Lipid membranes function

Lipid bilayer membrane systems, having gel (solvated crystalline state)-to-liquid crystalline phase transitions are attractive as specific organic media for separation chemistry. The first approach in HPLC was direct immobilization of a phosphatidylcholine lipid onto silica. This modified silica shows interesting selectivity against amino acids, but the separation mode is too complicated, due to the zwitter-ionic property of the immobilized molecule. In addition, no lipid membrane function is realized on the silica because of the direct immobilization with covalent bonding, which prohibits lateral diffusion of lipids from forming highly-ordered structures that lead to supramolecular functions of lipid membrane systems. 

Biological membranes provide the essential barrier between cells and the organelles of which cells are composed. Cellular membranes are complicated extensive biomolecular sheetlike structures, mostly fonned by lipid molecules held together by cooperative nonco-valent interactions. A membrane is not a static structure, but rather a complex dynamical two-dimensional liquid crystalline fluid mosaic of oriented proteins and lipids. A number of experimental approaches can be used to investigate and characterize biological membranes. However, the complexity of membranes is such that experimental data remain very difficult to interpret at the microscopic level. In recent years, computational studies of membranes based on detailed atomic models, as summarized in Chapter 21, have greatly increased the ability to interpret experimental data, yielding a much-improved picture of the structure and dynamics of lipid bilayers and the relationship of those properties to membrane function [21]. 

Mitochondria are surrounded by a simple outer membrane and a more complex inner membrane (Figure 21.1). The space between the inner and outer membranes is referred to as the intermembrane space. Several enzymes that utilize ATP (such as creatine kinase and adenylate kinase) are found in the intermembrane space. The smooth outer membrane is about 30 to 40% lipid and 60 to 70% protein, and has a relatively high concentration of phos-phatidylinositol. The outer membrane contains significant amounts of porin —a transmembrane protein, rich in /3-sheets, that forms large channels across the membrane, permitting free diffusion of molecules with molecular weights of about 10,000 or less. Apparently, the outer membrane functions mainly to... 

Because skin exhibits many of the properties of a lipid membrane, dermal penetration can often be enhanced by increasing a molecule s lipophilicity. Preparation of an ester of an alcohol is often used for this purpose since this stratagem simultaneously time covers a hydrophilic group and provides a hydrophobic moiety the ready cleavage of this function by the ubiquitous esterase enzymes assures availability of the parent drug molecule. Thus acylation of the primary alcohol in flucinolone (65) with propionyl chloride affords procinonide (66) the same transform... 

VIII. S-LAYER AS SUPPORTING STRUCTURE FOR FUNCTIONAL LIPID MEMBRANES... 

The use of Upid bilayers as a relevant model of biological membranes has provided important information on the structure and function of cell membranes. To utilize the function of cell membrane components for practical applications, a stabilization of Upid bilayers is imperative, because free-standing bilayer lipid membranes (BLMs) typically survive for minutes to hours and are very sensitive to vibration and mechanical shocks [156,157]. The following concept introduces S-layer proteins as supporting structures for BLMs (Fig. 15c) with largely retained physical features (e.g., thickness of the bilayer, fluidity). Electrophysical and spectroscopical studies have been performed to assess the appUcation potential of S-layer-supported lipid membranes. The S-layer protein used in aU studies on planar BLMs was isolated fromB. coagulans E38/vl. 

It has been proposed " that the mechanism(s) of action of gymnemic acids and ziziphins is a biphasic, model-membrane penetration-process. The model suggested that the modifier molecules interact first with the receptor-cell plasma-membrane surface. It was postulated that this initial interaction involves a selective effect on taste perception, including the transduction and quality specification of the sweet stimuli, and selective depression of sweetness perception. Following the initial interaction, the modifier molecules interact with the membrane-lipid interior to produce a general disruption of membrane function and a nonselective effect on taste... 

The ion channel receptors are relatively simple in functional terms because the primary response to receptor activation is generated by the ion channel which is an integral part of the protein. Therefore, no accessory proteins are needed to observe the response to nicotinic AChR activation and the full functioning of the receptor can be observed by isolating and purifying the protein biochemically and reconstituting the protein in an artificial lipid membrane. In contrast, the G-protein-coupled receptors require both G-proteins and those elements such as phospholipase-C illustrated in Fig. 3.1, in order to observe the response to receptor activation (in this case a rise in intracellular calcium concentration resulting from the action of IP3 on intracellular calcium stores). 

Figures 4.2b, 4.3b, and 4.4b are log-log speciation plots, indicating the concentrations of species in units of the total aqueous sample concentration. (Similar plots were described by Scherrer [280].) The uppermost curve in Fig. 4.2b shows the concentration of the uncharged species in octanol, as a function of pH. If only uncharged species permeate across lipid membranes, as the pH-partition hypothesis...

