Crystal structure and mutagenic analysis of GDOsp, a gentisate 1,2-dioxygenase from Silicibacter pomeroyi

Bioremediation of Polycyclic Aromatic Hydrocarbons by Means of Bacteria and Bacterial Enzymes

Polycyclic aromatic hydrocarbons (PAHs) are widespread, persistent, and toxic environmental pollutants. Many anthropogenic and some natural factors contribute to the spread and accumulation of PAHs in aquatic and soil systems. The effective and environmentally friendly remediation of these chemical compounds is an important and challenging problem that has kept scientists busy over the last few decades. This review briefly summarizes data on the main sources of PAHs, their toxicity to living organisms, and physical and chemical approaches to the remediation of PAHs. The basic idea behind existing approaches to the bioremediation of PAHs is outlined with an emphasis on a detailed description of the use of bacterial strains as individual isolates, consortia, or cell-free enzymatic agents.

Proton Transfer via Arginine with Suppressed pKa Mediates Catalysis by Gentisate and Salicylate Dioxygenase

Gentisate and salicylate 1,2-dioxygenases (GDO and SDO) facilitate aerobic degradation of aromatic rings by inserting both atoms of dioxygen into their substrates, thereby participating in global carbon cycling. The role of acid-base catalysts in the reaction cycles of these enzymes is debatable. We present evidence of the participation of a proton shuffler during catalysis by GDO and SDO. The pH dependence of Michaelis-Menten parameters demonstrates that a single proton transfer is mandatory for the catalysis. Measurements at variable temperatures and pHs were used to determine the standard enthalpy of ionization (ΔHion°) of 51 kJ/mol for the proton transfer event. Although the observed apparent pKa in the range of 6.0-7.0 for substrates of both enzymes is highly suggestive of a histidine residue, ΔHion° establishes an arginine residue as the likely proton source, providing phylogenetic relevance for this strictly conserved residue in the GDO family. We propose that the atypical 3-histidine ferrous binding scaffold of GDOs contributes to the suppression of arginine pKa and provides support for this argument by employing a 2-histidine-1-carboxylate variant of the enzyme that exhibits elevated pKa. A reaction mechanism considering the role of the proton source in stabilizing key reaction intermediates is proposed.

Crystal structure of the monocupin ring-cleaving dioxygenase 5-nitrosalicylate 1,2-dioxygenase from Bradyrhizobium sp

5-Nitrosalicylate 1,2-dioxygenase (5NSDO) is an iron(II)-dependent dioxygenase involved in the aerobic degradation of 5-nitroanthranilic acid by the bacterium Bradyrhizobium sp. It catalyzes the opening of the 5-nitrosalicylate aromatic ring, a key step in the degradation pathway. Besides 5-nitrosalicylate, the enzyme is also active towards 5-chlorosalicylate. The X-ray crystallographic structure of the enzyme was solved at 2.1 Å resolution by molecular replacement using a model from the AI program AlphaFold. The enzyme crystallized in the monoclinic space group P21, with unit-cell parameters a = 50.42, b = 143.17, c = 60.07 Å, β = 107.3°. 5NSDO belongs to the third class of ring-cleaving dioxygenases. Members of this family convert para-diols or hydroxylated aromatic carboxylic acids and belong to the cupin superfamily, which is one of the most functionally diverse protein classes and is named on the basis of a conserved β-barrel fold. 5NSDO is a tetramer composed of four identical subunits, each folded as a monocupin domain. The iron(II) ion in the enzyme active site is coordinated by His96, His98 and His136 and three water molecules with a distorted octahedral geometry. The residues in the active site are poorly conserved compared with other dioxygenases of the third class, such as gentisate 1,2-dioxygenase and salicylate 1,2-dioxygenase. Comparison with these other representatives of the same class and docking of the substrate into the active site of 5NSDO allowed the identification of residues which are crucial for the catalytic mechanism and enzyme selectivity.

Spectroscopic analysis of the mammalian enzyme cysteine dioxygenase

l-Cysteine (Cys) is an essential building block for the synthesis of new proteins and serves as a precursor for several biologically important sulfur-containing molecules, such as coenzyme A, taurine, glutathione, and inorganic sulfate. However, organisms must tightly regulate the concentration of free Cys, as elevated levels of this semi-essential amino acid can be extremely harmful. The non-heme iron enzyme cysteine dioxygenase (CDO) serves to maintain the proper levels of Cys by catalyzing its oxidation to cysteine sulfinic acid. Crystal structures of resting and substrate-bound mammalian CDO revealed two surprising structural motifs in the first and second coordination spheres of the Fe center. The first is the existence of a neutral three histidine (3-His) facial triad that coordinates the Fe ion, as opposed to an anionic 2-His-1-carboxylate facial triad that is typically observed in mononuclear non-heme Fe(II) dioxygenases. The second unusual structural feature exhibited by mammalian CDO is the presence of a covalent crosslink between the sulfur of a Cys residue and an ortho-carbon of a tyrosine residue. Spectroscopic studies of CDO have provided invaluable insights into the roles that these unusual features play with regards to substrate Cys and co-substrate O2 binding and activation. In this chapter, we summarize results obtained from electronic absorption, electron paramagnetic resonance, magnetic circular dichroism, resonance Raman, and Mössbauer spectroscopic studies of mammalian CDO carried out in the last two decades. Pertinent results obtained from complementary computational studies are also briefly summarized.

