Interactions of the antimicrobial peptide lactoferricin B with neutral POPC and charged POPG model membrane systems
Molecular dynamics (MD) simulations are used to study the interaction of a zwitterionic palmitoyl-oleoyl-phosphatidylcholine (POPC) bilayer and anionic palmitoyloleoyl-phosphatidylglycerol (POPG) bilayer with the cationic antimicrobial peptide bovine lactoferricin B (LFCinB) in a 100 mM NaCl solution at 310 K. The interaction of LFCinB with POPC and POPG is used as a model system for studying the details of membrane adsorption selectivity and understanding the mechanism of the antimicrobial nature of LFCinB. Seventy-two 3 ns MD simulations, with six orientations of LFCinB at twelve different distances from a POPC membrane, and seventy-eight 4 ns MD simulations, with six orientations of LFCinB at thirteen different distances from a POPG membrane, are carried out to determine the potential of mean force (PMF) or free energy profile of the peptide as a function of the distance between LFCinB and the membrane surface. To calculate the PMF for this relatively large system, a new variant of constrained MD and thermodynamic integration is developed. A simplified method for relating the PMF to the LFCinB-membrane binding free energy is described and used to predict a free energy of adsorption (or binding) of -1.05 ± 0.39 kcal/mol, with a corresponding maximum binding force of about 20 pN, for LFCinB-POPC, and -5.37 ± 1.25 kcal/mol, with a corresponding maximum binding force of about 58 pN, for LFCinB-POPG. The contributions of ions and water to the LFCinB-POPC PMF are discussed. The calculation of the forces from different components of the LFCinB-POPC system shows that the only source of the attractive force acting on the LFCinB is the direct membrane-peptide interaction. Ion-peptide and water-peptide interactions provide a net repulsive force acting on LFCinB. For the LFCinB-POPG system Poisson-Boltzman (PB) theory is used for analyzing the PMF. It is found that the entropic contribution due to counterion-release from the region between the charged membrane and LFCinB plays a major role in the peptide attraction for LFCinB-POPG. In conclusion, the data suggested that the binding for LFCinB-POPG is thus entropically driven, while the binding for LFCinB-POPC is enthalpically driven.