Substrate Specificity of Sphingobium chlorophenolicum 2,6-Dichlorohydroquinone 1,2-Dioxygenase

Discovery and characterization of l-DOPA 2,3-dioxygenase from Streptomyces hygroscopicus jingganensis

The largest natural reservoir of untapped carbon can be found in the cell-wall strengthening, plant woody-tissue polymer, lignin - a polymer of catechols or 1,2-dihydroxybenzene monomers. The catecholic carbon of lignin could be valorized into feedstocks and natural products by way of catabolic and biosynthetic transformations, including the oxygen-dependent cleavage reaction of extradiol dioxygenase (EDX) enzymes. The EDX l-DOPA 2,3-dioxygenase was first discovered as part of a biosynthetic gene cluster to the natural product antibiotic, lincomycin, and also contributes to the biosyntheses of anthramycin, sibiromycin, tomaymycin, porothramycin and hormaomycin. Using these l-DOPA 2,3-dioxygenases as a starting point, we searched sequence space in order to identify new sources of dioxygenase driven natural product diversity. A "vicinal-oxygen-chelate (VOC) family protein" from Streptomyces hygroscopicus jingganensis was identified using bioinformatic methods and biochemically investigated for dioxygenase activity against a suite of natural and synthetic catechols. Steady-state oxygen consumption assays were used to screen and identify substrates, and a steady-state kinetic model of oxygen consumption was developed to evaluate activity of the S. hygroscopicus jingganensis VOC-family-protein with respect to activity of l-DOPA 2,3-dioxygenases from Streptomyces lincolnensis and Streptomyces sclerotialus. Lastly, these data were integrated with steady-state kinetic methods to observe the formation of the EDX cleavage product with UV-visible spectroscopy. The genomic context and enzymatic activity of the S. hygroscopicus jingganensis VOC family protein are consistent with a l-DOPA 2,3-dioxygenase contained within a cryptic biosynthetic pathway.

Sulfadiazine chlorination disinfection by-products in constructed wetlands: Identification of biodegradation products and inference of transformation pathways

Disinfection by-products (DBPs) formed from chlorination of antibiotics have greater toxicity than their parent compounds. Herein, this study investigated the biotransformation process of sulfadiazine Cl-DBPs in constructed wetlands (CWs). Results showed that, S atom on sulfonyl group, and N atoms on primary and secondary amine groups were the most reactive sites of sulfadiazine molecule. S1-N4 and S1-C8 of sulfadiazine are the most vulnerable bonds to cleave, followed by C14-N4 and C11-N5 bonds. In the chlorination process, sulfadiazine went through C-N bond cleavage, N-reductive alkylation, halogenation, and desulfonation to produce two aromatic Cl-DBPs. In the biodegradation process in CWs, sulfadiazine Cl-DBPs went through processes mainly including dechlorination, S-N bond cleavage, aniline-NH2 oxidation, desulfonation, phenol-OH oxidation, benzene ring cleavage, C-N bond cleavage, and β-oxidation of fatty acids under the action of a variety of oxidoreductases and hydrolases, during which a total of ten biodegradation products was identified. Moreover, sulfadiazine affected the biodegradation rather than the adsorption process in CWs. The two aromatic sulfadiazine Cl-DBPs had much higher bioaccumulation potentials than their parent sulfadiazine, but for the ten biodegradation products of sulfadiazine Cl-DBPs in CWs, 70% and almost 100% of them had lower bioaccumulation potentials than sulfadiazine and their parent sulfadiazine Cl-DBPs, respectively. The CWs were effective in reducing the environmental risk of sulfadiazine Cl-DBPs.

Cloning, overexpression, purification, characterization and structural modelling of a metabolically active Fe2+ dependent 2,6-dichloro-p-hydroquinone 1,2-dioxygenase (CpsA) from Bacillus cereus strain AOA-CPS_1

