Mechanical Properties of Lipid Bilayers and Protein-Lipid Interactions

Abstract

Protein-lipid interactions play an essential role in the functioning of biomembranes and in the establishing the membrane structure. The functioning of membrane proteins is accompanied by changes in their conformation. These changes could influence the structure and physical properties of surrounding lipid environment. On the other hand, lipids can also influence the protein function. This is due to certain specificity of protein-lipid interactions. As a rule, integral proteins require specific lipids for optimal functioning and can be inhibited by other lipids1. Both peripheral and integral proteins contribute to protein-lipid interactions. Peripheral proteins are associated with membrane largely electrostatically. However nascent peripheral proteins and many soluble proteins must pass through one or more membranes during biosynthesis and therefore should interact also with inner part of the membrane. Integral proteins span the membrane transversely, establishing contacts between their hydrophobic moieties and the hydrocarbon chains of lipids. Owing to the different geometry of the hydrophobic moiety of proteins and that of lipids, as well as due to the action of electrostatic and elastic forces, regions of altered structure may arise around protein molecules. The hydrophobic interactions between integral protein and lipids are considered as a dominant. In this case the influence of the protein on its lipid environment is determined by the size of so-called hydrophobic mismatch2, i.e. the difference between hydrophobic length of protein and lipids. This reflects, e.g., in changes of phase transition temperature of lipids. On the other hand, the appearance of distorted regions around integral proteins is accompanied by changes of surface tension and the mechanical parameters of the membrane3,4. If conformational changes of integral proteins and binding of the peripheral proteins actually affects its lipid environment, then we can expect that these changes could be associated with changes of the microscopic structural state of lipid molecules as well as with the macroscopic, i.e. mechanical parameters of the membrane as a whole.