Fatty acid desaturation control

Signalling Pathways Controlling Fatty Acid Desaturation... 

See also Acetyl-CoA, Fats, Albumin, Fatty Acid Activation, Oxidation of Saturated Fatty Acids, Oxidation of Unsaturated Fatty Acids, Fatty Acid Biosynthesis Strategy, Palmitate Synthesis from Acetyl-CoA, Fatty Acid Desaturation, Essential Fatty Acids, Control of Fatty Acid Synthesis, Molecular Structures and Properties of Lipids (from Chapter 10)... 

In order to study the processes of plant fatty acid desaturation and glycerolipid biosynthesis and to develop a crop seed oil with reduced level of saturated fatty acids, a rat liver stearyl-CoA A-9 desaturase (D9DS) gene was introduced into Nicotiana tahacum under the control of the 35S promoter via Agrobacterium transformation (3) and soybean somatic embryos with the seed-specific phaseolin promoter by particle bombardment. In this article, we decribe the effects of the mammalian A-9 desaturase on fatty acid composition of membrane and storage lipids. 

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. 

First, the use of two specific reactions — All desaturation and controlled 2 carbon chain shortening of fatty acid precursors to account for the biosynthesis of a large number of pheromones — has been an extremely fruitful approach. Even in a case where it seemed uncertain if this approach was appropriate (22)r it turned out that it was (23.). Other reactions should now be added to increase the range of products accounted for. Examples already mentioned include the A10 desaturase and the chain elongation of branched-chain starting materials. Other functional groups that appear in sex pheromones should also be accounted for, such as epoxides. 

A-ACPs in higher plants are homodimeric soluble proteins of about 70 kDa. They catalyze the first and most important desaturation of stearoyl-acyl carrier protein, resulting in an oleoyl-acyl carrier protein, which is a major precursor in fatty acid biosynthesis in plants. A-ACP is involved in controlling the ratio of saturated to unsaturated... 

In animals, desaturation of fatty acids requires a fatty acyl-CoA desaturase (Figure 18.32). The enzyme that creates oleic acid and palmitoleic acid from stearate and palmitate, respectively, is called a A-9 enzyme, because it creates a double bond nine carbons from the carboxyl group of the fatty acids. Similar enzymes in mammalian systems include A5 and A6 desaturases, which are under complex hormonal control. 

In summary, it is now clear that unsaturated fatty acids serve as substrates for the synthesis of a variety of different prostaglandins, hydroxy fatty acids, and leukotrienes. The types and amounts of these compounds produced will in part depend on what type of dietary fat is included in the diet. In turn, the factors regulating the desaturation and chain elongation of fatty acids for subsequent incorporation into and release from phospholipids will contribute in defining what types and amounts of prostaglandins are produced to mediate and control physiological processes. 

These considerations render 3 (n-x) classification of double bond position in fatty acids introduced by Thomasson (1953) less useful since new double bonds seem to be introduced in animals only with respect to the carboxyl end rather than the methyl end of the molecule. I would therefore suggest that this nomenclature be dropped since although it identifies fatty acids in their chain elongation or shortening sequences it conceals the important controlling effects of the carboxyl (A) specification of desaturation. 

Apparently, the capacity of the desaturation enzyme system to convert saturated Into monounsaturated fatty acids depends on the amount of the terminal protein component and Its control Is mediated by protein synthesis and degradation (Oshlno and Sato, 1972). This fact could account for the liver s adaptability to different physiological conditions In which a definite microsomal desaturation activity Is required. Probably one of the best examples of this Is the response of the A9 desaturase enzyme to fasting and refeedlng. 

The liver A9 desaturase activity measured in vivo in control, refed, and diabetic rats is given in Table 1. Clearly, the fatty acid desaturase enzyme is significantly depressed in the diabetic state. These results agree with previous data (Mercuri et al., 1974). On the other hand a very significant increase in the liver s capacity to desaturate stearic acid in vivo was elicited by feeding animals a fat-free diet. 

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