Cholesterol structure
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Cholesterol Molecular Structure and Crystal Forms
Cholesterol is a steroid molecule that is almost insoluble in water and is commonly found in the body as part of lipoproteins, lipid aggregates, or within biomembranes. When cholesterol levels are high, it can crystallize as cholesterol monohydrate, which is known to deposit in gallstones and atherosclerotic lesions. The crystal structure of cholesterol monohydrate is unusually complex, with eight independent molecules per triclinic cell, showing local pseudo-symmetry and forming stacked bilayers of about 33.9 Å thickness. Anhydrous cholesterol also crystallizes in a triclinic form with eight independent molecules per cell, providing further insight into how cholesterol molecules pack together and how this relates to their behavior in biological membranes 12.
Cholesterol Structure in Membranes and Lipid Rafts
In cell membranes, cholesterol plays a key structural role, making up about 30% of total lipids. It is positioned at the interface between polar and nonpolar regions due to its hydrophilic hydroxyl group and hydrophobic body. Cholesterol is essential for the formation of lipid rafts—small, ordered domains in the plasma membrane that are important for processes like signal transduction and protein sorting. Studies using molecular dynamics simulations and neutron diffraction have shown that cholesterol can form ordered pairs and bilayer structures within these rafts, supporting the "umbrella model" where cholesterol is shielded by phospholipid headgroups. However, other research supports a "template mechanism," where cholesterol acts as a planar hydrophobic template, maximizing interactions with surrounding lipids 35.
Cholesterol Self-Association and Oligomeric Structures
Cholesterol does not exist as isolated molecules in membranes. Instead, it tends to self-associate, forming dimers and, at higher concentrations, tetramers. These clusters are observed in both phase-separated and disordered membranes, suggesting that dimers are the basic structural unit of cholesterol in phospholipid bilayers. This self-association is important for understanding how cholesterol interacts with membrane proteins and its role in disease .
Cholesterol Conformation and Orientation in Membranes
Cholesterol's conformation in membranes is influenced by the surrounding lipid environment. Density functional theory and Raman spectroscopy have identified multiple cholesterol conformers, with the iso-octyl chain aligning with saturated phospholipid chains and adopting a more flexible distribution with unsaturated chains. The rigid ring structure and the top of the iso-octyl chain remain relatively inflexible .
The location and orientation of cholesterol within the membrane depend on the thickness and composition of the lipid bilayer. In most phosphatidylcholine bilayers, cholesterol's hydroxyl group is near the membrane surface, and the molecule stands upright. However, in membranes rich in polyunsaturated fatty acids, cholesterol can lie flat in the middle of the bilayer due to the high disorder of the surrounding lipids. The hydrophobic thickness of the membrane is a primary factor determining cholesterol's position 89.
Cholesterol's Effect on Membrane Structure
Cholesterol fluidizes membranes by promoting packing defects and increasing interdigitation of lipid tails, which can reduce bilayer thickness. Its presence also leads to the formation of laterally segregated domains, as seen in monolayer studies with different phospholipids. Cholesterol is pushed away from low-melting phospholipids and pulled toward high-melting lipids, supporting the formation of lipid rafts and influencing membrane organization 5710.
Conclusion
Cholesterol's structure is defined by its complex crystal forms and its dynamic behavior in biological membranes. It self-associates into dimers and tetramers, adopts different conformations depending on the lipid environment, and plays a crucial role in membrane organization and function. Its position and orientation in membranes are determined by lipid composition and bilayer thickness, highlighting its adaptability and importance in cellular processes 1234+6 MORE.
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