NADPH-to-NADH conversion by mitochondrial transhydrogenase is indispensable for sustaining anaerobic metabolism in Euglena gracilis

Euglena gracilis produces ATP in the anaerobic mitochondria with concomitant wax ester formation, and NADH is essential for ATP formation and fatty acid synthesis in the mitochondria. This study demonstrated that mitochondrial cofactor conversion by nicotinamide nucleotide transhydrogenase (NNT), converting NADPH/NAD(+) to NADP(+)/NADH, is indispensable for sustaining anaerobic metabolism. Silencing of NNT genes significantly decreased wax ester production and cellular viability during anaerobiosis but had no such marked effects under aerobic conditions. An analogous phenotype was observed in the silencing of the gene encoding a mitochondrial NADP(+)-dependent malic enzyme. These results suggest that the reducing equivalents produced in glycolysis are shuttled to the mitochondria as malate, where cytosolic NAD(+) regeneration is coupled with mitochondrial NADPH generation.

Automated summary

This study revealed the crucial role of NNT in ATP production and wax ester formation in Euglena gracilis during anaerobic metabolism, with silencing of NNT genes leading to significant decreases in these processes. Additionally, a similar phenotype was observed when the gene encoding mitochondrial NADP(+)dependent malic enzyme was silenced.