Lipid membranes proteins

Generally, the largest membrane In a eukaryotic cell encloses the endoplasmic reticulum (ER)—an extensive network of closed, flattened membrane-bounded sacs called cisternae (see Figure 5-19). The endoplasmic reticulum has a number of functions in the cell but Is particularly Important In the synthesis of lipids, membrane proteins, and secreted proteins. The smooth endoplasmic reticulum Is smooth because it lacks ribosomes. In contrast, the c rt osolIc face of the rough endoplasmic reticulum Is studded with ribosomes. 

Proteins are polymeric amino acids and are the major component of dried living material. Some proteins are glycosylated (glycoproteins), still others are closely associated with lipids (membrane proteins, and lipoproteins). Each amino acid has a different chemical characteristic so that when they are assembled in a protein, they give the protein a unique series of chemical properties. Each protein varies by the... 

Staining Applications Cardiac tissues lipid membranes proteins neurons ... 

Biological membranes are complex, multicomponent systems consisting of a wide variety of lipids, membrane proteins, and, to a lesser degree, carbohydrates. The main structural features of membranes are determined by the arrangement of lipids in bilayers, such that the polar lipid head groups are exposed to water while the hydrocarbon tails form the nonpolar interior of the bilayer. The widths of cellular membranes vary... 

The spatial arrangement of atoms in two-dimensional protein arrays can be detennined using high-resolution transmission electron microscopy [20]. The measurements have to be carried out in high vacuum, but since tire metliod is used above all for investigating membrane proteins, it may be supposed tliat tire presence of tire lipid bilayer ensures tliat tire protein remains essentially in its native configuration. 

The modem era of biochemistry and molecular biology has been shaped not least by the isolation and characterization of individual molecules. Recently, however, more and more polyfunctional macromolecular complexes are being discovered, including nonrandomly codistributed membrane-bound proteins [41], These are made up of several individual proteins, which can assemble spontaneously, possibly in the presence of a lipid membrane or an element of the cytoskeleton [42] which are themselves supramolecular complexes. Some of these complexes, e.g. snail haemocyanin [4o], are merely assembled from a very large number of identical subunits vimses are much larger and more elaborate and we are still some way from understanding the processes controlling the assembly of the wonderfully intricate and beautiful stmctures responsible for the iridescent colours of butterflies and moths [44]. 

In biological systems molecular assemblies connected by non-covalent interactions are as common as biopolymers. Examples arc protein and DNA helices, enzyme-substrate and multienzyme complexes, bilayer lipid membranes (BLMs), and aggregates of biopolymers forming various aqueous gels, e.g, the eye lens. About 50% of the organic substances in humans are accounted for by the membrane structures of cells, which constitute the medium for the vast majority of biochemical reactions. Evidently organic synthesis should also develop tools to mimic the Structure and propertiesof biopolymer, biomembrane, and gel structures in aqueous media. 

Phosphatidylcholine is an important component of cell membranes but cell mem branes are more than simply lipid bilayers Although their composition varies with their source a typical membrane contains about equal amounts of lipid and protein and the amount of cholesterol m the lipid fraction can approximate that of phosphatidylcholine The lipid fraction is responsible for the structure of the membrane Phosphatidyl choline provides the bilayer that is the barrier between what is inside the cell and what IS outside Cholesterol intermingles with the phosphatidylcholine to confer an extra measure of rigidity to the membrane... 

These difficulties have prompted a search for novel techniques for crystallization of membrane proteins. Two approaches have given promising results one using antibodies to solubilize the proteins and the second using continuous lipidic phases as crystallization media. Complexes with specific antibodies have larger polar surfaces than the membrane protein itself and are therefore likely to form crystals more easily in an aqueous enviroment. A recent example of an antibody-membrane protein complex utilized an Fv... 

A continuous lipidic cubic phase is obtained by mixing a long-chain lipid such as monoolein with a small amount of water. The result is a highly viscous state where the lipids are packed in curved continuous bilayers extending in three dimensions and which are interpenetrated by communicating aqueous channels. Crystallization of incorporated proteins starts inside the lipid phase and growth is achieved by lateral diffusion of the protein molecules to the nucleation sites. This system has recently been used to obtain three-dimensional crystals 20 x 20 x 8 pm in size of the membrane protein bacteriorhodopsin, which diffracted to 2 A resolution using a microfocus beam at the European Synchrotron Radiation Facility. 

Landau, E.M., Rosenbuch, J.R Lipid cubic phases a concept for the crystallization of membrane proteins. 

Plant cell membranes are similar in overall structure and organization to animal cell membranes but differ in lipid and protein composition. 

In 1972, S. J. Singer and G. L. Nicolson proposed the fluid mosaic model for membrane structure, which suggested that membranes are dynamic structures composed of proteins and phospholipids. In this model, the phospholipid bilayer is a fluid matrix, in essence, a two-dimensional solvent for proteins. Both lipids and proteins are capable of rotational and lateral movement. 

FIGURE 9.6 The fluid mosaic model of membrane structure proposed by S. J. Singer and G. L. Nicolsou. In this model, the lipids and proteins are assumed to be mobile, so that they can move rapidly and laterally in the plane of the membrane. Transverse motion may also occur, but it is much slower. 

Lipids also undergo rapid lateral motion in membranes. A typical phospholipid can diffuse laterally in a membrane at a linear rate of several microns per second. At that rate, a phospholipid could travel from one end of a bacterial ceil to the other in less than a second or traverse a typical animal ceil in a few minutes. On the other hand, transverse movement of lipids (or proteins) from one face of the bilayer to the other is much slower (and much less likely). For example, it can take as long as several days for half the phospholipids in a bilayer vesicle to flip from one side of the bilayer to the other. 

Biological membranes are asymmetric structures. There are several kinds of asymmetry to consider. Both the lipids and the proteins of membranes exhibit lateral and transverse asymmetries. Lateral asymmetry arises when lipids or proteins of particular types cluster in the plane of the membrane. 

There are other ways in which the lateral organization (and asymmetry) of lipids in biological membranes can be altered. Eor example, cholesterol can intercalate between the phospholipid fatty acid chains, its polar hydroxyl group associated with the polar head groups. In this manner, patches of cholesterol and phospholipids can form in an otherwise homogeneous sea of pure phospholipid. This lateral asymmetry can in turn affect the function of membrane proteins and enzymes. The lateral distribution of lipids in a membrane can also be affected by proteins in the membrane. Certain integral membrane proteins prefer associations with specific lipids. Proteins may select unsaturated lipid chains over saturated chains or may prefer a specific head group over others. 

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