Interaction of Azole-Based Environmental Pollutants with the Coelomic Hemoglobin from Amphitrite ornata: A Molecular Basis for Toxicity

Comparison of the Backbone Dynamics of Dehaloperoxidase-Hemoglobin Isoenzymes

Dehaloperoxidase (DHP) is a multifunctional hemeprotein with a functional switch generally regulated by the chemical class of the substrate. Its two isoforms, DHP-A and DHP-B, differ by only five amino acids and have an almost identical protein fold. However, the catalytic efficiency of DHP-B for oxidation by a peroxidase mechanism ranges from 2- to 6-fold greater than that of DHP-A depending on the conditions. X-ray crystallography has shown that many substrates and ligands have nearly identical binding in the two isoenzymes, suggesting that the difference in catalytic efficiency could be due to differences in the conformational dynamics. We compared the backbone dynamics of the DHP isoenzymes at pH 7 through heteronuclear relaxation dynamics at 11.75, 16.45, and 19.97 T in combination with four 300 ns MD simulations. While the overall dynamics of the isoenzymes are similar, there are specific local differences in functional regions of each protein. In DHP-A, Phe35 undergoes a slow chemical exchange between two conformational states likely coupled to a swinging motion of Tyr34. Moreover, Asn37 undergoes fast chemical exchange in DHP-A. Given that Phe35 and Asn37 are adjacent to Tyr34 and Tyr38, it is possible that their dynamics modulate the formation and migration of the active tyrosyl radicals in DHP-A at pH 7. Another significant difference is that both distal and proximal histidines have a 15-18% smaller S2 value in DHP-B, thus their greater flexibility could account for the higher catalytic activity. The distal histidine grants substrate access to the distal pocket. The greater flexibility of the proximal histidine could also accelerate H2O2 activation at the heme Fe by increased coupling of an amino acid charge relay to stabilize the ferryl Fe(IV) oxidation state in a Poulos-Kraut "push-pull"-type peroxidase mechanism.

Direct Electrophilic Attack of Compound I on the Indole Ring in the Peroxygenase Mechanism of Dehaloperoxidase DHP B in Degrading Haloindole: Electron Transfer Promotes the Reaction

The H2O2-dependent degradation of haloindole catalyzed by the dehaloperoxidase (DHP) from Amphitrite ornate has been reported to employ the peroxygenase mechanism, and the two oxidized products 5-halo-2-oxindole and 5-halo-3-oxindole have a similar amount. According to a previous experimental study, compound I (Cpd I) was suggested to be responsible for triggering the reaction, and the reaction may undergo three possible intermediates; however, the reaction details are still unclear. To clarify the reaction mechanism of DHP, the computational model was constructed on the basis of the high-resolution crystal structure, and a series of the quantum mechanical/molecular mechanical calculations were performed. Based on our calculation results, it is confirmed that the reaction starts from the direct electrophilic attack of Cpd I on the indole ring of the substrate, and the resulted intermediate contains both a carbocation and an oxygen anion, whereas the common hydrogen abstraction by Cpd I was calculated to correspond to a relatively higher barrier. In addition, a net electron transfer from the substrate to the iron center is observed during the attack of Cpd I on the indole ring; therefore, the carbocation/oxygen anion intermediate can easily undergo an intramolecular hydride transfer to form the product 5-halo-2-oxindole or isomerize to the epoxide intermediate which finally generates another product 5-halo-3-oxindole. It is the zwitterionic characteristic of the intermediate that makes the intermolecular hydride transfer quite easy, and it is the high electron affinity of the iron center that promotes the single-electron oxidation of the reaction intermediate. Our calculations well explain the formation of two oxidized products 5-halo-2-oxindole and 5-halo-3-oxindole.

A review on structural mechanisms of protein-persistent organic pollutant (POP) interactions

With the advent of the industrial revolution, the accumulation of persistent organic pollutants (POPs) in the environment has become ubiquitous. POPs are halogen-containing organic molecules that accumulate, and remain in the environment for a long time, thus causing toxic effects in living organisms. POPs exhibit a high affinity towards biological macromolecules such as nucleic acids, proteins and lipids, causing genotoxicity and impairment of homeostasis in living organisms. Proteins are essential members of the biological assembly, as they stipulate all necessary processes for the survival of an organism. Owing to their stereochemical features, POPs and their metabolites form energetically favourable complexes with proteins, as supported by biological and dose-dependent toxicological studies. Although individual studies have reported the biological aspects of protein-POP interactions, no comprehensive study summarizing the structural mechanisms, thermodynamics and kinetics of protein-POP complexes is available. The current review identifies and classifies protein-POP interaction according to the structural and functional basis of proteins into five major protein targets, including digestive and other enzymes, serum proteins, transcription factors, transporters, and G-protein coupled receptors. Further, analysis detailing the molecular interactions and structural mechanism evidenced that H-bonds, van der Waals, and hydrophobic interactions essentially mediate the formation of protein-POP complexes. Moreover, interaction of POPs alters the protein conformation through kinetic and thermodynamic processes like competitive inhibition and allostery to modulate the cellular signalling processes, resulting in various pathological conditions such as cancers and inflammations. In summary, the review provides a comprehensive insight into the critical structural/molecular aspects of protein-POP interactions.

