Effects of lipid phase transition and membrane surface charge on the interfacial activation of phospholipase A(2)

Phospholipase A(2) (PLA(2)) 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 PLA(2) 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 PLA(2). The temperature dependence of the initial catalytic rate of PLA(2) peaks around the lipid phase transition temperature (T-m) when T-m is not too far from physiological temperatures (30-40 degrees C), and the peak is higher in the presence of anionic membranes. High PLA(2) 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 PLA(2) activity, the membrane insertion depth of PLA(2) 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 PLA(2) activity