Stearoyl-acyl carrier protein (ACP) desaturase (EC 1.14.99.6) catalyzes the principal conversion of saturated fatty acids to unsaturated fatty acids in the synthesis of vegetable oils. Stearoyl-ACP desaturase was purified from developing embryos of safflower seed, and extensive amino acid sequence was determined. The amino acid sequence was used in conjunction with polymerase chain reactions to clone a full-length cDNA. The primary structure of the protein, as deduced from the nucleotide sequence of the eDNA, includes a 33-amino-acid transit peptide not found in the purified enzyme. Expression in Escherichia coli of a gene encoding the mature form of stearoyl-ACP desaturase did not result in an altered fatty acid composition. However, active enzyme was detected when assayed in vitro with added spinach ferredoxin. The lack of significant activity in vitro without added ferredoxin and the lack of observed change in fatty acid composition indicate that ferredoxin is a required cofactor for the enzyme and that E. coli ferredoxin functions poorly, if at all, as an electron donor for the plant enzyme.Membrane fluidity and function are greatly influenced by the ratios of saturated to unsaturated fatty acids in the membrane lipids. In plants (1) and bacteria (2), the saturated fatty acids are synthesized in two-carbon increments as acyl thioesters of acyl carrier protein (ACP). In enteric bacteria such as Escherichia coli, the primary unsaturated fatty acids are cis-All C18:1 (vaccenic acid) and cis-A9 C16:1 (palmitoleic acid). Vaccenic and palmitoleic acids are synthesized by elongation of precursor monounsaturated acyl-ACPs; the saturated 16-and 18-carbon fatty acids (palmitic and stearic acids) are synthesized from precursor saturated acyl-ACPs. In higher plants, however, the unsaturated 16-carbon transhexadec-9-enoic acid and 18-carbon oleic acid (cis-A9 C18:1) are formed directly from palmitic and stearic acids esterified to specific glycerol lipids or to ACP (3). These reactions take place in the chloroplast (or proplastid in nonphotosynthetic tissues). Further double bonds are introduced into the monounsaturated acyl-lipids, typically at the 12 position followed by desaturation at the 15 or 6 positions of the diunsaturated species; saturated acyl groups generally do not serve as substrates for desaturation at the 6, 12, or 15 position in the carbon chain. Thus in higher plants, the ratio of saturated fatty acids to unsaturated fatty acids in membrane lipids is directly regulated by the enzymes that catalyze the conversion of saturated species to monounsaturated ones.Our interests lie in the regulation of the enzymatic steps in higher plants that determine the relative amounts of specific saturated and unsaturated fatty acids in neutral storage lipids. Unsaturated fatty acids in seed oils are predominantly 18 carbons or more in length and are derived by a series of enzymatic steps following the conversion of stearoyl-ACP to oleoyl-ACP. Stearoyl-ACP desaturase (EC 1.14.99.6) is therefore a necessary enzyme ...
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