Functional role of aspartic acid-27 in dihydrofolate reductase revealed by mutagenesis

Secondary Amine Catalysis in Enzyme Design: Broadening Protein Template Diversity through Genetic Code Expansion

Secondary amines, due to their reactivity, can transform protein templates into catalytically active entities, accelerating the development of artificial enzymes. However, existing methods, predominantly reliant on modified ligands or N-terminal prolines, impose significant limitations on template selection. In this study, genetic code expansion was used to break this boundary, enabling secondary amines to be incorporated into alternative proteins and positions of choice. Pyrrolysine analogues carrying different secondary amines could be incorporated into superfolder green fluorescent protein (sfGFP), multidrug-binding LmrR and nucleotide-binding dihydrofolate reductase (DHFR). Notably, the analogue containing a D-proline moiety demonstrated both proteolytic stability and catalytic activity, conferring LmrR and DHFR with the desired transfer hydrogenation activity. While the LmrR variants were confined to the biomimetic 1-benzyl-1,4-dihydronicotinamide (BNAH) as the hydride source, the optimal DHFR variant favorably used the pro-R hydride from NADPH for stereoselective reactions (e.r. up to 92 : 8), highlighting that a switch of protein template could broaden the nucleophile option for catalysis. Owing to the cofactor compatibility, the DHFR-based secondary amine catalysis could be integrated into an enzymatic recycling scheme. This established method shows substantial potential in enzyme design, applicable from studies on enzyme evolution to the development of new biocatalysts.

Solvent isotope and mutagenesis studies on the proton relay system in yeast alcohol dehydrogenase 1

Alcohol dehydrogenase catalyzes the reversible transfer of a hydride directly from an alcohol to the nicotinamide ring of NAD+ to form an aldehyde and NADH, and the proton from the alcohol probably is transferred through a hydrogen-bonded system to the imidazole of His-48. Studies of the pH dependencies, and solvent and substrate isotope effects on the wild-type and the enzyme with His-48 substituted with Gln-48 were used to demonstrate a role for the proton relay system. The H48Q substitution increases affinities for NAD+ and NADH by ∼2-fold, suggesting that the overall protein structure is maintained. In contrast, catalytic efficiencies (V/Km) on ethanol and acetaldehyde and affinity for 2,2,2-trifluoroethanol are decreased by about 10-fold. The pH dependencies for catalytic efficiencies on ethanol and acetaldehyde (log V/Km versus pH), show pK values of about 7.5 for wild-type enzyme, but ethanol oxidation by H48Q ADH is essentially linear over the pH range from 5.5 to 9.2 with a slope of 0.47. Steady-state kinetics and substrate isotope effects suggest that the kinetic mechanism of H48Q ADH has become partly random for oxidation of ethanol. Both wild-type and H48Q ADHs have pH-independent isotope effects for oxidation (V1/Kb) of 1-butanol/1-butanol-d9 of 4, suggesting that hydride transfer is a major rate-limiting step. The pH dependence for butanol oxidation by wild type ADH shows a wavy profile over the pH range from pH 6 to 10, with a ∼2.3-fold larger V1/Kb in D2O than in H2O, an "inverse" isotope effect. The substrate isotope effect of 4 is not altered by the solvent isotope effect, suggesting concerted proton/hydride transfer. The solvent isotope effect can be explained by a ground state with a water bound to the catalytic zinc in the enzyme-NAD+ complex, and a transition state that resembles a complex with NADH and aldehyde. In contrast, the H48Q enzyme has a diminished inverse solvent isotope effect of ∼1.3 and an essentially linear pH dependence with a slope of log V1/Kb against pH of 0.49 for oxidation of 1-butanol, which together are consistent with a transition state where hydroxide ion directly accepts a proton from the 2'-hydroxyl group of the nicotinamide ribose in the proton relay system in the enzyme-NAD+-alcohol complex. The results support a catalytic role for His-48 in the proton relay system.

From a binding module to essential catalytic activity: how nature stumbled on a good thing

Enzymes are complex macromolecules capable of catalyzing a wide variety of chemical reactions with high efficiency. Nonetheless, biological catalysis can be rudimentary. Here, we describe an enzyme that is built from a simple protein fold. This short protein sequence - almost a peptide - belongs to the ancient SH3 family of binding modules. Surprisingly, this binding module catalyzes the specific reduction of dihydrofolate using NADPH as a reducing cofactor, making this a dihydrofolate reductase. Too small to provide all the required binding and catalytic machinery on its own, it homotetramerizes, thus creating a large, central active site environment. Remarkably, none of the active site residues is essential to the catalytic function. Instead, backbone interactions juxtapose the reducing cofactor proximal to the target imine of the folate substrate, and a specific motion of the substrate promotes formation of the transition state. In this feature article, we describe the features that make this small protein a functional enzyme capable of catalyzing a metabolically essential reaction, highlighting the characteristics that make it a model for the evolution of primitive enzymes from binding modules.

Specific base catalysis by yeast alcohol dehydrogenase I with substitutions of histidine-48 by glutamate or serine residues in the proton relay system

His-48 in yeast alcohol dehydrogenase I (His 51 in horse liver alcohol dehydrogenase) is a highly conserved residue in the active sites of many alcohol dehydrogenases. The imidazole group of His-48 may participate in base catalysis of proton transfer as it is linked by hydrogen bonds through the 2'-hydroxyl group of the nicotinamide ribose and the hydroxyl group of Thr-45 to the hydroxyl group of the alcohol bound to the catalytic zinc. In this study, His-48 was substituted with a glutamic acid residue to determine if a carboxylate could replace imidazole or to a serine residue to determine if the exposure of the 2'-hydroxyl group of the ribose to solvent would allow proton transfer to water without base catalysis. At pH 7.3, the H48E substitution increases affinity for NAD+ and NADH 17- or 2.6-fold, but decreases catalytic efficiency (V/Km) on ethanol by 70-fold and on acetaldehyde by 6-fold relative to wild-type enzyme. The H48S substitution increases affinity for coenzymes by 2-fold and decreases (V/Km) on ethanol and acetaldehyde only by ∼3-fold. The substituted enzymes show substrate deuterium isotope (H/D) effects of 3-4 for turnover number (V1) and catalytic efficiency (V1/Kb) for ethanol oxidation, indicating that hydrogen transfer is partially rate-limiting and suggesting a somewhat more random mechanism for binding of ethanol and NAD+. For reduction of acetaldehyde, the deuterium isotope effects are small, and the kinetic mechanism appears to be ordered for binding of NADH first and acetaldehyde next. The pH dependencies for H48E and H48S ADHs can be described by a mechanism with pK values of about 6-7 and 9. However, the pH dependencies for oxidation of ethanol and butanol by the H48S enzyme are also simply described by a straight line, with slopes of log V1/Kb against pH of 0.37 or 0.43, respectively. The linear dependence apparently represents catalysis by hydroxide that has a low activity coefficient due to the protein environment, or to a kinetically complex proton transfer. The effects of the substitutions of His-48 show that this residue contributes to catalysis, although many dehydrogenases also have other residues.

