase through ubiquinone biosynthesis) and two in photosynthesis, through their roles in carotenoid and chlorophyll biosynthesis (PTOX and MME hydroxylase, respectively). We discuss what is known about each enzyme as well as our expectations based on their identification as interfacially bound proteins with a diiron carboxylate active site.

Oxygen-oxygen bond homolysis in a novel titanium(IV) alkylperoxide complex, Cp2Ti(OOtBu)Cl

Cp2TiCl2 reacts with NaOOtBu to form the new titanium peroxide complex, Cp2Ti(OOtBu)Cl (1), which has been characterized both in solution and in the solid state. This complex is surprisingly unreactive towards olefins and phosphines, as it does not directly transfer an oxygen atom. Instead, decomposition occurs via initial homolysis of the oxygen-oxygen bond, yielding a tert-butoxyl radical. Decomposition of 1 in the presence of phosphines yields either phosphine oxides (e.g., OPPh3) or phosphinites (e.g., tBuOPEt2), products that result from tBuO* + PR3. O-O bond homolysis is surprising because the Ti(IV) center is d0 and cannot be oxidized, where all previous clear examples of homolytic cleavage of metal peroxide complexes are facilitated by oxidation of the metal center.

Fluorescence anisotropy studies of enzyme-substrate complex formation in stearoyl-ACP desaturase

Stearoyl-acyl carrier protein Delta(9)-desaturase (delta9D) catalyzes regio- and stereospecific insertion of cis double bonds into acyl chains attached to acyl carrier protein. Steady-state and stopped-flow fluorescence anisotropy measurements using acylated forms of dansyl- and fluoresceinyl-ACPs revealed equilibrium dissociation constants and dissociation rate constants for 16:0-, 17:0-, and 18:0-ACPs with resting and chemically 4e(-) reduced delta9D. Binding of 1 nM 18:0-fluoresceinyl-ACP to one subunit of the dimeric resting delta9D was observed with K(D1) = 13 +/- 3 nM. No significant difference in the K(D1) value was observed for 4e(-) delta9D. An approximately 4-fold increase in K(D1) per methylene group was observed upon shortening the acyl chain from 18:0 to 17:0 and then 16:0. In different experiments performed with 850 nM 18:0-dansyl-ACP, binding to the second subunit of resting delta9D was estimated to have K(D2) approximately 350 +/- 40 nM. The K(D2) values exhibited a similar dependence on acyl chain length as observed for the K(D1) values. The k(off) values measured by stopped-flow anisotropy measurements for reversal of the enzyme-substrate complex were also acyl-chain length dependent and increased 130-fold for 16:0-ACP (130 s(-)(1)) relative to 18:0-ACP (1 s(-)(1)). Increases in acyl chain length are thus associated with the presently reported increases in the K(D) and k(off) values. These results indicate that acyl chain length selectivity derives in major part from partition of the enzyme-substrate complex between substrate release and subsequent steps in catalysis.

Metal-hydride bond activation and metal-metal interaction in dinuclear iron complexes with linking dinitriles: a synthetic, electrochemical, and theoretical study

The dinuclear iron(II)-hydride complexes [[FeH(dppe)(2)](2)(mu-LL)][BF(4)](2) (LL = NCCH=CHCN (1a), NCC(6)H(4)CN (1b), NCCH(2)CH(2)CN (1c); dppe = Ph(2)PCH(2)CH(2)PPh(2)) and the corresponding mononuclear ones, trans-[FeH(LL)(dppe)(2)][BF(4)] (2a-c) were prepared by treatment of trans-[FeHCl(dppe)(2)], in tetrahydrofuran (thf) and in the presence of Tl[BF(4)], with the appropriate dinitrile (in molar deficiency or excess, respectively). Metal-metal interaction was detected by cyclic voltammetry for 1a, which, upon single-electron reversible oxidation, forms the mixed valent Fe(II)/Fe(III) 1a(+) complex. The latter either undergoes heterolytic Fe-H bond cleavage (loss of H(+)) or further oxidation, at a higher potential, also followed by hydride-proton evolution, according to ECECE or EECECEC mechanistic processes, respectively, which were established by digital simulation. Anodically induced Fe-H bond rupture was also observed for the other complexes and the detailed electrochemical behavior, as well as the metal-metal interaction (for 1a), were rationalized by ab initio calculations for model compounds and oxidized derivatives. These calculations were used to generate the structural parameters (full geometry optimization), the most stable isomeric forms, the ionization potentials, the effective atomic charges, and the molecular orbital diagrams, as well as to predict the nature of the other electron-transfer induced chemical steps, i.e. geometric isomerization and nucleophilic addition, by BF(4)(-), to the unsaturated iron center resulting from hydride-proton loss. From the values of the oxidation potential of the complexes, the electrochemical P(L) and E(L) ligand parameters were also estimated for the dinitrile ligands (LL) and for their mononuclear complexes 2 considered as ligands toward a second binding metal center.

