Binding of nisin Z to bilayer vesicles as determined with isothermal titration calorimetry

Nisin Z, a 34-residue lantibiotic, is secreted by some lactic acid bacteria and exerts its antibacterial activity against various Gram-positive bacteria by permeabilizing the cell membrane. It is a cationic amphiphilic peptide with several unusual dehydro residues and thioether-bridged lanthionines. Isothermal titration calorimetry was used to provide a quantitative thermodynamic description for nisin Z adsorption to and penetration into negatively charged and neutral lipid bilayers. The binding of the cationic peptide (electric charge z similar to 3.8) to anionic membranes was found to be dominated by electrostatic forces which could be described with the Gouy-Chapman theory. For biologically relevant conditions with a membrane surface potential of -40 mV, the peptide concentration near the membrane surface increases by about 2-3 orders of magnitude compared to the bulk concentration. The binding step proper, i.e., the transition from the lipid-water interface into the membrane, is almost exclusively driven by the high surface concentration. Binding can be described by a partition equilibrium of the form X-b = KCM = KCp,f exp(-z(p)psi(0)F(0)/RT), where C-M is the peptide surface concentration, C-p,C-f the bulk concentration, and psi(0) the membrane surface potential. The intrinsic partition coefficient (K = 1.8 M-1) is remarkably small, indicating a correspondingly small hydrophobic energy contribution to the binding process. The electrostatic model was confirmed with nisin Z mutants in which valine-32 was replaced with either lysine (V32K) or glutamate (V32E), increasing or decreasing the electric charge by 1 unit. The extent of peptide binding increased for V32K and decreased for V32E as predicted by the electrostatic theory. In contrast, electrostatic effects were almost negligible for the binding of nisin Z to neutral membranes. However, the binding isotherms were characterized by a distinctly larger intrinsic binding constant K-0 of similar to 540 M-1 and an enhanced hydrophobic free energy of binding. The binding of nisin Z to sonicated lipid vesicles is exothermic with a Delta H-o of ca. -9 and -3.4 kcal/mol for charged and neutral membranes, respectively.