Enzymatic basis underlying the biosynthesis of Δ7-desaturated C₂₂ polymethylene-interrupted fatty acids across marine...

Polymethylene-interrupted fatty acids (PMI-FA) represent a structurally unusual class of polyunsaturated fatty acids characterised by double bonds separated by two or more methylene groups. Among them, Δ7-desaturated C₂₂ PMI-FA are widely detected in marine invertebrates. Previous metabolic studies and fatty acid compositional data suggest that these compounds are produced via elongation of Δ5-desaturated C₂₀ PMI-FA, but the specific enzymes and molecular mechanisms responsible for this pathway remain unclear. In this study, we investigated the enzymatic basis underlying the biosynthesis of Δ7-desaturated C₂₂ PMI-FA across marine invertebrates. Candidate Δ5 desaturases (FadsA) and fatty acid elongases (Elovl) were identified from representative species of echinoderms, molluscs and annelids, and their activities were functionally characterised using a heterologous yeast expression system. By reconstituting the sequential Δ5 desaturation-elongation steps through co-expression of FadsA and Elovl enzymes, we demonstrate that invertebrate enzymes possess the capability to convert common C₂₀ fatty acid precursors (20:1n-7, 20:1n-9, 20:2n-6, and 20:3n-3) into C₂₂ PMI-FA. Our results provide direct functional support for proposed biosynthetic pathways of Δ7-desaturated C₂₂ PMI-FA and reveal substantial interspecific variation in biosynthetic capacity among marine invertebrate lineages. These findings establish a molecular framework for understanding the widespread occurrence of PMI-FA in marine invertebrates and highlight the evolutionary flexibility and pleiotropy of fatty acid biosynthetic enzymes. Collectively, this study advances current knowledge of fatty acid biosynthesis beyond canonical ω3 and ω6 pathways and expands our understanding of fatty acid biosynthetic diversity in marine invertebrates.