Relating Protein Motion to Catalysis

Guidelines for tunneling in enzymes

Here we extend the engineering descriptions of simple, single-electron-tunneling chains common in oxidoreductases to quantify sequential oxidation-reduction rates of two-or-more electron cofactors and substrates. We identify when nicotinamides may be vulnerable to radical mediated oxidation-reduction and merge electron-tunneling expressions with the chemical rate expressions of Eyring. The work provides guidelines for the construction of new artificial oxidoreductases inspired by Nature but adopting independent design and redox engineering.

At the dawn of the 21st century: Is dynamics the missing link for understanding enzyme catalysis?

Enzymes play a key role in almost all biological processes, accelerating a variety of metabolic reactions as well as controlling energy transduction, the transcription, and translation of genetic information, and signaling. They possess the remarkable capacity to accelerate reactions by many orders of magnitude compared to their uncatalyzed counterparts, making feasible crucial processes that would otherwise not occur on biologically relevant timescales. Thus, there is broad interest in understanding the catalytic power of enzymes on a molecular level. Several proposals have been put forward to try to explain this phenomenon, and one that has rapidly gained momentum in recent years is the idea that enzyme dynamics somehow contributes to catalysis. This review examines the dynamical proposal in a critical way, considering basically all reasonable definitions, including (but not limited to) such proposed effects as "coupling between conformational and chemical motions," "landscape searches" and "entropy funnels." It is shown that none of these proposed effects have been experimentally demonstrated to contribute to catalysis, nor are they supported by consistent theoretical studies. On the other hand, it is clarified that careful simulation studies have excluded most (if not all) dynamical proposals. This review places significant emphasis on clarifying the role of logical definitions of different catalytic proposals, and on the need for a clear formulation in terms of the assumed potential surface and reaction coordinate. Finally, it is pointed out that electrostatic preorganization actually accounts for the observed catalytic effects of enzymes, through the corresponding changes in the activation free energies.

Dynamics based alignment of proteins: an alternative approach to quantify dynamic similarity

Background

The dynamic motions of many proteins are central to their function. It therefore follows that the dynamic requirements of a protein are evolutionary constrained. In order to assess and quantify this, one needs to compare the dynamic motions of different proteins. Comparing the dynamics of distinct proteins may also provide insight into how protein motions are modified by variations in sequence and, consequently, by structure. The optimal way of comparing complex molecular motions is, however, far from trivial. The majority of comparative molecular dynamics studies performed to date relied upon prior sequence or structural alignment to define which residues were equivalent in 3-dimensional space.

Results

Here we discuss an alternative methodology for comparative molecular dynamics that does not require any prior alignment information. We show it is possible to align proteins based solely on their dynamics and that we can use these dynamics-based alignments to quantify the dynamic similarity of proteins. Our method was tested on 10 representative members of the PDZ domain family.

Conclusions

As a result of creating pair-wise dynamics-based alignments of PDZ domains, we have found evolutionarily conserved patterns in their backbone dynamics. The dynamic similarity of PDZ domains is highly correlated with their structural similarity as calculated with Dali. However, significant differences in their dynamics can be detected indicating that sequence has a more refined role to play in protein dynamics than just dictating the overall fold. We suggest that the method should be generally applicable.

Nucleotide-dependent conformational change governs specificity and analog discrimination by HIV reverse transcriptase

Single turnover studies on HIV reverse transcriptase suggest that nucleoside analogs bind more tightly to the enzyme than normal substrates, contrary to rational structural predictions. Here we resolve these controversies by monitoring the kinetics of nucleotide-induced changes in enzyme structure. We show that the specificity constant for incorporation of a normal nucleotide (dCTP) is determined solely by the rate of binding (including isomerization) because isomerization to the closed complex commits the substrate to react. In contrast, a nucleoside analog (3TC-TP, triphosphate form of lamivudine) is incorporated slowly, allowing the conformational change to come to equilibrium and revealing tight nucleotide binding. Our data reconcile previously conflicting reports suggesting that nucleotide analogs bind tighter than normal nucleotides. Rather, dCTP and 3TC-TP bind with nearly equal affinities, but the binding of dCTP never reaches equilibrium. Discrimination against 3TC-TP is based on the slower rate of incorporation due to misalignment of the substrate and/or catalytic residues.

