Major pathways of energy metabolism in mitochondria of the peanut worm, Sipunculus nudus. Major pathways of energy metabolism in mitochondria of the peanut worm, Sipunculus nudus. The map is redrawn based on data reported previously (169). This free-living worm inhabits sandy marine sediments ranging from intertidal to subtidal zones down to a 900-m depth, where it often encounters hypoxia. S. nudus deals with low oxygen by using the same pathways as those found in other marine invertebrates facing hypoxic or anoxic conditions. Fumarate serves as the terminal electron acceptor in the anaerobic branch of the respiratory chain and enables the synthesis of ATP not only via oxidative phosphorylation but also through propionate production from succinate. Additional ATP can be generated from succinyl-CoA in the pyruvate degradation pathway, leading to acetate as an end product (169). The gray shading of lines is described in the legend of Fig. 4. Abbreviations: CI to CIV, respiratory complexes I to IV; UQ, ubiquinone; RQ, rhodoquinone; C, cytochrome c; A, ATPase; FRD, fumarate reductase; [2B], acetate:succinate CoA-transferase (subfamily 1B); [3], succinyl-CoA thiokinase; [5], malic enzyme; [6], pyruvate dehydrogenase complex; [8], fumarase; [9], pyruvate kinase; [11], malate dehydrogenase; [14], methylmalonyl-CoA mutase; [15], methylmalonyl-CoA epimerase; [16], propionyl-CoA carboxylase; [17], phosphoenolpyruvate carboxykinase (ITP/GTP dependent); [18], glutamate:pyruvate transaminase; [19], glutamate:oxaloacetate transaminase; [26], strombine dehydrogenase; [27], octopine dehydrogenase. The photograph shows Sipunculus nudus at the adult stage, with a length of ca. 15 cm. (Photograph courtesy of Matt du Fort.)‏ Miklós Müller et al. Microbiol. Mol. Biol. Rev. 2012; doi:10.1128/MMBR.05024-11