Structure-guided functional studies of plasmid-encoded dihydrofolate reductases reveal a common mechanism of trimethoprim resistance in Gram-negative pathogens

Two plasmid-encoded dihydrofolate reductase (DHFR) isoforms, DfrA1 and DfrA5, that give rise to high levels of resistance in Gram-negative bacteria were structurally and biochemically characterized to reveal the mechanism of TMP resistance and to support phylogenic groupings for drug development against antibiotic resistant pathogens. Preliminary screening of novel antifolates revealed related chemotypes that showed high levels of inhibitory potency against Escherichia coli chromosomal DHFR (EcDHFR), DfrA1, and DfrA5. Kinetics and biophysical analysis, coupled with crystal structures of trimethoprim bound to EcDHFR, DfrA1 and DfrA5, and two propargyl-linked antifolates (PLA) complexed with EcDHFR, DfrA1 and DfrA5, were determined to define structural features of the substrate binding pocket and guide synthesis of pan-DHFR inhibitors.

Critical residue variations in two of the most clinically prevalent DHFR isoforms are identified as a common structural element in trimethoprim-resistant DHFR which impose changes on enzyme catalysis and ligand-cofactor cooperativity.

Effects of Distal Mutations on Ligand-Binding Affinity in E. coli Dihydrofolate Reductase

Mutations far from the center of chemical activity in dihydrofolate reductase (DHFR) can affect several steps in the catalytic cycle. Mutations at highly conserved positions and the distal distance of the catalytic center (Met-42, Thr-113, and Gly-121) were designed, including single-point and double-point mutations. Upon ligand binding, the fluorescence of the intrinsic optical probe, tryptophan, decreases due to either fluorescence quenching or energy transfer. We demonstrated an optical approach in measuring the equilibrium dissociation constant for enzyme-cofactor, enzyme-substrate, and enzyme-product complexes in wildtype ecDHFR and each mutant. We propose that the effects of these distal mutations on ligand-binding affinity stem from the spatial steric hindrance, the disturbance on the hydrogen network, or the modification of the protein flexibility. The modified N-terminus tag in DHFR acts as a cap on the entrance of the substrate-binding cavity, squeezes the adenosine binding subdomain, and influences the binding of NADPH in some mutants. If the mutation positions are away from the N-terminus tag and the adenosine binding subdomain, the additive effects due to the N-terminus tag were not observed. In the double-mutant-cycle analysis, double mutations show nonadditive properties upon either cofactor or substrate binding. Also, in general, the first point mutation strongly affects the ligand binding compared to the second one.

Structurally distributed surface sites tune allosteric regulation

Our ability to rationally optimize allosteric regulation is limited by incomplete knowledge of the mutations that tune allostery. Are these mutations few or abundant, structurally localized or distributed? To examine this, we conducted saturation mutagenesis of a synthetic allosteric switch in which Dihydrofolate reductase (DHFR) is regulated by a blue-light sensitive LOV2 domain. Using a high-throughput assay wherein DHFR catalytic activity is coupled to E. coli growth, we assessed the impact of 1548 viable DHFR single mutations on allostery. Despite most mutations being deleterious to activity, fewer than 5% of mutations had a statistically significant influence on allostery. Most allostery disrupting mutations were proximal to the LOV2 insertion site. In contrast, allostery enhancing mutations were structurally distributed and enriched on the protein surface. Combining several allostery enhancing mutations yielded near-additive improvements to dynamic range. Our results indicate a path toward optimizing allosteric function through variation at surface sites.

Many proteins exhibit a property called ‘allostery’. In allostery, an input signal at a specific site of a protein – such as a molecule binding, or the protein absorbing a photon of light – leads to a change in output at another site far away. For example, the protein might catalyze a chemical reaction faster or bind to another molecule more tightly in the presence of the input signal. This protein ‘remote control’ allows cells to sense and respond to changes in their environment. An ability to rapidly engineer new allosteric mechanisms into proteins is much sought after because this would provide an approach for building biosensors and other useful tools. One common approach to engineering new allosteric regulation is to combine a ‘sensor’ or input region from one protein with an ‘output’ region or domain from another.

When researchers engineer allostery using this approach of combining input and output domains from different proteins, the difference in the output when the input is ‘on’ versus ‘off’ is often small, a situation called ‘modest allostery’. McCormick et al. wanted to know how to optimize this domain combination approach to increase the difference in output between the ‘on’ and ‘off’ states.

More specifically, McCormick et al. wanted to find out whether swapping out or mutating specific amino acids (each of the individual building blocks that make up a protein) enhances or disrupts allostery. They also wanted to know if there are many possible mutations that change the effectiveness of allostery, or if this property is controlled by just a few amino acids. Finally, McCormick et al. questioned where in a protein most of these allostery-tuning mutations were located.

To answer these questions, McCormick et al. engineered a new allosteric protein by inserting a light-sensing domain (input) into a protein involved in metabolism (a metabolic enzyme that produces a biomolecule called a tetrahydrofolate) to yield a light-controlled enzyme. Next, they introduced mutations into both the ‘input’ and ‘output’ domains to see where they had a greater effect on allostery.

After filtering out mutations that destroyed the function of the output domain, McCormick et al. found that only about 5% of mutations to the ‘output’ domain altered the allosteric response of their engineered enzyme. In fact, most mutations that disrupted allostery were found near the site where the ‘input’ domain was inserted, while mutations that enhanced allostery were sprinkled throughout the enzyme, often on its protein surface. This was surprising in light of the commonly-held assumption that mutations on protein surfaces have little impact on the activity of the ‘output’ domain. Overall, the effect of individual mutations on allostery was small, but McCormick et al. found that these mutations can sometimes be combined to yield larger effects.

