Biomedical subjects

B M Pettitt

Publications and source records attributed to B M Pettitt.

At least 37 records · Page 2Linked to original sources

PubMed1998

Protein hydration density: theory, simulations and crystallography.

Models of protein hydration are becoming increasingly more accurate in comparison with experimental data. The recent success of these models implies that the major features of the solvation layers are dominated by local correlations and that such correlations are universal. The excellent agreement between theoretical and experimental solvent electron density radial distributions marks a significant success in our ability to accurately model macromolecular hydration.

Crystallography↗

PubMed1998

NMR and quenched molecular dynamics studies of superpotent linear and cyclic alpha-melanotropins.

Conformational searching, computer simulations, synthesis and NMR are used on a variety of alpha melanocyte-stimulating hormone (alpha-MSH) analogues to understand the physical characteristics required for biological potency. Peptides I (Ac-[Nle4,Asp5,D-Phe7,Lys10]alpha-MSH(4-10)-NH2), II (Ac-c[Nle4,Asp5,D-Phe7,Lys10]alpha-MSH(4-10)-NH2) and III (Ac-[Nle4,Asp5,D-Phe7,Dap10]alpha-MSH(4-10)-NH2 all show very similar conformational properties (backbone and side-chain torsional angles), and all display high biological potencies. The modeling results for these compounds are supported by the NMR data. Peptide IV (Ac-c[Nle4,Asp5,D-Phe7,Dap10]alpha-MSH(4-10)-NH2) appears to have a markedly different conformation and has decreased biological potency.

Animals↗

PubMed1997

Modeling the DNA-solvent interface.

We extend the technique of using perpendicular distribution functions to salt solutions around nucleic acids. Both solute density averaged and nonaveraged reference frames are considered and compared. Using a previous simulation of DNA in salt water of over a nanosecond in duration, the aqueous distribution functions were found to be well coveraged, whereas the salt perpendicular distribution functions were less well determined. Three-dimensional density reconstructions reliably showed the prominent solvation features with transferable functions. The number of solute atom types needed for reconstructions of a given precision was determined in the context of the reference simulation data set with the goal of achieving a required level of reconstruction quality.

Computer Simulation↗

PubMed1997

A simple two-dimensional representation for the common secondary structural elements of polypeptides and proteins.

A simple method is presented for projecting the conformation of extended secondary structure elements of peptides and proteins that extend over four C alpha atoms onto a simple two-dimensional surface. A new set of two degrees of freedom is defined, a pseudodihedral involving four sequential C alpha atoms, as well as the triple scalar product for the vectors describing the orientation of the three intervening peptide groups. The method provides a reduction in dimensionality, from the usual combination of multiple phi,psi pairs to a single pair, yielding valuable information concerning the structure and dynamics of these important elements. The new two-dimensional surface is explored by reference to 63 selected protein crystal structures together with a comparison of model built peptides representing the common secondary structural elements. Dynamical aspects on this new surface are examined using a molecular dynamics trajectory of Basic Pancreatic Trypsin Inhibitor.

Aprotinin↗

PubMed1996

Salt effects on peptide conformers: a dielectric study of tuftsin.

Four 1-ns molecular dynamics computer simulations of tuftsin, Thr-Lys-Pro-Arg, are analyzed: (1) cis tuftsin in water, (2) trans tuftsin in water, (3) cis tuftsin in 1 M NaCl, and (4) trans tuftsin in 1 M NaCl. Independently of the salt concentration, the trans conformer has a higher dielectric constant than the cis conformer because the former exhibits a more widely distributed charge distribution in space. Independently of the peptide conformation, the presence of salt reduces the dielectric constants of both the peptide and the solvating water molecules because ions, on binding, restrict the motion of other atoms. In contrast to the dielectric constants, neither the peptide conformation nor the salt concentration shows a significant influence on the dielectric relaxation time of water molecules.

Electrochemistry↗

PubMed1995

Structure and stability of a model pyrimidine-purine-purine DNA triple helix with a GC.T mismatch by simulation.

