A virtual screening study of the acetylcholine binding protein using a relaxed-complex approach

The nicotinic acetylcholine receptor (nAChR) is a member of the ligand-gated ion channel family and is implicated in many neurological events. Yet, the receptor is difficult to target without high-resolution structures. In contrast, the structure of the acetylcholine binding protein (AChBP) has been solved to high resolution, and it serves as a surrogate structure of the extra-cellular domain in nAChR. Here we conduct a virtual screening study of the AChBP using the relaxed-complex method, which involves a combination of molecular dynamics simulations (to achieve receptor structures) and ligand docking. The library screened through comes from the National Cancer Institute, and its ligands show great potential for binding AChBP in various manners. These ligands mimic the known binders of AChBP; a significant subset docks well against all species of the protein and some distinguish between the various structures. These novel ligands could serve as potential pharmaceuticals in the AChBP/nAChR systems.

Coupling the Level-Set Method with Molecular Mechanics for Variational Implicit Solvation of Nonpolar Molecules

We construct a variational explicit-solute implicit-solvent model for the solvation of molecules. Central in this model is an effective solvation free-energy functional that depends solely on the position of solute-solvent interface and solute atoms. The total free energy couples altogether the volume and interface energies of solutes, the solute-solvent van der Waals interactions, and the solute-solute mechanical interactions. A curvature dependent surface tension is incorporated through the so-called Tolman length which serves as the only fitting parameter in the model. Our approach extends the original variational implicit-solvent model of Dzubiella, Swanson, and McCammon [Phys. Rev. Lett. 2006, 96, 087802 and J. Chem. Phys. 2006, 124, 084905] to include the solute molecular mechanics. We also develop a novel computational method that combines the level-set technique with optimization algorithms to determine numerically the equilibrium conformation of nonpolar molecules. Numerical results demonstrate that our new model and methods can capture essential properties of nonpolar molecules and their interactions with the solvent. In particular, with a suitable choice of the Tolman length for the curvature correction to the surface tension, we obtain the solvation free energy for a benzene molecule in a good agreement with experimental results.

Finite element analysis of drug electrostatic diffusion: inhibition rate studies in N1 neuraminidase

This article describes a numerical solution of the steady-state Poisson-Boltzmann-Smoluchowski (PBS) and Poisson-Nernst-Planck (PNP) equations to study diffusion in biomolecular systems. Specifically, finite element methods have been developed to calculate electrostatic interactions and ligand binding rate constants for large biomolecules. The resulting software has been validated and applied to the wild-type and several mutated avian influenza neurominidase crystal structures. The calculated rates show very good agreement with recent experimental studies. Furthermore, these finite element methods require significantly fewer computational resources than existing particle-based Brownian dynamics methods and are robust for complicated geometries. The key finding of biological importance is that the electrostatic steering plays the important role in the drug binding process of the neurominidase.

E9-Im9 colicin DNase-immunity protein biomolecular association in water: a multiple-copy and accelerated molecular dynamics simulation study

Protein-protein transient and dynamic interactions underlie all biological processes. The molecular dynamics (MD) of the E9 colicin DNase protein, its Im9 inhibitor protein, and their E9-Im9 recognition complex are investigated by combining multiple-copy (MC) MD and accelerated MD (aMD) explicit-solvent simulation approaches, after validation with crystalline-phase and solution experiments. Im9 shows higher flexibility than its E9 counterpart. Im9 displays a significant reduction of backbone flexibility and a remarkable increase in motional correlation upon E9 association. Im9 loops 23-31 and 54-64 open with respect to the E9-Im9 X-ray structure and show high conformational diversity. Upon association a large fraction (approximately 20 nm2) of E9 and Im9 protein surfaces become inaccessible to water. Numerous salt bridges transiently occurring throughout our six 50 ns long MC-MD simulations are not present in the X-ray model. Among these Im9 Glu31-E9 Arg96 and Im9 Glu41-Lys89 involve interface interactions. Through the use of 10 ns of Im9 aMD simulation, we reconcile the largest thermodynamic impact measured for Asp51Ala mutation with Im9 structure and dynamics. Lys57 acts as an essential molecular switch to shift Im9 surface loop towards an ideal configuration for E9 inhibition. This is achieved by switching Asp60-Lys57 and Asp62-Lys57 hydrogen bonds to Asp51-Lys57 salt bridge. E9-Im9 recognition involves shifts of conformational distributions, reorganization of intramolecular hydrogen bond patterns, and formation of new inter- and intramolecular interactions. The description of key transient biological interactions can be significantly enriched by the dynamic and atomic-level information provided by computer simulations.