McCormick et al.’s results suggest a new approach for optimizing engineered allosteric proteins: by introducing mutations on the protein surface. It also opens up new questions: mechanically, how do surface sites affect allostery? In the future, it will be important to characterize how combinations of mutations can optimize allosteric regulation, and to determine what evolutionary trajectories to high performance allosteric ‘switches’ look like.

Distal Regions Regulate Dihydrofolate Reductase-Ligand Interactions

Protein motions play a fundamental role in enzyme catalysis and ligand binding. The relationship between protein motion and function has been extensively investigated in the model enzyme dihydrofolate reductase (DHFR). DHFR is an essential enzyme that catalyzes the reduction of dihydrofolate to tetrahydrofolate. Numerous experimental and computational methods have been used to probe the motions of DHFR through the catalytic cycle and to investigate the effect of distal mutations on DHFR motions and ligand binding. These experimental investigations have pushed forward the study of protein motions and their role in protein-ligand interactions. The introduction of mutations distal to the active site has been shown to have profound effects on ligand binding, hydride transfer rates and catalytic efficacy and these changes are captured by enzyme kinetics measurements. Distal mutations have been shown to exert their effects through a network of correlated amino acids and these effects have been investigated by NMR, protein dynamics, and analysis of coupled amino acids. The experimental methods and the findings that are reviewed here have broad implications for our understanding of enzyme mechanisms, ligand binding and for the future design and discovery of enzyme inhibitors.

Structure-based mechanisms: On the way to apply alcohol dehydrogenases/reductases to organic-aqueous systems

Alcohol dehydrogenases/reductases catalyze enantioselective syntheses of versatile chiral compounds relying on direct hydride transfer from cofactor to substrates, or to an intermediate and then to substrates. Since most of the substrates catalyzed by alcohol dehydrogenases/reductases are insoluble in aqueous solutions, increasing interest has been turning to organic-aqueous systems. However, alcohol dehydrogenases/reductases are normally instable in organic solvents, leading to the unsatisfied enantioselective synthesis efficiency. The behaviors of these enzymes in organic solvents at an atomic level are unclear, thus it is of great importance to understand its structure-based mechanisms in organic-aqueous systems to improve their relative stability. Here, we summarized the accessible structures of alcohol dehydrogenases/reductases in Protein Data Bank crystallized in organic-aqueous systems, and compared the structures of alcohol dehydrogenases/reductases which have different tolerance towards organic solvents. By understanding the catalytic behaviors and mechanisms of these enzymes in organic-aqueous systems, the efficient enantioselective syntheses mediated by alcohol dehydrogenases/reductases and further challenges are also discussed through solvent engineering and enzyme-immobilization in the last decade.

Mutational analysis confirms the presence of distal inhibitor-selectivity determining residues in B. stearothermophilus dihydrofolate reductase

Many antibacterial and antiparasitic drugs work by competitively inhibiting dihydrofolate reductase (DHFR), a vital enzyme in folate metabolism. The interactions between inhibitors and DHFR active site residues are known in many homologs but the contributions from distal residues are less understood. Identifying distal residues that aid in inhibitor binding can improve targeted drug development programs by accounting for distant influences that may be less conserved and subject to frequent resistance causing mutations. Previously, a novel, homology-based, computational approach that mines ligand inhibition data was used to predict residues involved in inhibitor selectivity in the DHFR family. Expectedly, some inhibitor selectivity determining residue positions were predicted to lie in the active site and coincide with experimentally known inhibitor selectivity determining positions. However, other residues that group spatially in clusters distal to the active site have not been previously investigated. In this study, the effect of introducing amino acid substitutions at one of these predicted clusters (His38-Ala39-Ile40) on the inhibitor selectivity profile in Bacillus stearothermophilus dihydrofolate reductase (Bs DHFR) was investigated. Mutations were introduced into these cluster positions to change sidechain chemistry and size. We determined kcat and KM values and measured KD values at equilibrium for two competitive DHFR inhibitors, trimethoprim (TMP) and pyrimethamine (PYR). Mutations in the His38-Ala39-Ile40 cluster significantly impacted inhibitor binding and TMP/PYR selectivity - seven out of nine mutations resulted in tighter binding to PYR when compared to TMP. These data suggest that the His38-Ala39-Ile40 cluster is a distal inhibitor selectivity determining region that favors PYR binding in Bs DHFR and, possibly, throughout the DHFR family.

Extensive Assessment of Various Computational Methods for Aspartate's pKa Shift

A series of computational methods for pK_a shift prediction are extensively tested on a set of benchmark protein systems, aiming at identifying pitfalls and evaluating their performance on high variants. Including 19 ASP residues in 10 protein systems, the benchmark set consists of both residues with highly shifted pK_a values as well as those varying little from the reference value, with an experimental RMS free energy differences of 2.49 kcal/mol with respect to blocked amino acid, namely the RMS pK_a shift being 1.82 pK_a units. The constant pH molecular dynamics (MD), alchemical methods, PROPKA3.1, and multiconformation continuum electrostatics give RMSDs of 1.52, 2.58, 1.37, and 3.52 pK_a units, respectively, on the benchmark set. The empirical scoring method is the most accurate one with extremely low computational cost, and the pH-dependent model is also able to provide accurate results, while the accuracy of MD sampling incorporating alchemical free energy simulation is prohibited by convergence achievement and the performance of conformational search incorporating multiconformation continuum electrostatics is bad. Former research works did not define statistical uncertainty with care and yielded the questionable conclusion that alchemical methods perform well in most benchmarks. In this work the traditional alchemical methods are thoroughly tested for high variants. We also performed the first application of nonequilibrium alchemical methods to the pK_a cases.

Insights on the Auxochromic Properties of the Guanidinium Group

UV/vis spectra of phenylguanidine (PHGU) in the gas phase and in acetonitrile have been simulated by TD-DFT calculations. Several DFT hybrid and long-range corrected functionals were tested with respect to CASPT2 gas phase calculations. Solvent effects were considered using polarizable continuum model (PCM) and compared with the measured data in acetonitrile. Comparison with isoelectronic phenylurea and related phenyltiourea was done as well. The PBE0 and long-range corrected CAM-B3LYP functionals were selected to investigate the effect of protonation on the excitation energies and absorption intensities of PHGU and several guanidine derivatives with different aromatic chromophoric groups (naphthyl, anthracenyl, quinolinyl, anthraquinonyl, and coumarinyl). Also, the effect of complexation and specific interactions through hydrogen bonds with different anions was examined. It was shown that the protonation of the guanidine subunit shifts the low energy absorption bands toward higher energies (hypsochromic shift). The shift is reduced upon complexation with anions. In phenylguanidine salts, λmax values are correlated to the anion basicity and strength of H-bonding. The observed changes diminish upon increase of chromophoric size (naphthyl, anthracenyl). Theoretical predictions of UV/vis spectra correlate well with experimentally measured spectra of selected guanidine derivatives and their salts.

