Amino Acid Sequence and Membrane Binding for a Series of Closely related Amphipathic Peptides

Abstract

We have investigated the dependence of peptide activity on the amino acid sequence for a series of synthetic peptides derived from δ-lysin. δ-Lysin is a 26 amino acid, N-terminally formylated, hemolytic peptide that forms an amphipathic α-helix bound at membrane-water interfaces. A shortened peptide, lysette, was derived from δ-lysin by deletion of the 4 N-terminal amino acid residues. Five variants of lysette were synthesized by altering the amino acid sequence under the constraint that the overall hydrophobic moment be essentially the same for all peptides. Peptide-lipid equilibrium dissociation constants and helicities of peptides bound to zwitterionic lipid vesicles were determined by stopped-flow fluorescence and circular dichroism. We then compared the thermodynamics of peptide binding calculated using the Wimley-White hydrophobicity scale with the experimentally determined free energy of binding. We found a systematic deviation of the experimentally determined dissociation constant and that predicted by the Wimley-White scale. Molecular dynamics simulations suggest two factors that account for the very favorable experimental binding free energy. (1), in all δ-lysin-derived peptides simulated, the initial bilayer contact is made with the polar peptide face, allowing charged residues to establish strong interactions with the bilayer headgroup region early on. (2), if the two aspartate residues contained in the lysette sequences are located at the C-terminus, they remain exposed to water and are, thus, effectively removed from the bilayer headgroup region.