Adsorption of glyceraldehyde 3 phosphate dehydrogenase on condensed monolayers of phospholipid

Abstract

The adsorption of [14C]alkylated glyceraldehyde 3 phosphate dehydrogenase from rabbit muscle to condensed monolayers of phosphatidic acid was investigated under a variety of conditions. The rate constant for association at 20°C depended on ionic strength. At I/2=60 mM the rate constant was 0.39 min-1. At I/2=260 mM it decreased to 0.27 min-1. The apparent association constant K(ass) for adsorption at I/2=60 mM was 1.06x106M-1 and was strongly influenced by subphase changes in pH and ionic strength. Measurements of K(ass), at 20° and 5°C gave a value for the apparent enthalpy change on adsorption of -33kJ mol-1. Calculations of the apparent change in free energy and apparent entropy change for the adsorption process gave values of -34kJ mol-1 and +2J K-1 mol-1 respectively. Decreasing the amount of phosphatidic acid in the monolayer by replacement with phosphatidylcholine caused the shape of the adsorption isotherm to change from apparent hyperbolic to sigmoid. Subphase changes in pH or ionic strength did not affect the shape of the adsorption isotherm. However, adsorption of enzyme on monolayers of 100% phosphatidic acid in the presence of 1 mM CaCl2 was sigmoid in nature. It is concluded that glyceraldehyde 3 phosphate dehydrogenase binds to condensed charged monolayers by multiple electrostatic interactions. At low concentrations of phosphatidic acid in the monolayer or in the presence of Ca2+, this occurs in a two step process and depends on lateral diffusion of phosphatidic acid for strong binding to take place.