Oxidation of bisphenol A (BPA) and related compounds by the multifunctional catalytic globin dehaloperoxidase

Dehaloperoxidase (DHP) from the marine polychaete Amphitrite ornata is a multifunctional enzyme that possesses peroxidase, peroxygenase, oxidase and oxygenase activities. Herein, we investigated the reactivity of DHP B with bisphenol A (BPA) and related compounds (bisphenol E, bisphenol F, tetrachlorobisphenol A, 2,2'-biphenol, 3,3'-biphenol, 4,4'-biphenol, and 3,3'-dibromo-4,4'-biphenol). As a previously unknown substrate for DHP B, BPA (as a representative substrate) is an endocrine disruptor widely used in polycarbonate and epoxy resins, thus resulting in human exposure. Reactivity studies with these substrates were investigated using high performance liquid chromatography (HPLC), and their corresponding oxidation products were determined by mass spectrometry (GC-MS/ LC-MS). BPA undergoes oxidation in the presence of DHP B and hydrogen peroxide yielding two cleavage products (4-isopropenylphenol and 4-(2-hydroxypropan-2-yl)phenol), and oligomers with varying degrees of oxidation. ¹⁸O-labeling studies confirmed that the O-atom incorporated into the products was derived exclusively from water, consistent with substrate oxidation via a peroxidase-based mechanism. The X-ray crystal structures of DHP bound with bisphenol E (1.48 Å), bisphenol F (1.75 Å), 2,2'-biphenol (1.90 Å) and 3,3'-biphenol (1.30 Å) showed substrate binding sites are in the distal pocket of the heme cofactor, similar to other previously studied DHP substrates. Stopped-flow UV-visible spectroscopy was utilized to investigate the mechanistic details and enzyme oxidation states during substrate turnover, and a reaction mechanism is proposed. The data presented here strongly suggest that DHP B can catalyze the oxidation of bisphenols and biphenols, thus providing evidence of how infaunal invertebrates can contribute to the biotransformation of these marine pollutants.

Bridging the functional gap between reactivity and inhibition in dehaloperoxidase B from Amphitrite ornata: Mechanistic and structural studies with 2,4- and 2,6-dihalophenols

The multifunctional catalytic globin dehaloperoxidase (DHP) from the marine worm Amphitrite ornata was shown to catalyze the H₂O₂-dependent oxidation of 2,4- and 2,6-dihalophenols (DXP; X = F, Cl, Br). Product identification by LC-MS revealed multiple monomeric products with varying degrees of oxidation and/or dehalogenation, as well as oligomers with n up to 6. Mechanistic and ¹⁸O-labeling studies demonstrated sequential dihalophenol oxidation via peroxidase and peroxygenase activities. Binding studies established that 2,4-DXP (X = Cl, Br) have the highest affinities of any known DHP substrate. X-ray crystallography identified different binding positions for 2,4- and 2,6-DXP substrates in the hydrophobic distal pocket of DHP. Correlation between the number of halogens and the substrate binding orientation revealed a halogen-dependent binding motif for mono- (4-halophenol), di- (2,4- and 2,6-dihalophenol) and trihalophenols (2,4,6-trihalopenol). Taken together, the findings here on dihalophenol reactivity with DHP advance our understanding of how these compounds bridge the inhibitory and oxidative functions of their mono- and trihalophenol counterparts, respectively, and provide further insight into the protein structure-function paradigm relevant to multifunctional catalytic globins in comparison to their monofunctional analogs.

Complementarity of neutron, XFEL and synchrotron crystallography for defining the structures of metalloenzymes at room temperature

Room-temperature macromolecular crystallography allows protein structures to be determined under close-to-physiological conditions, permits dynamic freedom in protein motions and enables time-resolved studies. In the case of metalloenzymes that are highly sensitive to radiation damage, such room-temperature experiments can present challenges, including increased rates of X-ray reduction of metal centres and site-specific radiation-damage artefacts, as well as in devising appropriate sample-delivery and data-collection methods. It can also be problematic to compare structures measured using different crystal sizes and light sources. In this study, structures of a multifunctional globin, dehaloperoxidase B (DHP-B), obtained using several methods of room-temperature crystallographic structure determination are described and compared. Here, data were measured from large single crystals and multiple microcrystals using neutrons, X-ray free-electron laser pulses, monochromatic synchrotron radiation and polychromatic (Laue) radiation light sources. These approaches span a range of 18 orders of magnitude in measurement time per diffraction pattern and four orders of magnitude in crystal volume. The first room-temperature neutron structures of DHP-B are also presented, allowing the explicit identification of the hydrogen positions. The neutron data proved to be complementary to the serial femtosecond crystallography data, with both methods providing structures free of the effects of X-ray radiation damage when compared with standard cryo-crystallography. Comparison of these room-temperature methods demonstrated the large differences in sample requirements, data-collection time and the potential for radiation damage between them. With regard to the structure and function of DHP-B, despite the results being partly limited by differences in the underlying structures, new information was gained on the protonation states of active-site residues which may guide future studies of DHP-B.

