Membranes, biological cholesterol

Langmuir-Blodgett films (LB) and self assembled monolayers (SAM) deposited on metal surfaces have been studied by SERS spectroscopy in several investigations. For example, mono- and bilayers of phospholipids and cholesterol deposited on a rutile prism with a silver coating have been analyzed in contact with water. The study showed that in these models of biological membranes the second layer modified the fluidity of the first monolayer, and revealed the conformation of the polar head close to the silver [4.300]. 

There are other ways in which the lateral organization (and asymmetry) of lipids in biological membranes can be altered. Eor example, cholesterol can intercalate between the phospholipid fatty acid chains, its polar hydroxyl group associated with the polar head groups. In this manner, patches of cholesterol and phospholipids can form in an otherwise homogeneous sea of pure phospholipid. This lateral asymmetry can in turn affect the function of membrane proteins and enzymes. The lateral distribution of lipids in a membrane can also be affected by proteins in the membrane. Certain integral membrane proteins prefer associations with specific lipids. Proteins may select unsaturated lipid chains over saturated chains or may prefer a specific head group over others. 

Phospholipids are a major component of all biological membranes together with glycolipids and cholesterol. Due to their polar nature, i.e. hydrophilic head and hydrophobic tail, phospholipids form in water vesicles or liposomes. 

Cholesterol is found in many biological membrane and is the main sterol of animal organisms. It is eqnimolar with phospholipids in membranes of liver cell, erythrocytes, and myelin, whereas in human stratum comeum it lies in the outermost layer of the epidermis... 

On the other hand, cellular membranes are composed of chiral molecules such as phospholipids and cholesterol, but the homochirality of these constituents is not obviously manifested in the membrane s structure. However, in certain cases biological membranes exhibit a distinct helical or twisted structure, which is a very conspicuous sign of the chirality of the supramolec-ular aggregate. These chiral supramolecular aggregates are the subject of this chapter. 

Biological membranes Fluidity and order parameters Determination of the phase transition temperature Effect of additives (e.g. cholesterol)... 

The serious drawback of the methods of evaluation of fluidity based on intermolecular quenching or excimer formation is that the translational diffusion can be perturbed in constrained media. It should be emphasized that, in the case of biological membranes, problems in the estimation of fluidity arise from the presence of proteins and possible additives (e.g. cholesterol). Nevertheless, excimer formation with pyrene or pyrene-labeled phospholipids can provide interesting in-... 

Natural biological membranes consist of lipid bilayers, which typically comprise a complex mixture of phospholipids and sterol, along with embedded or surface associated proteins. The sterol cholesterol is an important component of animal cell membranes, which may consist of up to 50 mol% cholesterol. As cholesterol can significantly modify the bilayer physical properties, such as acyl-chain orientational order, model membranes containing cholesterol have been studied extensively. Spectroscopic and diffraction experiments reveal that cholesterol in a lipid-crystalline bilayer increases the orientational order of the lipid acyl-chains without substantially restricting the mobility of the lipid molecules. Cholesterol thickens a liquid-crystalline bilayer and increases the packing density of lipid acyl-chains in the plane of the bilayer in a way that has been referred to as a condensing effect. 

Figure19.1 A schematic diagram of a plasma membrane. Integral proteins are embedded in a bilayer composed of phospholipids (shown, for clarity, in much greater proportion than they have in biological membranes) and cholesterol. The carbohydrate components of glycoproteins and glycolipids occur only on the external face of the membrane. (Reproduced from D. Voet and J. G. Voet, Biochemistry, 3rd edn, 2004. 2004, Donald and Judith G Voet. Reprinted with permission of John Wiley and Sons, Inc.)...

Steroids are derived bio synthetically from cholesterol. They play multiple roles in human physiology sex characteristics, control of inflammation, embryo implantation, and control of salt and water balance. Cholesterol itself is an indispensable constituent of biological membranes. 

Cholesterol is absolutely required in human physiology (a) as a constituent of biological membranes and (b) as the metabolic precursor to all other steroids. 

