Role of the arginine finger in Ras.RasGAP revealed by QM/MM calculations

Differential roles of putative arginine fingers of AAA+ ATPases Rvb1 and Rvb2

The evolutionarily conserved AAA+ ATPases Rvb1 and Rvb2 proteins form a heteromeric complex (Rvb1/2) required for assembly or remodeling of macromolecular complexes in essential cellular processes ranging from chromatin remodeling to ribosome biogenesis. Rvb1 and Rvb2 have a high degree of sequence and structural similarity, and both contain the classical features of ATPases of their clade, including an N-terminal AAA+ subdomain with the Walker A motif, an insertion domain that typically interacts with various binding partners, and a C-terminal AAA+ subdomain containing a Walker B motif, the Sensor I and II motifs, and an arginine finger. In this study, we find that despite the high degree of structural similarity, Rvb1 and Rvb2 have distinct active sites that impact their activities and regulation within the Rvb1/2 complex. Using a combination of biochemical and genetic approaches, we show that replacing the homologous arginine fingers of Rvb1 and Rvb2 with different amino acids not only has distinct effects on the catalytic activity of the complex, but also impacts cell growth, and the Rvb1/2 interactions with binding partners. Using molecular dynamics simulations, we find that changes near the active site of Rvb1 and Rvb2 cause long-range effects on the protein dynamics in the insertion domain, suggesting a molecular basis for how enzymatic activity within the catalytic site of ATP hydrolysis can be relayed to other domains of the Rvb1/2 complex to modulate its function. Further, we show the impact that the arginine finger variants have on snoRNP biogenesis and validate the findings from molecular dynamics simulations using a targeted genetic screen. Together, our results reveal new aspects of the regulation of the Rvb1/2 complex by identifying a relay of long-range molecular communication from the ATPase active site of the complex to the binding site of cofactors. Most importantly, our findings suggest that despite high similarity and cooperation within the same protein complex, the two proteins have evolved with unique properties critical for the regulation and function of the Rvb1/2 complex.

Unveiling the State Transition Mechanisms of Ras Proteins through Enhanced Sampling and QM/MM Simulations

In cells, wild-type RasGTP complexes exist in two distinct states: active State 2 and inactive State 1. These complexes regulate their functions by transitioning between the two states. However, the mechanisms underlying this state transition have not been clearly elucidated. To address this, we conducted a detailed simulation study to characterize the energetics of the stable states involved in the state transitions of the HRasGTP complex, specifically from State 2 to State 1. This was achieved by employing multiscale quantum mechanics/molecular mechanics and enhanced sampling molecular dynamics methods. Based on the simulation results, we constructed the two-dimensional free energy landscapes that provide crucial information about the conformational changes of the HRasGTP complex from State 2 to State 1. Furthermore, we also explored the conformational changes from the intermediate state to the product state during guanosine triphosphate hydrolysis. This study on the conformational changes involved in the HRas state transitions serves as a valuable reference for understanding the corresponding events of both KRas and NRas as well.

Comparative MD simulations and advanced analytics based studies on wild-type and hot-spot mutant A59G HRas

The Ras family of proteins is known to play an important role in cellular signal transduction. The oncoprotein Ras is also found to be mutated in ~90% of the pancreatic cancers, of which G12V, G13V, A59G and Q61L are the known hot-spot mutants. These ubiquitous proteins fall in the family of G-proteins, and hence switches between active GTP bound and inactive GDP bound states, which is hindered in most of its oncogenic mutant counterparts. Moreover, Ras being a GTPase has an intrinsic property to hydrolyze GTP to GDP, which is obstructed due to mutations and lends the mutants stuck in constitutively active state leading to oncogenic behavior. In this regard, the present study aims to understand the dynamics involved in the hot-spot mutant A59G-Ras using long 10μs classical MD simulations (5μs for each of the wild-type and mutant systems) and comparing the same with its wild-type counterpart. Advanced analytics using Markov State Model (MSM) based approach has been deployed to comparatively understand the transition path for the wild-type and mutant systems. Roles of crucial residues like Tyr32, Gln61 and Tyr64 have also been established using multivariate PCA analyses. Furthermore, this multivariate PCA analysis also provides crucial features which may be used as reaction coordinates for biased simulations for further studies. The absence of formation of pre-hydrolysis network is also reported for the mutant conformation, using the distance-based analyses (between crucial residues) of the conserved regions. The implications of this study strengthen the hypothesis that the disruption of the pre-hydrolysis network in the mutant A59G ensemble might lead to permanently active oncogenic conformation in the mutant conformers.

Biology, pathology, and therapeutic targeting of RAS

RAS was identified as a human oncogene in the early 1980s and subsequently found to be mutated in nearly 30% of all human cancers. More importantly, RAS plays a central role in driving tumor development and maintenance. Despite decades of effort, there remain no FDA approved drugs that directly inhibit RAS. The prevalence of RAS mutations in cancer and the lack of effective anti-RAS therapies stem from RAS' core role in growth factor signaling, unique structural features, and biochemistry. However, recent advances have brought promising new drugs to clinical trials and shone a ray of hope in the field. Here, we will exposit the details of RAS biology that illustrate its key role in cell signaling and shed light on the difficulties in therapeutically targeting RAS. Furthermore, past and current efforts to develop RAS inhibitors will be discussed in depth.

Exploiting RAS Nucleotide Cycling as a Strategy for Drugging RAS-Driven Cancers

Oncogenic mutations in RAS genes result in the elevation of cellular active RAS protein levels and increased signal propagation through downstream pathways that drive tumor cell proliferation and survival. These gain-of-function mutations drive over 30% of all human cancers, presenting promising therapeutic potential for RAS inhibitors. However, many have deemed RAS “undruggable” after nearly 40 years of failed drug discovery campaigns aimed at identifying a RAS inhibitor with clinical activity. Here we review RAS nucleotide cycling and the opportunities that RAS biochemistry presents for developing novel RAS inhibitory compounds. Additionally, compounds that have been identified to inhibit RAS by exploiting various aspects of RAS biology and biochemistry will be covered. Our current understanding of the biochemical properties of RAS, along with reports of direct-binding inhibitors, both provide insight on viable strategies for the discovery of novel clinical candidates with RAS inhibitory activity.