2,6-Dichloro-p-hydroquinone (DiCHQ) aromatic-ring cleavage by DiCHQ 1,2-dioxygenase (CpsA) is very crucial for complete transformation of pentachlorophenol (PCP) to 2-chloromaleylacetate in Bacillus cereus AOA-CPS_1 (BcAOA). The 978 bp gene (cpsA) was detected and amplified in the genome of BcAOA; cloned, overexpressed and purified to homogeneity. CpsA showed a single ≅36.9 kDa protein band on SDS-PAGE and exhibited optimum activity at 30 °C and pH 9.0. CpsA was stable between 20 °C and 40 °C, and also retained about 90% of its activity at 60 °C for 120 min. The enzyme retained about 90% activity between pH 9.0 and 11.5 and 60% activity at pH 13.0. CpsA was found to be Fe²⁺ dependent as about 90% increased activity was observed in the presence of FeSO₄. CpsA showed apparent v_max, K_m, k_cat and k_cat/K_m of 27.77 ± 0.9 μMs^-1, 0.990 ± 0.03 mM, 4.20 ± 0.04 s^-1 and 4.24 ± 0.03 s^-1 mM^-1, respectively at pH 9.0. Analysis of the reaction products via GC-MS confirmed 2-chloromaleylacetate as the ring-cleavage product. CpsA 3D structure revealed a conserved 2-His-1-carboxylate facial triad motif (His 9, His 244 and Thr 11), with Fe³⁺ at the centre. Findings from this study provide new insights into the involvement of this enzyme in PCP degradation and suggests alternate possible mechanism of ring-cleavage by dioxygenases.

Cobalt Superoxo and Alkylperoxo Complexes Derived from Reaction of Ring-Cleaving Dioxygenase Models with O2

The syntheses and O₂ reactivities of active-site models of cobalt-substituted ring-cleaving dioxygenases are presented. The pentacoordinate cobalt(II)-aminophenolate complex, [Co(Tp^Me2)(^tBu2APH)], gives rise to two distinct dioxygen adducts at reduced temperatures. The first is a paramagnetic (S = 1/2) cobalt(III)-superoxo species that was characterized with spectroscopic and computational techniques. The identity of the second Co/O₂ adduct was elucidated by X-ray crystallography, which revealed an unprecedented cobalt(III)-alkylperoxo structure generated by O₂ addition to the metal ion and ligand. These results provide synthetic precedents for proposed intermediates in the catalytic cycles of O₂-activating cobalt enzymes.

Oxidative opening of the aromatic ring: Tracing the natural history of a large superfamily of dioxygenase domains and their relatives

A diverse collection of enzymes comprising the protocatechuate dioxygenases (PCADs) has been characterized in several extradiol aromatic compound degradation pathways. Structural studies have shown a relationship between PCADs and the more broadly-distributed, functionally enigmatic Memo domain linked to several human diseases. To better understand the evolution of this PCAD-Memo protein superfamily, we explored their structural and functional determinants to establish a unified evolutionary framework, identifying 15 clearly-delineable families, including a previously-underappreciated diversity in five Memo clade families. We place the superfamily's origin within the greater radiation of the nucleoside phosphorylase/hydrolase-peptide/amidohydrolase fold prior to the last universal common ancestor of all extant organisms. In addition to identifying active-site residues across the superfamily, we describe three distinct, structurally-variable regions emanating from the core scaffold often housing conserved residues specific to individual families. These were predicted to contribute to the active-site pocket, potentially in substrate specificity and allosteric regulation. We also identified several previously-undescribed conserved genome contexts, providing insight into potentially novel substrates in PCAD clade families. We extend known conserved contextual associations for the Memo clade beyond previously-described associations with the AMMECR1 domain and a radical S-adenosylmethionine family domain. These observations point to two distinct yet potentially overlapping contexts wherein the elusive molecular function of the Memo domain could be finally resolved, thereby linking it to nucleotide base and aliphatic isoprenoid modification. In total, this report throws light on the functions of large swaths of the experimentally-uncharacterized PCAD-Memo families.

The role of halogen substituents and substrate pKa in defining the substrate specificity of 2,6-dichlorohydroquinone 1,2-dioxygenase (PcpA)

