Role of peptide peptide interactions in aggregation: protonectins observed in equilibrium and replica exchange molecular dynamics simulations

Abstract

Protonectin (ILGTILGLLKGL-NH2), a peptide extracted from the venom of the wasp Agelaia pallipes pallipes, promotes mast cell degranulation activity, antibiosis against Gram-positive and -negative bacteria, and chemotaxis in polymorphonucleated leukocytes. Another peptide from the same venom, Protonectin (1-6), corresponding to the first six residues of Protonectin, exhibits only chemotaxis. A 1:1 mixture of these two peptides showed positive synergistic antimicrobial effects, attributed to the formation of a heterodimer.16 The antimicrobial activity is probably related to the peptides interaction with membrane phospholipids. Equilibrium and replica exchange molecular dynamics simulations were used to investigate two systems: the interaction of Protonectins (two molecules) and that of a mixture Protonectin and Protonectin (1-6) in the environment of sodium dodecyl sulfate (SDS) micelles, which mimic bacterial membranes and are also highly anionic. We found that in both systems the peptides tend to aggregate in the aqueous environment and are held together by hydrophobic interactions and hydrogen bonds. In the equilibrium simulations, aggregated Protonectin/Protonectin (1-6) dissociates after penetrating the SDS micelle, whereas the two Protonectins remain associated throughout the simulation time. Also, in the replica exchange simulations, the Protonectins remain closer, associating through a greater number of hydrogen bonds, and were found at only one free energy minimum, whereas the peptides in the mixture display other probable distances from each other, which are significantly longer than those observed with two Protonectin molecules. Coulomb contributions and the free energy of the systems containing micelles were calculated and show that the interactions of the mixed peptides are favored, whereas the interactions between pure Protonectins are more probable. As a consequence of the preferential interaction with the micelle, the Protonectin molecule of the mixed system presents a higher helical structure content. The enhancement of the amphipathic features caused by Protonectin (1-6) can be related to the increase in the antimicrobial activity experimentally observed.