The Mitochondrial Respiratory Chain Is Partially Organized in a Supercomplex Assembly

Abstract

The model of the respiratory chain in which the enzyme complexes are independently embedded in the lipid bilayer of the inner mitochondrial membrane and connected by randomly diffusing coenzyme Q and cytochrome c is mostly favored. However, multicomplex units can be isolated from mammalian mitochondria, suggesting a model based on direct electron channeling between complexes. Kinetic testing using metabolic flux control analysis can discriminate between the two models: the former model implies that each enzyme may be rate-controlling to a different extent, whereas in the latter, the whole metabolic pathway would behave as a single supercomplex and inhibition of any one of its components would elicit the same flux control. In particular, in the absence of other components of the oxidative phosphorylation apparatus (i.e. ATP synthase, membrane potential, carriers), the existence of a supercomplex would elicit a flux control coefficient near unity for each respiratory complex, and the sum of all coefficients would be well above unity. Using bovine heart mitochondria and submitochondrial particles devoid of substrate permeability barriers, we investigated the flux control coefficients of the complexes involved in aerobic NADH oxidation (I, III, IV) and in succinate oxidation (II, III, IV). Both Complexes I and III were found to be highly rate-controlling over NADH oxidation, a strong kinetic evidence suggesting the existence of functionally relevant association between the two complexes, whereas Complex IV appears randomly distributed. Moreover, we show that Complex II is fully rate-limiting for succinate oxidation, clearly indicating the absence of substrate channeling toward Complexes III and IV.Considerable information exists on the structure at atomic resolution of most of the transmembrane protein complexes forming the mitochondrial respiratory chain; there is, however, still little direct information on the supramolecular organization of the enzymatic complexes in the inner mitochondrial membrane. Two extreme models for their arrangement in the membrane are conceivable: the model of a random organization of the individual respiratory complexes and that of a supercomplex assembly formed by stable association between proteins.The original solid-state model of Chance and Williams (1) changed gradually because the oxidative phosphorylation enzymes were found functionally active when isolated as individual complexes (2) Despite the acceptance of the idea that electron transfer in mitochondrial membranes depends on random collisions between small diffusing molecules (coenzyme Q and cytochrome c) and complexes

Genetic and epigenetic factors are associated with expression of respiratory chain component NDUFB6 in human skeletal muscle

“…We therefore attempted to determine which mechanisms regulate OXPHOS gene expression in human muscle. A nuclear-encoded gene, NDUFB6 (from complex 1 in the respiratory chain), was selected for this analysis because its expression is significantly reduced in diabetic muscle, the NDUFB6 gene is located under a linkage peak for T2DM on chromosome 9, and complex 1 is suggested to be rate limiting for NADH oxidation (3,9,11,12). The key finding of the present study was that genetic (polymorphism), epigenetic (DNA methylation), and nongenetic (e.g., age) factors were associated with the expression level of NDUFB6 in human skeletal muscle.…”

“…We hypothesized that the effect of age on mitochondrial function can be determined by both genetic and epigenetic factors. To address this we selected the NDUFB6 gene from the first respiratory complex for further studies, because NDUFB6 is among the set of OXPHOS genes showing significant reduction in muscle from patients with T2DM compared with healthy control subjects and because the first complex has a key role in regulating OXPHOS (3,9). We specifically examined whether polymorphisms in the promoter of the gene were associated with an age-dependent impact on gene expression and/or risk of T2DM and whether this was modified by epigenetic factors like DNA methylation.…”

Kinetic evidence against partitioning of the ubiquinone pool and the catalytic relevance of respiratory-chain supercomplexes

“…It has been proposed that the Q pool is partitioned such that the ubiquinol produced by complex I is channeled to complex III within supercomplex assemblies, whereas the ubiquinol produced by complex II diffuses into the membrane to access a separate complex III population (13,15). If this is the case, then adding either NADH or succinate to mitochondrial membranes or SMPs should reduce only part of the complex III population, whereas adding both simultaneously should reduce the whole population.…”

“…The most widely cited evidence for a catalytic role, from flux control analysis (15), was considered to support substrate channeling between enzymes present within respirasomes. More recently, the existence of dedicated Q and cyt c pools within supercomplex assemblies was proposed to determine electron transport chain flux, based on the relative rates of complex I-and complex II-linked respiration observed when complexes I and/or III are ablated (13).…”

Mitochondrial respiratory chain dysfunction: Implications in neurodegeneration

“…This is highly relevant because RC complexes associate in higher order assemblies called supercomplexes or respirasomes (Schagger and Pfeiffer, 2000;Schagger and Pfeiffer 2001;Schagger, 2002;Bianchi et al, 2004;Schagger et al, 2004;Dudkina et al, 2005, Boekema andBraun, 2007;Acin-Perez et al, 2008;Dudkina et al, 2011), which have been suggested to offer structural and functional advantages to the system, such as the prevention of the destabilization and degradation of RC complexes, the enhancement of electron transport efficiency and substrate channeling, or the reduction of electron or proton leakages Koopman et al, 2010;Lenaz and Genova, 2010). As a consequence of their organization in the respirasomes, structural interdependences amongst the individual RC complexes exist (Acin-Perez et al, 2004;Schagger et al, 2004;D'Aurelio et al, 2006;Diaz et al, 2006;Li et al, 2007;Saddar et al, 2008;Soto et al, 2009;Vempati et al, 2009).…”