4-Hydroxynonenal impairs miRNA maturation in heart failure via Dicer post-translational modification.

BACKGROUND AND AIMS: Developing novel therapies to battle the global public health burden of heart failure remains challenging. This study investigates the underlying mechanisms and potential treatment for 4-hydroxynonenal (4-HNE) deleterious effects in heart failure. METHODS: Biochemical, functional, and histochemical measurements were applied to identify 4-HNE adducts in rat and human failing hearts. In vitro studies were performed to validate 4-HNE targets. RESULTS: 4-HNE, a reactive aldehyde by-product of mitochondrial dysfunction in heart failure, covalently inhibits Dicer, an RNase III endonuclease essential for microRNA (miRNA) biogenesis. 4-HNE inhibition of Dicer impairs miRNA processing. Mechanistically, 4-HNE binds to recombinant human Dicer through an intermolecular interaction that disrupts both activity and stability of Dicer in a concentration- and time-dependent manner. Dithiothreitol neutralization of 4-HNE or replacing 4-HNE-targeted residues in Dicer prevents 4-HNE inhibition of Dicer in vitro. Interestingly, end-stage human failing hearts from three different heart failure aetiologies display defective 4-HNE clearance, decreased Dicer activity, and miRNA biogenesis impairment. Notably, boosting 4-HNE clearance through pharmacological re-activation of mitochondrial aldehyde dehydrogenase 2 (ALDH2) using Alda-1 or its improved orally bioavailable derivative AD-9308 restores Dicer activity. ALDH2 is a major enzyme responsible for 4-HNE removal. Importantly, this response is accompanied by improved miRNA maturation and cardiac function/remodelling in a pre-clinical model of heart failure. CONCLUSIONS: 4-HNE inhibition of Dicer directly impairs miRNA biogenesis in heart failure. Strikingly, decreasing cardiac 4-HNE levels through pharmacological ALDH2 activation is sufficient to re-establish Dicer activity and miRNA biogenesis; thereby representing potential treatment for patients with heart failure.

Dissecting Reaction Mechanisms and Catalytic Contributions in Flavoprotein Fumarate Reductases.

The interconversion between fumarate and succinate is fundamental to the energy metabolism of nearly all organisms. This redox reaction is catalyzed by a large family of enzymes, fumarate reductases and succinate dehydrogenases, using hydride and proton transfers from a flavin cofactor and a conserved Arg side-chain. These flavoenzymes also have substantial biomedical and biotechnological importance. Therefore, a detailed understanding of their catalytic mechanisms is valuable. Here, calibrated electronic structure calculations in a cluster model of the active site of the Fcc3 fumarate reductase were employed to investigate various reaction pathways and possible intermediates in the enzymatic environment and to dissect interactions that contribute to catalysis of fumarate reduction. Carbanion, covalent adduct, carbocation, and radical intermediates were examined. Significantly lower barriers were obtained for mechanisms via carbanion intermediates, with similar activation energies for hydride and proton transfers. Interestingly, the carbanion bound to the active site is best described as an enolate. Hydride transfer is stabilized by a preorganized charge dipole in the active site and by the restriction of the C1-C2 bond in a twisted conformation of the otherwise planar fumarate dianion. But, protonation of a fumarate carboxylate and quantum tunneling effects are not critical for catalysis of the hydride transfer. Calculations also suggest that the driving force for enzyme turnover is provided by regeneration of the catalytic Arg, either coupled with flavin reduction and decomposition of a proposed transient state or directly from the solvent. The detailed mechanistic description of enzymatic reduction of fumarate provided here clarifies previous contradictory views and provides new insights into catalysis by essential flavoenzyme reductases and dehydrogenases.

Mechanism of rotenone binding to respiratory complex I depends on ligand flexibility.

Respiratory complex I is a major cellular energy transducer located in the inner mitochondrial membrane. Its inhibition by rotenone, a natural isoflavonoid, has been used for centuries by indigenous peoples to aid in fishing and, more recently, as a broad-spectrum pesticide or even a possible anticancer therapeutic. Unraveling the molecular mechanism of rotenone action will help to design tuned derivatives and to understand the still mysterious catalytic mechanism of complex I. Although composed of five fused rings, rotenone is a flexible molecule and populates two conformers, bent and straight. Here, a rotenone derivative locked in the straight form was synthesized and found to inhibit complex I with 600-fold less potency than natural rotenone. Large-scale molecular dynamics and free energy simulations of the pathway for ligand binding to complex I show that rotenone is more stable in the bent conformer, either free in the membrane or bound to the redox active site in the substrate-binding Q-channel. However, the straight conformer is necessary for passage from the membrane through the narrow entrance of the channel. The less potent inhibition of the synthesized derivative is therefore due to its lack of internal flexibility, and interconversion between bent and straight forms is required to enable efficient kinetics and high stability for rotenone binding. The ligand also induces reconfiguration of protein loops and side-chains inside the Q-channel similar to structural changes that occur in the open to closed conformational transition of complex I. Detailed understanding of ligand flexibility and interactions that determine rotenone binding may now be exploited to tune the properties of synthetic derivatives for specific applications.

