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The metal- and substrate-dependences of 2,4'-dihydroxyacetophenone dioxygenase.

ABSTRACT: While the enzyme, 2,4'-dihydroxyacetophenone dioxygenase (DAD), has been known for decades, very little has been characterized of the mechanism of the DAD-catalyzed oxidative cleavage of its reported substrate, 2,4'-dihydroxyacetophenone (DHA). The purpose of this study was to identify the active metal center and to characterize the substrate-dependence of the kinetics of the reaction to lay the foundation for deeper mechanistic investigation. To this, the DAD V1M mutant (bDAD) was overexpressed, purified, and reconstituted with various metal ions. Kinetic assays evaluating the activity of the reconstituted enzyme as well as the substrate- and product-dependences of the reaction kinetics were performed. The results from reconstitution of the apoprotein with a variety of metal ions support the requirement for an Fe³⁺ center for enzyme activity. Reaction rates showed simple saturation kinetics for DHA with values for k_cat and K_DHA of 2.4 s^-1 and 0.7 μM, respectively, but no significant dependence on the concentration of O₂. A low-level inhibition (K_I = 1100 μM) by the 4HB product was observed. The results support a minimal kinetic model wherein DHA binds to resting ferric enzyme followed by rapid addition of O₂ to yield an intermediate complex that irreversibly collapses to products.

SUBMITTER: Roberts KM

PROVIDER: S-EPMC7484352 | biostudies-literature |

REPOSITORIES: biostudies-literature

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

Project description: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.

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Extension of resolution and oligomerization-state studies of 2,4'-dihydroxyacetophenone dioxygenase from Alcaligenes sp. 4HAP.

Project description:The enzyme 2,4'-dihydroxyacetophenone dioxygenase (DAD) catalyses the conversion of 2,4'-dihydroxyacetophenone to 4-hydroxybenzoic acid and formic acid. This enzyme is a very unusual dioxygenase in that it cleaves a C-C bond in a substituent of the aromatic ring rather than within the ring itself. Whilst it has been shown that DAD is a tetramer in solution, the recently solved crystal structure of the Alcaligenes sp. 4HAP enzyme was in fact dimeric rather than tetrameric. Since the use of limited chymotrypsinolysis, which apparently results in removal of the first 20 or so N-terminal residues of DAD, was necessary for crystallization of the protein, it was investigated whether this was responsible for the change in its oligomerization state. Gel-filtration and analytical ultracentrifugation studies were conducted, which confirmed that chymotrypsinolysed DAD has an apparent molecular weight of around 40?kDa, corresponding to a dimer. In contrast, the native enzyme has a molecular weight in the 70-80?kDa region, as expected for the tetramer. The structural basis for tetramerization has been investigated by the use of several docking servers, and the results are remarkably consistent with the tetrameric structure of a homologous cupin protein from Ralstonia eutropha (PDB entry 3ebr).

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2,4'-Dihydroxyacetophenone dioxygenase (EC 1.13.11.41) from Alcaligenes sp. 4HAP: a novel enzyme with an atypical dioxygenase sequence.

Project description:2,4'-Dihydroxyacetophenone dioxygenase (EC 1.13.11.41) was purified to homogeneity from Alcaligenes sp. 4HAP grown on 4-hydroxyacetophenone. Measurements of the M(r) of the native enzyme ranged from 81600 to 87000, whereas values of 21000 and 20379 were given by SDS/PAGE and electrospray MS respectively. The enzyme is a homotetramer and contains one atom of iron per molecule of enzyme. From C- and N-terminal analyses, primers for PCR were designed and the dad gene cloned and sequenced. The predicted amino acid sequence of dad, deduced from the nucleotide sequence, confirms the N-terminal amino acid sequencing data and contains the sequence of an internal tryptic peptide. It gave a calculated M(r) of 20364. The gene was expressed in Escherichia coli and yielded active enzyme. The derived amino acid sequence does not show significant similarity to other dioxygenases or any strong similarity to protein sequences presently available in the databases.

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2,4-Dinitrotoluene dioxygenase from Burkholderia sp. strain DNT: similarity to naphthalene dioxygenase.

