Supercomplex Assembly Determines Electron Flux in the Mitochondrial Electron Transport Chain

Abstract

The textbook description of mitochondrial respiratory complexes (RCs) views them as free-moving entities linked by the mobile carriers coenzyme Q (CoQ) and cytochrome c (cyt c). This model (known as the fluid model) is challenged by the proposal that all RCs except complex II can associate in supercomplexes (SCs). The proposed SCs are the respirasome (complexes I, III, and IV), complexes I and III, and complexes III and IV. The role of SCs is unclear, and their existence is debated. By genetic modulation of interactions between complexes I and III and III and IV, we show that these associations define dedicated CoQ and cyt c pools and that SC assembly is dynamic and organizes electron flux to optimize the use of available substrates.

Granzyme B-induced mitochondrial ROS are required for apoptosis

“…We did not observe SC I+III2+IV probably because the mitochondria were purified from C57B6 mice that are deficient for Cox7a2l (Cyt c oxidase subunit VIIa polypeptide 2 like) and therefore cannot have III+IV association (Supplementary Figure 6e). 28 GB induced a significant loss in monomeric complex I and III activities, as shown previously (Figures 6c and f). In D-solubilized samples, complex I and III activities of SC I+III 2 were not significantly changed by GB, even though much more SC was formed after GB treatment, indicating that the specific activity of these complexes was reduced by GB treatment.…”

“…In the presence of cycloheximide that inhibits protein synthesis and de novo complex I production, GB and P still cleave complex I subunits, indicating that GB is acting on fully assembled complex I (Supplementary Figure 2d). [28][29][30] NDUFS3 and NDUFA9 complex I subunits not cleaved by GB were still present long after the cells had died following 4 h of GB treatment (Supplementary Figure 2e). Altogether, GB, independently of caspases, cleaves NDUFV1, NDUFS2 and NDUFS1 in cells undergoing killer cell attack.…”

“…Mitochondrial ETC complexes organize into supercomplexes (SCs) that channel electrons in response to changes in substrate availability. 28 The alteration in complex III activity we measured might be because of changes in SC formation caused by GB cleavage of complex I subunits. In lauryl maltoside (LM)-solubilized mitochondria, which preserves monomeric ETC complexes but disrupts SCs, GB reduced NDUFV1, NDUFS2 and NDUFS1, but had no effect on the complex III proteins UQCRC1, UQCR2 or UQCRS1 (Figure 6e).…”

Combined defects in oxidative phosphorylation and fatty acid β-oxidation in mitochondrial disease

“…Flux experiments have shown that the formation of the respirasome allows for substrate channelling by decreasing the distance in which electrons travel between mobile electron carriers and the OXPHOS complexes. This finding is supported by in-gel enzymatic assays that demonstrate respirasome catalytic activity in a range of eukaryotes [16,24,25]. In addition, the formation of the respirasome has also been proposed to limit oxidative stress [26].…”

Severe encephalopathy associated to pyruvate dehydrogenase mutations and unbalanced coenzyme Q10 content

“…1 It has antioxidant properties, regulates both transition pore and uncoupling proteins, and it is an obligated substrate for both β-oxidation of fatty acids and pyrimidine nucleotide biosynthesis. 2 CoQ 10 deficiency impairs oxidative phosphorylation and causes clinically heterogeneous mitochondrial diseases, 3 including an encephalomyopathy form with muscle disorders and myoglobinuria; an ataxic form with atrophic cerebellum; a severe infantile form with renal failure; an isolated myopathy, and nephrotic syndrome, have been so far described.…”