Differences in the Second Coordination Sphere Tailor the Substrate Specificity and Reactivity of Thiol Dioxygenases

In recent years, considerable progress has been made toward elucidating the geometric and electronic structures of thiol dioxygenases (TDOs). TDOs catalyze the conversion of substrates with a sulfhydryl group to their sulfinic acid derivatives via the addition of both oxygen atoms from molecular oxygen. All TDOs discovered to date belong to the family of cupin-type mononuclear nonheme Fe(II)-dependent metalloenzymes. While most members of this enzyme family bind the Fe cofactor by two histidines and one carboxylate side chain (2-His-1-carboxylate) to provide a monoanionic binding motif, TDOs feature a neutral three histidine (3-His) facial triad. In this Account, we present a bioinformatics analysis and multiple sequence alignment that highlight the significance of the secondary coordination sphere in tailoring the substrate specificity and reactivity among the different TDOs. These insights provide the framework within which important structural and functional features of the distinct TDOs are discussed.The best studied TDO is cysteine dioxygenase (CDO), which catalyzes the conversion of cysteine to cysteine sulfinic acid in both eukaryotes and prokaryotes. Crystal structures of resting and substrate-bound mammalian CDOs revealed two surprising structural motifs in the first- and second coordination spheres of the Fe center. The first is the presence of the abovementioned neutral 3-His facial triad that coordinates the Fe ion. The second is the existence of a covalent cross-link between the sulfur of Cys93 and an ortho carbon of Tyr157 (mouse CDO numbering scheme). While the exact role of this cross-link remains incompletely understood, various studies established that it is needed for proper substrate Cys positioning and gating solvent access to the active site. Intriguingly, bacterial CDOs lack the Cys-Tyr cross-link; yet, they are as active as cross-linked eukaryotic CDOs.The other known mammalian TDO is cysteamine dioxygenase (ADO). Initially, it was believed that ADO solely catalyzes the oxidation of cysteamine to hypotaurine. However, it has recently been shown that ADO additionally oxidizes N-terminal cysteine (Nt-Cys) peptides, which indicates that ADO may play a much more significant role in mammalian physiology than was originally anticipated. Though predicted on the basis of sequence alignment, site-directed mutagenesis, and spectroscopic studies, it was not until last year that two crystal structures, one of wild-type mouse ADO (solved by us) and the other of a variant of nickel-substituted human ADO, finally provided direct evidence that this enzyme also features a 3-His facial triad. These structures additionally revealed several features that are unique to ADO, including a putative cosubstrate O2 access tunnel that is lined by two Cys residues. Disulfide formation under conditions of high O2 levels may serve as a gating mechanism to prevent ADO from depleting organisms of Nt-Cys-containing molecules.The combination of kinetic and spectroscopic studies in conjunction with structural characterizations of TDOs has furthered our understanding of enzymatic sulfhydryl substrate regulation. In this article, we take advantage of the fact that the ADO X-ray crystal structures provided the final piece needed to compare and contrast key features of TDOs, an essential family of metalloenzymes found across all kingdoms of life.

Characterization of Gentisate 1,2-Dioxygenase from Pseudarthrobacter phenanthrenivorans Sphe3 and Its Stabilization by Immobilization on Nickel-Functionalized Magnetic Nanoparticles

The aim of this study was the biochemical and kinetic characterization of the gentisate 1,2-dioxygenase (GDO) from Pseudarthrobacter phenanthrenivorans Sphe3 and the development of a nanobiocatalyst by its immobilization on Ni2+-functionalized Fe3O4-polydopamine magnetic nanoparticles (Ni2+-PDA-MNPs). This is the first GDO to be immobilized. The gene encoding the GDO was cloned with an N-terminal His-tag and overexpressed in E. coli. The nanoparticles showed a high purification efficiency of GDO from crude cell lysates with a maximum activity recovery of 97%. The immobilized enzyme was characterized by Fourier transform infrared spectroscopy (FTIR). The reaction product was identified by 1H NMR. Both free and immobilized GDO exhibited Michaelis–Menten kinetics with Km values of 25.9 ± 4.4 and 82.5 ± 14.2 μM and Vmax values of 1.2 ± 0.1 and 0.03 ± 0.002 mM*s−1, respectively. The thermal stability of the immobilized GDO was enhanced at 30 °C, 40 °C, and 50 °C, compared to the free GDO. Stored at −20 °C, immobilized GDO retained more than 60% of its initial activity after 30 d, while the free enzyme completely lost its activity after 10 d. Furthermore, the immobilized nanoparticle–enzyme conjugate retained more than 50% enzyme activity up to the fifth cycle.