Computational Study of the Peroxygenase Mechanism Catalyzed by Hemoglobin Dehaloperoxidase Involved in the Degradation of Chlorophenols

The biochemical evidence showed that hemoglobin dehaloperoxidase (DHP B) from Amphitrite Ornata is a multifunctional hemoprotein that catalyzes both dehalogenation and hydroxylation of halophenols via the peroxidase and peroxygenase mechanism, respectively, which sets the basis for the degradation of halophenols. In the peroxygenase mechanism, the reaction was previously suggested to be triggered either by the hydrogen atom abstraction by the Fe═O center or by the proton abstraction by His55. To illuminate the peroxygenase mechanism of DHP B at the atomistic level, on the basis of the high-resolution crystal structure, computational models were constructed, and a series of quantum mechanical/molecular mechanical calculations have been performed. According to the calculation results, the pathway (Path a) initiated by the H-abstraction by the Fe═O center is feasible. In another pathway (Path b), His55 can abstract the proton from the hydroxyl group of the substrate (4-Cl-o-cresol) to initiate the reaction; however, its feasibility depends on the prior electron transfer from the substrate to the porphyrin group. The rate-limiting step of Path a is the OH-rebound, which corresponds to an energy barrier of 14.7 kcal/mol at the quartet state. His55 acts as an acid-base catalyst and directly involves in the catalysis. Our mutant study indicates that His55 can be replaced by other titratable residues. These findings may provide useful information for further understanding of the catalytic reaction of DHP B and for the design of enzymes in the degradation of pollutants, in particular, halophenols.

A new inhibition mechanism in the multifunctional catalytic hemoglobin dehaloperoxidase as revealed by the DHP A(V59W) mutant: A spectroscopic and crystallographic study

As multifunctional catalytic hemoglobins, dehaloperoxidase isoenzymes A and B (DHP A and B) are among the most versatile hemoproteins in terms of activities displayed. The ability of DHP to bind over twenty different substrates in the distal pocket might appear to resemble the promiscuousness of monooxygenase enzymes, yet there are identifiable substrate-specific interactions that can steer the type of oxidation (O-atom vs. electron transfer) that occurs inside the DHP distal pocket. Here, we have investigated the DHP A(V59W) mutant in order to probe the limits of conformational flexibility in the distal pocket as it relates to the genesis of this substrate-dependent activity differentiation. The X-ray crystal structure of the metaquo DHP A(V59W) mutant (PDB 3K3U) and the V59W mutant in complex with fluoride [denoted as DHP A(V59W-F)] (PDB 7MNH) show significant mobility of the tryptophan in the distal pocket, with two parallel conformations having W59-N[Formula: see text] H-bonded to a heme-bound ligand (H2O or F[Formula: see text], and another conformation [observed only in DHP A(V59W-F)] that brings W59 sufficiently close to the heme as to preclude axial ligand binding. UV-vis and resonance Raman spectroscopic studies show that DHP A(V59W) is 5-coordinate high spin (5cHS) at pH 5 and 6-coordinate high spin (6cHS) at pH 7, whereas DHP A(V59W-F) is 6cHS from pH 5 to 7. Enzyme assays confirm robust peroxidase activity at pH 5, but complete loss of activity at pH 7. We find no evidence that tryptophan plays a role in the oxidation mechanism ([Formula: see text]. radical formation). Instead, the data reveal a new mechanism of DHP inhibition, namely a shift towards a non-reactive form by OH[Formula: see text] ligation to the heme-Fe that is strongly stabilized (presumably through H-bonding interactions) by the presence of W59 in the distal cavity.

