Abstract
Docosahexaenoic acid (DHA) plays important physiological roles in vertebrates. Studies in rats and rainbow trout confirmed that DHA biosynthesis proceeds through the so-called “Sprecher pathway”, a biosynthetic process requiring a Δ6 desaturation of 24:5n−3 to 24:6n−3. Alternatively, some teleosts possess fatty acyl desaturases 2 (Fads2) that enable them to biosynthesis DHA through a more direct route termed the “Δ4 pathway”. In order to elucidate the prevalence of both pathways among teleosts, we investigated the Δ6 ability towards C24 substrates of Fads2 from fish with different evolutionary and ecological backgrounds. Subsequently, we retrieved public databases to identify Fads2 containing the YXXN domain responsible for the Δ4 desaturase function, and consequently enabling these species to operate the Δ4 pathway. We demonstrated that, with the exception of Δ4 desaturases, fish Fads2 have the ability to operate as Δ6 desaturases towards C24 PUFA enabling them to synthesise DHA through the Sprecher pathway. Nevertheless, the Δ4 pathway represents an alternative route in some teleosts and we identified the presence of putative Δ4 Fads2 in a further 11 species and confirmed the function as Δ4 desaturases of Fads2 from medaka and Nile tilapia. Our results demonstrated that two alternative pathways for DHA biosynthesis exist in teleosts.
Highlights
Long chain (≥C20) polyunsaturated fatty acids (LC-PUFA) including arachidonic acid (ARA, 20:4n−6), eicosapentaenoic acid (EPA, 20:5n−3) and docosahexaenoic acid (DHA, 22:6n−3) play numerous physiologically important roles essential to health in humans[1, 2]
It was subsequently demonstrated that the same ∆6 Fatty acyl desaturases (Fads)-like enzyme that acts on C18 PUFA precursors at the initiation of the LC-PUFA biosynthesis (Fig. 1) was responsible for the desaturation of 24:5n−3 required in the Sprecher pathway[20, 22]
Our results show that the two pathways of Docosahexaenoic acid (DHA) biosynthesis, namely the Sprecher and ∆4 pathways, co-exist within some species such as S. canaliculatus and C. estor since, in addition to the role of their ∆6∆5 Fads[2] in the Sprecher pathway uncovered in the present study, the existence of ∆4 desaturases in their genomes potentially enables them to further operate via the ∆4 pathway[27, 29]
Summary
Long chain (≥C20) polyunsaturated fatty acids (LC-PUFA) including arachidonic acid (ARA, 20:4n−6), eicosapentaenoic acid (EPA, 20:5n−3) and docosahexaenoic acid (DHA, 22:6n−3) play numerous physiologically important roles essential to health in humans[1, 2]. The biosynthesis of C20–22 LC-PUFA in vertebrates including fish involves alternating steps of desaturation and elongation of the dietary essential C18 fatty acids (FA), LOA and ALA. The “∆4 pathway”, first described in the marine protist Thraustochytrium sp.[26], is a more direct pathway involving one single elongation of EPA to docosapentaenoic acid (DPA, 22:5n−3), which is subsequently desaturated at the ∆4 position to produce DHA. We have taken advantage of the known key amino acid (aa) residues determining Δ4 desaturase ability of Fads[34] to identify teleost taxa, with publically available genomic or transcriptomic databases, in which their desaturase repertoire enables them to biosynthesise DHA through the more direct Δ4 pathway
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