Gentisate 1,2-dioxygenase from the gram-positive bacteria Rhodococcus opacus 1CP: Identical active sites vs. different substrate selectivities

Gentisate 1,2-dioxygenases belong to the class III ring-cleaving dioxygenases catalyzing key reactions of aromatic compounds degradation by aerobic microorganisms. In the present work, the results of complete molecular, structural, and functional investigations of the gentisate 1,2-dioxygenase (rho-GDO) from a gram-positive bacterium Rhodococcus opacus 1CP growing on 3-hydroxybenzoate as a sole source of carbon and energy are presented. The purified enzyme showed a narrow substrate specificity. Among fourteen investigated substrate analogues only gentisate was oxidized by the enzyme, what can be potentially applied in biosensor technologies. The rho-GDO encoding gene was identified in the genomic DNA of the R. opacus 1CP. According to phylogenetic analysis, the rho-GDO belongs to the group of apparently most recently acquired activities in bacterial genera Rhodococcus, Arthrobacter, Corynebacterium, Nocardia, Amycolatopsis, Comamonas, and Streptomyces. Homology modeling the rho-GDO 3D-structure demonstrates the composition identity of the first-sphere residues of the active site of rho-GDO and salicylate 1,2-dioxygenase from Pseudaminobacter salicylatoxidans (RCSB PDB: 2PHD), despite of their different substrate specificities. The phenomenon described for the first time for this family of enzymes supposes a more complicated mechanism of substrate specificity than previously imagined, and makes the rho-GDO a convenient model for a novel direction of structure-function relationship studies.

Roles of the Gentisate 1,2-Dioxygenases DsmD and GtdA in the Catabolism of the Herbicide Dicamba in Rhizorhabdus dicambivorans Ndbn-20

3-Chlorogentisate is a key intermediate in the catabolism of the herbicide dicamba in R. dicambivorans Ndbn-20. In this study, we identified two gentisate 1,2-dioxygenases (GDOs), DsmD and GtdA, from Ndbn-20. The amino acid sequence similarity between DsmD and GtdA is 51%. Both of them are dimers and showed activities to gentisate and 3-chlorogentisate but not 3,6-dichlorogentisate (3,6-DCGA) or 6-chlorogentisate in vitro. The k_cat/K_m of DsmD for 3-chlorogentisate was 28.7 times higher than that of GtdA, whereas the k_cat/K_m of DsmD for gentisate was only one-fourth of that of GtdA. Transcription of dsmD was dramatically induced by 3-chlorogentisate but not gentisate, whereas gtdA was not induced. Disruption of dsmD resulted in a significant decline in the degradation rates of 3-chlorogentisate and dicamba but had no effect on the degradation of gentisate, whereas the result of disruption of gtdA was converse; the disruption of both dsmD and gtdA led to the inability to degrade 3-chlorogentisate and gentisate. This study revealed that 3-chlorogentisate but not gentisate or 3,6-DCGA is the ring-cleavage substrate in the dicamba degradation pathway in R. dicambivorans Ndbn-20; DsmD is specifically responsible for cleavage of 3-chlorogentisate, whereas GtdA is a general GDO involved in the catabolism of various natural aromatic compounds.

Spectroscopic Investigation of Cysteamine Dioxygenase

Thiol dioxygenases are mononuclear non-heme Fe^II-dependent metalloenzymes that initiate the oxidative catabolism of thiol-containing substrates to their respective sulfinates. Cysteine dioxygenase (CDO), the best characterized mammalian thiol dioxygenase, contains a three-histidine (3-His) coordination environment rather than the 2-His-1-carboxylate facial triad seen in most mononuclear non-heme Fe^II enzymes. A similar 3-His active site is found in the bacterial thiol dioxygenase 3-mercaptopropionate dioxygenase (MDO), which converts 3-mercaptopropionate into 3-sulfinopropionic acid as part of the bacterial sulfur metabolism pathway. In this study, we have investigated the active site geometric and electronic structures of a third non-heme Fe^II-dependent thiol dioxygenase, cysteamine dioxygenase (ADO), by using a spectroscopic approach. Although a 3-His facial triad had previously been implicated on the basis of sequence alignment and site-directed mutagenesis studies, little is currently known about the active site environment of ADO. Our magnetic circular dichroism and electron paramagnetic resonance data provide compelling evidence that ADO features a 3-His facial triad, like CDO and MDO. Despite this similar coordination environment, spectroscopic results obtained for ADO incubated with various substrate analogues are distinct from those obtained for the other Fe^II-dependent thiol dioxygenases. This finding suggests that the secondary coordination sphere of ADO is distinct from those of CDO and MDO, demonstrating the significant role that secondary-sphere residues play in dictating substrate specificity.

