Design and synthesis of novel antimicrobial peptides on the basis of alpha helical domain of Tenecin 1, an insect defensin protein, and structure-activity relationship study.

Tenecin 1, a peptide consisting of 43 amino acids, exhibits a potent bactericidal activity against various Gram-positive bacteria and shares a common structural feature of insect defensin family corresponding to cysteine stabilized alpha/beta motif. Our previous research indicated that an active fragment was successfully extracted from C-terminal beta sheet domain of Tenecin 1, whereas the fragment corresponding to the alpha helical region of the protein had no antibacterial activity. We chose this inactive fragment corresponding to alpha helical region of Tenecin 1 and synthesized derivatives with a different net positive charge by using rational design. Interestingly, we successfully endowed antibacterial activity as well as antifungal activity to the inactive alpha helical fragment by single or double amino acid replacement(s) without an increase of hemolytic activity. The leakage of dye from vesicles induced by the active peptides suggested that these peptides act on the membranes of pathogen as a primary mode of action. Structure-activity relationship study of a series of the active derivatives revealed that amphiphilic structure and high net positive charge were prerequisite factors for the activity and that there was a relationship between the antibacterial activity and the isoelectric point of the active peptides. In this work, we showed an efficient method to endow the antibacterial activity as well as antifungal activity to the inactive fragment derived from a cyclic insect defensin protein and suggested a facile method to screen for active fragments from cyclic host defense peptides.