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MetaCyc Pathway: α-linolenate biosynthesis I (plants and red algae)
Inferred from experiment

Enzyme View:

Pathway diagram: alpha-linolenate biosynthesis I (plants and red algae)

This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.

Superclasses: BiosynthesisFatty Acid and Lipid BiosynthesisFatty Acid BiosynthesisUnsaturated Fatty Acid Biosynthesisalpha-Linolenate Biosynthesis

Some taxa known to possess this pathway include : Actinidia chinensis, Arabidopsis thaliana col, Brassica napus, Cannabis sativa, Cyanidium caldarium, Glycine max, Hippophae rhamnoides, Linum usitatissimum, Perilla frutescens, Portulaca oleracea, Salvia hispanica, Vaccinium vitis-idaea

Expected Taxonomic Range: Rhodophyta, Viridiplantae

General Background

α-linolenate (often refferred to as simply linolenate) is a polyunsaturated fatty acid (18:3) that is an important constituent of plant membranes. It is also known as an ω-3 fatty acid, referring to the position of the first double bond from the methyl end of the fatty acid. Seed oils are the richest sources of α-linolenate, especially those of rapeseed (canola), soybeans, walnuts, flaxseed (Linseed), perilla, chia and hemp. It is also common in the lipids that are found in thylakoid membranes in the green leaves of broadleaf plants, and in cyanobacteria and alga [Kenyon72, Bedord78]. α-linolenate is an essential fatty acid for animals, and is considered an essential dietary requirement for all mammals.

About This Pathway

In all plant tissues, the major glycerolipids are first synthesized using only palmityl (16:0) and oleoyl (18:1) acyl groups, the products of the plastidic saturated fatty acid synthesis (see palmitate biosynthesis II (bacteria and plants), stearate biosynthesis II (bacteria and plants) and oleate biosynthesis I (plants)). Subsequent desaturation of the fatty acids to the highly unsaturated forms typical of the membranes of plant cells is carried while they are already attached to lipids.

Most of the oleoyl-[acp] is released from the [acp]-moiety and exported from the plastid to the endoplasmic reticulum (ER), and is converted to oleoyl-CoA in the process by the plastidic long-chain acyl-CoA synthetase 9 (see oleate biosynthesis I (plants)). The oleoyl-CoA is incorporated into lipids, forming a 1,2-diacyl-sn-glycerol 3-phosphate, a diglyceride, and a phosphatidylcholine (see linoleate biosynthesis I (plants)). Further desaturation of the oleoyl groups to linoleoyl groups in the ER occurrs while incorporated into lipids. The first step, which produces a [glycerolipid]-linoleate, is catalyzed by the acyl-lipid ω-3 desaturase (endoplasmic reticulum), encoded by the FAD2 gene. The second step, which produces a [glycerolipid]-α-linolenate, is catalyzed by the FAD3-encoded acyl-lipid ω-3 desaturase (endoplasmic reticulum).

A smaller portion of the oleoyl-[acp] molecules can be incorporated into diacylglycerol and a diglyceride within the chloroplast. In this case, the desaturation to a [glycerolipid]-linoleate occurs by a plastidic desaturase, which is encoded by the FAD6 gene, and further desaturation into a [glycerolipid]-α-linolenate is catalyzed by two plastidic desaturases, encoded by the FAD7 and FAD8 genes [McConn94, Wallis02].

Variants: α-linolenate biosynthesis II (cyanobacteria)

Created 05-Aug-2008 by Caspi R, SRI International


Bedord78: Bedord CJ, McMahon V, Adams B (1978). "alpha-linolenic acid biosynthesis in Cyanidium caldarium." Arch Biochem Biophys 185(1);15-20. PMID: 414660

Eckert06: Eckert H, La Vallee B, Schweiger BJ, Kinney AJ, Cahoon EB, Clemente T (2006). "Co-expression of the borage Delta 6 desaturase and the Arabidopsis Delta 15 desaturase results in high accumulation of stearidonic acid in the seeds of transgenic soybean." Planta 224(5);1050-7. PMID: 16718484

