The random collision model and a critical assessment of diffusion and collision in mitochondrial electron transport

Understanding coenzyme Q

Coenzyme Q (CoQ), also known as ubiquinone, comprises a benzoquinone head group and a long isoprenoid side chain. It is thus extremely hydrophobic and resides in membranes. It is best known for its complex function as an electron transporter in the mitochondrial electron transport chain (ETC) but is also required for several other crucial cellular processes. In fact, CoQ appears to be central to the entire redox balance of the cell. Remarkably, its structure and therefore its properties have not changed from bacteria to vertebrates. In metazoans, it is synthesized in all cells and is found in most, and maybe all, biological membranes. CoQ is also known as a nutritional supplement, mostly because of its involvement with antioxidant defenses. However, whether there is any health benefit from oral consumption of CoQ is not well established. Here we review the function of CoQ as a redox-active molecule in the ETC and other enzymatic systems, its role as a prooxidant in reactive oxygen species generation, and its separate involvement in antioxidant mechanisms. We also review CoQ biosynthesis, which is particularly complex because of its extreme hydrophobicity, as well as the biological consequences of primary and secondary CoQ deficiency, including in human patients. Primary CoQ deficiency is a rare inborn condition due to mutation in CoQ biosynthetic genes. Secondary CoQ deficiency is much more common, as it accompanies a variety of pathological conditions, including mitochondrial disorders as well as aging. In this context, we discuss the importance, but also the great difficulty, of alleviating CoQ deficiency by CoQ supplementation.

Mitochondrial puzzle in muscle: Linking the electron transport system to overweight

Human skeletal muscle mitochondria regulate energy expenditure. Research has shown that the functionality of muscle mitochondria is altered in subjects with overweight, as well as in response to nutrient excess and calorie restriction. Two metabolic features of obesity and overweight are (1) incomplete muscular fatty acid oxidation and (2) increased circulating lactate levels. In this study, I propose that these metabolic disturbances may originate from a common source within the muscle mitochondrial electron transport system. Specifically, a reorganization of the supramolecular structure of the electron transport chain could facilitate the maintenance of readily accessible coenzyme Q pools, which are essential for metabolizing lipid substrates. This approach is expected to maintain effective electron transfer, provided that there is sufficient complex III to support the Q-cycle. Such an adaptation could enhance fatty acid oxidation and prevent mitochondrial overload, thereby reducing lactate production. These insights advance our understanding of the molecular mechanisms underpinning metabolic dysregulation in overweight states. This provides a basis for targeted interventions in the quest for metabolic health.

CoQ10 and Mitochondrial Dysfunction in Alzheimer's Disease

The progress in understanding the pathogenesis and treatment of Alzheimer's disease (AD) is based on the recognition of the primary causes of the disease, which can be deduced from the knowledge of risk factors and biomarkers measurable in the early stages of the disease. Insights into the risk factors and the time course of biomarker abnormalities point to a role for the connection of amyloid beta (Aβ) pathology, tau pathology, mitochondrial dysfunction, and oxidative stress in the onset and development of AD. Coenzyme Q10 (CoQ10) is a lipid antioxidant and electron transporter in the mitochondrial electron transport system. The availability and activity of CoQ10 is crucial for proper mitochondrial function and cellular bioenergetics. Based on the mitochondrial hypothesis of AD and the hypothesis of oxidative stress, the regulation of the efficiency of the oxidative phosphorylation system by means of CoQ10 can be considered promising in restoring the mitochondrial function impaired in AD, or in preventing the onset of mitochondrial dysfunction and the development of amyloid and tau pathology in AD. This review summarizes the knowledge on the pathophysiology of AD, in which CoQ10 may play a significant role, with the aim of evaluating the perspective of the pharmacotherapy of AD with CoQ10 and its analogues.

Understanding differential aspects of microdiffusion (channeling) in the Coenzyme Q and Cytochrome c regions of the mitochondrial respiratory system

Over the past decades, models of the organization of mitochondrial respiratory system have been controversial. The goal of this perspective is to assess this "conflict of models" by focusing on specific kinetic evidence in the two distinct segments of Coenzyme Q- and Cytochrome c-mediated electron transfer. Respiratory supercomplexes provide kinetic advantage by allowing a restricted diffusion of Coenzyme Q and Cytochrome c, and short-range interaction with their partner enzymes. In particular, electron transfer from NADH is compartmentalized by channeling of Coenzyme Q within supercomplexes, whereas succinate oxidation proceeds separately using the free Coenzyme Q pool. Previous evidence favoring Coenzyme Q random diffusion in the NADH-dependent electron transfer is due to downstream flux interference and misinterpretation of results. Indeed, electron transfer by complexes III and IV via Cytochrome c is less strictly dependent on substrate channeling in mammalian mitochondria. We briefly describe these differences and their physiological implications.

Structure and function of the S. pombe III-IV-cyt c supercomplex

The respiratory chain in aerobic organisms is composed of a number of membrane-bound protein complexes that link electron transfer to proton translocation across the membrane. In mitochondria, the final electron acceptor, complex IV (CIV), receives electrons from dimeric complex III (CIII2), via a mobile electron carrier, cytochrome c. In the present study, we isolated the CIII2CIV supercomplex from the fission yeast Schizosaccharomyces pombe and determined its structure with bound cyt. c using single-particle electron cryomicroscopy. A respiratory supercomplex factor 2 was found to be bound at CIV distally positioned in the supercomplex. In addition to the redox-active metal sites, we found a metal ion, presumably Zn2+, coordinated in the CIII subunit Cor1, which is encoded by the same gene (qcr1) as the mitochondrial-processing peptidase subunit β. Our data show that the isolated CIII2CIV supercomplex displays proteolytic activity suggesting a dual role of CIII2 in S. pombe. As in the supercomplex from S. cerevisiae, subunit Cox5 of CIV faces towards one CIII monomer, but in S. pombe, the two complexes are rotated relative to each other by ~45°. This orientation yields equal distances between the cyt. c binding sites at CIV and at each of the two CIII monomers. The structure shows cyt. c bound at four positions, but only along one of the two symmetrical branches. Overall, this combined structural and functional study reveals the integration of peptidase activity with the CIII2 respiratory system and indicates a two-dimensional cyt. c diffusion mechanism within the CIII2-CIV supercomplex.

