Lipid Bilayer Phase Behavior - Cholesterol

Cholesterol

The presence of cholesterol exerts a profound but complicated influence on lipid bilayer properties because of its unique physical characteristics. Although it is a lipid, cholesterol bears little resemblance to a phospholipid. The hydrophilic domain of cholesterol is quite small, consisting of a single alcohol group. Adjacent to this alcohol is a rigid planar structure composed of several fused rings. At the opposite end of the ring structure is a short single chain tail. It has been known for decades that the addition of cholesterol to a fluid phase bilayer decreases its permeability to water. The mode of this interaction has more recently been shown to be due to cholesterol intercalating between lipid molecules, filling in free space and decreasing the flexibility of surrounding lipid chains. This interaction also increases the mechanical rigidity of fluid bilayers and decreases their lateral diffusion coefficient. In contrast, the addition of cholesterol to gel phase bilayers disrupts local packing order, increasing the diffusion coefficient and decreasing the elastic modulus. Interactions of cholesterol with multi-component systems are even more complicated, as these can result in intricate phase diagrams. One lipid-cholesterol system that has recently been studied intently is the lipid raft. Lipid rafts are cholesterol-enriched gel domains that have been potentially implicated in certain cell signaling processes, but the subject remains controversial, with some researchers doubting even their existence in vivo.