Three-dimensional solution structure of apo-neocarzinostatin.

Two and three-dimensional solution nuclear magnetic resonance studies of the 11K apoprotein from natural antitumor agent neocarzinostatin (NCS) were extended to elucidation of the high-resolution structure by the use of restrained molecular dynamics computations. The refined structures attained convergency upon three steps of iterative calculations, in which more distance restraints were extracted from experimental data, and the existing distance bounds were optimized on the basis of computed structures. The solution structures of apo-NCS contain seven antiparallel beta-strands, which form two closely located beta-sheets and a short beta-segment. This protein lacks any alpha-helical component. The alignment of the seven beta-strands gives rise to a beta-barrel with an elongated diameter in one direction. The global structure of apo-NCS resembles that of the Ig-fold domain found in immunoglobulins and other structurally related beta-proteins. Residues responsible for side-chain packing and the possible salt-bridge formation important for protein folding were identified. Neocarzinostatin and the analogous proteins are known to exert their biological activity through the interaction of DNA with a chromophoric molecule, which is non-covalently bound to the apo-proteins. This molecular chromophore-binding site in apo-NCS is made of a cavity consisting of residues from the four-beta-stranded sheet and the short beta-segment. Although the solution structures of apo-NCS are similar to that of the analogous apoauromomycin in the crystalline state, difference in the shape of the binding cavities between the two was found. This study provides a structural basis for characterization of the specific recognition and molecular mechanism of the antitumor NCS chromophore binding to its host protein.     

Conformational flexibility of N-glycans in solution studied by REMD simulations

Protein–glycan recognition regulates a wide range of biological and pathogenic processes. Conformational diversity of glycans in solution is apparently incompatible with specific binding to their receptor proteins. One possibility is that among the different conformational states of a glycan, only one conformer is utilized for specific binding to a protein. However, the labile nature of glycans makes characterizing their conformational states a challenging issue. All-atom molecular dynamics (MD) simulations provide the atomic details of glycan structures in solution, but fairly extensive sampling is required for simulating the transitions between rotameric states. This difficulty limits application of conventional MD simulations to small fragments like di- and tri-saccharides. Replica-exchange molecular dynamics (REMD) simulation, with extensive sampling of structures in solution, provides a valuable way to identify a family of glycan conformers. This article reviews recent REMD simulations of glycans carried out by us or other research groups and provides new insights into the conformational equilibria of N-glycans and their alteration by chemical modification. We also emphasize the importance of statistical averaging over the multiple conformers of glycans for comparing simulation results with experimental observables. The results support the concept of “conformer selection” in protein–glycan recognition.     

Design, production, and characterization of recombinant neocarzinostatin apoprotein in Escherichia coli

Neocarzinostatin (NCS) is the first discovered anti-tumor antibiotic having an enediyne-containing chromophore and an apoprotein with a 1:1 complex. An artificial gene library for NCS apoprotein (apo-NCS) production in Escherichia coli was designed and constructed on a phage-display vector, pJuFo. The recombinant phages expressing pre-apo-NCS protein were enriched with a mouse anti-apo-NCS monoclonal antibody, 1C7D4. The apo-NCS gene (encsA) for E. coli was successfully cloned, and then re-cloned into the pRSET A vector. After the his-tagged apo-NCS protein had been purified and cleaved with enterokinase, the binding properties of the recombinant protein as to ethidium bromide (EtBr) were studied by monitoring of total fluorescence intensity and fluorescence polarization with a BEACON 2000 system and GraphPad Prism software. A dissociation constant of 4.4 +/- 0.3 microM was obtained for recombinant apo-NCS in the fluorescence polarization study. This suggests that fluorescence polarization monitoring with EtBr as a chromophore mimic may be a simplified method for the characterization of recombinant apo-NCS binding to the NCS chromophore. When Phe78 on apo-NCS was substituted with Trp78 by site-directed mutagenesis using a two stage megaprimer polymerase chain reaction, the association of the apo-NCS mutant and EtBr observed on fluorescence polarization analysis was of the same degree as in the case of the wild type, although the calculated maximum change (DeltaIT(max)) in total fluorescence intensity decreased from 113.9 to 31.3. It was suggested that an environmental change of the bound EtBr molecule on F78W might have dramatically occurred as compared with in the case of wild type apo-NCS. This combination of monitoring of fluorescence polarization and total fluorescence intensity will be applicable for determination and prediction of the ligand state bound or associated with the target protein. The histone-specific proteolytic activity was also re-investigated using this recombinant apo-NCS preparation, and calf thymus histone H1, H2A, H2B, H3, and H4. The recombinant apo-NCS does not act as a histone protease because a noticeable difference was not observed between the incubation mixtures with and without apo-NCS under our experimental conditions.     

