Transmembrane α helices

This chapter discusses effects of intrinsic membrane proteins on lipid bilayers and model transmembrane α helices. Incorporation of a protein into a lipid bilayer has significant effects on the properties of the bilayer. The rough surface presented by a protein to the surrounding lipid bilayer tends to produce poor packing unless the lipid fatty acyl chains distort to match the surface of the protein. In a liquid crystalline bilayer the lipid fatty acyl chains are disordered, because the chains undergo extensive wobbling fluctuations. The presence of a rigid protein surface reduces the extent of these motional fluctuations. However, the chains tilt and become conformationally disordered to maximize contact with the rough surface of the protein. The net result is that the presence of a protein leads to decreased order for the chains, with a wide range of chain orientations relative to the bilayer normal, but with reduced extent and rate of motion. Because of the reduced motion, lipids adjacent to membrane proteins are often referred to as being motionally restricted. It is clear that the reasons for the disorder of the bulk lipids and the disorder of the lipids adjacent to the protein are different; for the bulk phospholipids, the disorder is dynamic, whereas, for the boundary lipids the disorder is static.