The functional significance of mitochondrial respiratory chain supercomplexes

The mitochondrial respiratory chain (MRC) is a key energy transducer in eukaryotic cells. Four respiratory chain complexes cooperate in the transfer of electrons derived from various metabolic pathways to molecular oxygen, thereby establishing an electrochemical gradient over the inner mitochondrial membrane that powers ATP synthesis. This electron transport relies on mobile electron carries that functionally connect the complexes. While the individual complexes can operate independently, they are in situ organized into large assemblies termed respiratory supercomplexes. Recent structural and functional studies have provided some answers to the question of whether the supercomplex organization confers an advantage for cellular energy conversion. However, the jury is still out, regarding the universality of these claims. In this review, we discuss the current knowledge on the functional significance of MRC supercomplexes, highlight experimental limitations, and suggest potential new strategies to overcome these obstacles.

Mitochondrial sAC-cAMP-PKA Axis Modulates the ΔΨm-Dependent Control Coefficients of the Respiratory Chain Complexes: Evidence of Respirasome Plasticity

The current view of the mitochondrial respiratory chain complexes I, III and IV foresees the occurrence of their assembly in supercomplexes, providing additional functional properties when compared with randomly colliding isolated complexes. According to the plasticity model, the two structural states of the respiratory chain may interconvert, influenced by the intracellular prevailing conditions. In previous studies, we suggested the mitochondrial membrane potential as a factor for controlling their dynamic balance. Here, we investigated if and how the cAMP/PKA-mediated signalling influences the aggregation state of the respiratory complexes. An analysis of the inhibitory titration profiles of the endogenous oxygen consumption rates in intact HepG2 cells with specific inhibitors of the respiratory complexes was performed to quantify, in the framework of the metabolic flux theory, the corresponding control coefficients. The attained results, pharmacologically inhibiting either PKA or sAC, indicated that the reversible phosphorylation of the respiratory chain complexes/supercomplexes influenced their assembly state in response to the membrane potential. This conclusion was supported by the scrutiny of the available structure of the CI/CIII2/CIV respirasome, enabling us to map several PKA-targeted serine residues exposed to the matrix side of the complexes I, III and IV at the contact interfaces of the three complexes.

Structural rather than catalytic role for mitochondrial respiratory chain supercomplexes

Mammalian mitochondrial respiratory chain (MRC) complexes are able to associate into quaternary structures named supercomplexes (SCs), which normally coexist with non-bound individual complexes. The functional significance of SCs has not been fully clarified and the debate has been centered on whether or not they confer catalytic advantages compared with the non-bound individual complexes. Mitochondrial respiratory chain organization does not seem to be conserved in all organisms. In fact, and differently from mammalian species, mitochondria from Drosophila melanogaster tissues are characterized by low amounts of SCs, despite the high metabolic demands and MRC activity shown by these mitochondria. Here, we show that attenuating the biogenesis of individual respiratory chain complexes was accompanied by increased formation of stable SCs, which are missing in Drosophila melanogaster in physiological conditions. This phenomenon was not accompanied by an increase in mitochondrial respiratory activity. Therefore, we conclude that SC formation is necessary to stabilize the complexes in suboptimal biogenesis conditions, but not for the enhancement of respiratory chain catalysis.

Regulation of electron transfer in the terminal step of the respiratory chain

In mitochondria, electrons are transferred along a series of enzymes and electron carriers that are referred to as the respiratory chain, leading to the synthesis of cellular ATP. The series of the interprotein electron transfer (ET) reactions is terminated by the reduction in molecular oxygen at Complex IV, cytochrome c oxidase (CcO) that is coupled with the proton pumping from the matrix to the inner membrane space. Unlike the ET reactions from Complex I to Complex III, the ET reaction to CcO, mediated by cytochrome c (Cyt c), is quite specific in that it is irreversible with suppressed electron leakage, which characterizes the ET reactions in the respiratory chain and is thought to play a key role in the regulation of mitochondrial respiration. In this review, we summarize the recent findings regarding the molecular mechanism of the ET reaction from Cyt c to CcO in terms of specific interaction between two proteins, a molecular breakwater, and the effects of the conformational fluctuation on the ET reaction, conformational gating. Both of these are essential factors, not only in the ET reaction from Cyt c to CcO, but also in the interprotein ET reactions in general. We also discuss the significance of a supercomplex in the terminal ET reaction, which provides information on the regulatory factors of the ET reactions that are specific to the mitochondrial respiratory chain.

A molecular rotor FLIM probe reveals dynamic coupling between mitochondrial inner membrane fluidity and cellular respiration

The inner mitochondrial membrane (IMM), housing components of the electron transport chain (ETC), is the site for respiration. The ETC relies on mobile carriers; therefore, it has long been argued that the fluidity of the densely packed IMM can potentially influence ETC flux and cell physiology. However, it is unclear if cells temporally modulate IMM fluidity upon metabolic or other stimulation. Using a photostable, red-shifted, cell-permeable molecular-rotor, Mitorotor-1, we present a multiplexed approach for quantitatively mapping IMM fluidity in living cells. This reveals IMM fluidity to be linked to cellular-respiration and responsive to stimuli. Multiple approaches combining in vitro experiments and live-cell fluorescence (FLIM) lifetime imaging microscopy (FLIM) show Mitorotor-1 to robustly report IMM 'microviscosity'/fluidity through changes in molecular free volume. Interestingly, external osmotic stimuli cause controlled swelling/compaction of mitochondria, thereby revealing a graded Mitorotor-1 response to IMM microviscosity. Lateral diffusion measurements of IMM correlate with microviscosity reported via Mitorotor-1 FLIM-lifetime, showing convergence of independent approaches for measuring IMM local-order. Mitorotor-1 FLIM reveals mitochondrial heterogeneity in IMM fluidity; between-and-within cells and across single mitochondrion. Multiplexed FLIM lifetime imaging of Mitorotor-1 and NADH autofluorescence reveals that IMM fluidity positively correlates with respiration, across individual cells. Remarkably, we find that stimulating respiration, through nutrient deprivation or chemically, also leads to increase in IMM fluidity. These data suggest that modulating IMM fluidity supports enhanced respiratory flux. Our study presents a robust method for measuring IMM fluidity and suggests a dynamic regulatory paradigm of modulating IMM local order on changing metabolic demand.

