Enzymes enabling the biosynthesis of various C₂₀polyunsaturated fatty acids in a sea urchinHemicentrotus pulcherrimus

Sea urchins, integral to marine ecosystems and valued as a delicacy in Asia and Europe, contain physiologically important long-chain (>C 20 ) polyunsaturated fatty acids (PUFA) in their gonads, including arachidonic acid (ARA, 20:4n-6), eicosapentaenoic acid (EPA, 20:5n-3), and unusual non-methylene interrupted fatty acids (NMI-FA) such as 20:2 Δ5,11 . Although these fatty acids may partially be derived from their diet, such as macroalgae, the present study on Hemicentrotus pulcherrimus has uncovered multiple genes encoding enzymes involved in LC-PUFA biosynthesis. Specifically, three fatty acid desaturases (FadsA, FadsC1 and FadsC2) and thirteen elongation of very-long chain fatty acids proteins (Elovl-like, Elovl1/7-like, Elovl2/5-like, Elovl4-like, Elovl8-like, Elovl6-like A to H) were identified in their genome and transcriptomes. Functional analysis showed that FadsA and FadsC2 function as a Δ5 desaturase and a Δ8 desaturase, respectively, enabling the conversion of 18:2n-6 and 18:3n-3 into ARA and EPA, respectively, along with Elovl, particularly Elovl6-like C. Elovl6-like C demonstrates elongase activity towards both C 18 PUFA and monounsaturated fatty acids (MUFA). Consequently, FadsA and Elovl6-like C enable to synthesise several NMI-FA, including 20:2 Δ5,11 and 20:3 Δ5,11,14 , from C 18 precursors. This indicates that H. pulcherrimus can endogenously synthesise a wide variety of C 20 PUFA and NMI-FA, highlighting active biosynthesis pathways within sea urchins.