Biological membranes, lipid compositions

It has been postulated that Pgp acts as a so-called vacuum cleaner (49), moving compounds from the lipid bilayer into the extracellular space. A second hypothesis has been postulated where the transporter acts as a flippase (50), either moving the substrate from the inner to the outer leaflet of the membrane or locally altering membrane lipid composition such that the substrate detaches. These mechanisms support the observation that Pgp effluxes amphipatic peptides, proteins lacking signal sequences, or lipid-modified proteins from biological membranes (51). 

Leeder and Rippon [85] have analyzed the lipid composition of wool fibers after removing surface grease. Continued extraction with solvent removed the beta layers evidenced by electron microscopy however, the extract contained free cholesterol and free fatty acid and triglycerides but negligible quantities of phospholipid normally associated with biological membrane lipids. Koch [86], in his work with internal lipid of human hair, did not report significant quantities of phospholipid. These lipid-protein layers of hair are most likely related structurally to those of the epicuticle. 

Physiology and Cell Biology of Membrane Lipid Composition... 

Native biological membranes also display characteristic phase transitions, but these are broad and strongly dependent on the lipid and protein composition of the membrane. 

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. 

Myelin in situ has a water content of about 40%. The dry mass of both CNS and PNS myelin is characterized by a high proportion of lipid (70-85%) and, consequently, a low proportion of protein (15-30%). By comparison, most biological membranes have a higher ratio of proteins to lipids. The currently accepted view of membrane structure is that of a lipid bilayer with integral membrane proteins embedded in the bilayer and other extrinsic proteins attached to one surface or the other by weaker linkages. Proteins and lipids are asymmetrically distributed in this bilayer, with only partial asymmetry of the lipids. The proposed molecular architecture of the layered membranes of compact myelin fits such a concept (Fig. 4-11). Models of compact myelin are based on data from electron microscopy, immunostaining, X-ray diffraction, surface probes studies, structural abnormalities in mutant mice, correlations between structure and composition in various species, and predictions of protein structure from sequencing information [4]. 

It is necessary to elaborate on yet another essential aspect of biological membranes, i.e. their complexity . This keyword points to the large number of different molecules that are usually found in the biological membrane. First of all, there is a large variety of lipid molecules. The lipid composition of the biological membranes varies from one species to another, and is adapted to meet the needs of organs, cells, organelles, etc. The variations in the head-tail... 

In many biological systems the biological membrane is a type of surface on which hydrophilic molecules can be attached. Then a microenvironment is created in which the ionic composition can be tuned in a controlled way. Such a fluffy polymer layer is sometimes called a slimy layer. Here we report on the first attempt to generate a realistic slimy layer around the bilayer. This is done by grafting a polyelectrolyte chain on the end of a PC lipid molecule. When doing so, it was found that the density in which one can pack such a polyelectrolyte layer depends on the size of the hydrophobic anchor. For this reason, we used stearoyl Ci8 tails. The results of such a calculation are given in Figure 26. 

Biological membranes consist of lipids, proteins, and carbohydrates (see p. 214). These components occur in varying proportions (left). Proteins usually account for the largest proportion, at around half. By contrast, carbohydrates, which are only found on the side facing away from the cytoplasm, make up only a few percent. An extreme composition is seen in myelin, the insulating material in nerve cells, three-quarters of which consists of lipids. By contrast, the inner mitochondrial membrane is characterized by a very low proportion of lipids and a particularly high proportion of proteins. 

An important approach to the study of biological membranes has been the preparation and study of model membranes. According to current usage, model membranes include lipid bilayers and lipid bilayers into which have been incorporated additional components such as one or more membrane proteins. It is through the study of such model membranes that one has the best opportunity to isolate and study fundamental physical chemical and biophysical processes, and it is for this reason that the present report emphasizes these systems. A discussion of model membranes necessarily starts with a description of the chemical compositions and physical properties of lipid molecules. 

One approach to understanding membrane function is to study membrane composition—to determine, for example, which components are common to all membranes and which are unique to membranes with specific functions. So before describing membrane structure and function we consider the molecular components of membranes proteins and polar lipids, which account for almost all the mass of biological membranes, and carbohydrates, present as part of glycoproteins and glycolipids. 

Lipids in a biological membrane can exist in liquid-ordered or liquid-disordered states in the latter state, thermal motion of acyl chains makes the interior of the bilayer fluid. Fluidity is affected by temperature, fatty acid composition, and sterol content. 

The second major type of lipid found in some biological membranes is cholesterol. Cholesterol (fig. 17.5) is an isoprenoid compound with four fused rings, a short aliphatic chain, and a single hydroxyl group. It occurs in membranes both in its free form and esterified with long-chain fatty acids. Table 17.3 compares the lipid compositions of mem-... 

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