Single-molecule force spectroscopy approach to enzyme catalysis

Enzyme catalysis has been traditionally studied using a diverse set of techniques such as bulk biochemistry, x-ray crystallography, and NMR. Recently, single-molecule force spectroscopy by atomic force microscopy has been used as a new tool to study the catalytic properties of an enzyme. In this approach, a mechanical force ranging up to hundreds of piconewtons is applied to the substrate of an enzymatic reaction, altering the conformational energy of the substrate-enzyme interactions during catalysis. From these measurements, the force dependence of an enzymatic reaction can be determined. The force dependence provides valuable new information about the dynamics of enzyme catalysis with sub-angstrom resolution, a feat unmatched by any other current technique. To date, single-molecule force spectroscopy has been applied to gain insight into the reduction of disulfide bonds by different enzymes of the thioredoxin family. This minireview aims to present a perspective on this new approach to study enzyme catalysis and to summarize the results that have already been obtained from it. Finally, the specific requirements that must be fulfilled to apply this new methodology to any other enzyme will be discussed.

On the Origin of the Chemical Barrier and Tunneling in Enzymes

This paper presents both a review of some recent results from our group and experimental groups, and some new theoretical results all of which are helping to form a more physically rigorous picture of the process of enzymatic catalysis. A common classical picture of enzymatic catalysis is the transition state tight binding model. Schwartz and Schramm1 have recently argued from both theoretical and experimental results that this picture is incorrect. We now investigate what the nature of barriers might be in enzymatic reactions, and what this viewpoint might imply for tunneling in a hydrogen transfer enzyme. For lactate dehydrogenase we conclude that the enzymes role in catalysis is at least partially to hunt through configuration space for those configurations that minimize chemical free energy barriers. Those configurations do not seem to be stable basins on the free energy surface, and in fact the overall free energy barrier to reaction may well largely be due to this stochastic hunt - both probabilistically and energetically. We suggest further computations to test this hypothesis.

Temperature and pH dependence of enzyme-catalyzed hydrolysis of trans-methylstyrene oxide. A unifying kinetic model for observed hysteresis, cooperativity, and regioselectivity

The underlying enzyme kinetics behind the regioselective promiscuity shown by epoxide hydrolases toward certain epoxides has been studied. The effects of temperature and pH on regioselectivity were investigated by analyzing the stereochemistry of hydrolysis products of (1R,2R)-trans-2-methylstyrene oxide between 14-46 degrees C and pH 6.0-9.0, either catalyzed by the potato epoxide hydrolase StEH1 or in the absence of enzyme. In the enzyme-catalyzed reaction, a switch of preferred epoxide carbon that is subjected to nucleophilic attack is observed at pH values above 8. The enzyme also displays cooperativity in substrate saturation plots when assayed at temperatures < or = 30 degrees C and at intermediate pH. The cooperativity is lost at higher assay temperatures. Cooperativity can originate from a kinetic mechanism involving hysteresis and will be dependent on the relationship between k(cat) and the rate of interconversion between two different Michaelis complexes. In the case of the studied reactions, the proposed different Michaelis complexes are enzyme-substrate complexes in which the epoxide substrate is bound in different binding modes, allowing for separate pathways toward product formation. The assumption of separated, but interacting, reaction pathways is supported by that formation of the two product enantiomers also displays distinct pH dependencies of k(cat)/K(M). The thermodynamic parameters describing the differences in activation enthalpy and entropy suggest that (1) regioselectivity is primarily dictated by differences in activation entropy with positive values of both DeltaDeltaH(++) and DeltaDeltaS(++) and (2) the hysteretic behavior is linked to an interconversion between Michaelis complexes with rates increasing with temperature. From the collected data, we propose that hysteresis, regioselectivity, and, when applicable, hysteretic cooperativity are closely linked properties, explained by the kinetic mechanism earlier introduced by our group.

A tetrahedral transition state at the active sites of the 20S proteasome is coupled to opening of the alpha-ring channel

Intrinsic conformational transitions contribute to the catalytic action of many enzymes. Here we use a single-molecule approach to demonstrate how such transitions are linked to the catalytic sites of the eukaryotic proteasome, an essential protease of the ubiquitin pathway. The active sites of the cylindrical proteasomal core particle are located in a central chamber accessible through gated entry channels. By using atomic force microscopy, we found continual alternation between open and closed gate conformations. We analyzed the relative abundance of these conformers in wild-type and mutated yeast core particles upon exposure to substrates or inhibitors. Our data indicate that the dynamic gate can be opened by allosteric coupling to a tetrahedral transition state at any of the working active centers. The results point to the N(alpha)-amine of the N-terminal active site threonyl residue as the major effector group responsible for triggering the essential conformational switch.

