Phospholipase A(2) interactions with model lipid monolayer membranes at the air-water interface

Mechanisms of domain formation by the membrane-active enzyme, phospholipase A2 (PLA(2)), in hydrolyzed phospholipid monolayers at the air-water interface have been investigated. PLA(2)-catalyzed hydrolysis of phospholipid membrane substrates yields both free fatty acid and lyse-lipid reaction products. After a certain extent of monolayer hydrolysis, PLA(2) forms large, regular two-dimensional aggregates at the monolayer interface. PLA(2)-catalyzed monolayer hydrolysis has been investigated using monolayer isotherm techniques, surface potential, and dual-label fluorescence microscopy at the air-water interface, as well as electron microscopy. Our results indicate that PLA(2)-produced fatty acid reaction products laterally phase separate from remaining enzyme substrate and lyse-lipid, forming two-dimensional, anionic domains. Results support our hypothesis that enzyme domains are formed by PLA(2) binding electrostatically to phase separated fatty acid through basic amino acid residues located on the enzyme's interfacial binding surface.