Hot-spot residues at the E9/Im9 interface help binding via different mechanisms

Protein-protein association involves many interface interactions, but they do not contribute equally. Ala scanning experiments reveal that only a few mutations significantly lower binding affinity. These key residues, which appear to drive protein-protein association, are called hot-spot residues. Molecular dynamics simulations of the Colicin E9/Im9 complex show Im9 Glu41 and Im9 Ser50, both hot-spots, bind via different mechanisms. The results suggest that Im9 Ser50 restricts Glu41 in a conformation auspicious for salt-bridge formation across the interface. This type of model may be helpful in engineering hot-spot clusters at protein-protein interfaces and, consequently, the design of specificity.

Intrinsic conformational flexibility of acetylcholinesterase

Proteins have been metaphorically described--due to the introduction and extraordinary advances in biomolecular dynamics and computational biophysics over the past decades--as "kicking and screaming" molecules [G. Weber, Adv. Protein Chem. 29 (1975) 1-83]. In fact, dynamic fluctuations in protein structural conformation have been known to play an important role in protein function. However, fundamental mechanisms by which protein fluctuations couple with catalytic function of particular enzymes remain poorly understood. To understand the dynamical properties of acetylcholinesterase (AChE) in rapid termination of cationic neurotransmitter, acetylcholine at neurosynaptic junctions, multiple molecular dynamics (MD) trajectories of AChE in the presence and absence of its inhibitors [J.M. Bui, J.A. McCammon, Proc. Natl. Acad. Sci. U.S.A. 103 (2006) 15451-15456; J.M. Bui, Z. Radic, P. Taylor, J.A. McCammon, Biophys. J. 90 (2006) 3280-3287; J.M. Bui, K. Tai, J.A. McCammon, J. Am. Chem. Soc. 126 (2004) 7198-7205; J.M. Bui, R.H. Henchman, J.A. McCammon, Biophys. J. 85 (2003) 2267-2272] have been conducted and correlated with its inhibitory mechanisms. The intrinsic flexibilities of AChE, particularly of the long omega loop, are important in facilitating the ligand's inhibition of the enzyme.

Similar membrane affinity of mono- and Di-S-acylated ras membrane anchors: a new twist in the role of protein lipidation

The functionally required membrane attachment of Ras is achieved through an invariant isoprenylation of a C-terminal Cys, supplemented by further lipid modification of adjacent Cys residues by one (N-ras) or two (H-ras) palmitoyls. However, whether the triply lipidated membrane anchor of H-ras has a higher membrane affinity than its doubly lipidated counterpart, or whether the affinity contribution of the two palmitates and the farnesyl is additive, was not known. To address this issue, we carried out potential of mean force (PMF or free energy profile) calculations on a hexadecylated but nonpalmitoylated anchor (Cys186-HD), hexadecylated and monopalmitoylated anchors (Cys181-monopalmitate and Cys184-monopalmitate), and a nonlipid-modified anchor. We found that the overall insertion free energy follows the trend Cys181/Cys184-bipalmitate (wild type) approximately Cys181-monopalmitate > Cys184-monopalmitate >> nonpalmitoylated anchor. Consistent with suggestions from recent cell biological experiments, the computed PMFs, coupled with structural analysis, demonstrate that membrane affinity of the Ras anchor depends on both the hydrophobicity of the palmitate and the prenyl groups and the spacing between them. The data further suggest that while Cys181-palmitate and Cys186-farnesyl together provide sufficient hydrophobic force for tight membrane binding, the palmitoyl at Cys184 is likely designed to serve another, presumably functional, role.

