Positively charged bilayers composed of phosphatidylcholine (PC) and stearylamine (SA) in a 4:1 ratio reduce the effectiveness of a protease from Mucor miehei to produce milk clotting. This is related to the adsorption of the protein, which at pH 7 is negatively charged, by electrostatic forces. However, an increase in SA, which increases the membrane packing parallel to the increase in the surface charge density, counteracts the protein membrane association. This is in agreement with the fact that the protease can also adsorb on pure phosphadylcholine bilayers in the fluid state but not in the gel state. In addition, the presence of phosphatidylethanolamine also inhibits protease adsorption. It is concluded that the protein affects the membrane interface of fluid PC membranes because the electrostatic charges pull the protein to the bilayer interface causing changes in hydration and area per molecule. The adsorption is only at the level of the polar head groups since no effects were observed in the hydrocarbon core region.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada
