Desaturation mechanism

This desaturation mechanism may explain the presence of cis-vac-cenic acid in animal tissues. This acid, which is more frequently... 

A shift in temperature from 38 to 22 °C leads to desaturation of fatty acids in Anabaena variabilis [110], resulting in control of the fluidity of the plasma membrane. Mutants have been isolated in Synechocystis PCC 6803 that were defective in desaturation of fatty acids, and the growth rate of one of these mutants was much lower than that of the wild-type at 22 °C [112]. It turned out that the mutant strain had a mutation in the gene desA, and when the wild-type allele was introduced into the chilling-sensitive cyanobacterium Anacystis nidulans, it resulted in increasing the tolerance of that strain to low temperature [113]. These experiments nicely demonstrate the existence of a mechanism of adaptation to low temperature in a chilling-tolerant cyanobacterium. 

Fig. 3 Proposed biosynthetic pathways for the production of the sex pheromone components in the indicated insects. Common mechanisms include fatty acid synthesis, desaturation, chain elongation, and decarboxylation...

Biosynthesis and Metabolism.—Pathways and Reactions. Two reviews of carotenoid biosynthesis discuss, respectively, the early steps and the later reactions." The former paper deals with the mechanism of formation of phytoene and the series of desaturation reactions by which phytoene is converted into lycopene, and also describes in detail the biosynthesis of bacterial C30 carotenoids. The second paper" presents details of the mechanism and stereochemistry of cyclization and the other reactions that involve the carotenoid C-1 —C-2 double bond and the later modifications, especially the introduction of oxygen functions. 

Linoleic (C18 2) and linolenic (C18 3) acids cannot be synthesized by mammals and must be supplied in the diet, i.e. they are essential fatty acids (linoleic is the only true essential acid). These two polyenoic acids may then be elongated and/or further desaturated by mechanisms similar to stearic - oleic, to provide a full range of polyenoic acids. A summary of these reactions is given in Figure 3.12a, b. 

Desaturation of alkyl groups. This novel reaction, which converts a saturated alkyl compound into a substituted alkene and is catalyzed by cytochromes P-450, has been described for the antiepileptic drug, valproic acid (VPA) (2-n-propyl-4-pentanoic acid) (Fig. 4.29). The mechanism proposed involves formation of a carbon-centered free radical, which may form either a hydroxy la ted product (alcohol) or dehydrogenate to the unsaturated compound. The cytochrome P-450-mediated metabolism yields 4-ene-VPA (2-n-propyl-4pentenoic acid), which is oxidized by the mitochondrial p-oxidation enzymes to 2,4-diene-VPA (2-n-propyl-2, 4-pentadienoic acid). This metabolite or its Co A ester irreversibly inhibits enzymes of the p-oxidation system, destroys cytochrome P-450, and may be involved in the hepatotoxicity of the drug. Further metabolism may occur to give 3-keto-4-ene-VPA (2-n-propyl-3-oxo-4-pentenoic acid), which inhibits the enzyme 3-ketoacyl-CoA thiolase, the terminal enzyme of the fatty acid oxidation system. 

In contrast to the anaerobic pathway found in E. coli, the aerobic pathway in eukaryotic cells introduces double bonds after the saturated fatty acid has been synthesized. Stearoyl-CoA (18 0) is the major substrate for desaturation. Stearic acid is made by the fatty acid synthase as a minor product, the major product being palmitic acid, and is activated to its CoA derivative by acyl-CoA synthase. In eukaryotic cells an enzyme complex associated with the endoplasmic reticulum desaturates stearoyl-CoA to oleoyl-CoA (18 1A9). This remarkable reaction requires NADH and 02 and results in the formation of a double bond in the middle of an acyl chain with no activating groups nearby. The chemical mechanism for desaturation of long-chain acyl-CoAs remains unclear. 

The cholinesterase inhibitor physostigmine has been investigated in a small blinded, placebo-controlled study of moderate to severe OSA patients, and via steady-state intravenous infusion been shown to modestly decrease the overall AHI and severity of oxygen desaturation, predominantly in REM compared with NREM sleep [81]. The exact mechanism of the beneficial action of physostigmine in sleep apnea is not clear. 

