Enthalpic and Entropic Contributions to Interleaflet Coupling Drive Domain Registration and Anti-registration in Biological Membrane

Biological membrane is a complex self-assembly of lipids, sterols and proteins organized as a fluid bilayer of two closely stacked lipid leaflets. Differential molecular interactions among its diverse constituents give rise to heterogeneities in the membrane lateral organization. Under certain conditions, heterogeneities in the two leaflets can be spatially synchronised and exist as registered domains across the bilayer. Several contrasting theories behind mechanisms that induce registration of nanoscale domains have been suggested[1–3]. Following a recent study[4] showing the effect of position of lipid tail unsaturation on domain registration behavior, we decided to develop an analytical theory to elucidate the driving forces that create and maintain domain registry across leaflets. Towards this, we formulated a Hamiltonian for a stacked lattice system where site variables encapsulate the lipid molecular properties including the position of unsaturation and various other interactions that could drive phase separation and interleaflet coupling. We solve the Hamiltonian using Monte Carlo simulations and create a complete phase diagram that reports the presence or absence of registered domains as a function of various Hamiltonian parameters. We find that the interleaflet coupling should be described as a competing enthalpic contribution due to interaction of lipid tail termini, primarily due to saturated-saturated interactions, and an interleaflet entropic contribution from overlap of unsaturated tail termini. We find that higher position of unsaturation provides weaker interleaflet coupling. We also find points in our parameter space that allow thermodynamically stable nanodomains in our bilayer model, which we have verified by carrying out extended Monte Carlo simulations. These persistent non-coalescing registered nanodomains close to the lower end of the accepted nanodomain size range also point towards a possible “nanoscale” emulsion description of lateral heterogeneities in biological membrane leaflets.

Energy-entropy prediction of octanol–water logP of SAMPL7 N-acyl sulfonamide bioisosters

Partition coefficients quantify a molecule’s distribution between two immiscible liquid phases. While there are many methods to compute them, there is not yet a method based on the free energy of each system in terms of energy and entropy, where entropy depends on the probability distribution of all quantum states of the system. Here we test a method in this class called Energy Entropy Multiscale Cell Correlation (EE-MCC) for the calculation of octanol–water logP values for 22 N-acyl sulfonamides in the SAMPL7 Physical Properties Challenge (Statistical Assessment of the Modelling of Proteins and Ligands). EE-MCC logP values have a mean error of 1.8 logP units versus experiment and a standard error of the mean of 1.0 logP units for three separate calculations. These errors are primarily due to getting sufficiently converged energies to give accurate differences of large numbers, particularly for the large-molecule solvent octanol. However, this is also an issue for entropy, and approximations in the force field and MCC theory also contribute to the error. Unique to MCC is that it explains the entropy contributions over all the degrees of freedom of all molecules in the system. A gain in orientational entropy of water is the main favourable entropic contribution, supported by small gains in solute vibrational and orientational entropy but offset by unfavourable changes in the orientational entropy of octanol, the vibrational entropy of both solvents, and the positional and conformational entropy of the solute.

Entropic effect and residue specific entropic contribution to the cooperativity in streptavidin–biotin binding

Molecular dynamics (MD) simulations were performed employing the polarized protein-specific charge (PPC) to explore the origin of the cooperativity in streptavidin–biotin systems (wild type, two single mutations and one double-mutation).

An accurate free energy estimator: based on MM/PBSA combined with interaction entropy for protein–ligand binding affinity

Modifying the energy term and considering the entropic contribution by IE method significantly improve the accuracy of predicted binding free energy in MM/PBSA method.

Application of ESMACS Binding Free Energy Protocols to Diverse Datasets: Bromodomain-Containing Protein 4

<div>We investigate the robustness of our ensemble molecular dynamics binding free energy protocols, known as ESMACS, to different choices of forcefield, starting structure and analysis. ESMACS is based on MMPBSA and we examinge the influence of multiple trajectories, explicit water molecules and estimates of the entropic contribution to the binding free energy.</div><div><br></div><div>Simulation input and binding affinity calculation data:</div>https://doi.org/10.5281/zenodo.1484050

Application of ESMACS Binding Free Energy Protocols to Diverse Datasets: Bromodomain-Containing Protein 4

<div>We investigate the robustness of our ensemble molecular dynamics binding free energy protocols, known as ESMACS, to different choices of forcefield, starting structure and analysis. ESMACS is based on MMPBSA and we examinge the influence of multiple trajectories, explicit water molecules and estimates of the entropic contribution to the binding free energy.</div><div><br></div><div>Simulation input and binding affinity calculation data:</div>https://doi.org/10.5281/zenodo.1484050

Application of ESMACS Binding Free Energy Protocols to Diverse Datasets: Bromodomain-Containing Protein 4

<div>We investigate the robustness of our ensemble molecular dynamics binding free energy protocols, known as ESMACS, to different choices of forcefield, starting structure and analysis. ESMACS is based on MMPBSA and we examinge the influence of multiple trajectories, explicit water molecules and estimates of the entropic contribution to the binding free energy.</div><div><br></div><div>Simulation input and binding affinity calculation data:</div>https://doi.org/10.5281/zenodo.1484050

Entropic contribution to enhanced thermal stability in the thermostable P450 CYP119

The enhanced thermostability of thermophilic proteins with respect to their mesophilic counterparts is often attributed to the enthalpy effect, arising from strong interactions between protein residues. Intuitively, these strong interresidue interactions will rigidify the biomolecules. However, the present work utilizing neutron scattering and solution NMR spectroscopy measurements demonstrates a contrary example that the thermophilic cytochrome P450, CYP119, is much more flexible than its mesophilic counterpart, CYP101A1, something which is not apparent just from structural comparison of the two proteins. A mechanism to explain this apparent contradiction is that higher flexibility in the folded state of CYP119 increases its conformational entropy and thereby reduces the entropy gain during denaturation, which will increase the free energy needed for unfolding and thus stabilize the protein. This scenario is supported by thermodynamic data on the temperature dependence of unfolding free energy, which shows a significant entropic contribution to the thermostability of CYP119 and lends an added dimension to enhanced stability, previously attributed only to presence of aromatic stacking interactions and salt bridge networks. Our experimental data also support the notion that highly thermophilic P450s such as CYP119 may use a mechanism that partitions flexibility differently from mesophilic P450s between ligand binding and thermal stability.