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AcrAB(Z)-TolC is the main drug efflux transporter complex in Escherichia coli. The extrusion of various toxic compounds depends on several drug binding sites within the trimeric AcrB transporter. Membrane-localized carboxylated substrates, such as fusidic acid and hydrophobic β-lactams, access the pump via a groove between the transmembrane helices TM1 and TM2. In this article, the transport route from the initial TM1/TM2 groove binding site toward the deep binding pocket located in the periplasmic part has been addressed via molecular modeling studies followed by functional and structural characterization of several AcrB variants. We propose that membrane-embedded drugs bind initially to the TM1/TM2 groove, are oriented by the AcrB PN2 subdomain, and are subsequently transported via a PN2/PC1 interface pathway directly toward the deep binding pocket. Our work emphasizes the exploitation of multiple transport pathways by AcrB tuned to substrate physicochemical properties related to the polyspecificity of the pump.

The drug/proton antiporter AcrB, which is part of the major efflux pump AcrABZ-TolC in Escherichia coli, is the paradigm transporter of the resistance-nodulation-cell division (RND) superfamily. Despite the impressive ability of AcrB to transport many chemically unrelated compounds, only a few of these ligands have been co-crystallized with the protein. Therefore, the molecular features that distinguish good substrates of the pump from poor ones have remained poorly understood to date. In this work, a thorough in silico protocol was employed to study the interactions of a series of congeneric compounds with AcrB to examine how subtle chemical differences affect the recognition and transport of substrates by this protein. Our analysis allowed us to discriminate among different compounds, mainly in terms of specific interactions with diverse sub-sites within the large distal pocket of AcrB. Our findings could provide valuable information for the design of new antibiotics that can evade the antimicrobial resistance mediated by efflux pump machinery.

The drug/proton antiporter AcrB, part of the major efflux pump AcrABZ-TolC in Escherichia coli, is characterized by its impressive ability to transport chemically diverse compounds, conferring a multidrug resistance phenotype. However, the molecular features differentiating between good and poor substrates of the pump have yet to be identified. In this work, we combined molecular docking with molecular dynamics simulations to study the interactions between AcrB and two representative cephalosporins, cefepime and ceftazidime (a good and poor substrate of AcrB, respectively). Our analysis revealed different binding preferences of the two compounds toward the subsites of the large deep binding pocket of AcrB. Cefepime, although less hydrophobic than ceftazidime, showed a higher affinity than ceftazidime for the so-called hydrophobic trap, a region known for binding inhibitors and substrates. This supports the hypothesis that surface complementarity between the molecule and AcrB, more than the intrinsic hydrophobicity of the antibiotic, is a feature required for the interaction within this region. Oppositely, the preference of ceftazidime for binding outside the hydrophobic trap might not be optimal for triggering allosteric conformational changes needed to the transporter to accomplish its function. Altogether, our findings could provide valuable information for the design of new antibiotics less susceptible to the efflux mechanism.

Antibiotic resistance poses an immediate and growing threat to human health. Multidrug efflux pumps are promising targets for overcoming antibiotic resistance with small-molecule therapeutics. Previously, we identified a diaminoquinoline acrylamide, NSC-33353, as a potent inhibitor of the AcrAB-TolC efflux pump in Escherichia coli. This inhibitor potentiates the antibacterial activities of novobiocin and erythromycin upon binding to the membrane fusion protein AcrA. It is also a substrate for efflux and lacks appreciable intrinsic antibacterial activity of its own in wild-type cells. Here, we have modified the substituents of the cinnamoyl group of NSC-33353, giving rise to analogs that retain the ability to inhibit efflux, lost the features of the efflux substrates, and gained antibacterial activity in wild-type cells. The replacement of the cinnamoyl group with naphthyl isosteres generated compounds that lack antibacterial activity but are both excellent efflux pump inhibitors and substrates. Surprisingly, these inhibitors potentiate the antibacterial activity of novobiocin but not erythromycin. Surface plasmon resonance experiments and molecular docking suggest that the replacement of the cinnamoyl group with naphthyl shifts the affinity of the compounds away from AcrA to the AcrB transporter, making them better efflux substrates and changing their mechanism of inhibition. These results provide new insights into the duality of efflux substrate/inhibitor features in chemical scaffolds that will facilitate the development of new efflux pump inhibitors.

International audience; Human Hedgehog receptor Patched1 (PTCH1) is able to efflux chemotherapeutics of different chemical structure out of cancer cells thus contributing to multidrug resistance phenomena in tumor treatment. A screening of natural compounds purified from marine sponges led to the identification of the first PTCH1 efflux inhibitor, panicein A hydroquinone (PAH), demonstrated to increase doxorubicin toxicity in vitro and vemurafenib toxicity in vitro and in vivo. In this work we combined different computational techniques to gain molecular insights of the inhibitory activity of PAH and some of its active and inactive analogues. We first performed a thorough characterization and druggability analysis of the main putative substrate binding pockets known from available cryo-electron microscopy structures. Further, dynamical descriptors of the active and inactive PAH analogues were extracted from microsecond-long all-atom molecular dynamics simulations in water solution. Finally, a blind ensemble docking methodology coupled with the conformational analysis of compounds enabled rationalization of the interaction between PTCH1 and PAH and derivatives in terms of their intrinsic physico-chemical properties. Our results suggest that the Neck pocket is the preferential binding site for PAH analogues on PTCH1, and that compounds assuming an open cylindric-like shape in solution are most likely to be good binders for PTCH1.