2,6-Dichlorohydroquinone 1,2-dioxygenase (PcpA) is a non-heme Fe(II) enzyme that is specific for ortho-dihalohydroquinones. Here we deconvolute the role of halogen polarizability vs. substrate pK_a in defining this specificity, and show how substrate binding compares to the structurally homologous catechol extradiol dioxygenases. The substrates 2,6-dichloro- and 2,6-dibromohydroquinone (polarizable halogens, pK_a1 = 7.3), 2,6-difluorohydroquinone (nonpolarizable halogens, pK_a1 = 7.5), and 2-chloro-6-methylhydroquinone (polarizable halogen, pK_a1 = 9.0) were examined through spectrophotometric titrations and steady-state kinetics. The results show that binding of the substrates to the enzyme decreased [Formula: see text] by about 0.5, except for 2,6-difluorohydroquinone, which showed no change. Additionally, the K_d values of 2,6-dichloro- and 2,6-dibromohydroquinone are about equal to their respective [Formula: see text]. For comparison, with catechol 2,3-dioxygenase (XylE), the substrates 4-methyl- and 3-bromocatechol are bound to the enzyme exclusively in the monoanion form over a wide pH range, indicating a [Formula: see text] of at least - 2.9 and - 1.2, respectively. The steady-state kinetic studies showed that 2,6-difluorohydroquinone is a poor substrate, with [Formula: see text] approximately 40-fold lower and [Formula: see text] 20-fold higher than 2,6-dichlorohydroquinone, despite its similar pK_a1. Likewise, the pH dependence of [Formula: see text] for 2-chloro-6-methylhydroquinone is nearly identical to that of 2,6-dichlorohydroquinone, despite its very different pK_a1. These results show that (1) it is clearly the halogen polarizability and not the lower substrate pK_a that determines the substrate specificity of PcpA, and (2) that PcpA, unlike the catechol extradiol dioxygenases, lacks an active site base that assists with substrate deprotonation, highlighting a key functional difference in what are otherwise similar active sites that defines their different reactivity.

Substrate-dependent aromatic ring fission of catechol and 2-aminophenol with O2 catalyzed by a nonheme iron complex of a tripodal N4 ligand

The catalytic reactivity of an iron(ii) complex [(TPA)Fe(II)(CH3CN)2](2+) (1) (TPA = tris(2-pyridylmethyl)amine) towards oxygenative aromatic C-C bond cleavage of catechol and 2-aminophenol is presented. Complex 1 exhibits catalytic and regioselective C-C bond cleavage of 3,5-di-tert-butylcatechol (H2DBC) to form intradiol products, whereas it catalyzes extradiol-type C-C bond cleavage of 2-amino-4,6-di-tert-butylphenol (H2AP). The catalytic reactions are found to be pH-dependent and the complex exhibits maximum turnovers at pH 5 in acetonitrile-phthalate buffer. An iron(iii)-catecholate complex [(TPA)Fe(III)(DBC)](+) (2) is formed in the ring cleavage of catechol. In the extradiol-type cleavage of H2AP, an iron(iii)-2-iminobenzosemiquinonate complex [(TPA)Fe(III)(ISQ)](2+) (3) (ISQ = 4,6-di-tert-butyl-2-iminobenzosemiquinonate radical anion) is observed in the reaction pathway. This work shows the importance of the nature of 'redox non-innocent' substrates in governing the mode of ring fission reactivity.

The coffee-machine bacteriome: biodiversity and colonisation of the wasted coffee tray leach

Microbial communities are ubiquitous in both natural and artificial environments. However, microbial diversity is usually reduced under strong selection pressures, such as those present in habitats rich in recalcitrant or toxic compounds displaying antimicrobial properties. Caffeine is a natural alkaloid present in coffee, tea and soft drinks with well-known antibacterial properties. Here we present the first systematic analysis of coffee machine-associated bacteria. We sampled the coffee waste reservoir of ten different Nespresso machines and conducted a dynamic monitoring of the colonization process in a new machine. Our results reveal the existence of a varied bacterial community in all the machines sampled, and a rapid colonisation process of the coffee leach. The community developed from a pioneering pool of enterobacteria and other opportunistic taxa to a mature but still highly variable microbiome rich in coffee-adapted bacteria. The bacterial communities described here, for the first time, are potential drivers of biotechnologically relevant processes including decaffeination and bioremediation.

Structural Basis for Substrate and Oxygen Activation in Homoprotocatechuate 2,3-Dioxygenase: Roles of Conserved Active Site Histidine 200