Molecular properties and tautomeric equilibria of isolated flavins.

Flavins are employed as redox cofactors and chromophores in a plethora of flavoenzymes. Their versatility is an outcome of intrinsic molecular properties of the isoalloxazine ring modulated by the protein scaffold and surrounding solvent. Thus, an investigation of isolated flavins with high-level electronic-structure methods and with error assessment of the calculated properties will contribute to building better models of flavin reactivity. Here, we benchmarked ground-state properties such as electron affinity, gas-phase basicity, dipole moment, torsion energy, and tautomer stability for lumiflavins in all biologically relevant oxidation and charge states. Overall, multiconfigurational effects are small and chemical accuracy is achieved by coupled-cluster treatments of energetic properties. Augmented basis sets and extrapolations to the complete basis-set limit are necessary for consistent agreement with experimental energetics. Among DFT functionals tested, M06-2X shows the best performance for most properties, except gas-phase basicity, in which M06 and CAM-B3LYP perform better. Moreover, dipole moments of radical flavins show large deviations for all functionals studied. Tautomers with noncanonical protonation states are significantly populated at normal temperatures, adding to the complexity of modeling flavins. These results will guide future computational studies of flavoproteins and flavin chemistry by indicating the limitations of electronic-structure methodologies and the contributions of multiple tautomeric states.

Cryo-EM structures define ubiquinone-10 binding to mitochondrial complex I and conformational transitions accompanying Q-site occupancy.

Mitochondrial complex I is a central metabolic enzyme that uses the reducing potential of NADH to reduce ubiquinone-10 (Q10) and drive four protons across the inner mitochondrial membrane, powering oxidative phosphorylation. Although many complex I structures are now available, the mechanisms of Q10 reduction and energy transduction remain controversial. Here, we reconstitute mammalian complex I into phospholipid nanodiscs with exogenous Q10. Using cryo-EM, we reveal a Q10 molecule occupying the full length of the Q-binding site in the 'active' (ready-to-go) resting state together with a matching substrate-free structure, and apply molecular dynamics simulations to propose how the charge states of key residues influence the Q10 binding pose. By comparing ligand-bound and ligand-free forms of the 'deactive' resting state (that require reactivating to catalyse), we begin to define how substrate binding restructures the deactive Q-binding site, providing insights into its physiological and mechanistic relevance.

Tunneling and Nonadiabatic Effects on a Proton-Coupled Electron Transfer Model for the Qo Site in Cytochrome bc1.

Cytochrome bc1 is a fundamental enzyme for cellular respiration and photosynthesis. This dimeric protein complex catalyzes a proton-coupled electron transfer (PCET) from the reduced coenzyme-Q substrate (Q) to a bimetallic iron-sulfur cluster in the Qo active site. Herein, we combine molecular dynamics simulations of the complete cytochrome bc1 protein with electronic-structure calculations of truncated models and a semiclassical tunneling theory to investigate the electron-proton adiabaticity of the initial reaction catalyzed in the Qo site. After sampling possible orientations between the Q substrate and a histidine side chain that functions as hydrogen acceptor, we find that a truncated model composed by ubiquinol-methyl and imidazole-iron(III)-sulfide captures the expected changes in oxidation and spin states of the electron donor and acceptor. Diabatic electronic surfaces obtained for this model with multiconfigurational wave function calculations demonstrate that this reaction is electronic nonadiabatic, and proton tunneling is faster than mixing of electronic configurations. These results indicate the formalism that should be used to calculate vibronic couplings and kinetic parameters for the initial reaction in the Qo site of cytochrome bc1. This framework for molecular simulation may also be applied to investigate other PCET reactions in the Q-cycle or in various metalloproteins that catalyze proton translocation coupled to redox processes.

Mechanisms for Flavin-Mediated Oxidation: Hydride or Hydrogen-Atom Transfer?

