Membrane lipids amphipathicity

Amphipathic molecules can form bilayered lamellar structures spontaneously if they have an appropriate geometry. Most of the major cell membrane lipids have a polar head, most commonly a glycerophosphorylester moiety, and a hydrocarbon tail, usually consisting of two... 

Heme dissociates from methemoglobin or metmyoglobin in the circulation and can be boimd by hemopexin or albumin, a heme binding plasma protein of lower avidity than hemopexin (49). It is important that the heme be controlled, since this amphipathic, oxidatively active compound can nonspecifically associate with membrane lipids or lipoproteins and cause oxidative damage of vital biomolecules, including DNA (50, 51). 

Nutrients. Amphipathic lipids are used by cells to build membranes (see p. 214). Typical membrane lipids include phospholipids, glycolipids, and cholesterol. Fats are only weakly amphiphilic and are therefore not suitable as membrane components. 

The naturally occurring fatty acids are carboxylic acids with unbranched hydrocarbon chains of 4-24 carbon atoms. They are present in all organisms as components of fats and membrane lipids, in these compounds, they are esterified with alcohols (glycerol, sphingosine, or cholesterol). However, fatty acids are also found in small amounts in unesterified form. In this case, they are known as free fatty adds (FFAs). As free fatty acids have strongly amphipathic properties (see p. 28), they are usually present in protein-bound forms. 

Membrane lipids are strongly amphipathic molecules with a polar hydrophilic head group and an apolar hydrophobic tail. in membranes, they are primarily held together by the hydrophobic effect (see p. 28) and weak Van der Waals forces, and are therefore mobile relative to each other. This gives membranes a more or less fluid quality. 

Hydrophobic and Hydrophilic Components of Membrane Lipids A common structural feature of membrane lipids is their amphipathic nature. For example, in phosphatidylcholine, the two fatty acid chains are hydrophobic and the phosphocholine head group is hydrophilic. For each of the following membrane lipids, name the components that serve as the hydrophobic and hydrophilic units (a) phos-phatidylethanolamine (b) sphingomyelin (c) galactosyl-cerebroside (d) ganglioside (e) cholesterol. 

Membranes Membranes contain amphipathic lipid molecules that... 

The tetrameric state of a-LTX is divalent cation-dependent. Accordingly, treatment with EDTA renders purified a-LTX mostly dimeric, while subsequent addition of millimolar Ca2+ or Mg2+ promptly restores tetramerization (Ashton et al. 2000). This transition, requiring conformation changes (Orlova et al. 2000) and catalyzed by divalent cations, may explain the dependence of a-LTX pore-mediated actions on Mg2+ (e.g. Misler and Hurlbut 1979). Tetramerization can also be triggered by amphipathic molecules (Ashton et al. 2000), possibly membrane lipids. 

The plasma membrane of epithelial cells, in common with other cell types, is selectively permeable, allowing the penetration of some substances but not others. The construction of the membrane from amphipathic lipid molecules forms a highly impermeable barrier to most polar and charged molecules, thereby preventing the loss of most water-soluble contents of the cell. This selective permeability presents a physical barrier to drag absorption, limiting absorption to specific routes and mechanisms, as described below (see Section 1.3.3). 

For proteins with multiple transmembrane domains, it is not necessary to have exclusively hydrophobic amino acids a pair of amino acids with opposite charges may be present in the lipophilic environment of the membrane. Therefore a search for amphipathic a-helices must be undertaken. Amphipathic helices have well-defined hydrophobic character, the hydrophobic face which would project towards the membrane/lipid environment, and a hydrophilic face, which would project out into the aqueous phase or towards the core of a helix bundle. Often times the distinction is not clear and there are regions of mixed hydrophobic/hydrophilic character. Graphically this can be realized with a helical-wheel representation in which the amino acid side chains project out, at 100 degree intervals, from the view along the long, helical axis. 

An obvious hypothesis is that this unusual membrane lipid composition is related directly to membrane function in some way. Within the restricted area of lipid bilayers, lipid composition is known to be an important determinant of physical properties. There are several prominent examples. First, the temperature at which the hydrocarbon chains melt when assembled in bilayers (the gel-to-liquid-crystalline transition temperature, marks an abrupt change in many of the physical properties of such bilayer systems for example, water permeability through such bilayers increases by several orders of magnitude above the transition. Second, the presence of cholesterol within bilayers composed of amphipathic lipids has a profound effect on lipid motion, mechanical properties (such as resistance to shear), and permeability to water. 

Figure 1 Physical and chemical stimuli affecting the gating of bacterial MS channels. (A) The structure of the pentameric MscL channel (left) and a channel monomer (right) from Mycobacterium tuberculosis according to the 3-D structural model of a closed channel (7). MscL is activated by membrane stretch, amphipaths (e.g., lysophopholipids, chlorpromazine, and trinitrophenol) and parabens. The channel activity is inhibited by Gd + and static magnetic fields (SMF) and is modulated by temperature and intracellular pH (3). (B) The structure of the MscS heptamer (left) and the channel monomer (right) from E. coli based on the 3-D structural model of MscS (8) most likely depicting an inactive or desensitized functional state of the channel (3). MscS is activated by membrane stretch, amphipaths, and parabens and is modulated by voltage. The activity of the channel is inhibited by Gd + and high hydrostatic pressure (HHP) (3). The arrows point at membrane structures (i.e., channel protein and/or lipid bilayer) affected by the specific stimuli.

A Membrane Lipid Is an Amphipathic Molecule Containing a Hydrophilic and a Hydrophobic Moiety... 

The repertoire of membrane lipids is extensive, perhaps even bewildering, at first sight. However, they possess a critical common structural theme membrane lipids are amphipathic molecules (amphiphilic molecules). A membrane lipid contains both a hydrophilic and a hydrophobic moiety. 

However, the most important use for lipids in the cell is in the formation of membranes, membranes contain amphipathic molecules, i.e., ones with a hydrophobic end and a hydrophilic end. 

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. 

MEMBRANE LIPIDS When amphipathic molecules are suspended in water, they spontaneously rearrange into ordered structures (Figure 11.6). As these structures form, hydrophobic groups become buried in the water-depleted interior. Simultaneously, hydrophilic groups become oriented so that they are exposed to water. Phospholipids form into bimolecular layers when sufficiently concentrated. This property of phospholipids (and other amphipathic lipid molecules) is the basis of membrane structure. 

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