A GAP-GTPase-GDP-Pi Intermediate Crystal Structure Analyzed by DFT Shows GTP Hydrolysis Involves Serial Proton Transfers

Cell signaling by small G proteins uses an ON to OFF signal based on conformational changes following the hydrolysis of GTP to GDP and release of dihydrogen phosphate (Pi ). The catalytic mechanism of GTP hydrolysis by RhoA is strongly accelerated by a GAP protein and is now well defined, but timing of inorganic phosphate release and signal change remains unresolved. We have generated a quaternary complex for RhoA-GAP-GDP-Pi . Its 1.75 Å crystal structure shows geometry for ionic and hydrogen bond coordination of GDP and Pi in an intermediate state. It enables the selection of a QM core for DFT exploration of a 20 H-bonded network. This identifies serial locations of the two mobile protons from the original nucleophilic water molecule, showing how they move in three rational steps to form a stable quaternary complex. It also suggests how two additional proton transfer steps can facilitate Pi release.

Controlling the Revolving and Rotating Motion Direction of Asymmetric Hexameric Nanomotor by Arginine Finger and Channel Chirality

Nanomotors in nanotechnology are as important as engines in daily life. Many ATPases are nanoscale biomotors classified into three categories based on the motion mechanisms in transporting substrates: linear, rotating, and the recently discovered revolving motion. Most biomotors adopt a multisubunit ring-shaped structure that hydrolyzes ATP to generate force. How these biomotors control the motion direction and regulate the sequential action of their multiple subunits is intriguing. Many ATPases are hexameric with each monomer containing a conserved arginine finger. This review focuses on recent findings on how the arginine finger controls motion direction and coordinates adjacent subunit interactions in both revolving and rotating biomotors. Mechanisms of intersubunit interactions and sequential movements of individual subunits are evidenced by the asymmetrical appearance of one dimer and four monomers in high-resolution structural complexes. The arginine finger is situated at the interface of two subunits and extends into the ATP binding pocket of the downstream subunit. An arginine finger mutation results in deficiency in ATP binding/hydrolysis, substrate binding, and transport, highlighting the importance of the arginine finger in regulating energy transduction and motor function. Additionally, the roles of channel chirality and channel size are discussed as related to controlling one-way trafficking and differentiating the revolving and rotating mechanisms. Finally, the review concludes by discussing the conformational changes and entropy conversion triggered by ATP binding/hydrolysis, offering a view different from the traditional concept of ATP-mediated mechanochemical energy coupling. The elucidation of the motion mechanism and direction control in ATPases could facilitate nanomotor fabrication in nanotechnology.

Importance of an Orchestrate Participation of each Individual Residue Present at a Catalytic Site

GTP hydrolysis is indispensable to keep a living cell healthy. Nature has evolved so many enzymes to enhance the slow GTP hydrolysis. Rab GTPases are evolved to regulate vesicle trafficking. GTPase activating proteins (GAPs) accelerates their intrinsic slow GTP hydrolysis in order to maintain the sustainability between cellular events. Any malfunction/interference in this hydrolysis disrupts normal cellular events and causes severe diseases. In this study, GTP hydrolysis mechanism of Rab33B catalyzed by TBC-domain GAP protein Gyp1p has been decoded using extensive ab initio QM/MM metadynamics simulations. An organized coupled movement of individual residues present at the catalytic site is found to be the key factor for this reaction. An unorganized coupled movement leads the hydrolysis through very high energy pathways. This also reveals that the chemical transformations occurring at a catalytic site are residue specific.

Assessing the Influence of Mutation on GTPase Transition States by Using X-ray Crystallography, 19 F NMR, and DFT Approaches

We report X-ray crystallographic and ¹⁹ F NMR studies of the G-protein RhoA complexed with MgF₃^- , GDP, and RhoGAP, which has the mutation Arg85'Ala. When combined with DFT calculations, these data permit the identification of changes in transition state (TS) properties. The X-ray data show how Tyr34 maintains solvent exclusion and the core H-bond network in the active site by relocating to replace the missing Arg85' sidechain. The ¹⁹ F NMR data show deshielding effects that indicate the main function of Arg85' is electronic polarization of the transferring phosphoryl group, primarily mediated by H-bonding to O^3G and thence to P^G . DFT calculations identify electron-density redistribution and pinpoint why the TS for guanosine 5'-triphosphate (GTP) hydrolysis is higher in energy when RhoA is complexed with RhoGAP_Arg85'Ala relative to wild-type (WT) RhoGAP. This study demonstrates that ¹⁹ F NMR measurements, in combination with X-ray crystallography and DFT calculations, can reliably dissect the response of small GTPases to site-specific modifications.

Differentiating the pre-hydrolysis states of wild-type and A59G mutant HRas: An insight through MD simulations

The most representative member of the Ras subfamily is its HRas isoform. Ras proteins being GTPases, possess an intrinsic activity to hydrolyze the GTP molecule to GDP. During the transition phases, between active and inactive states, P-loop and switch regions show maximum variations. Various hot-spot Ras mutants (G12V, A59G, Q61L etc) have been reported, that limit the protein's conformation in the permanent active state. In the present study, we aim to explore the structural dynamics of one such crucial mutant of Ras namely A59G which belongs to the conserved Switch II region of the protein. Approximately ∼15μs of Classical Molecular Dynamics (CMD) simulations have been carried out on the mutant and wild-type complexes. Further, a metadynamics simulation of 500ns was also carried out, which suggests an energy barrier of ∼9.56kcal/mol between wild-type and mutant conformation. We demonstrate the role of water molecule in maintaining the required interaction networks in the pre-hydrolysis state, its impact on A59G mutation, distinct orientation of the Gln61 residue in two conformations, disruption of crucial Gly60 and γ phosphate and the change in the Switch II region. The outcome of our study captures the pre-hydrolysis state of the HRas protein. It also establishes the fact that this mutation makes the movement of Switch II region and the conserved DXXGQ motif highly constrained, which is known to be an important requirement for hydrolysis. This suggests that the A59G mutation may decrease the rate of intrinsic hydrolysis as well as GAP-mediated hydrolysis.

