What if molecular modeling

Skip to main content Skip to table of contents. Advertisement Hide. This service is more advanced with JavaScript available. Molecular Modeling of Proteins. Editors view affiliations Andreas Kukol. Includes cutting-edge methods and protocols involving the molecular modeling of proteins Provides step-by-step detail essential for reproducible results Contains key notes and implementation advice from the experts.

Front Matter Pages i-x. Front Matter Pages Molecular Dynamics Simulations. Pages Pedro E. Lopes, Olgun Guvench, Alexander D. This is usually done by analogy for bonded terms and assigning charges by a procedure consistent with the used force field. There are many levels of theory in which computational models of 3D structures can be constructed. The overall aim of modelling methods is often to try to relate biological activity to structure.

An important step towards this goal is to be able to compute the potential energy of the molecule as a function of the position of the constituent atoms. Once a method for evaluating the molecular potential energy is available, it is natural to search for an optimum molecular geometry by minimising the energy of the system.

In a biological macromolecule, the potential energy surface is a complicated one, in which there are many local energy minima as well as a single overall energy minimum. All the energy minimisation algorithms commonly used have a marked tendency to locate only a local energy minimum that is close to the starting conformation.

For a biological macromolecule, the number of conformations that have to be searched rises exponentially with the size of the molecule; hence, systematic searching is not a practical method for large molecules. Depending on the simulated temperature of the system, the macromolecule can then overcome barriers at the potential energy surface in a way that is not possible with a minimisation procedure. This method uses a molecular dynamics calculation in which the system temperature is raised to a high value to allow for a widespread exploration of the available conformational space.

The system temperature is then gradually decreased as further dynamics are performed. Finally, a minimisation phase may be used to select a minimum energy molecular conformation. One of the most important applications of molecular modelling techniques in structural biology is the simulation of the docking of a ligand molecule onto a receptor.

These methods often search to identify the location of the ligand binding site and the geometry of the ligand in the active site, to get the correct ranking when considering a series of related ligands in terms of their affinity, or to evaluate the absolute binding free energy as accurately as possible.

To select a force field and the adequate modelling methodology for a given task, it is important to appreciate the range of molecular systems to which it is applicable and the types of simulations that can be performed. It uses five valence terms, one of which is electrostatic and is a basis for other force fields e. It uses five valence terms, one of which is electrostatic [van Gunsteren and Berendsen.

Its parameters are currently being updated [ Horta et al. MM Molecular Mechanics developed by Allinger [ ] are general purpose force fields for monofunctional organic molecules. The first version of this method was the MM1 [ Allinger, ]. MM4 is the latest version with several improvements [ Allinger et al. MMFF94 [ Halgren, ] was originally designed for molecular dynamics simulations, but has also been widely used for geometrical optimisation.

It uses five valence terms, one of which is electrostatic and another is a cross term. MMFF was parameterised based on high level ab initio calculations.

It uses five valence terms, one of which is an electrostatic term and none of them is a cross term. It is often used for CoMFA analysis and uses five valence terms, one of which is an electrostatic term. CVFF Consistent Valence Force Field developed by Dauber-Osguthorpe is a method parameterised for small organic amides and carboxylic acids, among others crystals and gas phase structures [ Dauber-Osguthorpe et al.

It handles peptides, proteins and a wide range of organic systems. It was primarily intended for studies of structures and binding energies, although it predicts vibrational frequencies and conformational energies reasonably well. One of the most important and useful areas of application of molecular modelling is the approach of docking a protein onto a second molecule, typically a small ligand. This is of interest because it models the possible interactions between the protein and the ligand in the formation of a biologically important protein-ligand complex.

To perform a computational docking, experimental or model 3D structures of both the protein and ligand molecules are required together with the charge distribution for each molecule.

There are several software programs that are available for carrying out docking calculations, only some of them will be considered here. First of all, a set of overlapping spheres are used in the program to construct a negative image of a specified site on the protein or another macromolecule, and the negative image is then matched against structures of potential ligands. Matches can be scored in this program by the quality of the geometric fit, as well as by the molecular mechanics interaction energy [ Meng et al.