Flavins are versatile biological cofactors which catalyze proton-coupled electron transfers (PCET) with varying number and coupling of electrons. Flavin-mediated oxidations of nicotinamide adenine dinucleotide (NADH) and of succinate, initial redox reactions in cellular respiration, were examined here with multiconfigurational quantum chemical calculations and a simple analysis of the wave function proposed to quantify electron transfer along the proton reaction coordinate. The mechanism of NADH oxidation is a prototypical hydride transfer, with two electrons moving concerted with the proton to the same acceptor group. However, succinate oxidation depends on the elimination step and can proceed through the transfer of a hydride or hydrogen atom, with proton and electrons moving to different groups in both cases. These results help to determine the mechanism of fundamental but still debated biochemical reactions and illustrate a new diagnostic tool for electron transfer that can be useful to characterize a broad class of PCET processes.

Combining Free Energy Simulations and NMR Chemical-Shift Perturbation To Identify Transient Cation-π Contacts in Proteins.

Flexible protein regions containing cationic and aromatic side-chains exposed to solvent may form transient cation-π interactions with structural and functional roles. To evaluate their stability and identify important intramolecular cation-π contacts, a combination of free energy profiles estimated from umbrella sampling with molecular dynamics simulations and chemical shift perturbations (CSP) obtained from nuclear magnetic resonance (NMR) experiments is applied here to the complete catalytic domain of human phosphatase Cdc25B. This protein is a good model system for transient cation-π interactions as it contains only one Trp residue (W550) in the disordered C-terminal segment and a total of 17 Arg residues, many exposed to solvent. Eight putative Arg-Trp pairs were simulated here. Only R482 and R544 show bound profiles corresponding to important transient cation-π interactions, while the others have dissociative or almost flat profiles. These results are corroborated by CSP analysis of three Cdc25B point mutants (W550A, R482A, and R544A) disrupting cation-π contacts. The proposed validation of statistically representative molecular simulations by NMR spectroscopy could be applied to identify transient contacts of proteins in general but carefully, as NMR chemical shifts are sensitive to changes in both molecular contacts and conformational distributions.

Modeling the Hydrolysis of Iron-Sulfur Clusters.

Iron-sulfur (FeS) clusters are essential metal cofactors involved in a wide variety of biological functions. Their catalytic efficiency, biosynthesis, and regulation depend on FeS stability in aqueous solution. Here, molecular modeling is used to investigate the hydrolysis of an oxidized (ferric) mononuclear FeS cluster by bare dissociation and water substitution mechanisms in neutral and acidic solution. First, approximate electronic structure descriptions of FeS reactions by density functional theory are validated against high-level wave function CCSD(T) calculations. Solvation contributions are included by an all-atom model with hybrid quantum chemical/molecular mechanical (QM/MM) potentials and enhanced sampling molecular dynamics simulations. The free energy profile obtained for FeS cluster hydrolysis indicates that the hybrid functional M06 together with an implicit solvent correction capture the most important aspects of FeS cluster reactivity in aqueous solution. Then, 20 reaction channels leading to two consecutive Fe-S bond ruptures were explored with this calibrated model. For all protonation states, nucleophilic substitution with concerted bond breaking and forming to iron is the preferred mechanism, both kinetic and thermodynamically. In neutral solution, proton transfer from water to the sulfur leaving group is also concerted. Dissociative reactions show higher barriers and will not be relevant for FeS reactivity when exposed to solvent. These hydrolysis mechanisms may help to explain the stability and catalytic mechanisms of FeS clusters of multiple sizes and proteins.

Mutations close to a hub residue affect the distant active site of a GH1 ß-glucosidase.

The tertiary structure of proteins has been represented as a network, in which residues are nodes and their contacts are edges. Protein structure networks contain residues, called hubs or central, which are essential to form short connection pathways between any pair of nodes. Hence hub residues may effectively spread structural perturbations through the protein. To test whether modifications nearby to hub residues could affect the enzyme active site, mutations were introduced in the ß-glycosidase Sfßgly (PDB-ID: 5CG0) directed to residues that form an α-helix (260-265) and a ß-strand (335-337) close to one of its main hub residues, F251, which is approximately 14 Å from the Sfßgly active site. Replacement of residues A263 and A264, which side-chains project from the α-helix towards F251, decreased the rate of substrate hydrolysis. Mutation A263F was shown to weaken noncovalent interactions involved in transition state stabilization within the Sfßgly active site. Mutations placed on the opposite side of the same α-helix did not show these effects. Consistently, replacement of V336, which side-chain protrudes from a ß-strand face towards F251, inactivated Sfßgly. Next to V336, mutation S337F also caused a decrease in noncovalent interactions involved in transition state stabilization. Therefore, we suggest that mutations A263F, A264F, V336F and S337F may directly perturb the position of the hub F251, which could propagate these perturbations into the Sfßgly active site through short connection pathways along the protein network.