Efflux pump avoidance and inhibition are desired properties for the optimization of antibacterial activities against Gram-negative bacteria. However, molecular and physicochemical interactions defining the interface between compounds and efflux pumps remain poorly understood. We identified properties that correlate with efflux avoidance and inhibition, are predictive of similar features in structurally diverse compounds, and allow researchers to distinguish between efflux substrates, inhibitors, and avoiders in P. aeruginosa., Antibiotic-resistant bacteria rapidly spread in clinical and natural environments and challenge our modern lifestyle. A major component of defense against antibiotics in Gram-negative bacteria is a drug permeation barrier created by active efflux across the outer membrane. We identified molecular determinants defining the propensity of small peptidomimetic molecules to avoid and inhibit efflux pumps in Pseudomonas aeruginosa, a human pathogen notorious for its antibiotic resistance. Combining experimental and computational protocols, we mapped the fate of the compounds from structure-activity relationships through their dynamic behavior in solution, permeation across both the inner and outer membranes, and interaction with MexB, the major efflux transporter of P. aeruginosa. We identified predictors of efflux avoidance and inhibition and demonstrated their power by using a library of traditional antibiotics and compound series and by generating new inhibitors of MexB. The identified predictors will enable the discovery and optimization of antibacterial agents suitable for treatment of P. aeruginosa infections.

Efflux pumps of the resistance nodulation-cell division (RND) superfamily are major contributors to multidrug resistance for most of the Gram-negative ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) pathogens. The development of inhibitors of these pumps would be highly desirable; however, several issues have thus far hindered all efforts at designing new efflux inhibitory compounds devoid of adverse effects. An alternative route to de novo design relies on the use of marketed drugs, for which side effects on human health have been already assessed. In this work, we provide experimental evidence that the antipsychotic drugs chlorpromazine and amitriptyline are inhibitors of the AcrB transporter, the engine of the major RND efflux pumps in Escherichia coli and Salmonella enterica serovar Typhimurium. Furthermore, in silico calculations have provided a molecular-level picture of the inhibition mechanism, allowing rationalization of experimental data and paving the way for similar studies with other classes of marketed compounds., Efflux is an important mechanism in Gram-negative bacteria conferring multidrug resistance. Inhibition of efflux is an encouraging strategy to restore the antibacterial activity of antibiotics. Chlorpromazine and amitriptyline have been shown to behave as efflux inhibitors. However, their mode of action is poorly understood. Exposure of Salmonella enterica serovar Typhimurium and Escherichia coli to chlorpromazine selected for mutations within genes encoding RamR and MarR, regulators of the multidrug tripartite efflux pump AcrAB-TolC. Further experiments with S. Typhimurium containing AcrB D408A (a nonfunctional efflux pump) and chlorpromazine or amitriptyline resulted in the reversion of the mutant acrB allele to the wild type. Together, this suggests these drugs are AcrB efflux substrates. Subsequent docking studies with AcrB from S. Typhimurium and E. coli, followed by molecular dynamics simulations and free energy calculations showed that chlorpromazine and amitriptyline bind at the hydrophobic trap, a preferred binding site for substrates and inhibitors within the distal binding pocket of AcrB. Based on these simulations, we suggest that chlorpromazine and amitriptyline inhibit AcrB-mediated efflux by interfering with substrate binding. Our findings provide evidence that these drugs are substrates and inhibitors of AcrB, yielding molecular details of their mechanism of action and informing drug discovery of new efflux inhibitors.

Salmonella is an important genus of Gram-negative pathogens, treatment of which has become problematic due to increases in antimicrobial resistance. This is partly attributable to the overexpression of tripartite efflux pumps, particularly the constitutively expressed AcrAB-TolC. Despite its clinical importance, the structure of the Salmonella AcrB transporter remained unknown to-date, with much of our structural understanding coming from the Escherichia coli orthologue. Here, by taking advantage of the styrene maleic acid (SMA) technology to isolate membrane proteins with closely associated lipids, we report the very first experimental structure of Salmonella AcrB transporter. Furthermore, this novel structure provides additional insight into mechanisms of drug efflux as it bears the mutation (G288D), originating from a clinical isolate of Salmonella Typhimurium presenting an increased resistance to fluoroquinolones. Experimental data are complemented by state-of-the-art molecular dynamics (MD) simulations on both the wild type and G288D variant of Salmonella AcrB. Together, these reveal several important differences with respect to the E. coli protein, providing insights into the role of the G288D mutation in increasing drug efflux and extending our understanding of the mechanisms underlying antibiotic resistance.

Resistance–nodulation–division efflux pumps play a key role in inherent and evolved multidrug resistance in bacteria. AcrB, a prototypical member of this protein family, extrudes a wide range of antimicrobial agents out of bacteria. Although high-resolution structures exist for AcrB, its conformational fluctuations and their putative role in function are largely unknown. Here, we determine these structural dynamics in the presence of substrates using hydrogen/deuterium exchange mass spectrometry, complemented by molecular dynamics simulations, and bacterial susceptibility studies. We show that an efflux pump inhibitor potentiates antibiotic activity by restraining drug-binding pocket dynamics, rather than preventing antibiotic binding. We also reveal that a drug-binding pocket substitution discovered within a multidrug resistant clinical isolate modifies the plasticity of the transport pathway, which could explain its altered substrate efflux. Our results provide insight into the molecular mechanism of drug export and inhibition of a major multidrug efflux pump and the directive role of its dynamics., AcrB is a prototypical resistance–nodulation–division (RND) bacterial transporter, conferring resistance to a variety of antibiotics. HDX-MS and other, complementary approaches offer insight into AcrB structural dynamics and suggest the molecular mechanisms underlying drug export and inhibition of this multidrug-resistance conferring pump.

