Hydrophobic acyl fatty acid chains

N-Myristoylation is achieved by the covalent attachment of the 14-carbon saturated myristic acid (C14 0) to the N-terminal glycine residue of various proteins with formation of an irreversible amide bond (Table l). 10 This process is cotranslational and is catalyzed by a monomeric enzyme called jV-myri s toy 11ransferase. 24 Several proteins of diverse families, including tyrosine kinases of the Src family, the alanine-rich C kinase substrate (MARKS), the HIV Nef phosphoprotein, and the a-subunit of heterotrimeric G protein, carry a myr-istoylated N-terminal glycine residue which in some cases is in close proximity to a site that can be S-acylated with a fatty acid. Functional studies of these proteins have shown an important structural role for the myristoyl chain not only in terms of enhanced membrane affinity of the proteins, but also of stabilization of their three-dimensional structure in the cytosolic form. Once exposed, the myristoyl chain promotes membrane association of the protein. 5 The myristoyl moiety however, is not sufficiently hydrophobic to anchor the protein to the membrane permanently, 25,26 and in vivo this interaction is further modulated by a variety of switches that operate through covalent or noncovalent modifications of the protein. 4,5,27 In MARKS, for example, multiple phosphorylation of a positively charged domain moves the protein back to the cytosolic compartment due to the mutated electrostatic properties of the protein, a so-called myristoyl-electrostatic switch. 28 ... 

It is well-established that in the case of short lipid chains such as famesyl and myristoyl, the binding affinity of monolipidated proteins to membranes, is not sufficient to promote stable association. 25 26 36 8°1 Among the various intracellular proteins that are covalently modified by an acyl group from a fatty acid or a prenyl group, several carry more than one such hydrophobic substituent per protein molecule. The addition of a second hydrophobic chain to a... 

Lipid moieties can impart good stability to polymeric micelles since the presence of two fatty acid acyls increases the hydrophobic interactions between polymeric chains in the micelle core. Indeed, no dissociation into individual polymeric chain was observed upon the chromatography of serial dilutions of diacyllipid-PEO conjugates (Trubetskoy and Torchilin, 1995). 

For this purpose liposomes are used as lipid phase. Unilamellar liposomes are artificial lipid bilayer vesicles. They can be considered as real model bilayer membranes as they ideally consist of a circular bilayer membrane. The hydrophobic acyl chains are assembled in the hydrophobic core of the liposome whereas the hydrophilic head groups point to the water in the inside and outside of the vesicle. Liposomes can be produced from a variety of lipids and from mixtures of lipids. This possibility allows studying the influence of membrane constituents on the partition of solutes. Kramer et al. (1997) studied the influence of the presence of free fatty acids in membranes on the partition behaviour of propranolol. The influence on a-Tocopherol in membranes on the partition behaviour of desipramine has been reported recently (Marenchino et al. 2004) using a liposome model. 

Acylate ions are amphiphilic, and the hydrocarbon chains are able to penetrate fatty (hydrophobic) particles, leaving the surface of the particle ionic. (See Fig. 6-26.) Thus, the particle behaves as a micelle and is readily soluble in water. The sodium and potassium salts of fatty acids are soaps. Soaps have poor detergent properties in hard water because the calcium present in such water causes the micelles to aggregate and precipitate. The divalent calcium ion can act as a bridge between two micelles, but since a micelle is polyvalent, a small amount of calcium relative to the amount of the soap can cause all the micelles to aggregate. 

Membranes exhibit a common stmcture, with lipid molecules arranged in the form of one or more bilayers, or lamellae. Since lipids are generally nonfluo-rescent, lipid-bound fluorophores are an excellent tool to study this environment. These membrane probes are poorly soluble in water, and hence they partition readily into the hydrophobic regions of the membranes. The derivatives of anthroyloxy fatty acids (AF), with the fluorophore 9-anthroic acid esterfied to the 2, 6,9, 12, or 16 position along a fatty acid acyl chain (stearic acid or palmitic acid), are frequently used. The stmcture of an AF probe is shown schematically in Fig. 1... 

The high water solubility of these adamantyl GSL derivatives was surprising considering that the adamantane frame is at least as hydrophobic as the fatty acid it replaced. It is possible, however, that the adamantane frame together with the sphingosine acyl chain may pack sufficiently to be below the hydrophobic size that is necessary to disturb the molecular organization of water (211). This situation may provide the basis of their aqueous solubility characteristics. 

Figure 6-15. Synthesis of sphingolipids. NANA = W-acetylneuraminic acid G/c = glucose Gal = galactose GalNAc = N-acetylgalactosamine PIP = pyridoxal phosphate FA = fatty acyl groups derived from fatty acids = hydrophobic chains of ceramide. The dashed box contains the portion of ceramide derived from serine.

It would be expected that hydrophobic bonding of a substrate to a polymer carrying catalytic groups could also serve to enhance the catalytic efficiency and an attempt to demonstrate such an effect has recently been described (52). It was found that when p-nitrophenyl laurate was added to polyethyleneimine partially acylated with a long-chain fatty acid, p-nitrophenol was extremely rapidly released. However, it seems likely that this reaction involves aminolysis of the ester rather than catalysis of a hydrolytic process and it remains to be seen whether true catalysis can be achieved in this manner. 

Acyl-CoA molecules are desaturated in ER membrane in the presence of NADH and 02. All components of the desaturase system are integral membrane proteins that are apparently randomly distributed on the cytoplasmic surface of the ER. The association of cytochrome b5 reductase (a flavoprotein), cytochrome b5, and oxygen-dependent desaturases constitutes an electron transport system. This system efficiently introduces double bonds into long-chain fatty acids (Figure 12.15). Both the flavoprotein and cytochrome b5 (found in a ratio of approximately 1 30) have hydrophobic peptides that anchor the proteins into the microsomal membrane. Animals typically have A9, A6, and A5 desaturases that use electrons supplied by NADH via the electron transport system to activate the oxygen needed to create the double bond. Plants contain additional desaturases for the A12 and A15 positions. 

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