TAG Synthesis

TAG synthesis is a complex, multistep pathway involving multiple cellular compartments (Fig. 7.3). Plastids, whether the chloroplasts of photosynthetic organs or the tiny proplastids of typical oilseeds, build 2-carbon units into fatty acids with up to 18 carbons and 1 double bond. Two of these acyl units are then esterified to glycerol-3-phosphate, producing phosphatidic acid. The endoplasmic reticulum (ER) is the major site of phosphatidic acid synthesis for TAG; however, plastids likewise generate phosphatidic acid, and flow of glycerolipid backbones from the plastids into storage oils has been observed. Fatty acids ultimately incorporated into TAG can undergo further desaturation, elongation, or other modifications, often while the acyl units are esterified to phosphatidylcholine (PC) or coenzyme A. Finally, phosphatidic acid is dephosphorylated at the ER to form diacylglycerol (DAG), and a diacylglycerol acyltransferase (DGAT) adds the final fatty acid, forming TAG that is sequestered from the ER into lipid bodies for storage. Alternative mechanisms for transfer of fatty acids to TAG are also possible, as will be discussed below.

FIGURE 7.3 Triacylglycerol (TAG) synthesis, highlighting points in the pathway at which genetic engineering and/or mutagenesis have been used to modify fatty acid composition of the resulting oil. The upper left portion of the diagram shows synthesis of malonyl-CoA by ACCase, and the cyclic nature of the reactions catalyzed by fatty acyl synthase (FAS). FAS is composed of malonyl-CoA:malonyl-ACP acyltransferase (AT), 3-ketoacyl-ACP synthase (KAS), 3-ketoacyl-ACP reductase, 3-hydroxyacyl-ACP dehydratase, and enoyl-ACP reductase. As shown on the right of the diagram, the products of FAS depend on the contributions of various KASes, the substrate and double bond specificities of acyl-ACP desaturases, and the substrate specificities of thioesterases that release fatty acids for export from the plastids. In the ER, phosphatidic acid (PA) is assembled by sequential activities of glycerol-3-phosphate acyltransferase (G3P-AT) and lysophosphatidic acid-acyltransferase (LPAAT). Diacylglycerol (DAG) units released from lyso-PA by phosphatidate phosphatase may be converted directly to triacylglycerol by DGAT. However, a large proportion