Three-dimensional geometric modeling of membrane-bound organelles in ventricular myocytes: bridging the gap between microscopic imaging and mathematical simulation

A general framework of image-based geometric processing is presented to bridge the gap between three-dimensional (3D) imaging that provides structural details of a biological system and mathematical simulation where high-quality surface or volumetric meshes are required. A 3D density map is processed in the order of image pre-processing (contrast enhancement and anisotropic filtering), feature extraction (boundary segmentation and skeletonization), and high-quality and realistic surface (triangular) and volumetric (tetrahedral) mesh generation. While the tool-chain described is applicable to general types of 3D imaging data, the performance is demonstrated specifically on membrane-bound organelles in ventricular myocytes that are imaged and reconstructed with electron microscopic (EM) tomography and two-photon microscopy (T-PM). Of particular interest in this study are two types of membrane-bound Ca(2+)-handling organelles, namely, transverse tubules (T-tubules) and junctional sarcoplasmic reticulum (jSR), both of which play an important role in regulating the excitation-contraction (E-C) coupling through dynamic Ca(2+) mobilization in cardiomyocytes.

Replica-Exchange Accelerated Molecular Dynamics (REXAMD) Applied to Thermodynamic Integration

Accelerated molecular dynamics (AMD) is an efficient strategy for accelerating the sampling of molecular dynamics simulations, and observable quantities such as free energies derived on the biased AMD potential can be reweighted to yield results consistent with the original, unmodified potential. In conventional AMD the reweighting procedure has an inherent statistical problem in systems with large acceleration, where the points with the largest biases will dominate the reweighted result and reduce the effective number of data points. We propose a replica exchange of various degrees of acceleration (REXAMD) to retain good statistics while achieving enhanced sampling. The REXAMD method is validated and benchmarked on two simple gas-phase model systems, and two different strategies for computing reweighted averages over a simulation are compared.

An improved relaxed complex scheme for receptor flexibility in computer-aided drug design

The interactions among associating (macro)molecules are dynamic, which adds to the complexity of molecular recognition. While ligand flexibility is well accounted for in computational drug design, the effective inclusion of receptor flexibility remains an important challenge. The relaxed complex scheme (RCS) is a promising computational methodology that combines the advantages of docking algorithms with dynamic structural information provided by molecular dynamics (MD) simulations, therefore explicitly accounting for the flexibility of both the receptor and the docked ligands. Here, we briefly review the RCS and discuss new extensions and improvements of this methodology in the context of ligand binding to two example targets: kinetoplastid RNA editing ligase 1 and the W191G cavity mutant of cytochrome c peroxidase. The RCS improvements include its extension to virtual screening, more rigorous characterization of local and global binding effects, and methods to improve its computational efficiency by reducing the receptor ensemble to a representative set of configurations. The choice of receptor ensemble, its influence on the predictive power of RCS, and the current limitations for an accurate treatment of the solvent contributions are also briefly discussed. Finally, we outline potential methodological improvements that we anticipate will assist future development.

Mapping the nucleotide and isoform-dependent structural and dynamical features of Ras proteins

Ras GTPases are conformational switches controlling cell proliferation, differentiation, and development. Despite their prominent role in many forms of cancer, the mechanism of conformational transition between inactive GDP-bound and active GTP-bound states remains unclear. Here we describe a detailed analysis of available experimental structures and molecular dynamics simulations to quantitatively assess the structural and dynamical features of active and inactive states and their interconversion. We demonstrate that GTP-bound and nucleotide-free G12V H-ras sample a wide region of conformational space, and show that the inactive-to-active transition is a multiphase process defined by the relative rearrangement of the two switches and the orientation of Tyr32. We also modeled and simulated N- and K-ras proteins and found that K-ras is more flexible than N- and H-ras. We identified a number of isoform-specific, long-range side chain interactions that define unique pathways of communication between the nucleotide binding site and the C terminus.

Intrinsic Free Energy of the Conformational Transition of the KcsA Signature Peptide from Conducting to Nonconducting State

We explore a conformational transition of the TATTVGYG signature peptide of the KcsA ion selectivity filter and its GYG to AYA mutant from the conducting α-strand state into the nonconducting pII-like state using a novel technique for multidimensional optimization of transition path ensembles and free energy calculations. We find that the wild type peptide, unlike the mutant, intrinsically favors the conducting state due to G77 backbone propensities and additional hydrophobic interaction between the V76 and Y78 side chains in water. The molecular mechanical free energy profiles in explicit water are in very good agreement with the corresponding adiabatic energies from the Generalized Born Molecular Volume (GBMV) implicit solvent model. However comparisons of the energies to higher level B3LYP/6-31G(d) Density Functional Theory calculations with Polarizable Continuum Model (PCM) suggest that the nonconducting state might be more favorable than predicted by molecular mechanics simulations. By extrapolating the single peptide results to the tetrameric channel, we propose a novel hypothesis for the ion selectivity mechanism.

