Lipids in biological membranes

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. 

Vitamin E is a generic term that represents four tocopherols and four tocotrienols of varying biological potency. The term tocopherol correctly refers to the methyl-substituted derivatives of to-col and is not synonymous with the term vitamin E. The tocopherols and tocotrienols may be referred to collectively as tocochromanols. Many of the diverse deficiency syndromes observed in animals experimentally deprived of vitamin E can be explained by the vitamin s acting as an antioxidant in stabilizing unsaturated lipids in biological membranes. 

There are two main types of lipids in biological membranes Phospholipids and sterols. The predominant phospholipids in most membranes are phosphoglycerides, which are phosphate esters... 

Cullis, P. R. and de Kmijff, B. (1979). Lipid polymorphism and the functional roles of lipids in biological membranes. Biochim Biophys. Acta 559 399. 

The important structural lipid in biological membrane is glycero phospholipid which contains glycerol, fatty acids, phosphoric acid and a nitrogenous base. Two fatty acids and a phosphate esterified to glycerol... 

These unique molecules have many important biological roles. They can serve as a source of energy, signal molecules, and components of various cell membranes. The primary lipids in biological membranes are phospholipids, cholesterol, and glycolipids (24),... 

Molecular Structures and Properties of Lipids Lipids in Biological Membranes Membrane Proteins Membrane Fluidity Membrane Asymmetry Erythrocyte Membrane... 

The presence of unsaturated lipids in biological membranes confers a fluid character on the structure and this is integral to its function. Evidence for this has been established by homogeneous catalytic hydrogenation of these lipids in membrane... 

As will become apparent, the role of lipids in biological membranes is a major point of contention. 

Hydrated bilayers containing one or more lipid components are commonly employed as models for biological membranes. These model systems exhibit a multiplicity of structural phases that are not observed in biological membranes. In the state that is analogous to fluid biological membranes, the liquid crystal or La bilayer phase present above the main bilayer phase transition temperature, Ta, the lipid hydrocarbon chains are conforma-tionally disordered and fluid ( melted ), and the lipids diffuse in the plane of the bilayer. At temperatures well below Ta, hydrated bilayers exist in the gel, or Lp, state in which the mostly all-trans chains are collectively tilted and pack in a regular two-dimensional... 

Miller, I. R. Structural and energetic aspects of charge transport in lipid layers and in biological membranes, in Topics in Bioelectrochemistry and Bioenergetics, Vol. 4 (ed.) Milazzo, G., New York, Wiley 1981... 

Vladimirov, Yu. A. Archakov, A. I. Lipid Peroxidation in Biological Membranes (in Russian), Nauka, Moscow, 1972... 

In biological membranes, the class of fully saturated PC lipids is just one of many. Indeed, variations in the lipid architecture are frequently encountered. There are variations with respect to the head-group structure as well as variations in the tail architecture. For example, in the tails, single or multiple unsaturated bonds are very common. There is relatively little knowledge about the role of lipid unsaturation in biomembranes. There are relatively few MD simulations on bilayers composed of lipids with unsaturated bonds. Nevertheless, single unsaturation as well as multiple unsaturated chains have been of some interest. For a recent review see reference [102], We will now present a selection of the results. We will also take this opportunity to compare the results from the MD simulations with those from the SCF calculations. 

Table 4. Major building blocks of lipid bilayers in biological membranes and their speciation and acidity constants...

The studies on phospholipid bilayers with defined amounts of charged component are helpful to explain the partition characteristics in biological membranes. Liposome water partition data of propranolol in lipids from kidney epithelial cells (a common model system in pharmaceutical sciences for the uptake into the gastrointestinal tract) have been successfully described with partition models developed for pure bilayers or defined mixtures [159]. Since lipophilic cations and anions can be used as probes for the membrane potential, their interaction with microbial and mitochondrial membranes has been studied... 

While examples such as these provide evidence that strong interactions of negatively-charged membrane lipids with membrane proteins the role in maintaining asymmetric distributions of lipids aaoss biological membranes is unclear. In any event such effects are likely to be of minor importance relative to actively mediated phospholipid translocation processes. 

Vitamin E (tocopherol) and related compounds only occur in plants (e.g., wheat germ). They contain what is known as a chro-man ring. In the lipid phase, vitamin E is mainly located in biological membranes, where as an antioxidant it protects unsaturated lipids against ROS (see p. 284) and other radicals. 

The rates of lateral diffusion of phospholipids in lipid bilayer membranes, and in biological membranes, were first measured using spin-labeled lipids.26 50 10 11 9 In general, these rates have been determined by incorporating spin-labeled lipids such as (V) and (VI) in phospholipid bilayers, or multilayers. The paramagnetic resonance spectra of labels such as (V), as well as the nuclear resonance spectra of other lipids in membranes containing (V), depend on the concentration c of the label in the membrane and the rate of lateral motion of the lipids. Two methods... 

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