Structural and thermodynamic aspects of the interaction between heparan sulfate and analogues of melittin

Melittin is an amphipathic cationic peptide derived from honeybee venom with well-known cytolytic and antimicrobial properties. When coupled to cationic polymers or lipid molecules, it forms conjugates with high transfection efficiency and low toxicity with promising applications in gene therapy. A first step in the internalization of melittin and its conjugates is their binding to the cell surface, a reaction likely to involve heparan sulfate proteoglycans (HSPG). In the present work, we characterize the binding equilibrium of heparan sulfate (HS) with two melittin analogues, [Cys(1)]melittin (mel-SH) and retro-inverso [Cys(1)]melittin (ri-mel-SH). The terminal cysteine found in these peptides replaces the N-terminal glycine present in native melittin and allows covalent binding to other molecules. Isothermal titration calorimetry (ITC) reveals a high affinity of each melittin analogue to HS. Association constants of 4.7 x 10(4) and 3.5 x 10(5) M-1 are found at physiological ionic strength and 15 degrees C for ri-mel-SH and mel-SH, respectively. The reaction enthalpy measured under these conditions is Delta H degrees(pep) = 4.2 kcal/mol for ri-mel-SH and Delta H degrees(pep) = 1.1 kcal/mol for mel-SH. The peptide-to-HS stoichiometry is similar to 20 for ri-mel-SH and similar to 14 for mel-SH under the same conditions. Temperature dependence studies using ri-mel-SH (mel-SH) show that Delta H degrees(pep) decreases in magnitude upon increase in temperature, which results in a molar heat capacity of Delta H degrees(pep) = -322 cal mol(-1) K-1 (-45 cal mol(-1) K-1). Such a negative heat capacity change is not expected for a purely electrostatic interaction and indicates that hydrophobic and other interactions are also involved in the binding equilibrium. Salt dependence studies of the binding constants confirm that nonelectrostatic forces are an important component of the HS-melittin interaction. Binding to HS induces conformational changes in both peptides, with ri-mel-SH showing a 6-fold increase of the alpha-helix content when incubated with HS under saturation conditions.