Induced allostery in the directed evolution of an enantioselective Baeyer-Villiger monooxygenase

The molecular basis of allosteric effects, known to be caused by an effector docking to an enzyme at a site distal from the binding pocket, has been studied recently by applying directed evolution. Here, we utilize laboratory evolution in a different way, namely to induce allostery by introducing appropriate distal mutations that cause domain movements with concomitant reshaping of the binding pocket in the absence of an effector. To test this concept, the thermostable Baeyer-Villiger monooxygenase, phenylacetone monooxygenase (PAMO), was chosen as the enzyme to be employed in asymmetric Baeyer-Villiger reactions of substrates that are not accepted by the wild type. By using the known X-ray structure of PAMO, a decision was made regarding an appropriate site at which saturation mutagenesis is most likely to generate mutants capable of inducing allostery without any effector compound being present. After screening only 400 transformants, a double mutant was discovered that catalyzes the asymmetric oxidative kinetic resolution of a set of structurally different 2-substituted cyclohexanone derivatives as well as the desymmetrization of three different 4-substituted cyclohexanones, all with high enantioselectivity. Molecular dynamics (MD) simulations and covariance maps unveiled the origin of increased substrate scope as being due to allostery. Large domain movements occur that expose and reshape the binding pocket. This type of focused library production, aimed at inducing significant allosteric effects, is a viable alternative to traditional approaches to "designed" directed evolution that address the binding site directly.

Analysis of Hydrogen Tunneling in an Enzyme Active Site using von Neumann Measurements

We build on our earlier quantum wavepacket study of hydrogen transfer in the biological enzyme, soybean lipoxygenase-1, by using von Neumann quantum measurement theory to gain qualitative insights into the transfer event. We treat the enzyme active site as a measurement device which acts on the tunneling hydrogen nucleus via the potential it exerts at each configuration. A series of changing active site geometries during the tunneling process effects a sequential projection of the initial, reactant state onto the final, product state. We study this process using several different kinds of von Neumann measurements and show how a discrete sequence of such measurements not only progressively increases the projection of the hydrogen nuclear wavepacket onto the product side but also favors proton over deuteron transfer. Several qualitative features of the hydrogen tunneling problem found in wavepacket dynamics studies are also recovered here. These include the shift in the "transition state" towards the reactant as a result of nuclear quantization, greater participation of excited states in the case of deuterium, and presence of critical points along the reaction coordinate that facilitate hydrogen and deuterium transfer and coincide with surface crossings. To further "tailor" the dynamics, we construct a perturbation to the sequence of measurements, that is a perturbation to the dynamical sequence of active site geometry evolution, which leads us to insight on the existence of sensitive regions of the reaction profile where subtle changes to the dynamics of the active site can have an effect on the hydrogen and deuterium transfer process.

Measurement of ligand binding to tubulin by sulfhydryl reactivity

Ligand binding can induce shifts in protein conformation. In the case of tubulin, these drug-induced confirmational changes can prevent or stabilize microtubule polymerization. 5',5'-Dithiobis(2-nitrobenzoate) (DTNB) reacts with free and accessible sulfhydryls and stoichiometrically produces a detectable product, which allows an exact measurement of reacted thiols. Since binding of small ligands may alter conformational dynamics, it may also affect the reactivity of thiols on tubulin. Differences in DTNB reactivity with thiols upon ligand binding can therefore be used to deduce binding characteristics. We will describe two methods that use tubulin cysteine reactivity with DTNB in the presence of drug to define ligand-binding characteristics.

Conformational transitions in DNA polymerase I revealed by single-molecule FRET

The remarkable fidelity of most DNA polymerases depends on a series of early steps in the reaction pathway which allow the selection of the correct nucleotide substrate, while excluding all incorrect ones, before the enzyme is committed to the chemical step of nucleotide incorporation. The conformational transitions that are involved in these early steps are detectable with a variety of fluorescence assays and include the fingers-closing transition that has been characterized in structural studies. Using DNA polymerase I (Klenow fragment) labeled with both donor and acceptor fluorophores, we have employed single-molecule fluorescence resonance energy transfer to study the polymerase conformational transitions that precede nucleotide addition. Our experiments clearly distinguish the open and closed conformations that predominate in Pol-DNA and Pol-DNA-dNTP complexes, respectively. By contrast, the unliganded polymerase shows a broad distribution of FRET values, indicating a high degree of conformational flexibility in the protein in the absence of its substrates; such flexibility was not anticipated on the basis of the available crystallographic structures. Real-time observation of conformational dynamics showed that most of the unliganded polymerase molecules sample the open and closed conformations in the millisecond timescale. Ternary complexes formed in the presence of mismatched dNTPs or complementary ribonucleotides show unique FRET species, which we suggest are relevant to kinetic checkpoints that discriminate against these incorrect substrates.