Kinetic and spectroscopic studies have shown that the conserved active site residue His200 of the extradiol ring-cleaving homoprotocatechuate 2,3-dioxygenase (FeHPCD) from Brevibacterium fuscum is critical for efficient catalysis. The roles played by this residue are probed here by analysis of the steady-state kinetics, pH dependence, and X-ray crystal structures of the FeHPCD position 200 variants His200Asn, His200Gln, and His200Glu alone and in complex with three catecholic substrates (homoprotocatechuate, 4-sulfonylcatechol, and 4-nitrocatechol) possessing substituents with different inductive capacity. Structures determined at 1.35-1.75 Å resolution show that there is essentially no change in overall active site architecture or substrate binding mode for these variants when compared to the structures of the wild-type enzyme and its analogous complexes. This shows that the maximal 50-fold decrease in kcat for ring cleavage, the dramatic changes in pH dependence, and the switch from ring cleavage to ring oxidation of 4-nitrocatechol by the FeHPCD variants can be attributed specifically to the properties of the altered second-sphere residue and the substrate. The results suggest that proton transfer is necessary for catalysis, and that it occurs most efficiently when the substrate provides the proton and His200 serves as a catalyst. However, in the absence of an available substrate proton, a defined proton-transfer pathway in the protein can be utilized. Changes in the steric bulk and charge of the residue at position 200 appear to be capable of altering the rate-limiting step in catalysis and, perhaps, the nature of the reactive species.

Catabolism of Phenol and Its Derivatives in Bacteria: Genes, Their Regulation, and Use in the Biodegradation of Toxic Pollutants

Phenol and its derivatives (alkylphenols, halogenated phenols, nitrophenols) are natural or man-made aromatic compounds that are ubiquitous in nature and in human-polluted environments. Many of these substances are toxic and/or suspected of mutagenic, carcinogenic, and teratogenic effects. Bioremediation of the polluted soil and water using various bacteria has proved to be a promising option for the removal of these compounds. In this review, we describe a number of peripheral pathways of aerobic and anaerobic catabolism of various natural and xenobiotic phenolic compounds, which funnel these substances into a smaller number of central catabolic pathways. Finally, the metabolites are used as carbon and energy sources in the citric acid cycle. We provide here the characteristics of the enzymes that convert the phenolic compounds and their catabolites, show their genes, and describe regulatory features. The genes, which encode these enzymes, are organized on chromosomes and plasmids of the natural bacterial degraders in various patterns. The accumulated data on similarities and the differences of the genes, their varied organization, and particularly, an astonishingly broad range of intricate regulatory mechanism may be read as an exciting adventurous book on divergent evolutionary processes and horizontal gene transfer events inscribed in the bacterial genomes. In the end, the use of this wealth of bacterial biodegradation potential and the manipulation of its genetic basis for purposes of bioremediation is exemplified. It is envisioned that the integrated high-throughput techniques and genome-level approaches will enable us to manipulate systems rather than separated genes, which will give birth to systems biotechnology.

Exploring allosteric activation of LigAB from Sphingobium sp. strain SYK-6 through kinetics, mutagenesis and computational studies

The protocatechuate 4,5-dioxygenase (LigAB) from Sphingobium sp. strain SYK-6 is the defining member of the Type II extradiol dioxygenase superfamily (a.k.a. PCA Dioxygenase Superfamily or PCADSF) and plays a key aromatic ring-opening role in the metabolism of several lignin derived aromatic compounds. In our search for alternate substrates and inhibitors of LigAB, we discovered allosteric rate enhancement in the presence of non-substrate protocatechuate-like aldehydes such as vanillin. LigAB has the broadest substrate utilization profile of all protocatechuate (PCA) 4,5-dioxygenase described in the literature, however, the rate enhancement is only observed with PCA, with vanillin increasing kcat for LigAB by 36%. Computational docking has identified a potential site of allosteric binding near the entrance to the active site. Examination of a multiple sequence alignment reveals that many of the residues contributing to this newly identified allosteric pocket are highly conserved within the LigB family of the PCADSF. Point mutants of Phe103α and Ala18β, two residues located in the putative allosteric pocket, display altered rate enhancement as compared to LigAB-WT, providing support for the computationally identified allosteric binding site. Further investigation of this binding site may provide insight into the mechanism of this never before observed allosteric activation in extradiol dioxygenases.

Synthesis and spectroscopic characterization of high-spin mononuclear iron(II) p-semiquinonate complexes

Two mononuclear iron(II) p-semiquinonate (pSQ) complexes have been generated via one-electron reduction of precursor complexes containing a substituted 1,4-naphthoquinone ligand. Detailed spectroscopic and computational analysis confirmed the presence of a coordinated pSQ radical ferromagnetically coupled to the high-spin Fe(II) center. The complexes are intended to model electronic interactions between (semi)quinone and iron cofactors in biology.