The program AutoDock developed by Morris et al. In a recent study, comparison of seven popular docking programs [ Plewczynski et al. Yet, there is still the lack of universal scoring function for all types of molecules and protein families. One can always hope that incremental improvements in current techniques will gradually lead to major advances in this field.

Solvation plays an important role in ligand-protein association and has a strong impact on comparisons of binding energies for dissimilar molecules. Various methods have been proposed to evaluate or estimate these terms. The problem is difficult because the energy of each component on the right hand side of Equation 1 is large while the difference between them is small. An accurate way to calculate relative binding energies is with free-energy perturbation techniques, although they are usually restricted to calculating the differential binding of similar compounds and require extensive computation, making it impractical as an initial screen, but quite useful sometimes [ Buch et al.

Several authors have described force fields that consider the bound and solvated states [see for example Chen et al. This is particularly true when the calculation is done partitioning the electrostatic free energy contributions into a van der Waals term from the molecular mechanics force field, and an electrostatic contribution computed using a continuum method [ Checa et al. In most cases, these calculations of molecular mechanics are performed on a structure that is taken to represent the ensemble average of each complex.

Entropy contributions are usually ignored although solvation terms are sometimes added to the scoring function by calculating changes in buried nonpolar surface area [ Viswanadhan et al. In the last years, theoretical methods have been developed to calculate fragment contributions to the solvation free energy, particularly in the framework of quantum mechanical QM continuum solvation methods [ Klamt et al.

An explicit solvent model includes individual solvent molecules and calculates the free energy of solvation by simulating solute-solvent interactions. MC calculations can be used to compute free energy differences and radial distribution functions, among others, and cannot be used to compute time-dependent properties such as diffusion coefficients or viscosity.

MD simulations, on the other hand, can be used to compute free energies and time-dependent properties, transport properties, correlation functions, and others. The charge distribution of the solute polarises the solvent, producing a reaction potential that alters the solute. This interaction is represented by a solvent reaction potential introduced into the Hamiltonian.

As interactions should be self consistently computed, they are also known as self-consistent reaction field SCRF methods [ Onsager, ]. These models are significantly easier than explicit solvent models, but cannot model specific interactions such as hydrogen bonds. With the use of methods to predict binding free energies becoming common-place in the field of drug design, there is still a need for solvation methods that are both quick and accurate [ Mancera, ], although much research has been carried out on the improvement of existing methods and development of new solvation models at many levels of theory [ Chambers et al.

Implicit solvation models offer a faster alternative to explicit models by replacing the individual water molecules with a continuous medium [ Baker, ; Chen et al. For small organic molecules, the loss of molecular detail of the solvent results in relatively small differences between hydration free energy prediction accuracies calculated with explicit solvent models relative to the explicit treatment [ Mobley et al. It is sometimes possible to get quite accurate results with very simple models, such as the case of the molecular modellisation of phenethylamine carriers conducted in our lab.

Calculations were carried out using chloride anion to mimic the picrate anion used in experimental measurements and with no explicit solvent molecules. The chloroform environment was simulated by a constant dielectric factor, as this solvent has a low dielectric constant and thus, interactions should not end quickly with the distance [ Campayo et al. When complexation takes place in water as the solvent, the environment is simulated by a distance-dependent dielectric factor, as it takes into account the fact that the intermolecular electrostatic interactions should vanish with distance faster than in the gas phase.

This assumption proves to work as it gives theoretical results in good agreement with experimental transportation values [ Miranda et al. Latest articles Effects of geometrical parameters and functionalization percentage on the mechanical properties of oxygenated single-walled carbon nanotubes Authors Mohesn Eghbalian Reza Ansari Saeed Rouhi Content type: Original Paper Published: 12 November Article: Improving the antioxidant activity of natural antioxidant honokiol by introducing the amino group Authors first, second and last of 6 Xiaohu Liu Yuanzuo Li Xiuhua Zhao Content type: Original Paper Published: 10 November Article: This journal has open access articles.

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