Lipids in membrane

Cullis PR, Fenske DB, Fiope MJ Physical properties and functional roles of lipids in membranes. In Biochemistry of Lipids, Lipoproteins and Membranes. Vance DE, Vance JE (editors). Elsevier, 1996. 

Material barriers Contours of land Immiscible substances (lipids) in membranes Earth s mantle (oxides)... 

Lipids are biological molecules that are soluble in certain organic solvents (whether or not something is a lipid is operationally defined by the solubility). Lipids include a variety of molecules such as triglycerides, phospholipids, and cholesterol. The major type of lipid in membranes is the phospholipid. They re called phospholipids because they all contain a phosphate diester. 

The importance of lipids in membrane structure was established early in the 20th century when pioneering biophysicists established positive correlations between cell membrane permeabilities to small non-electrolytes and the oil/water partition coefficients of these molecules. Contemporary measurements of the electrical impedance of cell suspensions suggested that cells are surrounded by a hydrocarbon barrier, which was first estimated to be about 3.3 nm thick. This was originally thought to be a lipid monolayer. Among the pioneering biophysical experiments were those that established that the ratio of the area of a monolayer formed from erythrocyte... 

When the individual proportions of lipids in membranes are examined more closely (right part of the illustration), typical patterns for particular cells and tissues are also found. The illustration shows the diversity of the membrane lipids and their approximate quantitative composition. Phospholipids are predominant in membrane lipids in comparison with glycolipids and cholesterol. Triacyl-glycerols (neutral fats) are not found in membranes. 

Much research has been invested to identify a common and validated test method that may be used in all industrial laboratories concerned by phototoxicology. Cultured mammalian cells constitute an essential model for the evaluation of phototoxicity. Such systems include all biological targets (lipids in membranes, proteins, nucleic acids) as well as active pathways likely to modulate the phototoxic impact (apoptotic pathways, cellular defenses, endogenous antioxidants, repair pathways, metabolism). 

Cholesterol is the prototypic steroid lipid. It was first isolated in 1770 in the 1920s, the German chemists Windaus and Wieland deduced the structure for cholesterol, receiving Nobel Prizes for their work in 1927 and 1928. Cholesterol is important in medicinal chemistry, not only for its role in atherosclerosis, but also because it is an important lipid in membrane structure. 

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... 

The linear polypeptide chains of a protein fold in a highly specific way that is determined by the sequence of amino acids in the chains. Many proteins are composed of two or more polypeptides. Certain proteins function in structural roles. Some structural proteins interact with lipids in membrane structures. Others aggregate to form part of the cytoskeleton that helps to give the cell its shape. Still others are the chief components of muscle or connective tissue. Enzymes constitute yet another major class of proteins, which function as catalysts that accelerate and direct biochemical reactions. 

Two questions concerning the distribution of lipids in membranes remain unanswered ... 

CARS microscopy has emerged as a highly sensitive analytical tool for vibrational bioimaging, predominantly, of lipids in membrane model systems [69, 81-84], live unstained cells [85-95, 43], and both ex vivo and in vivo tissues [26, 96-103, 43]. Examples of CARS imaging applications in the physical and material sciences include the study of fracture dynamics in drying silica nanoparticle suspensions [104], patterned polymeric photoresist film [105], drug molecules in a polymer matrix [106], and liquid crystals [107, 108],... 

While there is no doubt that the major attractive forces maintaining lipids in membranes are noncovalent in nature, there is excellent evidence in the literature showing that a small percentage of the membrane lipids is covalently linked to membrane proteins. These lipids are highly specific in nature and will be discussed briefly below. Normally these lipid protein complexes are not found in the organic solvent phase of a typical (lipid) extraction procedure. Rather they would be found in the water-rich phase of such an extraction approach. Basically there are four specific classes of lipid covalent binding to protein ... 

Morii, H., and Koga, Y., 1994, Asymmetrical topology of diether- and tetraether-type polar lipids in membranes of Methanobacterium thermoautotrophicum cells. J. Biol. Chem. 269 10492-10497. 

One caveat of the studies of nuclei is that the ER is closely associated with the nuclear envelope. Lipid kinase activity has been associated with ER and the role of the inositol lipids in membrane trafficking is well accepted (Cockcroft and De Matteis, 2001 Godi et al., 2004 Roth, 2004 van Meer and Sprang,... 

E.J.M. Helmreich and E. L.Bson. Mobility of proteins and lipids in membranes. In Advances in cydic nucleotide and protein phosphorylation research, Vol.18, (ed. P. Greengard and G.A. Robison). Raven Press, New York, pp. 1—61, 1984. 

A biologic reason for the abundance of nonlamellar lipids in membranes is that they possess the ability to modulate the activities of membrane proteins (15, 16). It has been recognized that membranes exist in a state of curvature frustration, which may be sufficiently large to have significant effect on certain protein conformations (17). Many examples show that the lipid bilayer elastic curvature stress indeed couples to conformational changes of membrane proteins (15, 18, 19). Protein kinase C is one such example of an enzyme activated by lipids that exhibit a propensity for nonlamellar phase formation (20). The activity of Ca " -ATPase from sarcoplasmic reticulum membranes also strongly correlates with the occurrence of nonbilayer lipids in the membrane and increases with the increase of their amount. It is noteworthy that the protein activity does not depend on the chemical structure of the lipids but only on their phase propensity thus specific binding interactions are ruled out. The list of proteins with activities that depend on the phase properties... 

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