Resistance-nodulation-division (RND) efflux pumps play a key role in inherent and evolved multidrug-resistance (MDR) in bacteria. AcrB is the prototypical member of the RND family and acts to recognise and export a wide range of chemically distinct molecules out of bacteria, conferring resistance to a variety of antibiotics. Although high resolution structures exist for AcrB, its conformational fluctuations and their putative role in function are largely unknown, preventing a complete mechanistic understanding of efflux and inhibition. Here, we determine these structural dynamics in the presence of AcrB substrates using hydrogen/deuterium exchange mass spectrometry, complemented by molecular modelling, drug binding and bacterial susceptibility studies. We show that the well-studied efflux pump inhibitor phenylalanine-arginine-β-naphthylamide (PAβN) potentiates antibiotic activity by restraining drug-binding pocket dynamics, rather than preventing antibiotic binding. We also reveal that a drug-binding pocket substitution discovered within an MDR clinical isolate, AcrBG288D, modifies the plasticity of the transport pathway, which could explain its altered substrate specificity. Our results provide molecular insight into drug export and inhibition of a major MDR-conferring efflux pump and the important directive role of its dynamics.

Secondary multidrug (Mdr) transporters utilize ion concentration gradients to actively remove antibiotics and other toxic compounds from cells. The model Mdr transporter MdfA from Escherichia coli exchanges dissimilar drugs for protons. The transporter should open at the cytoplasmic side to enable access of drugs into the Mdr recognition pocket. Here we show that the cytoplasmic rim around the Mdr recognition pocket represents a previously overlooked important regulatory determinant in MdfA. We demonstrate that increasing the positive charge of the electrically asymmetric rim dramatically inhibits MdfA activity and sometimes even leads to influx of planar, positively charged compounds, resulting in drug sensitivity. Our results suggest that unlike the mutants with the electrically modified rim, the membrane-embedded wild-type MdfA exhibits a significant probability of an inward-closed conformation, which is further increased by drug binding. Since MdfA binds drugs from its inward-facing environment, these results are intriguing and raise the possibility that the transporter has a sensitive, drug-induced conformational switch, which favors an inward-closed state.

Understanding the molecular determinants for recognition, binding and transport of antibiotics by multidrug efflux systems is important for basic research and useful for the design of more effective antimicrobial compounds. Imipenem and meropenem are two carbapenems whose antibacterial activity is known to be poorly and strongly affected by MexAB-OprM, the major efflux pump transporter in Pseudomonas aeruginosa. However, not much is known regarding recognition and transport of these compounds by AcrAB-TolC, which is the MexAB-OprM homologue in Escherichia coli and by definition the paradigm model for structural studies on efflux pumps. Prompted by this motivation, we unveiled the molecular details of the interaction of imipenem and meropenem with the transporter AcrB by combining computer simulations with biophysical experiments. Regarding the interaction with the two main substrate binding regions of AcrB, the so-called access and deep binding pockets, molecular dynamics simulations revealed imipenem to be more mobile than meropenem in the former, while comparable mobilities were observed in the latter. This result is in line with isothermal titration calorimetry, differential scanning experiments, and binding free energy calculations, indicating a higher affinity for meropenem than imipenem at the deep binding pocket, while both sharing similar affinities at the access pocket. Our findings rationalize how different physico-chemical properties of compounds reflect on their interactions with AcrB. As such, they constitute precious information to be exploited for the rational design of antibiotics able to evade efflux pumps.

Secondary multidrug transporters of the resistance-nodulation-cell division (RND) superfamily contribute crucially to antibiotic resistance in Gram-negative bacteria. Compared to the most studied transporter AcrB of Escherichia coli, little is known about the molecular determinants of distinct polyspecificities of the most important RND transporters MexB and MexY of Pseudomonas aeruginosa. In an effort to add knowledge on this topic, we performed an exhaustive atomic-level comparison of the main putative recognition sites (access and deep binding pockets) in these two Mex transporters. We identified an underlying link between some structural, chemical and dynamical features of the binding pockets and the physicochemical nature of the corresponding substrates recognized by either one or both pumps. In particular, mosaic-like lipophilic and electrostatic surfaces of the binding pockets provide for both proteins several multifunctional sites for diffuse binding of diverse substrates. Specific lipophilicity signatures of the weakly conserved deep pocket suggest a key role of this site as a selectivity filter as in Acr transporters. Finally, the different dynamics of the bottom-loop in MexB and MexY support its possible role in binding of large substrates. Our work represents the first comparative study of the major RND transporters in P. aeruginosa and also the first structure-based study of MexY, for which no experimental structure is available yet.

Background Efflux pumps of the Resistance-Nodulation-cell Division superfamily confer multi-drug resistance to Gram-negative bacteria. The most-studied polyspecific transporter belonging to this class is the inner-membrane trimeric antiporter AcrB of Escherichia coli. In previous studies, a functional rotation mechanism was proposed for its functioning, according to which the three monomers undergo concerted conformational changes facilitating the extrusion of substrates. However, the molecular determinants and the energetics of this mechanism still remain unknown, so its feasibility must be proven mechanistically. Methods A computational protocol able to mimic the functional rotation mechanism in AcrB was developed. By using multi-bias molecular dynamics simulations we characterized the translocation of the substrate doxorubicin driven by conformational changes of the protein. In addition, we estimated for the first time the free energy profile associated to this process. Results We provided a molecular view of the process in agreement with experimental data. Moreover, we showed that the conformational changes occurring in AcrB enable the formation of a layer of structured waters on the internal surface of the transport channel. This water layer, in turn, allows for a fairly constant hydration of the substrate, facilitating its diffusion over a smooth free energy profile. Conclusions Our findings reveal a new molecular mechanism of polyspecific transport whereby water contributes by screening potentially strong substrate-protein interactions. General significance We provided a mechanistic understanding of a fundamental process related to multi-drug transport. Our results can help rationalizing the behavior of other polyspecific transporters and designing compounds avoiding extrusion or inhibitors of efflux pumps.

