Molecular dynamics simulation of melittin in a dimyristoylphosphatidylcholine bilayer membrane

Molecular dynamics trajectories of melittin in an explicit dimyristoyl phosphatidylcholine (DMPC) bilayer are generated to study the details of lipid-protein interactions at the microscopic lever. Melittin, a small amphipathic peptide round in bee venom, is known to have a pronounced effect on the lysis of membranes. The peptide is initially set parallel to the membrane-solution interfacial region in an a-helical conformation with unprotonated N-terminus. Solid-state nuclear magnetic resonance (NMR) and polarized attenuated total internal reflectance Fourier transform infrared (PATIR-FTIR) properties of melittin are calculated from the trajectory to characterize the orientation of the peptide relative to the bilayer. The residue Lys(7) located in the hydrophobic moiety of the helix and residues Lys(23), Arg(24), Gln(25), and Gln(26) at the C-terminus hydrophilic form hydrogen bonds with water molecules and with the ester carbonyl groups of the lipids, suggesting their important contribution to the stability of the helix in the bilayer. Lipid acyl chains are closely packed around melittin, contributing to the stable association with the membrane. Calculated density profiles and order parameters of the lipid acyl chains averaged over the molecular dynamics trajectory indicate that melittin has effects on both layers of the membrane. The presence of melittin in the upper layer causes a local thinning of the bilayer that favors the penetration of water through the lower layer. The energetic factors involved in the association of melittin at the membrane surface are characterized using an implicit mean-field model in which the membrane and the surrounding solvent are represented as structureless continuum dielectric material. The results obtained by solving the Poisson-Bolztmann equation numerically are in qualitative agreement with the detailed dynamics. The influence of the protonation state of the N-terminus of melittin is examined. After 600 ps, the N-terminus of melittin is protonated and the trajectory is continued for 400 ps, which leads to an important penetration of water molecules into the bilayer. These observations provide insights into how melittin interacts with membranes and the mechanism by which it enhances their lysis.