Hidden alternative structures of proline isomerase essential for catalysis

A long-standing challenge is to understand at the atomic level how protein dynamics contribute to enzyme catalysis. X-ray crystallography can provide snapshots of conformational substates sampled during enzymatic reactions, while NMR relaxation methods reveal the rates of interconversion between substates and the corresponding relative populations. However, these current methods cannot simultaneously reveal the detailed atomic structures of the rare states and rationalize the finding that intrinsic motions in the free enzyme occur on a timescale similar to the catalytic turnover rate. Here we introduce dual strategies of ambient-temperature X-ray crystallographic data collection and automated electron-density sampling to structurally unravel interconverting substates of the human proline isomerase, cyclophilin A (CYPA, also known as PPIA). A conservative mutation outside the active site was designed to stabilize features of the previously hidden minor conformation. This mutation not only inverts the equilibrium between the substates, but also causes large, parallel reductions in the conformational interconversion rates and the catalytic rate. These studies introduce crystallographic approaches to define functional minor protein conformations and, in combination with NMR analysis of the enzyme dynamics in solution, show how collective motions directly contribute to the catalytic power of an enzyme.

Dynamic disorder-driven substrate inhibition and bistability in a simple enzymatic reaction

Conformations and catalytic rates of enzymes (biological catalysts) fluctuate over a wide range of time scales. Recent experimental and theoretical investigations demonstrated case studies where the enzymatic catalysis rate follows the Michaelis-Menten (MM) rate law despite molecular fluctuations. In this paper, we investigate deviations from MM law and their effects on the dynamical behavior of the enzymatic network. We consider a simple kinetic scheme for a single substrate enzymatic reaction in which the product release step is treated explicitly. We examine how conformational fluctuations affect the underlying rate law in the quasi-static limit when conformational dynamics is very slow in one of the states. Our numerical results and analytically solvable model indicate that slow conformational fluctuations of the enzyme-substrate complex lead to non-MM behavior, substrate inhibition, and possible bistability of the reaction network.

Extracting the causality of correlated motions from molecular dynamics simulations

The information theory measure of transfer entropy is used to extract the causality of correlated motions from molecular dynamics simulations. For each pair of correlated residues, the method quantifies which residue drives the correlated motions, and which residue responds. The measure reveals how correlated motions are used to transmit information through the system, and helps to clarify the link between correlated motions and biological function in biomolecular systems. The method is illustrated by its application to the Ets-1 transcription factor, which partially unfolds upon binding DNA. The calculations show dramatic changes in the direction of information flow upon DNA binding, and elucidate how the presence of DNA is communicated from the DNA binding H1 and H3 helices to inhibitory helix HI-1. Helix H4 is shown to act as a relay, which is attenuated in the apo state.

Examination of enzymatic H-tunneling through kinetics and dynamics

In recent years, kinetic measurements of isotope effects of enzyme-catalyzed reactions and their temperature dependence led to the development of theoretical models that were used to rationalize the findings. These models suggested that motions at the femto- to picosecond (fs to ps) time scale modulate the environment of the catalyzed reaction. Due to the fast nature of motions that directly affect the cleavage of a covalent bond, it is challenging to correlate the enzyme kinetics and dynamics related to that step. We report a study of formate dehydrogenase (FDH) that compares the temperature dependence of intrinsic kinetic isotope effects (KIEs) to measurements of the environmental dynamics at the fs-ps time scale (Bandaria et al. J. Am. Chem. Soc. 2008, 130, 22-23). The findings from this comparison of experimental kinetics and dynamics are consistent with models of environmentally coupled H-tunneling models, also known as Marcus-like models. Apparently, at tunneling ready conformations, the donor-acceptor distance, orientation, and fluctuations seems to be well tuned for H-transfer and are not affected by thermal fluctuations slower than 10 ps. This phenomenon has been suggested in the past to be quite general in enzymatic reactions. Here, the kinetics and the dynamics measurements on a single chemical step and on fs-ps time scale, respectively, provide new insight and support for the relevant theoretical models. Furthermore, this methodology could be applied to other systems and be used to examine mutants for which the organization of the donor and acceptor is not ideal, or enzymes with different rigidity and different temperature optimum.

Theoretical studies of RNA catalysis: hybrid QM/MM methods and their comparison with MD and QM

Hybrid QM/MM methods combine the rigor of quantum mechanical (QM) calculations with the low computational cost of empirical molecular mechanical (MM) treatment allowing to capture dynamic properties to probe critical atomistic details of enzyme reactions. Catalysis by RNA enzymes (ribozymes) has only recently begun to be addressed with QM/MM approaches and is thus still a field under development. This review surveys methodology as well as recent advances in QM/MM applications to RNA mechanisms, including those of the HDV, hairpin, and hammerhead ribozymes, as well as the ribosome. We compare and correlate QM/MM results with those from QM and/or molecular dynamics (MD) simulations, and discuss scope and limitations with a critical eye on current shortcomings in available methodologies and computer resources. We thus hope to foster mutual appreciation and facilitate collaboration between experimentalists and theorists to jointly advance our understanding of RNA catalysis at an atomistic level.