Conformational flexibility of the complete catalytic domain of Cdc25B phosphatases.

Cdc25B phosphatases are involved in cell cycle checkpoints and have become a possible target for developing new anticancer drugs. A more rational design of Cdc25B ligands would benefit from detailed knowledge of its tertiary structure. The conformational flexibility of the C-terminal region of the Cdc25B catalytic domain has been debated recently and suggested to play an important structural role. Here, a combination of experimental NMR measurements and molecular dynamics simulations for the complete catalytic domain of the Cdc25B phosphatase is presented. The stability of the C-terminal α-helix is confirmed, but the last 20 residues in the complete catalytic domain are very flexible, partially occlude the active site and may establish transient contacts with the protein core. This flexibility in the C-terminal tail may modulate the molecular recognition of natural substrates and competitive inhibitors by Cdc25B. Proteins 2016; 84:1567-1575. © 2016 Wiley Periodicals, Inc.

Protein thermal denaturation is modulated by central residues in the protein structure network.

Network structural analysis, known as residue interaction networks or graphs (RIN or RIG, respectively) or protein structural networks or graphs (PSN or PSG, respectively), comprises a useful tool for detecting important residues for protein function, stability, folding and allostery. In RIN, the tertiary structure is represented by a network in which residues (nodes) are connected by interactions (edges). Such structural networks have consistently presented a few central residues that are important for shortening the pathways linking any two residues in a protein structure. To experimentally demonstrate that central residues effectively participate in protein properties, mutations were directed to seven central residues of the ß-glucosidase Sfßgly (ß-D-glucoside glucohydrolase; EC 3.2.1.21). These mutations reduced the thermal stability of the enzyme, as evaluated by changes in transition temperature (Tm ) and the denaturation rate at 45 °C. Moreover, mutations directed to the vicinity of a central residue also caused significant decreases in the Tm of Sfßgly and clearly increased the unfolding rate constant at 45 °C. However, mutations at noncentral residues or at surrounding residues did not affect the thermal stability of Sfßgly. Therefore, the data reported in the present study suggest that the perturbation of the central residues reduced the stability of the native structure of Sfßgly. These results are in agreement with previous findings showing that networks are robust, whereas attacks on central nodes cause network failure. Finally, the present study demonstrates that central residues underlie the functional properties of proteins.

Force-induced chemical reactions on the metal centre in a single metalloprotein molecule.

Metalloproteins play indispensable roles in biology owing to the versatile chemical reactivity of metal centres. However, studying their reactivity in many metalloproteins is challenging, as protein three-dimensional structure encloses labile metal centres, thus limiting their access to reactants and impeding direct measurements. Here we demonstrate the use of single-molecule atomic force microscopy to induce partial unfolding to expose metal centres in metalloproteins to aqueous solution, thus allowing for studying their chemical reactivity in aqueous solution for the first time. As a proof-of-principle, we demonstrate two chemical reactions for the FeS4 centre in rubredoxin: electrophilic protonation and nucleophilic ligand substitution. Our results show that protonation and ligand substitution result in mechanical destabilization of the FeS4 centre. Quantum chemical calculations corroborated experimental results and revealed detailed reaction mechanisms. We anticipate that this novel approach will provide insights into chemical reactivity of metal centres in metalloproteins under biologically more relevant conditions.

Theoretical modeling of low-energy electronic absorption bands in reduced cobaloximes.

The reduced Co(I) states of cobaloximes are powerful nucleophiles that play an important role in the hydrogen-evolving catalytic activity of these species. In this work we analyze the low-energy electronic absorption bands of two cobaloxime systems experimentally and use a variety of density functional theory and molecular orbital ab initio quantum chemical approaches. Overall we find a reasonable qualitative understanding of the electronic excitation spectra of these compounds but show that obtaining quantitative results remains a challenging task.

A component of the Xanthomonadaceae type IV secretion system combines a VirB7 motif with a N0 domain found in outer membrane transport proteins.

