(671j) Translocation Energy Calculation on Human ? Defensin Type 3 through Bacterial Lipid Membranes

Human β Defensin type 3 (hBD-3), mainly secreted by the human epithelial cells, is a cysteine-rich small peptide, which has 45 residues and a charge density of +11. It can form a dimer or higher order oligomer. It has antimicrobial activity even at high salt concentrations, of which hBD-3 is believed to interact with the bacterial membrane, breaking the membrane, causing cell leakage, thus killing the bacterial cell. To better understand the anti-bacterial mechanism of hBD-3, we have calculated free energies of hBD-3 monomer, dimer and tetramer through two different types of bacterial lipid bilayers. One kind is represented by 1-Palmitoyl-2-oleoyl-sn-glycero-3–phosphatidylserine (POPS) bilayer, and the other is the mixture of POPS with 1-Palmitoyl-2-oleoyl-sn-glycero-3 –phosphatidylcholine (POPC), to consider the lipid membrane chemical composition effect. Besides that, hBD-3 dimer through POPC bilayer was also performed to as a comparison, with POPC bilayer resembling neutrally charged normal cell membrane. Umbrella sampling simulations along with coarse-grained molecular dynamics were performed to calculate the potential of mean force (PMF) associated with hBD-3 monomer and oligomers translocating through different kinds of membranes. Based on PMF results up to now, hBD-3 dimer cannot trans-pass the neutrally charged lipid membrane, while only needs to overcome a much lower energy barrier to transpass the negatively charged lipid membrane. hBD-3 dimer needs to encounter a similar free energy barrier to hBD-3 monomer to cross the POPS lipid bilayer. Besides that, due to the electrostatic interaction between positively charged hBD-3 and the negatively charged head of POPS lipid, hBD-3 inserting into the negatively charged membrane by no more than 16.2 Šis energetically favorable.