LOVely enzymes - towards engineering light-controllable biocatalysts

Light control over enzyme function represents a novel and exciting field of biocatalysis research. Blue‐light photoreceptors of the Light, Oxygen, Voltage (LOV) family have recently been investigated for their applicability as photoactive switches. We discuss here the primary photochemical events leading to light activation of LOV domains as well as the proposed signal propagation mechanism to the respective effector domain. Furthermore, we describe the construction of LOV fusions to different effector domains, namely a dihydrofolate reductase from Escherichia coli and a lipase from Bacillus subtilis. Both fusion partners retained functionality, and alteration of enzyme activity by light was also demonstrated. Hence, it appears that fusion of LOV photoreceptors to functional enzyme target sites via appropriate linker structures may represent a straightforward strategy to design light controllable biocatalysts.

Enzymatic transition states and dynamic motion in barrier crossing

What are the atomic motions at enzymatic catalytic sites on the timescale of chemical change? Combined experimental and computational chemistry approaches take advantage of transition-state analogs to reveal dynamic motions linked to transition-state formation. QM/MM transition path sampling from reactive complexes provides both temporal and dynamic information for barrier crossing. Fast (femtosecond to picosecond) dynamic motions provide essential links to enzymatic barrier crossing by local or promoting-mode dynamic searches through bond-vibrational space. Transition-state lifetimes are within the femtosecond timescales of bond vibrations and show no manifestations of stabilized, equilibrated complexes. The slow binding and protein conformational changes (microsecond to millisecond) also required for catalysis are temporally decoupled from the fast dynamic motions forming the transition state. According to this view of enzymatic catalysis, transition states are formed by fast, coincident dynamic excursions of catalytic site elements, while the binding of transition-state analogs is the conversion of the dynamic excursions to equilibrated states.

Computational methods for predicting sites of functionally important dynamics

Understanding and controlling biological function of proteins at the atomic level is of great importance; allosteric mechanisms provide such an interface. Experimental and computational methods have been developed to search for residue mutations that produce changes in function by altering sites of correlated motion. These methods are often observational in that altered motions are achieved by random sampling without revealing the underlying mechanism(s). We present two deterministic methods founded on structure-function relationships that predict dynamic control sites (i.e., locations that experience correlated motions as a result of altered dynamics). The first method ("static") is based on a single structure conformation (e.g., the wild type (WT)) and utilizes a graph description of atomic connectivity. The local atomic interactions are used to compute the propagation of contact paths. This description of structure connectivity reveals flexible locations that are susceptible to altered dynamics. The second method ("dynamic") is a comparative analysis between the normal modes of a WT structure and a mutant structure. A mapping function is defined that quantifies the significance of the motions in one structure projected onto the motions of the other. Each mode is considered up- or down-regulated according to its change in relative significance. This description of altered dynamics is the basis for a motion correlation analysis, from which the dynamic control sites are readily identified. The methods are theoretically derived and applied using the canonical system dihydrofolate reductase (DHFR). Both methods demonstrate a very high predictive value (p<0.005) in identifying known dynamic control sites. The dynamic method also produces a new hypothesis regarding the mechanism by which the DHFR mutant achieves hyperactivity. These tools are suitable for allosteric investigations and may greatly enhance the speed and effectiveness of other computational and experimental methods.

Communication breakdown: protein dynamics and drug design

Mauldin et al. (2009) use NMR to show that drug binding can break up collective protein motions necessary for function. We discuss their findings in the context of drug discovery in pharmaceutical research.

Dynamic dysfunction in dihydrofolate reductase results from antifolate drug binding: modulation of dynamics within a structural state

The arduous task of rationally designing small-molecule enzyme inhibitors is complicated by the inherent flexibility of the protein scaffold. To gain insight into the changes in dynamics associated with small-molecule-based inhibition, we have characterized, using NMR spectroscopy, Escherichia coli dihydrofolate reductase in complex with two drugs: methotrexate and trimethoprim. The complexes allowed the intrinsic dynamic effects of drug binding to be revealed within the context of the "closed" structural ensemble. Binding of both drugs results in an identical decoupling of global motion on the micro- to millisecond timescale. Consistent with a change in overall dynamic character, the drugs' perturbations to pico- to nanosecond backbone and side-chain methyl dynamics are also highly similar. These data show that the inhibitors simultaneously modulate slow concerted switching and fast motions at distal regions of dihydrofolate reductase, providing a dynamic link between the substrate binding site and distal loop residues known to affect catalysis.

