Micelle, lipid structure

In the bilayer or upon interaction with detergent micelles, a structural reorganization of pardaxin aggregates takes place, in which the polar side chains interact with themselves and the hydrophobic residues are externally oriented in the pardaxin aggregate, therefore allowing interactions with the lipid backbone hydrocarbons. 

This broad class of hydrolases constitutes a special category of enzymes which bind to and conduct their catalytic functions at the interface between the aqueous solution and the surface of membranes, vesicles, or emulsions. In order to explain the kinetics of lipolysis, one must determine the rates and affinities that govern enzyme adsorption to the interface of insoluble lipid structures -. One must also account for the special properties of the lipid surface as well as for the ability of enzymes to scooC along the lipid surface. See specific enzyme Micelle Interfacial Catalysis... 

Film penetration studies show unequivocally that lecithin-cholesterol mixtures containing from 0 to 50 mole % cholesterol and lecithin—lactoside mixtures containing from 0 to 80 mole % Ci6-dihydroceramide lactoside have the same effect as pure lecithin. This suggests the presence of a lipid complex in which lecithin prevents the interaction of the cholesterol or ceramide lactoside with globulin. Over these ranges of composition the lipid film would consist of a mixture of the lecithin-cholesterol or the lecithin-lactoside complex with excess lecithin. One may picture two models in which the protein contact is restricted to molecules of lecithin. In one, individual polar groups of the protein interact with the excess lecithin molecules as well as with the lecithin portions of the complex. In the other model, the protein as a whole interacts with the lecithin sites of polymeric lipid structures. The latter, which could be referred to as surface micelles (I), are visualized also through the term "mono-... 

There is a close resemblance between fatty-acid salts and phospholipids (p. 790) in that both possess long hydrocarbon tails and a polar head. Phospholipids also aggregate in a polar medium to form micelles and continuous bilayer structures such as shown in Figure 18-5. The bilayer lipid structure is very important to the self-sealing function of membranes and their impermeability to very polar molecules. 

Disrupting lipid structure, e.g., solubilization by formation of micelles to create aqueous channels... 

Tengamnuay, P., and A.K. Mitra. 1990. Bile-salt fatty acid mixed micelles as nasal absorption promoters of peptides. I. Effects of ionic strength, adjuvant composition and lipid structure on the nasal absorption of [D-Arg2]kyotorphin. Pharm Res 7 127. 

Lipid. Lipids or surfactants had been used widely in the delivery of pharmacologically active materials in the form of liposomes, emulsions, or micelles. Since the first description of their potential for exogenous gene transfer, much progress has been made in the development of improved cationic lipid structures and formulations with enhanced gene transfection activity. 

Bile salts readily form mixed micelles with lipid-like molecules such as lecithins or fatty acids. These mixed micelles are structurally very different from the simple micelles and generally have a much greater solubilizing capacity for hydrophobic molecules, both biological and synthetic. The solubility of DDT, a non-polar, water insoluble molecule, for example, in bile salt micellar solution can be increased to a far greater extent by the addition of unsaturated long chain fatty acids, probably because of mixed micelle formation. 

Emulsifiers are necessary to allow water and lipids to combine. A surfactant is an amphiphilic molecule that has affinities for fats as well as water and that can be incorporated into lamellar lipid structures (e.g. cell walls). Surfactants increase the fluidity of the lipid structures by partitioning into the lipid membranes, as their lateral interactions with the membrane-forming lipids reduce the force of their attractive interaction. The mobility of the membrane lipids increases considerably in a similar manner to when a liquid crystal is converted into a gel. Finally, lipids can be seen to micellize or simply dissolve. Membranes lose their relative impermeability. See Figure 5.16. 

There are a variety of other types of nonbilayer lipid structures such as reversed micelles sandwiched between monolayers of the lipid bilayers in vivo, while the main structural pattern of biological membranes is the flat bilayer of lipid molecules. These nonbilayer structures can explain many processes occurring in the living cell, such as fusion, and exo- and endo-cytosis. Because the water in the reversed micelle resembles that adjacent to biological membranes or biological reversed micelle-like microcompartments, reversed micelles may be an appropriate model for investigating biological catalysis at the molecular level [3-5]. 

A Protein Example Phoshpholipase A2 Pancreatic phospholipase A2 is an enzyme of molecular weight 14,000, which catalyses the hydrolysis of 2-acyl ester bonds in a variety of naturally occurring phospholipids. The enzyme is secreted as a zymogen, which is activated by tryptic cleavage of the N-terminal heptapeptide. Both the enzyme and its precursor show catalytic activity towards monomeric lipids, but in contrast to the precursor the active enzyme shows a tremendous rate increase when it acts upon organized lipid structures such as micelles and bilayers (23). 