Coupling Accelerated Molecular Dynamics Methods with Thermodynamic Integration Simulations

In this work we propose a straightforward and efficient approach to improve accuracy and convergence of free energy simulations in condensed-phase systems. We also introduce a new accelerated Molecular Dynamics (MD) approach in which molecular conformational transitions are accelerated by lowering the energy barriers while the potential surfaces near the minima are left unchanged. All free energy calculations were performed on the propane-to-propane model system. The accuracy of free energy simulations was significantly improved when sampling of internal degrees of freedom of solute was enhanced. However, accurate and converged results were only achieved when the solvent interactions were taken into account in the accelerated MD approaches. The analysis of the distribution of boost potential along the free energy simulations showed that the new accelerated MD approach samples efficiently both low- and high-energy regions of the potential surface. Since this approach also maintains substantial populations in regions near the minima, the statistics are not compromised in the thermodynamic integration calculations, and, as a result, the ensemble average can be recovered.

Thermodynamics of peptide insertion and aggregation in a lipid bilayer

A variety of biomolecules mediate physiological processes by inserting and reorganizing in cell membranes, and the thermodynamic forces responsible for their partitioning are of great interest. Recent experiments provided valuable data on the free energy changes associated with the transfer of individual amino acids from water to membrane. However, a complete picture of the pathways and the associated changes in energy of peptide insertion into a membrane remains elusive. To this end, computational techniques supplement the experimental data with atomic-level details and shed light on the energetics of insertion. Here, we employed the technique of umbrella sampling in a total 850 ns of all-atom molecular dynamics simulations to study the free energy profile and the pathway of insertion of a model hexapeptide consisting of a tryptophan and five leucines (WL5). The computed free energy profile of the peptide as it travels from bulk solvent through the membrane core exhibits two minima: a local minimum at the water−membrane interface or the headgroup region and a global minimum at the hydrophobic−hydrophilic interface close to the lipid glycerol region. A rather small barrier of roughly 1 kcal mol⁻¹ exists at the membrane core, which is explained by the enhanced flexibility of the peptide when deeply inserted. Combining our results with those in the literature, we present a thermodynamic model for peptide insertion and aggregation which involves peptide aggregation upon contact with the membrane at the solvent−lipid headgroup interface.

Molecular dynamics of a kappaB DNA element: base flipping via cross-strand intercalative stacking in a microsecond-scale simulation

The sequence-dependent structural variability and conformational dynamics of DNA play pivotal roles in many biological milieus, such as in the site-specific binding of transcription factors to target regulatory elements. To better understand DNA structure, function, and dynamics in general, and protein···DNA recognition in the ‘κB’ family of genetic regulatory elements in particular, we performed molecular dynamics simulations of a 20-bp DNA encompassing a cognate κB site recognized by the proto-oncogenic ‘c-Rel’ subfamily of NF-κB transcription factors. Simulations of the κB DNA in explicit water were extended to microsecond duration, providing a broad, atomically detailed glimpse into the structural and dynamical behavior of double helical DNA over many timescales. Of particular note, novel (and structurally plausible) conformations of DNA developed only at the long times sampled in this simulation—including a peculiar state arising at ≈0.7 μs and characterized by cross-strand intercalative stacking of nucleotides within a longitudinally sheared base pair, followed (at ≈1 μs) by spontaneous base flipping of a neighboring thymine within the A-rich duplex. Results and predictions from the microsecond-scale simulation include implications for a dynamical NF-κB recognition motif, and are amenable to testing and further exploration via specific experimental approaches that are suggested herein.

