Two alternative pathways for docosahexaenoic acid (DHA, 22:6n-3) biosynthesis are widespread among teleost fish

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.

A complete enzymatic capacity for long-chain polyunsaturated fatty acid biosynthesis is present in the Amazonian teleost tambaqui, Colossoma macropomum

“…Unlike Elovl5 though, the tambaqui Elovl2 had efficiency to operate towards C22 substrates. For instance, 22:5n-3 was efficiently elongated to 24:5n-3, a key intermediate component of the so-called "Sprecher pathway", a metabolic route that accounts for DHA biosynthesis in vertebrates (Buzzi et al, 1996(Buzzi et al, , 1997Sprecher, 2000) and recently demonstrated to be widespread among teleosts (Oboh et al, 2017).…”

“…Yeast were transformed with the C. macropomum fads2 ORF and grown in the presence of Δ6 (18:2n-6 and 18:3n-3), Δ8 (20:2n-6 and 20:3n-3), Δ5 (20:3n-6 and 20:4n-3), and Δ4 (22:4n-6 and 22:5n-3) fatty acid (FA) substrates. Desaturation of 24:5n-3 was tested by co-expressing both D. rerio elovl2 and C. macropomum fads2 in S. cerevisiae (Oboh et al, 2017) Expressed with D. rerio elovl2 18:3n-3 18:4n-3 20.5 Δ6 24:5n-3 24:6n-3 21.9 Δ6 a Conversions of Δ8 substrates (20:2n-6 and 20:3n-3) include stepwise reactions due to multifunctional desaturation abilities. Thus, the conversions of the C. macropomum Fads2 on 20:2n-6 and 20:3n-3 include the Δ8 desaturation toward 20:3n-6 and 20:4n-3, respectively, and their subsequent Δ5 desaturations to 20:4n-6 and 20:5n-3, respectively.…”

“…While the majority of teleost Fads2 enzymes functionally characterized to date present Δ6 desaturase activity (Zheng et al, 2004González-Rovira et al, 2009;Mohd-Yusof et al, 2010;Monroig et al, 2013Monroig et al, , 2010aKabeya et al, 2018), an increasingly high number of teleost Fads2 have alternative desaturase substrate specificities including both Δ6 and Δ5 activities within the same enzyme (Hastings et al, 2001;Tanomman et al, 2013;Kuah et al, 2016;Oboh et al, 2016), Δ5 (Abdul Hamid et al, 2016) and Δ4, the latter often exhibiting some residual activity as Δ5 desaturase (Li et al, 2010;Morais et al, 2012;Fonseca-Madrigal et al, 2014;Oboh et al, 2017). Consistent with the mammalian FADS2 (Park et al, 2009), fish Fads2 typically possess Δ8 desaturation capacity (Monroig et al, 2011a), enabling the "Δ8 pathway", an alternative route to the most prominent "Δ6 pathway" leading to the biosynthesis of 20:3n-6 and 20:4n-3, immediate precursors of ARA and EPA, respectively .…”

Comparison of Fatty Acid Contents in Major Lipid Classes of Seven Salmonid Species from Siberian Arctic Lakes

“…In CCA biplots of total lipids, PL and TAG, charr had a particular position due to higher levels of minor n-3 PUFA, like 22:4-3, C24 PUFA and 20:3n-3. These FA were not assigned as trophic markers, whereas some of them, C24 PUFA, were considered as intermediate compounds indicative for conversion of C20 to C22 PUFA [34,35]. In TAG of charr the percentage of 22:4n-3 accounted for 1.3% of FA sum, being absent or found in traces in other studied fish.…”

Functional characterization of fatty acyl desaturase Fads2 and Elovl5 elongase in the Boddart’s goggle-eyed goby,Boleophthalmus boddarti(Gobiidae) suggest an incapacity for long-chain polyunsaturated fatty acid biosynthesis

“…As for Fads2, majority of the Teleostei groups not only retain the Δ6 capacity but undergo functionalisation for Δ4, Δ5 and Δ8 desaturation capabilities (Zheng et al, 2004, Zheng et al, 2009a, Tanomman et al, 2013, Fonseca-Madrigal et al, 2014). Many of the characterised Fads2 sequences also have bifunctional desaturation activities (Monroig et al, 2011, Kabeya et al, 2015, Oboh et al, 2017). The Fads from the four mudskipper species shares a common ancestor with Fads2 of various percomorpha fishes.…”