Molecular Dynamics Simulations of Human Beta-Defensin Type 3 Crossing Different Lipid Bilayers

Human beta defensin type 3 (hBD-3) is a small cationic cysteine-rich peptide. It has a broad spectrum of antimicrobial activities. However, at high concentrations, it also shows hemolytic activity by interrupting red blood cells. To understand the selectivity of hBD-3 disrupting cell membranes, investigating the capability of hBD-3 translocating through different membranes is important. Since hBD-3 in the analogue form in which all three pairs of disulfide bonds are broken has similar antibacterial activities to the wild-type, this project investigates the structure and dynamics of an hBD-3 analogue in monomer, dimer, and tetramer forms through both zwitterionic and negatively charged lipid bilayers using molecular dynamics (MD) simulations. One tetramer structure of hBD-3 was predicted by running all-atom MD simulations on hBD-3 in water at a high concentration, which was found to be stable in water during 400 ns all-atom simulations based on root-mean-squared deviation, root-mean-squared fluctuation, buried surface area, and binding interaction energy calculations. After that, hBD-3 in different forms was placed inside different membranes, and then steered MD simulation was conducted to pull the hBD-3 out of the membrane along the z-direction to generate different configurational windows to set up umbrella-sampling (US) simulations. Because extensive sampling is important to obtain accurate free energy barriers, coarse-grained US MD simulations were performed in each window. Based on the long-term simulation result, membrane thinning was found near hBD-3 in different lipid bilayers and in different hBD-3 oligomer systems. By calculating the root-mean-squared deviation of the z-coordinate of hBD-3 molecules, rotation of the oligomer inside the bilayer and stretching of the oligomer structure along the z-direction were observed. Although reorientation of lipid heads toward the hBD-3 tetramer was observed based on the density profile calculation, the order parameter calculation shows that hBD-3 disrupts 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine (POPS) lipids more significantly and makes it less ordered than on 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipids. Calculating the free energy of hBD-3 through different lipid bilayers, it was found that generally hBD-3 encounters a lower energy barrier through negatively charged lipid membranes than the zwitterionic membrane. hBD-3 in different forms needs to overcome a lower energy barrier crossing the combined POPC+POPS bilayer through the POPS leaflet than through the POPC leaflet. Besides that, the potential of mean force result suggests that hBD-3 forms an oligomer translocating negatively charged lipid membranes at a low concentration. This study supplied new insight into the antibacterial mechanism of hBD-3 through different membranes.