Constrained Unfolding of a Helical Peptide: Implicit versus Explicit Solvents

A theoretical study on the activity and selectivity of IDO/TDO inhibitors

Indoleamine 2,3-dioxygenase 1 (IDO) is a tryptophan (Trp) metabolic enzyme along the kynurenine (NFK) pathway. Under pathological conditions, IDO overexpressed by tumor cells causes depletion of tryptophan and the accumulation of metabolic products, which inhibit the local immune response and form immune escape. Therefore, the suppression of IDO activity is one of the strategies for tumor immunotherapy, and drug design for this target has been the focus of research for more than two decades. Apart from IDO, tryptophan dioxygenase (TDO) of the same family can also catalyze the same biochemical reaction in the human body, but it has different tissue distribution and substrate selectivity from IDO. Based on the principle of drug design with high potency and low cross-reactivity to specific targets, in this subject, the activity and selectivity of IDO and TDO toward small molecular inhibitors were studied from the perspective of thermodynamics and kinetics. The aim was to elucidate the structural requirements for achieving favorable biological activity and selectivity of IDO and TDO inhibitors. Specifically, the interactions of inhibitors from eight families with IDO and TDO were initially investigated through molecular docking and molecular dynamics simulations, and the thermodynamic data for binding of inhibitors were predicted by the molecular mechanics/generalized Born surface area (MM/GBSA) method. Secondly, we explored the free energy landscape of JKloops, the kinetic control element of IDO/TDO, using temperature replica exchange molecular dynamics (T-REMD) simulations and elucidated the connection between the rules of IDO/TDO conformational changes and the inhibitor selectivity mechanism. Furthermore, the binding and dissociation processes of the C1 inhibitor (NLG919) were simulated by the adaptive steering molecular dynamics (ASMD) method, which not only addressed the possible stable, metastable, and transition states for C1 inhibitor-IDO/TDO interactions, but also accurately predicted kinetic data for C1 inhibitor binding and dissociation. In conclusion, we have constructed a complete process from enzyme (IDO/TDO) conformational activation to inhibitor binding/dissociation and used the thermodynamic and kinetic data of each link as clues to verify the control mechanism of IDO/TDO on inhibitor selectivity. This is of great significance for us to understand the design principles of tumor immunotherapy drugs and to avoid drug resistance of immunotherapy drugs.

Molecular Mechanisms Underlying Medium-Chain Free Fatty Acid-Regulated Activity of the Phospholipase PlaF from Pseudomonas aeruginosa

PlaF is a membrane-bound phospholipase A1 from Pseudomonas aeruginosa that is involved in remodeling membrane glycerophospholipids (GPLs) and modulating virulence-associated signaling and metabolic pathways. Previously, we identified the role of medium-chain free fatty acids (FFAs) in inhibiting PlaF activity and promoting homodimerization, yet the underlying molecular mechanism remained elusive. Here, we used unbiased and biased molecular dynamics simulations and free energy computations to assess how PlaF interacts with FFAs localized in the water milieu surrounding the bilayer or within the bilayer and how these interactions regulate PlaF activity. Medium-chain FFAs localized in the upper bilayer leaflet can stabilize inactive dimeric PlaF, likely through interactions with charged surface residues, as has been experimentally validated. Potential of mean force (PMF) computations indicate that membrane-bound FFAs may facilitate the activation of monomeric PlaF by lowering the activation barrier for changing into a tilted, active configuration. We estimated that the coupled equilibria of PlaF monomerization-dimerization and tilting at the physiological concentration of PlaF lead to the majority of PlaF forming inactive dimers when in a cell membrane loaded with decanoic acid (C10). This is in agreement with a suggested in vivo product feedback loop and gas chromatography-mass spectrometry profiling results, indicating that PlaF catalyzes the release of C10 from P. aeruginosa membranes. Additionally, we found that C10 in the water milieu can access the catalytic site of active monomeric PlaF, contributing to the competitive component of C10-mediated PlaF inhibition. Our study provides mechanistic insights into how medium-chain FFAs may regulate the activity of PlaF, a potential bacterial drug target.

