Major pathways of energy metabolism in mitochondria of the free-living annelid polychaete Arenicola marina (lugworm or sandworm). Major pathways of energy metabolism in mitochondria of the free-living annelid polychaete Arenicola marina (lugworm or sandworm). The worm inhabits intertidal sediments and produces ATP predominantly by an aerobic respiratory chain during high tide. At low tide, it is exposed for several hours to hypoxia (445) and also has to deal with high concentrations of H2S (up to 2 mM), which is a potent inhibitor of respiratory complex IV (495, 496). A. marina protects itself intrinsically by means of membrane-bound flavoprotein sulfide:quinone oxidoreductase, which oxidizes hydrogen sulfide and transfers the electrons to the quinone pool. The generation of the final oxidized sulfur species, thiosulfate, can occur in an oxygen-dependent manner in cooperation with a sulfur dioxygenase and a sulfur transferase (189). The animal can also reduce fumarate in anaerobic respiration by utilizing rhodoquinone (531) and can produce acetate and ATP via substrate-level phosphorylation. The gray shading of lines is described in the legend of Fig. 4; the light gray rectangle highlights metabolic pathways preferentially employed during the early phase, as described in the legend of Fig. 6. Abbreviations: CI to CIV, respiratory complexes I to IV; UQ, ubiquinone; RQ, rhodoquinone; C, cytochrome c; A, ATPase; FRD, fumarate reductase; SQR, sulfide:quinone oxidoreductase; ST, sulfur transferase; SD, sulfur dioxygenase; [2B], acetate:succinate CoA-transferase (subfamily 1B); [3], succinyl-CoA synthetase; [5], malic enzyme; [6], pyruvate dehydrogenase complex; [8], fumarase; [9], pyruvate kinase; [10], phosphoenolpyruvate carboxykinase (ATP dependent); [11], malate dehydrogenase; [14], methylmalonyl-CoA mutase; [15], methylmalonyl-CoA epimerase; [16], propionyl-CoA carboxylase; [18], alanine aminotransferase; [19], aspartate aminotransferase; [20], alanopine dehydrogenase; [26], strombine dehydrogenase. The photograph shows Arenicola marina at the adult stage, with a length of ca. 20 cm. (Photograph by Auguste Le Roux [http://commons.wikimedia.org/wiki/File:Arenicola_marina.JPG] [Wikimedia Commons].)‏ Miklós Müller et al. Microbiol. Mol. Biol. Rev. 2012; doi:10.1128/MMBR.05024-11