Desaturases membranes

This impressive reaction is catalyzed by stearoyl-CoA desaturase, a 53-kD enzyme containing a nonheme iron center. NADH and oxygen (Og) are required, as are two other proteins cytochrome 65 reductase (a 43-kD flavo-protein) and cytochrome 65 (16.7 kD). All three proteins are associated with the endoplasmic reticulum membrane. Cytochrome reductase transfers a pair of electrons from NADH through FAD to cytochrome (Figure 25.14). Oxidation of reduced cytochrome be, is coupled to reduction of nonheme Fe to Fe in the desaturase. The Fe accepts a pair of electrons (one at a time in a cycle) from cytochrome b and creates a cis double bond at the 9,10-posi-tion of the stearoyl-CoA substrate. Og is the terminal electron acceptor in this fatty acyl desaturation cycle. Note that two water molecules are made, which means that four electrons are transferred overall. Two of these come through the reaction sequence from NADH, and two come from the fatty acyl substrate that is being dehydrogenated. 

Cyanobacteria, prokaryotic algae that perform oxygenic photosynthesis, respond to a decrease in ambient growth temperature by desaturating the fatty acids of membrane lipids to compensate for the decrease in the molecular motion of the membrane lipids at low temperatures. During low-temperature acclimation of cyanobacterial cells, the desaturation of fatty acids occurs without de novo synthesis of fatty acids [110, 111]. All known cyanobacterial desaturases are intrinsic membrane proteins that act on acyl-Hpid substrates. 

Clearly, the control of gene expression at the transcriptional level is a key regulatory mechanism controlling carotenogenesis in vivo. However, post-transcriptional regulation of carotenoid biosynthesis enzymes has been found in chromoplasts of the daffodil. The enzymes phytoene synthase (PSY) and phytoene desaturase (PDS) are inactive in the soluble fraction of the plastid, but are active when membrane-bound (Al-Babili et al, 1996 Schledz et al, 1996). The presence of inactive proteins indicates that a post-translational regulation mechanism is present and is linked to the redox state of the membrane-bound electron acceptors. In addition, substrate specificity of the P- and e-lycopene cyclases may control the proportions of the p, P and P, e carotenoids in plants (Cunningham et al, 1996). 

AL-BABiLi s, VON LiNTiG J, HAUBRUCK H and BEYER p (1996) A novel, soluble form of phytoene desaturase from Narcissus pseudonarcissus chromoplasts is Hsp70-complexed and competent for flavinylation, membrane association and enzymatic activation . Plant J,9, 601-12. 

Solubilize desaturases with 1 % CHAPS at a detergent-membrane protein ratio of equal to one, 5°C for 30 min with gentle stirring. 

In higher plants, animals, protozoa, and fungi, saturated fatty acids are acted upon by desaturases to introduce double bonds, usually of the cis (Z) configuration. The substrates may be fatty acyl-ACP, fatty acyl-CoA molecules, membrane phospholipids,97 or glycolipids.98 The A9 desaturase, isolated from liver or from yeast, converts stearoyl-CoA to oleoyl-CoA (Eq. 21-3).99-102 This membrane-associated enzyme system... 

Malvaceae). Malvalic acid is produced from sterculic acid by chain shortening from the carboxyl end (Figure 3.14). Sterculic acid is an inhibitor of the A9-desaturase which converts stearic acid into oleic acid and is potentially harmful to humans in that it can alter membrane permeability and inhibit reproduction. 

Hagar, A.F. and Hazel, J.R. (1985). Changes in desaturase activity and the fatty acid composition of microsomal membranes from liver tissue of thermally-acclimating rainbow trout. Journal of Comparative Physiology 156B, 35-42. 

Knipple D. C., Rosenfield C.-L., You K. M. and Jeong S. E. (2002) Evolution of the integral membrane desaturase gene family in moths and flies. Genetics 162, 1737-1752. 

Liu W., Jiao H., Murray N. C., O Conner M. and Roelofs W. L. (2002) Gene characterized for membrane desaturase that produces ( )-11 isomers of mono- and diunsaturated fatty acids. Proc. Natl. Acad. Sci. USA 99, 620-624. 

Shanklin J., Whittle E. and Fox B. G. (1994) Eight histidine residues are catalytically essential in a membrane associated iron enzyme, stearoyl-CoA desaturase and are conserved in alkane hydroxylase and xylene monooxygenase. Biochem. 33, 12787-12794. 

Similar to Scd2 in mice (Kaestner et al., 1989), desatl mRNA is found at higher levels in brain. The role of the desaturase in brain is to supply unsaturated fatty acids necessary for the synthesis of membrane phospholipids, particularly for myelination. 

This reaction is catalyzed by a membrane-bound complex of three enzymes NADH-cytochrome b5 reductase, cytochrome b5 and a desaturase. The overall reaction is ... 

Terova, B., Petersen, G., Hansen, H. S., and Slotte, J. P. (2005). V-Acyl phosphatidylethanolamines affect the lateral distribution of cholesterol in membranes. Biochim. Biophys. Acta 1715, 49 56. Terrazzino, S., Berto, F., Carbonare, M. D., Fabris, M., Guiotto, A., Bernardini, D., and Leon, A. (2004). Stearoylethanolamide exerts anorexic effects in mice via down-regulation ofliver stearoyl-coenzyme A desaturase-1 mma expression. FASEBJ. 18, 1580-1582. 

Fatty acyl-CoA desaturases are terminal oxidases of a membrane-bound enzyme complex that also includes cytochrome b5 and cytochrome b5 reductase (Bloomfield and Bloch, 1960). They remove substrate hydrogen atoms at a position determined by the specificity of the enzyme. They play essential roles in regulating membrane fluidity and are also involved in insect lipid and pheromone metabolism. They share the presence of three highly conserved histidine-rich sequences (H-boxes) that coordinate the diiron-oxo structure at the active sites (Shanklin and Cahoon, 1998) and four hydrophobic a helices that appear to anchor the protein into the lipid bilayer and situate the H-boxes in their correct position in the active site. 

The association of proton movement with electron transport is not reflected in the fatty acyl desaturase system universal to endomembranes. In these enzymes the dehydrogenase, NADH cyt b5 reductase and the cytochrome b5 (a single heme cytochrome) are associated exclusively with the cytosolic side of the membrane by acyl groups and have no transmembrane segment. The cytochrome b5 oxidase associated with the desaturation of fatty acyl CoA may be transmembranous but has not been associated with proton movement. It is an iron-containing protein. The other type of endomembrane cytochromes are the P-450 group of cytochrome bs... 

Sterculic acid is a potent inhibitor of several desaturases, which are the enzymes responsible for the formation of double bonds in long-chain acids used as fuels, membrane components, and other critical biological molecules. Consequently, vegetable oils containing sterculic acid must be hydrogenated or processed at high temperatures to reduce or destroy the cyclopropene ring. 

Pyridoxal phosphate has a clear role in lipid metabolism as the coenzyme for the decarboxylation of phosphatidylserine, leading to the formation of phosphatidylethanolamine, and then phosphatidylcholine (Section 14.2.1), and membrane lipids from vitamin Bg-deficient animals are low in phosphatidylcholine (She et al., 1995). It also has a role, less well defined, in the metabolism of polyunsaturated fatty acids vitamin Bg deficiency results in reduced activity of A desaturase and impairs the synthesis of eicosapentanoic and docosahexanoic acids (Tsuge et al., 2000). 

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