Protein mosaic model, membrane

Fig. 2, The lipid-globular protein mosaic model with a lipid matrix (fluid mosaic model1) schematic three dimensional and cross-sectional view. The solid bodies with stippled surfaces represent the globular integral proteins, which are randomly distributed in the plane of the membrane...

Figure 1. A schematic representation of the cross section of the lipid-globular protein mosaic model of membrane structure. The globular proteins (with dark lines denoting the polypeptide chain) are amphipathic molecules with their ionic and highly polar groups exposed at the exterior surfaces of the membranes the degree to which these molecules are embedded in the membrane is under thermodynamic control. The bulk of the phospholipids (with filled circles representing their polar head groups and thin wavy lines their fatty acid chains) is organized as a discontinuous bilayer.

Most of the properties attributed to living organisms (e.g., movement, growth, reproduction, and metabolism) depend, either directly or indirectly, on membranes. All biological membranes have the same general structure. As previously mentioned (Chapter 2), membranes contain lipid and protein molecules. In the currently accepted concept of membranes, referred to as the fluid mosaic model, membrane is a bimolecular lipid layer (lipid bilayer). The proteins, most of which float within the lipid bilayer, largely determine a membrane s biological functions. Because of the importance of membranes in biochemical processes, the remainder of Chapter 11 is devoted to a discussion of their structure and functions. 

Figure 4. Membrane models. A. Black, or bimolecular, lipid membrane. Adapted from Reference (28) with permission. B. Lipid-globular protein mosaic model. Adapted from Reference (27) with permission.

When placed in aqueous solution, phospholipids spontaneously form lipid bilayers. According to the fluid-mosaic model, membrane phospholipids form lipid bilayers with membrane proteins associated with the bilayer as both peripheral and integral proteins. 

The Lipid-Globular Protein Mosaic model is based on experimental data on the conformation of proteins in intact membranes and on general thermodynamic considerations (maximization of hydrophilic interactions and minimization of hydrophobic interactions of the lip-... 

The lipid-globular protein mosaic model now represents the best overall picture of membrane structure... 

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. 

Membrane proteins in many cases are randomly distributed through the plane of the membrane. This was one of the corollaries of the fluid mosaic model of Singer and Nicholson and has been experimentally verified using electron microscopy. Electron micrographs show that integral membrane proteins are often randomly distributed in the membrane, with no apparent long-range order. 

Figure 41-7. The fluid mosaic model of membrane structure. The membrane consists of a bimolecu-lar lipid layer with proteins inserted in it or bound to either surface. Integral membrane proteins are firmly embedded in the lipid layers. Some of these proteins completely span the bilayer and are called transmembrane proteins, while others are embedded in either the outer or inner leaflet of the lipid bilayer. Loosely bound to the outer or inner surface of the membrane are the peripheral proteins. Many of the proteins and lipids have externally exposed oligosaccharide chains. (Reproduced, with permission, from Junqueira LC, Carneiro J Basic Histology. Text Atlas, 10th ed. McGraw-Hill, 2003.)...

While the fluid mosaic model of membrane stmcture has stood up well to detailed scrutiny, additional features of membrane structure and function are constantly emerging. Two structures of particular current interest, located in surface membranes, are tipid rafts and caveolae. The former are dynamic areas of the exo-plasmic leaflet of the lipid bilayer enriched in cholesterol and sphingolipids they are involved in signal transduction and possibly other processes. Caveolae may derive from lipid rafts. Many if not all of them contain the protein caveolin-1, which may be involved in their formation from rafts. Caveolae are observable by electron microscopy as flask-shaped indentations of the cell membrane. Proteins detected in caveolae include various components of the signal-transduction system (eg, the insutin receptor and some G proteins), the folate receptor, and endothetial nitric oxide synthase (eNOS). Caveolae and lipid rafts are active areas of research, and ideas concerning them and their possible roles in various diseases are rapidly evolving. 

Fig. 7 Diagrammatic representation of the fluid mosaic model of the cell membrane. The basic structure of the membrane is that of a lipid bilayer in which the lipid portion (long tails) points inward and the polar portion (round head ) points outward. The membrane is penenetrated by transmembrane (or integral) proteins. Attached to the surface of the membrane are peripheral proteins (inner surface) and carbohydrates that bind to lipid and protein molecules (outer surface). (Modified from Ref. 14.)...

