Oleoyl desaturase

FIGURE 25.14 The conversion of stearoyl-CoA to oleoyl-CoA in eukaryotes is catalyzed by stearoyl-CoA desaturase in a reaction sequence that also involves cytochrome -65 and cytochrome -65 reductase. Two electrons are passed from NADH through the chain of reactions as shown, and two electrons are also derived from the fatty acyl substrate. 

This enzyme [EC 1.14.99.5], also known as stearoyl-CoA desaturase, fatty acid desaturase, and A -desaturase, will catalyze the reaction of stearoyl-CoA with a hydrogen donor and dioxygen to produce oleoyl-CoA, water, and... 

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... 

In plants a similar enzyme catalyzes formation of the first double bond in a fatty acyl group converting stearoyl-ACP into oleoyl-ACP in the chloroplasts.72 753/105 108 The soluble A9 stearoyl-ACP desaturase has a diiron-oxo active site (Fig. 16-20, B, C).i°9 no Electrons are donated from light-generated reduced ferredoxin (see Chapter 23). In addition to the A9 desaturase both plants and cyanobacteria usually desaturate C18 acids also at the A12 and A15 positions and C16 acids at the A7, A20, and A13 (co3) positions.iii ii2 Desaturation of oleate occurs primari-... 

Triricinolein (RRR) constitutes about 70% of TAG in castor oil (Lin et al., 2003). Recently, a new tetraacylglycerol, (12-ricinoleoylricinoleoyl)diricinoleo ylglycerol (RRRR), in castor oil was identified and its content was about 0.5% (Lin et al., 2006). These two AG contain ricinoleate only without other FA. The biosynthetic pathway of RRR and RRRR in the castor bean has been established and the key enzymatic steps driving ricinoleate into RRR and RRRR have been identified (Lin et al., 1998, 2000, 2002, 2006) as shown in Figure 25.1. Oleate of oleoyl-CoA is incorporated into 2-oleoyl-phosphatidylcholine (2-oleoyl-PC) by lysophosphatidylcholine-acyltransfer-ase and then hydroxylated to 2-ricinoleoyl-PC by 12-oleoyl-hydroxylase in castor microsomes (Fig. 25.1) (Lin et al., 1998). 2-Oleoyl-PC is also desaturated to 2-linoleoyl-PC and then to 2-linolenoyl-PC (Lin et al, 1998) by desaturases. 

Schwartzbeck JL, Jung S, Abbott AG, Mosley E, Lewis S, Pries GL, Powell GL. Endoplasmic oleoyl-PC desaturase references the second double bond. Phytochemistry 2001 57 643-652. 

Miyazaki, M., Kim, H.J., Man, W.C., Ntambi, J.M. 2001. Oleoyl-CoA is the major de novo product of stearoyl-CoA desaturase 1 gene isoform and substrate for the biosynthesis of the Harderian gland 1-alkyl-2,3-diacylglycerol. J. Biol. Chem. 276 39455-39461. 

ACP, with subsequent dehydration to enoyl-ACP, which is further reduced to butyryl-ACP (Voelker Kinney, 2001). Later steps of 2-carbon unit additions are catalyzed by KAS I (Shimakata Stumpf, 1983). Once palmitoyl-ACP is formed, KAS II may add an extra 2-carbon unit to form stearoyl-ACP (Shimakata Stumpf, 1982). The formation of oleoyl-ACP is catalyzed by stearoyl-ACP desaturase, an enzyme that promotes the formation of double bonds in the position (Shanklin Gaboon, 1998). This enzyme also desaturates palmitoyl-ACP in position A . However, the apparent specificity factor for stearoyl-ACP is much greater than for palmitoyl-ACP (Gibson, 1993). The presence of two A stearoyl-ACP desaturases coded by two different genes has been reported in soybean (Byfield et al., 2006). 

In plants a similar enzyme catalyzes formation of the first double bond in a fatty acyl group converting stearoyl-ACP into oleoyl-ACP in the chloroplasts. ° ° The soluble A stearoyl-ACP desaturase has a diiron-oxo active site (Fig. 16-20, B,... 