The drug/proton antiporter AcrB, engine of the major efflux pump AcrAB(Z)-TolC of Escherichia coli and other bacteria, is characterized by its impressive ability to transport chemically diverse compounds, conferring a multi-drug resistance (MDR) phenotype. Although hundreds of small molecules are known to be AcrB substrates, only a few co-crystal structures are available to date. Computational methods have been therefore intensively employed to provide structural and dynamical fingerprints related to transport and inhibition of AcrB. In this work, we performed a systematic computational investigation to study the interaction between representative carbapenem antibiotics and AcrB. We focused on the interaction of carbapenems with the so-called distal pocket, a region known for its importance in binding inhibitors and substrates of AcrB. Our findings reveal how the different physico-chemical nature of these antibiotics is reflected on their binding preference for AcrB. The molecular-level information provided here could help design new antibiotics less susceptible to the efflux mechanism.

We report the first case of cosegregation of two haemoglobins (Hbs): HbG-Philadelphia [α68(E17)Asn→Lys] and HbDuarte [β62(E6)Ala→Pro]. The proband is a young patient heterozygous also for β∘-thalassaemia. We detected exclusively two haemoglobin variants: HbDuarte and HbG-Philadelphia/Duarte. Functional study of the new double variant HbG-Philadelphia/Duarte exhibited an increase in oxygen affinity, with a slight decrease of cooperativity and Bohr effect. This functional behaviour is attributed to β62Ala→Pro instead of α68Asn→Lys substitution. Indeed, HbG-Philadelphia isolated in our laboratory from blood cells donor carrier for this variant is not affected by any functional modification, whereas purified Hb Duarte showed functional properties very similar to the double variant. NMR and MD simulation studies confirmed that the presence of Pro instead of Ala at the β62 position produces displacement of the E helix and modifications of the tertiary structure. The substitution α68(E17)Asn→Lys does not cause significant structural and dynamical modifications of the protein. A possible structure-based rational of substitution effects is suggested.

The major efflux system in Escherichia coli is the tripartite complex AcrAB-TolC. Its homotrimeric transporter AcrB is polyspecific and extrudes antibiotics out of the bacterium. This extrusion is performed via a functional rotation, in which each monomer assumes a particular conformation. In the present study, targeted molecular dynamics simulations have been employed to obtain a molecular level understanding of the transport process. A particular focus is put on the role of water molecules in this extrusion process. It is shown that the water flows from the binding pocket toward the exit gate in the extrusion step and helps the substrate to move along this path. These results are underpinned by a detailed analysis of the electrostatic interaction energy. Furthermore, the role of water for the polyspecificity of the transporter is discussed.

Our aim in this study was to provide an atomic description of ampicillin translocation through OmpF, the major outer membrane channel in Escherichia coli and main entry point for beta-lactam antibiotics. By applying metadynamics simulations, we also obtained the energy barriers along the diffusion pathway. We then studied the effect of mutations that affect the charge and size at the channel constriction zone, and found that in comparison to the wild-type, much lower energy barriers are required for translocation. The expected higher translocation rates were confirmed on the macroscopic scale by liposome-swelling assays. A microscopic view on the millisecond timescale was obtained by analysis of temperature-dependent ion current fluctuations in the presence of ampicillin and provide the enthalpic part of the energy barrier. By studying antibiotic translocation over various timescales and length scales, we were able to discern its molecular mechanism and rate-limiting interactions, and draw biologically relevant conclusions that may help in the design of drugs with enhanced permeation rates.

Water plays a pivotal role in the correct functioning of proteins. Hydration is fundamental to their stability and flexibility, to folding process and specific functions, and to protein protein interactions. In this work, the effects of solvation on proteins dynamics have been investigated by employing molecular dynamics simulations and using myoglobin as a model system. The investigation has been focused on solvent waters residing around/inside the protein, with average times of up to tens of nanoseconds, revealing that these slow waters may have significant effects on biological functioning of the protein. Our study pointed out that water is able to interact with proteins in diverse ways, leading to different kinds of perturbations in their intrinsic dynamic behavior. In particular, for myoglobin it was found that a water molecule can (i) "block" entry/escape of ligands to/from a particular docking site, (ii) act as a "wedge" modulating the dynamics of internal cavities, or (iii) join a "flow" of waters taking a ligand into (or "washing" a ligand away from) the protein interior. The information gathered in this work allowed us to provide a fingerprint of protein solvation state, the hydration sites map, which may represent a novel tool for comparing different forms/species of globular proteins.

The facilitated influx of moxifloxacin through the most abundant channel in the outer cell wall of gram-negative bacteria was investigated. Molecular modeling provided atomic details of the interaction with the channel surface, revealed the preferred orientation of the antibiotic along its pathway, and gave an estimated time necessary for translocation. High-resolution conductance measurements on single OmpF trimers allowed the passages of individual moxifloxacin molecules to be counted. The average mean residence time of 50 micros is in agreement with the predicted strong interaction from the modeling. In contrast, control measurements with nalidixic acid, a hydrophobic antibiotic that rather permeates across the lipid membrane, revealed a negligible interaction. The spectral overlap of tryptophan with moxifloxacin was suitable for a FRET study of the protein-antibiotic interaction. Combining molecular dynamics simulations with selective quenching identified an interaction of moxifloxacin with Trp61 inside the OmpF channel, whereas nalidixic acid showed preferential interaction with Trp214 on the channel exterior. An understanding of the detailed molecular interactions between the antibiotic and its preferred channel may be used to develop new antibiotics with improved uptake kinetics.

Currently, little is known about the molecular recognition pathways between DNA-alkylating anticancer drugs and their targets despite their pharmacological relevance. In the framework of classical molecular dynamics simulations, here we use umbrella sampling to map the potential of mean force (PMF) associated with sliding along the DNA minor groove of two of these compounds. These are an indole derivative of duocarmycin (DSI) and the putative reactive form of anthramycin (anhydro-anthramycin, IMI). Twenty-three configurations were considered for each drug/DNA complex, corresponding to a movement along similar to 3 basepairs. The alkylation site turns out to be the most favorable for DSI, while a barrier of similar to 6 kcal/ mol separates the reactive configuration of IMI center dot DNA from the absolute minimum. An analysis of various contributions to the PMF reveals that solvent effects play an important role for the largest and more flexible drug DSI. Instead, the PMF of IMI center dot DNA overall correlates with changes in the binding enthalpy. Implications of these results on the sequence selectivity of the two drugs are discussed.