The Interplay between Dysregulated Metabolism and Epigenetics in Cancer

Cellular metabolism (or energetics) and epigenetics are tightly coupled cellular processes. It is arguable that of all the described cancer hallmarks, dysregulated cellular energetics and epigenetics are the most tightly coregulated. Cellular metabolic states regulate and drive epigenetic changes while also being capable of influencing, if not driving, epigenetic reprogramming. Conversely, epigenetic changes can drive altered and compensatory metabolic states. Cancer cells meticulously modify and control each of these two linked cellular processes in order to maintain their tumorigenic potential and capacity. This review aims to explore the interplay between these two processes and discuss how each affects the other, driving and enhancing tumorigenic states in certain contexts.

Traumatic Brain Injury Alters Cerebral Concentrations and Redox States of Coenzymes Q9 and Q10 in the Rat

To date, there is no information on the effect of TBI on the changes in brain CoQ levels and possible variations in its redox state. In this study, we induced graded TBIs (mild TBI, mTBI and severe TBI, sTBI) in male rats, using the weight-drop closed-head impact acceleration model of trauma. At 7 days post-injury, CoQ₉, CoQ₁₀ and α-tocopherol were measured by HPLC in brain extracts of the injured rats, as well as in those of a group of control sham-operated rats. In the controls, about the 69% of total CoQ was in the form of CoQ₉ and the oxidized/reduced ratios of CoQ₉ and CoQ₁₀ were, respectively, 1.05 ± 0.07 and 1.42 ± 0.17. No significant changes in these values were observed in rats experiencing mTBI. Conversely, in the brains of sTBI-injured animals, an increase in reduced and a decrease in oxidized CoQ₉ produced an oxidized/reduced ratio of 0.81 ± 0.1 (p < 0.001 compared with both controls and mTBI). A concomitant decrease in both reduced and oxidized CoQ₁₀ generated a corresponding oxidized/reduced ratio of 1.38 ± 0.23 (p < 0.001 compared with both controls and mTBI). An overall decrease in the concentration of the total CoQ pool was also found in sTBI-injured rats (p < 0.001 compared with both controls and mTBI). Concerning α-tocopherol, whilst no differences compared with the controls were found in mTBI animals, a significant decrease was observed in rats experiencing sTBI (p < 0.01 compared with both controls and mTBI). Besides suggesting potentially different functions and intracellular distributions of CoQ₉ and CoQ₁₀ in rat brain mitochondria, these results demonstrate, for the first time to the best of knowledge, that sTBI alters the levels and redox states of CoQ₉ and CoQ₁₀, thus adding a new explanation to the mitochondrial impairment affecting ETC, OXPHOS, energy supply and antioxidant defenses following sTBI.

Mechano-control of Extracellular Electron Transport Rate via Modification of Inter-heme Coupling in Bacterial Surface Cytochrome

Bacterial outer-membrane multi-heme cytochromes (OMCs) mediate extracellular electron transport (EET). While heme alignment dictates the rate of EET, control of inter-heme coupling in a single OMC remains challenging, especially in intact cells. Given that OMCs diffuse and collide without aggregation on the cell surface, the overexpression of OMCs could increase such mechanical stress to impact the OMCs' protein structure. Here, the heme coupling is modified via mechanical interactions among OMCs by controlling their concentrations. Employment of whole-cell circular dichroism (CD) spectra of genetically engineered Escherichia coli reveals that the OMC concentration significantly impacts the molar CD and redox property of OMCs, resulting in a 4-fold change of microbial current production. The overexpression of OMCs increased the conductive current across the biofilm on an interdigitated electrode, indicating that a higher concentration of OMCs causes more lateral inter-protein electron hopping via collision on the cell surface. The present study would open a novel strategy to increase microbial current production by mechanically enhancing the inter-heme coupling.

Two separate pathways underlie NADH and succinate oxidation in swine heart mitochondria: Kinetic evidence on the mobile electron carriers

We have investigated NADH and succinate aerobic oxidation in frozen and thawed swine heart mitochondria. Simultaneous oxidation of NADH and succinate showed complete additivity under a variety of experimental conditions, suggesting that the electron fluxes originating from NADH and succinate are completely independent and do not mix at the level of the so-called mobile diffusible components. We ascribe the results to mixing of the fluxes at the level of cytochrome c in bovine mitochondria: the Complex IV flux control coefficient in NADH oxidation was high in swine mitochondria but very low in bovine mitochondria, suggesting a stronger interaction of cytochrome c with the supercomplex in the former. This was not the case in succinate oxidation, in which Complex IV exerted little control also in swine mitochondria. We interpret the data in swine mitochondria as restriction of the NADH flux by channelling within the I-III₂-IV supercomplex, whereas the flux from succinate shows pool mixing for both Coenzyme Q and probably cytochrome c. The difference between the two types of mitochondria may be ascribed to different lipid composition affecting the cytochrome c binding properties, as suggested by breaks in Arrhenius plots of Complex IV activity occurring at higher temperatures in bovine mitochondria.