Expression, purification and enzymatic characterization of Brugia malayi dihydrofolate reductase

Brugia malayi (B. malayi) is one of the three causative agents of lymphatic filariasis, a neglected parasitic disease. Current literature suggests that dihydrofolate reductase is a potential drug target for the elimination of B. malayi. Here we report the recombinant expression and purification of a ∼20 kDa B. malayi dihydrofolate reductase (BmDHFR). A His6-tagged construct was expressed in E. coli and purified by affinity chromatography to yield active and homogeneous enzyme for steady-state kinetic characterization and inhibition studies. The catalytic activity kcat was found to be 1.4 ± 0.1 s(-1), the Michaelis Menten constant KM for dihydrofolate 14.7 ± 3.6 μM, and the equilibrium dissociation constant KD for NADPH 25 ± 24 nM. For BmDHFR, IC50 values for a six DHFR inhibitors were determined to be 3.1 ± 0.2 nM for methotrexate, 32 ± 22 μM for trimethoprim, 109 ± 34 μM for pyrimethamine, 154 ± 46 μM for 2,4-diaminoquinazoline, 771 ± 44 μM for cycloguanil, and >20,000 μM for 2,4-diaminopyrimidine. Our findings suggest that antifolate compounds can serve as inhibitors of BmDHFR.

Efficient implementation of constant pH molecular dynamics on modern graphics processors

The treatment of pH sensitive ionization states for titratable residues in proteins is often omitted from molecular dynamics (MD) simulations. While static charge models can answer many questions regarding protein conformational equilibrium and protein-ligand interactions, pH-sensitive phenomena such as acid-activated chaperones and amyloidogenic protein aggregation are inaccessible to such models. Constant pH molecular dynamics (CPHMD) coupled with the Generalized Born with a Simple sWitching function (GBSW) implicit solvent model provide an accurate framework for simulating pH sensitive processes in biological systems. Although this combination has demonstrated success in predicting pKa values of protein structures, and in exploring dynamics of ionizable side-chains, its speed has been an impediment to routine application. The recent availability of low-cost graphics processing unit (GPU) chipsets with thousands of processing cores, together with the implementation of the accurate GBSW implicit solvent model on those chipsets (Arthur and Brooks, J. Comput. Chem. 2016, 37, 927), provide an opportunity to improve the speed of CPHMD and ionization modeling greatly. Here, we present a first implementation of GPU-enabled CPHMD within the CHARMM-OpenMM simulation package interface. Depending on the system size and nonbonded force cutoff parameters, we find speed increases of between one and three orders of magnitude. Additionally, the algorithm scales better with system size than the CPU-based algorithm, thus allowing for larger systems to be modeled in a cost effective manner. We anticipate that the improved performance of this methodology will open the door for broad-spread application of CPHMD in its modeling pH-mediated biological processes. © 2016 Wiley Periodicals, Inc.

Escherichia coli dihydrofolate reductase catalyzed proton and hydride transfers: temporal order and the roles of Asp27 and Tyr100

The reaction catalyzed by Escherichia coli dihydrofolate reductase (ecDHFR) has become a model for understanding enzyme catalysis, and yet several details of its mechanism are still unresolved. Specifically, the mechanism of the chemical step, the hydride transfer reaction, is not fully resolved. We found, unexpectedly, the presence of two reactive ternary complexes [enzyme:NADPH:7,8-dihydrofolate (E:NADPH:DHF)] separated by one ionization event. Furthermore, multiple kinetic isotope effect (KIE) studies revealed a stepwise mechanism in which protonation of the DHF precedes the hydride transfer from the nicotinamide cofactor (NADPH) for both reactive ternary complexes of the WT enzyme. This mechanism was supported by the pH- and temperature-independent intrinsic KIEs for the C-H→C hydride transfer between NADPH and the preprotonated DHF. Moreover, we showed that active site residues D27 and Y100 play a synergistic role in facilitating both the proton transfer and subsequent hydride transfer steps. Although D27 appears to have a greater effect on the overall rate of conversion of DHF to tetrahydrofolate, Y100 plays an important electrostatic role in modulating the pKa of the N5 of DHF to enable the preprotonation of DHF by an active site water molecule.

Toward resolving the catalytic mechanism of dihydrofolate reductase using neutron and ultrahigh-resolution X-ray crystallography

Dihydrofolate reductase (DHFR) catalyzes the NADPH-dependent reduction of dihydrofolate (DHF) to tetrahydrofolate (THF). An important step in the mechanism involves proton donation to the N5 atom of DHF. The inability to determine the protonation states of active site residues and substrate has led to a lack of consensus regarding the catalytic mechanism involved. To resolve this ambiguity, we conducted neutron and ultrahigh-resolution X-ray crystallographic studies of the pseudo-Michaelis ternary complex of Escherichia coli DHFR with folate and NADP(+). The neutron data were collected to 2.0-Å resolution using a 3.6-mm(3) crystal with the quasi-Laue technique. The structure reveals that the N3 atom of folate is protonated, whereas Asp27 is negatively charged. Previous mechanisms have proposed a keto-to-enol tautomerization of the substrate to facilitate protonation of the N5 atom. The structure supports the existence of the keto tautomer owing to protonation of the N3 atom, suggesting that tautomerization is unnecessary for catalysis. In the 1.05-Å resolution X-ray structure of the ternary complex, conformational disorder of the Met20 side chain is coupled to electron density for a partially occupied water within hydrogen-bonding distance of the N5 atom of folate; this suggests direct protonation of substrate by solvent. We propose a catalytic mechanism for DHFR that involves stabilization of the keto tautomer of the substrate, elevation of the pKa value of the N5 atom of DHF by Asp27, and protonation of N5 by water that gains access to the active site through fluctuation of the Met20 side chain even though the Met20 loop is closed.