Antimicrobial resistance is a key public health concern of our era due to an ever-increasing number of drug-resistant pathogens, including several Gram-negative bacilli. The latter are endowed with a low permeable outer membrane and with numerous chromosomally encoded multidrug efflux pumps, which are not only ubiquitous but also polyspecific, thus recognizing a broad range of compounds. Efflux pumps are a major defense mechanism of these organisms against antimicrobials as they can significantly increase the levels of resistance by allowing time for the organisms to develop specific resistance mechanisms. One of the potential strategies to reinvigorate the efficacy of antimicrobials is by joint administration with efflux pump inhibitors, which either block the substrate binding and/or hinder any of the transport-dependent steps of the pumps. In this chapter, we provide an overview of multidrug resistance efflux pumps, their inhibition strategies, and the important findings from the various computational simulation studies reported to date with respect to the rational design of inhibitors and on deciphering their mechanism of action.

The relatively small size of myoglobin makes it suitable for the investigation of the ligand escape process in respiratory proteins and, in general, an ideal model system for the study of the more general structure-function paradigm. In this work, we use Molecular Dynamics simulations combined with an accelerated algorithm, the metadynamics, to probe the escape of CO from myoglobin. Our approach permits to quantitatively describe the escape process via the reconstruction of the associated free energy surface. Additionally, hints on the involvement of a larger numbers of residues than hitherto assumed in the gating process are extracted from our data. Proteins 2008. © 2007 Wiley-Liss, Inc.

Density functional theory (DFT)-based Car-Parrinello molecular dynamics (CPMD) simulations describe the time evolution of molecular systems without resorting to a predefined potential energy surface. CPMD and hybrid molecular mechanics/CPMD schemes have recently enabled the calculation of redox properties of electron transfer proteins in their complex biological environment. They provided structural and spectroscopic information on novel platinum-based anticancer drugs that target DNA, also setting the basis for the construction of force fields for the metal lesion. Molecular mechanics/CPMD also lead to mechanistic hypotheses for a variety of metalloenzymes. Recent advances that increase the accuracy of DFT and the efficiency of investigating rare events are further expanding the domain of CPMD applications to biomolecules.

The structure-activity relationships of four hydroxycoumarins, two with the hydroxyl group on the aromatic ring of the molecule and two with the hydroxyl group replacing hydrogen of the pyrone ring, and their interactions with mushroom tyrosinase were studied. These compounds displayed different behaviors upon action of the enzyme. The two compounds, ar-hydroxylated 6-hydroxycoumarin and 7-hydroxycoumarin, were both weak substrates of the enzyme. Interestingly, in both cases, the product of the catalysis was the 6,7-hydroxycoumarin, although 5,6- and 7,8-isomers could also theoretically be formed. Additionally, both were able to reduce the formation of dopachrome when tyrosinase acted on its typical substrate, L-tyrosine. Although none of the compounds that contained a hydroxyl group on the pyrone ring were substrates of tyrosinase, the 3-hydroxycoumarin was a potent inhibitor of the enzyme, and the 4-hydroxycoumarin was not an inhibitor. These results were compared with those obtained by in silico molecular docking predictions to obtain potentially useful information for the synthesis of new coumarin-based inhibitors that resemble the structure of the 3-hydroxycoumarin.

We present an on-line database of all-atom force-field parameters and molecular properties of compounds with antimicrobial activity (mostly antibiotics and some beta-lactamase inhibitors). For each compound, we provide the General Amber Force Field parameters for the major species at physiological pH, together with an analysis of properties of interest as extracted from μs-long molecular dynamics simulations in explicit water solution. The properties include number and population of structural clusters, molecular flexibility, hydrophobic and hydrophilic molecular surfaces, the statistics of intra- and inter-molecular H-bonds, as well as structural and dynamical properties of solvent molecules within first and second solvation shells. In addition, the database contains several key molecular parameters, such as energy of the frontier molecular orbitals, vibrational properties, rotational constants, atomic partial charges and electric dipole moment, computed by Density Functional Theory. The present database (to our knowledge the first extensive one including dynamical properties) is part of a wider project aiming to build-up a database containing structural, physico-chemical and dynamical properties of medicinal compounds using different force-field parameters with increasing level of complexity and reliability. The database is freely accessible at http://www.dsf.unica.it/translocation/db/.

Our study examines the functional and structural effects of amino acid substitution in the distal side of β-chains of human Hb Duarte (\documentclass[10pt]{article} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \pagestyle{empty} \oddsidemargin -1.0in \begin{document} \begin{equation*}{\alpha}_{2}{\beta}_{2}^{62{\mathrm{Ala}}{\rightarrow}{\mathrm{Pro}}}\end{equation*}\end{document}). We have compared the functional properties of the purified Hb Duarte with those of HbA, and through proton NMR and molecular dynamics simulations we have investigated their tertiary and quaternary structures. The variant exhibits an increased oxygen affinity with a normal Hill coefficient and Bohr effect. The abnormal function of Hb Duarte is attributed to the presence of a proline residue at the β62 position, since the functional properties of another Hb variant in the same position, Hb J-Europa (β62Ala→Asp), have been described as normal. Thereafter 1H-NMR studies have shown that the β62 Ala→Pro substitution causes structural modifications of the tertiary structure of the β globins, leaving the quaternary structure unaltered. These results have been confirmed by extensive all-atom molecular dynamics simulations. All these findings lead to the conclusion that the β62 Ala→Pro substitution produces a destabilization of the E-helix extending downward to the CD corner. Particularly, a cavity near the distal histidine of the β-chains, connecting the heme pocket to the solvent, is affected, altering the functional properties of the protein molecule.