Structural and spectroscopic characterization of iron(II), cobalt(II), and nickel(II) ortho-dihalophenolate complexes: insights into metal-halogen secondary bonding

Metal complexes incorporating the tris(3,5-diphenylpyrazolyl)borate ligand (Tp(Ph2)) and ortho-dihalophenolates were synthesized and characterized in order to explore metal-halogen secondary bonding in biorelevant model complexes. The complexes Tp(Ph2)ML were synthesized and structurally characterized, where M was Fe(II), Co(II), or Ni(II) and L was either 2,6-dichloro- or 2,6-dibromophenolate. All six complexes exhibited metal-halogen secondary bonds in the solid state, with distances ranging from 2.56 Å for the Tp(Ph2)Ni(2,6-dichlorophenolate) complex to 2.88 Å for the Tp(Ph2)Fe(2,6-dibromophenolate) complex. Variable temperature NMR spectra of the Tp(Ph2)Co(2,6-dichlorophenolate) and Tp(Ph2)Ni(2,6-dichlorophenolate) complexes showed that rotation of the phenolate, which requires loss of the secondary bond, has an activation barrier of ~30 and ~37 kJ/mol, respectively. Density functional theory calculations support the presence of a barrier for disruption of the metal-halogen interaction during rotation of the phenolate. On the other hand, calculations using the spectroscopically calibrated angular overlap method suggest essentially no contribution of the halogen to the ligand-field splitting. Overall, these results provide the first quantitative measure of the strength of a metal-halogen secondary bond and demonstrate that it is a weak noncovalent interaction comparable in strength to a hydrogen bond. These results provide insight into the origin of the specificity of the enzyme 2,6-dichlorohydroquinone 1,2-dioxygenase (PcpA), which is specific for ortho-dihalohydroquinone substrates and phenol inhibitors.

Characterizing the promiscuity of LigAB, a lignin catabolite degrading extradiol dioxygenase from Sphingomonas paucimobilis SYK-6

LigAB from Sphingomonas paucimobilis SYK-6 is the only structurally characterized dioxygenase of the largely uncharacterized superfamily of Type II extradiol dioxygenases (EDO). This enzyme catalyzes the oxidative ring-opening of protocatechuate (3,4-dihydroxybenzoic acid or PCA) in a pathway allowing the degradation of lignin derived aromatic compounds (LDACs). LigAB has also been shown to utilize two other LDACs from the same metabolic pathway as substrates, gallate, and 3-O-methyl gallate; however, kcat/KM had not been reported for any of these compounds. In order to assess the catalytic efficiency and get insights into the observed promiscuity of this enzyme, steady-state kinetic analyses were performed for LigAB with these and a library of related compounds. The dioxygenation of PCA by LigAB was highly efficient, with a kcat of 51 s(-1) and a kcat/KM of 4.26 × 10(6) M(-1)s(-1). LigAB demonstrated the ability to use a variety of catecholic molecules as substrates beyond the previously identified gallate and 3-O-methyl gallate, including 3,4-dihydroxybenzamide, homoprotocatechuate, catechol, and 3,4-dihydroxybenzonitrile. Interestingly, 3,4-dihydroxybenzamide (DHBAm) behaves in a manner similar to that of the preferred benzoic acid substrates, with a kcat/Km value only ∼4-fold lower than that for gallate and ∼10-fold higher than that for 3-O-methyl gallate. All of these most active substrates demonstrate mechanistic inactivation of LigAB. Additionally, DHBAm exhibits potent product inhibition that leads to an inactive enzyme, being more highly deactivating at lower substrate concentration, a phenomena that, to our knowledge, has not been reported for another dioxygenase substrate/product pair. These results provide valuable catalytic insight into the reactions catalyzed by LigAB and make it the first Type II EDO that is fully characterized both structurally and kinetically.

Synthetic, spectroscopic, and DFT studies of iron complexes with iminobenzo(semi)quinone ligands: implications for o-aminophenol dioxygenases