Simulations of remote mutants of dihydrofolate reductase reveal the nature of a network of residues coupled to hydride transfer

Recent experimental and theoretical studies have proposed that enzymes involve networks of coupled residues throughout the protein that participate in motions accompanying chemical barrier crossing. Here, we have examined portions of a proposed network in dihydrofolate reductase (DHFR) using quantum mechanics/molecular mechanics simulations. The simulations use a hybrid quantum mechanics-molecular mechanics approach with a recently developed semiempirical AM1-SRP Hamiltonian that provides accurate results for this reaction. The simulations reproduce experimentally determined catalytic rates for the wild type and distant mutants of E. coli DHFR, underscoring the accuracy of the simulation protocol. Additionally, the simulations provide detailed insight into how residues remote from the active site affect the catalyzed chemistry, through changes in the thermally averaged properties along the reaction coordinate. The mutations do not greatly affect the structure of the transition state near the bond activation, but we observe differences somewhat removed from the point of C-H cleavage that affect the rate. The mutations have global effects on the thermally averaged structure that propagate throughout the enzyme and the current simulations highlight several interactions that appear to be particularly important.

Investigation of osmolyte effects on FolM: comparison with other dihydrofolate reductases

A weak association between osmolytes and dihydrofolate (DHF) decreases the affinity of the substrate for the Escherichia coli chromosomal and R67 plasmid dihydrofolate reductase (DHFR) enzymes. To test whether the osmolyte-DHF association also interferes with binding of DHF to FolM, an E. coli enzyme that possesses weak DHFR activity, ligand binding was monitored in the presence of osmolytes. The affinity of FolM for DHF, measured by kcat/Km(DHF), was decreased by the addition of an osmolyte. Additionally, binding of the antifolate drug, methotrexate, to FolM was weakened by the addition of an osmolyte. The changes in ligand binding with water activity were unique for each osmolyte, indicating preferential interaction between the osmolyte and folate and its derivatives; however, additional evidence provided support for further interactions between FolM and osmolytes. Binding of the reduced nicotinamide adenine dinucleotide phosphate (NADPH) cofactor to FolM was monitored by isothermal titration calorimetry as a control for protein-osmolyte association. In the presence of betaine (proposed to be the osmolyte most excluded from protein surfaces), the NADPH Kd decreased, consistent with dehydration effects. However, other osmolytes did not tighten binding to the cofactor. Rather, dimethyl sulfoxide (DMSO) had no effect on the NADPH Kd, while ethylene glycol and polyethylene glycol 400 weakened cofactor binding. Differential scanning calorimetry of FolM in the presence of osmolytes showed that both DMSO and ethylene glycol decreased the stability of FolM, while betaine increased the stability of the protein. These results suggest that some osmolytes can destabilize FolM by preferentially interacting with the protein. Further, these weak attractions can impede ligand binding. These various contributions have to be considered when interpreting osmotic pressure results.

Constant pH molecular dynamics of proteins in explicit solvent with proton tautomerism

pH is a ubiquitous regulator of biological activity, including protein-folding, protein-protein interactions, and enzymatic activity. Existing constant pH molecular dynamics (CPHMD) models that were developed to address questions related to the pH-dependent properties of proteins are largely based on implicit solvent models. However, implicit solvent models are known to underestimate the desolvation energy of buried charged residues, increasing the error associated with predictions that involve internal ionizable residue that are important in processes like hydrogen transport and electron transfer. Furthermore, discrete water and ions cannot be modeled in implicit solvent, which are important in systems like membrane proteins and ion channels. We report on an explicit solvent constant pH molecular dynamics framework based on multi-site λ-dynamics (CPHMD(MSλD)). In the CPHMD(MSλD) framework, we performed seamless alchemical transitions between protonation and tautomeric states using multi-site λ-dynamics, and designed novel biasing potentials to ensure that the physical end-states are predominantly sampled. We show that explicit solvent CPHMD(MSλD) simulations model realistic pH-dependent properties of proteins such as the Hen-Egg White Lysozyme (HEWL), binding domain of 2-oxoglutarate dehydrogenase (BBL) and N-terminal domain of ribosomal protein L9 (NTL9), and the pKa predictions are in excellent agreement with experimental values, with a RMSE ranging from 0.72 to 0.84 pKa units. With the recent development of the explicit solvent CPHMD(MSλD) framework for nucleic acids, accurate modeling of pH-dependent properties of both major class of biomolecules-proteins and nucleic acids is now possible.

Capturing molten globule state of α-lactalbumin through constant pH molecular dynamics simulations

The recently developed methods of constant pH molecular dynamics directly captures the correlation between protonation and conformation to probe protein structure, function, and dynamics. In this work, we investigate the effect of pH on the conformational properties of the protein human α-lactalbumin. Constant pH simulations at both acidic and alkaline medium indicate the formation of the molten globule state, which is in accordance with the previous experimental observations (especially, in acidic medium). The size of the protein measured by its radius of gyration (RG) exhibits a marked increase in both acidic and alkaline medium, which matches with the corresponding experimentally observed value of RG found in the molten globule. The probability of native contacts is also considerably reduced at acidic and basic pH as compared to that of native structure crystallized at neutral pH. The mean fractal dimension D2 of the protein records a sharp increase in basic medium as compared to those in neutral and acidic solutions implying a significant pH induced conformational change. The mean square fluctuations of all residues of the entire protein are found to increase by several folds in both acidic and basic medium, which may be correlated with the normalized solvent accessibility of the residues indicating role of solvent accessible surface area on protein internal dynamics. The helices comprising the α-domain of the protein are moderately preserved in the acidic and alkaline pH. However, the β-sheet structures present in the β-domain are completely disrupted in both acidic as well as basic pH.

Hybrid Quantum and Classical Simulations of the Dihydrofolate Reductase Catalyzed Hydride Transfer Reaction on an Accurate Semi-Empirical Potential Energy Surface

Dihydrofolate reductase (DHFR) catalyzes the reduction of 7,8-dihydrofolate by nicotinamide adenine dinucleotide phosphate hydride (NADPH) to form 5,6,7,8-tetrahydrofolate and oxidized nicotinamide. DHFR is a small, flexible, monomeric protein with no metals or SS bonds and serves as one of the enzymes commonly used to examine basic aspects in enzymology. In the current work, we present extensive benchmark calculations for several model reactions in the gas phase that are relevant to the DHFR catalyzed hydride transfer. To this end, we employ G4MP2 and CBS-QB3 ab initio calculations as well as numerous density functional theory methods. Using these results, we develop two specific reaction parameter (SRP) Hamiltonians based on the semiempirical AM1 method. The first generation SRP Hamiltonian does not account for dispersion, while the second generation SRP accounts for dispersion implicitly via the AM1 core-repulsion functions. These SRP semiempirical Hamiltonians are subsequently used in hybrid quantum mechanics/molecular mechanics simulations of the DHFR catalyzed reaction. Finally, kinetic isotope effects are computed using a mass-perturbation-based path-integral approach.

