Biosynthesis of long-chain polyunsaturated fatty acids in the razor clam Sinonovacula constricta: characterization of four fatty acyl elongases and a novel desaturase capacity

As an unusual economically important aquaculture species, Sinonovacula constricta possesses high levels of long-chain polyunsaturated fatty acids (LC-PUFA). Previously, our group identified fatty acyl desaturases (Fad) with Δ5 and Δ6 activities in S. constricta, which was the first report of Δ6 Fad in a marine mollusc. Here, we further successfully characterize elongases of very long-chain fatty acids (Elovl) in this important bivalve species, including one Elovl2/5, two Elovl4 isoforms (a and b) and a novel Elovl (c) with Elovl4 activity. In addition, we also determined the desaturation activity of S. constricta Δ6 Fad toward 24:5n-3 to give 24:6n-3, a key intermediate in docosahexaenoic acid (DHA) biosynthesis. Therefore, S. constricta is the first marine mollusc reported to possess all Fad and Elovl activities required for LC-PUFA biosynthesis via the ‘Sprecher pathway’. This finding greatly increases our understanding of LC-PUFA biosynthesis in marine molluscs. Phylogenetic analysis by interrogating six marine molluscan genomes, and previously functionally characterized Elovl and Fad from marine molluscs, suggested that DHA biosynthetic ability was limited to a few species, due to the general lack of Δ4 or Δ6 Fad in most molluscs

Biosynthesis of Polyunsaturated Fatty Acids in the Razor Clam <i>Sinonovacula constricta</i>: Characterization of Δ5 and Δ6 Fatty Acid Desaturases

To investigate the endogenous long-chain polyunsaturated fatty acid (LC-PUFA) biosynthetic ability in <i>Sinonovacula constricta</i>, fatty acid desaturases (Fads) of this bivalve, namely, <i>Scfad5a</i>, <i>Scfad5b</i>, and <i>Scfad6</i>, were cloned and characterized in the current study. Meanwhile, the tissue distributions of <i>S. constricta</i> Fads and fatty acids (FAs) were examined. Heterologous expression in yeasts confirmed that <i>Scfad5a</i> and <i>Scfad5b</i> were both Δ5 Fads, while <i>Scfad6</i> was a Δ6 Fad. However, compared with Fads in other organisms, the desaturation activities of <i>S. constricta</i> Fads were relatively low (especially for <i>Scfad6</i>), indicating an adaptation to living conditions. <i>S. constricta</i> Fads were expressed in all tissues examined, and particularly high expressions were found in intestine and gonad. Moreover, FAs were differently distributed among tissues, which might be correlated with their corresponding physiological roles. Taken together, the results provided an insight into LC-PUFA biosynthesis in <i>S. constricta</i>. Notably, <i>Scfad6</i> was the first functionally characterized Δ6 Fad in marine molluscs to date