Peripheral membrane proteins lipid bilayer surface

The central stmctural feature of almost all biological membranes is a continuous and fluid lipid bilayer that serves as the major permeability barrier of the cell or intracellular compartment (1) and as a scaffold for the attachment and organization of other membrane constituents (2, 3). In particular, peripheral membrane proteins are bound to the surface of lipid bilayers primarily by electrostatic and hydrogen-bonding interactions, whereas integral membrane proteins penetrate into, and usually span, the lipid bilayer, and are stabilized by hydrophobic and van der Waal s interactions with the lipid hydrocarbon chains in the interior of the lipid bilayer as well as by polar interactions... 

Figure 12.17. Integral and Peripheral Membrane Proteins. Integral membrane proteins (a, b, and c) interact extensively with the hydrocarbon region of the bilayer. Nearly all known integral membrane proteins traverse the lipid bilayer. Peripheral membrane proteins d and e) bind to the surfaces of integral proteins. Some peripheral membrane proteins interact with the polar head groups of the lipids (not shown).

Specific proteins mediate distinctive membrane functions such as transport, communication, and energy transduction. Many integral membrane proteins span the lipid bilayer, whereas others are only partly embedded in the membrane. Peripheral membrane proteins are bound to membrane surfaces by electrostatic and hydrogen-bond... 

Proteins and a lipid bilayer. Proteins are found in membranes, either spanning the membrane (transmembrane proteins), buried in the membrane (integral membrane proteins), or attached to the membrane surface (peripheral membrane proteins), as shown from top... 

The fluid mosaic iiiodclpictures biological membranes as dynamic, two-dimensional seas of lipid bilayer within which float the multitude of proteins and other molecules. These membrane-associated macromolecules may be partially submerged in the lipid bilayer, or may traverse the entire membrane. Other peripheral membrane proteins may be more loosely associated at the surface of the bilayer. 

So far, we were concerned with site-directed solid-state NMR studies on fully hydrated integral membrane proteins in which anisotropic spin interactions leading to broadened line widths are not averaged as they are in solution NMR but a variety of motions with various timescale from millisecond to microsecond are persistent because of integration into the lipid bilayer. It is still difficult to obtain structural information for peripheral membrane proteins which are not integrated but bound to a membrane surface, because the persistent anisotropic interaction with the membrane surface still hampers the utilization of the solution NMR approach. 

Membrane proteins fall into two classes peripheral and integral. Peripheral membrane proteins are associated with the membrane, but may be removed by high concentrations of salt or metal chelators such as EDTA. In most aspects the stmctures of peripheral membrane proteins are very similar to water-soluble proteins. Integral membrane proteins differ in that they are very difficult to extract from the lipid bilayer and require detergents for solubilization. Detergents disrupt the lipid bilayer and bind to the hydrophobic surfaces of the protein that are buried within the membrane. 

The major components of biological membranes are lipids and proteins. The lipids are arranged in a bimoleciilar leaflet. The proteins are either incorporated into the bilayer or are bound to the surface by electrostatic interactions, the two protein classes being called intrinsic (or integral) and extrinsic (or peripheral) membrane proteins. The cuiTent membrane model is still based on the proposal by Singer and Nicolson from 1972 [I]. Figure 1 shows a cartoon of a biological membrane with its intrinsic proteins. 

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.)...

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.)...

Several glycoproteins, which are present in the lipid bilayer of the virus, are necessary for infection. One is known as GP120. It binds to the CD4 protein on the surface of the Th lymphocyte (i.e. the CD4-I- ceU). This initiates fusion with the plasma membrane of the CD4-I- cell so that the viral RNA and its proteins enter the cell (i.e. it infects the CD4-t cell). The original infection probably occurs in the peripheral circulation but the lymphocytes will be transported by the blood to the spleen, other lymph nodes and the brain, where the microglia become infected (Figure 17.46). 

Figure A3.1 The fluid mosaic model of membranes. Note Integral proteins either pass right through or are deeply embedded in the lipid bilayer. Peripheral proteins are attached to the surface of the lipid bilayer. Anchored proteins are attached to a so called anchor molecule embedded in the lipid bilayer.

Cell membrane Lipid bilayer, containing surface proteins (peripheral proteins), proteins totally embedded in the membrane (intregal proteins), and glycoproteins partially embedded in the membrane Maintains ionic and chemical concentration gradients, cell-specific markers, intercellular communication, regulates cell growth and proliferation... 

Fig. 3 Structural model of the cell membrane. The membrane is composed of a bimolecular leaflet of phospholipid with the polar head groups facing the extracellular and cytosolic compartments and the acyl groups in the middle of the bilayer. Integral membrane proteins are embedded in the lipid bilayer. Integral proteins are glycosylated on the exterior surface and may be phosphorylated on the cytoplasmic surface. Extrinsic membrane proteins, peripheral proteins, are linked to the cytosolic surface of the intrinsic proteins by electrostatic interactions. (From Ref. l)...

All mammalian cells are enclosed by a plasma membrane composed of a lipid bilayer (two layers) containing embedded proteins (Fig. 10.2). The membranes are continuous and sealed so that the hydrophobic lipid bilayer selectively restricts the exchange of polar compounds between the external fluid and the intracellular compartment. The membrane is referred to as a fluid mosaic because it consists of a mosaic of proteins and lipid molecules that can, for the most part, move laterally in the plane of the membrane. The proteins are classified as integral proteins, which span the cell membrane, or peripheral proteins, which are attached to the membrane surface through electrostatic bonds to lipids or integral proteins. Many of the proteins and lipids on the external leaflet contain covalently bound carbohydrate chains and therefore are glycoproteins and glycolipids. This layer of carbohydrate on the outer surface of the cell is called the gly cocalyx. 

How are proteins associated with the bilayer in membranes The proteins that occur in membranes can be peripheral proteins, which are found on the surface of the membrane, or integral proteins, which lie within the lipid bilayer. Various structural motifs, such as bundles of seven a-helices, occur in proteins that span membranes. 

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