Interactions of N-terminal fragments of groups I and II phospholipases A 2 with phospholipid bilayers and their surface recognition properties

Abstract

In order to elucidate the roles of the N-terminal segments of groups I and II phospholipases A 2 (PLA 2s) which have been known to have α-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 PLA 2 ( Naja naja atra) and group II PLA 2s ( 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 PLA 2s took α-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 PLA 2s formed α-helical structure, while that of group I PLA 2 remained unaffected. The hydrophobic moments showed that amphipathicities of N-teriminal fragments of group II PLA 2s are evidently larger than those of N-terminal fragments of group I PLA 2s. The leakage of carboxyfluorescein from acidic liposomes was induced only with group II PLA 2 peptides. Large blue shift and increase in intensity of tryptophan fluorescence were also observed for group II PLA 2 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 PLA 2s appears to be due in large part to the difference in intrinsic α-helix forming properties of their amino acid sequences. It is inferred that N-terminal amphipathic α-helical structures of group I PLA 2s are possibly formed by assistance of a neighboring chain bridged by Cys-11 and Cys-77.