Synthetic models for the cysteinate-ligated non-heme iron enzyme superoxide reductase: observation and structural characterization by XAS of an Fe(III)-OOH intermediate

Superoxide reductases (SORs) belong to a new class of metalloenzymes that degrade superoxide by reducing it to hydrogen peroxide. These enzymes contain a catalytic iron site that cycles between the Fe(II) and Fe(III) states during catalysis. A key step in the reduction of superoxide has been suggested to involve HO(2) binding to Fe(II), followed by innersphere electron transfer to afford an Fe(III)-OO(H) intermediate. In this paper, the mechanism of the superoxide-induced oxidation of a synthetic ferrous SOR model ([Fe(II)(S(Me2)N(4)(tren))](+) (1)) to afford [Fe(III)(S(Me2)N(4)(tren)(solv))](2+) (2-solv) is reported. The XANES spectrum shows that 1 remains five-coordinate in methanolic solution. Upon reaction of 1 with KO(2) in MeOH at -90 degrees C, an intermediate (3) is formed, which is characterized by a LMCT band centered at 452(2780) nm, and a low-spin state (S = 1/2), based on its axial EPR spectrum (g(perpendicular) = 2.14; g(parallel) = 1.97). Hydrogen peroxide is detected in this reaction, using both (1)H NMR spectroscopy and a catalase assay. Intermediate 3 is photolabile, so, in lieu of a Raman spectrum, IR was used to obtain vibrational data for 3. At low temperatures, a nu(O-O) Fermi doublet is observed in the IR at 788(2) and 781(2) cm(-)(1), which collapses into a single peak at 784 cm(-1) upon the addition of D(2)O. This vibrational peak diminishes in intensity over time and essentially disappears after 140 s. When 3 is generated using an (18)O-labeled isotopic mixture of K(18)O(2)/K(16)O(2) (23.28%), the vibration centered at 784 cm(-1) shifts to 753 cm(-1). This new vibrational peak is close to that predicted (740 cm(-1)) for a diatomic (18)O-(18)O stretch. In addition, a nu(O-O) vibrational peak assigned to free hydrogen peroxide is also observed (nu(O-O) = 854 cm(-1)) throughout the course of the reaction between Fe(II)-1 and superoxide and is strongest after 100 s. XAS studies indicate that 3 possesses one sulfur scatterer at 2.33(2) A and four nitrogen scatterers at 2.01(1) A. Addition of two Fe-O shells, each containing one oxygen, one at 1.86(3) A and one at 2.78(3) A, improved the EXAFS fits, suggesting that 3 is an end-on peroxo or hydroperoxo complex, [Fe(III)(S(Me2)N(4)(tren))(OO(H))](+). Upon warming above -50 degrees C, 3 is converted to 2-MeOH. In methanol and methanol:THF (THF = tetrahydrofuran) solvent mixtures, 2-MeOH is characterized by a LMCT band at lambda(max) = 511(1765) nm, an intermediate spin-state (S = 3/2), and, on the basis of EXAFS, a relatively short Fe-O bond (assigned to a coordinated methanol or methoxide) at 1.94(10) A. Kinetic measurements in 9:1 THF:MeOH at 25 degrees C indicate that 3 is formed near the diffusion limit upon addition of HO(2) to 1 and converts to 2-MeOH at a rate of 65(1) s(-1), which is consistent with kinetic studies involving superoxide oxidation of the SOR iron site.