Unraveling evolutionary constraints: A heterogeneous conservation in dynamics of the titin Ig domains

The giant protein titin, which comprises immunoglobulin (Ig) domains, acts as a bidirectional spring in muscle. The unfolding of Ig domains has been extensively studied, but their dynamics under native states have not been well-characterized. We performed molecular dynamics simulation on a single titin Ig domain and multi-domains. Mobile regions displaying concerted motions were identified. The dynamics of Ig domains are constrained by evolutionary pressures, in such a way that global dominant motion is conserved, yet different flexibilities within Ig domains and in linkers connecting neighbouring domains were observed. We explain these heterogeneous conserved dynamics in relation to sequence conservation across species and the sequence diversity among neighbouring Ig domains.     

Multiscale modeling of cellular actin filaments: from atomistic molecular to coarse-grained dynamics

In this article, we present a computational multiscale model for the characterization of subcellular proteins. The model is encoded inside a simulation tool that builds coarse-grained (CG) force fields from atomistic simulations. Equilibrium molecular dynamics simulations on an all-atom model of the actin filament are performed. Then, using the statistical distribution of the distances between pairs of selected groups of atoms at the output of the MD simulations, the force field is parameterized using the Boltzmann inversion approach. This CG force field is further used to characterize the dynamics of the protein via Brownian dynamics simulations. This combination of methods into a single computational tool flow enables the simulation of actin filaments with length up to 400 nm, extending the time and length scales compared to state-of-the-art approaches. Moreover, the proposed multiscale modeling approach allows to investigate the relationship between atomistic structure and changes on the overall dynamics and mechanics of the filament and can be easily (i) extended to the characterization of other subcellular structures and (ii) used to investigate the cellular effects of molecular alterations due to pathological conditions.     

Global changes in local protein dynamics reduce the entropic cost of carbohydrate binding in the arabinose-binding protein

Protein dynamics make important but poorly understood contributions to molecular recognition phenomena. To address this, we measure changes in fast protein dynamics that accompany the interaction of the arabinose-binding protein (ABP) with its ligand, d-galactose, using NMR relaxation and molecular dynamics simulation. These two approaches present an entirely consistent view of the dynamic changes that occur in the protein backbone upon ligand binding. Increases in the amplitude of motions are observed throughout the protein, with the exception of a few residues in the binding site, which show restriction of dynamics. These counter-intuitive results imply that a localised binding event causes a global increase in the extent of protein dynamics on the pico- to nanosecond timescale. This global dynamic change constitutes a substantial favourable entropic contribution to the free energy of ligand binding. These results suggest that the structure and dynamics of ABP may be adapted to exploit dynamic changes to reduce the entropic costs of binding.     

Molecular dynamics simulations on SDF-1alpha: binding with CXCR4 receptor

Insights into the interacting mode of CXCR4 with SDF-1alpha are crucial in understanding the structural and functional characteristics of CXCR4 receptor. In this paper a computational pipeline, integrating protein structure prediction, molecular dynamics simulations, automated molecular docking, and Brownian dynamics simulations were employed to investigate the dynamic and energetic aspects of CXCR4 associating with SDF-1alpha. The entire simulation revealed the surface distribution feature of electrostatic potentials and conformational "open-close" process of the receptor. The possible binding conformation of CXCR4 was identified, and the CXCR4-SDF-1alpha binding complex was generated. Arg188-Glu277 salt bridge plays an important role for both the extracellular domain conformational change and SDF-1alpha binding. Two binding sites were mapped at the extracellular domain (Site 1) and inside the transmembrane domain (Site 2), which are composed of conserved residues. Sites 1 and 2 contribute approximately 60% and 40% to the binding affinity with SDF-1alpha, respectively. The binding model is in agreement with most of the experimental data. Transmembrane VI has more significant motion in the harmonious conformational transition of CXCR4 during SDF-1alpha binding, which may be possibly associated with signal transduction. Based on the modeling and simulation, a binding mechanism hypothesis between CXCR4 and SDF-1alpha and its relationship to the signal transduction has been proposed.     

