Effect of Amidation on the Antimicrobial Peptide Aurein 2.5 from Australian Southern Bell Frogs

Aurein 2.5 is a naturally C-terminally amidated amphibian antimicrobial peptide. C-terminal amidation can increase efficacy and hence a comparison was made between aurein 2.5-CONH2 and its nonamidated analogue. Amidation of the C-terminal carboxyl of aurein 2.5 enhanced antimicrobial activity 2.5- fold against Klebsiella pneumonia. Our results demonstrate that both peptide analogues had high surface activities (23 mN m-1for aurein 2.5-COOH and 26 mN m-1 aurein 2.5-CONH2). Circular dichroism measurements suggest that the helical content of the amidated form, in the presence of trifluoroethanol, was significantly enhanced (33.66 % for aurein 2.5-COOH and 60.89 % aurein 2.5-CONH2). The interaction of aurein 2.5 with bacterial cell membrane mimics was investigated using Langmuir monolayers. Aurein 2.5-CONH2 induced stable surface pressure changes in monolayers formed from K. pneumonia (circa 4.7 mN m-1), however, lower surface pressure changes were observed for aurein 2.5- COOH (circa 3.8 mN m-1). The data shows that in the case of aurein 2.5, amidation is able to enhance antibacterial activity and it is proposed that the increase in effectiveness is due to stabilization of the α-helical structure at the membrane interface.