High-resolution cryo-EM structures of plant cytochrome b6f at work

Plants use solar energy to power cellular metabolism. The oxidation of plastoquinol and reduction of plastocyanin by cytochrome b₆f (Cyt b₆f) is known as one of the key steps of photosynthesis, but the catalytic mechanism in the plastoquinone oxidation site (Q_p) remains elusive. Here, we describe two high-resolution cryo-EM structures of the spinach Cyt b₆f homodimer with endogenous plastoquinones and in complex with plastocyanin. Three plastoquinones are visible and line up one after another head to tail near Q_p in both monomers, indicating the existence of a channel in each monomer. Therefore, quinones appear to flow through Cyt b₆f in one direction, transiently exposing the redox-active ring of quinone during catalysis. Our work proposes an unprecedented one-way traffic model that explains efficient quinol oxidation during photosynthesis and respiration.

Alternative respiratory oxidases to study the animal electron transport chain

Oxidative phosphorylation is a common process to most organisms in which the main function is to generate an electrochemical gradient across the inner mitochondrial membrane (IMM) and to make energy available to the cell. However, plants, many fungi and some animals maintain non-energy conserving oxidases which serve as a bypass to coupled respiration. Namely, the alternative NADH:ubiquinone oxidoreductase NDI1, present in the complex I (CI)-lacking Saccharomyces cerevisiae, and the alternative oxidase, ubiquinol:oxygen oxidoreductase AOX, present in many organisms across different kingdoms. In the last few years, these alternative oxidases have been used to dissect previously indivisible processes in bioenergetics and have helped to discover, understand, and corroborate important processes in mitochondria. Here, we review how the use of alternative oxidases have contributed to the knowledge in CI stability, bioenergetics, redox biology, and the implications of their use in current and future research.

Mathematical Modeling of ROS Production and Diode-like Behavior in the SDHA/SDHB Subcomplex of Succinate Dehydrogenases in Reverse Quinol-Fumarate Reductase Direction

Succinate dehydrogenase (SDH) plays an important role in reverse electron transfer during hypoxia/anoxia, in particular, in ischemia, when blood supply to an organ is disrupted, and oxygen is not available. It was detected in the voltammetry studies about three decades ago that the SDHA/SDHB subcomplex of SDH can have such a strong nonlinear property as a "tunnel-diode" behavior in reverse quinol-fumarate reductase direction. The molecular and kinetic mechanisms of this phenomenon, that is, a strong drop in the rate of fumarate reduction as the driving force is increased, are still unclear. In order to account for this property of SDH, we developed and analyzed a mechanistic computational model of reverse electron transfer in the SDHA/SDHB subcomplex of SDH. It was shown that a decrease in the rate of succinate release from the active center during fumarate reduction quantitatively explains the experimentally observed tunnel-diode behavior in SDH and threshold values of the electrode potential of about -80 mV. Computational analysis of ROS production in the SDHA/SDHB subcomplex of SDH during reverse electron transfer predicts that the rate of ROS production decreases when the tunnel-diode behavior appears. These results predict a low rate of ROS production by the SDHA/SDHB subcomplex of SDH during ischemia.

Probing the Link between Pancratistatin and Mitochondrial Apoptosis through Changes in the Membrane Dynamics on the Nanoscale

Pancratistatin (PST) is a natural antiviral alkaloid that has demonstrated specificity toward cancerous cells and explicitly targets the mitochondria. PST initiates apoptosis while leaving healthy, noncancerous cells unscathed. However, the manner by which PST induces apoptosis remains elusive and impedes the advancement of PST as a natural anticancer therapeutic agent. Herein, we use neutron spin-echo (NSE) spectroscopy, molecular dynamics (MD) simulations, and supporting small angle scattering techniques to study PST's effect on membrane dynamics using biologically representative model membranes. Our data suggests that PST stiffens the inner mitochondrial membrane (IMM) by being preferentially associated with cardiolipin, which would lead to the relocation and release of cytochrome c. Second, PST has an ordering effect on the lipids and disrupts their distribution within the IMM, which would interfere with the maintenance and functionality of the active forms of proteins in the electron transport chain. These previously unreported findings implicate PST's effect on mitochondrial apoptosis.

VJ21.089The subcompartmented oxphosomic model of the phosphorylating system organization in mitochondria

The oxidative phosphorylation (OXPHOS) system of mitochondria supports all the vitally important energy-consuming processes in eukaryotic cells, providing them with energy in the form of ATP. OXPHOS enzymes (complexes I-V) are located in the inner mitochondrial membrane, mainly in the cristae subcompartment. At present, there is a large body of data evidencing that the respiratory complexes I, III2 and IV under in vivo conditions can physically interact with each other in diverse stoichiometry, thereby forming supercomplexes. Despite active accumulation of knowledge about the structure of the main supercomplexes of the OXPHOS system, its physical and functional organization in vivo remains unclear. Contemporary models of the OXPHOS system's organization in the inner membrane of mitochondria are contradictory and presume the existence of either highly organized respiratory strings, or, by contrast, a set of randomly dispersed respiratory supercomplexes and complexes. Furthermore, it is assumed that ATP-synthase (complex V) does not form associations with respiratory enzymes and operates autonomously. Our latest data obtained on mitochondria of etiolated shoots of pea evidence the possibility of physical association between the respiratory supercomplexes and dimeric ATP-synthase. These data have allowed us to reconsider the contemporary concept of the phosphorylation system organization and propose a new subcompartmented oxphosomic model. According to this model, a substantial number of the OXPHOS complexes form oxphosomes, which in a def inite stoichiometry include complexes I-V and are located predominantly in the cristae subcompartment of mitochondria in the form of highly organized strings or patches. These suprastructures represent "mini-factories" for ATP production. It is assumed that such an organization (1) contributes to increasing the eff iciency of the OXPHOS system operation, (2) involves new levels of activity regulation, and (3) may determine the inner membrane morphology to some extent. The review discusses the proposed model in detail. For a better understanding of the matter, the history of development of concepts concerning the OXPHOS organization with the emphasis on recent contemporary models is brief ly considered. The principal experimental data accumulated over the past 40 years, which conf irm the validity of the oxphosomic hypothesis, are also provided.