Metal Fluorides: Tools for Structural and Computational Analysis of Phosphoryl Transfer Enzymes

The phosphoryl group, PO₃^-, is the dynamic structural unit in the biological chemistry of phosphorus. Its transfer from a donor to an acceptor atom, with oxygen much more prevalent than nitrogen, carbon, or sulfur, is at the core of a great majority of enzyme-catalyzed reactions involving phosphate esters, anhydrides, amidates, and phosphorothioates. The serendipitous discovery that the phosphoryl group could be labeled by "nuclear mutation," by substitution of PO₃^- by MgF₃^- or AlF₄^-, has underpinned the application of metal fluoride (MF _x ) complexes to mimic transition states for enzymatic phosphoryl transfer reactions, with sufficient stability for experimental analysis. Protein crystallography in the solid state and ¹⁹F NMR in solution have enabled direct observation of ternary and quaternary protein complexes embracing MF _x transition state models with precision. These studies have underpinned a radically new mechanistic approach to enzyme catalysis for a huge range of phosphoryl transfer processes, as varied as kinases, phosphatases, phosphomutases, and phosphohydrolases. The results, without exception, have endorsed trigonal bipyramidal geometry (tbp) for concerted, "in-line" stereochemistry of phosphoryl transfer. QM computations have established the validity of tbp MF _x complexes as reliable models for true transition states, delivering similar bond lengths, coordination to essential metal ions, and virtually identical hydrogen bond networks. The emergence of protein control of reactant orbital overlap between bond-forming species within enzyme transition states is a new challenging theme for wider exploration.

Local Mode Analysis: Decoding IR Spectra by Visualizing Molecular Details

Integration of experimental and computational approaches to investigate chemical reactions in proteins has proven to be very successful. Experimentally, time-resolved FTIR difference-spectroscopy monitors chemical reactions at atomic detail. To decode detailed structural information encoded in IR spectra, QM/MM calculations are performed. Here, we present a novel method which we call local mode analysis (LMA) for calculating IR spectra and assigning spectral IR-bands on the basis of movements of nuclei and partial charges from just a single QM/MM trajectory. Through LMA the decoding of IR spectra no longer requires several simulations or optimizations. The novel approach correlates the motions of atoms of a single simulation with the corresponding IR bands and provides direct access to the structural information encoded in IR spectra. Either the contributions of a particular atom or atom group to the complete IR spectrum of the molecule are visualized, or an IR-band is selected to visualize the corresponding structural motions. Thus, LMA decodes the detailed information contained in IR spectra and provides an intuitive approach for structural biologists and biochemists. The unique feature of LMA is the bidirectional analysis connecting structural details to spectral features and vice versa spectral features to molecular motions.

Theoretical vibrational spectroscopy of intermediates and the reaction mechanism of the guanosine triphosphate hydrolysis by the protein complex Ras-GAP

The structures and vibrational spectra of the reacting species upon guanosine triphosphate (GTP) hydrolysis to guanosine diphosphate and inorganic phosphate (Pi) trapped inside the protein complex Ras-GAP were analyzed following the results of QM/MM simulations. The frequencies of the phosphate vibrations referring to the reactants and to Pi were compared to those observed in the experimental FTIR studies. A good correlation between the theoretical and experimental vibrational data provides a strong support to the reaction mechanism of GTP hydrolysis by the Ras-GAP enzyme system revealed by the recent QM/MM modeling. Evolution of the vibrational bands associated with the inorganic phosphate Pi during the elementary stages of GTP hydrolysis is predicted.

Reaction Mechanism of Guanosine Triphosphate Hydrolysis by the Vision-Related Protein Complex Arl3-RP2

Complexes of small GTPases with GTPase-activating proteins have been intensively studied with the main focus on the complex of H-Ras with p120GAP (Ras-GAP). The detailed mechanism of GTP hydrolysis is still unresolved. To clarify it, we calculated the energy profile of GTP hydrolysis in the active site of a recently characterized vision-related member of this family, the Arl3-RP2 complex. The mechanism suggested in this study retains the main features of GTP hydrolysis by the Ras-GAP complex, but the relative energies of the corresponding intermediates are different and an additional intermediate exists in the Arl3-RP2 complex compared with the Ras-GAP. These differences arise from small deviations in the catalytic arginine conformation of the active site. In the Arl3-RP2 complex, the first two intermediates, corresponding to the Pγ-Oβγ bond cleavage and the glutamine-assisted proton transfer, are almost isoenergetic with the ES complex. Numerical simulations of the kinetic curves demonstrate that the concentrations of these intermediates are comparable with that of ES during the reaction. The calculated IR spectra reveal specific vibrational bands, corresponding to these intermediates. These specific features of the Arl3-RP2 complex open the opportunity to identify spectroscopically two more reaction intermediates in GTP hydrolysis in addition to the ES and EP complexes.

(19)F NMR and DFT Analysis Reveal Structural and Electronic Transition State Features for RhoA-Catalyzed GTP Hydrolysis

Molecular details for RhoA/GAP catalysis of the hydrolysis of GTP to GDP are poorly understood. We use (19)F NMR chemical shifts in the MgF3(-) transition state analogue (TSA) complex as a spectroscopic reporter to indicate electron distribution for the γ-PO3(-) oxygens in the corresponding TS, implying that oxygen coordinated to Mg has the greatest electron density. This was validated by QM calculations giving a picture of the electronic properties of the transition state (TS) for nucleophilic attack of water on the γ-PO3(-) group based on the structure of a RhoA/GAP-GDP-MgF3(-) TSA complex. The TS model displays a network of 20 hydrogen bonds, including the GAP Arg85' side chain, but neither phosphate torsional strain nor general base catalysis is evident. The nucleophilic water occupies a reactive location different from that in multiple ground state complexes, arising from reorientation of the Gln-63 carboxamide by Arg85' to preclude direct hydrogen bonding from water to the target γ-PO3(-) group.

