Figure 2.

Lipid-cholesterol interactions. In the plasma membrane, cholesterol (Chol) can interact with phosphatidylcholine, e.g., palmitoyl-oleyl-phosphatidylcholine (POPC) (panel A) or sphingolipids such as sphingomyelin (panel B). When cholesterol interacts with POPC, its OH group is not buried in the complex, and both its α and β faces are available for TM domains of proteins (A). However, when cholesterol interacts with SM, a hydrogen bond (H bond) is formed between the OH group of cholesterol and the NH group of the sphingolipid. This H bond orientates cholesterol with respect to SM so that only its β face remains available for TM domains. The OH group of cholesterol is masked by the polar head of sphingomyelin in a typical “umbrella” effect.