The Roles of Coenzyme Q in Disease: Direct and Indirect Involvement in Cellular Functions

Coenzyme Q (CoQ) is a key component of the respiratory chain of all eukaryotic cells. Its function is closely related to mitochondrial respiration, where it acts as an electron transporter. However, the cellular functions of coenzyme Q are multiple: it is present in all cell membranes, limiting the toxic effect of free radicals, it is a component of LDL, it is involved in the aging process, and its deficiency is linked to several diseases. Recently, it has been proposed that coenzyme Q contributes to suppressing ferroptosis, a type of iron-dependent programmed cell death characterized by lipid peroxidation. In this review, we report the latest hypotheses and theories analyzing the multiple functions of coenzyme Q. The complete knowledge of the various cellular CoQ functions is essential to provide a rational basis for its possible therapeutic use, not only in diseases characterized by primary CoQ deficiency, but also in large number of diseases in which its secondary deficiency has been found.

Characterization of oxidation of glutathione by cytochrome c

Cytochrome c is a member of the respiratory chain of the mitochondria. Non-membrane-bound (free) cytochrome c can be reduced by gluthatione as well as ascorbic acid. We investigated the effect of pH, Ca²⁺, Mg²⁺ and anionic phospholipids on the reduction of cytochrome c by glutathione.The reduction of cytochrome c by thiols was measured using photometry. Mitochondrial oxygen consumption was detected by use of oxygen electrode. Glutathione does not reduce cytochrome c at pH = 7.0 in the absence of Ca²⁺ and Mg²⁺. The reduction of cytochrome c by glutathione is inhibited by anionic lipids, especially cardiolipin. The typical conditions of apoptosis-elevated pH, Ca²⁺ level and Mg²⁺-increases the reduction of cytochrome c. Glutathione (5 mM) causes increased mitochondrial O₂ consumption at pH = 8.0, in the presence of ADP either 1 mM Mg²⁺ or 1 mM Ca²⁺. Our results suggest that membrane bound cyt c does not oxidize glutathione. Free (not membrane bound) cytochrome c can oxidize glutathione. In mitochondria, O₂ is depleted only in the presence of ADP, so the O₂ depletion observed in the presence of glutathione can be related to the respiratory chain. Decreased glutathione levels play a role in apoptosis. Therefore, membrane unbound cyt c can contribute to apoptosis by oxidation of glutathione.

Regulation and functional role of the electron transport chain supercomplexes

Mitochondria are one of the most exhaustively investigated organelles in the cell and most attention has been paid to the components of the mitochondrial electron transport chain (ETC) in the last 100 years. The ETC collects electrons from NADH or FADH₂ and transfers them through a series of electron carriers within multiprotein respiratory complexes (complex I to IV) to oxygen, therefore generating an electrochemical gradient that can be used by the F₁-F₀-ATP synthase (also named complex V) in the mitochondrial inner membrane to synthesize ATP. The organization and function of the ETC is a continuous source of surprises. One of the latest is the discovery that the respiratory complexes can assemble to form a variety of larger structures called super-complexes (SCs). This opened an unexpected level of complexity in this well-known and fundamental biological process. This review will focus on the current evidence for the formation of different SCs and will explore how they modulate the ETC organization according to the metabolic state. Since the field is rapidly growing, we also comment on the experimental techniques used to describe these SC and hope that this overview may inspire new technologies that will help to advance the field.

Extended supercomplex contains type-II NADH dehydrogenase, cytochrome bcc complex, and aa3 oxidase in the respiratory chain of Corynebacterium glutamicum

To clarify the precise subunit composition of the respiratory supercomplex of Corynebacterium glutamicum, several wash conditions were examined. MEGA (9 + 10) wash-buffer (0.5%) was used for this purpose and two-step column chromatography was performed. Almost equal amounts of cytochrome c, b, and a were observed in the purified fraction, estimated by their different absorption spectra. The 833 kDa and 685 kDa bands were observed in the clear native polyacrylamide gel electrophoresis (CN-PAGE) of the purified fraction. Both bands were stained using N,N',N',N-tetramethyl-p-phenylenediamine (TMPD) oxidase dye, and the 833 kDa band was also stained using NADH oxidase dye. The 3D map reconstructed from the 833 kDa band indicated that the bcc complex and aa₃ oxidase are heterodimers. Lastly, electron transfer from NADH to the bcc-aa₃ supercomplex was observed. The 833 kDa band is the supercomplex, which includes the heterodimer cytochrome bcc complex and cytochrome aa₃ oxidase, as well as the monomer NDH-II. Hence, we termed the 833 kDa band the extended supercomplex (ESC).

Cryo-EM structure and kinetics reveal electron transfer by 2D diffusion of cytochrome c in the yeast III-IV respiratory supercomplex

Energy conversion in aerobic organisms involves an electron current from low-potential donors, such as NADH and succinate, to dioxygen through the membrane-bound respiratory chain. Electron transfer is coupled to transmembrane proton transport, which maintains the electrochemical proton gradient used to produce ATP and drive other cellular processes. Electrons are transferred from respiratory complexes III to IV (CIII and CIV) by water-soluble cytochrome (cyt.) c In Saccharomyces cerevisiae and some other organisms, these complexes assemble into larger CIII₂CIV_1/2 supercomplexes, the functional significance of which has remained enigmatic. In this work, we measured the kinetics of the S. cerevisiae supercomplex cyt. c-mediated QH₂:O₂ oxidoreductase activity under various conditions. The data indicate that the electronic link between CIII and CIV is confined to the surface of the supercomplex. Single-particle electron cryomicroscopy (cryo-EM) structures of the supercomplex with cyt. c show the positively charged cyt. c bound to either CIII or CIV or along a continuum of intermediate positions. Collectively, the structural and kinetic data indicate that cyt. c travels along a negatively charged patch on the supercomplex surface. Thus, rather than enhancing electron transfer rates by decreasing the distance that cyt. c must diffuse in three dimensions, formation of the CIII₂CIV_1/2 supercomplex facilitates electron transfer by two-dimensional (2D) diffusion of cyt. c This mechanism enables the CIII₂CIV_1/2 supercomplex to increase QH₂:O₂ oxidoreductase activity and suggests a possible regulatory role for supercomplex formation in the respiratory chain.

