Conformational analysis of the broad-spectrum antibacterial peptide, ranatuerin-2CSa: Identification of a full length helix-turn-helix motif

Design of clinically valuable antibacterial agents based upon naturally occurring peptides requires the use of spectroscopic methods, particularly NMR, to determine the three-dimensional structure of the native peptide so that analogues with improved therapeutic properties can be made. Ranatuerin-2CSa (GILSSFKGVAKGVAKDLAG KLLETLKCKITGC), first isolated from skin secretions of the Cascades frog, Rana cascadae, represents a promising candidate for drug development. The peptide shows potent growth inhibitory activity against Escherichia coli (MIC = 5 μM) and Staphylococcus aureus (MIC = 10 μM) but displays haemolytic activity against human erythrocytes (LC₅₀ = 160 μM). The solution structure of ranatuerin-2CSa was investigated by proton NMR spectroscopy and molecular modelling. In aqueous solution, the peptide lacks secondary structure but, in a 2,2,2-trifluoroethanol (TFE-d₃)-H₂O solvent mixture, the structure is characterised by a full length helix-turn-helix conformation between residues I²-L²¹, L²²-L²⁵ and K²⁶-T³⁰ respectively. This structural information will facilitate the design of novel therapeutic agents based upon the ranatuerin-2CSa structure with improved antimicrobial potencies but decreased cytolytic activities against mammalian cells.