A 1.5 ns long molecular dynamics simulation was conducted to compare the structure and stability of a model DNA triplex in saline solution with that found from experiments. The model DNA was an antiparallel py.pu.pu (CG.G) 7-mer structure which contained a GC.T mismatch triplet at the middle of the sequence. The local conformation of the mismatch triplet and the effects of this triplet on the global helical structure suggest that the GC.T triplet forms stable hydrogen bonds and shows distortions from an in-plane alignment. The overall rms deviation of the triplex is similar to one without a mismatch, although the thymine base in the mismatch triplet shows significantly higher mobility. A high coordination probability for water between the G and T bases in the mismatch triplet was observed to have an effect on the stability of non-hydrogen-bonded base pairs. Average helical parameters, sugar pucker, and backbone dihedral angles indicate that the CG.G triplets on the 3' side of the mismatch triplet possess different structural and dynamical properties than that of the 5' side. These observations are consistent with recently available experimental results and provide an interpretation of the observed experimental structure. They also suggest that inclusion of explicit water molecules is necessary in order to understand and predict the interaction between the third strand and duplex DNA.

Computer Simulation↗

PubMed1995

A sampling problem in molecular dynamics simulations of macromolecules.

Correlations in low-frequency atomic displacements predicted by molecular dynamics simulations on the order of 1 ns are undersampled for the time scales currently accessible by the technique. This is shown with three different representations of the fluctuations in a macromolecule: the reciprocal space of crystallography using diffuse x-ray scattering data, real three-dimensional Cartesian space using covariance matrices of the atomic displacements, and the 3N-dimensional configuration space of the protein using dimensionally reduced projections to visualize the extent to which phase space is sampled.

Computer Simulation↗

PubMed1995

Solvent effects on model d(CG.G)7 and d(TA.T)7 DNA triple helices.

Using free energy molecular mechanics, we find that the molecular effects of solvent are critical in determining relative stabilities in DNA triple helices or triplexes. The continuum solvent model is unable to differentiate the thermodynamics reflecting the basic solvation differences around the occupied major groove in triplexes. In order to avoid the local minimum problem, which is a major limitation of any modeling study, we started our computations with multiple structures rather than relying on the optimization of a single reference structure. By constructing triplex models with different initial helical twists, helical rises, and sugar-pucker permutations, we explore the potential surface and the structural preference with respect to these variations. We find that in order to accommodate a third strand in triplex formation, the backbone geometry of the B-DNA duplex target has to be adjusted into A-DNA-like form with a deep major groove. This is achieved by concerted adjustment in torsions beta, epsilon, and zeta around the phosphate groups. However, the sugar pucker displays a more rich variation, resulting in conformations not usually associated with the canonical duplex structures.

DNA↗

PubMed1995

Conformational states governing the rates of spontaneous transition mutations.

We report the results of a theoretical study, combining the results of sequence analysis and integral equation structural methods for nucleic acids in aqueous solutions, on the effects of nearest neighbors on the (T.G) mispair in solution, for 12 nearest neighbor contexts. Attempts have been made to classify the structural and energetic effects of the 5' and 3' neighbors with respect to the observed spontaneous mutation rates in vertebrates. It is found that 5' nearest neighbor is probably the most critical structural factor in facilitating or discouraging mutations. Local conformational states correlate with discrimination of bases to be excised in mispairs. Our study confirms the role of the flexibility of the DNA molecule in governing the rates of spontaneous mutations.

Base Composition↗

PubMed1995

Structure and dynamics of the water around myoglobin.

The interplay between simulations at various levels of hydration and experimental observables has led to a picture of the role of solvent in thermodynamics and dynamics of protein systems. One of the most studied protein-solvent systems is myoglobin, which serves as a paradigm for the development of structure-function relationships in many biophysical studies. We review here some aspects of the solvation of myoglobin and the resulting implications. In particular, recent theoretical and simulation studies unify much of the diverse set of experimental results on water near proteins.

Crystallography, X-Ray↗

PubMed1995

Dielectric response of triplex DNA in ionic solution from simulations.