The oxidative C-C bond cleavage of o-aminophenols by nonheme Fe dioxygenases is a critical step in both human metabolism (the kynurenine pathway) and the microbial degradation of nitroaromatic pollutants. The catalytic cycle of o-aminophenol dioxygenases (APDOs) has been proposed to involve formation of an Fe(II)/O2/iminobenzosemiquinone complex, although the presence of a substrate radical has been called into question by studies of related ring-cleaving dioxygenases. Recently, we reported the first synthesis of an iron(II) complex coordinated to an iminobenzosemiquinone (ISQ) ligand, namely, [Fe((Ph2)Tp)((tBu)ISQ)] (2a; where (Ph2)Tp=hydrotris(3,5-diphenylpyrazol-1-yl)borate and (tBu)ISQ is the radical anion derived from 2-amino-4,6-di-tert-butylphenol). In the current manuscript, density functional theory (DFT) calculations and a wide variety of spectroscopic methods (electronic absorption, Mössbauer, magnetic circular dichroism, and resonance Raman) were employed to obtain detailed electronic-structure descriptions of 2a and its one-electron oxidized derivative [3a](+). In addition, we describe the synthesis and characterization of a parallel series of complexes featuring the neutral supporting ligand tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine ((Ph2)TIP). The isomer shifts of about 0.97 mm s(-1) obtained through Mössbauer experiments confirm that 2a (and its (Ph2)TIP-based analogue [2b](+)) contain Fe(II) centers, and the presence of an ISQ radical was verified by analysis of the absorption spectra in light of time-dependent DFT calculations. The collective spectroscopic data indicate that one-electron oxidation of the Fe(II)-ISQ complexes yields complexes ([3a](+) and [3b](2+)) with electronic configurations between the Fe(III)-ISQ and Fe(II)-IBQ limits (IBQ=iminobenzoquinone), highlighting the ability of o-amidophenolates to access multiple oxidation states. The implications of these results for the mechanism of APDOs and other ring-cleaving dioxygenases are discussed.

Preparation of a semiquinonate-bridged diiron(II) complex and elucidation of its geometric and electronic structures

The synthesis and crystal structure of a diiron(II) complex containing a bridging semiquinonate radical are presented. The unique electronic structure of this S = 7/2 complex is examined with spectroscopic (absorption, EPR, resonance Raman) and computational methods.

Structural characterization of 2,6-dichloro-p-hydroquinone 1,2-dioxygenase (PcpA) from Sphingobium chlorophenolicum, a new type of aromatic ring-cleavage enzyme

PcpA (2,6-dichloro-p-hydroquinone 1,2-dioxygenase) from Sphingobium chlorophenolicum, a non-haem Fe(II) dioxygenase capable of cleaving the aromatic ring of p-hydroquinone and its substituted variants, is a member of the recently discovered p-hydroquinone 1,2-dioxygenases. Here we report the 2.6 Å structure of PcpA, which consists of four βαβββ motifs, a hallmark of the vicinal oxygen chelate superfamily. The secondary co-ordination sphere of the Fe(II) centre forms an extensive hydrogen-bonding network with three solvent exposed residues, linking the catalytic Fe(II) to solvent. A tight hydrophobic pocket provides p-hydroquinones access to the Fe(II) centre. The p-hydroxyl group is essential for the substrate-binding, thus phenols and catechols, lacking a p-hydroxyl group, do not bind to PcpA. Site-directed mutagenesis and kinetic analysis confirm the critical catalytic role played by the highly conserved His10, Thr13, His226 and Arg259. Based on these results, we propose a general reaction mechanism for p-hydroquinone 1,2-dioxygenases.

Structural, spectroscopic, and electrochemical properties of nonheme Fe(II)-hydroquinonate complexes: synthetic models of hydroquinone dioxygenases

Using the tris(3,5-diphenylpyrazol-1-yl)borate ((Ph2)Tp) supporting ligand, a series of mono- and dinuclear ferrous complexes containing hydroquinonate (HQate) ligands have been prepared and structurally characterized with X-ray crystallography. The monoiron(II) complexes serve as faithful mimics of the substrate-bound form of hydroquinone dioxygenases (HQDOs) - a family of nonheme Fe enzymes that catalyze the oxidative cleavage of 1,4-dihydroxybenzene units. Reflecting the variety of HQDO substrates, the synthetic complexes feature both mono- and bidentate HQate ligands. The bidentate HQates cleanly provide five-coordinate, high-spin Fe(II) complexes with the general formula [Fe((Ph2)Tp)(HL(X))] (1X), where HL(X) is a HQate(1-) ligand substituted at the 2-position with a benzimidazolyl (1A), acetyl (1B and 1C), or methoxy (1D) group. In contrast, the monodentate ligand 2,6-dimethylhydroquinone (H(2)L(F)) exhibited a greater tendency to bridge between two Fe(II) centers, resulting in formation of [Fe(2)((Ph2)Tp)(2)(μ-L(F))(MeCN)]·[2F(MeCN)]. However, addition of one equivalent of "free" pyrazole ((Ph2)pz) ligand provided the mononuclear complex, [Fe((Ph2)Tp)(HL(F))((Ph2)pz)]·[1F((Ph2)pz)], which is stabilized by an intramolecular hydrogen bond between the HL(F) and (Ph2)pz donors. Complex 1F((Ph2)pz) represents the first crystallographically-characterized example of a monoiron complex bound to an untethered HQate ligand. The geometric and electronic structures of the Fe/HQate complexes were further probed with spectroscopic (UV-vis absorption, (1)H NMR) and electrochemical methods. Cyclic voltammograms of complexes in the 1X series revealed an Fe-based oxidation between 0 and -300 mV (vs. Fc(+/0)), in addition to irreversible oxidation(s) of the HQate ligand at higher potentials. The one-electron oxidized species (1X(OX)) were examined with UV-vis absorption and electron paramagnetic resonance (EPR) spectroscopies.

