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Zebrafish fads2 is a bifunctional front-end desaturase with both Delta-6 and
Delta-5 activities, validated by heterologous expression in yeast.
"The target protein is *Danio rerio* **fads2** (UniProt Q9DEX7), historically cloned and functionally validated as a **bifunctional front-end desaturase** with **Δ6 and Δ5** activities in zebrafish."
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The yeast assay directly demonstrated Delta-6 desaturation of C18 PUFA and
Delta-5 desaturation of C20 PUFA, with products validated by GC-MS.
"The seminal yeast heterologous expression study demonstrated that zebrafish Fads2 catalyzes:"
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Delta-6 desaturation of C18 PUFA is the rate-limiting first step in converting
dietary essential PUFA precursors into longer-chain PUFA.
"introduces a double bond at the 6th carbon from the carboxyl end and is commonly the **rate-limiting first step** for converting dietary essential PUFA precursors into longer-chain PUFA."
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Delta-5 desaturation converts C20 intermediates to arachidonic acid (ARA) and
eicosapentaenoic acid (EPA).
"introduces a double bond at the 5th carbon and typically converts C20 intermediates (e.g., DGLA/ETA analogs) to **arachidonic acid (ARA)** and **eicosapentaenoic acid (EPA)**."
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Teleosts often lack a canonical Fads1 (Delta-5) ortholog; zebrafish carry a
single bifunctional Delta-6/Delta-5 Fads2 covering multiple desaturation steps.
"Zebrafish are a canonical example: they carry a single **bifunctional Δ6/Δ5 Fads2** that supports multiple LC-PUFA biosynthetic steps."
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Zebrafish Fads2 can act on C24 PUFA, supporting the Sprecher pathway for DHA
biosynthesis (Delta-6 desaturation of C24 followed by peroxisomal beta-oxidation).
"These findings support that zebrafish Fads2 can participate in the C24 Δ6 step required for Sprecher-pathway DHA biosynthesis."
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Partial in-vivo CRISPR knockout reroutes LC-PUFA synthesis through an
alternative Delta-8 pathway, so the enzyme behaves as Delta-6/Delta-5/Delta-8
in the in-vivo pathway context.
"The authors interpret this as evidence that zebrafish Fads2 behaves as **Δ6/Δ5/Δ8** within the in vivo pathway context."
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Fads2 is a non-heme diiron front-end desaturase using the NADH-cytochrome b5
reductase / cytochrome b5 electron-transfer chain and molecular oxygen for
double-bond insertion.
"Front-end desaturases are non-heme diiron enzymes that require an electron-transfer chain involving **NADH–cytochrome b5 reductase**, **cytochrome b5**, and the desaturase, using **molecular oxygen** during double-bond insertion."
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Zebrafish Fads2 co-localizes with the ER marker ERp57 and is in FRET proximity
to CYB5R2/3 reductases and ELOVL elongases in an ER lipid-synthetic module.
"Co-localization with an ER marker (ERp57) supports ER association."
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Most biochemical and review evidence supports Fads2 as an ER membrane enzyme;
a mitochondrial signal seen in heterologous HeLa cells is interpreted cautiously.
"Most biochemical and review evidence supports Fads2 as an **ER membrane** enzyme."
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Partial CRISPR disruption of fads2 in adult female zebrafish altered egg
LC-PUFA composition and produced poor-quality eggs, affecting reproduction.
"Partial CRISPR disruption of fads2 in adult females altered egg LC-PUFA signatures and was associated with **poor-quality eggs**, consistent with a requirement for properly balanced LC-PUFA production/availability during oogenesis and egg provisioning."