Desaturation phosphatidylcholine involved

The most abundant membranes in nature are the thylakoids inside chloroplasts of green plants. A surprising amount of lipid traffic is involved in the assembly of these membranes. Almost all the acyl chains that form the core of the photosynthetic membranes are first produced by fatty acid synthase in the chloroplast. In most plants these acyl chains are then exported to the ER where they become esterified to glycerol, desaturated while they are part of phosphatidylcholine and then are returned to the plastid. The exact mechanisms for the export and return of acyl chains are still uncertain although much has been learned (Chapter 17) [10]. The export from plastids across the chloroplast envelope membranes is known to involve a fatty acid intermediate, and probably is a channeled or facilitated process rather than free diffusion because only a tiny pool of free fatty acid is ever detected (A. Koo, 2004). An acyl-CoA synthetase on the envelope membrane is believed to quickly convert the exported fatty acid to a thioester form that is then a substrate for acyltransferases. Transfer of acyl groups to the ER may occur via diffusion of the acyl-CoAs however, recent evidence suggests this initial acyl transfer reaction involves acylation of lyso-phosphatidylcholine and may occur at the chloroplast envelope. 

Fig. 11. Various hypotheses proposed by which higher plants may attain high levels of unsaturated fatty acids in their chloroplast membrane galactolipids. (a) Phosphatidylcholine acts as a carrier molecule involved in the desaturation, (b) Desaturation of fatty acids occurs after formation of the galactolipid molecule, (c) Desaturation occurs before formation of the galactolipid molecule. In the first hypothesis, all the desaturases involved are confined in the chloroplast in the second hypothesis, the conversion of 18 1 to 18 2 is maximal in microsomes," whereas desaturation of 18 2 to 18 3 is highest in chloroplast membranes, (d) Deacylation-reacylation mechanism in which X can be a CoA-thioester, a polar lipid, etc. D, Desaturases T, acyl-ACP thioesterase e.r., endoplasmic reticulum.

Fatty acid synthase (FAS) constitutes a multisubunit complex in the plastid where it catalyzes ordered synthesis of fatty acids, initiated from acetyl CoA and malonyl CoA [146, 147]. Stepwise FAS activity generates the products 16 0-acyl carrier protein (AGP) and 18 0-ACP. Most of the 18 0-ACP is desaturated by a soluble stearoyl-ACP desaturase, yielding 18 1 D9-ACP [148]. Acyl-ACP thioesterases release AGP from 16 0-AGP and 18 0-AGP the deacylated fatty acids exit the plastid and are then esterified with coenzyme A (GoA) to form respective acyl-GoAs [148]. These acyl moieties are then esterified to phosphatidylcholine (PQ and then undergo desaturation by D12- and D15-desaturases to yield the essential fatty acids, linoleic acid, and a-linolenic acid [148-150], All higher plants have the enzymes for synthesizing the G18 PUFAs linoleic acid and a-linolenic acid. The primary genes involved in PUFA biosynthesis have been reviewed [151],... 

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