Why does mutation of Gln61 in Ras by the nitro analog NGln maintain activity of Ras-GAP in hydrolysis of guanosine triphosphate?

Interpretation of the experiments showing that the Ras-GAP protein complex maintains activity in guanosine triphosphate (GTP) hydrolysis upon replacement of Glu61 in Ras with its unnatural nitro analog, NGln, is an important issue for understanding details of chemical transformations at the enzyme active site. By using molecular modeling we demonstrate that both glutamine and its nitro analog in the aci-nitro form participate in the reaction of GTP hydrolysis at the stages of proton transfer and formation of inorganic phosphate. The computed structures and the energy profiles for the complete pathway from the enzyme-substrate to enzyme-product complexes for the wild-type and mutated Ras suggest that the reaction mechanism is not affected by this mutation.

Hydrolysis of Guanosine Triphosphate (GTP) by the Ras·GAP Protein Complex: Reaction Mechanism and Kinetic Scheme

Molecular mechanisms of the hydrolysis of guanosine triphosphate (GTP) to guanosine diphosphate (GDP) and inorganic phosphate (Pi) by the Ras·GAP protein complex are fully investigated by using modern modeling tools. The previously hypothesized stages of the cleavage of the phosphorus-oxygen bond in GTP and the formation of the imide form of catalytic Gln61 from Ras upon creation of Pi are confirmed by using the higher-level quantum-based calculations. The steps of the enzyme regeneration are modeled for the first time, providing a comprehensive description of the catalytic cycle. It is found that for the reaction Ras·GAP·GTP·H2O → Ras·GAP·GDP·Pi, the highest barriers correspond to the process of regeneration of the active site but not to the process of substrate cleavage. The specific shape of the energy profile is responsible for an interesting kinetic mechanism of the GTP hydrolysis. The analysis of the process using the first-passage approach and consideration of kinetic equations suggest that the overall reaction rate is a result of the balance between relatively fast transitions and low probability of states from which these transitions are taking place. Our theoretical predictions are in excellent agreement with available experimental observations on GTP hydrolysis rates.

Computational characterization of the chemical step in the GTP hydrolysis by Ras-GAP for the wild-type and G13V mutated Ras

The free energy profiles for the chemical reaction of the guanosine triphosphate hydrolysis GTP + H2O → GDP + Pi by Ras-GAP for the wild-type and G13V mutated Ras were computed by using molecular dynamics protocols with the QM(ab initio)/MM potentials. The results are consistent with the recent measurements of reaction kinetics in Ras-GAP showing about two-order reduction of the rate constant upon G13V mutation in Ras: the computed activation barrier on the free energy profile is increased by 3 kcal/mol upon the G13V replacement. The major reason for a higher energy barrier is a shift of the "arginine finger" (R789 from GAP) from the favorable position in the active site. The results of simulations provide support for the mechanism of the reference reaction according to which the Q61 side chain directly participates in chemical transformations at the proton transfer stage.

Modeling the mechanisms of biological GTP hydrolysis

Enzymes that hydrolyze GTP are currently in the spotlight, due to their molecular switch mechanism that controls many cellular processes. One of the best-known classes of these enzymes are small GTPases such as members of the Ras superfamily, which catalyze the hydrolysis of the γ-phosphate bond in GTP. In addition, the availability of an increasing number of crystal structures of translational GTPases such as EF-Tu and EF-G have made it possible to probe the molecular details of GTP hydrolysis on the ribosome. However, despite a wealth of biochemical, structural and computational data, the way in which GTP hydrolysis is activated and regulated is still a controversial topic and well-designed simulations can play an important role in resolving and rationalizing the experimental data. In this review, we discuss the contributions of computational biology to our understanding of GTP hydrolysis on the ribosome and in small GTPases.

Modeling the role of G12V and G13V Ras mutations in the Ras-GAP-catalyzed hydrolysis reaction of guanosine triphosphate

Cancer-associated point mutations in Ras, in particular, at glycine 12 and glycine 13, affect the normal cycle between inactive GDP-bound and active GTP-bound states. In this work, the role of G12V and G13V replacements in the GAP-stimulated intrinsic GTP hydrolysis reaction in Ras is studied using molecular dynamics (MD) simulations with quantum mechanics/molecular mechanics (QM/MM) potentials. A model molecular system was constructed by motifs of the relevant crystal structure (Protein Data Bank entry 1WQ1 ). QM/MM optimization of geometry parameters in the Ras-GAP-GTP complex and QM/MM-MD simulations were performed with a quantum subsystem comprising a large fraction of the enzyme active site. For the system with wild-type Ras, the conformations fluctuated near the structure ready to be involved in the efficient chemical reaction leading to the cleavage of the phosphorus-oxygen bond in GTP upon approach of the properly aligned catalytic water molecule. Dynamics of the system with the G13V mutant is characterized by an enhanced flexibility in the area occupied by the γ-phosphate group of GTP, catalytic water, and the side chains of Arg789 and Gln61, which should somewhat hinder fast chemical steps. Conformational dynamics of the system with the G12V mutant shows considerable displacement of the Gln61 side chain and catalytic water from their favorable arrangement in the active site that may lead to a marked reduction in the reaction rate. The obtained computational results correlate well with the recent kinetic measurements of the Ras-GAP-catalyzed hydrolysis of GTP.

