Linoleic acid biosynthesis

Guiet et al. (2003) demonstrated that deuterium (2H) distribution in fatty acids was non-statistical and could be related to isotopic discrimination during chain extension and desaturation. Petroselinic acid (C18 1A6) (Fig. 21.4), a fatty acid characteristic of the seeds of the Apiaceae, has been shown to be biosynthesized from palmitoyl-ACP (C16 0) by two steps, catalysed by a dedicated A4-desaturase and an elongase. The isotopic profile resulting from this pathway is similar to the classical plant fatty acid pathway, but the isotopic fingerprint from both the desaturase and elongase steps shows important differences relative to oleic and linoleic acid biosynthesis. 

Conjugated Linoleic Acid Biosynthesis and Nutritional Significance... 

Coakley M, Ross RP, Nordgren M, Fitzgerald G, Devery R, Stanton C. Conjugated linoleic acid biosynthesis by human-derived Bifidobacterium species. / Appl Microbiol. 2003 94 138-145. 

The common fatty acids have a linear chain containing an even number of carbon atoms, which reflects that the fatty acid chain is built up two carbon atoms at a time during biosynthesis. The structures and common names for several common fatty acids are provided in table 18.1. Fatty acids such as palmitic and stearic acids contain only carbon-carbon single bonds and are termed saturated. Other fatty acids such as oleic acid contain a single carbon-carbon double bond and are termed monounsaturated. Note that the geometry around this bond is cis, not trans. Oleic acid is found in high concentration in olive oil, which is low in saturated fatty acids. In fact, about 83% of all fatty acids in olive oil is oleic acid. Another 7% is linoleic acid. The remainder, only 10%, is saturated fatty acids. Butter, in contrast, contains about 25% oleic acid and more than 35% saturated fatty acids. 

In human being, arachidonic acid is the most important precursor for the biosynthesis of eicosanoids. Arachidonic acid is formed from linoleic acid in most mammalians by desaturation and carbon elongation to dihomog-linolenic acid and subsequent desaturation. 

Li, Y. and Watkins, B.A. 1998. Conjugated linoleic acids alter bone fatty acid composition and reduce ex vivo prostaglandin E2 biosynthesis in rats fed n-6orn-3 fatty acids. Lipids 33 417-425. 

Enzyme complexes occur in the endoplasmic reticulum of animal cells that desaturate at A5 if there is a double bond at the A8 position, or at A6 if there is a double bond at the A9 position. These enzymes are different from each other and from the A9-desaturase discussed in the previous section, but the A5 and A6 desaturases do appear to utilize the same cytochrome b5 reductase and cytochrome b5 mentioned previously. Also present in the endoplasmic reticulum are enzymes that elongate saturated and unsaturated fatty acids by two carbons. As in the biosynthesis of palmitic acid, the fatty acid elongation system uses malonyl-CoA as a donor of the two-carbon unit. A combination of the desaturation and elongation enzymes allows for the biosynthesis of arachidonic acid and docosahexaenoic acid in the mammalian liver. As an example, the pathway by which linoleic acid is converted to arachidonic acid is shown in figure 18.17. Interestingly, cats are unable to synthesize arachidonic acid from linoleic acid. This may be why cats are carnivores and depend on other animals to make arachidonic acid for them. Also note that the elongation system in the endoplasmic reticulum is important for the conversion of palmitoyl-CoA to stearoyl-CoA. 

A considerable amount of knowledge has accumulated about how pheromone components are produced in female moths since the first pathway was identified some 20 years ago. It appears that most female moths produce their pheromone through modifications of fatty acid biosynthesis pathways. For moths that utilize aldehydes, alcohols, or esters biosynthesis occurs in the pheromone gland. The exceptions are those that utilize linoleic or linolenic acids, which must be obtained from the diet. However, modifications of these fatty acids occur in the gland. For moths that utilize hydrocarbons or epoxides of hydrocarbons, the hydrocarbon is produced in oenocyte cells and then transported to the pheromone gland where the epoxidation step takes place. 

J. (1987) Biosynthesis of linoleic acid in insects. Trends Biochem. Science 12, 364— 366. 

The mammalian organism is unable to introduce double bonds at fatty acids, and this is probably why these families must be present in the diet. These fatty acids can be desaturated and elongated (see Chapter 19) to form derived essential fatty acids, dihomo-T-linoleic acid (20 3o>6), arachidonic acid (20 4ft)6), and eicosapentaenoic acid (20 5ft>3), the three direct precursor acids of PGs. Dihomo-r-linoleic acid, an intermediate in the biosynthesis of arachidonic acid from linoleic acid, is the precursor of PGs of the 1 series. Arachidonic acid and eicosapentaenoic acid are precursors of PGs of the 2 series and 3 series, respectively. 

In human adipose tissue, palmitoyl-CoA is usually used in the first glycerol-3-phosphate acylation reaction. The next two acyl residues are normally unsaturated fatty acids oleic acid and, less commonly, linoleic acid. Triglyceride biosynthesis is stimulated by insulin, most likely via its activation of lipoprotein lipase and its activity in moving glucose into the cells. 

Griinari, J.M., Bauman, D.E. 1999. Biosynthesis of conjugated linoleic acid and its incorporation into meat and milk in ruminants. In Advances in Conjugated Linoleic Acid Research, Vol. 1 (M.P. Yurawecz, M.M. Mossoba, J.K.G. Kramer, M.W. Pariza, G. Nelson, eds.), AOCS Press, Champaign, IL, pp. 180-200. 

Brodowsky, I. D., Hamberg, M., and Oliw, E. H. 1992. A Linoleic-Acid (8r)-Dioxy-genase and Hydroperoxide Isomerase of the Fungus Gaeumannomyces-Graminis. Biosynthesis of (8r)-Hydroxylinoleic Acid and (7s,8s)-Dihydroxylinoleic Acid from (8r)-Hydroperoxylinoleic Acid. J. Biol. Chem., 267, 4738-14745. 

Hosokawa, M., Hou, C. T., Weisleder, D., and Brown, W. 2003c. Biosynthesis of tet-rahydrofuranyl fatty acids from linoleic acid by Clavibacter sp ALA2. J. Am. Oil Chem. Soc., 80,145-149. 

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