Fatty acids, activation hydroxy

A convenient way to summarize the reactions of coenzyme A thioesters is by reviewing the )3-oxidation pathway for fatty adds". Fatty acid activation occurs by acylation of the coenzyme A thiol by way of an acyl adenylate. This is then dehydrogenated to an o,/3-enoyl acyl coenzyme A derivative by a flavin-dependent dehydrogenase. The ability of the adjacent carbonyl to provide resonance stabilization of the product appears to be an important aspect of this reaction. Such flavin-dependent dehydro- nations occur in other reaction sequences, but only where carbonyl resonance stabilization is possible. Water adds to the a,j8-enoyl thioester to generate a j8-hydroxy fatty acid derivative, a reaction facilitated by j8-carbonium ion stabilization in enoyl thioesters. The j3-hydroxyl is next... 

Had sjmthesis occurred via psychosine, the presence of a-hydroxy-lignoceryl-SCoA or (x-hydroxy-lignoceric acid, ATP, and the fatty acid activating system would have been required to convert the psychosine to the cerebroside. In addition more recent experiments have shown a stimulation of the incorporation of radioactivity from UDP-galactose-1- C into cerebroside by the addition of exogenous ceramide (isolated from cerebroside) in a reaction mixture containing brain microsomes and 2% n-butanol (Burton, unpublished data). 

Free radicals are by-products of prostaglandin metabolism and may even regulate the activity of the arachidonate pathway. Arachidonic acid, released from lipids as a result of activation of phospholipases by tissue injury or by hormones, may be metabolized by the prostaglandin or leu-kotriene pathways. The peroxidase-catalysed conversion of prostaglandin G2 to prostaglandin H2 (unstable prostanoids) and the mechanism of hydroperoxy fatty acid to the hydroxy fatty acid conversion both yield oxygen radicals, which can be detected by e.s.r. (Rice-Evans et al., 1991). 

In addition to p-oxidation, two other oxidation routes are known for fatty acids, referred to as a- and co-oxidation. However, they exhibit a lower activity and initially involve the formation of a- and o-hydroxy acids, with subsequent con-versions thereof. These oxidation routes are of inferior energetic value as compared with p-oxidation presumably, they are implicated in special functions of the cell. 

How the aliphatic monomers are incorporated into the suberin polymer is not known. Presumably, activated co-hydroxy acids and dicarboxylic acids are ester-ified to the hydroxyl groups as found in cutin biosynthesis. The long chain fatty alcohols might be incorporated into suberin via esterification with phenylpro-panoic acids such as ferulic acid, followed by peroxidase-catalyzed polymerization of the phenolic derivative. This suggestion is based on the finding that ferulic acid esters of very long chain fatty alcohols are frequently found in sub-erin-associated waxes. The recently cloned hydroxycinnamoyl-CoA tyramine N-(hydroxycinnamoyl) transferase [77] may produce a tyramide derivative of the phenolic compound that may then be incorporated into the polymer by a peroxidase. The glycerol triester composed of a fatty acid, caffeic acid and a>-hydroxy acid found in the suberin associated wax [40] may also be incorporated into the polymer by a peroxidase. 

The polymerase is stereospecific. It accepts only the D-(-)-stereoisomer which is generally formed by the NADPH-linked reductase. With respect to chain length of the activated fatty acids the specificity of the polymerase varies in different organisms. It links not only C4 3-acyl moieties but also C5 compounds when forming the polyester molecule [26]. It also polymerizes 3-hy-droxy-, 4-hydroxy-, and 5-hydroxyalkanoates from C3 to C5 monomers, but not C6 or higher (e.g., in R. eutropha) [27-31]. In pseudomonads, in contrast, it links C6 to C14 3-hydroxyalkanoyl-CoA [32]. 

Their hydrophobicity and their plasticity were appreciated and used for a long time in a wide range of activities. To our knowledge, the first wax to have been exploited is beeswax. Beeswax is produced by various species of bees in the world, and it has a melting point between 62°C and 64°C. It mainly contains homologous series of even-numbered fatty acids (C22 C34, C2 being the predominat compound), odd-numbered ra-alkanes (C2i C33, C27 being the major compound) and even-numbered palmitic esters from C40 to C52 (Tulloch and Hoffman, 1972 Kolattukudy, 1976). Hydroxy esters, diesters and hydroxy diesters also form part of beeswax to a lesser extent. 

Esterification of glycerol 3-phosphate with a long-chain fatty acid produces a strongly amphipathic lysophosphatidate (enzyme glycerol-3-phosphate acyltransferase 2.3.1.15). In this reaction, an acyl residue is transferred from the activated precursor acyl-CoA to the hydroxy group at C-1. 

A simple approach for lipidation of peptides with di-fatty acid substituted glycerol moieties is based on the use of glyceric acid.119" For this purpose (2i )-glyceric acid is esterified at the two hydroxy groups with fatty acid acyl chlorides and the resulting lipophilic synthon (18) is used directly as an active ester, e.g. Pfp ester, to acylate selected amino groups of peptides, or is used to acylate suitably functionalized spacers. 

Another process patented by Givaudan uses Mucor circinelloides as a biocatalyst for the production of 4-decanolide [228]. Here the natural substrate is the ethyl ester of decanoic acid which is isolated from coconut oil. The key microbial activity harnessed in this process is the stereoselective and regioselective hydroxylation of the fatty acid in the y-position, which is followed by spontaneous lactonisation of the hydroxy fatty acid under acidic conditions and results in yields of up to 10.5 g 4-decanolide after 60 h. 

LCAT catalyzes the transfer of a preferentially unesterified fatty acid from the sn-2 position of phosphatidylcholine to the 3/i-hydroxy group of cholesterol, and thereby produces lysophosphatidylcholine and a cholesteryl ester [50]. Depending on the mutation in the LCAT gene, homozygous or compound heterozygous patients present with one of two clinical phenotypes, classical LCAT deficiency or fish-eye disease [58, 85]. Classical LCAT deficiency is caused by a broad spectrum of missense and non-sense mutations that interfere with the synthesis or secretion or affect the catalytic activity of LCAT [10]. Fish-eye disease is caused by a limited number of missense point mutations that alter the surface polarity, and thereby interfere with the binding of the enzyme to apoA-I containing lipoproteins [77]). 

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