Oligomeric assembly regulating mitochondrial HtrA2 function as examined by methyl-TROSY NMR

Human High temperature requirement A2 (HtrA2) is a mitochondrial protease chaperone that plays an important role in cellular proteostasis and in regulating cell-signaling events, with aberrant HtrA2 function leading to neurodegeneration and parkinsonian phenotypes. Structural studies of the enzyme have established a trimeric architecture, comprising three identical protomers in which the active sites of each protease domain are sequestered to form a catalytically inactive complex. The mechanism by which enzyme function is regulated is not well understood. Using methyl transverse relaxation optimized spectroscopy (TROSY)-based solution NMR in concert with biochemical assays, a functional HtrA2 oligomerization/binding cycle has been established. In the absence of substrates, HtrA2 exchanges between a heretofore unobserved hexameric conformation and the canonical trimeric structure, with the hexamer showing much weaker affinity toward substrates. Both structures are substrate inaccessible, explaining their low basal activity in the absence of the binding of activator peptide. The binding of the activator peptide to each of the protomers of the trimer occurs with positive cooperativity and induces intrasubunit domain reorientations to expose the catalytic center, leading to increased proteolytic activity. Our data paint a picture of HtrA2 as a finely tuned, stress-protective enzyme whose activity can be modulated both by oligomerization and domain reorientation, with basal levels of catalysis kept low to avoid proteolysis of nontarget proteins.

Structure and Mechanism of Respiratory III-IV Supercomplexes in Bioenergetic Membranes

In the final steps of energy conservation in aerobic organisms, free energy from electron transfer through the respiratory chain is transduced into a proton electrochemical gradient across a membrane. In mitochondria and many bacteria, reduction of the dioxygen electron acceptor is catalyzed by cytochrome c oxidase (complex IV), which receives electrons from cytochrome bc1 (complex III), via membrane-bound or water-soluble cytochrome c. These complexes function independently, but in many organisms they associate to form supercomplexes. Here, we review the structural features and the functional significance of the nonobligate III2IV1/2 Saccharomyces cerevisiae mitochondrial supercomplex as well as the obligate III2IV2 supercomplex from actinobacteria. The analysis is centered around the Q-cycle of complex III, proton uptake by CytcO, as well as mechanistic and structural solutions to the electronic link between complexes III and IV.

The mitochondrial peptidase, neurolysin, regulates respiratory chain supercomplex formation and is necessary for AML viability

Neurolysin (NLN) is a zinc metallopeptidase whose mitochondrial function is unclear. We found that NLN was overexpressed in almost half of patients with acute myeloid leukemia (AML), and inhibition of NLN was selectively cytotoxic to AML cells and stem cells while sparing normal hematopoietic cells. Mechanistically, NLN interacted with the mitochondrial respiratory chain. Genetic and chemical inhibition of NLN impaired oxidative metabolism and disrupted the formation of respiratory chain supercomplexes (RCS). Furthermore, NLN interacted with the known RCS regulator, LETM1, and inhibition of NLN disrupted LETM1 complex formation. RCS were increased in patients with AML and positively correlated with NLN expression. These findings demonstrate that inhibiting RCS formation selectively targets AML cells and stem cells and highlights the therapeutic potential of pharmacologically targeting NLN in AML.

Comparative analysis of CI- and CIV-containing respiratory supercomplexes at single-cell resolution

Mitochondria sustain the energy demand of the cell. The composition and functional state of the mitochondrial oxidative phosphorylation system are informative indicators of organelle bioenergetic capacity. Here, we describe a highly sensitive and reproducible method for a single-cell quantification of mitochondrial CI- and CIV-containing respiratory supercomplexes (CI∗CIV-SCs) as an alternative means of assessing mitochondrial respiratory chain integrity. We apply a proximity ligation assay (PLA) and stain CI∗CIV-SCs in fixed human and mouse brains, tumorigenic cells, induced pluripotent stem cells (iPSCs) and iPSC-derived neural precursor cells (NPCs), and neurons. Spatial visualization of CI∗CIV-SCs enables the detection of mitochondrial lesions in various experimental models, including complex tissues undergoing degenerative processes. We report that comparative assessments of CI∗CIV-SCs facilitate the quantitative profiling of even subtle mitochondrial variations by overcoming the confounding effects that mixed cell populations have on other measurements. Together, our PLA-based analysis of CI∗CIV-SCs is a sensitive and complementary technique for detecting cell-type-specific mitochondrial perturbations in fixed materials.

Molecular and Functional Effects of Loss of Cytochrome c Oxidase Subunit 8A

In this work we studied molecular and functional effects of the loss of the smallest nuclear encoded subunit of cytochrome c oxidase COX8A in fibroblasts from a patient with a homozygous splice site mutation and in CRISPR/Cas9 genome-edited HEK293T cells. In both cellular model systems, between 20 to 30% of the residual enzymatic activity of cytochrome c oxidase (COX) was detectable. In immunoblots of BN-PAGE separated mitochondria from both cellular models almost no monomers and dimers of the fully assembled COX could be visualized. Interestingly, supercomplexes of COX formed with complex III and also with complexes I and III retained considerable immunoreactivity, while nearly no immunoreactivity attributable to subassemblies was found. That indicates that COX lacking subunit 8A is stabilized in supercomplexes, while monomers and dimers are rapidly degraded. With transcriptome analysis by 3'-RNA sequencing we failed to detect in our cellular models of COX8A deficiency transcriptional changes of genes involved in the mitochondrial unfolded protein response (mtUPR) and the integrated stress response (ISR). Thus, our data strongly suggest that the smallest subunit of cytochrome c oxidase COX8A is required for maintenance of the structural stability of COX monomers and dimers.

