Membrane Binding and Pore Formation of the Antibacterial Peptide PGLa: Thermodynamic and Mechanistic Aspects

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Abstract
The antibacterial peptide PGLa exerts its activity by permeabilizing bacterial membranes whereas eukaryotic membranes are not affected. To provide insight into the selectivity and the permeabilization mechanism, the binding of PGLa to neutral and negatively charged model membranes was studied with high-sensitivity isothermal titration calorimetry (ITC), circular dichroism (CD), and solid-state deuterium nuclear magnetic resonance (2H NMR). The binding of PGLa to negatively charged phosphatidylcholine (PC)/phosphatidylglycerol (PG) (3:1) vesicles was by a factor of ∼50 larger than that to neutral PC vesicles. The negatively charged membrane accumulates the cationic peptide at the lipid−water interface, thus facilitating the binding to the membrane. However, if bulk concentrations are replaced by surface concentrations, very similar binding constants are obtained for neutral and charged membranes (K ∼ 800−1500 M-1). Membrane selectivity is thus caused almost exclusively by electrostatic attraction to the membrane surface and not by hydrophobic insertion. Membrane insertion is driven by an exothermic enthalpy (ΔH ∼ −11 to −15 kcal/mol) but opposed by entropy. An important contribution to the binding process is the membrane-induced random coil → α-helix transition of PGLa. The peptide is random coil in solution but adopts an ∼80% α-helical conformation when bound to the membrane. Helix formation is an exothermic process, contributing ∼70% to the binding enthalpy and ∼30% to the free energy of binding. The 2H NMR measurements with selectively deuterated lipids revealed small structural changes in the lipid headgroups and in the hydrocarbon interior upon peptide binding which were continuous over the whole concentration range. In contrast, isothermal titration calorimetry of PGLa solutions with PC/PG(3:1) vesicles gave rise to two processes: (i) an exothermic binding of PGLa to the membrane followed by (ii) a slower endothermic process. The latter is only detected at peptide-to-lipid ratios >17 mmol/mol and is paralleled by the induction of membrane leakiness. Dye efflux measurements are consistent with the critical limit derived from ITC measurements. The endothermic process is assigned to peptide pore formation and/or lipid perturbation. The enthalpy of pore formation is 9.7 kcal/mol of peptide. If the same excess enthalpy is assigned to the lipid phase, the lipid perturbation enthalpy is 180 cal/mol of lipid. The functional synergism between PGLa and magainin 2 amide could also be followed by ITC and dye release experiments and is traced back to an enhanced pore formation activity of a peptide mixture.

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