Efflux pumps of the Resistance Nodulation Division (RND) superfamily play a major role in multi-drug resistance (MDR) to antibiotics in Gram-negatives. Understanding the molecular mechanism by which these pumps expel so many unrelated drugs is essential to develop antibiotics able to escape efflux machineries, as well as efflux pumps inhibitors. The drug/proton antiporter AcrB, part of the major efflux pump AcrAB-TolC in Escherichia coli, is the paradigm RND transporter. Unfortunately, only few compounds have been co-crystallized with AcrB, and thus computational approaches are essential in deciphering determinants of the interactions between diverse ligands and this promiscous protein.Previously we used a truncated model of AcrB to characterize the binding of several substrates and inhibitors. Here we performed docking calculations, molecular dynamics (MD) simulations and free energy calculations to investigate the interaction of 10 congeneric molecules with AcrB. Importantly, we use a full model of the protein in the membrane in order to exploit interactions with all the possible binding sites.Flexible ensemble-docking was performed using almost 40 non-redundant conformations of extracted from experiments and extensive MD simulations of AcrB and different protonation states for each compound. The top poses for each ligand (for a total of 200 AcrB-ligand structures) were then subjected to MD μs- long simulations and analyzed with respect to descriptors of their structure, dynamics, and thermodynamics of binding. The outcome of this study, unprecedented for its extensiveness, will provide key information to medicinal chemists for the design of new compounds with improved efficiency.

The AcrB multidrug efflux transporter of E. coli is a giant homotrimeric membrane protein able to recognize and expel a wide variety of compounds out of the cell [1]. Obtaining the co-crystal structures of AcrB bound to its substrates has proven to be difficult, and to date only a few antibiotics have been successfully co-crystallized with the protein. From these studies, two main recognition sites were identified, but the small number of available co-crystal structures still hinders a full understanding of the molecular recognition mechanism. As a part of an extensive computational research activity on efflux pumps, we performed a systematic docking campaign to characterize the putative binding sites of antimicrobial compounds from different classes, namely carbapenems, cephalosporins, fluoroquinolones, penicillines, tetracyclines, and beta-lactamase inhibitors. In particular, for each compound we screened for possible binding sites on the whole protein, including thus possible binding pockets other than those already known. Flexibility has been taken into account by considering ensembles of rigid structures for both the ligand and the receptor. Ligand conformations have been extracted from microsecond-long molecular dynamics (MD) simulations in explicit water [2]. For the protein we considered a large set of conformations, including structures from both high-resolution X-ray and MD simulations. We present the results of this systematic investigation by comparing the patterns of interactions observed for the different compounds. The binding properties of the transporter are discussed in terms of statistics of the most contacted residues.1. P. Ruggerone, S. Murakami, K. M. Pos and A.V. Vargiu, Curr. Top. Med. Chem.13, 3079 (2013).2. G. Malloci, A. V. Vargiu, G. Serra, A. Bosin, P. Ruggerone, M. Ceccarelli, Molecules20, 13997 (2015).

For an antibiotic to be effective, it needs to cross the outer membrane barrier and reach the target inside the cell. Hydrophilic antibiotics, e.g. β-lactams, use porin channels to cross the outer membrane and accumulate in the periplasm. Experimental determination of antibiotic interactions with porin is performed by using electrophysiology on a single channel level by noise analysis or single event analysis methods. We report a novel framework for analyzing the ion-current noise, taking into account the corrections due to the analogous filter and the sampling procedure, with the goal of extending the time resolution to a range previously inaccessible by event analysis or by conventional noise analysis. The new method allows one to analyse fast binding events and/or the case when the single channel is not completely blocked by the substrate. We demonstrate the power of this approach by using as an example the interactions of meropenem, an antibiotic of the carbapenem family, with the OmpF porin that is considered to be one of the main pathways for antibiotics to enter Escherichia coli. The presence of meropenem in OmpF is detected by ion current blockages, and the on and off rates are estimated from the concentration dependence of the average ion current and of its power spectral density. The obtained average residence time of the antibiotic inside the channel is in the range of a few microseconds, i.e. more than 50 times smaller than the inverse cut-off frequency of the analogous filter.

Recently, we reported on a potent benzimidazole derivative (227G) that inhibits the growth of the bovine viral diarrhea virus (BVDV) in cell-based and enzyme assays at nanomolar concentrations. The target of 227G is the viral RNA-dependent RNA polymerase (RdRp), and the I261M mutation located in motif I of the RdRp finger domain was found to induce drug resistance. Here we propose a molecular mechanism for the retained functionality of the enzyme in the presence of the inhibitor, on the basis of a thorough computational study of the apo and holo forms of the BVDV RdRp either in the wild type (wt) or in the form carrying the I261M mutation. Our study shows that although the mutation affects to some extent the structure of the apoenzyme, the functional dynamics of the protein appear to be largely maintained, which is consistent with the retained functionality of this natural mutant. Despite the binding site of 227G not collapsing or undergoing drastic structural changes upon introduction of the I261M substitution, these alterations reflect crucially on the binding mode of 227G, which is significantly different from that found in wt RdRp. In particular, while in the wt system the four loops lining the template entrance site embrace 227G and close the template passageway, in the I261M variant the template entrance is only marginally occluded, allowing in principle the translocation of the template to the interior of the enzyme. In addition, the mutated enzyme in the presence of 227G retains several characteristics of the wt apoprotein. Our work provides an original molecular picture of a resistance mechanism that is consistent with published experimental data.

