Identification and characterization of cytosolic fructose-1,6-bisphosphatase in <i>Euglena gracilis</i>

<p><i>Euglena gracilis</i> has the ability to accumulate a storage polysaccharide, a β-1,3-glucan known as paramylon, under aerobic conditions. Under anaerobic conditions, <i>E. gracilis</i> cells degrade paramylon and synthesize wax esters. Cytosolic fructose-1,6-bisphosphatase (FBPase) appears to be a key enzyme in gluconeogenesis and position branch point of carbon partitioning between paramylon and wax ester biosynthesis. We herein identified and characterized cytosolic FBPase from <i>E. gracilis</i>. The <i>K</i>_m and <i>V</i>_max values of EgFBPaseIII were 16.5 ± 1.6 μM and 30.4 ± 7.2 μmol min⁻¹ mg protein⁻¹, respectively. The activity of EgFBPaseIII was not regulated by AMP or reversible redox modulation. No significant differences were observed in the production of paramylon in transiently suppressed <i>EgFBPaseIII</i> gene expression cells by RNAi (KD-<i>EgFBPaseIII</i>); nevertheless, FBPase activity was markedly decreased in KD-<i>EgFBPaseIII</i> cells. On the other hand, the growth of KD-<i>EgFBPaseIII</i> cells was slightly higher than that of control cells.</p> <p>EgFBPaseIII, a cytosolic FBPase, may play an important role in carbon partitioning between paramylon and wax ester biosynthesis in <i>Euglena gracilis</i>.