Overview of simulation studies on the enzymatic activity and conformational dynamics of the GTPase Ras

Over the last 40 years, we have learnt a great deal about the Ras onco-proteins. These intracellular molecular switches are essential for the function of a variety of physiological processes, including signal transduction cascades responsible for cell growth and proliferation. Molecular simulations and free energy calculations have played an essential role in elucidating the conformational dynamics and energetics underlying the GTP hydrolysis reaction catalysed by Ras. Here we present an overview of the main lessons from molecular simulations on the GTPase reaction and conformational dynamics of this important anti-cancer drug target. In the first part, we summarise insights from quantum mechanical and combined quantum mechanical/molecular mechanical simulations as well as other free energy methods and highlight consensus viewpoints as well as remaining controversies. The second part provides a very brief overview of new insights emerging from large-scale molecular dynamics simulations. We conclude with a perspective regarding future studies of Ras where computational approaches will likely play an active role.

Probing the wild-type HRas activation mechanism using steered molecular dynamics, understanding the energy barrier and role of water in the activation

Ras is one of the most common oncogenes in human cancers. It belongs to a family of GTPases that functions as binary conformational switches by timely switching of their conformations from GDP to GTP and vice versa. It attains the final active state structure via an intermediate GTP-bound state. The transition between these states is a millisecond-time-scale event. This makes studying this mechanism beyond the scope of classical molecular dynamics. In the present study, we describe the activation pathway of the HRas protein complex along the distance-based reaction coordinate using steered molecular dynamics. Approximately ~720 ns of MD simulations using CMD and SMD was performed. We demonstrated the change in orientation and arrangement of the two switch regions and the role of various hydrogen bonds during the activation process. The weighted histogram analysis method was also performed, and the potential of mean force was calculated between the inactive and active via the intermediate state (state 1) of HRas. The study indicates that water seems to play a crucial role in the activation process and to transfer the HRas protein from its intermediate state to the fully active state. The implications of our study hereby suggest that the HRas activation mechanism is a multistep process. It starts from the inactive state to an intermediate state 1 followed by trapping of water molecules and flipping of the Thr35 residue to form a fully active state (state 2). This state 2 also comprises Gly60, Thr35, GTP, Mg(2+) and water-forming stable interactions.

Lessons from computer simulations of Ras proteins in solution and in membrane

BACKGROUND

A great deal has been learned over the last several decades about the function of Ras proteins in solution and membrane environments. While much of this knowledge has been derived from a plethora of experimental techniques, computer simulations have also played a substantial role.

SCOPE OF REVIEW

Our goal here is to summarize the contribution of molecular simulations to our current understanding of normal and aberrant Ras function. We focus on lessons from molecular dynamics simulations in aqueous and membrane environments.

MAJOR CONCLUSIONS

The central message is that a close interaction between theory and simulation on the one hand and cell-biological, spectroscopic and other experimental approaches on the other has played, and will likely continue to play, a vital role in Ras research.

GENERAL SIGNIFICANCE

Atomistic insights emerging from detailed simulations of Ras in solution and in bilayers may be the key to unlock the secret that to date prevented development of selective anti-Ras inhibitors for cancer therapy.

Substrate recognition by norovirus polymerase: microsecond molecular dynamics study

Molecular dynamics simulations of complexes between Norwalk virus RNA dependent RNA polymerase and its natural CTP and 2dCTP (both containing the O5'-C5'-C4'-O4' sequence of atoms bridging the triphosphate and sugar moiety) or modified coCTP (C5'-O5'-C4'-O4'), cocCTP (C5'-O5'-C4'-C4'') substrates were produced by means of CUDA programmable graphical processing units and the ACEMD software package. It enabled us to gain microsecond MD trajectories clearly showing that similar nucleoside triphosphates can bind surprisingly differently into the active site of the Norwalk virus RNA dependent RNA polymerase. It corresponds to their different modes of action (CTP-substrate, 2dCTP-poor substrate, coCTP-chain terminator, cocCTP-inhibitor). Moreover, extremely rare events-as repetitive pervasion of Arg182 into a potentially reaction promoting arrangement-were captured.

Why nature really chose phosphate

Phosphoryl transfer plays key roles in signaling, energy transduction, protein synthesis, and maintaining the integrity of the genetic material. On the surface, it would appear to be a simple nucleophile displacement reaction. However, this simplicity is deceptive, as, even in aqueous solution, the low-lying d-orbitals on the phosphorus atom allow for eight distinct mechanistic possibilities, before even introducing the complexities of the enzyme catalyzed reactions. To further complicate matters, while powerful, traditional experimental techniques such as the use of linear free-energy relationships (LFER) or measuring isotope effects cannot make unique distinctions between different potential mechanisms. A quarter of a century has passed since Westheimer wrote his seminal review, 'Why Nature Chose Phosphate' (Science 235 (1987), 1173), and a lot has changed in the field since then. The present review revisits this biologically crucial issue, exploring both relevant enzymatic systems as well as the corresponding chemistry in aqueous solution, and demonstrating that the only way key questions in this field are likely to be resolved is through careful theoretical studies (which of course should be able to reproduce all relevant experimental data). Finally, we demonstrate that the reason that nature really chose phosphate is due to interplay between two counteracting effects: on the one hand, phosphates are negatively charged and the resulting charge-charge repulsion with the attacking nucleophile contributes to the very high barrier for hydrolysis, making phosphate esters among the most inert compounds known. However, biology is not only about reducing the barrier to unfavorable chemical reactions. That is, the same charge-charge repulsion that makes phosphate ester hydrolysis so unfavorable also makes it possible to regulate, by exploiting the electrostatics. This means that phosphate ester hydrolysis can not only be turned on, but also be turned off, by fine tuning the electrostatic environment and the present review demonstrates numerous examples where this is the case. Without this capacity for regulation, it would be impossible to have for instance a signaling or metabolic cascade, where the action of each participant is determined by the fine-tuned activity of the previous piece in the production line. This makes phosphate esters the ideal compounds to facilitate life as we know it.