We have analyzed a 1.2-ns molecular dynamics simulation of 51 mM d(CG.G)7 with 21 Na+ counter-ions and 1 M NaCl in water. Via the dipole fluctuations, the dielectric constant for the DNA is found to be around 16, whereas that for the bases and sugars combined is only 3. The dielectric constant for water in this system is 41, which is much smaller than 71 for pure SPC/E water, because of the strong restriction imposed on the motion of water molecules by the DNA and the ions. Also addressed in the present work are several technical issues related to the calculation of the dipole moment of an ionic solution from molecular dynamics simulations using periodic boundary conditions.

DNA↗

PubMed1995

Simulations of conformers of tuftsin and a cyclic tuftsin analog.

The conformational properties of the configurational isomers of tuftsin, a linear tetrapeptide with the sequence Thr-Lys-Pro-Arg, were investigated with six 1 ns molecular dynamics simulations in explicit water and in a 1.0 m NaCl solution. The average conformation of the cis isomer is a type VI beta-turn. Our results indicate that water-peptide hydrogen bonding, in addition to intramolecular hydrogen bonds, stabilizes the cis conformer. The trans isomer is neither a beta- nor a gamma-turn. Results are compared with parallel studies on a cyclic analog of tuftsin, cyclo(Thr-Lys-Pro-Arg-Gly). The addition of salt does, not influence the backbone conformation of the peptide. Differences between the structures are confined to the side-chain orientations of the Lys and Arg residues.

Amino Acid Sequence↗

PubMed1994

A connected-cluster of hydration around myoglobin: correlation between molecular dynamics simulations and experiment.

An analysis of a molecular dynamics simulation of metmyoglobin in an explicit solvent environment of 3,128 water molecules has been performed. Both statics and dynamics of the protein-solvent interface are addressed in a comparison with experiment. Three-dimensional density distributions, temperature factors, and occupancy weights are computed for the solvent by using the trajectory coordinates. Analysis of the hydration leads to the localization of more than 500 hydration sites distributed into multiple layers of solvation located between 2.6 and 6.8 A from the atomic protein surface. After locating the local solvent density maxima or hydration sites we conclude that water molecules of hydration positions and hydration sites are distinct concepts. Both global and detailed properties of the hydration cluster around myoglobin are compared with recent neutron and X-ray data on myoglobin. Questions arising from differences between X-ray and neutron data concerning the locations of the protein-bound water are investigated. Analysis of water site differences found from X-ray and neutron experiments compared with our simulation shows that the simulation gives a way to unify the hydration picture given by the two experiments.

Amino Acid Sequence↗

PubMed1994

Distribution function implied dynamics versus residence times and correlations: solvation shells of myoglobin.

The dynamics of water at the protein-solvent interface is investigated through the analysis of a molecular dynamics simulation of metmyoglobin in explicit aqueous environment. Distribution implied dynamics, harmonic and quasi-harmonic, are compared with the simulated macroscopic dynamics. The distinction between distinguishable solvent molecules and hydration sites developed in the previous paper is used. The simulated hydration region within 7 A from the protein surface is analyzed using a set of 551 hydration sites characterized by occupancy weights and temperature B-factors determined from the simulation trajectory. The precision of the isotropic harmonic and anisotropic harmonic models for the description of proximal solvent fluctuations is examined. Residence times and dipole reorientation times of water around the protein surface are compared with NMR and ESR results. A correlation between diffraction experiment quantities such as the occupancy weights and temperature factors and the residence and correlation times resulting from magnetic resonance experiments is found via comparison with simulation.

Animals↗

PubMed1994

Integral equation approaches to structure and thermodynamics of aqueous salt solutions.

Results for free energy, entropy, enthalpy and internal energy of solvation for monovalent ions in water have been studied by comparing DRISM theory results to those of RISM and ARISM theories. The greatly improved dielectric behavior in the DRISM case enabled the examination of realistically modeled salts at finite concentrations. The link between solvent structure and the entropy of solvent co-spheres was examined. Finally comparison with the Born free energy equation shows its virtues and flaws due to ignoring cavity formation and asymmetric solvation terms which together always contribute significantly to the free energy of hydration of ions.