Mechanism of fatty acid decarboxylation catalyzed by a non-heme iron oxidase (UndA): a QM/MM study

UndA is a non-heme iron enzyme that was recognized to catalyze the decarboxylation of medium chain (C10-C14) fatty acids to produce trace amounts of 1-alkenes. Owing to the electron imbalance during the oxidative decarboxylation of the substrate and the reduction of O₂, only single turnover reactions were obtained in UndA in vitro assays. Unlike the general non-heme iron enzymes, the catalytic efficiency of UndA is quite low. According to the previous proposal, both Fe^III-OO˙^- and Fe^IV[double bond, length as m-dash]O complexes may abstract the β-H of fatty acids to trigger the oxidative decarboxylation reaction. Herein, on the basis of the crystal structures of UndA in complex with the substrate analogues, we constructed a series of computational models and performed quantum mechanics/molecular mechanics (QM/MM) calculations to explore the UndA-catalyzed decarboxylation using lauric acid as the substrate. Our calculation results reveal that only the Fe^III-OO˙^- complex can initiate the decarboxylation, and the substrate (lauric acid) should monodentately coordinate to the Fe center to facilitate the β-H abstraction. In addition, the monodentate coordination corresponds to higher relative energy than the bidentate mode, which may explain the low efficiency of UndA. It is also revealed that as long as the β-H is extracted by the Fe^III-OO˙^-, the decarboxylation of the substrate radical is quite easy, and an electron transfer from the substrate to the iron center is the prerequisite. For the Fe^IV[double bond, length as m-dash]O complex, since the β-H is far from the O_Fe atom and the angle of ∠Fe-O-H is 53.1°, the H-abstraction is calculated to be difficult.

Substrate promiscuity and active site differences in gentisate 1,2-dioxygenases: electron paramagnetic resonance study

Gentisate 1,2-dioxygenases (GDOs) are non-heme iron enzymes that catalyze the oxidation of dihydroxylated aromatic substrate, gentisate (2,5-dihydroxybenzoate). Salicylate 1,2-dioxygenase (SDO), a member of the GDO family, performs the ring scission of monohydroxylated substrates such as salicylate, thereby oxidizing a broader range of substrates compared to GDOs. Although the two types of enzymes share a high degree of sequence similarity, the origin of substrate specificity between SDO and GDOs is not understood. We present electron paramagnetic resonance (EPR) investigation of ferrous-nitrosyl complexes of SDO and a GDO from the bacterium Corynebacterium glutamicum (GDO_Cg). The EPR spectra of these complexes, which mimic the Fe-substrate-O₂ intermediates in the catalytic cycle, show unexpected differences in the substrate binding mode and the coordination geometry of the metal cofactor in the two enzymes. Binding of substrate to the ferrous center increases the symmetry of the Fe(II)-NO complex in SDO, while a reverse trend is observed in GDO_Cg where substrate ligation reduces the symmetry of the nitrosyl complex. Identical EPR spectra were obtained for the NO derivatives of a variant of GDO_Cg(A112G), which can oxidize salicylate, and wild-type GDO_Cg revealing that the A112G mutation does not alter the nature of the Fe-substrate-O₂ ternary complex.

Dioxygen reactivity of iron(ii)–gentisate/1,4-dihydroxy-2-naphthoate complexes of N4 ligands: oxidative coupling of 1,4-dihydroxy-2-naphthoate

Oxidative C–C coupling of iron-coordinated co-ligand: Iron(ii)-1,4-dihydroxy-2-naphthoate complexes of neutral N4 ligands react with dioxygen to display C–C coupling of 1,4-dihydroxy-2-naphthoate.

A Biomimetic System for Studying Salicylate Dioxygenase

We report the characterization of [Fe(T1Et4iPrIP)(sal)] (2) (T1Et4iPrIP = tris(1-ethyl-4-isopropyl-imidazolyl)phosphine; sal^2- = salicylate dianion), which serves as a model for substrate-bound salicylate dioxygenase (SDO). Complex 2 crystallizes in the monoclinic space group P2₁/n with a = 10.7853(12) Å, b = 16.5060(19) Å, c = 21.217(2) Å, β = 94.489(2)°, and V = 3765.5(7) ų. The structure consists of Fe^II bonded in distorted square pyramidal geometry (τ = 0.32) with two salicylate oxygens and two T1Et4iPrIP nitrogens serving as the base and the apical position occupied by the other ligand nitrogen. [Fe(T1Et4iPrIP)(OTf)₂] (1), the precursor for 2, catalyzes the cleavage of 1,4-dihydroxy-2-naphthoate in the presence of O₂. Complex 1 is also capable of cleaving the salicylate aromatic ring in the presence of H₂O₂. The progression of this reaction toward product formation involves an Fe^III-phenoxide species.