Tertiary Plasticity Drives the Efficiency of Enterocin 7B Interactions with Lipid Membranes

The ability of antimicrobial peptides to efficiently kill their bacterial targets depends on the efficiency of their binding to the microbial membrane. In the case of enterocins, there is a three-part interaction: initial binding, unpacking of helices on the membrane surface, and permeation of the lipid bilayer. Helical unpacking is driven by disruption of the peptide hydrophobic core when in contact with membranes. Enterocin 7B is a leaderless enterocin antimicrobial peptide produced from Enterococcus faecalis that functions alone, or with its cognate partner enterocin 7A, to efficiently kill a wide variety of Gram-stain positive bacteria. To better characterize the role that tertiary structural plasticity plays in the ability of enterocin 7B to interact with the membranes, a series of arginine single-site mutants were constructed that destabilize the hydrophobic core to varying degrees. A series of experimental measures of structure, stability, and function, including CD spectra, far UV CD melting profiles, minimal inhibitory concentrations analysis, and release kinetics of calcein, show that decreased stabilization of the hydrophobic core is correlated with increased efficiency of a peptide to permeate membranes and in killing bacteria. Finally, using the computational technique of adaptive steered molecular dynamics, we found that the atomistic/energetic landscape of peptide mechanical unfolding leads to free energy differences between the wild type and its mutants, whose trends correlate well with our experiment.

Toward a structural identification of metastable molecular conformations

Interpreting high-dimensional data from molecular dynamics simulations is a persistent challenge. In this paper, we show that for a small peptide, deca-alanine, metastable states can be identified through a neural net based on structural information alone. While processing molecular dynamics data, dimensionality reduction is a necessary step that projects high-dimensional data onto a low-dimensional representation that, ideally, captures the conformational changes in the underlying data. Conventional methods make use of the temporal information contained in trajectories generated through integrating the equations of motion, which forgoes more efficient sampling schemes. We demonstrate that EncoderMap, an autoencoder architecture with an additional distance metric, can find a suitable low-dimensional representation to identify long-lived molecular conformations using exclusively structural information. For deca-alanine, which exhibits several helix-forming pathways, we show that this approach allows us to combine simulations with different biasing forces and yields representations comparable in quality to other established methods. Our results contribute to computational strategies for the rapid automatic exploration of the configuration space of peptides and proteins.

Effect of the R126C mutation on the structure and function of the glucose transporter GLUT1: A molecular dynamics simulation study

Glucose transporter 1 (GLUT1) is responsible for basal glucose uptake and is expressed in most tissues under normal conditions. GLUT1 mutations can cause early-onset absence epilepsy and myoclonus dystonia syndrome (MDS), with MDS potentially lethal. In this study, the effect of the R126C mutation, which is associated with MDS, on structural stability and substrate transport of GLUT1 was investigated. Various bioinformatics tools were used to predict the stability of GLUT1, revealing that the R126C mutation reduces the structural stability of GLUT1. Molecular dynamics (MD) simulations were used to further characterize the effect of the R126C mutation on GLUT1 structural stability. Based on the MD simulations, specific conformational changes and dominant motions of the GLUT1 mutant were characterized by Principal component analysis (PCA). The mutation disrupts hydrogen bonds between substrate-binding residues and glucose, thus likely reducing substrate affinity. The R126C mutation reduces the conformational stability of the protein, and fewer intramolecular hydrogen bonds were present in the mutated GLUT1 when compared with that of wild-type GLUT1. The mutation increased the free energy of glucose transport through GLUT1 significantly, especially at the mutation site, indicating that passage of glucose through the channel is hindered, and this mutant may even release cytoplasmic glucose. This study provides a detailed atomic-level explanation for the reduced structural stability and substrate transport capacity of a GLUT1 mutant. The results aid our understanding of the structure of GLUT1 and provide a framework for developing drugs to treat GLUT1-related diseases, such as MDS.

Implementation of Telescoping Boxes in Adaptive Steered Molecular Dynamics

Long-time dynamical processes, such as those involving protein unfolding and ligand interactions, can be accelerated and realized through steered molecular dynamics (SMD). The challenge has been the extraction of information from such simulations that generalize for complex nonequilibrium processes. The use of Jarzynski's equality opened the possibility of determining the free energy along the steered coordinate, but sampling over the nonequilibrium trajectories is slow to converge. Adaptive steered molecular dynamics (ASMD) and other related techniques have been introduced to overcome this challenge through the use of stages. Here, we take advantage of these stages to address the numerical cost that arises from the required use of very large solvent boxes. We introduce telescoping box schemes within adaptive steered molecular dynamics (ASMD) in which we adjust the solvent box between stages and thereby vary (and optimize) the required number of solvent molecules. We have benchmarked the method on a relatively long α-helical peptide, Ala₃₀, with respect to the potential of mean force and hydrogen bonds. We show that the use of telescoping boxes introduces little numerical error while significantly reducing the computational cost.

