Jan Domański

Lipidbook: A Public Repository for Force-Field Parameters Used in Membrane Simulations

Lipidbook is a public database for force-field parameters with a special emphasis on lipids, detergents and similar molecules that are of interest when simulating biological membrane systems. It stores parameter files that are supplied by the community. Topologies, parameters and lipid or whole bilayer structures can be deposited in any format for any simulation code, preferably under a license that promotes “open knowledge.” A number of mechanisms are implemented to aid a user in judging the appropriateness of a given parameter set for a project. For instance, parameter sets are versioned, linked to the primary citation via PubMed identifier and digital object identifier (DOI), and users can publicly comment on deposited parameters. Licensing and, hence, the conditions for use and dissemination of academically generated data are often unclear. In those cases it is also possible to provide a link instead of uploading a file. A snapshot of the linked file is then archived using the WebCite® service without further involvement of the user or Lipidbook, thus ensuring a transparent and permanent history of the parameter set. Lipidbook can be accessed freely online at http://lipidbook.bioch.ox.ac.uk. Deposition of data requires online registration.

Convergence and Sampling in Determining Free Energy Landscapes for Membrane Protein Association

Potential of mean force (PMF) calculations are used to characterize the free energy landscape of protein–lipid and protein–protein association within membranes. Coarse-grained simulations allow binding free energies to be determined with reasonable statistical error. This accuracy relies on defining a good collective variable to describe the binding and unbinding transitions, and upon criteria for assessing the convergence of the simulation toward representative equilibrium sampling. As examples, we calculate protein–lipid binding PMFs for ANT/cardiolipin and Kir2.2/PIP2, using umbrella sampling on a distance coordinate. These highlight the importance of replica exchange between windows for convergence. The use of two independent sets of simulations, initiated from bound and unbound states, provide strong evidence for simulation convergence. For a model protein–protein interaction within a membrane, center-of-mass distance is shown to be a poor collective variable for describing transmembrane helix–helix dimerization. Instead, we employ an alternative intermolecular distance matrix RMS (DRMS) coordinate to obtain converged PMFs for the association of the glycophorin transmembrane domain. While the coarse-grained force field gives a reasonable Kd for dimerization, the majority of the bound population is revealed to be in a near-native conformation. Thus, the combination of a refined reaction coordinate with improved sampling reveals previously unnoticed complexities of the dimerization free energy landscape. We propose the use of replica-exchange umbrella sampling starting from different initial conditions as a robust approach for calculation of the binding energies in membrane simulations.

Balancing Force Field Protein–Lipid Interactions To Capture Transmembrane Helix–Helix Association

Atomistic simulations have recently been shown to be sufficiently accurate to reversibly fold globular proteins and have provided insights into folding mechanisms. Gaining similar understanding from simulations of membrane protein folding and association would be of great medical interest. All-atom simulations of the folding and assembly of transmembrane protein domains are much more challenging, not least due to very slow diffusion within the lipid bilayer membrane. Here, we focus on a simple and well-characterized prototype of membrane protein folding and assembly, namely the dimerization of glycophorin A, a homodimer of single transmembrane helices. We have determined the free energy landscape for association of the dimer using the CHARMM36 force field. We find that the native structure is a metastable state, but not stable as expected from experimental estimates of the dissociation constant and numerous experimental structures obtained under a variety of conditions. We explore two straightforward approaches to address this problem and demonstrate that they result in stable dimers with dissociation constants consistent with experimental data.

Ligandbook: an online repository for small and drug-like molecule force field parameters

Summary: Ligandbook is a public database and archive for force field parameters of small and drug‐like molecules. It is a repository for parameter sets that are part of published work but are not easily available to the community otherwise. Parameter sets can be downloaded and immediately used in molecular dynamics simulations. The sets of parameters are versioned with full histories and carry unique identifiers to facilitate reproducible research. Text‐based search on rich metadata and chemical substructure search allow precise identification of desired compounds or functional groups. Ligandbook enables the rapid set up of reproducible molecular dynamics simulations of ligands and protein‐ligand complexes. Availability and Implementation: Ligandbook is available online at https://ligandbook.org and supports all modern browsers. Parameters can be searched and downloaded without registration, including access through a programmatic RESTful API. Deposition of files requires free user registration. Ligandbook is implemented in the PHP Symfony2 framework with TCL scripts using the CACTVS toolkit. Contact: oliver.beckstein@asu.edu or bogdan.iorga@cnrs.fr; contact@ligandbook.org. Supplementary information: Supplementary data are available at Bioinformatics online.

Interactions of the EphA2 Kinase Domain with PIPs in Membranes: Implications for Receptor Function.

Summary EphA2 is a member of the receptor tyrosine kinase family. Interactions of the cytoplasmic region of EphA2 with the cell membrane are functionally important and yet remain incompletely characterized. Molecular dynamics simulations combined with biochemical studies reveal the interactions of the transmembrane, juxtamembrane (JM), and kinase domains with the membrane. We describe how the kinase domain is oriented relative to the membrane and how the JM region can modulate this interaction. We highlight the role of phosphatidylinositol phosphates (PIPs) in mediating the interaction of the kinase domain with the membrane and, conversely, how positively charged patches at the kinase surface and in the JM region induce the formation of nanoclusters of PIP molecules in the membrane. Integration of these results with those from previous studies enable computational reconstitution of a near complete EphA2 receptor within a membrane, suggesting a role for receptor-lipid interactions in modulation of EphA2.