A Structural Model for the Iron-Nitrosyl Adduct of Gentisate Dioxygenase

We present the synthesis, properties, and characterization of [Fe(T1Et4iPrIP)(NO)(H₂O)₂](OTf)₂ (1) (T1Et4iPrIP = Tris(1-ethyl-4-isopropyl-imidazolyl)phosphine) as a model for the nitrosyl adduct of gentisate 1,2-dioxygenase (GDO). The further characterization of [Fe(T1Et4iPrIP)(THF)(NO)(OTf)](OTf) (2) which was previously communicated (Inorg. Chem. 2014, 53, 5414) is also presented. The weighted average Fe-N-O angle of 162° for 1 is very close to linear (≥ 165°) for these types of complexes. The coordinated water ligands participate in hydrogen bonding interactions. The spectral properties (EPR, UV-vis, FTIR) for 1 are compared with 2 and found to be quite comparable. Complex 1 closely follows the relationship between the Fe-N-O angle and NO vibrational frequency which was previously identified for 6-coordinate {FeNO}⁷ complexes. Liquid FTIR studies on 2 indicate that the ν(NO) vibration position is sensitive to solvent shifting to lower energy (relative to the solid) in donor solvent THF and shifting to higher energy in dichloromethane. The basis for this behavior is discussed. The K _eq for NO binding in 2 was calculated in THF and found to be 470 M^-1. Density functional theory (DFT) studies on 1 indicate donation of electron density to the iron center from the π* orbitals of formally NO^-. Such a donation accounts for the near linearity of the Fe-N-O bond and the large ν(NO) value of 1791 cm^-1.

The crystal structures of native hydroquinone 1,2-dioxygenase from Sphingomonas sp. TTNP3 and of substrate and inhibitor complexes

The crystal structure of hydroquinone 1,2-dioxygenase, a Fe(II) ring cleaving dioxygenase from Sphingomonas sp. strain TTNP3, which oxidizes a wide range of hydroquinones to the corresponding 4-hydroxymuconic semialdehydes, has been solved by Molecular Replacement, using the coordinates of PnpCD from Pseudomonas sp. strain WBC-3. The enzyme is a heterotetramer, constituted of two subunits α and two β of 19 and 38kDa, respectively. Both the two subunits fold as a cupin, but that of the small α subunit lacks a competent metal binding pocket. Two tetramers are present in the asymmetric unit. Each of the four β subunits in the asymmetric unit binds one Fe(II) ion. The iron ion in each β subunit is coordinated to three protein residues, His258, Glu264, and His305 and a water molecule. The crystal structures of the complexes with the substrate methylhydroquinone, obtained under anaerobic conditions, and with the inhibitors 4-hydroxybenzoate and 4-nitrophenol were also solved. The structures of the native enzyme and of the complexes present significant differences in the active site region compared to PnpCD, the other hydroquinone 1,2-dioxygenase of known structure, and in particular they show a different coordination at the metal center.

Synthesis, X-ray Structures, Electronic Properties, and O2/NO Reactivities of Thiol Dioxygenase Active-Site Models

Mononuclear non-heme iron complexes that serve as structural and functional mimics of the thiol dioxygenases (TDOs), cysteine dioxygenase (CDO) and cysteamine dioxygenase (ADO), have been prepared and characterized with crystallographic, spectroscopic, kinetic, and computational methods. The high-spin Fe(II) complexes feature the facially coordinating tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine (^Ph2TIP) ligand that replicates the three histidine (3His) triad of the TDO active sites. Further coordination with bidentate l-cysteine ethyl ester (CysOEt) or cysteamine (CysAm) anions yielded five-coordinate (5C) complexes that resemble the substrate-bound forms of CDO and ADO, respectively. Detailed electronic-structure descriptions of the [Fe(^Ph2TIP)(L_S,N)]BPh₄ complexes, where L_S,N = CysOEt (1) or CysAm (2), were generated through a combination of spectroscopic techniques [electronic absorption, magnetic circular dichroism (MCD)] and density functional theory (DFT). Complexes 1 and 2 decompose in the presence of O₂ to yield the corresponding sulfinic acid (RSO₂H) products, thereby emulating the reactivity of the TDO enzymes and related complexes. Rate constants and activation parameters for the dioxygenation reactions were measured and interpreted with the aid of DFT calculations for O₂-bound intermediates. Treatment of the TDO models with nitric oxide (NO)-a well-established surrogate of O₂-led to a mixture of high-spin and low-spin {FeNO}⁷ species at low temperature (-70 °C), as indicated by electron paramagnetic resonance (EPR) spectroscopy. At room temperature, these Fe/NO adducts convert to a common species with EPR and infrared (IR) features typical of cationic dinitrosyl iron complexes (DNICs). To complement these results, parallel spectroscopic, computational, and O₂/NO reactivity studies were carried out using previously reported TDO models that feature an anionic hydrotris(3-phenyl-5-methyl-pyrazolyl)borate (^Ph,MeTp^-) ligand. Though the O₂ reactivities of the ^Ph2TIP- and ^Ph,MeTp-based complexes are quite similar, the supporting ligand perturbs the energies of Fe 3d-based molecular orbitals and modulates Fe-S bond covalency, suggesting possible rationales for the presence of neutral 3His coordination in CDO and ADO.

