Erythrocyte membrane proteins bilayers

There is also inside-outside (transverse) asymmetry of the phospholipids. The choline-containing phospholipids (phosphatidylcholine and sphingomyelin) are located mainly in the outer molecular layer the aminophospholipids (phosphatidylserine and phos-phatidylethanolamine) are preferentially located in the inner leaflet. Obviously, if this asymmetry is to exist at all, there must be limited transverse mobility (flip-flop) of the membrane phospholipids. In fact, phospholipids in synthetic bilayers exhibit an extraordinarily slow rate of flip-flop the half-life of the asymmetry can be measured in several weeks. However, when certain membrane proteins such as the erythrocyte protein gly-cophorin are inserted artificially into synthetic bilayers, the frequency of phospholipid flip-flop may increase as much as 100-fold. 

Plasma membrane lipids are asymmetrically distributed between the two monolayers of the bilayer, although the asymmetry, unlike that of membrane proteins, is not absolute. In the plasma membrane of the erythrocyte, for example, choline-containing lipids (phosphatidylcholine and sphingomyelin) are typically found in the outer (extracellular or exoplasmic) leaflet (Fig. 11-5), whereas phosphatidylserine, phosphatidyl-ethanolamine, and the phosphatidylinositols are much more common in the inner (cytoplasmic) leaflet. Changes in the distribution of lipids between plasma membrane leaflets have biological consequences. For example, only when the phosphatidylserine in the plasma membrane moves into the outer leaflet is a platelet able to play its role in formation of a blood clot. For many other cells types, phosphatidylserine exposure on the outer surface marks a cell for destruction by programmed cell death. 

Besides the so-called integral membrane proteins (which are embedded in the hydrophobic part of the bilayer) there are peripheral proteins adsorbed on the hydrophilic surface of the membrane. Some of these peripheral proteins act as support, because they are associated with several integral proteins. A well known example is spectrin, situated at the inside of the erythrocyte membrane Z). 

Lipids also show asymmetrical distributions between the inner and outer leaflets of the bilayer. In the erythrocyte plasma membrane, most of the phosphatidylethanolamine and phosphatidylserine are in the inner leaflet, whereas the phosphatidylcholine and sphingomyelin are located mainly in the outer leaflet. A similar asymmetry is seen even in artificial liposomes prepared from mixtures of phospholipids. In liposomes containing a mixture of phosphatidylethanolamine and phosphatidylcholine, phosphatidylethanolamine localizes preferentially in the inner leaflet, and phosphatidylcholine in the outer. For the most part, the asymmetrical distributions of lipids probably reflect packing forces determined by the different curvatures of the inner and outer surfaces of the bilayer. By contrast, the disposition of membrane proteins reflects the mechanism of protein synthesis and insertion into the membrane. We return to this topic in chapter 29. 

Figure 8.2. Schematic diagram of an erythrocyte membrane viewed from inside. The scale of the molecules has been expanded relative to the scale of the cell by about two orders of magnitude, a. Spectrin, b. Actin. c. Ankyrin. d. Anion transporter, e. Protein 4.1./. Glycophorin. g. Adducin. h. Lipid bilayer.

The membrane skeleton acts as an elastic semisolid, allowing brief periods of deformation followed by reestablishment of the original cell shape (reviewed by Bennett and Gilligan, 1993). Erythrocytes in the human bloodstream have to squeeze repeatedly through narrow capillaries of diameters smaller than their own dimensions while resisting rupture. A functional erythrocyte membrane is pivotal to maintaining the functional properties of the erythrocyte. This importance is apparent when examination is made of many hemolytic anemias, where mutation of proteins involved in the structure of the submembranous cytoskeleton, and its attachment to the lipid bilayer, result in a malformed or altered cytoskeletal architecture and a disease phenotype. 

All of the proteins just described are examples of integral membrane proteins. As such, they are firmly embedded in the phospholipid bilayer and actually span the membrane one side of the protein faces the cytoplasmic side of the membrane, the other side faces the outer surface of the cell. The erythrocyte membrane also contains a number of peripheral-membrane proteins. Unlike the membrane-spanning integral-membrane proteins, these peripheral-membrane proteins are tightly associated only with the cytoplasmic side of the phospholipid bilayer. Together, these peripheral-membrane proteins form a meshlike matrix or skeleton on the inner surface of the membrane that... 