N-Ras forms dimers at POPC membranes

Ras is a central regulator of cellular signaling pathways. It is mutated in 20-30% of human tumors. To perform its function, Ras has to be bound to a membrane by a posttranslationally attached lipid anchor. Surprisingly, we identified here dimerization of membrane anchored Ras by combining attenuated total reflectance Fourier transform infrared spectroscopy, biomolecular simulations, and Förster resonance energy transfer experiments. By analyzing x-ray structural models and molecular-dynamics simulations, we propose a dimerization interface between α-helices 4 and 5 and the loop between β2 and β3. This seems to explain why the residues D47, E49, R135, R161, and R164 of this interface are influencing Ras signaling in cellular physiological experiments, although they are not positioned in the catalytic site. Dimerization could catalyze nanoclustering, which is well accepted for membrane-bound Ras. The interface could provide a new target for a seemingly novel type of small molecule interfering with signal transduction in oncogenic Ras mutants.

Ras and GTPase-activating protein (GAP) drive GTP into a precatalytic state as revealed by combining FTIR and biomolecular simulations

Members of the Ras superfamily regulate many cellular processes. They are down-regulated by a GTPase reaction in which GTP is cleaved into GDP and P(i) by nucleophilic attack of a water molecule. Ras proteins accelerate GTP hydrolysis by a factor of 10(5) compared to GTP in water. GTPase-activating proteins (GAPs) accelerate hydrolysis by another factor of 10(5) compared to Ras alone. Oncogenic mutations in Ras and GAPs slow GTP hydrolysis and are a factor in many cancers. Here, we elucidate in detail how this remarkable catalysis is brought about. We refined the protein-bound GTP structure and protein-induced charge shifts within GTP beyond the current resolution of X-ray structural models by combining quantum mechanics and molecular mechanics simulations with time-resolved Fourier-transform infrared spectroscopy. The simulations were validated by comparing experimental and theoretical IR difference spectra. The reactant structure of GTP is destabilized by Ras via a conformational change from a staggered to an eclipsed position of the nonbridging oxygen atoms of the γ- relative to the β-phosphates and the further rotation of the nonbridging oxygen atoms of α- relative to the β- and γ-phosphates by GAP. Further, the γ-phosphate becomes more positive although two of its oxygen atoms remain negative. This facilitates the nucleophilic attack by the water oxygen at the phosphate and proton transfer to the oxygen. Detailed changes in geometry and charge distribution in the ligand below the resolution of X-ray structure analysis are important for catalysis. Such high resolution appears crucial for the understanding of enzyme catalysis.

The role of magnesium for geometry and charge in GTP hydrolysis, revealed by quantum mechanics/molecular mechanics simulations

The coordination of the magnesium ion in proteins by triphosphates plays an important role in catalytic hydrolysis of GTP or ATP, either in signal transduction or energy conversion. For example, in Ras the magnesium ion contributes to the catalysis of GTP hydrolysis. The cleavage of GTP to GDP and P(i) in Ras switches off cellular signaling. We analyzed GTP hydrolysis in water, Ras, and Ras·Ras-GTPase-activating protein using quantum mechanics/molecular mechanics simulations. By comparison of the theoretical IR-difference spectra for magnesium ion coordinated triphosphate to experimental ones, the simulations are validated. We elucidated thereby how the magnesium ion contributes to catalysis. It provides a temporary storage for the electrons taken from the triphosphate and it returns them after bond cleavage and P(i) release back to the diphosphate. Furthermore, the Ras·Mg(2+) complex forces the triphosphate into a stretched conformation in which the β- and γ-phosphates are coordinated in a bidentate manner. In this conformation, the triphosphate elongates the bond, which has to be cleaved during hydrolysis. Furthermore, the γ-phosphate adopts a more planar structure, driving the conformation of the molecule closer to the hydrolysis transition state. GTPase-activating protein enhances these changes in GTP conformation and charge distribution via the intruding arginine finger.

The specific vibrational modes of GTP in solution and bound to Ras: a detailed theoretical analysis by QM/MM simulations

The hydrolysis of guanosine triphosphate (GTP) in general, and especially by GTPases like the Ras protein, is in the focus of biological investigations. A huge amount of experimental data from Fourier-transformed infrared studies is currently available, and many vibrational bands of free GTP, GTP·Mg(2+), and Ras·GTP·Mg(2+) in solution have been assigned by isotopic labeling. In the Ras environment, bands between 800 cm(-1) and 1300 cm(-1) have already been assigned, but not those below 800 cm(-1). The combination of quantum and molecular mechanics (QM/MM) methods takes the quantum effects for selected relevant atoms into account. This provides structural details, vibrational frequencies and electron distributions of the region of interest. We therefore used MM and QM/MM simulations to investigate the normal vibrational modes of GTP, GTP·Mg(2+), and Ras·GTP·Mg(2+) in solution, and assigned the vibrational frequencies for each normal vibration mode. In this study, the quantum box contains the nucleoside and the Mg(2+). The comparison of calculated and experimental vibrational spectra provides a very good control for the quality of the calculations. Structurally, MM and QM/MM simulations reveal a stable tridentate coordination of the Mg(2+) by GTP in water, and a stable bidentate coordination by GTP in complex with Ras. For validation, we compare the calculated frequencies and isotopic shifts with the experimental results available in the range of 800 cm(-1) to 1300 cm(-1). For the first time we suggest band assignments of the vibrational modes below 800 cm(-1) by comparison of calculated and experimental spectra.