Ensemble-based virtual screening reveals potential novel antiviral compounds for avian influenza neuraminidase

Avian influenza virus subtype H5N1 is a potential pandemic threat with human-adapted strains resistant to antiviral drugs. Although virtual screening (VS) against a crystal or relaxed receptor structure is an established method to identify potential inhibitors, the more dynamic changes within binding sites are neglected. To accommodate full receptor flexibility, we use AutoDock4 to screen the NCI diversity set against representative receptor ensembles extracted from explicitly solvated molecular dynamics simulations of the neuraminidase system. The top hits are redocked to the entire nonredundant receptor ensemble and rescored using the relaxed complex scheme (RCS). Of the 27 top hits reported, half ranked very poorly if only crystal structures are used. These compounds target the catalytic cavity as well as the newly identified 150- and 430-cavities, which exhibit dynamic properties in electrostatic surface and geometric shape. This ensemble-based VS and RCS approach may offer improvement over existing strategies for structure-based drug discovery.

Control of cation permeation through the nicotinic receptor channel

We used molecular dynamics (MD) simulations to explore the transport of single cations through the channel of the muscle nicotinic acetylcholine receptor (nAChR). Four MD simulations of 16 ns were performed at physiological and hyperpolarized membrane potentials, with and without restraints of the structure, but all without bound agonist. With the structure unrestrained and a potential of −100 mV, one cation traversed the channel during a transient period of channel hydration; at −200 mV, the channel was continuously hydrated and two cations traversed the channel. With the structure restrained, however, cations did not traverse the channel at either membrane potential, even though the channel was continuously hydrated. The overall results show that cation selective transport through the nAChR channel is governed by electrostatic interactions to achieve charge selectivity, but ion translocation relies on channel hydration, facilitated by a trans-membrane field, coupled with dynamic fluctuations of the channel structure.

Communication between a cell and its environment relies on channel-forming proteins to provide a low energy pathway for ions to move in and out. Although channel-forming proteins are essential to all life forms, the atomic-scale mechanisms that enable ions to pass through the channel remain elusive due to the lack of experimental approaches to monitor the protein and ion in real time and at atomic resolution. A powerful alternative approach is molecular dynamics (MD) simulation based on the laws of physics applied to the increasing body of protein structures resolved at atomic resolution. Here we present all-atom MD simulations applied to the nicotinic acetylcholine receptor (nAChR) that initiates voluntary movement in skeletal muscle. By focusing on individual permeant cations, we find that selective cation translocation occurs in stages: cations are first selected through a series of oppositely charged residues within the protein vestibule leading to a narrow hydrophobic constriction, but then hydration of the narrow region and dynamic fluctuations of the protein enable the cation to pass through. The findings provide a general framework for understanding how ions are selected for transport based on charge, and how the dynamic interplay between water, the ion, and the channel protein enable rapid ion translocation through the broad class of channel-forming proteins with hydrophobic barriers.

Acetylcholinesterase: mechanisms of covalent inhibition of H447I mutant determined by computational analyses

The reaction mechanisms of two inhibitor TFK(+) and TFK(0) binding to H447I mutant mouse acetylcholinesterase (mAChE) have been investigated by using a combined ab initio quantum mechanical/molecular mechanical (QM/MM) approach and classical molecular dynamics (MD) simulations. TFK(+) binding to the H447I mutant may proceed with a different reaction mechanism from the wild-type. A water molecule takes over the role of His447 and participates in the bond breaking and forming as a "charge relayer". Unlike in the wild-type mAChE case, Glu334, a conserved residue from the catalytic triad, acts as a catalytic base in the reaction. The calculated energy barrier for this reaction is about 8kcal/mol. These predictions await experimental verification. In the case of the neutral ligand TFK(0), however, multiple MD simulations on the TFK(0)/H447I complex reveal that none of the water molecules can be retained in the active site as a "catalytic" water. Taken together our computational studies confirm that TFK(0) is almost inactive in the H447I mutant, and also provide detailed mechanistic insights into the experimental observations.

Computing accurate potentials of mean force in electrolyte solutions with the generalized gradient-augmented harmonic Fourier beads method

We establish the accuracy of the novel generalized gradient-augmented harmonic Fourier beads (ggaHFB) method in computing free-energy profiles or potentials of mean force (PMFs) through comparison with two independent conventional techniques. In particular, we employ umbrella sampling with one dimensional weighted histogram analysis method (WHAM) and free molecular dynamics simulation of radial distribution functions to compute the PMF for the Na(+)-Cl(-) ion-pair separation to 16 A in 1.0M NaCl solution in water. The corresponding ggaHFB free-energy profile in six dimensional Cartesian space is in excellent agreement with the conventional benchmarks. We then explore changes in the PMF in response to lowering the NaCl concentration to physiological 0.3 and 0.1M, and dilute 0.0M concentrations. Finally, to expand the scope of the ggaHFB method, we formally develop the free-energy gradient approximation in arbitrary nonlinear coordinates. This formal development underscores the importance of the logarithmic Jacobian correction to reconstruct true PMFs from umbrella sampling simulations with either WHAM or ggaHFB techniques when nonlinear coordinate restraints are used with Cartesian propagators. The ability to employ nonlinear coordinates and high accuracy of the computed free-energy profiles further advocate the use of the ggaHFB method in studies of rare events in complex systems.

