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Food Lipids: Chemistry, Nutrition, and Biotechnology

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Figure 1 Biosynthesis of fatty acids in plant cells. (A) Synthesis of fatty acids in canola<br />

pro-plastids: fatty acids, or acyl groups, are built up in two-carbon increments while bound<br />

as a thioester to acyl carrier protein (ACP). As the acyl-ACP molecules lengthen into C16:0-<br />

ACP, C18:0-ACP, <strong>and</strong> (after a desaturation step) C18:1-ACP, an acyl–ACP thioesterase<br />

cleaves the thioester bond to ACP <strong>and</strong> the complete fatty acid is available for triacylglycerol<br />

synthesis after conversion to an acyl-CoA in the cytoplasm (see Fig. 2). (B) Engineered laurate<br />

biosynthesis in transgenic canola: addition of a lauroyl–ACP thioesterase from Umbellularia<br />

californica (or other lauric acid producing plant) allows the thioeste bond to ACP to be cleaved<br />

at the C12:0 stage, thus making free lauric acid available for triacylglycerol synthesis.<br />

Figure 2 Biosynthesis of triacylglycerols in plants. Reactions 1, 2, <strong>and</strong> 4 each draw on<br />

acyl-CoA pools available in the plant cell. (1) Glycerol-3-phosphate is acylated to form 1acyl-sn-glycerol-3-phosphate,<br />

also known as lysophosphatidic acid. (2) 1-Acyl-sn-glycerol-3phosphate<br />

is acylated to form phosphatidic acid by the enzyme lysophosphatidic acid acyltransferase<br />

(LPAT). (3) A phosphatase activity removes the phosphate group to generate<br />

diacylglycerol. (4) Diacylglycerol acyltransferase attaches the third acyl group to create triacylglycerol.<br />

LPAT in most plants is very selective for unsaturated 18-carbon acyl-CoA,<br />

whereas the other enzymes in this pathway are typically nonselective. However, coconut LPAT<br />

in particular has high activity on shorter chain saturated fatty acid CoAs.<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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