Protein-lipid interactions in bilayer membranes: A lattice model

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Abstract
A lattice model was developed to study the effects of intrinsic membrane proteins upon the thermodynamic properties of a lipid bilayer membrane. Assuming that only nearest-neighbor van der Waals and steric interactions are important, and that the polar group interactions can be represented by effective pressure-area terms, phase diagrams, the temperature To which locates the gel-fluid melting, the transition enthalpy and correlations were calculated by mean field and cluster approximations. Average lipid chain areas and chain areas when the lipid is in a given protein environment were obtained. Proteins that have a smooth homogeneous surface (cholesterol-like) and those that have inhomogeneous surfaces or that bind lipids specifically were considered. To can vary depending upon the interactions, and another peak can appear upon the shoulder of the main peak which reflects the melting of a eutectic mixture. The transition enthalpy decreases generally, as was found before, but when a 2nd peak appears, departures from this behavior reflect aspects of the eutectic mixture. Proteins have significant nonzero probabilities for being adjacent to one another so that no unbroken annulus of lipid necessarily exists around a protein. If To does not increase much, or decreases, with increasing c (concentration of intrinsic molecules), then lipids adjacent to a protein cannot all be all-trans on the time scale (10-7 s) of the system. Around a protein the lipid correlation depth is about 1 lipid layer, and this increases with c. Possible consequences of ignoring changes in polar group interactions due to clustering of proteins are discussed.