Effects of Lipid Phase Transition and Membrane Surface Charge on the Interfacial Activation of Phospholipase A₂

Phospholipase A₂ (PLA₂) enzymes act at the membrane−water interface to access their phospholipid substrate from the membrane. They are regulated by diverse factors, including the membrane charge, fluidity, mode of membrane binding (insertion, orientation), and allosteric conformational effects. Relative contributions of these factors to the complex kinetics of PLA₂ activation are not well understood. Here we examine the effects of thermal phase transitions and the surface charge of phospholipid membranes on the activation of human pancreatic PLA₂. The temperature dependence of the initial catalytic rate of PLA₂ peaks around the lipid phase transition temperature (T_m) when T_m is not too far from physiological temperatures (30−40 °C), and the peak is higher in the presence of anionic membranes. High PLA₂ activity can be induced by thermal perturbations of the membrane. Temperature-dependent fluorescence quenching experiments show that despite dramatic effects of the lipid phase transition on PLA₂ activity, the membrane insertion depth of PLA₂ increases only modestly above T_m. The data show that membrane structural disorder, and not the depth of membrane insertion, plays a major role in PLA₂ activity.