Biomedical subjects

Kenneth M Merz

Publications and source records attributed to Kenneth M Merz.

35 records · Page 2Linked to original sources

PubMed2005

Molecular dynamics simulations of the TEM-1 beta-lactamase complexed with cephalothin.

Herein, we present theoretical results aimed at elucidating the origin of the kinetic preference for penicillins over cephalosporins characteristic of the TEM/SHV subgroup of class A beta-lactamases. First, we study the conformational properties of cephalothin showing that the C2-down conformer of the dihydrothiazine ring is preferred over the C2-up one by approximately 2 kcal/mol in solution (0.4-1.4 kcal/mol in the gas phase). Second, the TEM-1 beta-lactamase complexed with cephalothin is investigated by carrying out a molecular dynamics simulation. The DeltaG(binding) energy is then estimated using molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) and quantum chemical PBSA (QM-PBSA) computational schemes. The preferential binding of benzylpenicillin over cephalothin is reproduced by the different energetic calculations, which predict relative DeltaDeltaG(binding) energies ranging from 1.8 to 5.7 kcal/mol. The benzylpenicillin/cephalothin DeltaDeltaG(binding) energy is most likely due to the lower efficacy of cephalosporins than that of penicillins in order to simultaneously bind the "carboxylate pocket" and the "oxyanion hole" in the TEM-1 active site.

Anti-Bacterial Agents↗

PubMed2005

QMQSAR: utilization of a semiempirical probe potential in a field-based QSAR method.

A semiempirical quantum mechanical approach is described for the creation of molecular field-based QSAR models from a set of aligned ligand structures. Each ligand is characterized by a set of probe interaction energy (PIE) values computed at various grid points located near the surface of the ligand. Single-point PM3 calculations afford these PIE values, which represents a pool of independent variables from which multilinear regression models of activity are built. The best n-variable fit is determined by constructing an initial regression using standard forward stepwise selection, followed by refinement using a simulated annealing technique. The resulting fit provides an easily interpreted 3D physical model of ligand binding affinity. Validation against three literature datasets demonstrates the ability of the semiempirical potential to model critical binding interactions in diverse systems.

Binding Sites↗

PubMed2005

The Amber biomolecular simulation programs.

We describe the development, current features, and some directions for future development of the Amber package of computer programs. This package evolved from a program that was constructed in the late 1970s to do Assisted Model Building with Energy Refinement, and now contains a group of programs embodying a number of powerful tools of modern computational chemistry, focused on molecular dynamics and free energy calculations of proteins, nucleic acids, and carbohydrates.

Algorithms↗

PubMed2004

PM3-compatible zinc parameters optimized for metalloenzyme active sites.

Recent studies have shown that semiempirical methods (e.g., PM3 and AM1) for zinc-containing compounds are unreliable for modeling structures containing zinc ions with ligand environments similar to those observed in zinc metalloenzymes. To correct these deficiencies a reparameterization of zinc at the PM3 level was undertaken. In this effort we included frequency corrected B3LYP/6-311G* zinc metalloenzyme ligand environments along with previously utilized experimental data. Average errors for the heats of formation have been reduced from 46.9 kcal/mol (PM3) to 14.2 kcal/mol for this new parameter set, termed ZnB for "Zinc, Biological." In addition, the new parameter sets predict geometries for the Bacillus fragilis active site model and other zinc metalloenzyme mimics that are qualitatively in agreement with high-level ab initio results, something existing parameter sets failed to do.

Bacillus↗

PubMed2004

Enzymatic catalysis of urea decomposition: elimination or hydrolysis?

We present a high-level quantum chemical study of possible reaction mechanisms associated with the catalytic decomposition of urea by a bioinorganic mimetic of the dinickel active site of urease. We chose the phthalazine-dinickel complexes of Lippard and co-workers, because these mimetics have been shown to hydrolytically degrade urea. High-level quantum chemical methodologies were utilized to identify stable intermediates and transition-state structures along several possible reaction pathways. The computed results were then used to further analyze what may occur in the active site of urease. Valuable information on the latter has been extracted from experimental data, computational approaches, and unpublished molecular dynamics simulations. On the basis of these comparative studies, we propose that both the elimination and hydrolytic pathways may compete for urea decomposition in the active site of urease.

Ammonia↗

PubMed2004

Pose scoring by NMR.