We have applied a density-functional derived tight-binding method (DF-TBMD) to the study of the energetics and the dynamics of boron defects in silicon. This study is motivated by a number of interstitial-driven phenomena observed in experiments, as the transient enhanced diffusion of B atoms in implanted silicon samples together with the formation of immobile B precipitates. We discuss first the DF-TBMD results for equilibrium structures and formation energies of different defect configurations containing a single boron atom and a silicon self-interstitial. Moreover, DF-TBMD molecular dynamics simulations at finite temperature allow us to investigate boron diffusivity in a temperature range between 900 and 1500 K. We provide for the first time a dynamical picture of B diffusion in silicon characterized by a migration energy of 0.7 eV.

The GaAs(112)A and (112)B surfaces have been prepared by molecular-beam epitaxy (MBE) and analyzed in situ by low-energy electron diffraction (LEED) and surface core-level spectroscopy of the Ga 3d and As 3d core levels using synchrotron radiation. The morphology of these surfaces has been studied in situ by scanning tunneling microscopy (STM) and ex situ by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Neither of the surfaces is stable, but both decompose into facets under standard MBE preparation conditions. The (112)A surface is covered by regular depressions (inverted pyramids) with a pentagonal base and with side walls formed of low-energy (111), {110}, and as yet unobserved {124} facets. The (112)B surface exhibits depressions with (111), {011}, and (113) facets on a rectangular base. These results are in agreement with ab-initio calculations of surface energies based on density functional theory. The theoretical findings show that, by forming roof-like structures employing {110} or {111} and {113} surfaces, the surface free energy can be lowered below the values found for a large number of reasonable GaAs(112)A and (112)B model surfaces.

The GaAs(112)A and B surfaces were prepared by molecular beam epitaxy (MBE) and characterized in situ by scanning tunneling microscopy (STM). Both surfaces are unstable. On the (112)A surface five facets with the orientations (111), {110} and {124} appear, while on the ( 1 1 2 )B surface four facets with the orientations ( 1 1 1 )B, { 1 1 0 } and ( 1 1 3 )B were observed. In both cases, the facets form depressions covering the entire surface. In addition, total-energy calculations employing density-functional theory were carried out for these surfaces. For the (112)A surface a roof-like structure of {111} and {113} planes has the lowest calculated energy. According to the calculations, the ( 1 1 2 )B surface forms under As-rich conditions a roof-like structure of { 1 1 1 }B and { 1 1 3 }B planes, and it decomposes under Ga-rich conditions into { 1 1 0} planes. The experimental and theoretical findings for the ( 1 1 2 )B surface are in good agreement. The occurrence of {124}-facets on the (112)A surface was not taken into consideration in the calculations and needs to be investigated further.

Results of a combined experimental and theoretical study are presented. GaAs{113} and {112} surfaces have been prepared by molecular beam epitaxy and analyzed in situ by low-energy electron diffraction and ex situ by atomic-force microscopy. The experimentally found structures are in full agreement with surface energies calculated by density-functional theory. The {112} surfaces are unstable under faceting into low-index planes, whereas the {113} surfaces appear as singular surfaces. Particularly, for GaAs(113) the surface energy is comparable with the values for the low-index surfaces. The impacts of these results on the interface between InAs quantum dots and the embedding GaAs matrix are discussed.

Despite the careful experimental work on the kinetic origin of the island shapes during the growth, a complete qualitative and quantitative theoretical analysis is still missing. In this paper we present the results of realistic Monte Carlo simulations of the growth of Al on Al(111). Our simulations are based on microscopic well-founded parameters derived from ab initio calculations. Reliable general criteria for interpreting the role the substrate temperature during the homoepitaxial growth on metallic fcc (111) and hcp (1000) surfaces are extracted. In particular, the importance of the asymmetry in the jump probability of fourfold coordinated adatoms and the different diffusion mechanism of the adatoms along the close-packed steps are discussed.

The virus-encoded RNA-dependent RNA polymerase (RdRp) has emerged as a primary target in the search for selective inhibitors of Flaviviridae. Recently, we reported on the selective inhibition, in cell-based assays, of both BVDV (EC50 = 0.80 +/- 0.06 mu M) and HCV (EC50 = 1.11 +/- 0.15 mu M) by 2-{1-[2-(2,4-dimethoxyphenyl)-1H-benzimidazol-5-yl]ethylidene}hydrazinecarbothioamide (227G). Here we show that, in enzyme assays with recombinant enzymes, 227G inhibits, in a dose-dependent manner, the RdRp of both BVDV (IC50 = 0.0020 +/- 0.0004 mu M) and HCV (IC50 = 0.40 +/- 0.04 mu M). Furthermore, we report on the selection and molecular analysis of a BVDV-resistant mutant, characterized by the presence of the 1261M mutation. By applying a multilevel computational approach, we identified different 227G binding sites on the two RdRps. They were further validated by the good agreement between the calculated affinities and those extrapolated from IC50 values. Our findings suggest different molecular mechanisms of inhibition of the HCV and BVDV RdRps by 227G and indicate the importance of understanding ligand-enzyme interactions at the molecular level for the rational design of new and more potent leads.

Many vital cellular processes, such as protein translocation, proton transport or molecular recognition, are mediated by self assembling membrane proteins. We have investigated the Twin-arginine translocase (TatA) complex, which forms transient pores through which proteins are translocated through the membrane. We postulated that complex formation is electrostatically driven by formation of salt bridges between amphiphilic transmembrane segments of the individual monomers and developed a structure-based model for this process[1].We studied the formation of oligomers of different sizes by structure-based[2] MD simulations in combination with NMR constraints and a hydrophobic-slab implicit membrane model. Starting from isolated monomers, distributed far apart from each other, we observed the formation of stable TatA oligomers on the basis of the postulated interactions. The dimensions of the resulting TatA complex agreed well with experimental electron microscopy measurements[3] and the postulated interactions were confirmed by subsequent mutation studies.[1] T. Walther, C. Gottselig, S. Grage, M. Wolf, A. Vargiu, Marco J. Klein, S. Vollmer, S. Prock, M. Hartmann, S. Afonin, E. Stockwald, H. Heinzmann, W Wenzel, P. Ruggerone, A. Ulrich. Folding and self-assembly of the TatA translocation pore based on a novel charge zipper mechanism, Cell (accepted)[2] Whitford, P. C. et al. An all-atom structure-based potential for proteins: Bridging minimal models with all-atom empirical forcefields. Proteins 75, 430-441 (2009).[3] Gohlke, U. et al. The TatA component of the twin-arginine protein transport system forms channel complexes of variable diameter. Proc. Natl. Acad. Sci. U.S.A. 102, 10482-10486 (2005).