A Nonlinear Elasticity Model of Macromolecular Conformational Change Induced by Electrostatic Forces

In this paper we propose a nonlinear elasticity model of macromolecular conformational change (deformation) induced by electrostatic forces generated by an implicit solvation model. The Poisson-Boltzmann equation for the electrostatic potential is analyzed in a domain varying with the elastic deformation of molecules, and a new continuous model of the electrostatic forces is developed to ensure solvability of the nonlinear elasticity equations. We derive the estimates of electrostatic forces corresponding to four types of perturbations to an electrostatic potential field, and establish the existance of an equilibrium configuration using a fixed-point argument, under the assumption that the change in the ionic strength and charges due to the additional molecules causing the deformation are sufficiently small. The results are valid for elastic models with arbitrarily complex dielectric interfaces and cavities, and can be generalized to large elastic deformation caused by high ionic strength, large charges, and strong external fields by using continuation methods.

A novel switch region regulates H-ras membrane orientation and signal output

The plasma membrane nanoscale distribution of H-ras is regulated by guanine nucleotide binding. To explore the structural basis of H-ras membrane organization, we combined molecular dynamic simulations and medium-throughput FRET measurements on live cells. We extracted a set of FRET values, termed a FRET vector, to describe the lateral segregation and orientation of H-ras with respect to a large set of nanodomain markers. We show that mutation of basic residues in helix alpha4 or the hypervariable region (HVR) selectively alter the FRET vectors of GTP- or GDP-loaded H-ras, demonstrating a critical role for these residues in stabilizing GTP- or GDP-H-ras interactions with the plasma membrane. By a similar analysis, we find that the beta2-beta3 loop and helix alpha5 are involved in a novel conformational switch that operates through helix alpha4 and the HVR to reorient the H-ras G-domain with respect to the plasma membrane. Perturbation of these switch elements enhances MAPK activation by stabilizing GTP-H-ras in a more productive signalling conformation. The results illustrate how the plasma membrane spatially constrains signalling conformations by acting as a semi-neutral interaction partner.

Feature-preserving adaptive mesh generation for molecular shape modeling and simulation

We describe a chain of algorithms for molecular surface and volumetric mesh generation. We take as inputs the centers and radii of all atoms of a molecule and the toolchain outputs both triangular and tetrahedral meshes that can be used for molecular shape modeling and simulation. Experiments on a number of molecules are demonstrated, showing that our methods possess several desirable properties: feature-preservation, local adaptivity, high quality, and smoothness (for surface meshes). We also demonstrate an example of molecular simulation using the finite element method and the meshes generated by our method. The approaches presented and their implementations are also applicable to other types of inputs such as 3D scalar volumes and triangular surface meshes with low quality, and hence can be used for generation/improvement of meshes in a broad range of applications.

Molecular dynamics simulations of metalloproteinases types 2 and 3 reveal differences in the dynamic behavior of the S1' binding pocket

Matrix Metalloproteinases (MMPs) are zinc-containing proteinases that are responsible for the metabolism of extracellular matrix proteins. Overexpression of MMPs has been associated with a wide range of pathological diseases such as arthritis, cancer, multiple sclerosis and Alzheimer's disease. The excessive and unregulated activity of Matrix Metalloproteinases type 2 (MMP-2), also known as gelatinase A, has been identified in a numbers of cancer metastases. Several MMP inhibitors (MMPi) have been proposed in the literature aiming to interfere in the MMPs activity. In this work we performed long MD simulations in order to study the dynamical behavior of the binding pocket S1' in the apo forms of MMP type 2 and 3, and identify, at the molecular level, the structural properties relevant for the designing of specific inhibitor of MMP-2.