Benchmarking Adaptive Steered Molecular Dynamics (ASMD) on CHARMM Force Fields

The potentials of mean force (PMFs) along the end‐to‐end distance of two different helical peptides have been obtained and benchmarked using the adaptive steered molecular dynamics (ASMD) method. The results depend strongly on the choice of force field driving the underlying all‐atom molecular dynamics, and are reported with respect to the three most popular CHARMM force field versions: c22, c27 and c36. Two small peptides, ALA10 and 1PEF, serve as the particular case studies. The comparisons between the versions of the CHARMM force fields provides both a qualitative and quantitative look at their performance in forced unfolding simulations in which peptides undergo large changes in structural conformations. We find that ASMD with the underlying c36 force field provides the most robust results for the selected benchmark peptides.

Seeding the multi-dimensional nonequilibrium pulling for Hamiltonian variation: indirect nonequilibrium free energy simulations at QM levels

The combination of free energy simulations in the alchemical and configurational spaces provides a feasible route to access the thermodynamic profiles under a computationally demanding target Hamiltonian. Normally, due to the significant differences between the computational cost of ab initio quantum mechanics (QM) calculations and those of semi-empirical quantum mechanics (SQM) and molecular mechanics (MM), this indirect method could be used to obtain the QM thermodynamics by combining the SQM or MM results and the SQM-to-QM or MM-to-QM corrections. In our previous work, a multi-dimensional nonequilibrium pulling framework for Hamiltonian variations was introduced based on bidirectional pulling and bidirectional reweighting. The method performs nonequilibrium free energy simulations in the configurational space to obtain the thermodynamic profile along the conformational change pathway under a selected computationally efficient Hamiltonian, and uses the nonequilibrium alchemical method to correct or perturb the thermodynamic profile to that under the target Hamiltonian. The BAR-based method is designed to achieve the best generality and transferability and thus leads to modest (∼20 fold) speedup. In this work, we explore the possibility of further accelerating the nonequilibrium free energy simulation by employing unidirectional pulling and using the selection criterion to obtain the initial configurations used to initiate nonequilibrium trajectories following the idea of adaptive steered molecular dynamics (ASMD). A single initial condition is used to seed the whole multi-dimensional nonequilibrium free energy simulation and the sampling is performed fully in the nonequilibrium ensemble. Introducing very short ps-length equilibrium sampling to grab more initial seeds could also be helpful. The ASMD scheme estimates the free energy difference with the unidirectional exponential average (EXP), but it does not follow exactly the requirements of the EXP estimator. Another deficiency of the seeding simulation is the inherently sequential or serial pulling due to the inter-segment dependency, which triggers some problems in the parallelizability of the simulation. Numerical tests are performed to grasp some insights and guidelines for using this selection-criterion-based ASMD scheme. The presented selection-criterion-based multi-dimensional ASMD scheme follows the same perturbation network of the BAR-based method, and thus could be used in various Hamiltonian-variation cases.

Demystifying DPP III Catalyzed Peptide Hydrolysis—Computational Study of the Complete Catalytic Cycle of Human DPP III Catalyzed Tynorphin Hydrolysis

Dipeptidyl peptides III (DPP III) is a dual-domain zinc exopeptidase that hydrolyzes peptides of varying sequence and size. Despite attempts to elucidate and narrow down the broad substrate-specificity of DPP III, there is no explanation as to why some of them, such as tynorphin (VVYPW), the truncated form of the endogenous heptapeptide spinorphin, are the slow-reacting substrates of DPP III compared to others, such as Leu-enkephalin. Using quantum molecular mechanics calculations followed by various molecular dynamics techniques, we describe for the first time the entire catalytic cycle of human DPP III, providing theoretical insight into the inhibitory mechanism of tynorphin. The chemical step of peptide bond hydrolysis and the substrate binding to the active site of the enzyme and release of the product were described for DPP III in complex with tynorphin and Leu-enkephalin and their products. We found that tynorphin is cleaved by the same reaction mechanism determined for Leu-enkephalin. More importantly, we showed that the product stabilization and regeneration of the enzyme, but not the nucleophilic attack of the catalytic water molecule and inversion at the nitrogen atom of the cleavable peptide bond, correspond to the rate-determining steps of the overall catalytic cycle of the enzyme.

