Synthesis pathways glycerolipids

Wang, L., Shen, W., Kazachkov, M., Chen, G., Chen, Q., Carlsson, A.S., Stymne, S., Weselake, R.J., Zou, J., 2012. Metabolic interactions between the lands cycle and the Kennedy pathway of glycerolipid synthesis in Arabidopsis developing seeds. Plant Cell 24, 4652-4669. 

Figure 21-3 Major pathways of synthesis of fatty acids and glycerolipids in the green plant Arabidopsis. The major site of fatty acid synthesis is chloroplasts. Most is exported to the cytosol as oleic acid (18 1). After conversion to its coenzyme A derivative it is converted to phosphatidic acid (PA), diacylglycerol (DAG), and the phospholipids phosphatidylcholine (PC), phosphatidylinositol (PI), phosphatidylglycerol (PG), and phosphatidylethanolamine (PE). Desaturation also occurs, and some linoleic and linolenic acids are returned to the chloroplasts. See text also. From Sommerville and Browse.106 See also Figs. 21-4 and 21-5. Other abbreviations monogalactosyldiacylglycerol (MGD), digalactosyldiacylglycerol (DGD), sulfolipid (SL), glycerol 3-phosphate (G3P), lysophosphatidic acid (LPA), acyl carrier protein (ACP), cytidine diphosphate-DAG (CDP-DAG).

In the plastids, acyltransferases provide a direct route for entrance of acyl groups from ACP to membrane lipids. Since this is the standard pathway for phosphatidic acid synthesis in E. coli and cyanobacteria, both the enzymes of phosphatidic acid synthesis in plastids and the glycerolipid backbones they produce are termed prokaryotic . In both chloroplasts and non-green plastids, the glycerol-3-phosphate acyltransferase is a soluble enzyme that, unlike the E. coli enzyme, shows preference for 18 1-ACP over 16 0-ACP. The lysophosphatidic acid acyltransferase, which is a component of the inner envelope of plastids, is extremely selective for 16 0-ACP. The presence of a 16-carbon fatty acid at the... 

In plants, de novo fatty acid biosynthesis occurs exclusively in the stroma of plastids, whereas, with the exception of plastidial desaturation, modification of fatty acid residues including further desaturation and triacylglrycerol (TAG) assembly are localized in the cytosol/endoplasmic reticulum (ER). The primary fatty acids formed in the plastid (palmitic, stearic, and oleic acid) are used in the plastidic prokaryotic pathway for membrane lipid synthesis or diverted to the cytoplasmic eukaryotic pathway for the synthesis of membrane lipids or storage TAGs (1). Movement of glycerolipids is believed to occur in the reverse direction between the cytosol/ER and the plastids in the highly regulated manner (2). 

S.2 Fatty acyl-CoA transferases. The enzyme systems involved with fatty acyl-CoA utilization in the cytosol appear to be membrane-bound. Consequently, detailed knowledge of their individual structure, specificity and genetic control is generally lacking due to the particular inability to obtain ready isolation and purification of the relevant proteins. Studies, however, support the concept of the operation of the eukaryotic pathway for the production of glycerolipids and polyunsaturated fatty acid (Browse et al., 1990 Stymne et al., 1990). While this pathway may contribute a significant quantity of fatty acid for use in membrane synthesis in the plastid (chloroplast) (Browse et al., 1990), its major importance would seem to lie with the production of unsaturated oils (Frentzen, 1986). On the other hand the occurrence of the prokaryotic pathway in the plastid permits more direct membrane lipid formation in both 16 3 and 18 3 plants (Browse et al., 1990 Somerville and Browse, 1991). Different sets of acyltransferase may be associated with the two pathways (Hills and Murphy, 1991). 

Figure 3 Synthesis of glycerolipids from sn-glycerol 3-phosphate by oaulifloaer bud plastid. In experiment B, UDP-gal was added to the incubation mixture. The cauliflower bud plastids are non-green and are mostly devoid of internal membranes. However, they behave almost like chloroplasts with respect to the Komberg-Pricer pathway enzymes and MGDG formation.

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