The use of NMR chemical shifts to analyse the MD trajectories: simulation of bovine pancreatic trypsin inhibitor dynamics in water as a test case for solvent influences.

In this paper the NMR secondary chemical shifts, that are estimated from a set of 3D-structures, are compared with the observed ones to appraise the behaviour of a known x-ray diffraction structure (of the bovine pancreatic trypsin inhibitor protein) when various molecular dynamics are applied. The results of a 200 ps molecular dynamics under various conditions are analysed and different ways to modify the molecular dynamics are considered. With the purpose of avoiding the time-consuming explicit representation of the solvent (water) molecules, an attempt was made to understand the role of the solvent and to develop an implicit representation, which may be refined. A simulation of hydrophobic effects in an aqueous environment is also proposed which seems to provide a better approximation of the observed solution structure of the protein.     

Recent advances in maximum entropy biasing techniques for molecular dynamics

ABSTRACT

This review describes recent advances by the authors and others on the topic of incorporating experimental data into molecular simulations through maximum entropy methods. Methods which incorporate experimental data improve accuracy in molecular simulation by minimally modifying the thermodynamic ensemble. This is especially important where force fields are approximate, such as when employing coarse-grain models, or where high accuracy is required, such as when attempting to mimic a multiscale self-assembly process. The authors review here the experiment directed simulation (EDS) and experiment directed metadynamics (EDM) methods that allow matching averages and distributions in simulations, respectively. Important system-specific considerations are discussed such as using enhanced sampling simultaneously, the role of pressure, treating uncertainty, and implementations of these methods. Recent examples of EDS and EDM are reviewed including applications to ab initio molecular dynamics of water, incorporating environmental fluctuations inside of a macromolecular protein complex, improving RNA force fields, and the combination of enhanced sampling with minimal biasing to model peptides     

Assembly and stability of Salmonella enterica ser. Typhi TolC protein in POPE and DMPE

In this work we assessed the suitability of two different lipid membranes for the simulation of a TolC protein from Salmonella enterica serovar Typhi. The TolC protein family is found in many pathogenic Gram-negative bacteria including Vibrio cholera and Pseudomonas aeruginosa and acts as an outer membrane channel for expulsion of drug and toxin from the cell. In S. typhi, the causative agent for typhoid fever, the TolC outer membrane protein is an antigen for the pathogen. The lipid environment is an important modulator of membrane protein structure and function. We evaluated the conformation of the TolC protein in the presence of DMPE and POPE bilayers using molecular dynamics simulation. The S. typhi TolC protein exhibited similar conformational dynamics to TolC and its homologues. Conformational flexibility of the protein is seen in the C-terminal, extracellular loops, and α-helical region. Despite differences in the two lipids, significant similarities in the motion of the protein in POPE and DMPE were observed, including the rotational motion of the C-terminal residues and the partially open extracellular loops. However, analysis of the trajectories demonstrated effects of hydrophobic matching of the TolC protein in the membrane, particularly in the lengthening of the lipids and subtle movements of the protein’s β-barrel towards the lower leaflet in DMPE. The study exhibited the use of molecular dynamics simulation in revealing the differential effect of membrane proteins and lipids on each other. In this study, POPE is potentially a more suitable model for future simulation of the S. typhi TolC protein.     

Thermal unfolding simulations of apo-calmodulin using leap-dynamics

The simulation method leap-dynamics (LD) has been applied to protein thermal unfolding simulations to investigate domain-specific unfolding behavior. Thermal unfolding simulations of the 148-residue protein apo-calmodulin with implicit solvent were performed at temperatures 290 K, 325 K, and 360 K and compared with the corresponding molecular dynamics trajectories in terms of a number of calculated conformational parameters. The main experimental results of unfolding are reproduced in showing the lower stability of the C-domain: at 290 K, both the N- and C-domains are essentially stable; at 325 K, the C-domain unfolds, whereas the N-domain remains folded; and at 360 K, both domains unfold extensively. This behavior could not be reproduced by molecular dynamics simulations alone under the same conditions. These results show an encouraging degree of convergence between experiment and LD simulation. The simulations are able to describe the overall plasticity of the apo-calmodulin structure and to reveal details such as reversible folding/unfolding events within single helices. The results show that by using the combined application of a fast and efficient sampling routine with a detailed molecular dynamics force field, unfolding simulations of proteins at atomic resolution are within the scope of current computational power.     

