Fatty acid desaturase structure

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. 

Marquardt, A., Stohr, H., White, K., and Weber, B. H. cDNA cloning, genomic structure, and chromosomal locaUzation of three members of the human fatty acid desaturase family. Genomics 66 (2000) 175-183. 

Meesapyodsuk D, Reed DW, Covello PS, Qiu X. (2007) Primary structure, regioselectivity, and evolution of the membrane-bound fatty acid desaturases of Claviceps purpurea. J Biol Chem 282 20191-20199. 

The ferroxidase center, important for rapid oxidation of Fe to Fe, was discovered relatively recently in the history of research into the metal sites in ferritins. Ferroxidase activity within H subunits appears to occur at a dinuclear site situated within a four-helix bundle and resembling the dinuclear centers found in ribonucleotide reductase, methane monooxygenase, fatty acid desaturases, and ruberythrin (Chapter 8.11). In bacterioferritins, for which protein crystal structures have been reported for ferritin from Escherichia col and Rhodobacter capsulatus the overall motif of a shell of 24 subunits with relative masses of about 18,500 Da is preserved but there are also 12 protoporphyrin IX heme groups present with unknown function which might have a role in connecting the dimer units and are buried within the shell between identical subunits related by twofold symmetry. In these bacterioferritins the subunits are all identical and contain both ftrroxidase and nucleation sites. 

C.hj from the microsomal fraction of bird and mammal livers is thought to deliver electrons to the fatty acid desaturase system of the endoplasmic reticulum. The heme group is noncovalently bound to the protein via histidine and does not react with O . It also protects the C.b molecule from denaturation and proteolytic attack. C.b solubilized by detergent treatment is an oligomer (A/, 120,000) of several monomers (M, 16,000, 126 amino acids), while C.65 solubilized by protease or lipase treatment has only 82 to 98 amino acids, depending on the species. The primary sequences of these fragments are known. They have a-helix and 25% -structure. 

Slocombe, P. and Murphy, D.J. (1990) Biochemical and molecular biological studies of the electron transport components of plant fatty acid desaturases, in Plant Lipid Biochemistry, Structure and Utilization, eds. P.J. Quinn and J.L. Harwood, Portland, London, pp. 134-137. 

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. 

Both plants and animals use mixed function oxidases (simultaneously oxidize two substrates) Acyl-CoA desaturases localized on the ER. Similar mixed function oxidases are also used to modify structural components of cells, hormones etc. so we will use the acyl-CoA desaturase as an example for this group of enzymes. In the acyl-CoA desaturase reaction molecular oxygen is used to oxidize both a fatty acid and NADH, each providing two of the the four electrons needed by the oxygen ... 

The main difference between plants and animals is that in animals a single multifunctional protein known as fatty acid synthase carries out all of the synthetic steps [5]. In plants, the steps are broken down and carried out by a number of different enzymes [5]. The introduction of unsaturation into a triglyceride structure is generally carried out by a separate protein, such as the conversion of stearic acid to oleic acid by stearoyl-CoA desaturase (see Figure 6.6) [6]. 

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