Vibrational softening of a protein on ligand binding

Neutron scattering experiments have demonstrated that binding of the cancer drug methotrexate softens the low-frequency vibrations of its target protein, dihydrofolate reductase (DHFR). Here, this softening is fully reproduced using atomic detail normal-mode analysis. Decomposition of the vibrational density of states demonstrates that the largest contributions arise from structural elements of DHFR critical to stability and function. Mode-projection analysis reveals an increase of the breathing-like character of the affected vibrational modes consistent with the experimentally observed increased adiabatic compressibility of the protein on complexation.

Kinetic and structural characterization of dihydrofolate reductase from Streptococcus pneumoniae

Drug resistance associated with dihydrofolate reductase (DHFR) has emerged as a critical issue in the treatment of bacterial infections. In our efforts to understand the mechanism of a drug-resistant dihydrofolate reductase (DHFR) from a pathogenic bacterial source, we report the first kinetic characterization of Streptococcus pneumoniae DHFR (spDHFR) along with its X-ray structure. This study revealed that the kinetic properties of spDHFR were significantly different from those of Escherichia coli DHFR. The product (tetrahydrofolate) dissociation step that is the rate-limiting step in E. coli DHFR is significantly accelerated in spDHFR so that hydride transfer or a preceding step is rate-limiting. Comparison of the binding parameters of this enzyme to those of a mutant spDHFR (Sp9) confirmed that the Leu100 residue in spDHFR is the critical element for the trimethoprim (TMP) resistance. Steady-state kinetics exhibited a pH dependence in k(cat), which prompted us to elucidate the role of the new catalytic residue (His33) in the active site of spDHFR. Structural data of the Sp9 mutant in complex with NADPH and methotrexate confirmed the participation of His33 in a hydrogen bonding network involving a water molecule, the hydroxyl group of Thr119, and the carboxylate ion of Glu30. Sequence analysis of the DHFR superfamily revealed that the His residue is the major amino acid component at this position and is found mostly in pathogenic bacterial DHFRs. A mutation of Val100 to Leu demonstrated a steric clash of the leucine side chain with the side chains of Ile8 and Phe34, rationalizing weaker binding of trimethoprim to Leu100 DHFR. Understanding the role of specific amino acids in the active site coupled with detailed structural analysis will inform us on how to better design inhibitors targeting drug-resistant pathogenic bacterial DHFRs.

Substrate and inhibitor specificity of Mycobacterium avium dihydrofolate reductase

Dihydrofolate reductase (EC 1.5.1.3) is a key enzyme in the folate biosynthetic pathway. Information regarding key residues in the dihydrofolate-binding site of Mycobacterium avium dihydrofolate reductase is lacking. On the basis of previous information, Asp31 and Leu32 were selected as residues that are potentially important in interactions with dihydrofolate and antifolates (e.g. trimethoprim), respectively. Asp31 and Leu32 were modified by site-directed mutagenesis, giving the mutants D31A, D31E, D31Q, D31N and D31L, and L32A, L32F and L32D. Mutated proteins were expressed in Escherichia coli BL21(DE3)pLysS and purified using His-Bind resin; functionality was assessed in comparison with the recombinant wild type by a standard enzyme assay, and growth complementation and kinetic parameters were evaluated. All Asp31 substitutions affected enzyme function; D31E, D31Q and D31N reduced activity by 80-90%, and D31A and D31L by > 90%. All D31 mutants had modified kinetics, ranging from three-fold (D31N) to 283-fold (D31L) increases in K(m) for dihydrofolate, and 12-fold (D31N) to 223 077-fold (D31L) decreases in k(cat)/K(m). Of the Leu32 substitutions, only L32D caused reduced enzyme activity (67%) and kinetic differences from the wild type (seven-fold increase in K(m); 21-fold decrease in k(cat)/K(m)). Only minor variations in the K(m) for NADPH were observed for all substitutions. Whereas the L32F mutant retained similar trimethoprim affinity as the wild type, the L32A mutation resulted in a 12-fold decrease in affinity and the L32D mutation resulted in a seven-fold increase in affinity for trimethoprim. These findings support the hypotheses that Asp31 plays a functional role in binding of the substrate and Leu32 plays a functional role in binding of trimethoprim.

Conformational change of the methionine 20 loop of Escherichia coli dihydrofolate reductase modulates pKa of the bound dihydrofolate

We evaluate the pK(a) of dihydrofolate (H(2)F) at the N(5) position in three ternary complexes with Escherichia coli dihydrofolate reductase (ecDHFR), namely ecDHFR(NADP(+):H(2)F) in the closed form (1), and the Michaelis complexes ecDHFR(NADPH:H(2)F) in the closed (2) and occluded (3) forms, by performing free energy perturbation with molecular dynamics simulations (FEP/MD). Our simulations suggest that in the Michaelis complex the pK(a) is modulated by the Met20 loop fluctuations, providing the largest pK(a) shift in substates with a "tightly closed" loop conformation; in the "partially closed/open" substates, the pK(a) is similar to that in the occluded complex. Conducive to the protonation, tightly closing the Met20 loop enhances the interactions of the cofactor and the substrate with the Met20 side chain and aligns the nicotinamide ring of the cofactor coplanar with the pterin ring of the substrate. Overall, the present study favors the hypothesis that N(5) is protonated directly from solution and provides further insights into the mechanism of the substrate protonation.

