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We monitored the action of phospholipase A2 (PLA2) on L- andD-dipalmitoylphosphatidylcholine (DPPC) Langmuir monolayers by mounting aLangmuir-trough on a wide-field fluorescence microscope with single moleculesensitivity. This made it possible to directly visualize the activity anddiffusion behavior of single PLA2 molecules in a heterogeneous lipidenvironment during active hydrolysis. The experiments showed that enzymemolecules adsorbed and interacted almost exclusively with the fluid region ofthe DPPC monolayers. Domains of gel state L-DPPC were degraded exclusively fromthe gel-fluid interface where the build-up of negatively charged hydrolysisproducts, fatty acid salts, led to changes in the mobility of PLA2. Themobility of individual enzymes on the monolayers was characterized by singleparticle tracking (SPT). Diffusion coefficients of enzymes adsorbed to thefluid interface were between 3 mu m^2/s on the L-DPPC and 4.6 mu m^/s on theD-DPPC monolayers. In regions enriched with hydrolysis products the diffusiondropped to approx. 0.2 mu m^2/s. In addition, slower normal and anomalousdiffusion modes were seen at the L-DPPC gel domain boundaries where hydrolysistook place. The average residence times of the enzyme in the fluid regions ofthe monolayer and on the product domain were between approx. 30 and 220 ms. Atthe gel domains it was below the experimental time resolution, i.e. enzymeswere simply reflected from the gel domains back into solution.

Influence of Lipid Heterogeneity and Phase Behavior on Phospholipase A2 Action at the Single Molecule Level