Lipids density

Baumgartner and coworkers [145,146] study lipid-protein interactions in lipid bilayers. The lipids are modeled as chains of hard spheres with heads tethered to two virtual surfaces, representing the two sides of the bilayer. Within this model, Baumgartner [145] has investigated the influence of membrane curvature on the conformations of a long embedded chain (a protein ). He predicts that the protein spontaneously localizes on the inner side of the membrane, due to the larger fluctuations of lipid density there. Sintes and Baumgartner [146] have calculated the lipid-mediated interactions between cylindrical inclusions ( proteins ). Apart from the... 

Figure 5. Profiles across the bilayer of the total lipid density of DPPC, the water density and the densities of certain lipid groups as obtained from MD simulations by Berger et al. [58]. The profiles are found by taking the time average over the last 300 ps of the simulation. The densities for the lipid head-group components are only shown on one side for clarity. The origin of the z-axis is arbitrarily positioned on the left of the bilayer. On the y-axis, the atom density in atoms per nm3 is given. Redrawn from [58] by permission of the Biophysical Society...

Close modulation of the physical states of lipids is important in a variety of other contexts. Lipids play major roles as buoyancy-regulating devices in which temperature-induced alterations in lipid density play important functions in setting the overall buoyancy of the organism (e.g., the spermaceti organ of sperm whales see Clarke, 1979). Cuticular lipids function as important barriers to water movement in terrestrial arthropods (Gibbs, 1998 Hadley, 1981), and the chemical compositions and physical states of these lipids manifest temperature responses comparable in many ways to those observed for membrane lipids. 

Cholesterol and triglycerides, as the major plasma hpids, are essential substrates for cell membrane formation and hormone synthesis and provide a source of free fatty acids. Hyperlipidemia is defined as an elevation of one or more of cholesterol, cholesterol esters, phospholipids, or triglycerides. Lipids, being water immiscible, are not present in free form in the plasma but rather circulate as hpoproteins. Hyperlipoproteinemia describes an increased concentration of the lipoprotein macromolecules that transport lipids in the plasma. The density of plasma lipoproteins is determined by their relative content of protein and lipid. Density, composition, size, and electrophoretic mobility divide lipoproteins into four classes (Table 21-1). 

Because of the fluctuations of the flexible biological membranes, the structural characterization of lipid bilayers is an arduous task when atomic details are sought. Indeed, structural information about the membrane thicknesses, such as the hydrophobic thickness and head group separation, as well as the lipid density, are very difficult to quantify. This results in a large uncertainty in the experimentally determined structural parameters of lipid bilayers found in the literature. For example, values of the average area per phospholipid molecule measured in a single lipid system can vary by nearly 30... 

Wurpel, G.W.H., Rinia, H.A., Muller, M. Imaging orientational order and lipid density in multilamellar vesicles with multiplex CARS microscopy. J. Microscopy (Oxford) 218,37-45... 

Figure 9.23 Scale drawing of a MOPC micelle at the surface of a PEG-grafted bilayer. The PEG-lipid density shown is approximately 5mol%.

Lipid-Lipid Interactions Why 2.7 Key Experiments on Lipid Density 49... 

Fig. XV-4. Schematic drawing of four streptavidin molecules bound to biotinylated lipid in a monolayer above heavy water. The scattering length density for neutron reflectivity is shown at the side. (From Ref. 30.)...

Cholesterol is biosynthesized in the liver trans ported throughout the body to be used in a va riety of ways and returned to the liver where it serves as the biosynthetic precursor to other steroids But cholesterol is a lipid and isn t soluble in water How can it move through the blood if it doesn t dis solve in if The answer is that it doesn t dissolve but IS instead carried through the blood and tissues as part of a lipoprotein (lipid + protein = lipoprotein) The proteins that carry cholesterol from the liver are called low density lipoproteins or LDLs those that return it to the liver are the high-density lipoproteins or HDLs If too much cholesterol is being transported by LDL or too little by HDL the extra cholesterol builds up on the walls of the arteries caus mg atherosclerosis A thorough physical examination nowadays measures not only total cholesterol con centration but also the distribution between LDL and HDL cholesterol An elevated level of LDL cholesterol IS a risk factor for heart disease LDL cholesterol is bad cholesterol HDLs on the other hand remove excess cholesterol and are protective HDL cholesterol IS good cholesterol... 

