Structure, energetics, and dynamics of lipid-protein interactions: A molecular dynamics study of the gramicidin A channel in a DMPC bilayer.

The microscopic details of lipid-protein interactions are examined using molecular dynamics simulations of the gramicidin A channel embedded in a fully hydrated dimyristoyl phosphatidylcholine (DMPC) bilayer. A novel construction protocol was used to assemble the initial configurations of the membrane protein complex for the simulations. Three hundred systems were constructed with different initial lipid placement and conformations. Seven systems were simulated with molecular dynamics. One system was simulated for a total of 600 psec, four were simulated for 300 psec, and two for 100 psec. Analysis of the resulting trajectories shows that the bulk solvent-membrane interface region is much broader than traditionally pictured in simplified continuum theories: its width is almost 15 angstroms. In addition, lipid-protein interactions are far more varied, both structurally and energetically, than is usually assumed: the total interaction energy between the gramicidin A and the individual lipids varies from 0 to -50 kcal/mol. The deuterium quadrupolar splittings of the lipid acyl chains calculated from the trajectories are in good agreement with experimental data. The lipid chains in direct contact with the GA are ordered but the effect is not uniform due to the irregular surface of the protein. Energy decompositions shows that the most energetically favorable interactions between lipid and protein involve nearly equal contributions from van der Waals and electrostatic interactions. The tryptophans, located near the bulk-membrane interface, appear to be particularly important in mediating both hydrogen bonding interactions with the lipid glycerol backbone and water and also in forming favorable van der Waals contacts with the hydrocarbon chains. In contrast, the interactions of the leucine residues with the lipids, also located near the interface, are dominated by van der Waals interactions with the hydrocarbon lipid chains.