Question

Discuss the effect of (a) cholesterol addition, (b) free fatty acid addition, and (c) lysophospholipid addition to a membrane bilayer. Be sure to include an explanation of concentration dependence in biochemical and physical terms. Will your answers to (b) or (c) change if you consider saturated versus unsaturated fatty acids?

Answer 1

Ans (a) Cholesterol plays an important role in regulating the properties of phospholipid membranes. Cholesterol regulates the fluidity of the membrane and modulates the function of membrane proteins. The influence on the bilayer widths of various lipids upon inclusion of cholesterol was determined from X-ray diffraction studies of the cast films of the lipid–cholesterol coaggregates in water. When incerased amount of cholesterol is incorporated into the phospholipid vesicles, the carboxyfluorescein (CF) leakage rates were progressively reduced. Independent experiments measuring trans-membrane OH⁻ ion permeation rates from cholesterol-doped cationic lipid vesicles using entrapped dye riboflavin also demonstrated that the addition of cholesterol into the cationic lipid vesicles reduced the leakage rates irrespective of lipid molecular structure. It was found that the cholesterol induced changes on the membrane properties such as lipid order, linewidth broadening, efflux rates, bilayer widths, etc., did not depend on the ability of the lipids to participate in the hydrogen bonding interactions with the 3β-OH of cholesterol. These findings emphasize the importance of hydrophobic interaction between lipid and cholesterol and demonstrate that it is not necessary to explain the observed cholesterol induced effects on the basis of the presence of hydrogen bonding between the 3β-OH of cholesterol and the lipid chain–backbone linkage region or headgroup region.

(b) In addition to the phospholipids, the plasma membranes of animal cells contain glycolipids and cholesterol, which generally correspond to about 40% of the total lipid molecules. An important property of lipid bilayers is that they behave as two-dimensional fluids in which individual molecules (both lipids and proteins) are free to rotate and move in lateral directions. Such fluidity is a critical property of membranes and is determined by both temperature and lipid composition. The interactions between shorter fatty acid chains are weaker than those between longer chains, so membranes containing shorter fatty acid chains are less rigid and remain fluid at lower temperatures. Lipids containing unsaturated fatty acids similarly increase membrane fluidity because the presence of double bonds introduces kinks in the fatty acid chains, making them more difficult to pack together.

(c) Lysophospholipid is a key intermediate in phospholipid and occurs as minor constitute in various cell membranes. Abnormal accumulations of lysophospholipids in cell membranes induce morphological changes in cells, mediate cell fusion and cause hemolysis.Lysophospholipid has also been shown to affect the permeability properties of phospholipid model membrane.