Various sources of lipid have been incorporated into ruminant diets to increase the energy density and provide the large amount of energy needed for slaughter animals to achieve market weight or for dairy cows to produce milk (see Milk and milkproducts). Fats also reduce the dustiness of feeds, increase the feedstuffs abiUty to pellet, and improve feed acceptabiUty. 

Figure 3 Comparison of the densities (in g/cm ) of model compounds for membrane lipids computed from constant-pressure MD simulations with the coiTespondmg experimental values. The model compounds include solid octane and tricosane, liquid butane, octane, tetradecane, and eico-sane, and the glycerylphosphorylcholme, cyclopentylphosphorylcholme monohydrate, dilauroly-glycerol, anhydrous cholesterol, cholesterol monohydrate, and cholesterol acetate crystals. (Models from Refs. 18, 42, and 43).

Figure 5 Electron density distributions along the bilayer normal from an MD simulation of a fully hydrated liquid crystalline phase DPPC bilayer. (a) Total, lipid, and water contributions (b) contributions of lipid components in the interfacial region.

Whereas the main challenge for the first bilayer simulations has been to obtain stable bilayers with properties (e.g., densities) which compare well with experiments, more and more complex problems can be tackled nowadays. For example, lipid bilayers were set up and compared in different phases (the fluid, the gel, the ripple phase) [67,68,76,81]. The formation of large pores and the structure of water in these water channels have been studied [80,81], and the forces acting on lipids which are pulled out of a membrane have been measured [82]. The bilayer systems themselves are also becoming more complex. Bilayers made of complicated amphiphiles such as unsaturated lipids have been considered [83,84]. The effect of adding cholesterol has been investigated [85,86]. An increasing number of studies are concerned with the important complex of hpid/protein interactions [87-89] and, in particular, with the structure of ion channels [90-92]. 

The Pink model is found to exhibit a gel-fluid transition for lipids with sufficiently long chains, which is weakly first order. The transition disappears in bilayers of shorter lipids, but it leaves a signature in that one observes strong lateral density fluctuations in a narrow temperature region [200,201]. In later studies, the model has been extended in many ways in order to explore various aspects of gel-fluid transitions [202]. For example, Mouritsen et al. [203] have investigated the interplay between chain melting and chain crystallization by coupling a two-state Doniach model or a ten-state Pink model to a Potts model. (The use of Potts models as models for... 

LIPOPROTEINS. Blood plasma lipoproteins are prominent examples of the class of proteins conjugated with lipid. The plasma lipoproteins function primarily in the transport of lipids to sites of active membrane synthesis. Serum levels of low density lipoproteins (LDLs) are often used as a clinical index of susceptibility to vascular disease. 

Sucrose gradients for separation of membrane proteins must be able to separate proteins and protein-lipid complexes having a wide range of densities, typically 1.00 to 1.35 g/mL. 

When most lipids circulate in the body, they do so in the form of lipoprotein complexes. Simple, unesterified fatty acids are merely bound to serum albumin and other proteins in blood plasma, but phospholipids, triacylglycerols, cholesterol, and cholesterol esters are all transported in the form of lipoproteins. At various sites in the body, lipoproteins interact with specific receptors and enzymes that transfer or modify their lipid cargoes. It is now customary to classify lipoproteins according to their densities (Table 25.1). The densities are... 

Name Density (g/mL) % Lipid % Protein Optimal (mg/dL) Poor (mg/dL)... 

Cells in the atheroma derived from both macrophages and smooth muscle cells that have accumulated modified low-density lipoproteins. Their cytoplasm laden with lipid causes the foamy appearance on microscopy... 

---