Biosynthesis of long-chain polyunsaturated fatty acids in the nereid polychaete Hediste diversicolor

Póster presentado en la International Conference & Exposition Aquaculture Europe, celebrada en Funchal, Maderia (Portugal) del 04 al 07 de octubre de 2021[Introduction]: Aquaculture of carnivorous species is greatly dependent on the supply of marine ingredients, which provide adequate levels of essential nutrients including long-chain (C20¿24) polyunsaturated fatty acids (LC-PUFAs), particularly the ¿omega-3¿ (¿3) including eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3). Primary production of these compounds was thought to take place exclusively by the action of photosynthetic microalgae, heterotrophic protists and bacteria. However, a study has revealed that multiple aquatic invertebrates including polychaetes possess enzymes that enable them to produce ¿3 LC-PUFAs de novo and, consequently, their culture with an adequate supply of biosynthetic fatty acid (FA) precursors and enhancing such pathways emerges as a highly innovative strategy to produce nutritious and sustainably sourced ingredients for aquaculture. The capacity by which an animal species can biosynthesise LC-PUFAs depends upon the complement and function of two types of enzymes, namely fatty acyl elongases and desaturases. The aim of the present study was to characterise molecularly and functionally two elongases (Elo) and one front-end desaturase (Fed) involved in the LC-PUFA biosynthesis of the common ragworm Hediste diversicolor. Our results show that H. diversicolor has at least two Elo and one Fed with putative roles in the LC-PUFA biosynthetic pathways. We termed them as Elovl2/5 and Elovl4 based on their phylogenetic relationship with well characterised elongases from other animals. Functional characterisation results showed that the H. diversicolor Elovl2/5 is indeed a PUFA elongase since elongated PUFA substrates with C18 and C20. Similarly, the H. diversicolor Elovl4 showed elongation capacity towards C18 and C20 PUFA substrates but, additionally, C22 PUFA substrates. Functional characterisation of the H. diversicolor Fed1 confirmed its role in the LC¿PUFA biosynthesis as it showed this enzyme has ¿5 activity converting 20:3n-6 and 20:4n-3 into 20:4n-6 (arachidonic acid) and 20:5n-3 (EPA), respectively. No activities as ¿4, ¿6 or ¿8 were detected . Along PUFA substrates, the H. diversicolor Fed1 also had ¿5 activity towards saturated substrates such as 18:0, which was converted to 18:1n-13. The present work demonstrates that H. diversicolor possess two distinct elongase genes, namely Elovl2/5 and Elovl4, with putative roles in the biosynthesis of LC-PUFAs. Molecular and functional characterisation of the Elovl sequences from H. diversicolor revealed that Elovl2/5 have C18 and C20 PUFA as preferred substrates for elongation, that is very common in invertebrates. The Elovl4 is able to elongate C22 substrates to C24 products, and thus compensates for the lack of C22 activity within Elovl2/5, as occurs in mollusc [4]. Functional characterisation of the H. diversicolor Fed1 ¿5 confirm its role in LC¿PUFA biosynthesis. ¿5 desaturases have been previously demonstrated in some molluscs as well as echinoderms. The herein reported results on the functions of the Elovl2/5, Elovl4 and ¿5 desaturase, along with those of the previously characterised methyl-end desaturases, clearly show that H. diversicolor has a highly diverse repertoire of LC-PUFA biosynthesising genes with complementary activities enabling an active production of these essential compounds.This study was funded through the project IMPROMEGA of the Ministry of Science, Innovation and Universities, Spanish Government (RTI2018-095119-B-100, MCIU/AEI/FEDER/UE)Peer reviewe