Project description:2,4-Dinitrotoluene (DNT) dioxygenase from Burkholderia sp. strain DNT catalyzes the initial oxidation of DNT to form 4-methyl-5-nitrocatechol (MNC) and nitrite. The displacement of the aromatic nitro group by dioxygenases has only recently been described, and nothing is known about the evolutionary origin of the enzyme systems that catalyze these reactions. We have shown previously that the gene encoding DNT dioxygenase is localized on a degradative plasmid within a 6.8-kb NsiI DNA fragment (W.-C. Suen and J. C. Spain, J. Bacteriol. 175:1831-1837, 1993). We describe here the sequence analysis and the substrate range of the enzyme system encoded by this fragment. Five open reading frames were identified, four of which have a high degree of similarity (59 to 78% identity) to the components of naphthalene dioxygenase (NDO) from Pseudomonas strains. The conserved amino acid residues within NDO that are involved in cofactor binding were also identified in the gene encoding DNT dioxygenase. An Escherichia coli clone that expressed DNT dioxygenase converted DNT to MNC and also converted naphthalene to (+)-cis-(1R,2S)-dihydroxy-1,2-dihydronaphthalene. In contrast, the E. coli clone that expressed NDO did not oxidize DNT. Furthermore, the enzyme systems exhibit similar broad substrate specificities and can oxidize such compounds as indole, indan, indene, phenetole, and acenaphthene. These results suggest that DNT dioxygenase and the NDO enzyme system share a common ancestor.

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Set of Fe(II)-3-Hydroxyflavonolate Enzyme-Substrate Model Complexes of Atypically Coordinated Mononuclear Non-Heme Fe(II)-Dependent Quercetin 2,4-Dioxygenase.

Project description:With the aim of revealing the catalytic role of atypically coordinated (3His-1Glu) active site mononuclear non-heme Fe(II)-dependent quercetin 2,4-dioxygenase (Fe-2,4-QD) and the electronic effects of the model ligands on the reactivity toward dioxygen, a set of p/m-R-substituted carboxylate-containing ligand-supported Fe(II)-3-hydroxyflavonolate complexes, [FeIILR(fla)] (LRH: 2-{[bis(pyridin-2-ylmethyl)amino]methyl}-p/m-R-benzoic acid; R: p-OMe (1), p-Me (2), m-Br (4), and m-NO2 (5); fla: 3-hydroxyflavonolate), were synthesized and characterized as structural and functional models for the ES (enzyme-substrate) complexes of Fe-2,4-QD. [FeIILR(fla)] show relatively high enzyme-type reactivity (dioxygenative ring opening of the coordinated substrate fla, single-turnover reaction) at low temperatures (30-65 °C). The reaction shows a linear Hammett plot (ρ = -1.21), and electron donating groups enhance the reaction rates. The notable difference on the reactivity can be rationalized from the electronic nature of the substituent in the ligands, which could tune the reactivity via tuning Lewis acidity of the Fe(II) ion, electron density, and the redox potential of fla. The properties and the reactivity show approximately linear correlations between λmax or E 1/2 of fla and the reaction rate constant k. This work sheds light not only on understanding of electronic effects of the ligands and the property-reactivity relationship but also on the role of the catalytic reaction by Fe-2,4-QD.

| S-EPMC6644611 | biostudies-literature

Mechanism and substrate recognition of 2-hydroxyethylphosphonate dioxygenase.

Project description:HEPD belongs to the superfamily of 2-His-1-carboxylate non-heme iron-dependent dioxygenases. It converts 2-hydroxyethylphosphonate (2-HEP) to hydroxymethylphosphonate (HMP) and formate. Previously postulated mechanisms for the reaction catalyzed by HEPD cannot explain its conversion of 1-HEP to acetylphosphate. Alternative mechanisms that involve either phosphite or methylphosphonate as intermediates, which potentially explain all experimental studies including isotope labeling experiments and use of substrate analogues, were investigated. The results of these studies reveal that these alternative mechanisms are not correct. Site-directed mutagenesis studies of Lys16, Arg90, and Tyr98 support roles of these residues in binding of 2-HEP. Mutation of Lys16 to Ala resulted in an inactive enzyme, whereas mutation of Arg90 to Ala or Tyr98 to Phe greatly decreased k(cat)/K(m,2-HEP). Furthermore, the latter mutants could not be saturated in O(2). These results suggest that proper binding of 2-HEP is important for O(2) activation and that the enzyme uses a compulsory binding order with 2-HEP binding before O(2). The Y98F mutant produces methylphosphonate as a minor side product providing indirect support for the proposal that the last step during catalysis involves a ferric hydroxide reacting with a methylphosphonate radical.

| S-EPMC3143709 | biostudies-literature

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