Amphiphilic α‐helices are important structural motifs in the α and β peptides that constitute the bacteriocin lactococcin G

Abstract

Lactococcin G (LcnG) is an antimicrobial substance (bacteriocin) consisting of two peptides, LcnG-α and LcnG-β. The structures of intact LcnG-α and LcnG-β as well as various fragments of these peptides were studied by circular dichroism (CD) under several conditions. All peptides had a non-structured conformation in aqueous solutions. In the presence of trifluoroethanol, dodecylphosphocholine micelles and (negatively charged) dioleoylglycerophosphoglycerol (Ole₂GroPGro) liposomes, varying amounts of α-helical structure were induced. Comparisons of the various fragments showed that helicity was concentrated in those parts of LcnG-α and LcnG-β that would become amphiphilic if an α-helical structure was adopted. In the presence of zwitterionic dioleoylglycerophosphocholine (Ole₂GroPCho) liposomes, the peptides were much less (if at all) structured, suggesting that the excess of positive charge on the antimicrobial peptides needs to be compensated by an excess of negative charge on the membrane. The structuring of LcnG-α and LcnG-β in the presence of Ole₂GroPGro liposomes was considerably enhanced when both peptides were presented simultaneously to the membranes. Consecutive addition of the two peptides to Ole₂GroPGro liposomes did not give this additional structuring, indicating that the individual LcnG-α and LcnG-β peptides associate with the membrane in a virtually irreversible manner that makes them inaccessible for interaction with the complementary peptide. The results suggest that upon arrival at and interaction with the target membrane, LcnG-α and LcnG-β form a complex that consists of approximately 50 % amphiphilic α-helices.