FIGURE 2.1 A side view of the structure of the prototype G-protein-coupled, 7TM receptor rhodopsin. The x-ray structure of bovine rhodopsin is shown with horizontal gray lines, indicating the limits of the cellular lipid membrane. The retinal ligand is shown in a space-filling model as the cloud in the middle of the structure. The seven transmembrane (7TM) helices are shown in solid ribbon form. Note that TM-III is rather tilted (see TM-III at the extracellular and intracellular end of the helix) and that kinks are present in several of the other helices, such as TM-V (to the left), TM-VI (in front of the retinal), and TM-VII. In all of these cases, these kinks are due to the presence of a well-conserved proline residue, which creates a weak point in the helical structure. These kinks are believed to be of functional importance in the activation mechanism for 7TM receptors in general. Also note the amphipathic helix-VIII which is located parallel to the membrane at the membrane interface. 

Additional epithelial aqueous pathways of significantly smaller radius (<3 A) have also been documented utilizing both equivalent pore and circuit theory [25], These pathways may correspond to specific channels through lipid membranes as opposed to paracellular pathways. Osmotically activated ion channels [35] and even specific water channels [36] have been characterized in renal epithelia. In intestinal epithelia, mucosal chloride channels have been studied in secreting crypt cells, and basolateral potassium channels in colonic epithelia serve cellular ion and volume homeostatic functions. 

F. H. Westheimer (1987) has provided a detailed survey of the multifarious ways in which phosphorus derivatives function in living systems (Table 4.7). The particular importance of phosphorus becomes clear when we remember that the daily turnover of adenosine triphosphate (ATP) in the metabolic processes of each human being amounts to several kilograms Phosphate residues bond two nucleotides or deoxynucleotides in the form of a diester, thus making possible the formation of RNA and DNA the phosphate always contains an ionic moiety, the negative charge of which stabilizes the diester towards hydrolysis and prevents transfer of these molecules across the lipid membrane. 

The optimum water content of most cells is around 80%. Liquid water is absolutely necessary for the stability of the lipid membrane and the hydrophobic regions in proteins. The hydrophilic fractions of the nucleic acids and the proteins require liquid water for maintaining their three-dimensional structures and thus their functionality. 

As seen in Eq. (6), AGjw depends on the packing density, tim, of the lipid membrane which in turn depends on the lipid composition. According to the above results, Km is expected to increase with increasing packing density of the membrane, and this has indeed been demonstrated by functionally reconstituting P-gp in different lipid bilayers [62],... 

Other systems like electroporation have no lipids that might help in membrane sealing or fusion for direct transfer of the nucleic acid across membranes they have to generate transient pores, a process where efficiency is usually directly correlated with membrane destruction and cytotoxicity. Alternatively, like for the majority of polymer-based polyplexes, cellular uptake proceeds by clathrin- or caveolin-dependent and related endocytic pathways [152-156]. The polyplexes end up inside endosomes, and the membrane disruption happens in intracellular vesicles. It is noteworthy that several observed uptake processes may not be functional in delivery of bioactive material. Subsequent intracellular obstacles may render a specific pathway into a dead end [151, 154, 156]. With time, endosomal vesicles become slightly acidic (pH 5-6) and finally fuse with and mature into lysosomes. Therefore, polyplexes have to escape into the cytosol to avoid the nucleic acid-degrading lysosomal environment, and to deliver the therapeutic nucleic acid to the active site. Either the carrier polymer or a conjugated endosomolytic domain has to mediate this process [157], which involves local lipid membrane perturbation. Such a lipid membrane interaction could be a toxic event if occurring at the cell surface or mitochondrial membrane. Thus, polymers that show an endosome-specific membrane activity are favorable. 

A highly stable and shielded polyplex should circulate in the blood stream without undesired interactions until it reaches the target cell. At that location, specific interactions with the cell surface should trigger intracellular uptake. While lipid membrane interaction is undesired at the cell surface, it should happen subsequently within the endosomal vesicle and mediate polyplex delivery into the cytosol. During or after intracellular transport to the site of action, the polyplex stability should be weakened to an extent that the nucleic acid is accessible to exert its function. 

Fig. 11 Methods for the construction of PEGylated liposomes, (a) Liposomes possessing reactive groups, such as amino and carboxyl groups, can be prepared by incorporating lipophilic components containing these functional groups into a bilayer membrane. Functionalized liposomes can be PEGylated by reaction with activated PEG derivatives, (b) Preparation of PEGylated liposomes using PEG derivatives possessing lipid moieties...

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