An Insect Defensin-Derived β‑Hairpin Peptide with Enhanced Antibacterial Activity

Insect defensins are a class of small, cysteine-rich antimicrobial peptides primarily active on Gram-positive bacteria. Their roles in maggot therapy for treating chronic wound infection have been reported recently. However, a relatively narrow antibacterial spectrum together with the lack of a cost-effective means of commercial-scale production has limited their application. To further exploit the therapeutic potential of these molecules, we engineered the carboxyl-terminal β-sheet of navidefensin2-2, an insect defensin from Nasonia vitripennis, based on its structural similarity to naturally occurring microbicidal β-hairpin peptides. The designed peptide of 14 residues, referred to as NvBH, spans the β-sheet region of the defensin with two amino acids substituted for assembly of a disulfide-bonded amphipathic β-hairpin structure. Liquid chromatography–tandem mass spectrometry (LC–MS/MS) combined with circular dichroism (CD) analysis shows that the oxidized NvBH (oNvBH), produced from the synthetic peptide by air oxidization in an alkaline environment, folds into a typical β-hairpin structure linked by two disulfide bridges (Cys1–Cys4; Cys2–Cys3). However, such a structure appears not to be functionally necessary as synthetic NvBH with a spontaneously oxidized disulfide bridge (Cys2–Cys3) (termed poNvBH) displayed similar antibacterial potency to oNvBH. In comparison with oNvBH, poNvBH exhibited higher serum stability and more resistance on tryptic digestion. These two forms of peptides are capable of killing an array of Gram-positive (including antibiotic-resistant strains of Staphylococcus) and Gram-negative bacterial pathogens at low micromolar concentrations through a membrane disruptive mode of action. Our work indicates that the β-sheet region of insect defensins is a promising subdomain of proteins in anti-infective drug discovery.