The Position of Aib Residues Defines the Antimicrobial Activity of Aib-Containing Peptides

Abstract

Abstract 2-Aminoisobutyric acid (Aib,1 2-amino-2-methylpropionic acid) is an unusual amino acid, which is present in some antimicrobial peptides. To elucidate the impact of Aib residues on the conformation and biological activity of ion channel forming peptides, Aib residues in Ac-(Aib-Lys-Aib-Ala)5-NH2 were partially and fully replaced by Ala residues. A total of three analogs were thus synthesized. Two of these analogs, Ac-(Aib-Lys-Ala-Ala)5-NH2 (BKAA-20) and Ac-(Ala-Lys-Aib-Ala)5-NH2 (AKBA-20), had similar amino acid composition and were not hemolytic. BKAA-20 exhibited comparable antimicrobial activity to gramicidin S against Gram-positive bacteria (minimum inhibitory concentration: ∼3.13 µg ml-1), whereas AKBA-20 was not active. Single-channel measurements showed that there was a clear correlation between the antimicrobial activity and the ion channel activity in these analogs. The third analog, Ac-(Ala-Lys-Ala-Ala)5-NH2, was biologically inactive. Circular dichroism spectroscopy revealed that the helix forming propensities of BKAA-20 and AKBA-20 were independent of the position of Aib residues in negatively charged phospholipid vesicles, as mimics of external cell membranes of bacteria. However, different positions of Aib residues in BKAA-20 and AKBA-20 affected their antimicrobial activity. It is indicated that the introduction of Aib residues into the peptide backbone not only promotes the generation of stable helical conformations, but also modulates the antimicrobial activity. The results of this study can implicate a deeper understanding of the role of Aib residues in the structure-activity relationships of helical pore forming peptides, which in turn promotes the rational design of effective antimicrobial peptides.