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:||Biosynthesis → Fatty Acids and Lipids Biosynthesis → Fatty Acid 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
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 acyl-CoA synthetase. The oleoyl-CoA is incorporated into lipids, forming a 1,2-diacyl-sn-glycerol 3-phosphate, a diglyceride, and a phosphatidylcholine. Further desaturation of the oleoyl groups to linoleoyl groups in the ER occurrs while incorporated into lipids. The first step, which produces a lipid linoleoyl group, is catalyzed by the fatty acyl Δ15 desaturase (endoplasmic reticulum), encoded by the FAD2 gene. The second step, which produces a lipid α-linolenoyl group, is catalyzed by the FAD3-encoded fatty acyl Δ15 desaturase (endoplasmic reticulum).
A smaller portion of the oleoyl-[acp] molecules can be incorporated into a 1,2-diacyl-sn-glycerol 3-phosphate and a diglyceride within the chloroplast. In this case, the desaturation to a lipid linoleoyl group occurs by a plastidic desaturase, which is encoded by the FAD6 gene, and further desaturation into a lipid α-linolenoyl group is catalyzed by two plastidic desaturases, encoded by the FAD7 and FAD8 genes [McConn94, Wallis02].
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
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
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
Gibson94: 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
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
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