For electrons to continuously enter and flow through the mitochondrial electron transport chain (ETC), they must ultimately land on a terminal electron acceptor (TEA), which is known to be oxygen in mammals. Paradoxically, we find that complex I and dihydroorotate dehydrogenase (DHODH) can still deposit electrons into the ETC when oxygen reduction is impeded. Cells lacking oxygen reduction accumulate ubiquinol, driving the succinate dehydrogenase (SDH) complex in reverse to enable electron deposition onto fumarate. Upon inhibition of oxygen reduction, fumarate reduction sustains DHODH and complex I activities. Mouse tissues display varying capacities to use fumarate as a TEA, most of which net reverse the SDH complex under hypoxia. Thus, we delineate a circuit of electron flow in the mammalian ETC that maintains mitochondrial functions under oxygen limitation.

中文翻译：

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富马酸盐是哺乳动物电子传递链中的末端电子受体

为了让电子持续进入并流过线粒体电子传递链 (ETC)，它们最终必须降落在末端电子受体 (TEA) 上，在哺乳动物中已知它是氧气。矛盾的是，我们发现当氧还原受阻时，复合物 I 和二氢乳清酸脱氢酶 (DHODH) 仍然可以将电子沉积到 ETC 中。缺乏氧还原的细胞会积累泛醇，从而反向驱动琥珀酸脱氢酶 (SDH) 复合物，从而使电子沉积到富马酸盐上。在抑制氧还原后，富马酸盐还原维持 DHODH 和复合物 I 的活性。小鼠组织显示出使用富马酸盐作为 TEA 的不同能力，其中大多数在缺氧条件下净逆转 SDH 复合物。因此，