Abstract
A cecropin-like peptide, papiliocin, isolated from the swallowtail butterfly Papilio xuthus, possesses high selectivity against gram-negative bacteria. Since Trp2 and Phe5 are highly conserved residues in cecropin-like peptides, we investigated the role of Trp2 and Phe5 in antibacterial activity. Substitution of Trp2 and Phe5 in papiliocin with Ala (papiliocin-2A and papiliocin-5A) revealed that Trp2 is a key residue in its antibacterial activities. In order to understand the structural requirements for papiliocin function and to design shorter, but more potent, peptide antibiotics, we designed papiliocin constructs, PapN (residues Arg1-Ala22 from the N-terminal amphipathic helix). PapN exhibited significant broad-spectrum antibacterial activities without cytotoxicity. Bactericidal kinetics of peptides against E.coli showed that papiliocin completely and rapidly killed E.coli in less than 10 minutes at 2× MIC concentration, while papiliocin-2A and papiliocin-5A killed four times more slowly than papiliocin. The PapN series peptides permeabilized bacterial membranes less effectively than papiliocin, showing no antibacterial activities in an hour. The results imply that the Trp2 and Phe5 in the amphipathic N-terminal helix are important in the rapid permeabilization of the gram-negative bacterial membrane. The hydrophobic C-terminal residues permeabilize the hydrophobic bacterial cell membrane synergistically with these aromatic residues, providing selectivity against gram-negative bacteria.
Highlights
Increasing antibiotic resistance resulting from the widespread use of antibiotics has prompted the need for a novel antimicrobial agent that has not been exposed to microorganisms[1,2]
The therapeutic potential of peptide antibiotics lies in their ability to kill bacterial cells effectively without exhibiting cytotoxicity significantly toward mammalian cells
This property is assessed by the therapeutic index which can be defined as a ratio of the minimally effective concentration against human hRBCs to the Minimum inhibitory concentration (MIC) against bacterial cells
Summary
Increasing antibiotic resistance resulting from the widespread use of antibiotics has prompted the need for a novel antimicrobial agent that has not been exposed to microorganisms[1,2]. Insect cecropin-like peptides are highly positively charged AMPs and share over 50% sequence similarity, excluding moricin. We showed that papiliocin has high bacterial cell selectivity, against gram-negative bacteria, and potent anti-inflammatory activity. It is important to evaluate systematically the role of Trp and Phe residues in antimicrobial activities of AMPs. To understand further the structural requirements for papiliocin function and to design potent and short peptide antibiotics, we designed papiliocin analogs, PapN, composed of residues Arg1–Ala[22] from the amphipathic N-terminal helix and PapC, composed of residues Ala25–Lys[37] from the C-terminal hydrophobic helix. To examine the structure–function relationships underlying antimicrobial and anti-inflammatory activities, we investigated the interactions between the peptides and LPS using saturation transfer difference (STD)-NMR
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