Ensemble docking and molecular dynamics identify knoevenagel curcumin derivatives with potent anti-EGFR activity

Epidermal growth factor receptor tyrosine kinase (EGFR-TK) is an attractive target for cancer therapy. Despite a number of effective EGFR inhibitors that are constantly expanding and different methods being employed to obtain novel compounds, the search for newer EGFR inhibitors is still a major scientific challenge. In the present study, a molecular docking and molecular dynamics investigation has been carried out with an ensemble of EGFR-TK structures against a synthetically feasible library of curcumin analogs to discover potent EGFR inhibitors. To resolve protein flexibility issue we have utilized 5 EGFR wild type crystal structures during docking as this gives improved possibility of identifying an active compound as compared to using a single crystal structure. We then identified five curcumin analogs representing different scaffolds that can serve as lead molecules. Finally, the 5 ns molecular dynamics simulation shows that knoevenagel condensate of curcumin specifically C29 and C30 can be used as starting blocks for developing effective leads capable of inhibiting EGFR.     

Investigations of Takeout proteins’ ligand binding and release mechanism using molecular dynamics simulation

Takeout (To) proteins exist in a diverse range of insect species. They are involved in many important processes of insect physiology and behaviors. As the ligand carriers, To proteins can transport the small molecule to the target tissues. However, ligand release mechanism of To proteins is unclear so far. In this contribution, the process and pathway of the ligand binding and release are revealed by conventional molecular dynamics simulation, steered molecular dynamics simulation and umbrella sampling methods. Our results show that the α4-side of the protein is the unique gate for the ligand binding and release. The structural analysis confirms that the internal cavity of the protein has high rigidity, which is in accordance with the recent experimental results. By using the potential of mean force calculations in combination with residue cross correlation calculation, we concluded that the binding between the ligand and To proteins is a process of conformational selection. Furthermore, the conformational changes of To proteins and the hydrophobic interactions both are the key factors for ligand binding and release.     

Exploring the energy landscape of protein folding using replica-exchange and conventional molecular dynamics simulations

Two independent replica-exchange molecular dynamics (REMD) simulations with an explicit water model were performed of the Trp-cage mini-protein. In the first REMD simulation, the replicas started from the native conformation, while in the second they started from a nonnative conformation. Initially, the first simulation yielded results qualitatively similar to those of two previously published REMD simulations: the protein appeared to be over-stabilized, with the predicted melting temperature 50-150K higher than the experimental value of 315K. However, as the first REMD simulation progressed, the protein unfolded at all temperatures. In our second REMD simulation, which starts from a nonnative conformation, there was no evidence of significant folding. Transitions from the unfolded to the folded state did not occur on the timescale of these simulations, despite the expected improvement in sampling of REMD over conventional molecular dynamics (MD) simulations. The combined 1.42 micros of simulation time was insufficient for REMD simulations with different starting structures to converge. Conventional MD simulations at a range of temperatures were also performed. In contrast to REMD, the conventional MD simulations provide an estimate of Tm in good agreement with experiment. Furthermore, the conventional MD is a fraction of the cost of REMD and continuous, realistic pathways of the unfolding process at atomic resolution are obtained.     

The interaction of cofilin with the actin filament

Cofilin is a key actin-binding protein that is critical for controlling the assembly of actin within the cell. Here, we present the results of molecular docking and dynamics studies using a muscle actin filament and human cofilin I. Guided by extensive mutagenesis results and other biophysical and structural studies, we arrive at a model for cofilin bound to the actin filament. This predicted structure agrees very well with electron microscopy results for cofilin-decorated filaments, provides molecular insight into how the known F- and G-actin sites on cofilin interact with the filament, and also suggests new interaction sites that may play a role in cofilin binding. The resulting atomic-scale model also helps us understand the molecular function and regulation of cofilin and provides testable data for future experimental and simulation work.     