Mass-spectrometry-based proteomics reveals mitochondrial supercomplexome plasticity

Mitochondrial respiratory complex subunits assemble in supercomplexes. Studies of supercomplexes have typically relied upon antibody-based quantification, often limited to a single subunit per respiratory complex. To provide a deeper insight into mitochondrial and supercomplex plasticity, we combine native electrophoresis and mass spectrometry to determine the supercomplexome of skeletal muscle from sedentary and exercise-trained mice. We quantify 422 mitochondrial proteins within 10 supercomplex bands in which we show the debated presence of complexes II and V. Exercise-induced mitochondrial biogenesis results in non-stoichiometric changes in subunits and incorporation into supercomplexes. We uncover the dynamics of supercomplex-related assembly proteins and mtDNA-encoded subunits after exercise. Furthermore, exercise affects the complexing of Lactb, an obesity-associated mitochondrial protein, and ubiquinone biosynthesis proteins. Knockdown of ubiquinone biosynthesis proteins leads to alterations in mitochondrial respiration. Our approach can be applied to broad biological systems. In this instance, comprehensively analyzing respiratory supercomplexes illuminates previously undetectable complexity in mitochondrial plasticity.

Functional segmentation of CoQ and cyt c pools by respiratory complex superassembly

Electron transfer between respiratory complexes is an essential step for the efficiency of the mitochondrial oxidative phosphorylation. Until recently, it was stablished that ubiquinone and cytochrome c formed homogenous single pools in the inner mitochondrial membrane which were not influenced by the presence of respiratory supercomplexes. However, this idea was challenged by the fact that bottlenecks in electron transfer appeared after disruption of supercomplexes into their individual complexes. The postulation of the plasticity model embraced all these observations and concluded that complexes and supercomplexes co-exist and are dedicated to a spectrum of metabolic requirements. Here, we review the involvement of superassembly in complex I stability, the role of supercomplexes in ROS production and the segmentation of the CoQ and cyt c pools, together with their involvement in signaling and disease. Taking apparently conflicting literature we have built up a comprehensive model for the segmentation of CoQ and cyt c mediated by supercomplexes, discuss the current limitations and provide a prospect of the current knowledge in the field.

Organization of the Respiratory Supercomplexes in Cells with Defective Complex III: Structural Features and Metabolic Consequences

The mitochondrial respiratory chain encompasses four oligomeric enzymatic complexes (complex I, II, III and IV) which, together with the redox carrier ubiquinone and cytochrome c, catalyze electron transport coupled to proton extrusion from the inner membrane. The protonmotive force is utilized by complex V for ATP synthesis in the process of oxidative phosphorylation. Respiratory complexes are known to coexist in the membrane as single functional entities and as supramolecular aggregates or supercomplexes (SCs). Understanding the assembly features of SCs has relevant biomedical implications because defects in a single protein can derange the overall SC organization and compromise the energetic function, causing severe mitochondrial disorders. Here we describe in detail the main types of SCs, all characterized by the presence of complex III. We show that the genetic alterations that hinder the assembly of Complex III, not just the activity, cause a rearrangement of the architecture of the SC that can help to preserve a minimal energetic function. Finally, the major metabolic disturbances associated with severe SCs perturbation due to defective complex III are discussed along with interventions that may circumvent these deficiencies.

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

Molecular and Supramolecular Structure of the Mitochondrial Oxidative Phosphorylation System: Implications for Pathology

Under aerobic conditions, mitochondrial oxidative phosphorylation (OXPHOS) converts the energy released by nutrient oxidation into ATP, the currency of living organisms. The whole biochemical machinery is hosted by the inner mitochondrial membrane (mtIM) where the protonmotive force built by respiratory complexes, dynamically assembled as super-complexes, allows the F1FO-ATP synthase to make ATP from ADP + Pi. Recently mitochondria emerged not only as cell powerhouses, but also as signaling hubs by way of reactive oxygen species (ROS) production. However, when ROS removal systems and/or OXPHOS constituents are defective, the physiological ROS generation can cause ROS imbalance and oxidative stress, which in turn damages cell components. Moreover, the morphology of mitochondria rules cell fate and the formation of the mitochondrial permeability transition pore in the mtIM, which, most likely with the F1FO-ATP synthase contribution, permeabilizes mitochondria and leads to cell death. As the multiple mitochondrial functions are mutually interconnected, changes in protein composition by mutations or in supercomplex assembly and/or in membrane structures often generate a dysfunctional cascade and lead to life-incompatible diseases or severe syndromes. The known structural/functional changes in mitochondrial proteins and structures, which impact mitochondrial bioenergetics because of an impaired or defective energy transduction system, here reviewed, constitute the main biochemical damage in a variety of genetic and age-related diseases.

Supramolecular associations between atypical oxidative phosphorylation complexes of Euglena gracilis

In vivo associations of respiratory complexes forming higher supramolecular structures are generally accepted nowadays. Supercomplexes (SC) built by complexes I, III and IV and the so-called respirasome (I/III₂/IV) have been described in mitochondria from several model organisms (yeasts, mammals and green plants), but information is scarce in other lineages. Here we studied the supramolecular associations between the complexes I, III, IV and V from the secondary photosynthetic flagellate Euglena gracilis with an approach that involves the extraction with several mild detergents followed by native electrophoresis. Despite the presence of atypical subunit composition and additional structural domains described in Euglena complexes I, IV and V, canonical associations into III₂/IV, III₂/IV₂ SCs and I/III₂/IV respirasome were observed together with two oligomeric forms of the ATP synthase (V₂ and V₄). Among them, III₂/IV SC could be observed by electron microscopy. The respirasome was further purified by two-step liquid chromatography and showed in-vitro oxygen consumption independent of the addition of external cytochrome c.