Identification and Characterization of a Novel Gentisate 1,2-Dioxygenase Gene from a Halophilic Martelella Strain

Halophilic Martelella strain AD-3, isolated from highly saline petroleum-contaminated soil, can efficiently degrade polycyclic aromatic hydrocarbons (PAHs), such as phenanthrene and anthracene, in 3-5% salinity. Gentisic acid is a key intermediate in the microbial degradation of PAH compounds. However, there is little information on PAH degradation by moderately halophilic bacteria. In this study, a 1,077-bp long gene encoding gentisate 1,2-dioxygenase (GDO) from a halophilic Martelella strain AD-3 was cloned, sequenced, and expressed in Escherichia coli. The recombinant enzyme GDO was purified and characterized in detail. By using the (18)O isotope experiment and LC-MS analysis, the sources of the two oxygen atoms added onto maleylpyruvate were identified as H2O and O2, respectively. The Km and kcat values for gentisic acid were determined to be 26.64 μM and 161.29 s(-1), respectively. In addition, optimal GDO activity was observed at 30 °C, pH 7.0, and at 12% salinity. Site-directed mutagenesis demonstrated the importance of four highly conserved His residues at positions 155, 157, 167, and 169 for enzyme activity. This finding provides new insights into mechanism and variety of gentisate 1,2-dioxygenase for PAH degradation in high saline conditions.

Function of different amino acid residues in the reaction mechanism of gentisate 1,2-dioxygenases deduced from the analysis of mutants of the salicylate 1,2-dioxygenase from Pseudaminobacter salicylatoxidans

The genome of the α-proteobacterium Pseudaminobacter salicylatoxidans codes for a ferrous iron containing ring-fission dioxygenase which catalyzes the 1,2-cleavage of (substituted) salicylate(s), gentisate (2,5-dihydroxybenzoate), and 1-hydroxy-2-naphthoate. Sequence alignments suggested that the "salicylate 1,2-dioxygenase" (SDO) from this strain is homologous to gentisate 1,2-dioxygenases found in bacteria, archaea and fungi. In the present study the catalytic mechanism of the SDO and gentisate 1,2-dioxygenases in general was analyzed based on sequence alignments, mutational and previously performed crystallographic studies and mechanistic comparisons with "extradiol- dioxygenases" which cleave aromatic nuclei in the 2,3-position. Different highly conserved amino acid residues that were supposed to take part in binding and activation of the organic substrates were modified in the SDO by site-specific mutagenesis and the enzyme variants subsequently analyzed for the conversion of salicylate, gentisate and 1-hydroxy-2-naphthoate. The analysis of enzyme variants which carried exchanges in the positions Arg83, Trp104, Gly106, Gln108, Arg127, His162 and Asp174 demonstrated that Arg83 and Arg127 were indispensable for enzymatic activity. In contrast, residual activities were found for variants carrying mutations in the residues Trp104, Gly106, Gln108, His162, and Asp174 and some of these mutants still could oxidize gentisate, but lost the ability to convert salicylate. The results were used to suggest a general reaction mechanism for gentisate-1,2-dioxygenases and to assign to certain amino acid residues in the active site specific functions in the cleavage of (substituted) salicylate(s).

Nickel quercetinase, a "promiscuous" metalloenzyme: metal incorporation and metal ligand substitution studies

BACKGROUND

Quercetinases are metal-dependent dioxygenases of the cupin superfamily. While fungal quercetinases are copper proteins, recombinant Streptomyces quercetinase (QueD) was previously described to be capable of incorporating Ni(2+) and some other divalent metal ions. This raises the questions of which factors determine metal selection, and which metal ion is physiologically relevant.

RESULTS

Metal occupancies of heterologously produced QueD proteins followed the order Ni > Co > Fe > Mn. Iron, in contrast to the other metals, does not support catalytic activity. QueD isolated from the wild-type Streptomyces sp. strain FLA contained mainly nickel and zinc. In vitro synthesis of QueD in a cell-free transcription-translation system yielded catalytically active protein when Ni(2+) was present, and comparison of the circular dichroism spectra of in vitro produced proteins suggested that Ni(2+) ions support correct folding. Replacement of individual amino acids of the 3His/1Glu metal binding motif by alanine drastically reduced or abolished quercetinase activity and affected its structural integrity. Only substitution of the glutamate ligand (E76) by histidine resulted in Ni- and Co-QueD variants that retained the native fold and showed residual catalytic activity.

CONCLUSIONS

Heterologous formation of catalytically active, native QueD holoenzyme requires Ni(2+), Co(2+) or Mn(2+), i.e., metal ions that prefer an octahedral coordination geometry, and an intact 3His/1Glu motif or a 4His environment of the metal. The observed metal occupancies suggest that metal incorporation into QueD is governed by the relative stability of the resulting metal complexes, rather than by metal abundance. Ni(2+) most likely is the physiologically relevant cofactor of QueD of Streptomyces sp. FLA.