High-throughput structures of protein-ligand complexes at room temperature using serial femtosecond crystallography

High-throughput X-ray crystal structures of protein-ligand complexes are critical to pharmaceutical drug development. However, cryocooling of crystals and X-ray radiation damage may distort the observed ligand binding. Serial femtosecond crystallography (SFX) using X-ray free-electron lasers (XFELs) can produce radiation-damage-free room-temperature structures. Ligand-binding studies using SFX have received only modest attention, partly owing to limited beamtime availability and the large quantity of sample that is required per structure determination. Here, a high-throughput approach to determine room-temperature damage-free structures with excellent sample and time efficiency is demonstrated, allowing complexes to be characterized rapidly and without prohibitive sample requirements. This yields high-quality difference density maps allowing unambiguous ligand placement. Crucially, it is demonstrated that ligands similar in size or smaller than those used in fragment-based drug design may be clearly identified in data sets obtained from <1000 diffraction images. This efficiency in both sample and XFEL beamtime opens the door to true high-throughput screening of protein-ligand complexes using SFX.

The multifunctional globin dehaloperoxidase strikes again: Simultaneous peroxidase and peroxygenase mechanisms in the oxidation of EPA pollutants

The multifunctional catalytic hemoglobin dehaloperoxidase (DHP) from the terebellid polychaete Amphitrite ornata was found to catalyze the H₂O₂-dependent oxidation of EPA Priority Pollutants (4-Me-o-cresol, 4-Cl-m-cresol and pentachlorophenol) and EPA Toxic Substances Control Act compounds (o-, m-, p-cresol and 4-Cl-o-cresol). Biochemical assays (HPLC/LC-MS) indicated formation of multiple oxidation products, including the corresponding catechol, 2-methylbenzoquinone (2-MeBq), and oligomers with varying degrees of oxidation and/or dehalogenation. Using 4-Br-o-cresol as a representative substrate, labeling studies with ¹⁸O confirmed that the O-atom incorporated into the catechol was derived exclusively from H₂O₂, whereas the O-atom incorporated into 2-MeBq was from H₂O, consistent with this single substrate being oxidized by both peroxygenase and peroxidase mechanisms, respectively. Stopped-flow UV-visible spectroscopic studies strongly implicate a role for Compound I in the peroxygenase mechanism leading to catechol formation, and for Compounds I and ES in the peroxidase mechanism that yields the 2-MeBq product. The X-ray crystal structures of DHP bound with 4-F-o-cresol (1.42 Å; PDB 6ONG), 4-Cl-o-cresol (1.50 Å; PDB 6ONK), 4-Br-o-cresol (1.70 Å; PDB 6ONX), 4-NO₂-o-cresol (1.80 Å; PDB 6ONZ), o-cresol (1.60 Å; PDB 6OO1), p-cresol (2.10 Å; PDB 6OO6), 4-Me-o-cresol (1.35 Å; PDB 6ONR) and pentachlorophenol (1.80 Å; PDB 6OO8) revealed substrate binding sites in the distal pocket in close proximity to the heme cofactor, consistent with both oxidation mechanisms. The findings establish cresols as a new class of substrate for DHP, demonstrate that multiple oxidation mechanisms may exist for a given substrate, and provide further evidence that different substituents can serve as functional switches between the different activities performed by dehaloperoxidase. More broadly, the results demonstrate the complexities of marine pollution where both microbial and non-microbial systems may play significant roles in the biotransformations of EPA-classified pollutants, and further reinforces that heterocyclic compounds of anthropogenic origin should be considered as environmental stressors of infaunal organisms.

Oxidation of Pyrrole by Dehaloperoxidase-Hemoglobin: Chemoenzymatic Synthesis of Pyrrolin-2-Ones

The use of oxidoreductases as biocatalysts in the syntheses of functionalized, monomeric pyrroles has been a challenge owing to, among a number of factors, undesired polypyrrole formation. Here, we have investigated the ability of dehaloperoxidase (DHP), the coelomic hemoglobin from the terebellid polychaete Amphitrite ornata, to catalyze the H2O2-dependent oxidation of pyrroles as a new class of substrate for this enzyme. Substrate oxidation was observed for all compounds employed (pyrrole, N-methylpyrrole, 2-methylpyrrole, 3-methylpyrrole and 2,5-dimethylpyrrole) under both aerobic and anaerobic conditions. Using pyrrole as a representative substrate, only a single oxidation product, 4-pyrrolin-2-one, was observed, and notably without formation of polypyrrole. Reactivity could be initiated from all three biologically relevant oxidation states for this catalytic globin: ferric, ferrous and oxyferrous. Isotope labeling studies determined that the O-atom incorporated into the 4-pyrrolin-2-one product was derived exclusively from H2O2, indicative of a peroxygenase mechanism. Consistent with this observation, single- and double-mixing stopped-flow UV-visible spectroscopic studies supported Compound I, but not Compounds ES or II, as the catalytically-relevant ferryl intermediate involved in pyrrole oxidation. Electrophilic addition of the ferryl oxygen to pyrrole is proposed as the mechanism of O-atom transfer. The results demonstrate the breadth of chemical reactivity afforded by dehaloperoxidase, and provide further evidence for establishing DHP as a multifunctional globin with practical applications as a biocatalyst.