Structural and kinetic evidence for an extended hydrogen-bonding network in catalysis of methyl group transfer. Role of an active site asparagine residue in activation of methyl transfer by methyltransferases

The methyltetrahydrofolate (CH(3)-H(4)folate) corrinoid-iron-sulfur protein (CFeSP) methyltransferase (MeTr) catalyzes transfer of the methyl group of CH(3)-H(4)folate to cob(I)amide. This key step in anaerobic CO and CO(2) fixation is similar to the first half-reaction in the mechanisms of other cobalamin-dependent methyltransferases. Methyl transfer requires electrophilic activation of the methyl group of CH(3)-H(4)folate, which includes proton transfer to the N5 group of the pterin ring and poises the methyl group for reaction with the Co(I) nucleophile. The structure of the binary CH(3)-H(4)folate/MeTr complex (revealed here) lacks any obvious proton donor near the N5 group. Instead, an Asn residue and water molecules are found within H-bonding distance of N5. Structural and kinetic experiments described here are consistent with the involvement of an extended H-bonding network in proton transfer to N5 of the folate that includes an Asn (Asn-199 in MeTr), a conserved Asp (Asp-160), and a water molecule. This situation is reminiscent of purine nucleoside phosphorylase, which involves protonation of the purine N7 in the transition state and is accomplished by an extended H-bond network that includes water molecules, a Glu residue, and an Asn residue (Kicska, G. A., Tyler, P. C., Evans, G. B., Furneaux, R. H., Shi, W., Fedorov, A., Lewandowicz, A., Cahill, S. M., Almo, S. C., and Schramm, V. L. (2002) Biochemistry 41, 14489-14498). In MeTr, the Asn residue swings from a distant position to within H-bonding distance of the N5 atom upon CH(3)-H(4)folate binding. An N199A variant exhibits only approximately 20-fold weakened affinity for CH(3)-H(4)folate but a much more marked 20,000-40,000-fold effect on catalysis, suggesting that Asn-199 plays an important role in stabilizing a transition state or high energy intermediate for methyl transfer.

Mechanism of dihydroneopterin aldolase: functional roles of the conserved active site glutamate and lysine residues

Dihydroneopterin aldolase (DHNA) catalyzes the conversion of 7,8-dihydroneopterin (DHNP) to 6-hydroxymethyl-7,8-dihydropterin (HP) in the folate biosynthetic pathway. There are four conserved active site residues at the active site, E22, Y54, E74, and K100 in Staphylococcus aureus DHNA (SaDHNA), corresponding to E21, Y53, E73, and K98, respectively, in Escherichia coli DHNA (EcDHNA). The functional roles of the conserved glutamate and lysine residues have been investigated by site-directed mutagenesis in this work. E22 and E74 of SaDHNA and E21, E73, and K98 of EcDHNA were replaced with alanine. K100 of SaDHNA was replaced with alanine and glutamine. The mutant proteins were characterized by equilibrium binding, stopped-flow binding, and steady-state kinetic analyses. For SaDHNA, none of the mutations except E74A caused dramatic changes in the affinities of the enzyme for the substrate or product analogues or the rate constants. The Kd values for SaE74A were estimated to be >3000 microM, suggesting that the Kd values of the mutant are at least 100 times those of the wild-type enzyme. For EcDHNA, the E73A mutation increased the Kd values for the substrate or product analogues neopterin (MP), monapterin (NP), and 6-hydroxypterin (HPO) by factors of 340, 160, and 5600, respectively, relative to those of the wild-type enzyme. The K98A mutation increased the Kd values for NP, MP, and HPO by factors of 14, 3.6, and 230, respectively. The E21A mutation increased the Kd values for NP and HPO by factors of 2.2 and 42, respectively, but decreased the Kd value for MP by a factor of 3.3. The E22 (E21) and K100 (K98) mutations decreased the kcat values by factors of 1.3-2 x 10(4). The E74 (E73) mutation decreased in the kcat values by factors of approximately 10. The results suggested that E74 of SaDHNA and E73 of EcDHNA are important for substrate binding, but their roles in catalysis are minor. In contrast, E22 and K100 of SaDHNA are important for catalysis, but their roles in substrate binding are minor. On the other hand, E21 and K98 of EcDHNA are important for both substrate binding and catalysis.

Conformational heterogeneity and low-frequency vibrational modes of proteins

Molecular dynamics simulation and normal mode analysis are used to calculate the vibrational density of states of dihydrofolate reductase complexed with nicotinamide adenine dinucleotide phosphate at 120 K and the results are compared with the experimental spectrum derived from inelastic neutron scattering. The simulation results indicate that the experimental spectrum arises from an average over proteins trapped in different conformations with structural differences mainly in the loop regions, and that these conformations have significantly different low-frequency (<20 cm(-1)) spectra. Thus, the experimentally measured spectrum is an average over the vibrational modes of different protein conformations and is thus inhomogeneously broadened. The implications of this broadening for future neutron scattering experiments and ligand binding calculations are discussed.

Electron Transfer Driven by Proton Fluctuations in a Hydrogen-Bonded Donor−Acceptor Assembly

The temperature-isotope dependence of proton-coupled electron transfer (PCET) for a noncovalent molecular dyad is reported. The system consists of an excited-state Zn(II) porphyrin that transfers an electron to a naphthalene diimide acceptor through an amidinium-carboxylate interface. Two different isotope effects are observed for variant temperature regimes. A reverse isotope effect (i.e., kH/kD < 1) is observed as T approaches 120 K (kH/kD = 0.9, 120 K), whereas a normal isotope effect (i.e., kH/kD > 1) is recovered as the temperature is increased (kH/kD = 1.2, 300 K). The transition between these limits is smooth, with a crossover temperature of T approximately 160 K. These observations are in accordance with charge-transfer dynamics that are susceptible to bath-induced fluctuations in the proton coordinate.

Searching sequence space: two different approaches to dihydrofolate reductase catalysis

There are numerous examples of proteins that catalyze the same reaction while possessing different structures. This review focuses on two dihydrofolate reductases (DHFRs) that have disparate structures and discusses how the catalytic strategies of these two DHFRs are driven by their respective scaffolds. The two proteins are E. coli chromosomal DHFR (Ec DHFR) and a type II R-plasmid-encoded DHFR, typified by R67 DHFR. The former has been described as a very well evolved enzyme with an efficiency of 0.15, while the latter has been suggested to be a model for a "primitive" enzyme that has not yet been optimized by evolution. This comparison underlines what is important to catalysis in these two enzymes and concurrently highlights fundamental issues in enzyme catalysis.

