Questioning the functional relevance of mitochondrial supercomplexes by time-resolved analysis of the respiratory chain

Abstract

Mitochondria are the powerhouses of eukaryotic cells as they feed metabolism with its major substrate. Oxidative-phosphorylation relies on the generation, by an electron/proton transfer chain, of an electrochemical transmembrane potential utilized to synthesize ATP. Although these fundamental principles are not a matter of debate, the emerging picture of the respiratory chain diverges from the linear and fluid scheme. Indeed, a growing number of pieces of evidence point to membrane compartments that possibly restrict the diffusion of electron carriers, and to supramolecular assembly of various complexes within various kinds of supercomplexes that modulate the thermodynamic and kinetic properties of the components of the chain. Here, we describe a method that allows the unprecedented time-resolved study of the respiratory chain in intact cells that is aimed at assessing these hypotheses. We show that, in yeast, cytochrome c is not trapped within supercomplexes and encounters no particular restriction to its diffusion which questions the functional relevance of these supramolecular edifices.bioenergetics | electon transfer | respiration | compartmentalization

Mitochondrial Respiratory Supercomplex Association Limits Production of Reactive Oxygen Species from Complex I

“…Kinetic evidence in favor of Complex III and Complex IV forming a functional supercomplex is mostly obtained in yeast, whereas many conclusions for cytochrome c channeling in mammalian mitochondria are based on assumptions driven by the observation that a fraction of molecules appears physically associated in the respirasome. On the contrary, some studies by metabolic flux control (8) and time-resolved analysis of the respiratory chain (76) have questioned the functional role of mammalian supercomplexes in the transfer of electrons to Complex IV and supported the model of random collisions of free diffusing molecules of cytochrome c despite the presence of Complex IV units bound in the respirasome.…”

Electronic Connection Between the Quinone and CytochromecRedox Pools and Its Role in Regulation of Mitochondrial Electron Transport and Redox Signaling

“…According to this concept, quinone and cytochrome c molecules are confined to form small subpools operating just within boundaries of the supercomplexes. However, several studies, including recent time-resolved kinetics on intact cells, suggest that diffusion of cytochrome c is not restricted and the protein is not trapped within supercomplexes (119,252).…”

Respiratory complexes III and IV can each bind two molecules of cytochrome c at low ionic strength

“…This function is essential for mitochondrial respiration, the major source of energy in eukaryotic cells. The organization of the mitochondrial electron transport chain is a subject of intense debate at present, where two different models are being considered: the fluid model, that proposes a random organization for individual respiratory protein components, and the solid model, suggesting a stable association between individual complexes [6][7][8]. Apart their role in respiration, Cc and Cc 1 are clearly involved in the development of programmed cell death [9][10][11][12][13][14][15].…”