Molecular dynamics study of the internalization of cell-penetrating peptides containing unnatural amino acids across membranes

Peptide-based delivery systems that deliver target molecules into cells have been gaining traction. These systems need cell-penetrating peptides (CPPs), which have the remarkable ability to penetrate into biological membranes and help internalize different cargoes into cells through the cell membranes. The molecular internalization mechanism and structure-function relationships of CPPs are not clear, although the incorporation of nonproteinogenic amino acids such as α-aminoisobutyric acid (Aib) has been reported to increase their helicity, biostability and penetration efficiencies. Here, we used molecular dynamics to study two Aib-containing CPPs, poly(LysAibAla)₃ (KAibA) and poly(LysAibGly)₃ (KAibG), that previously showed high cell internalization efficiency. KAibA and KAibG displayed the lowest internalization energies among the studied CPPs, showing distinct internalization mechanisms depending on the lipid composition of the model membranes. The presence of Aib residues allows these CPPs to adopt amphipathic folding to efficiently penetrate through the membranes. Elucidating how Aib incorporation affects CPP-membrane binding and interactions is beneficial for the design of CPPs for efficient intracellular delivery.

Conserved FimH mutations in the global Escherichia coli ST131 multi-drug resistant lineage weaken interdomain interactions and alter adhesin function

The binding of the type 1 fimbrial adhesin FimH to mannosylated receptors is allosterically regulated to enhance the fitness of uropathogenic Escherichia coli (UPEC) during urinary tract infection (UTI). Mutations in the two FimH domains (pilin and lectin) located outside the mannose binding pocket have been shown to influence mannose binding affinity, yet the details of the allostery mechanism are not fully elucidated. Here we characterised different FimH conformational states (termed low-affinity tense and high-affinity relaxed conformations) of natural FimH variants using molecular dynamics (MD) simulation techniques and report key structural dynamics differences between them. The clinically dominant FimH30 variant from the pandemic multidrug resistant E. coli ST131 lineage contains an R166H mutation that weakens FimH interdomain interactions and allows enhanced mannose interactions with pre-existing high-affinity relaxed conformations. When expressed in an isogenic ST131 strain background, FimH30 mediated high human cell adhesion and invasion, and enhanced biofilm formation over other variants. Collectively, our computational and experimental findings support a model of FimH protein allostery that is mediated by shifts in the pre-existing conformational equilibrium of FimH, additional to the sequential step-wise process of structural perturbations transmitted from one site to another within the protein. Importantly, it is the first study to shed light into how natural mutations in a clinically dominant FimH variant influence the protein’s conformational landscape optimising its function for ST131 fitness at intestinal and extraintestinal niches.

Substrate Access Mechanism in a Novel Membrane-Bound Phospholipase A of Pseudomonas aeruginosa Concordant with Specificity and Regioselectivity

PlaF is a cytoplasmic membrane-bound phospholipase A₁ from Pseudomonas aeruginosa that alters the membrane glycerophospholipid (GPL) composition and fosters the virulence of this human pathogen. PlaF activity is regulated by a dimer-to-monomer transition followed by tilting of the monomer in the membrane. However, how substrates reach the active site and how the characteristics of the active site tunnels determine the activity, specificity, and regioselectivity of PlaF for natural GPL substrates have remained elusive. Here, we combined unbiased and biased all-atom molecular dynamics (MD) simulations and configurational free-energy computations to identify access pathways of GPL substrates to the catalytic center of PlaF. Our results map out a distinct tunnel through which substrates access the catalytic center. PlaF variants with bulky tryptophan residues in this tunnel revealed decreased catalysis rates due to tunnel blockage. The MD simulations suggest that GPLs preferably enter the active site with the sn-1 acyl chain first, which agrees with the experimentally demonstrated PLA₁ activity of PlaF. We propose that the acyl chain-length specificity of PlaF is determined by the structural features of the access tunnel, which results in favorable free energy of binding of medium-chain GPLs. The suggested egress route conveys fatty acid (FA) products to the dimerization interface and, thus, contributes to understanding the product feedback regulation of PlaF by FA-triggered dimerization. These findings open up opportunities for developing potential PlaF inhibitors, which may act as antibiotics against P. aeruginosa.