Type IV secretion systems (T4SS) are used by Gram-negative bacteria to translocate protein and DNA substrates across the cell envelope and into target cells. Translocation across the outer membrane is achieved via a ringed tetradecameric outer membrane complex made up of a small VirB7 lipoprotein (normally 30 to 45 residues in the mature form) and the C-terminal domains of the VirB9 and VirB10 subunits. Several species from the genera of Xanthomonas phytopathogens possess an uncharacterized type IV secretion system with some distinguishing features, one of which is an unusually large VirB7 subunit (118 residues in the mature form). Here, we report the NMR and 1.0 Å X-ray structures of the VirB7 subunit from Xanthomonas citri subsp. citri (VirB7(XAC2622)) and its interaction with VirB9. NMR solution studies show that residues 27-41 of the disordered flexible N-terminal region of VirB7(XAC2622) interact specifically with the VirB9 C-terminal domain, resulting in a significant reduction in the conformational freedom of both regions. VirB7(XAC2622) has a unique C-terminal domain whose topology is strikingly similar to that of N0 domains found in proteins from different systems involved in transport across the bacterial outer membrane. We show that VirB7(XAC2622) oligomerizes through interactions involving conserved residues in the N0 domain and residues 42-49 within the flexible N-terminal region and that these homotropic interactions can persist in the presence of heterotropic interactions with VirB9. Finally, we propose that VirB7(XAC2622) oligomerization is compatible with the core complex structure in a manner such that the N0 domains form an extra layer on the perimeter of the tetradecameric ring.

A microscopic view of substitution reactions solvated by ionic liquids.

The solvation effect of the ionic liquid 1-N-butyl-3-methylimidazolium hexafluorophosphate on nucleophilic substitution reactions of halides toward the aliphatic carbon of methyl p-nitrobenzenesulfonate (pNBS) was investigated by computer simulations. The calculations were performed by using a hybrid quantum-mechanical/molecular-mechanical (QM/MM) methodology. A semiempirical Hamiltonian was first parametrized on the basis of comparison with ab initio calculations for Cl(-) and Br(-) reaction with pNBS at gas phase. In condensed phase, free energy profiles were obtained for both reactions. The calculated reaction barriers are in agreement with experiment. The structure of species solvated by the ionic liquid was followed along the reaction progress from the reagents, through the transition state, to the final products. The simulations indicate that this substitution reaction in the ionic liquid is slower than in nonpolar molecular solvents proper to significant stabilization of the halide anion by the ionic liquid in comparison with the transition state with delocalized charge. Solute-solvent interactions in the first solvation shell contain several hydrogen bonds that are formed or broken in response to charge density variation along the reaction coordinate. The detailed structural analysis can be used to rationalize the design of new ionic liquids with tailored solvation properties.

Free-energy profiles for catalysis by dual-specificity phosphatases.

PTPs (protein tyrosine phosphatases) are fundamental enzymes for cell signalling and have been linked to the pathogenesis of several diseases, including cancer. Hence, PTPs are potential drug targets and inhibitors have been designed as possible therapeutic agents for Type II diabetes and obesity. However, a complete understanding of the detailed catalytic mechanism in PTPs is still lacking. Free-energy profiles, obtained by computer simulations of catalysis by a dual-specificity PTP, are shown in the present study and are used to shed light on the catalytic mechanism. A highly accurate hybrid potential of quantum mechanics/molecular mechanics calibrated specifically for PTP reactions was used. Reactions of alkyl and aryl substrates, with different protonation states and PTP active-site mutations, were simulated. Calculated reaction barriers agree well with experimental rate measurements. Results show the PTP substrate reacts as a bi-anion, with an ionized nucleophile. This protonation state has been a matter of debate in the literature. The inactivity of Cys-->Ser active-site mutants is also not fully understood. It is shown that mutants are inactive because the serine nucleophile is protonated. Results also clarify the interpretation of experimental data, particularly kinetic isotope effects. The simulated mechanisms presented here are better examples of the catalysis carried out by PTPs.

Solvent influence on the photophysical properties of 4-methoxy-N-methyl-1,8-naphthalimide.

4-Methoxy-N-methyl-1,8-naphthalimide (1) exhibits considerable solvatochromism and its UV-vis spectral properties have been studied in several polar/non-polar and protic/aprotic solvents, as well as in ethanol-water mixtures. The results reveal a strong influence of the solvent's polarity and its hydrogen-bond donor (HBD) capability on the photophysical properties of 1. For binary ethanol/water mixtures, preferential solvation models describe the band shifts in the probe's visible absorption spectrum well, but they fail to describe the corresponding shifts of the emission maxima. Pseudolinear approximations between solvent composition and molecule's transition energies, E(T), can be used to study the composition of ethanol-water mixtures, simplifying the mathematical treatment for eventual analytical applications.