Structure of the 2,4'-dihydroxyacetophenone dioxygenase from Alcaligenes sp. 4HAP

The enzyme 2,4'-dihydroxyacetophenone dioxygenase (DAD) catalyses the conversion of 2,4'-dihydroxyacetophenone to 4-hydroxybenzoic acid and formic acid with the incorporation of molecular oxygen. Whilst the vast majority of dioxygenases cleave within the aromatic ring of the substrate, DAD is very unusual in that it is involved in C-C bond cleavage in a substituent of the aromatic ring. There is evidence that the enzyme is a homotetramer of 20.3 kDa subunits, each containing nonhaem iron, and its sequence suggests that it belongs to the cupin family of dioxygenases. In this paper, the first X-ray structure of a DAD enzyme from the Gram-negative bacterium Alcaligenes sp. 4HAP is reported, at a resolution of 2.2 Å. The structure establishes that the enzyme adopts a cupin fold, forming dimers with a pronounced hydrophobic interface between the monomers. The catalytic iron is coordinated by three histidine residues (76, 78 and 114) within a buried active-site cavity. The iron also appears to be tightly coordinated by an additional ligand which was putatively assigned as a carbonate dianion since this fits the electron density optimally, although it might also be the product formate. The modelled carbonate is located in a position which is highly likely to be occupied by the α-hydroxyketone group of the bound substrate during catalysis. Modelling of a substrate molecule in this position indicates that it will interact with many conserved amino acids in the predominantly hydrophobic active-site pocket where it undergoes peroxide radical-mediated heterolysis.

Spectroscopic and computational investigation of iron(III) cysteine dioxygenase: implications for the nature of the putative superoxo-Fe(III) intermediate

Cysteine dioxygenase (CDO) is a mononuclear, non-heme iron-dependent enzyme that converts exogenous cysteine (Cys) to cysteine sulfinic acid using molecular oxygen. Although the complete catalytic mechanism is not yet known, several recent reports presented evidence for an Fe(III)-superoxo reaction intermediate. In this work, we have utilized spectroscopic and computational methods to investigate the as-isolated forms of CDO, as well as Cys-bound Fe(III)CDO, both in the absence and presence of azide (a mimic of superoxide). An analysis of our electronic absorption, magnetic circular dichroism, and electron paramagnetic resonance data of the azide-treated as-isolated forms of CDO within the framework of density functional theory (DFT) computations reveals that azide coordinates directly to the Fe(III), but not the Fe(II) center. An analogous analysis carried out for Cys-Fe(III)CDO provides compelling evidence that at physiological pH, the iron center is six coordinate, with hydroxide occupying the sixth coordination site. Upon incubation of this species with azide, the majority of the active sites retain hydroxide at the iron center. Nonetheless, a modest perturbation of the electronic structure of the Fe(III) center is observed, indicating that azide ions bind near the active site. Additionally, for a small fraction of active sites, azide displaces hydroxide and coordinates directly to the Cys-bound Fe(III) center to generate a low-spin (S = ¹/₂) Fe(III) complex. In the DFT-optimized structure of this complex, the central nitrogen atom of the azide moiety lies within 3.12 Å of the cysteine sulfur. A similar orientation of the superoxide ligand in the putative Fe(III)-superoxo reaction intermediate would promote the attack of the distal oxygen atom on the sulfur of substrate Cys.

Protocatechuate 3,4-dioxygenase: a wide substrate specificity enzyme isolated from Stenotrophomonas maltophilia KB2 as a useful tool in aromatic acid biodegradation

Protocatechuate 3,4-dioxygenases (P34Os) catalyze the reaction of the ring cleavage of aromatic acid derivatives. It is a key reaction in many xenobiotic metabolic pathways. P34Os characterize narrow substrate specificity. This property is an unfavorable feature in the biodegradation process because one type of pollution is rarely present in the environment. Thus, the following study aimed at the characterization of a P34O from Stenotrophomonas maltophilia KB2, being able to utilize a wide spectrum of aromatic carboxylic acids. A total of 3 mM vanillic acid and 4-hydroxybenzoate were completely degraded during 8 and 4.5 h, respectively. When cells of strain KB2 were grown on 9 mM 4-hydroxybenzoate, P34O was induced. Biochemical analysis revealed that the examined enzyme was similar to other known P34Os, but showed untypical wide substrate specificity. A high activity of P34O against 2,4- and 3,5-dihydroxybenzoate was observed. As these substrates do not possess ortho configuration hydroxyl groups, it is postulated that their cleavage could be connected with their monodentate binding of substrate to the active site. Since this enzyme characterizes untypical wide substrate specificity it makes it a useful tool in applications for environmental clean-up purposes.

Structure and function of atypically coordinated enzymatic mononuclear non-heme-Fe(II) centers

Mononuclear, non-heme-Fe(II) centers are key structures in O₂ metabolism and catalyze an impressive variety of enzymatic reactions. While most are bound via two histidines and a carboxylate, some show a different organization. A short overview of atypically coordinated O₂ dependent mononuclear-non-heme-Fe(II) centers is presented here Enzymes with 2-His, 3-His, 3-His-carboxylate and 4-His bound Fe(II) centers are discussed with a focus on their reactivity, metal ion promiscuity and recent progress in the elucidation of their enzymatic mechanisms. Observations concerning these and classically coordinated Fe(II) centers are used to understand the impact of the metal binding motif on catalysis.

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.

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.

