PUFA Biosynthesis Pathway in Marine Scallop Chlamys nobilis Reeve

Long-chain polyunsaturated fatty acids (LC-PUFAs) are essential in important physiological processes. However, the endogenous PUFA biosynthesis pathway is poorly understood in marine bivalves. Previously, a fatty acyl desaturase (Fad) with Δ5 activity was functionally characterized and an elongase termed Elovl2/5 was reported to efficiently elongate 18:2n–6 and 18:3n–3 to 20:2n–6 and 20:3n–3 respectively in Chlamys nobilis. In this study, another elongase and another Fad were identified. Functional characterization in recombinant yeast showed that the newly cloned elongase can elongate 20:4n–6 and 20:5n–3 to C22 and C24, while the newly cloned scallop Fad exhibited a Δ8-desaturation activity, and could desaturate exogenously added PUFA 20:3n–3 and 20:2n–6 to 20:4n–3 and 20:3n–6 respectively, providing the first compelling evidence that noble scallop could de novo biosynthesize 20:5n–3 and 20:4n–6 from PUFA precursors though the “Δ8 pathway”. No Δ6 or Δ4 activity was detected for this Fad. Searching against our scallop transcriptome database failed to find any other Fad-like genes, indicating that noble scallop might have limited ability to biosynthesize 22:6n–3. Interestingly, like previously characterized Elovl2/5, the two newly cloned genes showed less efficient activity toward n–3 PUFA substrates than their homologous n–6 substrates, resulting in a relatively low efficiency to biosynthesize n–3 PUFA, implying an adaption to marine environment.