Micelles amphipathic lipids forming

Further addition of fatty acid eventually results in the formation of micelles. Micelles formed from an amphipathic lipid in water position the hydrophobic tails in the center of the lipid aggregation with the polar head groups facing outward. Amphipathic molecules that form micelles are characterized by a unique critical micelle concentration, or CMC. Below the CMC, individual lipid molecules predominate. Nearly all the lipid added above the CMC, however, spontaneously forms micelles. Micelles are the preferred form of aggregation in water for detergents and soaps. Some typical CMC values are listed in Figure 9.3. 

Depending on the precise conditions and the nature of the lipids, three types of lipid aggregates can form when amphipathic lipids are mixed with water (Fig. 11-4). Micelles are spherical structures that contain anywhere from a few dozen to a few thousand amphipathic molecules. These molecules are arranged with... 

Figure 9.23 Two possible means of aggregation by amphipathic lipids. The micelle is formed by lysolecithin, for example, whereas the bilayer may be formed by lecithins, sphingomyelins, or cephalins.

When amphipathic molecules are dispersed in water, their hydrophobic parts (i.e., hydrocarbon chains) aggregate and become segregated from the solvent. This is a manifestation of the hydrophobic effect which comes about because of exclusion and hence ordering of water at the interface between these distinct types of molecule. Aggregates of amphipathic molecules can be located at a water-air boimdary (monolayers) (Fig. 3-24) however, only a small quantity of an amphipathic lipid dispersed in water can form a monolayer (unless the water is spread as a very thin film). The bulk of the lipid must then be dispersed in water as micelles (Fig. 3-24). In both of these structures the polar parts, or heads (O), of the lipid make contact with the water, while the nonpolar parts, or tails (=), are as far from the water as possible. Micelles can be spherical as shown in Fig. 3-24, but can also form ellipsoidal, discoidal, and cylindrical stmctures. 

In principle, amphipathic lipids can also form hilayers (Fig. 3-25), but some do so more readily than others this ability depends on the diameter of the head group relative to the cross-sectional area of the hydrocarbon chain(s). More wedge-shaped molecules tend to favor the formation of micelles while cylindrical molecules tend to form bilayers. The latter consist of two sheets of lipid with opposed hydrocarbon chains. An isolated bilayer cannot exist in water because exposed hydrocarbon tails would exist at the edges and ends of the sheet. However, this is obviated by the sheet cinwing to form a self-sealed, hollow sphere. This type of bilayered micelle is referred to as a vesicle (Fig. 3-25). 

The major lipoproteins of insect hemolymph, the lipophorins, transport diacylglycerols. The apolipo-phorins have molecular masses of -250, 80, and sometimes 18 kDa.34-37a The three-dimensional structure of a small 166-residue lipophorin (apolipophorin-III) is that of a four-helix bundle. It has been suggested that it may partially unfold into an extended form, whose amphipathic helices may bind to a phospholipid surface of the lipid micelle of the lipophorin 35 A similar behavior may be involved in binding of mammalian apolipoproteins. Four-helix lipid-binding proteins have also been isolated from plants.38 See also Box 21-A. Specialized lipoproteins known as lipovitellins... 

Hydrophobic forces The hydrophobic effect is the name given to those forces that cause nonpolar molecules to minimize their contact with water. This is clearly seen with amphipathic molecules such as lipids and detergents which form micelles in aqueous solution (see Topic El). Proteins, too, find a conformation in which their nonpolar side chains are largely out of contact with the aqueous solvent, and thus hydrophobic forces are an important determinant of protein structure, folding and stability. In proteins, the effects of hydrophobic forces are often termed hydrophobic bonding, to indicate the specific nature of protein folding under the influence of the hydrophobic effect. 

Figure 18.1 Models for different modes of peptide-lipid interaction of membrane-active peptides. The peptide remains unstructured in solution and acquires an amphipathic structure in the presence of a membrane. The hydrophobic face of the amphipathic peptide binds to the membrane, as represented by the grayscale. At low concentration, the peptide lies on the surface. At higher peptide concentrations the membrane becomes disrupted, either by the formation of transmembrane pores or by destabilization via the "carpet mechanism." In the "barrel-stave pore" the pore consists of peptides alone, whereas in the "toroidal wormhole pore" negatively charged lipids also line the pore, counteracting the electrostatic repulsion between the positively charged peptides. The peptide may also act as a detergent and break up the membrane to form small aggregates. Peptides can also induce inverted micelle structures in the membrane.

Lipids are amphipathic inolecules composed of a polar, hydrophilic head connected to a nonpolar, hydrophobic hydrocarbon tail. When in an aqueous environment, lipids tend to associate noncovalently. There are two driving forces for this association the hydrophobic effect due to the nonpolar tails, and the van der Waals interactions between the hydrocarbon portions of the molecules. This behavior in water can cause lipids to spontaneously form surface monolayers, bilayers, micelles, or vesicles, depending on the structures of the head and tail of the lipid molecule. We shall direct our attention here to the cell membrane bilayer, the most important of these biological assemblies. 

Recently biochemists have come to understand the importance of cell membranes in animal and vegetable cells. These membranes consist of molecules called //p/form surfactant monolayers and micelles in solution (Fig. 1.17(a) and (b)), and the cell membranes are constructed from bilayers of ions (Fig. 1.17(c)). The lipid membranes also form closed cellular regions which contain fluid, known as visicles, which have important biological functions. The lipids are amphipathic water-insoluble organic substances, found in all cell membranes, which are extractable by non-polar solvents such as chloroform, ether and benzene. 

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