Scale-free flow of life: on the biology, economics, and physics of the cell

The present work is intended to demonstrate that most of the paradoxes, controversies, and contradictions accumulated in molecular and cell biology over many years of research can be readily resolved if the cell and living systems in general are re-interpreted within an alternative paradigm of biological organization that is based on the concepts and empirical laws of nonequilibrium thermodynamics. In addition to resolving paradoxes and controversies, the proposed re-conceptualization of the cell and biological organization reveals hitherto unappreciated connections among many seemingly disparate phenomena and observations, and provides new and powerful insights into the universal principles governing the emergence and organizational dynamics of living systems on each and every scale of biological organizational hierarchy, from proteins and cells to economies and ecologies.

Evolutionary basis for the coupled-domain motions in Thermus thermophilus leucyl-tRNA synthetase

Aminoacyl-tRNA synthetases are multidomain proteins that catalyze the covalent attachment of amino acids to their cognate transfer RNA. Various domains of an aminoacyl-tRNA synthetase perform their specific functions in a highly coordinated manner to maintain high accuracy in protein synthesis in cells. The coordination of their function, therefore, requires communication between domains. In this study we explored the relevance of enzyme motion in domain-domain communications. Specifically, we attempted to probe whether the communication between distantly located domains of a multidomain protein is accomplished through a coordinated movement of structural elements. We investigated the collective motion in Thermus thermophilus leucyl-tRNA synthetase by studying the low frequency normal modes. We identified the mode that best described the experimentally observed conformational changes of T. thermophilus leucyl-tRNA synthetase upon substrate binding and analyzed the correlated and anticorrelated motions between different domains. Furthermore, we used statistical coupling analysis to explore if the amino acid pairs and/or clusters whose motions are thermally coupled have also coevolved. Our study demonstrates that a small number of residues belong to the category whose coupled thermal motions correspond to evolutionary coupling as well. These residue clusters constitute a distinguished set of interacting networks that are sparsely distributed in the domain interface. Residues of these networking clusters are within van der Waals contact, and we suggest that they are critical in the propagation of long range mechanochemical motions in T. thermophilus leucyl-tRNA synthetase.

Dynamical networks in tRNA:protein complexes

Community network analysis derived from molecular dynamics simulations is used to identify and compare the signaling pathways in a bacterial glutamyl-tRNA synthetase (GluRS):tRNA(Glu) and an archaeal leucyl-tRNA synthetase (LeuRS):tRNA(Leu) complex. Although the 2 class I synthetases have remarkably different interactions with their cognate tRNAs, the allosteric networks for charging tRNA with the correct amino acid display considerable similarities. A dynamic contact map defines the edges connecting nodes (amino acids and nucleotides) in the physical network whose overall topology is presented as a network of communities, local substructures that are highly intraconnected, but loosely interconnected. Whereas nodes within a single community can communicate through many alternate pathways, the communication between monomers in different communities has to take place through a smaller number of critical edges or interactions. Consistent with this analysis, there are a large number of suboptimal paths that can be used for communication between the identity elements on the tRNAs and the catalytic site in the aaRS:tRNA complexes. Residues and nucleotides in the majority of pathways for intercommunity signal transmission are evolutionarily conserved and are predicted to be important for allosteric signaling. The same monomers are also found in a majority of the suboptimal paths. Modifying these residues or nucleotides has a large effect on the communication pathways in the protein:RNA complex consistent with kinetic data.

Nontarget DNA binding shapes the dynamic landscape for enzymatic recognition of DNA damage

The DNA repair enzyme human uracil DNA glycosylase (UNG) scans short stretches of genomic DNA and captures rare uracil bases as they transiently emerge from the DNA duplex via spontaneous base pair breathing motions. The process of DNA scanning requires that the enzyme transiently loosen its grip on DNA to allow stochastic movement along the DNA contour, while engaging extrahelical bases requires motions on a more rapid timescale. Here, we use NMR dynamic measurements to show that free UNG has no intrinsic dynamic properties in the millisecond to microsecond and subnanosecond time regimes, and that the act of binding to nontarget DNA reshapes the dynamic landscape to allow productive millisecond motions for scanning and damage recognition. These results suggest that DNA structure and the spontaneous dynamics of base pairs may drive the evolution of a protein sequence that is tuned to respond to this dynamic regime.

Development and application of ab initio QM/MM methods for mechanistic simulation of reactions in solution and in enzymes

Determining the free energies and mechanisms of chemical reactions in solution and enzymes is a major challenge. For such complex reaction processes, combined quantum mechanics/molecular mechanics (QM/MM) method is the most effective simulation method to provide an accurate and efficient theoretical description of the molecular system. The computational costs of ab initio QM methods, however, have limited the application of ab initio QM/MM methods. Recent advances in ab initio QM/MM methods allowed the accurate simulation of the free energies for reactions in solution and in enzymes and thus paved the way for broader application of the ab initio QM/MM methods. We review here the theoretical developments and applications of the ab initio QM/MM methods, focusing on the determination of reaction path and the free energies of the reaction processes in solution and enzymes.

