Interactions of N-terminal fragments of groups I and II phospholipases A2 with phospholipid bilayers and their surface recognition properties.

In order to elucidate the roles of the N-terminal segments of groups I and II phospholipases A2 (PLA2s) which have been known to have alpha-helical structure and have been assumed to be involved in the water/lipid interface recognition site, the peptides corresponding to the N-terminal moieties of group I PLA2 (Naja naja atra) and group II PLA2s (Trimeresurus flavoviridis and Crotalus atrox) were synthesized and their interactions with model membranes were studied. Circular dichroism spectra showed that N-terminal peptides of both groups I and II PLA2s took alpha-helical structure in trifluoroethanol but no significant secondary structure in buffer (pH 8.0). In the presence of acidic liposomes, N-terminal fragments of group II PLA2s formed alpha-helical structure, while that of group I PLA2 remained unaffected. The hydrophobic moments showed that amphipathicities of N-terminal fragments of group II PLA2s are evidently larger than those of N-terminal fragments of group I PLA2s. The leakage of carboxyfluorescein from acidic liposomes was induced only with group II PLA2 peptides. Large blue shift and increase in intensity of tryptophan fluorescence were also observed for group II PLA2 peptides when interacting with acidic liposomes. Such difference in the modes of interactions with lipid bilayers between N-terminal peptides of groups I and II PLA2s appears to be due in large part to the difference in intrinsic alpha-helix forming properties of their amino acid sequences. It is inferred that N-terminal amphipathic alpha-helical structures of group I PLA2s are possibly formed by assistance of a neighboring chain bridged by Cys-11 and Cys-77.