【正文】 four residues from the carboxyl terminus. Following this prenylation, the terminal three amino acids are cleaved off and the new carboxyl terminus is methylated before insertion into the membrane. The structures of two lipid anchors are shown underneath: (C) a myristyl anchor (a 14carbon saturated fatty acid chain), and (D) a farnesyl anchor (a 15carbon unsaturated hydrocarbon chain). Figure 1015. A segment of a transmembrane polypeptide chain crossing the lipid bilayer as an a helix. Only the acarbon backbone of the polypeptide chain is shown, with the hydrophobic amino acids in green and yellow. (J. Deisenhofer et al., Nature 318:618624 and H. Michel et al., EMBO J. 5:11491158) Figure 1017. A typical singlepass transmembrane protein. Note that the polypeptide chain traverses the lipid bilayer as a righthanded a helix and that the oligosaccharide chains and disulfide bonds are all on the noncytosolic surface of the membrane. Disulfide bonds do not form between the sulfhydryl groups in the cytoplasmic domain of the protein because the reducing environment in the cytosol maintains these groups in their reduced (SH) form. Figure 1018. A detergent micelle in water, shown in crosssection. Because they have both polar and nonpolar ends, detergent molecules are amphipathic. Figure 1019. Solubilizing membrane proteins with a mild detergent. The detergent disrupts the lipid bilayer and brings the proteins into solution as proteinlipiddetergent plexes. The phospholipids in the membrane are also solubilized by the detergent. Figure 1020. The structures of two monly used detergents. Sodium dodecyl sulfate (SDS) is an anionic detergent, and Triton X100 is a nonionic detergent. The hydrophobic portion of each detergent is shown in green, and the hydrophilic portion is shown in blue. Note that the bracketed portion of Triton X100 is repeated about eight times. Figure 1021. The use of mild detergents for solubilizing, purifying, and reconstituting functional membrane protein systems. In this example functional Na+K+ ATPase molecules are purified and incorporated into phospholipid vesicles. The Na+K+ ATPase is an ion pump that is present in the plasma membrane of most animal cells。 Glc = glucose, GalNAc = Nacetylgalactosamine。 ? et al(1997): lipid rafts model。 Unit membrane model。Cell Membrane and Cell Surface I. Cell Membrane II. Cell Junctions III. Cell Adhesion IV. Extracellular Matrix I. Biomembranes: Their Structure, Chemistry and Functions Learning objectives: 1. A brief history of studies on the structrure of the plasma membrane 2. Model of membrane structure: an experimental perspective 3. The chemical position of membranes 4. Characteristics of biomembrane 5. An overview of the functions of biomembranes 1. 1. A brief history of studies on the structrure of the plasmic membrane A. Conception: Plasma membrane(cell membrane), Intracellular membrane, Biomembrane. B. The history of study ?Overton(1890s): Lipid nature of PM。 ? (1959): The TEM showing:the trilaminar appearance of PM。 ? and (1972): fluidmosaic model。 Functional rafts in Cell membranes. Nature 387:569572 2. Singer and Nicolson’s Model of membrane structure: The fluidmosaic model is the “central dogma” of membrane biology. A. The core lipid bilayer exists in a fluid state, capable of dynamic movement. B. Membrane proteins form a mosaic of particles perating the lipid to varying degrees. The Fluid Mosaic Model, proposed in 1972 by Singer and Nicolson, had two key features, both implied in its name. 3. The chemical position of membranes A. Membrane Lipids: The Fluid Part of the Model Phospholipids: Phosphoglyceride and sphingolipids Glycolipids Sterols ( is only found in animals) ?Membrane lipids are amphipathic. ?There are three major classes of lipids: Figure 102. The parts of a phospholipid molecule. Phosphatidylcholine, represented schematically (A), in formula (B), as a spacefilling model (C), and as a symbol (D). The kink due to the cisdouble bond is exaggerated in these drawings for emphasis. Figure 103. A lipid micelle and a lipid bilayer seen in crosssection. Lipid molecules form such structures spontaneously in water. The shape of the lipid molecule determines which of these structures is formed. Wedgeshaped lipid molecules (above) form micelles, whereas cylindershaped phospholipid molecules (below) form bilayers. Figure 104. Liposomes. (A) An electron micrograph of unfixed, unstained phospholipid vesicles (liposomes) in water. The bilayer structure of the vesicles is readily apparent. (B) A drawing of a small spherical liposome seen in crosssection. Liposomes are monly used as model membranes in experimental studies. (A, courtesy of Jean Lepault.) Figure 105. A crosssectional view of a synthetic lipid bilayer, called a black membrane. This planar bilayer is formed across a small hole in a partition separating two aqueous partments. Black membranes are used to measure the permeability properties of synthetic membranes. Figure 106. Phospholipid mobility. The types of movement possible for phospholipid molecules in a lipid bilayer. Figure 107. Influence of cisdouble bonds in hydrocarbon chains. The double bonds make it more difficult to pack the chains together and therefore make the lipid bilayer more difficult to freeze. Figure 108. The structure of cholesterol. Cholesterol is represented by a formula in (A), by a schematic drawing in (B), and as a spacefilling model in (C). Figure 109. Cholesterol in a lipid bilayer. Schematic drawing of a cholesterol molecule interacting with two phospholipid molecul