An enzyme caught in action: Direct imaging of hydrolytic function and domain formation of phospholipase A2 in phosphatidylcholine monolayers

AbstractPhospholipase A2, a ubiquitous lipolytic enzyme that actively catalyses hydrolysis of phospholipids, has been studied as a model for enzyme-substrate reactions, as a membrane structural probe, and as a model for lipid-protein interactions. Its mechanism of action remains largely controversial. We report here for the first time direct microscopic observation of the lipolytic action of fluorescently marked phospholipase A2 (Naja naja naja) against phosphatidylcholine monolayers in the lipid phase transition region. Under these conditions, phospholipase A2 is shown to target and hydrolyse solid-phase lipid domains of L-α-dipalmitoylphosphatidylcholine. In addition, after a critical extent of monolayer hydrolysis, the enzyme itself aggregates into regular, visible proteinaceous domains within the lipid monolayer. Solid-phase lipid hydrolysis indicates a preferential hydrolytic environment for phospholipase A2 while enzyme domain formation points to a possible allosteric inhibition mechanism by hydrolysis products