Propagation of dynamic changes in barnase upon binding of barstar: an NMR and computational study

NMR spectroscopy and computer simulations were used to examine changes in chemical shifts and in dynamics of the ribonuclease barnase that result upon binding to its natural inhibitor barstar. Although the spatial structures of free and bound barnase are very similar, binding results in changes of the dynamics of both fast side-chains, as revealed by (2)H relaxation measurements, and NMR chemical shifts in an extended beta-sheet that is located far from the binding interface. Both side-chain dynamics and chemical shifts are sensitive to variations in the ensemble populations of the inter-converting molecular states, which can escape direct structural observation. Molecular dynamics simulations of free barnase and barnase in complex with barstar, as well as a normal mode analysis of barnase using a Gaussian network model, reveal relatively rigid domains that are separated by the extended beta-sheet mentioned above. The observed changes in NMR parameters upon ligation can thus be rationalized in terms of changes in inter-domain dynamics and in populations of exchanging states, without measurable structural changes. This provides an alternative model for the propagation of a molecular response to ligand binding across a protein that is based exclusively on changes in dynamics.     

Structure prediction of a complex between the chromosomal protein HMG-D and DNA

Non-histone chromosomal proteins are an important part of nuclear structure and function due to their ability to interact with DNA to form and modulate chromatin structure and regulate gene expression. However, the understanding of the function of chromosomal proteins at the molecular level has been hampered by the lack of structures of chromosomal protein–DNA complexes. We have carried out a molecular dynamics modeling study to provide insight into the mode of DNA binding to the chromosomal HMG-domain protein, HMG-D. Three models of a complex of HMG-D bound to DNA were derived through docking the protein to two different DNA fragments of known structure. Molecular dynamics simulations of the complexes provided data indicating the most favorable model. This model was further refined by molecular dynamics simulation and extensively analyzed. The structure of the corresponding HMG-D-DNA complex exhibits many features seen in the NMR structures of the sequence-specific HMG-domain-DNA complexes, lymphoid enhancer factor 1 (LEF-1) and testis determining factor (SRY). The model reveals differences from these known structures that suggest how chromosomal proteins bind to many different DNA sequences with comparable affinity. Proteins 30:113–135, 1998. © 1998 Wiley-Liss, Inc.     

Heterogeneity even at the speed limit of folding: large-scale molecular dynamics study of a fast-folding variant of the villin headpiece

We have performed molecular dynamics simulations on a set of nine unfolded conformations of the fastest-folding protein yet discovered, a variant of the villin headpiece subdomain (HP-35 NleNle). The simulations were generated using a new distributed computing method, yielding hundreds of trajectories each on a time scale comparable to the experimental folding time, despite the large (10,000 atom) size of the simulation system. This strategy eliminates the need to assume a two-state kinetic model or to build a Markov state model. The relaxation to the folded state at 300 K from the unfolded configurations (generated by simulation at 373 K) was monitored by a method intended to reflect the experimental observable (quenching of tryptophan by histidine). We also monitored the relaxation to the native state by directly comparing structural snapshots with the native state. The rate of relaxation to the native state and the number of resolvable kinetic time scales both depend upon starting structure. Moreover, starting structures with folding rates most similar to experiment show some native-like structure in the N-terminal helix (helix 1) and the phenylalanine residues constituting the hydrophobic core, suggesting that these elements may exist in the experimentally relevant unfolded state. Our large-scale simulation data reveal kinetic complexity not resolved in the experimental data. Based on these findings, we propose additional experiments to further probe the kinetics of villin folding.     

Molecular dynamics simulation of a phosphatidylglycerol membrane

Although molecular dynamics simulations are an important tool for studying membrane systems, relatively few simulations have used anionic lipids. This paper reports the first simulation of a pure phosphatidylglycerol (PG) bilayer. The properties of this equilibrated palmitoyloleoylphosphatidylglycerol membrane agree with experimental observations of PG membranes and with previous simulations of monolayers and mixed bilayers containing PG lipids. These simulations also provide interesting insights into hydrogen bonding interactions in PG membranes. This equilibrated membrane will be a useful starting point for simulations of membrane proteins interacting with PG lipids.