Cholesterol the steroid precursor of all steroid hormones in humans and a critical component of biological membranes. 

McMullen, T.P.W., Lewis, R.N.A.H., McElhaney, R.N. Cholesterol-phospholipid interactions, the liquid-ordered phase and lipid rafts in model and biological membranes. Curr. Opin. Colloid Interface Sci. 2004, 8, 459-68. 

Berkowitz, M.L. Detailed molecular dynamics simulations of model biological membranes containing cholesterol. Biochim. Biophys. Acta 2009, 1788, 86-96. 

The spread mixed lipid monolayer studies provide information about the packing and orientation of such molecules at the water interface. These interfacial characteristics affect many other systems. For instance, mixed surfactants are used in froth flotation. The monolayer surface pressure of a pure surfactant is measured after the injection of the second surfactant. From the change in n, the interaction mechanism can be measured. The monolayer method has also been used as a model biological membrane system. In the latter BLM, lipids are found to be mixed with other lipidlike molecules (such as cholesterol). Hence, mixed monolayers of lipids + cholesterol have been found to provide much useful information on BLM. The most important BLM and temperature melting phenomena is the human body temperature regulation. Normal body temperature is 37°C (98°F), at which all BLM function efficiently. 

Our present ideas about the nature of biological membranes, which are so fundamental to all biochemical processes, are based on the Singer-Nicholson mosaic model. This model of the membrane is based on a phospholipid bilayer that is, however, asymmetrical. In the outside monolayer, phosphatidylcholine (lecithin) predominates, whereas the inner monolayer on the cytoplasmic side is rich in a mixture of phos-phatidylethanolamine, phosphatidylserine, and phosphatidylinositol. Cholesterol molecules are also inserted into the bilayer, with their 3-hydroxyl group pointed toward the aqueous side. The hydrophobic fatty acid tails and the steran skeleton of cholesterol... 

Since cholesterol is an important component of many biological membranes mixtures of polymerizable lipids with this sterol are of great interest. In mixed monolayers of natural lipids with cholesterol a pronounced condensation effect , i.e. a reduction of the mean area per molecule of phospholipid is observed68. This influence of cholesterol on diacetylenic lecithin (18, n = 12), however, is not very significant (Fig. 32). Photopolymerization indicates phase separation in this system. Apparently due to the large hydrophobic interactions between the long hydrocarbon chains of... 

Sterols and Cholesterol. Natural sterols are crystalline C76 C1(1 steroid alcohols containing an aliphatic side chain at C17. Sterols were first isolated as lionsaponifiable fractions of lipids from various plant and animal sources and have been identified in almost all types of living organisms. By far, the most common sterol in vertebrates is cholesterol (8). Cholesterol serves two principal functions in mammals. First, cholesterol plays a role in the structure and function of biological membranes.. Secondly, cholesterol serves as a central intermediate in the biosynthesis of many biologically active steroids, including bile acids, corticosteroids, and sex hormones. 

Structure and Functions of Biological Membranes 381 Metabolism of Fatty Acids 411 Biosynthesis of Membrane Lipids 436 Metabolism of Cholesterol 459... 

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-... 

Qeveral recent investigations using various physicochemical methods have provided convincing evidence to support the contention that the basic structure of most biological membranes consists of a phospholipid bilayer (1,2,3, 4). Studies on phospholipid model membranes can therefore be expected to yield relevant information on the role played by phospholipids in determining the characteristic properties of biological membranes (5). One important aspect of this problem concerns the mechanisms of interaction between the phospholipids and other membrane constituents such as cholesterol, proteins, and different inorganic... 

Lipids like phospholipids, triacylglycerols and cholesterol play various important roles in living organisms. Phospholipids and cholesterol are the main components of biological membranes, which not only form the boundaries of the various compartments inside the cells and organisms, but also act as the interface between the organism and the environment, and in case of parasitic flatworms, the site of interaction between parasite and host. 

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