Recently, we have developed a fast approach to calculate NMR chemical shifts using the divide and conquer method at the semiempirical level. To demonstrate the utility of this approach for characterizing protein-ligand interactions, we used the deviation of calculated chemical shift perturbations from experiment to determine the orientation of a ligand (GPI-1046) in the binding pocket of the FK506 binding protein (FKBP12). Moreover, we were able to select the native state of the ligand from a collection of decoy poses. A key hydrogen bond between O1 and HN in Ile56 was also identified. Our results suggest that ligand-induced chemical shift perturbations can be used to refine protein/ligand structures.

Binding Sites↗

PubMed2004

Fast semiempirical calculations for nuclear magnetic resonance chemical shifts: a divide-and-conquer approach.

A new approach to calculate nuclear magnetic resonance chemical shifts has been implemented at the semiempirical modified neglect of diatomic overlap level using gauge-including atomic orbitals. The perturbed density matrix with respect to the magnetic field is obtained by the diagonalization of the complex Fock matrix using the divide and conquer (DC) method, instead of by solving the computationally expensive coupled perturbed Hartree-Fock equations. Adopting the Patchkovskii and Thiel parameters [S. Patchkovskii and W. Thiel J. Comput. Chem. 20, 1220 (1999)], we were able to reproduce their results for small organic molecules. The errors introduced by DC method are negligible, as shown by the calculations on a series of polyalaine structures. Test calculations on proteins have demonstrated that our approach makes it possible to calculate chemical shifts routinely on systems with hundreds of atoms with good accuracy.

Journal Article↗

PubMed2004

The hydrolysis of urea and the proficiency of urease.

We present the results of a computational study of the solution phase decomposition of urea, which provides insight into probable reaction pathways for the urease-catalyzed reaction. Calculations, which were used to derive thermodynamic parameters that were further used for a kinetic analysis, have been done at the solvent-corrected MP2/6-311++G** level. Both elimination and hydrolytic pathways have been considered. Elimination is favored for the solution phase reaction, which proceeds by H-bond coordination of a water molecule to the amine nitrogen atoms. The coordination of one water molecule greatly facilitates the reaction by allowing it to proceed through a cyclic six-member transition state. Aspects of the water-urea H-bond interactions have also provided insights into critical aspects of the hydrogen bond pattern in the urease active site. On the basis of a kinetic analysis, we have estimated the proficiency of urease and have predicted that it is the most proficient enzyme identified to date.

Catalysis↗

PubMed2004

A quantum mechanics-based scoring function: study of zinc ion-mediated ligand binding.

In this communication, we report the development of a novel quantum mechanics-based scoring function to predict free energy of ligand binding in the zinc metalloenzymes carbonic anhydrase (CA) and carboxypeptidase A (CPA). In particular, the AM1 method is used in conjunction with solvation modeling to predict the relative binding affinities of 18 CA and 5 CPA inhibitors. The effect of metal-ligand charge transfer is also discussed and shown to be different in CPA and CA, providing a further challenge to computing metalloenzyme binding affinities.

Carbonic Anhydrase Inhibitors↗

PubMed2003

Ureases: quantum chemical calculations on cluster models.

Herein, we present results from a computational study of dinickel complexes that are relevant to the catalytic hydrolysis of urea exerted by the urease enzymes. The B3LYP density functional is used to characterize the equilibrium geometry, electronic and magnetic properties, and energies for a series of realistic complexes modeling the active site of ureases. The analysis of the theoretical results gives new insight into the structure, substrate binding, and catalytic mechanism. The water bridge between the two Ni(II) ions observed in the crystallographic structures of the ureases was assigned to a hydroxide bridge in agreement with the observed small antiferromagnetic coupling. Both monodentate and bidentate urea-bound complexes, in which urea had favorable orientations for catalysis, were characterized. Finally, two reaction mechanisms were investigated starting from the monodentate and bidentate urea-bound complexes, respectively. Both a Ni1...Ni2 bridging hydroxide and a Ni2-bound water molecule play crucial roles in the two mechanisms.

Bacillus↗

PubMed2003

Prediction of aqueous solubility of a diverse set of compounds using quantitative structure-property relationships.