Desaturation. Although the sequence produces stearate, there is very little stearic acid found in plant lipids. An active desatur-ase, the stearoyl ACP desaturase, also found in the chloro-plast or plastids, forms a cis double bond between Cq and Cj o of the carbon chain. This system requires both NADPH and ferredoxin. Stearoyl ACP is the physiological substrate. The major product is oleoy1-ACP, which may then be hydrolyzed or converted to oleoyl-CoA. Animals also produce oleic acid as a major product. 

It has been reported that in castor microsomal incubation, oleoyl-Co A was rapidly incorporated into 2-oleoyl-PC by acyl-CoA. lysoPC acyltransferase (EC 2.3.1.23) (8). Our previous study (2) indicated that the 2-oleoyl-PC was hydroxylated to 2-ricinoleoyl-PC by oleoyl-12-hydroxylase and desaturated to 2-linoleoyl-PC by oleoyl-12-desaturase (EC 1.3.1.35). 2-Rici-noleoyl-PC was then hydrolyzed by phospholipase A2 (EC... 

Stearoly-ACP ( . coli) is rapidly desaturated to oleoyl-ACP by plant stearoyl-ACP desaturases. However, antibodies to E. coli ACP cross-react very poorly with spinach ACP (Ohlrogge et al., 1979). Conversely antibodies to spinach ACP cross-react poorly with E. coli ACP. 

As noted in Table II, all proplastids or chloroplasts synthesize oleic acid as their main unsaturated fatty acid from [ Cjacetate. Since Ohlrogge et al. (1979) have shown that all the ACP of a leaf cell is localized in the chloro-plast, it follows that the only site of stearyl-ACP desaturase must be the chloroplast. In addition, the highly specific acyl-ACP hydrolase must also be localized in the same organelle. The most effective substrate for acyl-ACP hydrolase is oleoyl-ACP, with stearoyl-ACP, palmitoyl-ACP, myristoyl-ACP, and lauroyl-ACP decreasing in activity in that order (Ohlrogge et al., 1978b). The consistent presence of the desaturase and the hydrolase in chloroplasts will be discussed in Section III. 

The stearoyl-ACP desaturase-oleoyl-ACP hydrolase system has been examined recently in terms of the possibility that these two enzymes are directly involved in chain termination at the Cig level in plants (Ohlrogge et al., 1978b). The requirement for chain termination, namely, a high specificity for the chain length acyl moiety which is to be terminated, is met twice by... 

Plant desaturase systems appear to be soluble, may contain ferredoxin in place of cytochrome 65, and act upon fatty acyl-ACP or upon acyl groups already incorporated into membrane lipids (e.g. [l-oleoyl]-dia-cylgalactosylglycerol desaturated to [l-linoleoyl]-dia-cylgalactosyl ycerol and then to [l-linolenoyl]-diacyl-galactosyl ycerol by spinach chloroplast desaturase). 

GC growth chamber HO high oleic HT high ODS oleoyl- phosphatidyl-choline desaturase TAG ... 

Effects of Temperature ON Oleoyl-Phoshatidyl-Choline DESATURASE (ODS) AND ITS ACTIVITY... 

Microsomes catalysed the transfer of oleate from [ " C]oleoyl-CoA to the sn-2 position of PC. Desaturation of oleate to linoleate was observed on addition of reductant with NADH preferred to NADPH. There was little production of GLA. To show conclusively that the A12 desaturase was acting on oleate esterified to PC, microsomes were incubated with [ C]oleoyl-CoA, repelleted and washed to remove any remaining acyl-CoA. The labelled microsomes were incubated with NADH and desaturation of in situ labelled oleoyl-PC to produce linoleoyl-PC was observed (Figure 2). 

Smith, M.A., Cross, A.R., Jones, O.T.G., Griffiths, W.T., Stymne, S. and Stobart, K. Electron-transport components of the l-acyl-2-oleoyl-5 -glycero-3-phosphocholine A12-desaturase in microsomal preparations from developing safflower (Carthamus tinctorius L.) cotyledons. Biochem. J. Ill (1990), 23-29. 

Desaturase activity calculated from the amount endogenous oleoylgroups + [ C] oleoyl groups desaturated in microsomal membranes. 

Serghini-Caid, H., Demandre, C., Justin, A-M. and Mazliak, P. (1988) Oleoyl-phosphatidylcholine molecular species desaturated in pea leaf microsomes - possible substrates of oleate-desaturase in other green leaves. Plant Sci. 54, 93-101. 

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