27. Factors that affect membrane fluidity: Length of the fatty acyl chains Degree of saturation of the fatty acyl chains Presence of cholesterol The rigid cholesterol ring system interferes with close packing of phospholipid fatty acid tails and thus inhibits transition from liquid crystal to crystalline state upon temperature decrease. At the same time, the rigid cholesterol makes the membrane somewhat less fluid.
28. The effect of fatty acid composition on membrane fluidity Two aspects: the length and the degree of un-saturation Membrane lipids often contain one saturated and on unsaturated fatty acids The effect of sterols on membrane fluidity Sterols – cholesterol and phytosterols. Sterols account for up to 50% of total membrane lipid on a molar basis The cholesterol molecules form hydrogen bonds with the oxygen atom on its neighboring phospholipid molecule The paradoxical effect of cholesterol : decreasing membrane fluidity at high temperature (due to the rigid “rings”) and increasing it at low temperature (by preventing regular arrangement of neighboring hydrocarbon tails) However, sterols decrease the permeability of a lipid bilayer, by plugging in the spaces between phospholipid molecules Most organisms can regulate membrane fluidity Poikelotherms (bacteria, fungus, plant, amphibians etc.) vs. homeotherms (warm-blooded animal) Homeoviscous adaptation – keeping the viscosity of the membrane about the same despite changes in temperature, by removing the terminal carbons to shorten the fatty acid molecules or introducing unsaturated bonds into the existing saturated fatty acid molecules (by e.g. E. coli desaturasae). Lipids of 18 carbon 70 o C -11 o C 76 o C 32 o C
41. α-helix is the major structural element of membrane-bound proteins
42.
43. Membrane lipids: the “mosaic” part of the model Freezing fracturing: quick frozen (in liquid nitrogen) and then subjected to a sharp blow of a diamond knife. E face (exoplasmic sicde) and P face (protoplasmic side) Membrane contain integral, peripheral, and lipid-anchored proteins Integral membrane proteins: contain ~20 aa hydrophobic transmembrane segment and hard to isolate or extract and need detergent treatment. Intergral monotopic proteins Transmembrane protein: singlepass multipass multi-subunit Glycophorin of erythrocyte Bacteriorhodopsin The hydropathy plot