The first membrane model to be widely accepted was that proposed by Danielli and Davson in 1935 [528]. On the basis of the observation that proteins could be adsorbed to oil droplets obtained from mackerel eggs and other research, the two scientists at University College in London proposed the sandwich of lipids model (Fig. 7.2), where a bilayer is covered on both sides by a layer of protein. The model underwent revisions over the years, as more was learned from electron microscopic and X-ray diffraction studies. It was eventually replaced in the 1970s by the current model of the membrane, known as the fluid mosaic model, proposed by Singer and Nicolson [529,530]. In the new model (Fig. 7.3), the lipid bilayer was retained, but the proteins were proposed to be globular and to freely float within the lipid bilayer, some spanning the entire bilayer. 

Fig. 6.9 Characteristic structures of biological membranes. (A) The fluid mosaic model (S. J. Singer and G. L. Nicholson) where the phospholipid component is predominant. (B) The mitochondrial membrane where the proteins prevail over the phospholipids...

Membranes are asymmetric. Integral membrane proteins can t be washed off. Peripheral membrane proteins can be washed off. Membrane spanning segments and lipid modification (fatty acylation and prenylation), anchor proteins in a fluid bilayer (Singer fluid mosaic model). 

FLUID-MOSAIC MODEL of membrane structure. Proteins and lipids that are embedded in the lipid bilayer diffuse rapidly in the plane of the membrane. 

The most recent model of this membrane, the fluid mosaic model [13] is pictured in cartoon fashion in Fig. 2. In this model, the transduction proteins (complexes I-IV) are randomly dispersed in the membrane and redox equivalents are delivered from one complex to another via the mobile electron carriers cytochrome c and ubiquinone. It is necessary that cytochrome c be able to move relatively facilely from one complex to another. Thus the binding constants cannot be too high without making the associated OS rates too slow. Conversely, to prevent unproductive short circuits via cytochrome c from complex I directly to IV, there must exist molecular recognition which favors selective binding of cytochrome c to hcj and cytochrome oxidase (and perhaps disfavors binding to complex 1 or II). 

The structure of biological and model membranes is frequently viewed in the context of the fluid mosaic model [4], Since biological membranes are composed of a mixture of various lipids, proteins, and carbohydrates the supra-structure or lateral organization of the components is not necessarily random. In order to model biological membranes, lipid assemblies of increasing complexity were studied. Extensive investigation of multicomponent monolayers (at the air-water interface) as well as bilayers have been reported. 

The overall picture of a membrane then becomes one in which the surface presents a background of lipids in- and onto which are placed proteins with specific positions and functions. This is called the fluid mosaic model as proposed by Singer and Nicolson. 

Figure 8.15 is a sketch of one possible relationship between the lipid bilayer and the membrane proteins. Molecules are free to move laterally in these membranes hence the structure pictured in Figure 8.15 is called the fluid mosaic model of a cell membrane. 

FIGURE 11-3 Fluid mosaic model for membrane structure. The fatty acyl chains in the interior of the membrane form a fluid, hydrophobic region. Integral proteins float in this sea of lipid, held by hydrophobic interactions with their nonpolar amino acid side chains. Both proteins and lipids are free to move laterally in the plane of the... 

Membranes are composed of lipids and proteins in varying combinations particular to each species, cell type, and organelle. The fluid mosaic model describes features common to all biological membranes. The lipid bilayer is the basic structural unit. Fatty acyl chains of phospholipids and the steroid nucleus of sterols are oriented toward the interior of the bilayer their hydrophobic interactions stabilize the bilayer but give it flexibility. 

Figure 8-5 The fluid mosaic model of Singer and Nicolson.61 Some integral membrane proteins, which are shown as irregular solids, are dissolved in the bilayer. Transmembrane proteins protrude from both sides. One of these is pictured as a seven-helix protein, a common type of receptor for hormones and for light absorption by visual pigments. Other proteins adhere to either the outer or the inner surface. Many membrane proteins carry complex oligosaccharide groups which protrude from the outer surface (Chapter 4). A few of these are indicated here as chains of sugar rings.

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