Fe(II) complexes that mimic the active site structure of acetylacetone dioxygenase: O2 and NO reactivity

Acetylacetone dioxygenase (Dke1) is a bacterial enzyme that catalyzes the dioxygen-dependent degradation of β-dicarbonyl compounds. The Dke1 active site contains a nonheme monoiron(II) center facially ligated by three histidine residues (the 3His triad); coordination of the substrate in a bidentate manner provides a five-coordinate site for O(2) binding. Recently, we published the synthesis and characterization of a series of ferrous β-diketonato complexes that faithfully mimic the enzyme-substrate intermediate of Dke1 (Park, H.; Baus, J.S.; Lindeman, S.V.; Fiedler, A.T. Inorg. Chem.2011, 50, 11978-11989). The 3His triad was modeled with three different facially coordinating N3 supporting ligands, and substituted β-diketonates (acac(X)) with varying steric and electronic properties were employed. Here, we describe the reactivity of our Dke1 models toward O(2) and its surrogate nitric oxide (NO), and report the synthesis of three new Fe(II) complexes featuring the anions of dialkyl malonates. Exposure of [Fe((Me2)Tp)(acac(X))] complexes (where (R2)Tp = hydrotris(pyrazol-1-yl)borate with R-groups at the 3- and 5-positions of the pyrazole rings) to O(2) at -70 °C in toluene results in irreversible formation of green chromophores (λ(max) ∼750 nm) that decay at temperatures above -60 °C. Spectroscopic and computational analyses suggest that these intermediates contain a diiron(III) unit bridged by a trans μ-1,2-peroxo ligand. The green chromophore is not observed with analogous complexes featuring (Ph2)Tp and (Ph)TIP ligands (where (Ph)TIP = tris(2-phenylimidazoly-4-yl)phosphine), since the steric bulk of the phenyl substituents prevents formation of dinuclear species. While these complexes are largely inert toward O(2), (Ph2)Tp-based complexes with dialkyl malonate anions exhibit dioxygenase activity and thus serve as functional Dke1 models. The Fe/acac(X) complexes all react readily with NO to yield high-spin (S = 3/2) {FeNO}(7) adducts that were characterized with crystallographic, spectroscopic, and computational methods. Collectively, the results presented here enhance our understanding of the chemical factors involved in the oxidation of aliphatic substrates by nonheme iron dioxygenases.

Crystallization and preliminary X-ray crystallographic analysis of hydroquinone dioxygenase from Sphingomonas sp. TTNP3

Hydroquinone dioxygenase (HQDO), a novel Fe(II) ring-fission dioxygenase from Sphingomonas sp. strain TTNP3 which oxidizes a wide range of hydroquinones to the corresponding 4-hydroxymuconic semialdehydes, has been crystallized. The enzyme is an α(2)β(2) heterotetramer constituted of two subunits of 19 and 38 kDa. Diffraction-quality crystals of HQDO were obtained using the sitting-drop vapour-diffusion method at 277 K from a solution consisting of 16% PEG 4000, 0.3 M MgCl(2), 0.1 M Tris pH 8.5. The crystals belonged to the monoclinic space group P2(1), with unit-cell parameters a = 88.4, b = 125.4, c = 90.8 Å, β = 105.3°. The asymmetric unit contained two heterotetramers, i.e. four copies of each of the two different subunits related by noncrystallographic 222 symmetry. A complete data set extending to a maximum resolution of 2.5 Å was collected at 100 K using a wavelength of 0.980 Å.