Helix-coil transition and conformational deformity in Aβ42-monomer: a case study using the Zn2+ cation

The metal ions (like Fe2+, Zn²⁺, Cu2+) are known to influence the amyloid beta (Aβ) aggregation. In this study, we have examined the conformational and dynamical changes during the coordination of Aβ-monomer with the Zn2+ ion using all-atom molecular dynamics (MD) simulations using explicit solvent models. We have probed the unfolding of the full-length Aβ₄₂ monomer both inclusive and exclusive of the Zn2+ cation, with 1:1 ratio of the peptide and the Zn2+ cation. The inclusion of the Zn2+ cation shows differential intra-peptide interactions which has been probed using various analyses. The Helix - Coil transition of the wild type Aβ42 monomer is studied using the steered molecular dynamics simulations by taking the end-to-end C-α distance across the peptide. This gives an idea of the unequal intra - peptide and peptide - water interactions being found across the length of the Aβ monomer. The transition of an α-helix dominated wild-type (WT) Aβ structure to the unfolded coil structure gives significant evidence of the intra-peptide hydrogen bonding shifts in the presence of the Zn2+ cation. This accounts for the structural and the dynamical variations that take place in the Aβ monomer in the presence of the Zn2+ cation to mimic the conditions/environment at the onset of fibrillation.Communicated by Ramaswamy H. Sarma.

Energetics and structure of alanine-rich α-helices via adaptive steered molecular dynamics

The energetics and hydrogen bonding profiles of the helix-to-coil transition were found to be an additive property and to increase linearly with chain length, respectively, in alanine-rich α-helical peptides. A model system of polyalanine repeats was used to establish this hypothesis for the energetic trends and hydrogen bonding profiles. Numerical measurements of a synthesized polypeptide Ac-Y(AEAAKA)kF-NH2 and a natural α-helical peptide a2N (1-17) provide evidence of the hypothesis's generality. Adaptive steered molecular dynamics was employed to investigate the mechanical unfolding of all of these alanine-rich polypeptides. We found that the helix-to-coil transition is primarily dependent on the breaking of the intramolecular backbone hydrogen bonds and independent of specific side-chain interactions and chain length. The mechanical unfolding of the α-helical peptides results in a turnover mechanism in which a 310-helical structure forms during the unfolding, remaining at a near constant population and thereby maintaining additivity in the free energy. The intermediate partially unfolded structures exhibited polyproline II helical structure as previously seen by others. In summary, we found that the average force required to pull alanine-rich α-helical peptides in between the endpoints-namely the native structure and free coil-is nearly independent of the length or the specific primary structure.

Insight into the drug resistance mechanisms of GS-9669 caused by mutations of HCV NS5B polymerase via molecular simulation

GS-9669 is a non-nucleos(t)ide inhibitor (NNI) binding to the thumb site II of the Hepatitis C virus (HCV) NS5B polymerase and has advanced into phase II trials. To clarify the drug resistance mechanisms of GS-9669 caused by M423T/I/V, L419M, R422K, and I482L mutations of NS5B polymerase (GT1b) and the receptor-ligand interactions during the binding process, a series of molecular simulation methods including molecular dynamics (MD) simulations and adaptive steered molecular dynamics (ASMD) simulations were performed for the wild-type (WT) and six mutant NS5B/GS-9669 complexes. The calculated results indicate that the binding free energies of the mutant systems are less negative than that of the WT system, indicating that these mutations will indeed cause NS5B to produce different degrees of resistance to GS-9669. The mutation-induced drug resistances are mainly caused by the loss of binding affinities of Leu419 and Trp528 with GS-9669 or the formation of multiple solvent bridges. Moreover, the ASMD calculations show that GS-9669 binds to the thumb II sites of the seven NS5B polymerases in distinct pathways without any obvious energy barriers. Although the recognition methods and binding pathways are distinct, the binding processes of GS-9669 with the WT and mutant NS5B polymerases are basically controlled thermodynamically. This study clearly reveals the resistance mechanisms of GS-9669 caused by M423T/I/V, L419M, R422K, and I482L mutations of HCV NS5B polymerase and provides some valuable clues for further optimization and design of novel NS5B inhibitors.

