| Abstract
| - The catalytic activity of a glycosylphosphatidylinositol (GPI)-anchored alkaline phosphatase has beenstudied in Langmuir phospholipid monolayers at different surface pressures. The enzyme substrate,p-nitrophenyl phosphate, was injected into the subphase of mixed enzyme/lipid Langmuir monolayers. Itshydrolysis product was followed by monitoring the absorbance at 410 nm in situ in the monolayer subphaseof the Langmuir trough. Several surface pressures, corresponding to different molecular surface densities,were attained by lateral compression of the monolayers. The morphology of the monolayers, observed byfluorescence microscopy, showed three different types of domains owing to the heterogeneous partition ofthe enzyme within the mixed enzyme/lipid film. The catalytic activity was modulated by the enzymesurface density, and it increased until a pressure of 18 mN/m was reached, but it decreased significantlywhen the equilibrium in-plane elasticity (surface compressional modulus) increased more noticeably,resulting in alterations in the interface morphology. A model for the modulation of the enzyme orientationand catalytic activity by lipid/enzyme surface morphology and enzyme surface packing at the air/liquidinterface is proposed. The results might have an important impact on the comprehension of the enzymaticactivity regulation of GPI-anchored proteins in biomembranes.
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