Biochemical and Functional Profiling of Thioredoxin-Dependent Cytosolic GPX-like Proteins in Euglena gracilis .

Unlike plants and animals, the phytoflagellate Euglena gracilis lacks catalase and contains a non-selenocysteine glutathione peroxidase-like protein (EgGPXL), two peroxiredoxins (EgPrx1 and EgPrx4), and one ascorbate peroxidase in the cytosol to maintain reactive oxygen species (ROS) homeostasis. In the present study, the full-length cDNA of three cytosolic EgGPXLs was obtained and further characterized biochemically and functionally. These EgGPXLs used thioredoxin instead of glutathione as an electron donor to reduce the levels of H 2 O 2 and t -BOOH. The specific peroxidase activities of these enzymes for H 2 O 2 and t -BOOH were 1.3 to 4.9 and 0.79 to 3.5 µmol/min/mg protein, respectively. Cytosolic EgGPXL s and EgPrx1 / EgPrx4 were silenced simultaneously to investigate the synergistic effects of these genes on the physiological function of E. gracilis . The suppression of cytosolic EgGPXL genes was unable to induce any critical phenomena in Euglena under normal (100 μmol photons m -2 s -1 ) and high-light conditions (350 μmol photons m -2 s -1 ) at both autotrophic and heterotrophic states. Unexpectedly, the suppression of EgGPXL genes was able to rescue the EgPrx1/EgPrx4 -silenced cell line from a critical situation. This study explored the potential resilience of Euglena to ROS, even with restriction of the cytosolic antioxidant system, indicating the involvement of some compensatory mechanisms.