Binding Sites↗

PubMed1994

A global model of the protein-solvent interface.

The solvent structure and dynamics around myoglobin is investigated at the microscopic level of detail by computer simulation. We analyze a molecular dynamics trajectory in terms of solvent mobility and probability distribution. Local events, occurring in the protein-solvent interfacial region, which are often masked by other approaches are thus revealed. Specifically, the local solvent mobility is greatly enhanced for certain locations at the protein surface and in its interior. In addition, a strong correlation between the solvent mobility and density emerges on both global and local scales. We propose a simple model where the solvent distribution measured perpendicularly to the protein surface is utilized to reconstruct the simulated network of hydration within 6 A from the protein surface with a relative error of only 17%. The global precision of this solvation model matches results obtained with more complicated models usually used in refinement procedures in x-ray and neutron experiments but with far fewer parameters. The dramatically improved correspondence between observed and calculated x-ray intensities at low resolution relative to other methods both confirms the validity of the approach used in the MD (molecular dynamics) simulations and allows the results of this study to be implemented in solvent studies on real systems.

Biophysical Phenomena↗

PubMed1993

The relationship between mutation rates for the (C-G)-->(T-A) transition and features of T-G mispair structures in different neighbor environments, determined by free energy molecular mechanics.

The results of this theoretical study combining sequence analysis and minimization with integral equation liquid structural methods indicate that the local sequence context of a T-G wobble mismatch influences the local conformation of the helix, and that conformational alterations are correlated with mutational activity. Studies on the mismatch in four different 5' and 3' neighbor contexts indicate that the nature of the 5' base to the thymine of the mispair is probably the single most critical factor in determining the structural features that facilitate or discourage mutations. When cytosine is the 5' neighbor, the helix adopts a mostly BII conformation, whereas a 5' guanine preserves the canonical BI. Structures that vary little from the BI structure on the incorporation of the mismatch have sequences that correspond to lower rates of transition, whereas those with mostly BII conformations, have sequences with high mutation rates. Subtle variations in stacking patterns around the mismatch precipitate a structural Domino-effect, with a variety of changes in conformation. The helix opens at the mismatch with increased roll angle and propeller twist, causing the thymine to migrate into the major groove and the guanine into the minor groove, exposing the heteroatomic groups to the solvent in the major and minor grooves, respectively, and allowing for some unusual hydrogen bonds. These alterations show a tentative correlation with mutation rates, implying that stacking and structure around the mismatch are important features in the discrimination by proofreading activities of canonical W-C and wobble mismatch base pairs during replication-repair. Variations in the C1'-C1' distances, high propeller twists, changes in the electrostatic complementarity leading to unusual hydrogen bonding patterns probably all correlate with detectability.

Base Composition↗

PubMed1993

On the mechanism of HRV-14 antiviral compounds: "slow growth" as a conformational search procedure.

We report a novel conformational search procedure that is used to investigate the binding mechanism of a member of the WIN class of antiviral compounds. A simple hypothesis of important residues in the binding site based on differences in drug-free and drug-bound X-ray structures along with more elaborate models, ultimately including the entire virus, is considered. Our search method is a variant of slow-growth molecular dynamics used in free energy simulations and gives rise to local motion in the protein backbone of up to 3 A. This technique involves the scaling of drug-protein interaction energies over time periods of 10-100 ps and gives rise to local motion in the protein backbone. In addition, we have used high-temperature dynamics with periodic quenching to generate low-energy conformations with backbone displacements in the crystallographic binding region of up to 7 A from the native structure. Mechanism of binding, hydrogen-bond stabilization of active-site conformations, concerted drug-protein motions, and the mode of virion stabilization are addressed in relation to our ligand induced and high-temperature conformational search procedures. A loop-cap like mechanism is consistent with the results of our study. A large movement of the "active-site" residues is shown to be theoretically possible and provides a greater access for entry of the drug into its binding pocket than seen in the available crystal structures.

Amino Acid Sequence↗