The structure of biological membranes

Biological membranes consist primarily of proteins and lip-ids. The relative amounts of these materials vary considerably, depending on the source of the membrane. At one extreme, the inner mitochondrial membrane is about 80% protein and 20% lipid by weight at the other, the myelin sheath membrane is about 80% lipid and 20% protein. The plasma membrane of human erythrocytes contains about equal amounts of protein and lipid. Many membranes also contain small amounts of carbohydrates. These almost always are covalently attached to either proteins (as glycoproteins) or lipids (as glycolipids or lipopolysaccharides). The mitochondrial inner membrane has little or no carbohydrate, but the myelin membrane has about 3% carbohydrate by weight, and the erythrocyte plasma membrane about 8%. 

During recent years much information has been obtained about the structure of biological membranes. This information has come in part from biochemical studies (the isolation of different chemical compounds from the cell membranes), x-ray diffraction, electron-spin and nuclear-spin magnetic resonance, spectroscopy, and especially the use of the electron microscope. Cell membranes, such as the membrane of the red blood cell, consist of about equal amounts of lipids and proteins. There is 

Stereodrawing of the structure of crystalline 1,2-dilauryIphosphotidylethanol-aminoacetic acid. [From P. B. Hitchcock, R. Mason, and K. M. Thomas, Journal oj the Chemical Society of London, Communications 1974, 539.] 

Diagrammatic representation of a cell membrane, showing lipid and protein molecules. 

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The double bonds found in fatty acids are nearly always in the cis configuration. As shown in Figure 8.1, this causes a bend or kink in the fatty acid chain. This bend has very important consequences for the structure of biological membranes. Saturated fatty acid chains can pack closely together to form ordered, rigid arrays under certain conditions, but unsaturated fatty acids prevent such close packing and produce flexible, fluid aggregates. 

Fats and oils are among the structurally simpler lipids. Later we will have a brief look at some of the more complex classes. Before we get to that, let s consider some unusual structures formed by fatty acids and structurally related molecules. These structures provide an early look at some aspects of the structure of biological membranes. 

Integral membrane protein a protein molecule that forms an essential part of the structure of biological membranes. 

Describe the structure of biological membranes and the characteristic functions of lipid-, protein-, and carbohydrate-containing components. Describe the differences between inner and outer membrane surfaces. 

In an early inaccurate model for the structure of biological membranes, a phospholipid bilayer was coated on both sides by protein in an unfolded or jS-pleated sheet conformation. This model reflected the prevailing view of membrane structure from about 1940 until the early 1970s. 

The fluid mosaic model is now known to be correct for the structure of biological membranes, in which the membranes are considered as two-dimensional solutions of oriented lipids and globular proteins. 

In recent years there has been considerable interest in the structure of biological membranes, and photochemical reagents capable of yielding low resolution structural information about membrane proteins have been developed. Several examples of photochemical surface-labeling reagents have appeared, and much effort has been devoted to the development of photoactivatable hydrophobic reagents for labeling from within the lipid bilayer. 

Yeagle, P., ed. (1992). The Structure of Biological Membranes, CRC Press, Boca Raton, FL. 

A similar type of interaction is thought to occur in membrane lipoprotein molecules. The problem in the latter studies is that the membrane apoproteins are not easily solubilized. If further information on the structure of biological membranes is required, then it is recommended that a recent book by Petty (1993) and one edited by Wirtz et al. (1993) be placed on a must reading list. An older, but very good, short review on lipid-protein interaction possibilities in membranes is one presented by Danielli (1982), who is widely recognized as a pioneer as well as a legend in this field. 

Lewis RNAH, McElhaney RN. The mesomorphic phase behaviour of lipid bilayers. In The Structure of Biological Membranes. Yeagle PE, ed. 1991. CRC Press, Boca Raton, FL. pp. 73-155. Ipsen JH, Karlstrom G, Mouritsen OG, Wennerstrom H, Zuk-ermann MJ. Phase equilibria in phosphatidylcholine-cholesterol system. Biochim. Biophys. Acta 1987 905 162-172. 

The stereospecific numbering of acylglycerols has been introduced by H. Hirschmann (J. Biol. Chem. 235 (1960) 2762) and is widely accepted, although its application is limited. See H. Hauser. G. Poupart, in The Structure of Biological Membranes. P. Yeagle (Ed.), CRC Press, Boca Raton, Florida, 1992. 

Lipid bilayer The structure of biological membranes in which the hydrophobic tails of lipids aggregate in two layers. 

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