Ring-cleaving dioxygenases with a cupin fold

Ring-cleaving dioxygenases catalyze key reactions in the aerobic microbial degradation of aromatic compounds. Many pathways converge to catecholic intermediates, which are subject to ortho or meta cleavage by intradiol or extradiol dioxygenases, respectively. However, a number of degradation pathways proceed via noncatecholic hydroxy-substituted aromatic carboxylic acids like gentisate, salicylate, 1-hydroxy-2-naphthoate, or aminohydroxybenzoates. The ring-cleaving dioxygenases active toward these compounds belong to the cupin superfamily, which is characterized by a six-stranded β-barrel fold and conserved amino acid motifs that provide the 3His or 2- or 3His-1Glu ligand environment of a divalent metal ion. Most cupin-type ring cleavage dioxygenases use an Fe(II) center for catalysis, and the proposed mechanism is very similar to that of the canonical (type I) extradiol dioxygenases. The metal ion is presumed to act as an electron conduit for single electron transfer from the metal-bound substrate anion to O(2), resulting in activation of both substrates to radical species. The family of cupin-type dioxygenases also involves quercetinase (flavonol 2,4-dioxygenase), which opens up two C-C bonds of the heterocyclic ring of quercetin, a wide-spread plant flavonol. Remarkably, bacterial quercetinases are capable of using different divalent metal ions for catalysis, suggesting that the redox properties of the metal are relatively unimportant for the catalytic reaction. The major role of the active-site metal ion could be to correctly position the substrate and to stabilize transition states and intermediates rather than to mediate electron transfer. The tentative hypothesis that quercetinase catalysis involves direct electron transfer from metal-bound flavonolate to O(2) is supported by model chemistry.

Gentisate 1,2-dioxygenase, in the third naphthalene catabolic gene cluster of Polaromonas naphthalenivorans CJ2, has a role in naphthalene degradation

Polaromonas naphthalenivorans strain CJ2 metabolizes naphthalene via the gentisate pathway and has recently been shown to carry a third copy of gentisate 1,2-dioxygenase (GDO), encoded by nagI3, within a previously uncharacterized naphthalene catabolic gene cluster. The role of this cluster (especially nagI3) in naphthalene metabolism of strain CJ2 was investigated by documenting patterns in regulation, transcription and enzyme activity. Transcriptional analysis of wild-type cells showed the third cluster to be polycistronic and that nagI3 was expressed at a relatively high level. Individual knockout mutants of all three nagI genes were constructed and their influence on both GDO activity and cell growth was evaluated. Of the three knockout strains, CJ2ΔnagI3 showed severely diminished GDO activity and grew slowest on aromatic substrates. These observations are consistent with the hypothesis that nagI3 may prevent toxic intracellular levels of gentisate from accumulating in CJ2 cells. All three nagI genes from strain CJ2 were cloned into Escherichia coli: the nagI2 and nagI3 genes were successfully overexpressed. The subunit mass of the GDOs were ~36–39 kDa, and their structures were deduced to be dimeric. The K m values of NagI2 and NagI3 were 31 and 10 µM, respectively, indicating that the higher affinity of NagI3 for gentisate may protect the wild-type cells from gentisate toxicity. These results provide clues for explaining why the third gene cluster, particularly the nagI3 gene, is important in strain CJ2. The organization of genes in the third gene cluster matched that of clusters in Polaromonas sp. JS666 and Leptothrix cholodnii SP-6. While horizontal gene transfer (HGT) is one hypothesis for explaining this genetic motif, gene duplication within the ancestral lineage is equally valid. The HGT hypothesis was discounted by noting that the nagI3 allele of strain CJ2 did not share high sequence identity with its homologues in Polaromonas sp. JS666 and L. cholodnii SP-6.

Structural and functional comparison of 2-His-1-carboxylate and 3-His metallocentres in non-haem iron(II)-dependent enzymes

The canonical structural motif for co-ordination of non-haem ferrous iron in metal-dependent oxygenases is a facial triad of two histidine residues and one aspartate or glutamate residue. This so-called 2-His-1-carboxylate metallocentre is often accommodated in a double-stranded beta-helix fold with the iron-co-ordinating residues located in the rigid core structure of the protein. At the sequence level, the metal ligands are arranged in a HXD/E...H motif (where the distance between the conserved histidine residues is variable). Interestingly, cysteine dioxygenase, among a growing number of other iron(II) oxygenases, has the carboxylate residue replaced by another histidine. In the present review, we compare the properties of 3-His and 2-His-1-carboxylate sites based on current evidence from high-resolution crystal structures, spectroscopic characterization of the metal centres and results from mutagenesis studies. Although the overall conformation of the two metal sites is quite similar, the carboxylate residue seems to accommodate a slightly closer co-ordination distance than the counterpart histidine. The ability of the 2-His-1-carboxylate site to fit a site-directed substitution by an alternatively co-ordinating or non-co-ordinating residue with retention of metal-binding capacity and catalytic function varies among different enzymes. However, replacement by histidine disrupted the activity in the three iron(II) oxygenases examined so far.