Kenyon72: Kenyon CN (1972). "Fatty acid composition of unicellular strains of blue-green algae." J Bacteriol 109(2);827-34. PMID: 4621688

McConn94: McConn M, Hugly S, Browse J, Somerville C (1994). "A Mutation at the fad8 Locus of Arabidopsis Identifies a Second Chloroplast [omega]-3 Desaturase." Plant Physiol 106(4);1609-1614. PMID: 12232435

Wallis02: Wallis JG, Browse J (2002). "Mutants of Arabidopsis reveal many roles for membrane lipids." Prog Lipid Res 41(3);254-78. PMID: 11814526

Other References Related to Enzymes, Genes, Subpathways, and Substrates of this Pathway

Arondel92: Arondel V, Lemieux B, Hwang I, Gibson S, Goodman HM, Somerville CR (1992). "Map-based cloning of a gene controlling omega-3 fatty acid desaturation in Arabidopsis." Science 258(5086);1353-5. PMID: 1455229

Browse93: Browse J, McConn M, James D, Miquel M (1993). "Mutants of Arabidopsis deficient in the synthesis of alpha-linolenate. Biochemical and genetic characterization of the endoplasmic reticulum linoleoyl desaturase." J Biol Chem 1993;268(22);16345-51. PMID: 8102138

Gibson94a: Gibson S, Arondel V, Iba K, Somerville C (1994). "Cloning of a temperature-regulated gene encoding a chloroplast omega-3 desaturase from Arabidopsis thaliana." Plant Physiol 1994;106(4);1615-21. PMID: 7846164

Iba93: Iba K, Gibson S, Nishiuchi T, Fuse T, Nishimura M, Arondel V, Hugly S, Somerville C (1993). "A gene encoding a chloroplast omega-3 fatty acid desaturase complements alterations in fatty acid desaturation and chloroplast copy number of the fad7 mutant of Arabidopsis thaliana." J Biol Chem 1993;268(32);24099-105. PMID: 8226956

Kumar12: Kumar R, Tran LS, Neelakandan AK, Nguyen HT (2012). "Higher plant cytochrome b5 polypeptides modulate fatty acid desaturation." PLoS One 7(2);e31370. PMID: 22384013

Latendresse13: Latendresse M. (2013). "Computing Gibbs Free Energy of Compounds and Reactions in MetaCyc."

Lederer94: Lederer F (1994). "The cytochrome b5-fold: an adaptable module." Biochimie 76(7);674-92. PMID: 7893819

Miquel92: Miquel M, Browse J (1992). "Arabidopsis mutants deficient in polyunsaturated fatty acid synthesis. Biochemical and genetic characterization of a plant oleoyl-phosphatidylcholine desaturase." J Biol Chem 267(3);1502-9. PMID: 1730697

Napier03: Napier JA, Michaelson LV, Sayanova O (2003). "The role of cytochrome b5 fusion desaturases in the synthesis of polyunsaturated fatty acids." Prostaglandins Leukot Essent Fatty Acids 68(2);135-43. PMID: 12538077

Ohlrogge95: Ohlrogge J, Browse J (1995). "Lipid biosynthesis." Plant Cell 7(7);957-70. PMID: 7640528

Park06: Park YJ, Yoo CB, Choi SY, Lee HB (2006). "Purifications and characterizations of a ferredoxin and its related 2-oxoacid:ferredoxin oxidoreductase from the hyperthermophilic archaeon, Sulfolobus solfataricus P1." J Biochem Mol Biol 39(1);46-54. PMID: 16466637

Yadav93: Yadav NS, Wierzbicki A, Aegerter M, Caster CS, Perez-Grau L, Kinney AJ, Hitz WD, Booth JR, Schweiger B, Stecca KL (1993). "Cloning of higher plant omega-3 fatty acid desaturases." Plant Physiol 103(2);467-76. PMID: 8029334

Report Errors or Provide Feedback
Please cite the following article in publications resulting from the use of MetaCyc: Caspi et al, Nucleic Acids Research 42:D459-D471 2014
Page generated by Pathway Tools version 19.5 (software by SRI International) on Mon May 2, 2016, biocyc14.