Electrostatic and hydrophobic interactions governing the interaction and binding of β-lactoglobulin to membranes

The role of electrostatic and hydrophobic interactions in the binding and penetration of beta-lactoglobulin (beta LG) to preformed lipid membranes was studied using various phospholipid micelles and vesicles. Zwitterionic lysophospholipid micelles are able to induce the beta-sheet to alpha-helix transition, as judged by circular dichroism (CD), but the degree of transition is dramatically below and the amount of lipid required above that for anionic phospholipids with equivalent hydrocarbon chains. Anionic phospholipids with short hydrocarbon chains induce only low alpha-helical content in beta LG as compared to phospholipids with the same head group but longer hydrocarbon chains. These results suggest that both electrostatic and hydrophobic interactions are indispensable in beta LG-lipid interaction. Furthermore, air-water interface monolayer surface pressure and fluorescence anisotropy studies reveal that the membrane insertion of beta LG strongly depends on the nature of phospholipids, given the identical headgroup, particularly lipid packing. These results are supported by urea denaturation and acrylamide fluorescence quenching tests and by the FTIR-ATR polarization results for beta LG in multilayers on a surface. Under the same experimental conditions, the membrane binding and insertion of beta LG as well as the stability of the beta LG-lipid complexes can be enhanced by lowering the pH. Collectively, electrostatic interactions play a crucial role in all the processes involved in the beta LG-lipid interaction, while the presence of hydrophobic interaction remains necessary. Finally, beta LG biological function in the transport of fatty acids was tested by demonstrating the release of 2-AS from a 2-AS-beta LG complex on binding to lipids.