Micelles and Liposomes

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membrane by treatment with salt solutions of different ionic strength or extremes of pH, and named to imply a physical location on the surface of the membrane. Peripheral proteins, many of which are enzymes, are usually soluble in water and free of lipids. Integral (or intrinsic) proteins require rather drastic treatment, such as use of detergents or organic solvents, to be separated from a membrane They usually contain tightly bound lipid, which if removed leads to denaturation of the protein and loss of biological function. Integral proteins have sequences of hydrophobic amino acids, which create hydrophobic domains in the tertiary structure. These hydrophobic regions interact with the hydrophobic hydrocarbons of the lipids stabilizing the protein–lipid complex. Removal of integral proteins leads to disruption of the membrane, whereas peripheral proteins can be removed with little or no change in the integrity of the membrane.
Proteolipids are hydrophobic lipoproteins soluble in chloroform and methanol but insoluble in water. They are present in many membranes but particularly in myelin, where they represent about 50% of the protein component. An example is lipophilin, a major lipoprotein of brain myelin that contains over 65% hydrophobic amino acids and covalently bound fatty acids.
Another class of integral membrane proteins is the glycoproteins; plasma membranes of cells contain a number of different glycoproteins, each with its own unique carbohydrate content.
The complexity, variety, and interaction of membrane proteins with lipids are just being resolved. Many of the proteins are enzymes located within or on the cellular membranes. Membrane proteins have a role in transmembrane movement of molecules and as receptors for the binding of hormones and growth factors. In many cells, such as neurons and erythrocytes, membrane proteins have a structural role to maintain the shape of the cell. Thus individual membrane proteins can have a catalytic, transport, receptor, structural, or recognition role. It is not surprising to find a high protein content in a membrane being correlated with the complexity and variety of functions of a membrane.
Carbohydrates of Membranes Are Present As Glycoproteins or Glycolipids
Carbohydrates present in membranes are oligosaccharides covalently attached to proteins to form glycoproteins and to a lesser amount to lipids to form glycolipids. The sugars found in glycoproteins and glycolipids include glucose, galactose, mannose, fucose, N­acetylgalactosamine, N­acetylglucosamine, and sialic acid (see Figure 5.18 and the Appendix for structures). Structures of glycoproteins and glycolipids are presented on pages 348 and 422, respectively. The carbohydrate is on the exterior side of the plasma membrane or the luminal side of the endoplasmic reticulum. Roles for membrane carbohydrates include cell­cell recognition, adhesion, and receptor action.
Figure 5.18 Structures of some membrane carbohydrates.
5.3— Micelles and Liposomes
Lipids Form Vesicular Structures
The basic structural characteristic of membranes is derived from the physicochemical properties of the major lipid components, the glycerophospholipids and sphingolipids. These amphipathic compounds, with a hydrophilic head and a hydrophobic tail (Figure 5.19a), will at appropriate concentrations interact in an aqueous system to form spheres, termed micelles (Figure 5.19b). The hydrophobic tails interact to exclude water and charged polar head groups will be on the outside of the sphere. The specific concentration of lipid required for micelle formation is referred to as the critical micelle concentration. Micelles with a single lipid or a mixture of lipids can be made. Formation of
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Figure 5.19 Representations of the interactions of phospholipids in an aqueous medium. (a) Representation of an amphipathic lipid. (b) Cross­sectional view of the structure of a micelle. (c) Cross­sectional view of the structure of lipid bilayer. (d) Cross section of a liposome. Each structure has an inherent stability due to the hydrocarbon chains and the attraction of the polar head groups to water.
micelles depends also on the temperature of the system and, if a mixture of lipids are used, on the ratio of concentrations of the different lipids in the mixture (see p. 1079). The micelle structure is very stable because of hydrophobic interaction of hydrocarbon chains and attraction of polar groups to water. Micelles are important in the digestion of lipids (see p. 1081).
Liposomes Have a Membrane Structure Similar to Biological Membranes
Depending on conditions, amphipathic lipids such as glycerophospholipids will form a bimolecular leaf structure with two layers of lipid. The polar head groups will be at the interface between the aqueous medium and the lipid, and the hydrophobic tails will interact to form an environment that excludes water (Figure 5.19c). This bilayer conformation is the basic lipid structure of all biological membranes.
Lipid bilayers are extremely stable structures held together by noncovalent interactions of the hydrocarbon chains and ionic interactions of charged head groups with water. Hydrophobic interactions of the hydrocarbon chains lead to the smallest possible area for water to be in contact with the chains, and water is essentially excluded from the interior of the bilayer. If disrupted, bilayers will self­seal because hydrophobic groups will seek to establish a structure in which there is minimal contact of the hydrocarbon chains with water, a condition that is most favorable thermodynamically. A lipid bilayer will close in on itself, forming a spherical vesicle separating the external environment from an internal compartment. Such vesicles are termed liposomes. Because individual lipid–lipid interactions have low energies of activation, lipids in a bilayer have a circumscribed mobility, breaking and forming interactions with surrounding molecules but not readily escaping from the lipid bilayer (Figure 5.19d). Self­assembly of amphipathic lipids into bilayers is an important characteristic and is involved in formation of cell membranes.
Individual phospholipid molecules exchange places with neighboring molecules in a bilayer, leading to rapid lateral diffusion in the plane of the membrane (Figure 5.20). There is rotation around the carbon–carbon bonds in fatty acyl chains; in fact, there is a greater degree of rotation nearer the methyl end,
Figure 5.20 Mobility of lipid components in membranes.