Exploitation of dihydroorotate dehydrogenase (DHODH) and p53 activation as therapeutic targets: A case study in polypharmacology

The tenovins are a frequently studied class of compounds capable of inhibiting sirtuin activity, which is thought to result in increased acetylation and protection of the tumor suppressor p53 from degradation. However, as we and other laboratories have shown previously, certain tenovins are also capable of inhibiting autophagic flux, demonstrating the ability of these compounds to engage with more than one target. In this study, we present two additional mechanisms by which tenovins are able to activate p53 and kill tumor cells in culture. These mechanisms are the inhibition of a key enzyme of the de novo pyrimidine synthesis pathway, dihydroorotate dehydrogenase (DHODH), and the blockage of uridine transport into cells. These findings hold a 3-fold significance: first, we demonstrate that tenovins, and perhaps other compounds that activate p53, may activate p53 by more than one mechanism; second, that work previously conducted with certain tenovins as SirT1 inhibitors should additionally be viewed through the lens of DHODH inhibition as this is a major contributor to the mechanism of action of the most widely used tenovins; and finally, that small changes in the structure of a small molecule can lead to a dramatic change in the target profile of the molecule even when the phenotypic readout remains static.

Testing amino acid-codon affinity hypothesis using molecular docking

Genetic code refers to a set of rules that assign trinucleotides called codons to amino acids in the process of protein synthesis. Investigating the genetic code's logic and its evolutionary origin has always been both intriguing and challenging. While the correspondence rules between codons and amino acids in the genetic code are well-known, it is still unclear whether those assignments can be explained based on energetic or/and entropic arguments. As an attempt at deciphering basic thermodynamic rules governing DNA translation, we used molecular docking to investigate the ability of amino acids to bind to their corresponding anticodon compared to other codons. The total number of 1280 direct docking interactions have been performed for each amino acid-codon/anti-codon case to find whether the amino acids have a preference to bind to their cognate anticodons or codons. Based on docking scores which are expected to correlate with binding affinity, no correlation with genetic correspondence rules was observed suggesting a more subtle process, other than direct binding, to explain codon-amino-acid specificity.

Binding of dipeptidyl peptidase III to the oxidative stress cell sensor Kelch-like ECH-associated protein 1 is a two-step process

This work is about synergy of theory and experiment in revealing mechanism of binding of dipeptidyl peptidase III (DPP III) and Kelch-like ECH-associated protein 1 (KEAP1), the main cellular sensor of oxidative stress. The NRF2 ̶ KEAP1 signaling pathway is important for cell protection, but it is also impaired in many cancer cells where NRF2 target gene expression leads to resistance to chemotherapeutic drugs. DPP III competitively binds to KEAP1 in the conditions of oxidative stress and induces release of NRF2 and its translocation into nucleus. The binding is established mainly through the ETGE motif of DPP III and the Kelch domain of KEAP1. However, although part of a flexible loop, ETGE itself is firmly attached to the DPP III surface by strong hydrogen bonds. Using combined computational and experimental study, we found that DPP III ̶ Kelch binding is a two-step process comprising the endergonic loop detachment and exergonic DPP III ̶ Kelch interaction. Substitution of arginines, which keep the ETGE motif attached, decreases the work needed for its release and increases DPP III ̶ Kelch binding affinity. Interestingly, mutations of one of these arginine residues have been reported in cBioPortal for cancer genomics, implicating its possible involvement in cancer development. Communicated by Ramaswamy H. Sarma.