Constant pH molecular dynamics with proton tautomerism

The current article describes a new two-dimensional lambda-dynamics method to include proton tautomerism in continuous constant pH molecular dynamics (CPHMD) simulations. The two-dimensional lambda-dynamics framework is used to devise a tautomeric state titration model for the CPHMD simulations involving carboxyl and histidine residues. Combined with the GBSW implicit solvent model, the new method is tested on titration simulations of blocked histidine and aspartic acid as well as two benchmark proteins, turkey ovomucoid third domain (OMTKY3) and ribonuclease A (RNase A). A detailed analysis of the errors inherent to the CPHMD methodology as well as those due to the underlying solvation model is given. The average absolute error for the computed pKa values in OMTKY3 is 1.0 pK unit. In RNase A the average absolute errors for the carboxyl and histidine residues are 1.6 and 0.6 pK units, respectively. In contrast to the previous work, the new model predicts the correct sign for all the pKa shifts, but one, in the benchmark proteins. The predictions of the tautomeric states of His12 and His48 and the conformational states of His48 and His119 are in agreement with experiment. Based on the simulations of OMTKY3 and RNase A, the current work has demonstrated the capability of the CPHMD technique in revealing pH-coupled conformational dynamics of protein side chains.

Identification of amino acids required for the functional up-regulation of human dihydrofolate reductase protein in response to antifolate Treatment

Human dihydrofolate reductase (DHFR) protein levels rapidly increase upon exposure to methotrexate, a potent inhibitor of this enzyme. A model to explain this increase proposes that DHFR inhibits its own translation by binding to its cognate mRNA and that methotrexate disrupts the DHFR protein-mRNA complex allowing its translation to resume. In the present study, Chinese hamster ovary cells lacking DHFR were transfected with wild type and mutants of human DHFR to identify amino acids that are essential for increases in DHFR in response to methotrexate. Glu-30, Leu-22, and Ser-118 were involved in the up-regulation of DHFR protein levels by methotrexate and certain other antifolates. Cells transfected with E30A, L22R, and S118A mutants that did not respond to methotrexate up-regulation had higher basal levels of DHFR, consistent with the model, i.e. lack of feedback regulation of these enzymes. Although cells containing the S118A mutant enzyme had higher levels of DHFR and had catalytic activity similar to that of wild type DHFR, they had the same sensitivity to the cytotoxicity of methotrexate, as were cells with wild type DHFR. This finding provides evidence that the adaptive up-regulation of DHFR by methotrexate contributes to the decreased sensitivity to this drug. Based on these observations, a new model is proposed whereby DHFR exists in two conformations, one bound to DHFR mRNA and the other bound to NADPH. The mutants that are not up-regulated by methotrexate are unable to bind their cognate mRNA.

The roles of the essential Asp-48 and highly conserved His-43 elucidated by the pH dependence of the pseudouridine synthase TruB

All known pseudouridine synthases have a conserved aspartic acid residue that is essential for catalysis, Asp-48 in Escherichia coli TruB. To probe the role of this residue, inactive D48C TruB was oxidized to generate the sulfinic acid cognate of aspartic acid. The oxidation restored significant but reduced catalytic activity, consistent with the proposed roles of Asp-48 as a nucleophile and general base. The family of pseudouridine synthases including TruB also has a nearly invariant histidine residue, His-43 in the E. coli enzyme. To examine the role of this conserved residue, site-directed mutagenesis was used to generate H43Q, H43N, H43A, H43G, and H43F TruB. Except for phenylalanine, the substitutions seriously impaired the enzyme, but all of the altered TruB retained significant activity. To examine the roles of Asp-48 and His-43 more fully, the pH dependences of wild-type, oxidized D48C, and H43A TruB were determined. The wild-type enzyme displays a typical bell-shaped profile. With oxidized D48C TruB, logk(cat) varies linearly with pH, suggesting the participation of specific rather than general base catalysis. Substitution of His-43 perturbs the pH profile, but it remains bell-shaped. The ascending limb of the pH profile is assigned to Asp-48, and the descending limb is tentatively ascribed to an active site tyrosine residue, the bound substrate uridine, or the bound product pseudouridine.

Combinatorial exploration of the catalytic site of a drug-resistant dihydrofolate reductase: creating alternative functional configurations

We have applied a global approach to enzyme active site exploration, where multiple mutations were introduced combinatorially at the active site of Type II R67 dihydrofolate reductase (R67 DHFR), creating numerous new active site environments within a constant framework. By this approach, we combinatorially modified all 16 principal amino acids that constitute the active site of this enzyme. This approach is fundamentally different from active site point mutation in that the native active site context is no longer accounted for. Among the 1536 combinatorially mutated active site variants of R67 DHFR we created, we selected and kinetically characterized three variants with highly altered active site compositions. We determined that they are of high fitness, as defined by a complex function consisting jointly of catalytic activity and resistance to trimethoprim. The k(cat) and K(M) values were similar to those for the native enzyme. The favourable Delta(DeltaG) values obtained (ranging from -0.72 to -1.08 kcal/mol) suggest that, despite their complex mutational pattern, no fundamental change in the catalytic mechanism has occurred. We illustrate that combinatorial active site mutagenesis can allow for the creation of compensatory mutations that could not be predicted and thus provides a route for more extensive exploration of functional sequence space than is allowed by point mutation.

Structure, dynamics, and catalytic function of dihydrofolate reductase

Molecular motions are widely regarded as contributing factors in many aspects of protein function. The enzyme dihydrofolate reductase (DHFR), and particularly that from Escherichia coli, has become an important system for investigating the linkage between protein dynamics and catalytic function, both because of the location and timescales of the motions observed and because of the availability of a large amount of structural and mechanistic data that provides a detailed context within which the motions can be interpreted. Changes in protein dynamics in response to ligand binding, conformational change, and mutagenesis have been probed using numerous experimental and theoretical approaches, including X-ray crystallography, fluorescence, nuclear magnetic resonance (NMR), molecular dynamics simulations, and hybrid quantum/classical dynamics methods. These studies provide a detailed map of changes in conformation and dynamics throughout the catalytic cycle of DHFR and give new insights into the role of protein motions in the catalytic activity of this enzyme.

Direct determination of vibrational density of states change on ligand binding to a protein

The change in the vibrational density of states of a protein (dihydrofolate reductase) on binding a ligand (methotrexate) is determined using inelastic neutron scattering. The vibrations of the complex soften significantly relative to the unbound protein. The resulting free-energy change, which is directly determined by the density of states change, is found to contribute significantly to the binding equilibrium.