Dynamics of the conformational transitions in the assembling of the Michaelis complex of a bisubstrate enzyme: a (15)N relaxation study of Escherichia coli 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase

6-Hydroxymethyl-7,8-dihydropterin pyrophosphokinase (HPPK) catalyzes the transfer of pyrophosphate from ATP to 6-hydroxymethyl-7,8-dihydropterin (HP), which follows an ordered bi-bi kinetic mechanism with ATP binding to the enzyme first. HPPK undergoes dramatic conformational changes during its catalytic cycle as revealed by X-ray crystallography, and the conformational changes are essential for the enzymatic catalysis as shown by site-directed mutagenesis and biochemical and crystallographic analysis of the mutants. However, the dynamic properties of the enzyme have not been measured experimentally. Here, we report a (15)N NMR relaxation study of the dynamic properties of Escherichia coli HPPK from the apo form to the binary substrate complex with MgATP (represented by MgAMPCPP, an ATP analogue) to the Michaelis complex (ternary substrate complex) with MgATP (represented by MgAMPCPP) and HP (represented by 7,7-dimethyl-6-hydroxypterin, an HP analogue). The results show that the binding of the nucleotide to HPPK does not cause major changes in the dynamic properties of the enzyme. Whereas enzymes are often more rigid when bound to the ligand or the substrate, the internal mobility of HPPK is not reduced and is even moderately increased in the binary complex, particularly in the catalytic loops. The internal mobility of the catalytic loops is significantly quenched upon the formation of the ternary complex, but some mobility remains. The enhanced motions in the catalytic loops of the binary substrate complex may be required for the assembling of the ternary complex. On the other hand, some degrees of mobility in the catalytic loops of the ternary complex may be required for the optimal stabilization of the transition state, which may need the instantaneous adjustment and alignment of the side-chain positions of catalytic residues. Such dynamic behaviors may be characteristic of bisubstrate enzymes.

Atomic force microscopy as a tool to study the proteasome assemblies

Proteasome is an exceptional enzyme because of its essential physiological role, multiple activities, and structural complexity. It is, in fact, a family of enzymes sharing a common catalytic core and equipped with distinct protein attachments regulating the core and adding to its new functional capabilities. As a drug target and a major regulator of cellular processes, proteasome is extensively studied with tools of structural, biochemical, and molecular biology. Atomic force microscopy (AFM) besides X-ray crystallography and electron microscopy is one of the most attractive methods to study proteasome. The noninvasive nature of this method is particularly well suited for investigating the structure-function relationship within the core particle (CP) as well as in higher-order assemblies. Here we review, from the methodological point of view, AFM-based studies on the proteasome. First, we will present the application of height distribution analysis of proteasome complexes to dissect the subunit organization in the base of the regulatory particle (RP). The RP is considered the most physiologically important among all the attachments of the CP; however, its structure remains enigmatic. Then, we will outline the use of AFM imaging to research on structural dynamics of the proteasome, a phenomenon which is starting to gain a broad interest. We will finish with a brief presentation of nanotechnological studies performed using ordered proteasomes and nanolithography carried out with the particles. The presented AFM research offers a unique and often unexpected insight into the structure and function of the proteasome.

Independent saturation of three TrpRS subsites generates a partially assembled state similar to those observed in molecular simulations

Two new crystal structures of Bacillus stearothermophilus tryptophanyl-tRNA synthetase (TrpRS) afford evidence that a closed interdomain hinge angle requires a covalent bond between AMP and an occupant of either pyrophosphate or tryptophan subsite. They also are within experimental error of a cluster of structures observed in a nonequilibrium molecular dynamics simulation showing partial active-site assembly. Further, the highest energy structure in a minimum action pathway computed by using elastic network models for Open and Pretransition state (PreTS) conformations for the fully liganded TrpRS monomer is intermediate between that simulated structure and a partially disassembled structure from a nonequilibrium molecular dynamics trajectory for the unliganded PreTS. These mutual consistencies provide unexpected validation of inferences drawn from molecular simulations.

