{"uniprot":"Q8N9I5","gene":"FADS6","name":"Fatty acid desaturase 6","organism":"Homo sapiens (Human)","status":"dark","length":368,"sequence":"MEPTEPMEPTEPMEPTEPMEPTEPMEPTEPMEPARSAHRGGEALLRELEVLVQDVVRTSSWWERHGVDCAILALSLFALPAGFLCLRWENALVFASGITILGVCHYTLTVKGSHLATHGALTESKRWSKIWLLFFVEVCTAFTAEHATHGHVKMHHAYTNVVGLGDSSTWRLPCLNRYVYMFLAPFLLPIATPLVAVERLRKVELGTALRTLALISLGLYSHYWLLLNVSGFKNPSSALGCMFLTRSLLAHPYLHVNIFQHIGLPMFSRDNKPRRIHMMSLGVLNLARLPVLDWAFGHSIISCHVEHHLFPRLSDNMCLKVKPVVSQFLREKQLPYNEDSYLARFQLFLRRYEEFMVQAPPITELVGL","function_summary":"A multi-pass endoplasmic reticulum membrane enzyme that belongs to the fatty acid desaturase class but exhibits low intrinsic activity. It assembles into higher-order complexes that organize electron transfer via ER redox partners and handle acyl-lipid substrates. By forming tetramers and engaging ER lipid-handling factors, it contributes to remodeling of membrane fatty acids and fine-tunes lipid composition and signaling at the ER interface.","plain_language":"FADS6 has barely been studied. The model's best guess: A multi-pass endoplasmic reticulum membrane enzyme that belongs to the fatty acid desaturase class but exhibits low intrinsic activity. It assembles into higher-order complexes that organize electron transfer via ER redox partners and handle acyl-lipid substrates.","uniprot_summary":"Exhibits low fatty acid desaturase activity.","go_terms":{"molecular_function":[],"biological_process":[],"cellular_component":[]},"interpro":[{"id":"IPR005804","name":"Fatty acid desaturase domain","type":"domain","start":92,"end":338},{"id":"IPR012171","name":"Fatty acid desaturase","type":"family","start":42,"end":361}],"reasoning":"I begin with the InterPro architecture. The polypeptide carries IPR012171 (Fatty acid desaturase family) spanning residues 42–361, which envelopes a catalytic core annotated as IPR005804 (Fatty acid desaturase domain) from residues 92–338. This arrangement—an internal desaturase catalytic domain embedded within a broader desaturase family envelope—defines a membrane-bound non-heme diiron enzyme that introduces double bonds into fatty acyl chains. Such enzymes are typically multi-pass membrane proteins that coordinate a diiron center via conserved histidine motifs and catalyze oxygen-dependent desaturation reactions. The membrane-embedded catalytic core dictates a redox chemistry that requires electron transfer through cytochrome b5 and its reductase, and the extended family signature implies oligomerization interfaces characteristic of desaturase assemblies.\n\nFrom this domain logic, the primary molecular function is redox-coupled lipid desaturation. However, the provided molecular function label is GO:0005515 protein binding, which fits the oligomeric and partner-recruiting nature of membrane desaturases. The desaturase fold creates surfaces that mediate assembly with cytochrome b5 and with other subunits, supporting protein binding as a necessary facet of function. The oligomerization behavior of membrane desaturases is well documented; they often form higher-order complexes, and this dovetails with the biological process GO:0051262 protein tetramerization. The tetrameric assembly stabilizes the catalytic core and organizes electron-transfer geometry, thereby tuning activity.\n\nThe biological process inference proceeds from the enzyme class to pathway context. Membrane desaturases in animals channel acyl-CoA or acyl-ACP substrates through specific double-bond introductions that set the composition of membrane lipids and signaling precursors. Even if this particular enzyme is catalytically weak or substrate-selective, its architecture and propensity to tetramerize position it within lipid-modifying pathways that shape membrane fluidity and downstream signaling.\n\nCellular localization follows directly from the architecture and chemistry. Membrane-bound desaturase cores require a lipid bilayer and typically reside in the endoplasmic reticulum, where cytochrome b5 systems and acyl-lipid metabolism concentrate. This places the protein at the GO:0005789 endoplasmic reticulum membrane and explains the multi-pass topology inferred from the desaturase fold.\n\nMechanistically, I hypothesize that the enzyme assembles as a tetrameric ER-membrane complex that recruits electron donors via cytochrome b5 and couples weak or context-dependent desaturation to broader lipid remodeling. Its protein-binding surfaces and tetramerization propensity suggest transient assemblies with ER-resident redox partners and lipid-handling factors. Likely interaction partners include cytochrome b5 and NADH-cytochrome b5 reductase for electron supply; ER-resident acyl-CoA synthetases and acyl-CoA-binding protein that present substrates; and ER scaffolds that stabilize higher-order complexes. Together, these interactions would tune an otherwise low-activity desaturase into a regulatory node for membrane lipid composition at the ER.","category":"dark","control_evaluation":null,"validation_plan":null,"structure":{"source":"AlphaFold DB","pdb":"/structures/AF-Q8N9I5.pdb","alphafold":"https://alphafold.ebi.ac.uk/entry/Q8N9I5"},"links":{"uniprot":"https://www.uniprot.org/uniprotkb/Q8N9I5/entry","atlas":"/p/Q8N9I5"},"provenance":{"model":"BioReason-Pro (ESM3 esm3-sm-open-v1)","source_list":"Pharos Tdark","note":"AI prediction, not experimentally verified"}}