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Protein Science (2007), 16:1969-1976. Published by Cold Spring Harbor Laboratory Press. Copyright © 2007 The Protein Society
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Structure–activity study of the antibacterial peptide fallaxin

Sandra L. Nielsen1, Niels Frimodt-Møller2, Birthe B. Kragelund3, and Paul R. Hansen1

1 Department of Natural Sciences, Faculty of Life Sciences, University of Copenhagen, 1871 Copenhagen, Denmark
2 National Centre for Antimicrobials and Infection Control, Statens Serum Institut, 2300 Copenhagen S, Denmark
3 Structural Biology and NMR Laboratory, Department of Molecular Biology, Faculty of Science, University of Copenhagen, 2200 Copenhagen, Denmark

(RECEIVED April 25, 2007; FINAL REVISION June 18, 2007; ACCEPTED June 18, 2007)

Fallaxin is a 25-mer antibacterial peptide amide, which was recently isolated from the West Indian mountain chicken frog Leptodactylus fallax. Fallaxin has been shown to inhibit the growth of several Gram-negative bacteria including Enterobacter cloacae, Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa. Here, we report a structure–activity study of fallaxin based on 65 analogs, including a complete alanine scan and a full set of N- and C-terminal truncated analogs. The fallaxin analogs were tested for hemolytic activity and antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-intermediate resistant S. aureus, (VISA), methicillin-susceptible S. aureus (MSSA), E. coli, K. pneumoniae, and P. aeruginosa. We identified several analogs, which showed improved antibacterial activity compared to fallaxin. Our best candidate was FA12, which displayed MIC values of 3.12, 25, 25, and 50 µM against E. coli, K. pneumoniae, MSSA, and VISA, respectively. Furthermore, we correlated the antibacterial activity with various structural parameters such as charge, hydrophobicity <H>, mean hydrophobic moment <µH>, and {alpha}-helicity. We were able to group the active and inactive analogs according to mean hydrophobicity <H> and mean hydrophobic moment <µH>. Far-UV CD-spectroscopy experiments on fallaxin and several analogs in buffer, in TFE, and in membrane mimetic environments (small unilamellar vesicles) indicated that a coiled-coil conformation could be an important structural trait for antibacterial activity. This study provides data that support fallaxin analogs as promising lead structures in the development of new antibacterial agents.

Keywords: alanine scan; antibacterial activity; coiled-coil conformation; fallaxin; solid-phase peptide synthesis


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