The interaction of phospholipase A2 with micellar interfaces. The role of the N-terminal region.

The localization of the previously postulated interface recognition site (IRS) in porcine pancreatic phospholipase A2, required for a specific interaction between the enzyme and organized lipid-water interfaces, was investigated by ultraviolet difference spectroscopy, by measurements of the intrinsic fluorescence of the unique Trp residue, and by protection experiments against specific tryptic hydrolysis. Using the enzymically nondegradable substrate analogues: CnH(2n+1)(0-)OOCH2CH2N+(CH3)3-(H,OH), it is shown that the rather hydrophobic N-terminal sequence of the enzyme, viz., Ala-Leu-Trp-Gln-Phe-Arg, is directly involved in the interaction with the lipid-water interface. Besides hydrophobic probably also polar interactions contribute to the binding process. At neutral or acidic pH the presence of a salt bridge between the N-terminal alpha-NH3+ group and a negatively charged side chain stablizes the interface recognition site and allows the enzyme to penetrate micellar surfaces, even in the absence of metal ion. At alkaline pH, interaction of the enzyme with micellar interfaces requires the presence of Ca2+ (Ba2+) ions.