Physics of the activation of interfacial enzymes Professor Ole G. Mouritsen MEMPHYS-Center for Biomembrane Physics University of Southern Denmark Campusvej 55, DK-5230 Odense M, Denmark ogm@memphys.sdu.dk www.memphys.sdu.dk A host of water-soluble enzymes are active at membrane surfaces and in association with membranes. Some of these enzymes are involved in signaling and in the modification and remodeling of membranes. A special class of enzymes, the phospholipases and in particular secretory phospholipase A2, are predominantly activated at the interface between water and membrane surfaces, where they lead to a break-down of the lipid molecules into lysolipids and free fatty acids. The activation is critically dependent on the physical properties of the lipid-membrane substrate as shown by a wide range of experimental and theoretical data. A simple mechanism involving single-lipid protrusions modes is suggested to provide a unified picture of the activation of phospholipase A2.