Fatty acid desaturase mechanism

Role of Fatty Acid Desaturases and DHAP Reductase in Systemic Acquired Resistance Systemic acquired resistance (SAR) is an inducible defense mechanism in plants to confer resistance to a broad spectrum of pathogens [59, 60]. SAR is constitutively expressed in the A. thaliana SSI2 mutant. Moreover, the SSI2 gene encodes a stearoyl carrier protein desaturase that catalyzes the conversion of acyl carrier protein-conjugated stearic acid to oleic acid in the plastids [61]. The resulted oleoyl moiety of acyl carrier protein is subsequently transferred to the... 

As far as the manipulation of plant lipids is concerned, there remains much to be done on the biochemistry of lipid synthesis in oil seed plants. There have, nevertheless, been considerable advances in our knowledge of the biochemical processes of fatty acid synthesis but more yet is to be learnt about the enzymology and points of regulation of the process as well as mechanisms of specificity of fatty acid desaturases and elongation reactions. 

We believe that the modification and specifity of the fatty acid pattern of plant glycerolipids reflects the regulation of the biosynthetic pathways, especially at the level of desaturases involved in the mechanisms of fatty acid desaturation. Insertion of double bonds into an acyl chain is achieved by a family of fatty acid desaturases. In fact, all the double bonds (except the initial desaturation of stearic acid which occurs at the level of acyl-ACP), each of which is inserted by a different enzyme, are added after the fatty acid becomes esterified to a glycerolipid . We believe that the desaturation process in plant cells is catalyzed by membrane bound enzymes which use polar lipids for substrates these same lipids are structural components of the membrane. 

Soluble enzymes are much easier to study, so more is known of the first type, but from many studies with a variety of spectroscopic. X-ray and molecular biological techniques, it seems the mechanism of reaction is the same in both types. Although the full story of the enzymes is not yet known, the description here summarizes our present knowledge of membrane-bound fatty acid desaturases found in insects. The description is of a A9-desaturase, the most common type, which converts stearic acid to oleic acid. The location of the double bond is measured from the carboxylate end of the molecule. Palmitic acid with the same enzyme gives palmitoleic acid. If an unnatural Cjy or C19 acid is supplied to the... 

In green plants a soluble A9 stearoyl-acyl carrier protein desaturase uses 02 and NADH or NADPH to introduce a double bond into fatty acids. The structure of this protein (Fig. 16-20B,C) is related to those of methane oxygenase and ribonucleotide reductase.333347 Tire desaturase mechanism is discussed in Chapter 21. 

Synthesis in mammalian tissues of arachidonic acid from linoleic acid. The A5 and A6 desaturases are separate enzymes and are also different from the A9 desaturase (fig. 18.16). The mechanisms, however, seem to be the same, involving cytochrome b5 and cytochrome reductase. The enzymes for elongation of unsaturated fatty acid such as 18 3 to 20 3 occur on the endoplasmic reticulum. 

The mechanism of fatty acid unsaturation (e.g., stearic acid — oleic acid) utilizes a system similar to that involving cytochrome P-450 (Chapter 17) it is micro-some bound, and it includes a heme-containing protein (cytochrome b5), an FAD-containing reductase, and an iron-sulfur center-containing "desaturase." The electron source is NADH or NADPH. Equation (19.14) summarizes this process ... 

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]. 

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