The lipid-protein interface in biological membranes

Abstract

A significant fraction of the lipid in many biological membranes is at the lipid-protein interface. The ESR and NMR data are in basic agreement that there is a dynamic equilibrium between lipid at the interface and the bulk bilayer. The lipid contact with the hydrophobic surfaces of the protein is spatially disordered compared to the bilayer lipids. The spatial disordering on the protein surface leads to the prediction that cooperative chain melting would not occur between lipid tails directly contacting the protein. This is in agreement with most, but not all of the DSC data. While there are some disagreements in the ESR studies, most of the quantitative data support the conclusion that the protein-associated lipid is motionally restricted under physiologically relevant conditions. In general, the NMR data are in agreement that exchange between boundary and bilayer regions is rapid on the NMR time scale at physiological temperatures, although there are some differences in interpretation of the lipid dynamics. From the available data, several kinds of lipid binding sites may be involved. Most of these sites are probably nonspecific, but with some additional sites exhibiting specificity for the chemical properties of the polar head group. The relative binding constants can vary within the boundary layer with several exchange rates applying. Although most of the exchange rates are rapid, perhaps more rapid than specific mechanistic steps in the enzyme reaction, there is a characterizable set of thermodynamic parameters for the boundary and bilayer equilibrium. Although many of the lipid binding sites may have very low relative binding constants, they must be higher than the binding constants for nonspecific protein-protein contacts. One probable function of the boundary is to act as a molecular spacer, preventing indiscriminate protein-protein aggregation in the two-dimensional lipid solvent. Other roles are suggested by the higher relative binding constants of some specific lipids.