Modeling the Sterol-Binding Domain of Aster-A Provides Insight into Its Multiligand Specificity

Intracellular transport of cholesterol and related sterols relies to a large degree on nonvesicular mechanisms, which are only partly understood at the molecular level. Aster proteins belonging to the Lam family of sterol transfer proteins have recently been identified as important catalysts of nonvesicular sterol exchange between the plasma membrane (PM) and endoplasmic reticulum (ER). Here, we used a range of computational tools to study the molecular mechanisms underlying sterol binding as well as multisterol ligand specificity of Aster-A. Our study focused primarily on gaining atomistic insight into the bound ligand-protein complex and was, on this basis, performed in the absence of any membrane. Molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) calculations provide a rationale for the experimentally found ranking of binding affinities of various sterols to Aster-A. In particular, the polarity of the sterols and the length of their alkyl chain could be identified as being critical determinants of ligand affinity. A Gibbs free energy decomposition identified a charged residue, Glu444, at the base of the binding pose as an important control point for sterol binding. Removing its net charge via protonation was found to cause significant changes to the environment surrounding this residue. In addition, the protonation of Glu444 was found to be paralleled by a large redistribution of molecular flexibility in the Aster domain. This finding was supplemented by multiple branched adaptive steered molecular dynamics (MB-ASMD) simulations by which we defined a possible molecular path for sterol release and demonstrated the importance of Glu444 in this process.

The relative stability of trpzip1 and its mutants determined by computation and experiment

The single-point mutations of tprzip1 are indicated at left, and their relative energetics are compared at right.

What are the effects of the serine triad on proton conduction of an influenza B M2 channel? An investigation by molecular dynamics simulations

The tetrameric influenza B M2 channel (BM2), an acid activated proton channel, is important in the influenza virus B lifecycle. A conserved HxxxW motif is responsible for proton conduction and channel gating. In this study, to explore the effects of the serine triad (S9, S12 and S16) on proton conduction, we performed classical molecular dynamics (CMD) simulations and adaptive steered molecular dynamics (ASMD) simulations at different protonation states of the H19 tetrad. The results of the pore radius and the C-terminal tilt angle show that the electrostatic repulsion induced by protonated H19 is the key driving force for opening the BM2 channel. The open states could be stabilized by the hydrogen bonds between S16 and protonated H19. The solvent accessible surface area and water density indicate that the polar hydrophilic environment provided by the serine triad facilitates the formation of a water wire, and then exhibits favourable effects on proton conduction. The mutant research verifies and supports these views. Our work clarifies the effects of the serine triad on proton conduction in the BM2 channel, which would help us deeply understand the proton conduction mechanism in BM2 and provides a new perspective for antiviral drug design against BM2.

BAR-based optimum adaptive steered MD for configurational sampling

The equilibrium and nonequilibrium adaptive alchemical free energy simulation methods optimum Bennett's acceptance ratio and optimum crooks' equation (OCE), based on the statistically optimal bidirectional reweighting estimator named Bennett's Acceptance Ratio or Crooks' equation, perform initial sampling in the staging alchemical transformation and then determine the importance rank of different states via the time-derivative of the variance. The method is proven to give speedups compared with the equal time rule. In the current work, we extend the time derivative of variance guided adaptive sampling method to the configurational space, falling in the term of steered MD (SMD). The SMD approach biasing physically meaningful collective variable (CV) such as one dihedral or one distance to pulling the system from one conformational state to another. By minimizing the variance of the free energy differences along the pathway in an optimized way, a new type of adaptive SMD (ASMD) is introduced. As exhibits in the alchemical case, this adaptive sampling method outperforms the traditional equal-time SMD in nonequilibrium stratification. Also, the method gives much more efficient calculation of potential of mean force than the selection criterion-based ASMD scheme, which is proven to be more efficient than traditional SMD. The OCE workflow is periodicity-of-CV dependent while ASMD is not. The performance is demonstrated in a dihedral flipping case and two distance pulling cases, accounting for periodic and nonperiodic CVs, respectively. © 2019 Wiley Periodicals, Inc.

Mutational Analysis of Neuropeptide Y Reveals Unusual Thermal Stability Linked to Higher-Order Self-Association

