FAD2 is the endoplasmic-reticulum (microsomal) omega-6 / Delta(12) fatty acid desaturase of Arabidopsis. It introduces a second double bond into monounsaturated C18 fatty acids, converting oleic acid (18:1) to linoleic acid (18:2) on fatty acids esterified to phosphatidylcholine (PC), using cytochrome b5 as the electron donor. It is a polytopic membrane protein with conserved histidine-box motifs. The 18:2 product can be further desaturated to 18:3 by FAD3, and because PC is a major precursor for triacylglycerol assembly, FAD2 is a primary determinant of extraplastidic membrane and seed-oil polyunsaturated fatty acid composition; ER membrane unsaturation set by FAD2 also contributes to ER-stress and low-temperature responses.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Correct localization, independently confirmed experimentally (EXP below). FAD2 is an ER (microsomal) membrane desaturase. |
| GO:0006629 lipid metabolic process | IEA GO_REF:0000002 | ACCEPT | Summary: Correct but general. FAD2 functions in glycerolipid/fatty acid metabolism; the specific unsaturated fatty acid biosynthetic role is captured below. |
| GO:0016491 oxidoreductase activity | IEA GO_REF:0000002 | ACCEPT | Summary: Correct but general. The informative molecular function is the omega-6 (Delta-12) fatty acid desaturase activity captured by more specific terms below. |
| GO:0016717 oxidoreductase activity, acting on paired donors, with oxidation of a pair of donors resulting in the reduction of molecular oxygen to two molecules of water | IEA GO_REF:0000002 | ACCEPT | Summary: Correct enzyme-class molecular function for a membrane fatty acid desaturase, which uses O2 and an electron donor (cytochrome b5) to introduce a double bond. Consistent with the specific desaturase annotations. |
| GO:0050184 acyl-lipid omega-6 desaturase (cytochrome b5) activity | IEA GO_REF:0000120 | ACCEPT | Summary: Accurate core molecular function, also experimentally supported (EXP below). FAD2 desaturates oleoyl-phosphatidylcholine using cytochrome b5 as electron donor. |
| GO:0102985 acyl-CoA (9+3)-desaturase activity | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: FAD2 is canonically an acyl-LIPID desaturase that acts on oleate esterified to phosphatidylcholine, not on acyl-CoA (its primary characterization is as an oleoyl-phosphatidylcholine desaturase). The acyl-lipid omega-6 desaturase (GO:0050184) and omega-6 desaturase (GO:0045485) terms are the accurate molecular functions; this acyl-CoA term is retained as non-core. [PMID:1730697] Supporting Evidence: PMID:1730697 Biochemical and genetic characterization of a plant oleoyl-phosphatidylcholine desaturase |
| GO:0006636 unsaturated fatty acid biosynthetic process | IEA GO_REF:0000041 | ACCEPT | Summary: Correct core biological process. FAD2 produces polyunsaturated fatty acids (18:2, feeding into 18:3) in the extraplastidic pathway. |
| GO:0005789 endoplasmic reticulum membrane | EXP PMID:14690501 Membrane-bound fatty acid desaturases are inserted co-transl... | ACCEPT | Summary: Experimentally supported ER membrane localization. Membrane-bound fatty acid desaturases including FAD2 are co-translationally inserted into the ER and carry C-terminal ER-retrieval motifs. [PMID:14690501] Supporting Evidence: PMID:14690501 Membrane-bound fatty acid desaturases are inserted co-translationally into the ER and contain different ER retrieval motifs at their carboxy termini. |
| GO:0050184 acyl-lipid omega-6 desaturase (cytochrome b5) activity | EXP PMID:1730697 Arabidopsis mutants deficient in polyunsaturated fatty acid ... | ACCEPT | Summary: Core molecular function with direct experimental support. FAD2 is the plant oleoyl-phosphatidylcholine (acyl-lipid) omega-6 desaturase; fad2 mutants are deficient in polyunsaturated fatty acid synthesis. [PMID:1730697] Supporting Evidence: PMID:1730697 Arabidopsis mutants deficient in polyunsaturated fatty acid synthesis. |
| GO:0102985 acyl-CoA (9+3)-desaturase activity | EXP PMID:8685264 Functional expression of the extraplastidial Arabidopsis tha... | KEEP AS NON CORE | Summary: Same substrate-class caveat as the IEA instance above. FAD2 functionally expressed in yeast desaturates oleate to linoleate, but its physiological substrate is PC-esterified oleate (acyl-lipid), so the acyl-lipid omega-6 desaturase term is the accurate core function; retained as non-core. [PMID:8685264] Supporting Evidence: PMID:8685264 Functional expression of the extraplastidial Arabidopsis thaliana oleate desaturase gene (FAD2) in Saccharomyces cerevisiae. |
| GO:0005634 nucleus | ISM GO_REF:0000122 | REMOVE | Summary: Spurious computational (AtSubP sequence-model) prediction that contradicts the experimentally established ER membrane localization. FAD2 is a polytopic ER membrane desaturase; there is no evidence for nuclear localization. Supporting Evidence: PMID:14690501 Membrane-bound fatty acid desaturases are inserted co-translationally into the ER and contain different ER retrieval motifs at their carboxy termini. |
| GO:0045485 omega-6 fatty acid desaturase activity | IDA PMID:8685264 Functional expression of the extraplastidial Arabidopsis tha... | ACCEPT | Summary: Core molecular function, directly demonstrated. Functional expression of FAD2 confers omega-6 (Delta-12) desaturase activity, converting oleate to linoleate. [PMID:8685264] Supporting Evidence: PMID:8685264 Functional expression of the extraplastidial Arabidopsis thaliana oleate desaturase gene (FAD2) in Saccharomyces cerevisiae. |
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