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Chapter 10

Lipids

 
Lipids have diverse and essential roles in plants. As the hydrophobic barrier of membranes, they are essential for integrity of cells and organelles. In addition, they are a major form of chemical energy storage in seeds and are now recognized as a key component of some signal transduction pathways. Most lipids, but not all, contain fatty acids esterified to glycerol, and consideration of this area of metabolism involves, first, the synthesis of the fatty acid and, second, the synthesis of lipid after esterification of the fatty acid to form phosphatidic acid.
      Fatty acid biosynthesis in plants is very similar to that in bacteria and is carried out in the plastid. Fatty acid synthesis begins with the carboxylation of acetyl-CoA to mal-onyl- CoA by acetyl-CoA carboxylase. This is the first committed step of fatty acid synthesis and is a likely site for regulation of the whole pathway. Acetyl-CoA and malonyl-CoA are subsequently converted into fatty acids by a series of reactions that add two carbons at a time to a growing chain. Acyl-carrier protein is a 9-kDa protein that transports the intermediates of fatty acid synthesis through the pathway. In plants, each reaction is catalyzed by a separate gene product—in contrast to fatty acid synthesis in animals, which depends on a multifunctional protein.
      Fatty acid biosynthesis can be terminated by several different reactions, including hydrolysis of the thioester bond, transfer of the acyl group to a glycerolipid, and acyl desaturation. Plants contain an unusual stearoyl-ACP desaturase that is soluble and plastid-localized. Most fatty acyl desaturases are membrane proteins localized in the endoplasmic reticulum or the plastid.
      Plants are capable of synthesizing unusual fatty acids, and more than 200 different fatty acids having been found in plants. Some of the enzymes that synthesize unusual fatty acids bear close resemblance's to common fatty acid enzymes, such as membrane- bound desaturases. These unusual fatty acids are found almost exclusively in seed oils, and it is thought that they may serve a defense function.
      There are two distinct pathways for the synthesis of membrane glycerolipids. The prokaryotic pathway, located in the chloroplast inner envelope, uses 18:1-ACP and 16:0-ACP for the sequential acylation of glycerol 3-phosphate and synthesis of glycerolipid components of the chloroplast membranes. The eukaryotic pathway involves (a) export of 16:0 and 18:1 fatty acids from the chloroplast to the endoplasmic reticulum as acyl-CoAs and (b) their incorporation into phosphatidylcholine and other phospholipids that are the principal structural lipids of all the membranes of the cell except the chloroplast. In addition, the diacylglycerol moiety of phosphatidylcholine can be returned to the chloroplast envelope and used as a second source of precursors for the synthesis of chloroplast lipids.
      Membrane lipids serve as a hydrophobic barrier, delimiting the cell and dividing it into functional compartments. The membrane lipid composition also affects plant form as well as many cellular functions. For example, photosynthesis is impaired in plants lacking polyunsaturated membrane lipids, and lipid composition can affect chilling sensitivity and influence plant cell responses to freezing. In addition, membrane lipids function in signal transduction pathways and defensive processes.
      Storage lipids play a distinctly different role from membrane lipids. Storage lipids are almost exclusively triacylglycerols and accumulate in discrete subcellular organelles called oil bodies. The mobilization and catabolism of triacylglycerols provides energy for germination and pollination. The catabolism of the released fatty acids occurs via ß-oxidation in peroxisomes and glyoxysomes.

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