The Small Ribozymes: Common and Diverse Features Observed through the FRET Lens

The hammerhead, hairpin, HDV, VS and glmS ribozymes are the five known, naturally occurring catalytic RNAs classified as the "small ribozymes". They share common reaction chemistry in cleaving their own backbone by phosphodiester transfer, but are diverse in their secondary and tertiary structures, indicating that Nature has found at least five independent solutions to a common chemical task. Fluorescence resonance energy transfer (FRET) has been extensively used to detect conformational changes in these ribozymes and dissect their reaction pathways. Common and diverse features are beginning to emerge that, by extension, highlight general biophysical properties of non-protein coding RNAs.

Adaptive protein evolution grants organismal fitness by improving catalysis and flexibility

Protein evolution is crucial for organismal adaptation and fitness. This process takes place by shaping a given 3-dimensional fold for its particular biochemical function within the metabolic requirements and constraints of the environment. The complex interplay between sequence, structure, functionality, and stability that gives rise to a particular phenotype has limited the identification of traits acquired through evolution. This is further complicated by the fact that mutations are pleiotropic, and interactions between mutations are not always understood. Antibiotic resistance mediated by beta-lactamases represents an evolutionary paradigm in which organismal fitness depends on the catalytic efficiency of a single enzyme. Based on this, we have dissected the structural and mechanistic features acquired by an optimized metallo-beta-lactamase (MbetaL) obtained by directed evolution. We show that antibiotic resistance mediated by this enzyme is driven by 2 mutations with sign epistasis. One mutation stabilizes a catalytically relevant intermediate by fine tuning the position of 1 metal ion; whereas the other acts by augmenting the protein flexibility. We found that enzyme evolution (and the associated antibiotic resistance) occurred at the expense of the protein stability, revealing that MbetaLs have not exhausted their stability threshold. Our results demonstrate that flexibility is an essential trait that can be acquired during evolution on stable protein scaffolds. Directed evolution aided by a thorough characterization of the selected proteins can be successfully used to predict future evolutionary events and design inhibitors with an evolutionary perspective.

The ferritin Fe2 site at the diiron catalytic center controls the reaction with O2 in the rapid mineralization pathway

Oxidoreduction in ferritin protein nanocages occurs at sites that bind two Fe(II) substrate ions and O(2), releasing Fe(III)(2)-O products, the biomineral precursors. Diferric peroxo intermediates form in ferritins and in the related diiron cofactor oxygenases. Cofactor iron is retained at diiron sites throughout catalysis, contrasting with ferritin. Four of the 6 active site residues are the same in ferritins and diiron oxygenases; ferritin-specific Gln(137) and variable Asp/Ser/Ala(140) substitute for Glu and His, respectively, in diiron cofactor active sites. To understand the selective functions of diiron substrate and diiron cofactor active site residues, we compared oxidoreductase activity in ferritin with diiron cofactor residues, Gln(137) --> Glu and Asp(140) --> His, to ferritin with natural diiron substrate site variations, Asp(140), Ser(140), or Ala(140). In Gln(137) --> Glu ferritin, diferric peroxo intermediates were undetectable; an altered Fe(III)-O product formed, DeltaA(350) = 50% of wild type. In Asp(140) --> His ferritin, diferric peroxo intermediates were also undetectable, and Fe(II) oxidation rates decreased 40-fold. Ferritin with Asp(140), Ser(140), or Ala(140) formed diferric peroxo intermediates with variable kinetic stabilities and rates: t(1/2) varied 1- to 10-fold; k(cat) varied approximately 2- to 3-fold. Thus, relatively small differences in diiron protein catalytic sites determine whether, and for how long, diferric peroxo intermediates form, and whether the Fe-active site bonds persist throughout the reaction cycle (diiron cofactors) or break to release Fe(III)(2)-O products (diiron substrates). The results and the coding similarities for cofactor and substrate site residues-e.g., Glu/Gln and His/Asp pairs share 2 of 3 nucleotides-illustrate the potential simplicity of evolving active sites for diiron cofactors or diiron substrates.

Atomic detail of chemical transformation at the transition state of an enzymatic reaction

Transition path sampling (TPS) has been applied to the chemical step of human purine nucleoside phosphorylase (PNP). The transition path ensemble provides insight into the detailed mechanistic dynamics and atomic motion involved in transition state passage. The reaction mechanism involves early loss of the ribosidic bond to form a transition state with substantial ribooxacarbenium ion character, followed by dynamic motion from the enzyme and a conformational change in the ribosyl group leading to migration of the anomeric carbon toward phosphate, to form the product ribose 1-phosphate. Calculations of the commitment probability along reactive paths demonstrated the presence of a broad energy barrier at the transition state. TPS identified (i) compression of the O4'...O5' vibrational motion, (ii) optimized leaving group interactions, and (iii) activation of the phosphate nucleophile as the reaction proceeds through the transition state region. Dynamic motions on the femtosecond timescale provide the simultaneous optimization of these effects and coincide with transition state formation.