If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Biosynthesis → Fatty Acids and Lipids Biosynthesis → Fatty Acid Biosynthesis → Unusual Fatty Acid Biosynthesis|
Expected Taxonomic Range: Viridiplantae
Plant oils contribute to the caloric intake of humans and animal. Vegetable oils are extracted from a number of plant sources, soybean oil accounts for nearly 68% of vegetable oil production in the US. Apart from the major use as food, plant oils have several industrial uses. Laurate from coconut oil is extensively used in the manufacture of soaps and detergents. The kinds of fatty acid composition in different seed oils determine the quality and use of the oil.
Plants contain significant amounts of hydroxy fatty acids in their sphingolipids. Castor oil extracted from Ricinus communis contains almost 90% ricinoleate (D-(-)12-hydroxy-octadec-cis-9-enoic acid). The fungus Claviceps purpurea produces Ergot hydroxy fatty acids which are found as esterifed glycerol in triacylglycerols. This esterification extends the ricinoleate molecules through the ester bonds and the resulting lipids are referred to as estolids. Ricinoleate is under large scale production for its uses in a variety of compounds. Pyrolyzation of ricinoleate yields sebacate which is used to produce some types of nylon. Sebacate also finds use as high temperature greases for jet engines. The importance of hydroxylated fatty acids for commercial and nutritional purposes resulted in efforts to develop metabolic engineered pathways for these compounds [Somerville96a, Thelen02].
Ricinoleate occurs throughout the plant kingdom, close relatives of some plants however, are not able to synthesize ricinoleate indicating that the ability to synthesize ricinoleate has had multiple independent origins during evolution. The most enzymes involved in the formation of hydroxylated fatty acids have been characterized from species of the Brassicaceae family which contain a high percentage of hydroxylated fatty acids in their seed oil. The hydroxylated fatty acid biosynthesis takes place at the endoplasmatic reticulum of the cells and includes the microsomal fatty acid elongation (FAE) system [Browse91, Moon01a]. The pathway depicted here represents the conversion of the fatty acid thioesters as the principal metabolic substrates which generally applies to the FAE elongation reactions [Fehling91]. However, it should be mentioned that the respective fatty acids are often esterified to the sn-2 position of the membrane lipid phosphatidylcholin or other phospholipids before further metabolization, i.e. hydroxylation and desaturation can take place [Bafor91, Lin96a]. Although acyl-CoA hydrolases exist to release fatty acids from coenzyme A, free fatty acids are considered toxic and therefore not very abundant in plants and unlikely direct reaction partners (Ed Cahoon 2013, personal communication).
The seeds of the genus Physaria (synonym Lesquerella) contain high amounts of hydroxylated fatty acids such as lesquerolate (C20:1Δ11,14OH) and the more desaturated densipolate (C18:2Δ9Δ15, 12OH) and auricolate (C20:2Δ11Δ17, 14OH) [Hayes95, Dierig04] whereas Arabidopsis [Smith00b] and Ricinus [Moreau81] accumulate mainly ricinoleate (18:1Δ9, 12OH). The main biosynthetic route in Physaria is based upon labeling studies [Reed97] and proposes the first formation of ricinoleate by a oleate 12-hydroxylase [Broun98, Dauk07] followed by a desaturase which leads to the formation of densipolate [Broun98, Engeseth96]. The further steps towards the biosynthesis of lesquerolate may occur either through the elongation of ricinoleate or the hydroxylation of (11Z)-eicos-11-enoate [Lin96a, Broun97]. The elongation of oleoyl-CoA to form (11Z)-eicos-11-enoyl-CoA has been demonstrated in various species [Fehling91, Moon01a, Ghanevati01]. The final reactions leading to auricolate are unknown and remain to be demonstrated.
Bafor91: Bafor M, Smith MA, Jonsson L, Stobart K, Stymne S (1991). "Ricinoleic acid biosynthesis and triacylglycerol assembly in microsomal preparations from developing castor-bean (Ricinus communis) endosperm." Biochem J 280 ( Pt 2);507-14. PMID: 1747126
Broun97: Broun P, Somerville C (1997). "Accumulation of ricinoleic, lesquerolic, and densipolic acids in seeds of transgenic Arabidopsis plants that express a fatty acyl hydroxylase cDNA from castor bean." Plant Physiol 113(3);933-42. PMID: 9085577
Dierig04: Dierig DA, Tomasi PM, Salywon AM, Ray DT (2004). "Improvement in hydroxy fatty acid seed oil content and other traits from interspecific hybrids of three Lesquerella species: Lesquerella fendleri, L. pallida, and L. lindheimeri." Euphytica 139:199-206.
Fehling91: Fehling E, Mukherjee KD (1991). "Acyl-CoA elongase from a higher plant (Lunaria annua): metabolic intermediates of very-long-chain acyl-CoA products and substrate specificity." Biochim Biophys Acta 1082(3);239-46. PMID: 2029543
Ghanevati01: Ghanevati M, Jaworski JG (2001). "Active-site residues of a plant membrane-bound fatty acid elongase beta-ketoacyl-CoA synthase, FAE1 KCS." Biochim Biophys Acta 2001;1530(1);77-85. PMID: 11341960
Hayes95: Hayes DG, Kleiman R, Philipps BS (1995). "The triglyceride composition, structure, and presence of estolides in the oils of Lesquerella and related species." J. Am. Oil Chem. Soc. 5: 559-569.
Lin96a: Lin JT, McKeon TA, Goodrich-Tanrikulu M, Stafford AE (1996). "Characterization of oleoyl-12-hydroxylase in castor microsomes using the putative substrate, 1-acyl-2-oleoyl-sn-glycero-3-phosphocholine." Lipids 31(6);571-7. PMID: 8784737
Reed97: Reed DW, Taylor DC, Covello PS (1997). "Metabolism of Hydroxy Fatty Acids in Developing Seeds in the Genera Lesquerella (Brassicaceae) and Linum (Linaceae)." Plant Physiol 114(1);63-68. PMID: 12223689
Somerville96a: Somerville C, Broun P, Van De Loo FJ (1996). "Production of hydroxylated fatty acids in genetically modified plants." Patent Cooperation Treaty International Appl. No. PCT/ US95 / 11855.
James95: James DW, Lim E, Keller J, Plooy I, Ralston E, Dooner HK (1995). "Directed tagging of the Arabidopsis FATTY ACID ELONGATION1 (FAE1) gene with the maize transposon activator." Plant Cell 1995;7(3);309-19. PMID: 7734965
Oh97: Oh CS, Toke DA, Mandala S, Martin CE (1997). "ELO2 and ELO3, homologues of the Saccharomyces cerevisiae ELO1 gene, function in fatty acid elongation and are required for sphingolipid formation." J Biol Chem 272(28);17376-84. PMID: 9211877
Paul06: Paul S, Gable K, Beaudoin F, Cahoon E, Jaworski J, Napier JA, Dunn TM (2006). "Members of the Arabidopsis FAE1-like 3-ketoacyl-CoA synthase gene family substitute for the Elop proteins of Saccharomyces cerevisiae." J Biol Chem 281(14);9018-29. PMID: 16449229
Rubio06: Rubio S, Larson TR, Gonzalez-Guzman M, Alejandro S, Graham IA, Serrano R, Rodriguez PL (2006). "An Arabidopsis mutant impaired in coenzyme A biosynthesis is sugar dependent for seedling establishment." Plant Physiol 140(3);830-43. PMID: 16415216
©2014 SRI International, 333 Ravenswood Avenue, Menlo Park, CA 94025-3493