It is the view of the present authors that a physiological role for these inhibitors cannot be disregarded on the basis of these arguments rather, the question must remain open until more definitive experiments are performed. It seems reasonable that, in the intracellular environment, the critical micelle concentration for the fatty acyl-CoA derivatives may never be achieved and that the largest fraction of these inhibitory substances is bound at hydrophobic sites of intracellular proteins and lipid structures such as membranes. In this case, the degree of inhibition would depend on the relative affinity of hydrophobic sites on the carboxylase, the concentration of free fatty acyl-CoA, and that bound at other intracellular hydrophobic sites. 

Supramolecular amphiphilic assemblies Micelles and liposomes are supramolecular aggregates that are formed under aqueous conditions by spontaneous self-assembly of amphiphilic molecules that contain both hydrophilic and hydrophobic ends (Figure 11). Large payloads of contrast agent can be incorporated into liposome and micelle supramolecular structures for signal amplification. Often, the lipid shell can be further modified for targeting and accumulation at the site of interest. The self-assembly of amphiphilic molecules into liposomes and... 

FIGURE 2.10 Different types of lipid structuration in water micelles and bilayers. Sphingolipids (P values < 1) have an inverted cone shape, so that they will not form bilayers but micelles in water (left panel). Bilayer assemblies require cylindrical lipids (P = 1) such as POPC (palmytoyl-oleoyl-phosphatidylchoHne). 

Simplistically stated, the hydrophobic effect may be defined as the tendency of water to reject any contact with substances of a nonpolar or hydrocarbon nature. The existence of this effect was first recognized in the study of the extremely low solubility of hydrocarbons in water. The principles involved were later successfully applied to the elucidation of the native conformation of protein molecules by Kauz-mann The application of these ideas to the study of membrane structures has been advanced by Singer. Recently, Tanford published an entire book on the hydrophobic effect, including the influence of this interaction on the formation of micelles, lipid bilayers, membranes and other ordered structures. Aside from Singer s and Tanford s" statements on the decisive role of the hydrophobic effect on lyotropics, the lyotropic liquid-crystal literature seems peculiarly unaware of this phenomenon. Winsor s extensive review with its systematic analysis (R-theory) of the many lyotropic phases does not take the hydrophobic effect into account. More recent reviews of lyotropic liquid crystals do not mention the phenomenon. We hope that the present discussion will help to advance the realization of the importance of the hydrophobic effect to lyotropics. The material of the following sections is taken chiefly from Ref. [3] with some assistance from Refs. [2] and [4]. 

The structure of cholic acid helps us understand how bile salts such as sodium tauro cholate promote the transport of lipids through a water rich environment The bot tom face of the molecule bears all of the polar groups and the top face is exclusively hydrocarbon like Bile salts emulsify fats by forming micelles m which the fats are on the inside and the bile salts are on the outside The hydrophobic face of the bile salt associates with the fat that is inside the micelle the hydrophilic face is m contact with water on the outside... 

FIG. 1 Self-assembled structures in amphiphilic systems micellar structures (a) and (b) exist in aqueous solution as well as in ternary oil/water/amphiphile mixtures. In the latter case, they are swollen by the oil on the hydrophobic (tail) side. Monolayers (c) separate water from oil domains in ternary systems. Lipids in water tend to form bilayers (d) rather than micelles, since their hydrophobic block (two chains) is so compact and bulky, compared to the head group, that they cannot easily pack into a sphere [4]. At small concentrations, bilayers often close up to form vesicles (e). Some surfactants also form cyhndrical (wormlike) micelles (not shown). 

In some polysaccharides, the reducing terminal is linked, through a phosphoric diester linkage, to O-1 of a 2,3-di-6 -acylglycerol. This structural feature has been demonstrated for some capsular polysaccharides from E. coli and Neisseria species, - but is probably more common than that. Non-covalent linkage between the lipid part and the cell membrane may explain why extracellular polysaccharides often occur as capsules, and the high (apparent) molecular weight observed for these polysaccharides may be due to micelle formation in aqueous solution. 

Figure 41-4. Diagrammatic cross-section of a micelle. The polar head groups are bathed in water, whereas the hydrophobic hydrocarbon tails are surrounded by other hydrocarbons and thereby protected from water. Micelles are relatively small (compared with lipid bilayers) spherical structures.

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