Effect of molecular surface packing on the enzymatic activity modulation of an anchored protein on phospholipid Langmuir monolayers.

CASELI, L.; RAFAEL GUSTAVO OLIVEIRA; MASUI, D. C.; FURRIEL, R. P. M.; LEONE, F. A.; MAGGIO, B.; ZANIQUELLI, M. E. D.

LANGMUIR

American Chemical Society

Año: 2005 vol. 21 p. 4090 - 4095

0743-7463

The catalytic activity of a glycosylphosphatidylinositol (GPI)-anchored alkaline phosphatase has been studied 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. Its hydrolysis product was followed by monitoring the absorbance at 410nmin situ in the monolayer subphase of 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 by fluorescence microscopy, showed three different types of domains owing to the heterogeneous partition of the enzyme within the mixed enzyme/lipid film. The catalytic activity was modulated by the enzyme surface 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 orientation and catalytic activity by lipid/enzyme surface morphology and enzyme surface packing at the air/liquid interface is proposed. The results might have an important impact on the comprehension of the enzymatic activity regulation of GPI-anchored proteins in biomembranes.