Aquaporins (AQPs) are a family of at least 13 members of small membrane-spanning proteins that assemble in cell membranes as homotetramers (Verkman and Mitra, 2000 Agre et al 2002 Verkman, 2005). Each monomer is approximately 30 kDa and six a-helical domains with cytosolically oriented amino- and carboxy-termini surround the water pore (Verkman and Mitra, 2000). AQPs can transport water in both directions (Tail et al., 2008). Early experiments demonstrating that erythrocyte membranes are more permeable to water than expected from water diffusion through a lipid bilayer provided the first experimental evidence of the existence of AQPs (Sidel and Solomon, 1957). 

The membrane constituents are lipids (phospholipids, glycosphingolipids, and cholesterol Figure 10-5), carbohydrates, and proteins. The ratio of protein lipid carbohydrate on a weight basis varies considerably from membrane to membrane. For example, the human erythrocyte membrane has a ratio of about 49 43 8, whereas myelin has a ratio of 18 79 3. The composition of the normal human erythrocyte membrane is shown in Table 10-2. All membrane lipids are amphipathic (i.e., polar lipids). The polar heads of the phospholipids may be neutral, anionic, or dipolar. The surface of the membrane bears a net negative charge. The distribution of lipid constituents in the bilayer is asymmetrical. For example, in the erythrocyte membrane, phosphatidylethanolamine and phosphatidylserine are located primarily in the internal monolayer, whereas phosphatidylcholine and sphingomyelin are located in the external monolayer. 

A hybrid antioxidant of new generation - ichfan is suggested for the therapy of membrane-pathologies associated with neurodegenerative diseases. It was shown that the compound applied in a wide range of concentrations modifies the properties of erythrocyte membranes and cells of Ehrlich ascitic carcinoma and changes the functional state of cells. Incorporated in the lipid phase and near-protein lipids of membranes, the antioxidant affects the structural state of the lipid bilayer, the structure and functional activity of proteins, in particular, the functions of ionic channels. Recommendations were made as to the compound doses responsible for the pronounced antioxidant and stabilizing effects and the absence of unfavorable side-effects. 

Figure 1. Dose-response diagram for the effect of ichfan on micro viscosity of the erythrocyte (A), and of the cell of Ehrlich ascitic carcinoma (B). Probe I (light) (2,2,6,6-tetramethyl-4-capryloyl-oxypiperidin-l-oxyl) is localized in the surface layer of the membrane lipid bilayer Probe II (dark) (5,6-benzo-2,2,6,6-tetramethyl-l,2.3,4-tetrahydro-y-carbolin-3-oxyl) in the deep near-protein sites of lipids.

Is lipid-assisted folding a widespread phenomenon and possibly applicable to soluble proteins The erythrocyte membrane contains about 20-mole % of PE that is almost exclusively localized in the inner leaflet and is in contact with highly concentrated heme-containing proteins. The refolding of the denatured soluble and heme-containing enzyme horseradish peroxidase (HRP) was followed in the presence and absence of liposomes made up of different phospholipids (Debnath et al., 2003). Remarkably, dimyristoyl-PE (a bilayer-forming... 

Bidirectional transporters. The bidirectional transporters at the plasma membrane randomize the lipid distribution across the plane of the bilayer, and are commonly referred to as scramblases [17]. The action of scramblase is summarized in Fig. 5, and is similar to that of the previously described transbilayer transporter present in the ER. Scramblase protein was first functionally identified in erythrocytes but is also present in nucleated cells. The scramblase shows no lipid specificity and essentially collapses the asymmetry of lipids at the cell surface. Phospholipids, SM, and glycosphingolipids all serve as substrates. The randomizing function of the plasma membrane protein is activated by Ca " and does not require ATP. 

Human Erythrocyte Intrinsic Membrane Proteins and Glycoproteins in Monolayer and Bilayer Systems... 

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