The EFA metabolism is presented in several extensive reviews.9 16 17 Much of the information concerning EFA physiology and biochemistry has been derived from work in hepatocytes and may be of limited relevance to epidermis since a major role of the liver is to convert dietary lipids into energy stores. Meanwhile, keratinocytes are involved in the fatty acid metabolism required both for normal cellular processes and the specialized role in the permeability barrier. Unlike the liver, the epidermis does not possess the capacity to desaturate at the A5 or A6 position, and therefore the skin relies on a supply of AA, LA, and ALA from the bloodstream. There is evidence for a distinct fatty acid binding protein in keratinocyte plasma membranes that is involved in EFA uptake into the skin and also recycling of free fatty acids from the stratum corneum.18 The transport mechanism in epidermis differs from that in hepatocytes since there is preferential uptake of LA over OA, which may function to ensure adequate capture of LA for barrier lipid synthesis.18... 

Finally, although the All desaturase from cabbage looper is probably the most studied enzyme involved in these pathways, much more work should be done with it and related enzymes. For example, it only produces Z products and does not function efficiently with 14 carbon substrates, indicating a fundamental difference with the enzyme from redbanded leafroller moths, which desaturates 14-carbon acids to give a mixture of Z and E product (22). Projects such as this may well prove technically difficult, but will provide great insight into the enzymatic mechanisms. 

Using the DAG methodology, Noiret et al.142 synthesized the two enantiomeric sulfoxides 99(R) and 99(5) (Scheme 33) in high yield and selectivity, in order to study the mechanism of the in vivo desaturation of oleic acid.143... 

Broadwater, J. A., Ai, J., Loehr, T. M., Sanders-Loehr, J., and Fox, B. G., 1998, Peroxodiferric intermediate of stearoyl-acyl carrier protein A9 desaturase oxidase reactivity during single turnover and implications for the mechanism of desaturation. Biochemistry 37 14664nl4471. 

The mid-chain dehydrogenation of saturated fatty acyl derivatives is carried out by a large family of 02-dependent, nonheme diiron-containing enzymes known as desaturases. Both soluble and membrane-bound desaturases have been characterized. The mechanism of desaturation is thought to involve the stepwise syn removal of vicinal hydrogen atoms via a short-lived carbon-centered radical intermediate. The most common desaturase inserts a (Z)-double bond between the C-9,10 carbons of a stearoyl thioester however, many variations of this prototypical reaction have been discovered. Accounting for this diversity in terms of subtle alterations in active-site architecture constitutes a new frontier for research in this area. 

Figure 2 Consensus mechanism for fatty acid desaturation. The related minor hydroxylation pathway also is shown.

The enzymes in this section can incorporate either one or two atoms from dioxygen into the organic product(s) of their reaction. It is of note that many oxygenases also can catalyze oxidase like reactions, such as desaturation and ring closure. Figure 2 shows illustrative examples of the reactions catalyzed by some of these enzymes, whereas Fig. 3 shows partial reaction mechanisms, including key intermediates. 

Polyunsaturated fatty acid synthesis is catalyzed by acyl-lipid-desaturases, also named front-end desaturases due to their action mechanism, which proceeds via introduction of double bonds into preformed acyl chains by oxygen and electron-donor dependent desaturation, between the carboxyl group and the pre-existing unsaturation which acts as substrate. For many microsomal desaturases, the electron donors are cytochrome b5, and a small hemoprotein that operates in numerous redox reactions in plants, involving NADH-dependent acyl-group desaturation [200]. 

There are two major mechanisms by which bacteria synthesize UFAs mostly of them, including Escherichia coli, synthesize UFAs anaerobically (Mansilla et al., 2004) whereas some prokaryotes such as cyanobacteria, bacilli, mycobacteria and pseudomonads use an oxygen-dependent fatty acid desaturation pathway (Mansilla and de Mendoza, 2005 Phetsuksiri et al., 2003 Zhu et al., 2006). 

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