"Fail early and fail fast" is the current paradigm that the pharmaceutical industry has adopted widely. Removing non-drug-like compounds from the drug discovery lifecycle in the early stages can lead to tremendous savings of resources. Thus, fast screening methods are needed to profile the large collection of synthesized and virtual libraries involved in the early stage. Solubility is one of the filters that are applied extensively to ensure that the compounds are reasonably soluble so that synthesis of the compounds and assay studies of pharmacokinetics and toxicity are feasible. To address this need, we have developed a fast quantitative structure-property relationship (QSPR) model for the prediction of aqueous solubility (at 298 K, unbuffered solution) from the molecular structures. Multiple linear regressions and genetic algorithms were used to develop the models. The model was based on a set of diverse compounds including small organic molecules and drug and drug-like species. The predicted solubility for the training and test sets agrees well with the experimental values. The coefficient of determination is R(2) = 0.84 for the training set of 775 compounds and the RMS error = 0.87. This model was validated on four sets of compounds. The RMS error for the 1665 compounds from the four validation data sets (including compounds from the Physician's Desk References and Comprehensive Medicinal Chemistry databases) is 1 log unit and the unsigned error is 0.77. This model does not require 3-D structure generation which is rather time-consuming. Using 2-D structure as input, this model is able to compute solubility for 90 000-700 000 compounds/h on a SGI Origin 2000 workstation. This kind of fast calculation allows the model to be used in data mining and screening of large synthesized or virtual libraries.

Databases, Factual↗

PubMed2003

Molecular dynamics and quantum chemical studies on the catalytic mechanism of Delta5-3-ketosteroid isomerase: the catalytic diad versus the cooperative hydrogen bond mechanism.

To further understand Delta(5)-3-ketosteroid isomerase (KSI) catalysis, we carried out molecular dynamics (MD) simulations of the KSI dimer ligated with a substrate and reaction intermediate analogue and high level ab initio calculations on relevant enzymatic reaction models. Simulation of the enzyme-substrate complex dimer systems showed asymmetric dynamics between the two monomers, in which the hydrogen bond pattern between the substrate and active site residues in the first and the second subunits supported the cooperative hydrogen bond (CH) and the catalytic diad (CD) mechanisms, respectively. On the other hand, only the CH mechanism was supported in the MD simulation of the enzyme-intermediate complex dimer. From MP2/6-31+G**//RHF/6-31G** calculations, we found the kinetic barriers for the two reaction mechanisms were similar. The CH route afforded a greater stabilization to the enolate intermediate than did the CD counterpart. Thus, the present computational studies indicate that the CH mechanism would be favored over the CD one in the catalytic action of KSI. However, the latter could not be ruled out conclusively because of the explicit appearance of a CD configuration in the MD trajectories of the enzyme-substrate complex and because of the similar intrinsic activation barrier for the CH and CD mechanisms. The appearance of configurations that favor the CD pathway is rationalized in terms of a model in which the KSI-substrate complex does not have a strong preference for one hydrogen bonding pattern over another, while the KSI-intermediate complex favors a cooperative hydrogen bond pattern in order to stabilize the reaction intermediate. This hypothesis is supported by the ab initio calculations which indicate that the CH intermediate is more stable than the CD one by approximately 6.3 kcal/mol.

Catalysis↗

PubMed2003

Insights into the acylation mechanism of class A beta-lactamases from molecular dynamics simulations of the TEM-1 enzyme complexed with benzylpenicillin.

Herein, we present results from molecular dynamics MD simulations ( approximately 1 ns) of the TEM-1 beta-lactamase in aqueous solution. Both the free form of the enzyme and its complex with benzylpenicillin were studied. During the simulation of the free enzyme, the conformation of the Omega loop and the interresidue contacts defining the complex H-bond network in the active site were quite stable. Most interestingly, the water molecule connecting Glu166 and Ser70 does not exchange with bulk solvent, emphasizing its structural and catalytic relevance. In the presence of the substrate, Ser130, Ser235, and Arg244 directly interact with the beta-lactam carboxylate via H-bonds, whereas the Lys234 ammonium group has only an electrostatic influence. These interactions together with other specific contacts result in a very short distance ( approximately 3 A) between the attacking hydroxyl group of Ser70 and the beta-lactam ring carbonyl group, which is a favorable orientation for nucleophilic attack. Our simulations also gave insight into the possible pathways for proton abstraction from the Ser70 hydroxyl group. We propose that either the Glu166 carboxylate-Wat1 or the substrate carboxylate-Ser130 moieties could abstract a proton from the nucleophilic Ser70.

Acylation↗

PubMed2002

Insights into the structure and dynamics of the dinuclear zinc beta-lactamase site from Bacteroides fragilis.

