화학공학소재연구정보센터
Langmuir, Vol.18, No.16, 6356-6364, 2002
Membrane perturbations induced by integral proteins: Role of conformational restrictions of the lipid chains
Integral membrane proteins affect lipid bilayers in which they reside. That is, a protein induces an elastic response of the membrane which, for instance, may be caused by the so-called hydrophobic mismatch between the protein and the host bilayer. In addition to that, there are conformational restrictions imposed on those acyl chains that reside in immediate vicinity to the rigid protein surface. We suggest a method how this entropic confinement can approximatively be incorporated into membrane elasticity theory. To this end, we model a (sufficiently large) integral protein by an impenetrable wall, contacting a symmetric, fluidlike lipid bilayer. We represent the flexible lipid chains by fluctuating directors and calculate the entropy loss upon interaction with the wall. The elastic free energy stored in the membrane is calculated from the average director positions. We base elasticity theory on two order parameters, namely, stretching/compression of the lipid chains and their ability to tilt with respect to the hydrocarbon chain-water interface. We show that conformational restrictions of the lipid chains in the vicinity of the wall modify the microelastic behavior of lipid membranes. In particular, the wall induces an increase in the tilt modulus and a spontaneous tilt. Both quantities depend on the distance to the wall. Our combined model generally predicts nonmonotonic membrane perturbation profiles that are typically characterized by a local membrane thickening 1-2 nm away from the wall. Optimal lipid-protein interaction is achieved for proteins that have a small negative hydrophobic mismatch. Positive spontaneous curvature of the lipids generally lowers the free energy of inserting a rigid inclusion into a lipid membrane.