Characterization of fatty acid biosynthesis in microalga Scenedesmus - from the perspective of biofuel production

Scenedesmus quadricauda, a freshwater microalga, has gained attention for its high lipid accumulation potential. However, information on fatty acid (FA) biosynthesis pathways in Scenedesmus species remains limited. Biomass (1.010 gL⁻¹) and lipid content (404 mgL⁻¹) in S. quadricauda were found to be higher compared to other microalgal isolates under autotrophic nutrition. All biodiesel indices were found within the defined range of biodiesel (EN14214) and petro-diesel (EN590:2013) standards. The predominant fatty acid was palmitic acid (16:0), accounting for 33.201 % of the total. Further, homology study and 3D models of all subunits of S. quadricauda enzymes acetyl-CoA carboxylase (ACC) – the key enzyme of the FA biosynthetic pathway - identified distinct structural features. The biotin carboxylase (BC), biotin carboxyl carrier protein (BCCP), β-carboxyltransferases (β-CT) subunits of the heteromeric ACC of S. quadricauda showed homology with its eukaryotic counterparts, whereas the α -carboxyltransferases (α-CT) subunit showed homology with prokaryotic carboxyltransferase. In contrast, homologous enzymes in other Scenedesmus species were found to resemble prokaryotic forms exclusively. Conserved motifs such as the glycine-rich loop, ERYV motif, and AAAP motif were identified in the BC and BCCP enzymes. Malonyl-CoA:ACP transacylase (MAT) enzyme of S. quadricauda was of prokaryotic origin but showed structural divergence from its homologs in other Scenedesmus species. Fatty-acyl thioesterases (FAT) enzyme contained a duplication of two 4-hydroxybenzoyl-CoA thioesterase-like domains (4HBT). These unique sequences and binding sites in the fatty acid biosynthesis enzymes of S. quadricauda may contribute to the distinct regulation of carbon flux and lipid assembly compared to other species.