Kinetic and Thermodynamic Analysis of the Interactions of 23-Residue Peptides with Endotoxin

Many naturally occurring peptides exhibit lipopolysaccharide binding properties. In this work we describe the endotoxin binding properties of a series of 23-residue peptides based on the sequence corresponding to the antisense strand of the magainin gene. Biochemical and biophysical characterization of these peptides reveals that they have the tendency to perturb both the inner and outer membranes of test pathogens. Structurally these peptides are amphiphilic and adopt helical conformations in membranes. Three of the seven peptides tested have high affinities for endotoxin that approach the values shown by polymyxin B, a cyclic cationic acylated decapeptide, which is used clinically in treating extreme cases of sepsis. The kinetic parameters obtained using stopped-flow methods and BIAcore analysis, when considered in conjunction with the isothermal titration calorimetry-derived thermodynamic parameters, allow us to highlight the key structural features essential for lipopolysaccharide (LPS) recognition by these peptides. The studies stress the role of ionic forces in the initial recognition of LPS. The fortification of the strength of these ionic charges increases affinity for LPS, whereas the hydrophobic residues involved in interactions are more amenable to disruptions in contiguity. Peptides that improve these features further are expected to perform better as endotoxin-neutralizing agents.