Free energy simulations of a GTPase: GTP and GDP binding to archaeal initiation factor 2

Archaeal initiation factor 2 (aIF2) is a protein involved in the initiation of protein biosynthesis. In its GTP-bound, “ON” conformation, aIF2 binds an initiator tRNA and carries it to the ribosome. In its GDP-bound, “OFF” conformation, it dissociates from tRNA. To understand the specific binding of GTP and GDP and its dependence on the ON or OFF conformational state of aIF2, molecular dynamics free energy simulations (MDFE) are a tool of choice. However, the validity of the computed free energies depends on the simulation model, including the force field and the boundary conditions, and on the extent of conformational sampling in the simulations. aIF2 and other GTPases present specific difficulties; in particular, the nucleotide ligand coordinates a divalent Mg²⁺ ion, which can polarize the electronic distribution of its environment. Thus, a force field with an explicit treatment of electronic polarizability could be necessary, rather than a simpler, fixed charge force field. Here, we begin by comparing a fixed charge force field to quantum chemical calculations and experiment for Mg²⁺:phosphate binding in solution, with the force field giving large errors. Next, we consider GTP and GDP bound to aIF2 and we compare two fixed charge force fields to the recent, polarizable, AMOEBA force field, extended here in a simple, approximate manner to include GTP. We focus on a quantity that approximates the free energy to change GTP into GDP. Despite the errors seen for Mg²⁺:phosphate binding in solution, we observe a substantial cancellation of errors when we compare the free energy change in the protein to that in solution, or when we compare the protein ON and OFF states. Finally, we have used the fixed charge force field to perform MDFE simulations and alchemically transform GTP into GDP in the protein and in solution. With a total of about 200 ns of molecular dynamics, we obtain good convergence and a reasonable statistical uncertainty, comparable to the force field uncertainty, and somewhat lower than the predicted GTP/GDP binding free energy differences. The sign and magnitudes of the differences can thus be interpreted at a semiquantitative level, and are found to be consistent with the experimental binding preferences of ON- and OFF-aIF2.

Label-free screening of drug-protein interactions by time-resolved Fourier transform infrared spectroscopic assays exemplified by Ras interactions

Time-resolved Fourier transform infrared (FT-IR) spectroscopy can reveal molecular details of protein interactions. Analysis of difference spectra selects the absorptions of respective protein groups involved in an interaction against the background of the whole sample. By comparison of the same difference spectrum with and without a small molecule, one can determine whether the small molecule interferes with the protein or not. Usually a marker band of a specific residue of the protein is monitored. Here, we show three different time-resolved FT-IR assays detecting interactions of potential small molecules for molecular therapy with the GTPase Ras as an example for small GTPase binding proteins. Ras regulates signal transduction processes through a switching mechanism, cycling between an active "on" GTP-bound form and an inactive "off" GDP-bound state. Molecular defects in Ras can impair the ability of Ras and the Ras-RasGAP complex to hydrolyze GTP, contributing to uncontrolled cell growth and cancer. Oncogenic mutated Ras is found in about 30% of all cancer cells. We show in vitro assays, indicating (I) the shift of Ras into its "off" conformation, which inhibits the Ras pathway; (II) down-regulation of Ras signaling by changes in the Ras-Raf effector interaction; and (III) down-regulation of Ras signaling pathway by catalyzing GTP hydrolysis. Since almost all molecules have characteristic marker bands in the infrared, time-resolved FT-IR spectroscopy can be used label-free. No artificial nucleotides that could influence the interaction are needed. Both, sample preparation and evaluation can be automated in order to allow for high-throughput screening.

Tyr39 of ran preserves the Ran.GTP gradient by inhibiting GTP hydrolysis

Ran is a member of the superfamily of small GTPases, which cycle between a GTP-bound "on" and a GDP-bound "off" state. Ran regulates nuclear transport. In order to maintain a gradient of excess Ran.GTP within the nucleoplasm and excess Ran.GDP within the cytoplasm, the hydrolysis of Ran.GTP in the nucleoplasm should be prevented, whereas in the cytoplasm, hydrolysis is catalyzed by Ran.GAP (GTPase-activating protein). In this article, we investigate the GTPase reaction of Ran in complex with its binding protein Ran-binding protein 1 by time-resolved Fourier transform infrared spectroscopy: We show that the slowdown of the intrinsic hydrolysis of RanGTP is accomplished by tyrosine 39, which is probably misplacing the attacking water. We monitored the interaction of Ran with RanGAP, which reveals two reactions steps. By isotopic labeling of Ran and RanGAP, we were able to assign the first step to a small conformational change within the catalytic site. The following bond breakage is the rate-limiting step of hydrolysis. An intermediate of protein-bound phosphate as found for Ras or Rap systems is kinetically unresolved. This demonstrates that despite the structural similarity among the G-domain of the GTPases, different reaction mechanisms are utilized.

Simulated 18O kinetic isotope effects in enzymatic hydrolysis of guanosine triphosphate

We compare the computed on the base of quantum mechanical-molecular mechanical (QM/MM) modeling kinetic isotope effects (KIEs) for a series of the (18)O-labeled substrates in enzymatic hydrolysis of guanosine triphosphate (GTP) with those measured experimentally. Following the quantitative structure-activity relationship concept, we introduce the correlation between KIEs and structure of substrates with the help of a labeling index, which also aids better imaging of presentation of both experimental and theoretical data. An evident correlation of the computed and measured KIEs provides support to the predominantly dissociative-type reaction mechanism of enzymatic GTP hydrolysis predicted in QM/MM simulations.

The hydrolysis activity of adenosine triphosphate in myosin: a theoretical analysis of anomeric effects and the nature of the transition state

Combined quantum mechanical/molecular mechanical (QM/MM) calculations with density functional theory are employed to analyze two issues related to the hydrolysis activity of adenosine triphosphate (ATP) in myosin. First, we compare the geometrical properties and electronic structure of ATP in the open (post-rigor) and closed (pre-powerstroke) active sites of the myosin motor domain. Compared to both solution and the open active site cases, the scissile P(gamma)-O(3beta) bond of ATP in the closed active site is shown to be substantially elongated. Natural bond orbital (NBO) analysis clearly shows that this structural feature is correlated with the stronger anomeric effects in the closed active site, which involve charge transfers from the lone pairs in the nonbridging oxygen in the gamma-phosphate to the antibonding orbital of the scissile bond. However, an energetic analysis finds that the ATP molecule is not significantly destabilized by the P(gamma)-O(3beta) bond elongation. Therefore, despite the notable perturbations in the geometry and electronic structure of ATP as its environment changes from solution to the hydrolysis-competent active site, ground-state destabilization is unlikely to play a major role in enhancing the hydrolysis activity in myosin. Second, two-dimensional potential energy maps are used to better characterize the energetic landscape near the hydrolysis transition state. The results indicate that the transition-state region is energetically flat and a range of structures representative of different mechanisms according to the classical nomenclature (e.g., "associative", "dissociative", and "concerted") are very close in energy. Therefore, at least in the case of ATP hydrolysis in myosin, the energetic distinction between different reaction mechanisms following the conventional nomenclature is likely small. This study highlights the importance of (i) explicitly evaluating the relevant energetic properties for determining whether a factor is essential to catalysis and (ii) broader explorations of the energy landscape beyond saddle points (even on free-energy surface) for characterizing the molecular mechanism of catalysis.

