The energetics of peptide-lipid interactions: Modulation by interfacial dipoles and cholesterol
The goal of this work is to determine how specific compositional, structural, and mechanical properties of the lipid bilayer modulate the membrane binding and conformation of a biologically significant peptide, the signal sequence of the bacterial protein LamB. The combination of circular dichroism, microelectrophoresis, and fluorescence quenching data showed that this peptide converted from a random coil in the aqueous phase to a "hammock" configuration in bilayers, with both termini exposed to the aqueous phase on the outside of the lipid vesicle and a central hydrophobic a -helical segment located near the hydrocarbon-water interface. Although the free energy of binding ΔG depended on the charge of the lipid, indicating the importance of electrostatics, the peptide bound to zwitterionic phosphatidylcholine (PC) bilayers with ΔG = -6.3 kcal/mol. Analysis of isothermal titration calorimetry (ITC) data indicated that both conformational changes in the peptide and the hydrophobic effect contributed to this binding to PC bilayers. Peptide binding and α-helix formation were essentially eliminated by adding lipids containing large, oriented dipoles located a few angstroms into the bilayer hydrocarbon region, pointing to an important role for interfacial dipoles. That is, for PC bilayers containing either 6-ketocholestanol or PCs with nitroxide moieties at the 7-position in one of their acyl chains, ΔG was reduced to -3 kcal/mol. Binding experiments with polyunsaturated PCs showed that ΔG was not strongly dependent on molecular area. However, incorporation of cholesterol into the PC bilayer, which increases the area expansion or isothermal compressibility modulus, modified the energetics of binding. Increasing concentrations of cholesterol reduced ΔG to about -5 kcal/mol at equimolar cholesterol, but even more markedly reduced the enthalpy ΔH of binding from -8 kcal/mol in the absence of cholesterol to near 0 kcal/mol with equimolar cholesterol. Thus, peptide binding was enthalpically driven for PC bilayers, but entropically driven for equimolar PC: cholesterol bilayers. We conclude that variations in lipid composition that alter bilayer properties, including fixed charges, interfacial dipoles, and isothermal compressibility modulus, can strongly influence peptide binding. © 2002.
McIntosh, TJ; Vidal, A; Simon, SA
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