Wax ester fermentation in E. gracilis.

Under anaerobic conditions, mitochondrial wax ester fermentation proceeds through two routes: the C2-donor route supplies acetyl-CoA via the pyruvate:NADP⁺ oxidoreductase (PNO) reaction, and the C3-donor route includes anaerobic fumarate respiration to produce propionyl-CoA. The ¹³C-isotope from ¹³CO₂ (red circle) is incorporated into phosphoenolpyruvate (PEP) by the PEPCK reaction, and it is retained in propionyl-CoA that is used as the C3-donor in wax ester fermentation. The metabolic flow functioning under anaerobic conditions is indicated by green arrows [8, 15, 29, 31]. Another possible route for the carboxylation reaction is indicated by broken green arrows: α-ketoglutarate (α-KG) is carboxylated to yield citrate which ultimately serves as the precursor to produce oxaloacetate (OAA) in cytoplasm. Abbreviations for key enzymes: ACL, ATP citrate lyase; FUM, fumarase; FRD, fumarate reductase; IDH*, isocitrate dehydrogenase; KAT, ketoacyl-CoA thiolase; α-KGDH, α-ketoglutarate decarboxylase; MDH, malate dehydrogenase; ME, malic enzyme; MMC, methylmalonyl-CoA mutase; PK, pyruvate kinase; PNO, pyruvate:NADP⁺ oxidoreductase; PrCC, propionyl-CoA carboxylase; SSDH, succynyl semialdehyde dehydrogenase; RQ, rhodoquinone; SCS, succinyl-CoA synthetase. Abbreviations for metabolic intermediates: Cit, citrate; Fum, fumarate; Isocit, isocitorate; Mal, malate; OAA, oxaloacetate; Pyr, pyruvate; Suc, succinate; SucSA, succinate semialdehyde. *IDH has not been known in Euglena to produce isocitrate from α-KG.