Structural and thermodynamic analyses of the interaction between melittin and lipopolysaccharide

Lipopolysaccharide (LPS), the major constituent of the outer membrane of Gram-negative bacteria, is the very first site of interactions with the antimicrobial peptides. In this work, we have determined a solution conformation of melittin, a well-known membrane active amphiphilic peptide from honey bee venom, by transferred nuclear Overhauser effect (Tr-NOE) spectroscopy in its bound state with lipopolysaccharide. The LPS bound conformation of melittin is characterized by a helical structure restricted only to the C-terminus region (residues A15-R24) of the molecule. Saturation transfer difference (STD) NMR studies reveal that several C-terminal residues of melittin including Trp19 are in close proximity with LPS. Isothermal titration calorimetry (ITC) data demonstrates that melittin binding to LPS or lipid A is an endothermic process. The interaction between melittin and lipid A is further characterized by an equilibrium association constant (Ka) of 2.85 x 10(6) M(-1) and a stoichiometry of 0.80, melittin/lipid A. The estimated free energy of binding (delta G0), -8.8 kcal mol(-1), obtained from ITC experiments correlates well with a partial helical structure of melittin in complex with LPS. Moreover, a synthetic peptide fragment, residues L13-Q26 or mel-C, derived from the C-terminus of melittin has been found to contain comparable outer membrane permeabilizing activity against Escherichia coli cells. Intrinsic tryptophan fluorescence experiments of melittin and mel-C demonstrate very similar emission maxima and quenching in presence of LPS micelles. The Red Edge Excitation Shift (REES) studies of tryptophan residue indicate that both peptides are located in very similar environment in complex with LPS. Collectively, these results suggest that a helical conformation of melittin, at its C-terminus, could be an important element in recognition of LPS in the outer membrane.