Binding of globular proteins to lipid membranes studied by isothermal titration calorimetry and fluorescence

The interactions of bovine serum albumin, lysozyme and cytochrome c with phosphatidylcholine liposomes in liquid crystalline phase have been investigated using isothermal titration calorimetry in combination with steady state, and fluorescence measurements. Calorimetric titration studies of the binding of liposomes to the protein species indicate in all cases exothermic processes with single binding sites in the protein molecule. Distinct saturation of the protein-lipid binding processes was observed at high molar lipid/protein ratio of 200/1. The thermodynamic parameters, including association enthalpy and entropy, Gibbs free energy and binding constant were determined at a constant temperature of 30 degreesC. Another set of experiments was performed using a fluorescence probe to determine the membrane order parameters such as limiting anisotropy, differential tangent and emission lifetime of fluorophore. The binding of proteins to liposomes significantly alters all membrane order parameters. The thermodynamic and fluorescence data presented indicate differences in the binding behaviour of these proteins to liposome suspensions. Information about thermodynamic properties of binding of globular proteins to liposomes and protein-induced membrane ordering can thus provide some clues about the nature of the protein-lipid complexes. (C) 2002 Elsevier Science B.V. All rights reserved.