Abstract
Free fatty acids and cholesterol are essential components of biomembranes. They both play distinct roles in membrane structure. For instance, cholesterol increases the rigidity of biomembranes, while free fatty acid increases the fluidity of biomembranes. The interactions between cholesterol and free fatty acids in biomembranes are not well understood. Therefore, we performed molecular dynamics simulations to systematically investigate the interactions between free fatty acid (i.e. DHA and EPA), cholesterol and phosphatidylcholine (POPC) in lipid bilayer. Despite their differences in chemical structure, free fatty acid and cholesterol produce similar effects in POPC bilayer: decreasing area-per-lipid and volume-per-lipid; and increasing acyl chain order and bilayer thickness. In comparison, cholesterol is more effective than free fatty acid in producing the above effects. Our results show that in lipid bilayer, neighboring POPC headgroups provide coverage to cholesterol to avoid the unfavorable exposure of their hydrophobic parts to water. In contrast, free fatty acid does not require coverage from neighboring POPC headgroups. Instead, free fatty acid expands its chain inside the hydrophobic region of the bilayer to avoid unfavorable interaction with water. Additionally, our simulations show that adding cholesterol to POPC bilayer decreases the lateral diffusion of POPC, however, adding free fatty acid increases the lateral diffusion of POPC. Interestingly, the presence of free fatty acid and cholesterol in lipid bilayer increases the lateral diffusion of cholesterol and reduces the translocation of free fatty acid across the lipid bilayer. This indicates that cholesterol may regulate the translocation of free fatty acid across the cytoplasmic membrane.
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