A comprehensive assessment of the biosynthetic pathways of ascorbate, alpha-tocopherol and free amino acids in Euglena gracilis var. saccharophila

Euglena gracilis produces several important health-enhancing metabolites including ascorbate, α-tocopherol and free amino acids (faa). The yield of metabolites is dependent on the strain of E. gracilis and the metabolic growth condition. Here we investigated the effects of photoautotrophic (PT), mixotrophic (MT) and heterotrophic (HT) cultivation on the synthesis of ascorbate, α-tocopherol and faa in E. gracilis var. saccharophila, using label-free shotgun proteomics, and metabolite analysis using colourimetric assay, high-performance and ultra-performance liquid chromatography (HPLC/UPLC). PT cultivation resulted in the production of more antioxidants (up to 4.13 mg g^{− 1} ascorbate and 2.52 mg g^{− 1} α-tocopherol) than the MT and HT growth conditions (up to 0.97 and 0.50 mg g^{− 1} ascorbate, and 1.40 and 0.21 mg g^{− 1} α-tocopherol, respectively). The relative abundance of several faa varied between mid-log and initial stationary growth phases, but the total amount of faa remained about the same, with arginine as the most abundant amino acid. Proteomic analysis revealed a total of 3843 non-redundant proteins in E. gracilis var. saccharophila, of which 1890 were common among all cultivations. Gene ontology annotations suggested derivatisation of metabolic pathways from different organisms, such as lysine biosynthesis from fungi and serine biosynthesis from plants, while a few pathways were unique to Euglena, such as those of ascorbate and arginine. Some enzymes exhibited several isoforms that were influenced by the metabolic growth condition. For example, one of the isozymes of threonine aldolase was expressed in HT/MT cultures only, and one of the isozymes of phosphoglycerate dehydrogenase was expressed in PT cultures only. This is the first proteomic study of E. gracilis var. saccharophila, which provides a mechanistic insight into the biosynthetic pathway dynamics of primary metabolites (antioxidants and faa). This new information can serve as a framework for further development of Euglena as a producer of nutraceuticals.