Unsaturated desaturation

Organisms differ with respect to formation, processing, and utilization of polyunsaturated fatty acids. E. coli, for example, does not have any polyunsaturated fatty acids. Eukaryotes do synthesize a variety of polyunsaturated fatty acids, certain organisms more than others. For example, plants manufacture double bonds between the A and the methyl end of the chain, but mammals cannot. Plants readily desaturate oleic acid at the 12-position (to give linoleic acid) or at both the 12- and 15-positions (producing linolenic acid). Mammals require polyunsaturated fatty acids, but must acquire them in their diet. As such, they are referred to as essential fatty acids. On the other hand, mammals can introduce double bonds between the double bond at the 8- or 9-posi-tion and the carboxyl group. Enzyme complexes in the endoplasmic reticulum desaturate the 5-position, provided a double bond exists at the 8-position, and form a double bond at the 6-position if one already exists at the 9-position. Thus, oleate can be unsaturated at the 6,7-position to give an 18 2 d5-A ,A fatty acid. 

Mammals can add additional double bonds to unsaturated fatty acids in their diets. Their ability to make arachidonic acid from linoleic acid is one example (Figure 25.15). This fatty acid is the precursor for prostaglandins and other biologically active derivatives such as leukotrienes. Synthesis involves formation of a linoleoyl ester of CoA from dietary linoleic acid, followed by introduction of a double bond at the 6-position. The triply unsaturated product is then elongated (by malonyl-CoA with a decarboxylation step) to yield a 20-carbon fatty acid with double bonds at the 8-, 11-, and 14-positions. A second desaturation reaction at the 5-position followed by an acyl-CoA synthetase reaction (Chapter 24) liberates the product, a 20-carbon fatty acid with double bonds at the 5-, 8-, IT, and ITpositions. 

Isomerisation of 15-c/5-phytoene to the all-/ra x configuration must occur during the desaturation steps, since most desaturated carotenes are in the all-trans form. The CRTI type desaturases appear to be able to carry out this isomerisation themselves (Fraser et al, 1992 Bartley etal, 1999), but mutants of PDS/ZDS-type organisms accumulate cis isomers of unsaturated carotenes, suggesting the presence of a separate isomerase (Clough and Pattenden, 1983 Ernst and Sandmann, 1988). Three recent publications have reported the cloning of a carotene isomerase (CrtlSO) from tomato (Isaacson et al, 2002), Arabidopsis (Park et al, 2002) and Synechocystis 6803 (Breitenbach... 

If a fatty acid already has a double bond in it, the scheme by which the fatty acid is oxidized depends on where the double bond ends up after several of the C-2 fragments have been removed by normal p oxidation. With a double bond already present, the enzyme that catalyzes the first step (insertion of the double bond at C-2) gets confused when there is already a double bond at C-2 or at C-3. The fact that the double bonds in unsaturated fatty acids are invariably cis also complicates life since the double bond introduced at C-2 by the desaturating enzyme of p oxidation is a trans double bond. 

FIGURE 3-7 Pathways for the interconversion of brain fatty acids. Palmitic acid (16 0) is the main end product of brain fatty acid synthesis. It may then be elongated, desaturated, and/or P-oxidized to form different long chain fatty acids. The monoenes (18 1 A7, 18 1 A9, 24 1 A15) are the main unsaturated fatty acids formed de novo by A9 desaturation and chain elongation. As shown, the very long chain fatty acids are a-oxidized to form a-hydroxy and odd numbered fatty acids. The polyunsaturated fatty acids are formed mainly from exogenous dietary fatty acids, such as linoleic (18 2, n-6) and a-linoleic (18 2, n-3) acids by chain elongation and desaturation at A5 and A6, as shown. A A4 desaturase has also been proposed, but its existence has been questioned. Instead, it has been shown that unsaturation at the A4 position is effected by retroconversion i.e. A6 unsaturation in the endoplasmic reticulum, followed by one cycle of P-oxidation (-C2) in peroxisomes [11], This is illustrated in the biosynthesis of DHA (22 6, n-3) above. In severe essential fatty acid deficiency, the abnormal polyenes, such as 20 3, n-9 are also synthesized de novo to substitute for the normal polyunsaturated acids. 

The FAS multi-enzyme complex synthesizes saturated C16 fatty acids, but cells and tissues need unsaturated and longer chain fatty acids. The palmitoyl-CoA can be modified by either chain elongation and/or oxidation in order to produce different fatty acid molecules. Both elongation and desaturation occur within the smooth endoplasmic reticulum (SER, microsomal fraction) of the cell. 

Desaturation of a saturated bond to produce an unsaturated fatty acid (the conversion of -CH2-CH2- to -CH=CH-) is catalysed by enzymes known as acyl-CoA desaturases. 

If the substrate is fully saturated, the first double bond is always inserted at position 9 by the A desaturase (so that, for example, stearic acid (18 0) is converted to oleic acid (18 co-9). Thus the A desaturase requires the presence of a cis double bond at position 9 before it can catalyse desaturation at position 6. Animals do not possess a desaturase that can insert a double bond at a position greater than nine. (Such desaturations are present in plants). It is this fact that determines that such unsaturated fatty acids must be provided in the diet, i.e. they are essential. 

The enzymes catalyse desaturation in sequence for example, the A desaturase converts stearate to oleic acid, which is converted, by the A desaturase, to the di-unsaturated fatty acid (18 2n-9), which is elongated to form eicosadienoic acid, which can be converted by the A -desaturase to the tri-unsaturated fatty acid (20 3n-9) which is known as Mead acid (see below). 

Desaturation or the creation of double bonds for synthesis of unsaturated fats is performed by mixed-function oxidases in the endoplasmic reticulum. 

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. 

Potassium tetrafluorocobaltate(III) at 200°C reacts with tetrahydrofuran to give82 unsaturated products the major ones are 5 and 6, although the overall yield is poor (< 30%). Furan itself gives no products at all over cobalt(IIl) fluoride it presumably polymerizes. This does not, however, rule furan out as an intermediate in the tetrahydrofuran fluorinations (it could form by desaturation, as does benzene in the fluorination of cyclohexane, vide supra). 2-Methyl-and 2,5-dimethyltetrahydrofuran83 have also been fluorinated with similar results to tetrahydrofuran. 

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. 

Many unsaturated compounds found in nature contain one or more acetylenic bonds, and these are predominantly produced by further desaturation of olefinic systems in fatty acid-derived molecules. They are surprisingly widespread in nature, and are found in many organisms, but are especially common in plants of the Compositae/Asteraceae, the Umbelliferae/Apiaceae, and fungi of the group... 

Saturated fatty acids or unsaturated fatty acids, such as oleic acid (18 1, n-9), can be synthesized by normal mammalian cells that posses elongation and desaturation enzymes (Rosenthal, 1987). However, the polyunsaturated fatty acids of the n-3 and n-6 group, such as linoleic acid (18 2, n-6) or linolenic acid (18 3, n-3), are essential nutrients for animals because they are precursors for the synthesis of eicosanoid hormones such as prostaglandins (Needleman et al., 1986). 

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