Methyl-end desaturases with ¿12 and ¿3 regioselectivities enable the de novo PUFA biosynthesis in the cephalopod Octopus vulgaris

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anglais. The interest in understanding the capacity of aquatic invertebrates to biosynthesise omega-3 (ω3) long-chain(≥C20) polyunsaturated fatty acids (LC-PUFA) has increased in recent years. Using the common octopus Octopusvulgaris as a model species, we previously characterised a Δ5 desaturase and two elongases (i.e. Elovl2/5 andElovl4) involved in the biosynthesis of LC-PUFA in molluscs. The aim of this study was to characterise bothmolecularly and functionally, two methyl-end (or ωx) desaturases that have been long regarded to be absent inmost animals. O. vulgaris possess two ωx desaturase genes encoding enzymes with Δ12 and ω3 regioselectivitiesenabling the de novo biosynthesis of the C18 PUFA 18:2ω6 (LA, linoleic acid) and 18:3ω3 (ALA, α-linolenic acid),generally regarded as dietary essential for animals. The O. vulgaris Δ12 desaturase (“ωx2”) mediates the conversion of 18:1ω9 (oleic acid) into LA, and subsequently, the ω3 desaturase (“ωx1”) catalyses the Δ15 desaturation from LA to ALA. Additionally, the O. vulgaris ω3 desaturase has Δ17 capacity towards a variety of C20 ω6PUFA that are converted to their ω3 PUFA products. Particularly relevant was the affinity of the ω3 desaturasetowards 20:4ω6 (ARA, arachidonic acid) to produce 20:5ω3 (EPA, eicosapentaenoic acid), as supported by yeastheterologous expression, and enzymatic activity exhibited in vivo when paralarvae were incubated in the presence of [1-14C]20:4ω6. These results confirmed that several routes enabling EPA biosynthesis are operative inO. vulgaris whereas ARA and docosahexaenoic acid (DHA, 22:6ω3) should be considered essential fatty acidssince endogenous production appears to be limited.