Supercomplexes of the respiratory chain are established constituents of the oxidative phosphorylation system, but their role in mammalian metabolism has been hotly debated. Although recent studies have shown that different tissues/organs are equipped with specific sets of supercomplexes, depending on their metabolic needs, the notion that supercomplexes have a role in the regulation of metabolism has been challenged. However, irrespective of the mechanistic conclusions, the composition of various high molecular weight supercomplexes remains uncertain. Here, using cryogenic electron microscopy, we demonstrate that mammalian (mouse) tissues contain three defined types of ‘respirasome’, supercomplexes made of CI, CIII₂ and CIV. The stoichiometry and position of CIV differs in the three respirasomes, of which only one contains the supercomplex-associated factor SCAF1, whose involvement in respirasome formation has long been contended. Our structures confirm that the ‘canonical’ respirasome (the C-respirasome, CICIII₂CIV) does not contain SCAF1, which is instead associated to a different respirasome (the CS-respirasome), containing a second copy of CIV. We also identify an alternative respirasome (A-respirasome), with CIV bound to the ‘back’ of CI, instead of the ‘toe’. This structural characterization of mouse mitochondrial supercomplexes allows us to hypothesize a mechanistic basis for their specific role in different metabolic conditions.

呼吸链的超级复合物是氧化磷酸化系统的既定组成部分，但它们在哺乳动物代谢中的作用一直备受争议。尽管最近的研究表明，不同的组织/器官根据其代谢需求配备有特定的超级复合物组，但超级复合物在代谢调节中发挥作用的观点受到了挑战。然而，无论机理结论如何，各种高分子量超复合物的组成仍然不确定。在这里，我们使用低温电子显微镜证明哺乳动物（小鼠）组织含有三种定义类型的“呼吸体”，即由 CI、CIII ₂和 CIV 组成的超级复合物。三种呼吸体中 CIV 的化学计量和位置不同，其中只有一种含有超复合物相关因子 SCAF1，长期以来人们一直争论其参与呼吸体形成。我们的结构证实“规范”呼吸体（C-呼吸体，CICIII ₂ CIV）不包含 SCAF1，而是与包含 CIV 的第二个副本的不同呼吸体（CS-呼吸体）相关。我们还确定了另一种呼吸体（A-呼吸体），其中 CIV 结合在 CI 的“背部”，而不是“脚趾”。小鼠线粒体超复合物的这种结构特征使我们能够假设它们在不同代谢条件下的特定作用的机制基础。