Herein, we report quantum chemical calculations and molecular dynamics (MD) simulations of the dinuclear form of the Bacteroides fragilis zinc beta-lactamase. We studied four different configurations which differ in the protonation state of the Asp103 residue and in the presence or absence of a Zn1-OH-Zn2 bridge. The flexibility of the Zn1-OH-Zn2 bridge was studied by means of quantum mechanical (QM) calculations on cluster models while the relative stabilities of the different configurations were estimated from QM linear scaling calculations on the enzyme. Contacts between important residues (Cys104, Asp69, Lys185, etc.), the solvation of the zinc ions, and the conformation of the active site beta-hairpin loop were characterized by the MD analyses. The influence of the buried sodium ion close to the Zn2 position was investigated by carrying out a secondary simulation where the sodium ion was replaced with an internal water molecule. The comparative structural analyses among the different MD trajectories augmented with energetic calculations have demonstrated that the B. fragilis protein efficiently binds the internal Na(+) ion observed crystallographically. Moreover, we found that when Asp103 is unprotonated, a rigid Zn1-OH-Zn2 bridge results, while for neutral Asp103, a fluctuating Zn1-Zn2 distance was possible via the breaking and formation of the Zn1-OH-Zn2 bridge. The mechanistic implications of these observations are discussed in detail.

Aspartic Acid↗

PubMed2002

Computation of the physio-chemical properties and data mining of large molecular collections.

Very large data sets of molecules screened against a broad range of targets have become available due to the advent of combinatorial chemistry. This information has led to the realization that ADME (absorption, distribution, metabolism, and excretion) and toxicity issues are important to consider prior to library synthesis. Furthermore, these large data sets provide a unique and important source of information regarding what types of molecular shapes may interact with specific receptor or target classes. Thus, the requirement for rapid and accurate data mining tools became paramount. To address these issues Pharmacopeia, Inc. formed a computational research group, The Center for Informatics and Drug Discovery (CIDD).* In this review we cover the work done by this group to address both in silico ADME modeling and data mining issues faced by Pharmacopeia because of the availability of a large and diverse collection (over 6 million discrete compounds) of drug-like molecules. In particular, in the data mining arena we discuss rapid docking tools and how we employ them, and we describe a novel data mining tool based on a ID representation of a molecule followed by a molecular sequence alignment step. For the ADME area we discuss the development and application of absorption, blood-brain barrier (BBB) and solubility models. Finally, we summarize the impact the tools and approaches might have on the drug discovery process.

Algorithms↗

PubMed2002

Molecular dynamics simulations of the dinuclear zinc-beta-lactamase from Bacteroides fragilis complexed with imipenem.

Herein, we present results from MD simulations of the Michaelis complex formed between the dizinc beta-lactamase from B. fragilis and imipenem. We considered two catalytically important configurations, which differ in the presence or absence of a hydroxide bridge connecting the two zinc ions in the active site. The structural and dynamical effects induced by substrate binding, the specific roles of the conserved residues and the zinc-bound water molecules, the near attack conformers of the Michaelis complex, and so forth, are discussed in detail. The relative stability of the two configurations was estimated from QM linear scaling calculations on the enzyme-substrate complex combined with Poisson-Boltzmann electrostatic calculations and normal mode calculations. Importantly, we find that the two configurations have similar energies, indicating that these two structures could readily be interchanged, thereby facilitating catalysis. The configuration with the hydroxide bound to the two zinc ions is predicted to be the resting form of the enzyme, while the configuration without the bridge is the reactive form that was found to place the hydroxide in position to attack the carbonyl of the beta-lactam ring. Thus, we propose that the enzyme initiates catalysis by converting from the hydroxide bridge form into the configuration that lacks the hydroxide bridge. This interconversion increases the nucleophilicity of the hydroxide ion and exposes it to the beta-lactam carbonyl, which ultimately facilitates nucleophilic attack. The implications of the observed modes of binding, the possible influence of mutating the Lys184 and Asn193 residues on substrate binding, and the reaction mechanism are also discussed in detail.

Algorithms↗

PubMed2002

Can we separate active from inactive conformations?

Molecular modeling methodologies such as molecular docking, pharmacophore modeling, and 3D-QSAR, rely on conformational searches of small molecules as a starting point. All of these methodologies seek conformations of the small molecules as they bind to target proteins, i.e., their active conformations. Thus the question as to whether active conformations can be separated from inactive conformations is extremely relevant. In this paper, 3D-descriptors that separate random conformations from active conformations of small molecules are sought. To select appropriate descriptors, 65 protein-ligand complexes were taken from the protein data bank. For each ligand the active conformation was compared to randomly generated low energy conformations. Descriptors such as solvent accessible surface area, number of internal interactions and radius of gyration appear to be useful for separating the active conformations from the random conformations. The results with all these descriptors indicate that active conformations are less compact that random conformations, i.e., they have more solvent accessible surface area, fewer internal interactions and a larger radius of gyration than random conformations. Thus these descriptors could be useful as weights to bias conformational search procedures to conformations more likely to bind to proteins or as filters to eliminate conformations unlikely to bind to any protein.

Computer Simulation↗