Theoretical studies of lipid-protein interactions in biological membranes

Abstract

Monte Carlo simulations are used to investigate the conformational and orierjtational properties of lipids and proteins in a bilayer membrane. in the first instance, linear, hard-core tri-atomics are used to represent the two-dimensional projections of the lipid molecules.
Studies on lipid only systems show that the average number of gauche rotations and the cross-sectional area of the lipids decrease with increasing density. There is no long range orientational order within the lipids but the local orieritational order increases with increasing density. No first order phase transitions are observed though a glassy solid is observed at high densities.
The properties of the bulk lipid are unchanged upon the addition of protein molecules represented by hard discs of varying sizes. The nearest neighbour lipids are unchanged conformationally but are found to exhibit a high degree of orientational ordering around the proteins preferring to have the long axis of their projections parallel to the proteins surface. The degree of ordering increases with increas-ing density and decreasing curvature of the protein.
The lateral pressure is almost independent of protein size or concentration if expressed as a function of the bulk
lipid density. The hard-core of the lipids is softened to a site-site Lennard-Jones potential. The particles are found to cluster
within the periodic cell used. There is a critical density below which the average number of gauche rotations and the
cross-sectional area change little and the local orientational order increases. Above this density these properties are the same as for the hard-core systems. The structure, indicated by the radial distribution functions, is much greater than that for the hard-core systems and is almost independent of density.
The invariance of the lipid conformations and the observed lipid-protein orientational order resolve the conflict that
has arisen in the past regarding the presence or absence of an annulus of lipid around integral protein molecules. The
different experimental methods are seen to examine different lipid properties.