Neuropeptide Y (NPY) is a 36-residue peptide, abundant in the central and peripheral nervous system. The peptide interacts with membrane-bound receptors to control processes such as food intake, vasoconstriction, and memory retention. The N-terminal polyproline sequence of NPY folds back onto a C-terminal α-helix to form a hairpin structure. The hairpin undergoes transient unfolding to allow the monomer to interact with its target membranes and receptors and to form reversible dimers in solution. Using computational, functional, and biophysical approaches, we characterized the role of two conserved tyrosines (Y20 and Y27) located within the hydrophobic core of the hairpin fold. Successive mutation of the tyrosines to more hydrophobic phenylalanines increased the thermal stability of NPY and reduced functional activity, consistent with computational studies predicting a more stable hairpin structure. However, mutant stability was high relative to wild-type: melting temperatures increased by approximately 20 °C for the single mutants (Y20F and Y27F) and by 30 °C for the double mutant (Y20F + Y27F). These findings suggested that the mutations were not just simply enhancing hairpin structure stability, but might also be driving self-association to dimer. Using analytical ultracentrifugation, we determined that the mutations indeed increased self-association, but shifted the equilibrium toward hexamer-like species. Notably, these latter species were not unique to the NPY mutants, but were found to preexist at low levels in the wild-type population. Collectively, the findings indicate that NPY self-association is more complex than previously recognized and that the ensemble of NPY quaternary states is tunable by modulating hairpin hydrophobicity.

Exploring the structure characteristics and major channels of cytochrome P450 2A6, 2A13, and 2E1 with pilocarpine

The majority of cytochromes P450 play a critical role in metabolism of endogenous and exogenous substrates, some of its products are carcinogens. Therefore, inhibition of P450 enzymes activity can promote the detoxification and elimination of chemical carcinogens. In this study, molecular dynamics (MD) simulations and adaptive steered molecular dynamics (ASMD) simulations were performed to explore the structure features and channel dynamics of three P450 isoforms 2A6, 2A13, and 2E1 bound with the common inhibitor pilocarpine. The binding free energy results combined with the PMF calculations give a reasonable ranking of binding affinity, which are consistent with the experimental data. Our results uncover how a sequence divergence of different CYP2 enzymes causes individual variations in major channel selections. On the basis of channel bottleneck and energy decomposition analysis, we propose a gating mechanism of their respective major channels in three enzymes, which may be attributed to a reversal of Phe209 in CYP2A6/2A13, as well as the rotation of Phe116 and Phe298 in CYP2E1. The hydrophobic residues not only make strong hydrophobic interactions with inhibitor, but also act as gatekeeper to regulate the opening of channel. The present study provides important insights into the structure-function relationships of three cytochrome P450s and the molecular basis for development of potent inhibitors.

Molecular dynamics investigation of stereoselective inhibition mechanism of HIF-2α/ARNT heterodimer

Hypoxia-inducible factors (HIFs) are heterodimeric transcription factors related with the onset and progression of solid tumors. Studies demonstrated a class of tetrazole containing chiral inhibitors could stereoselectively disrupt the HIF-2 dimerization and reduce the target gene expression. However, the dynamical features and structural motifs of the HIF-2 heterodimer caused by the binding of enantiomers have not been rationalized at the atomistic level. In this work, molecular dynamics (MD) simulations combined with adaptive steered MD (ASMD) simulations were used to investigate stereoselective interrupting mechanism of HIF-2. Our results decipher that the binding of ligand A (S, R)-24 begets the significant conformation changes of β-sheets and interrupts the HIF-2α/ARNT heterodimerization, which may be attributed to the disruption of the hydrogen bond and salt bridge interactions formed by the 4 foremost residues (Asp240, Arg247, Glu362, and Arg366) and the destruction of hydrophobic interactions on the binding interface. By contrast, the binding of ligand B (R, S)-24 does not disrupt protein dimerization and causes the motion of Fα helix in HIF-2α PAS-B domain to further change the major tunnel for ligand ingress and engress. The present work provides important molecular-level insight into the effect of the binding enantiomers on HIF-2 heterodimerization and bridges the gap between theory and the experimental results, which may conduce to develop highly potent antagonists for intervening the HIF-2-driven tumors.

Determining the Energetics of Small β-Sheet Peptides using Adaptive Steered Molecular Dynamics

Mechanically driven unfolding is a useful computational tool for extracting the energetics and stretching pathway of peptides. In this work, two representative β-hairpin peptides, chignolin (PDB: 1UAO ) and trpzip1 (PDB: 1LE0 ), were investigated using an adaptive variant of the original steered molecular dynamics method called adaptive steered molecular dynamics (ASMD). The ASMD method makes it possible to perform energetic calculations on increasingly complex biological systems. Although the two peptides are similar in length and have similar secondary structures, their unfolding energetics are quite different. The hydrogen bonding profile and specific residue pair interaction energies provide insight into the differing stabilities of these peptides and reveal which of the pairs provides the most significant stabilization.