Ligand-receptor interactions are at the basis of the mediation of our physiological responses to a large variety of ligands, such as hormones, neurotransmitters and environmental stimulants, and their tuning represents the goal of a large variety of therapies. Several molecular details of these interactions are still largely unknown. In an effort to shed some light on this important issue, we performed a computational study on the interaction of two related compounds differing by a single methyl group (clozapine and desmethylclozapine) with a [Formula: see text]-opioid receptor. According to experiments, desmethylclozapine is more active than clozapine, providing a system well suited for a comparative study. We investigated stable configurations of the two drugs inside the receptor by simulating their escape routes by molecular dynamics simulations. Our results point out that the action of the compounds might be related to the spatial and temporal distribution of the affinity sites they visit during their permanency. Moreover, no particularly pronounced structural perturbations of the receptor were detected during the simulations, reinforcing the idea of a strong dynamical character of the interaction process, with an important role played by the solvent in addition.

Bovine viral diarrhoea virus (BVDV), a major pathogen in cattle and other ruminants, has been used as a surrogate in vitro model for the development of HCV inhibitors. Recently, we have identified a very potent and selective benzimidazole derivative (227G) that: i) targets the RNA-dependent RNA polymerase (RdRp) of both BVDV and HCV, ii) allows the selection of BVDV resistant mutants characterized by the point mutation I261M located in the finger domain of the enzyme.This work deals with the effects of 227G and I261M on the function of the wt and mutated BVDV RdRp, respectively, investigated by using non-conventional approaches that include molecular dynamics, cluster analysis, flexible docking and metadynamics. Briefly, binding of 227G to the wt RdRp induces conformational changes that prevent entrance of the RNA primer into the enzyme's polymerization cavity. Viceversa, the I261M mutation results in structural changes that allow access of the primer to the polymerization cavity and binding of 227G into a site different from that occupied by the inhibitor in the wt enzyme.

Basic understanding of the means by which multidrug efflux systems can efficiently recognize and transport drugs constitutes a fundamental step toward development of compounds able to tackle the continuous outbreak of new bacterial strains resistant to traditional antibiotics. We applied a series of computational techniques, from molecular docking to molecular dynamics simulations and free energy estimate methods, to determine the differences in the binding properties of imipenem and meropenem, two potent antibiotics of the carbapenem family, to MexB, the RND transporter of the major efflux system of Pseudomonas aeruginosa. We identified and characterized two affinity sites in the periplasmic domain of the transporter, sharing strong similarities with the distal and proximal binding pockets identified in AcrB, the homologue of MexB in Escherichia coli. According to our results, meropenem has a higher affinity to the distal binding pocket than imipenem while both compounds are weakly bound to the proximal pocket. This different behavior is mainly due to the hydration properties of the nonpharmacophore part of the two compounds, being that of imipenem less bulky and hydrophobic. Our data provide for the first time a rationale at molecular level for the experimental evidence indicating meropenem as a compound strongly affected by MexB contrary to imipenem, which is apparently poorly transported by the same pump.

The dispersion curves of charge density wave (CDW)-induced phonons have been measured in 1T-TaS 2 by inelastic He atom scattering along both crystallographic and CDW symmetry directions. Evidence is given for a strong coupling of the CDW to Rayleigh waves, i.e. to quasi-transverse acoustic modes. A pseudocharge model analysis of lattice dynamics accounts for the CDW-induced splitting of the Raman-active modes originated from the folding of transverse acoustic modes.

Layered compounds are characterized by special phenomena such as phase changes, critical phenomena and charge-density waves, in which the phonons play the very important role of fingerprints. Moreover, the surface lattice dynamics of layered compounds provides a deep insight into the structural anisotropy of these technologically important materials. Therefore, it is important to support the large amount of experimental data with a careful theoretical analysis. Due to the presence of large structures such as the ones associated to the charge-density waves, the layered compounds are systems which are still unaccessible to the powerful ab initio methods and are investigated by means of phenomenological models. The theoretical analysis of the surface-phonon modes in several layered crystals is reviewed in this article. The calculations are performed on the basis of both force-constant and bond-charge models for graphite, the multipolar model for TaS2, and with the dispersive linear-chain (DLC) method for GaSe and TaSe2. For all the systems the agreement between experimental and theoretical results is very good.

In this paper we investigate the structural and dynamical properties of the two major porins (OmpF and OmpC) in Escherichia coli, using molecular dynamics (MD) simulations. In particular we characterized the atomic fluctuations, correlated motions, temperature dependence, solvent-accessible cross-sectional area and water dynamics in the key regions of the two channels. Our in-depth analysis allows us to highlight the importance of both the key conserved and substituted residues between OmpF and OmpC. The latter is characterized by a narrower and longer constriction region with respect to OmpF. OmpC also showed a higher stability upon increasing temperature. We then present the results of transport properties by using accelerated MD simulations to probe the diffusion of norfloxacin (a fluoroquinolone antibiotic) through the two porins OmpF/OmpC. Our study constitutes a step forward towards understanding the structure-function relationship of the two porins' channels. This will benefit the research of antibacterials with improved permeation properties and nanopores that aim to use these porins as sensing systems.