Assembly of electron transport chain components in bovine mitochondrial supercomplex I1III2IV1

The respiratory chain in the inner mitochondrial membrane contains the large redox complexes NADH-dehydrogenase, cytochrome c-reductase and cytochrome c-oxidase that utilize the energy from electron transfer to establish a proton gradient across the membrane that subsequently fuels ATP synthesis by the ATP synthase. At least partially they assemble into stable supercomplexes. A 3D reconstruction of the 1.7 MDa amphipol-solubilized supercomplex I₁III₂IV₁ from bovine heart, composed of complex I, complex III and complex IV at 19 Å resolution was obtained by single particle cryo-EM. It represents a native state and shows details that were not visible in a previous negative stain model. The amphipols form a belt around the transmembrane region replacing the lipid bilayer. Clearly discernible domains enabled a precise fit of the X-ray structures of the individual complexes into the density map. The binding sites for the mobile electron carriers ubiquinol and cytochrome c are oriented towards each other and separated by short distances. Lipids between the protein complexes integrate the carriers into the supercomplex so that all electron transfer components are present. Tracing the pathway of the electrons in the supercomplex from their entry site to the terminal electron acceptor supports the solid state model of the respiratory chain with a directed substrate transfer over short distances. Of the possible pathways the shorter with less transfer reactions seems to be preferred.