Three-dimensional structure of leucocin A in trifluoroethanol and dodecylphosphocholine micelles: spatial location of residues critical for biological activity in type IIa bacteriocins from lactic acid bacteria

The first three-dimensional structure of a type IIa bacteriocin from lactic acid bacteria is reported. Complete 1H resonance assignments of leucocin A, a 37 amino acid antimicrobial peptide isolated from the lactic acid bacterium Leuconostoc gelidum UAL187, were determined in 90% trifluoroethanol (TFE)-water and in aqueous dodecylphosphocholine (DPC) micelles (1:40 ratio of leucocin A:DPC) using two-dimensional NMR techniques (e.g., DQF-COSY, TOCSY, NOESY). Circular dichroism spectra, NMR chemical shift indices, amide hydrogen exchange rates, and long-range nuclear Overhauser effects indicate that leucocin A adopts a reasonably well defined structure in both TFE and DPC micelle environments but exists as a random coil in water or aqueous DMSO. Distance geometry and simulated annealing calculations were employed to generate structures for leucocin A in both lipophilic media. While some differences were noted between the structures calculated for the two different solvent systems, in both, the region encompassing residues 17-31 assumes an essentially identical amphiphilic alpha-helix conformation. A three-strand antiparallel beta-sheet domain (residues 2-16), anchored by the disulfide bridge, is also observed in both media. In TFE, these two regions have a more defined relationship relative to each other, while, in DPC micelles, the C-terminus is folded back onto the alpha-helix. The implications of these structural features with regard to the antimicrobial mechanism of action and target recognition are discussed.