Membrane lipid bilayers phospholipid composition

In this chapter, the authors describe the composition, structural organization, and general functions of biological membranes. After outlining the common features of membranes, a new class of biomolecules, the lipids, are introduced in the context of their role as membrane components. The authors focus on the three main kinds of membrane lipids—the phospholipids, glycolipids, and cholesterol. The amphi-pathic nature of membrane lipids and their ability to organize into bilayers in water are then described. An important functional feature of membranes is their selective permeability to molecules, in particular the inability of ions and most polar molecules to cross membrane bilayers. This aspect of membrane function is discussed next and will be revisited when the mechanisms for transport of ions and polar molecules across membranes is discussed in Chapter 13. 

The chemistry of ROS has been studied by several laboratories, and recently reviewed by Daemen (1973). Over 90% of the protein is rhodopsin, a photosensitive glycoprotein of molecular weight around 35,000, which is imbedded in a lipid bilayer. Phospholipids make up about 96% of the lipids of cattle ROS and cholesterol is the major component of the neutral lipid fraction. Phosphatidyl choline (PC) and phosphatidyl ethanolamine (PE) are the major phospholipids in all species examined, with phosphatidyl serine (PS), phosphatidyl inositol (PI), and sphingomyelin (SPh) present in lesser amounts (Anderson and Maude, 1972). Detailed analysis of the photoreceptor membranes of vertebrate species ranging from frogs to humans have revealed a fairly constant phospholipid class and protein composition (Basinger and Anderson, unpublished). 

Abstract To understand how membrane-active peptides (MAPs) function in vivo, it is essential to obtain structural information about them in their membrane-bound state. Most biophysical approaches rely on the use of bilayers prepared from synthetic phospholipids, i.e. artificial model membranes. A particularly successful structural method is solid-state NMR, which makes use of macroscopically oriented lipid bilayers to study selectively isotope-labelled peptides. Native biomembranes, however, have a far more complex lipid composition and a significant non-lipidic content (protein and carbohydrate). Model membranes, therefore, are not really adequate to address questions concerning for example the selectivity of these membranolytic peptides against prokaryotic vs eukaryotic cells, their varying activities against different bacterial strains, or other related biological issues. 

Artificial membranes are used to study the influence of drug structure and of membrane composition on drug-membrane interactions. Artificial membranes that simulate mammalian membranes can easily be prepared because of the readiness of phospholipids to form lipid bilayers spontaneously. They have a strong tendency to self-associate in water. The macroscopic structure of dispersions of phospholipids depends on the type of lipids and on the water content. The structure and properties of self-assembled phospholipids in excess water have been described [74], and the mechanism of vesicle (synonym for liposome) formation has been reviewed [75]. While the individual components of membranes, proteins and lipids, are made up of atoms and covalent bonds, their association with each other to produce membrane structures is governed largely by hydrophobic effects. The hydrophobic effect is derived from the structure of water and the interaction of other components with the water structure. Because of their enormous hydrogen-bonding capacity, water molecules adopt a structure in both the liquid and solid state. 

Compared with lAM, which uses a monolayer of phospholipid, the liposomal phospholipid bilayers in ILC have the advantage of closely resembling biologic membrane bilayers and constitute a 2-D fluid in which lipid molecules and other components are free to diffuse (10). With this technique, the phospholipid composition can be changed to mimic the membrane of interest. Membrane lipids extracted from human cells also can be used the technique then is called immobilized biomembrane chromatography (IBC) (11). 

The outer membrane (OM) is an asymmetric lipid bilayer, with an inner leaflet composed of phospholipids and an outer leaflet composed of lipopolysaccharide (O Fig. 1) [6]. The phospholipid composition of the inner leaflet of the OM of E. coli is similar to that of the inner membrane predominantly phosphatidylethanolamine, with smaller amounts of phosphatidyl-... 

Synthetic phospholipids are commercially available and they contain polar heads occurring in nature and and the same type of fatty acid in both, 9 -l and sn-2 sites. The acyl chains may be mainly myristoyl (C14 0), palmitoyl (C16 0), olcoyl (C16 l) or stearoyl (C18 0) residues. The physicochemical characteristics of the lipidic bilayers that are prepared from synthetic phospholipids are well defined. The lipid composition influences many physical properties of model membranes and it has been shown that single synthetic lipids or mixtures of short and long fatty acid chains can produce stable lipidic bilayers and liposomes. 

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