Structure-based virtual screening for novel inhibitors of Japanese encephalitis virus NS3 helicase/nucleoside triphosphatase

Japanese encephalitis (JE) is a significant cause of human morbidity and mortality throughout Asia and Africa. Vaccines have reduced the incidence of JE in some countries, but no specific antiviral therapy is currently available. The NS3 protein of Japanese encephalitis virus (JEV) is a multifunctional protein combining protease, helicase and nucleoside 5′‐triphosphatase (NTPase) activities. The crystal structure of the catalytic domain of this protein has recently been solved using a roentgenographic method. This enabled structure‐based virtual screening for novel inhibitors of JEV NS3 helicase/NTPase. The aim of the present research was to identify novel potent medicinal substances for the treatment of JE. In the first step of studies, the natural ligand ATP and two known JEV NS3 helicase/NTPase inhibitors were docked to their molecular target. The refined structure of the enzyme was used to construct a pharmacophore model for JEV NS3 helicase/NTPase inhibitors. The freely available ZINC database of lead‐like compounds was then screened for novel inhibitors. About 1 161 000 compounds have been screened and 15 derivatives of the highest scores have been selected. These compounds were docked to the JEV NS3 helicase/NTPase to examine their binding mode and verify screening results by consensus scoring procedure.

QM/MM methods for biomolecular systems

Combined quantum-mechanics/molecular-mechanics (QM/MM) approaches have become the method of choice for modeling reactions in biomolecular systems. Quantum-mechanical (QM) methods are required for describing chemical reactions and other electronic processes, such as charge transfer or electronic excitation. However, QM methods are restricted to systems of up to a few hundred atoms. However, the size and conformational complexity of biopolymers calls for methods capable of treating up to several 100,000 atoms and allowing for simulations over time scales of tens of nanoseconds. This is achieved by highly efficient, force-field-based molecular mechanics (MM) methods. Thus to model large biomolecules the logical approach is to combine the two techniques and to use a QM method for the chemically active region (e.g., substrates and co-factors in an enzymatic reaction) and an MM treatment for the surroundings (e.g., protein and solvent). The resulting schemes are commonly referred to as combined or hybrid QM/MM methods. They enable the modeling of reactive biomolecular systems at a reasonable computational effort while providing the necessary accuracy.

Mechanism of the chemical step for the guanosine triphosphate (GTP) hydrolysis catalyzed by elongation factor Tu

Elongation factor Tu (EF-Tu), the protein responsible for delivering aminoacyl-tRNAs (aa-tRNAs) to ribosomal A site during translation, belongs to the group of guanosine-nucleotide (GTP/GDP) binding proteins. Its active 'on'-state corresponds to the GTP-bound form, while the inactive 'off'-state corresponds to the GDP-bound form. In this work we focus on the chemical step, GTP+H(2)O-->GDP+Pi, of the hydrolysis mechanism. We apply molecular modeling tools including molecular dynamics simulations and the combined quantum mechanical-molecular mechanical calculations for estimates of reaction energy profiles for two possible arrangements of switch II regions of EF-Tu. In the first case we presumably mimic binding of the ternary complex EF-Tu.GTP.aa-tRNA to the ribosome and allow the histidine (His85) side chain of the protein to approach the reaction active site. In the second case, corresponding to the GTP hydrolysis by EF-Tu alone, the side chain of His85 stays away from the active site, and the chemical reaction GTP+H(2)O-->GDP+Pi proceeds without participation of the histidine but through water molecules. In agreement with the experimental observations which distinguish rate constants for the fast chemical reaction in EF-Tu.GTP.aa-tRNA.ribosome and the slow spontaneous GTP hydrolysis in EF-Tu, we show that the activation energy barrier for the first scenario is considerably lower compared to that of the second case.

The GAP arginine finger movement into the catalytic site of Ras increases the activation entropy

Members of the Ras superfamily of small G proteins play key roles in signal transduction pathways, which they control by GTP hydrolysis. They are regulated by GTPase activating proteins (GAPs). Mutations that prevent hydrolysis cause severe diseases including cancer. A highly conserved "arginine finger" of GAP is a key residue. Here, we monitor the GTPase reaction of the Ras.RasGAP complex at high temporal and spatial resolution by time-resolved FTIR spectroscopy at 260 K. After triggering the reaction, we observe as the first step a movement of the switch-I region of Ras from the nonsignaling "off" to the signaling "on" state with a rate of 3 s(-1). The next step is the movement of the "arginine finger" into the active site of Ras with a rate of k(2) = 0.8 s(-1). Once the arginine points into the binding pocket, cleavage of GTP is fast and the protein-bound P(i) intermediate forms. The switch-I reversal to the "off" state, the release of P(i), and the movement of arginine back into an aqueous environment is observed simultaneously with k(3) = 0.1 s(-1), the rate-limiting step. Arrhenius plots for the partial reactions show that the activation energy for the cleavage reaction is lowered by favorable positive activation entropy. This seems to indicate that protein-bound structured water molecules are pushed by the "arginine finger" movement out of the binding pocket into the bulk water. The proposed mechanism shows how the high activation barrier for phosphoryl transfer can be reduced by splitting into partial reactions separated by a P(i)-intermediate.