Role of respiratory supercomplexes : Electronic connection between complexes III and IV

Sammanfattning: In the final step of cellular respiration, electrons are transferred through the respiratory chain to reduce molecular oxygen to water. The energy released in this chain is used to maintain a proton electrochemical gradient across the cell membrane, which is used, for example, by the ATP synthase to produce ATP. The enzyme complexes of the respiratory chain are known to organize in supramolecular assemblies, so-called respiratory supercomplexes.In this work we investigated the functional significance of respiratory supercomplexes consisting of complexes III and IV in mitochondria. By combining structural and kinetic studies we showed that at the commonly assumed "physiological" ionic strength of 150 mM monovalent salt, the water-soluble cyt. c associates with the negatively charged surface of III2-IV1-2 supercomplexes in the yeast species Saccharomyces cerevisiae and Schizosaccharomyces pombe. The data showed that one cyt. c diffuses in 2D, between complexes III and IV, indicating a kinetic advantage of forming supercomplexes. These studies also showed different relative orientation of the individual complexes in the supercomplexes from the two yeast species, indicating that 2D diffusion is a general mechanism, not limited to a specific relative orientation of complexes III and IV. More recent data in the literature indicate that a more realistic mimic of intracellular conditions is a monovalent salt concentration of 20 mM. We showed that under these conditions two cyt. c molecules bind simultaneously to the supercomplex. This result further supports a kinetic advantage of forming supercomplexes.We also determined the cryo-EM structure of the obligate III2-IV2 supercomplex from the Gram-positive bacterium Corynebacterium glutamicum. The structure revealed an electronic connection between complexes III and IV by a di-heme cyt. cc subunit. The structure also showed that complexes III and IV are structurally intertwined and strongly connected with unique features conserved in the phylum actinobacteria.