Mitochondrial Structure and Bioenergetics in Normal and Disease Conditions

Mitochondria are ubiquitous intracellular organelles found in almost all eukaryotes and involved in various aspects of cellular life, with a primary role in energy production. The interest in this organelle has grown stronger with the discovery of their link to various pathologies, including cancer, aging and neurodegenerative diseases. Indeed, dysfunctional mitochondria cannot provide the required energy to tissues with a high-energy demand, such as heart, brain and muscles, leading to a large spectrum of clinical phenotypes. Mitochondrial defects are at the origin of a group of clinically heterogeneous pathologies, called mitochondrial diseases, with an incidence of 1 in 5000 live births. Primary mitochondrial diseases are associated with genetic mutations both in nuclear and mitochondrial DNA (mtDNA), affecting genes involved in every aspect of the organelle function. As a consequence, it is difficult to find a common cause for mitochondrial diseases and, subsequently, to offer a precise clinical definition of the pathology. Moreover, the complexity of this condition makes it challenging to identify possible therapies or drug targets.

Mitochondrial respiratory supercomplexes in mammalian cells: structural versus functional role

Mitochondria are recognized as the main source of ATP to meet the energy demands of the cell. ATP production occurs by oxidative phosphorylation when electrons are transported through the electron transport chain (ETC) complexes and develop the proton motive force across the inner mitochondrial membrane that is used for ATP synthesis. Studies since the 1960s have been concentrated on the two models of structural organization of ETC complexes known as "solid-state" and "fluid-state" models. However, advanced new techniques such as blue-native gel electrophoresis, mass spectroscopy, and cryogenic electron microscopy for analysis of macromolecular protein complexes provided new data in favor of the solid-state model. According to this model, individual ETC complexes are assembled into macromolecular structures known as respiratory supercomplexes (SCs). A large number of studies over the last 20 years proposed the potential role of SCs to facilitate substrate channeling, maintain the integrity of individual ETC complexes, reduce electron leakage and production of reactive oxygen species, and prevent excessive and random aggregation of proteins in the inner mitochondrial membrane. However, many other studies have challenged the proposed functional role of SCs. Recently, a third model known as the "plasticity" model was proposed that partly reconciles both "solid-state" and "fluid-state" models. According to the "plasticity" model, respiratory SCs can co-exist with the individual ETC complexes. To date, the physiological role of SCs remains unknown, although several studies using tissue samples of patients or animal/cell models of human diseases revealed an associative link between functional changes and the disintegration of SC assembly. This review summarizes and discusses previous studies on the mechanisms and regulation of SC assembly under physiological and pathological conditions.

Curvature-Dependent Binding of Cytochrome c to Cardiolipin

Cytochrome c binds cardiolipin on the concave surface of the inner mitochondrial membrane, before oxidizing the lipid and initiating the apoptotic pathway. This interaction has been studied in vitro, where mimicking the membrane curvature of the binding environment is difficult. Here we report binding to concave, cardiolipin-containing, membrane surfaces and compare findings to convex binding under the same conditions. For binding to the convex outer surface of cardiolipin-containing vesicles, a two-step structural rearrangement is observed with a small rearrangement detectable by Soret circular dichroism (CD) occurring at an exposed lipid-to-protein ratio (LPR) near 10 and partial unfolding detectable by Trp59 fluorescence occurring at an exposed LPR near 23. On the concave inner surface of cardiolipin-containing vesicles, the structural transitions monitored by Soret CD and Trp59 fluorescence are coincident and occur at an exposed LPR near 58. On the concave inner surface of mitochondrial cristae, we estimate the LPR of cardiolipin to cytochrome c is between 50 and 100. Thus, cytochrome c may have adapted to its native environment so that it can undergo a conformational change that switches on its peroxidase activity when it binds to CL-containing membranes in the cristae early in apoptosis. Our results show that membrane curvature qualitatively affects peripheral protein-lipid interactions and also highlights the disparity between in vitro binding studies and their physiological counterparts where cone-shaped lipids, like cardiolipin, are involved.

Mitochondria orchestrate macrophage effector functions in atherosclerosis

Macrophages are pivotal in the initiation and development of atherosclerotic cardiovascular diseases. Recent studies have reinforced the importance of mitochondria in metabolic and signaling pathways to maintain macrophage effector functions. In this review, we discuss the past and emerging roles of macrophage mitochondria metabolic diversity in atherosclerosis and the potential avenue as biomarker. Beyond metabolic functions, mitochondria are also a signaling platform integrating epigenetic, redox, efferocytic and apoptotic regulations, which are exquisitely linked to their dynamics. Indeed, mitochondria functions depend on their density and shape perpetually controlled by mitochondria fusion/fission and biogenesis/mitophagy balances. Mitochondria can also communicate with other organelles such as the endoplasmic reticulum through mitochondria-associated membrane (MAM) or be secreted for paracrine actions. All these functions are perturbed in macrophages from mouse or human atherosclerotic plaques. A better understanding and integration of how these metabolic and signaling processes are integrated and dictate macrophage effector functions in atherosclerosis may ultimately help the development of novel therapeutic approaches.

New insights into the organisation of the oxidative phosphorylation system in the example of pea shoot mitochondria

The physical and functional organisation of the OXPHOS system in mitochondria in vivo remains elusive. At present, different models of OXPHOS arrangement, representing either highly ordered respiratory strings or, vice versa, a set of randomly dispersed supercomplexes and respiratory complexes, have been suggested. In the present study, we examined a supramolecular arrangement of the OXPHOS system in pea shoot mitochondria using digitonin solubilisation of its constituents, which were further analysed by classical BN-related techniques and a multidimensional gel electrophoresis system when required. As a result, in addition to supercomplexes I1III2, I1III2IVn and III2IV1-2, dimer V2, and individual complexes I-V previously detected in plant mitochondria, new OXPHOS structures were also revealed. Of them, (1) a megacomplex (IIxIIIyIVz)n including complex II, (2) respirasomes I2III4IVn with two copies of complex I and dimeric complex III2, (3) a minor new supercomplex IV1Va2 comigrating with I1III2, and (4) a second minor form of ATP synthase, Va, were found. The activity of singular complexes I, IV, and V was higher than the activity of the associated forms. The detection of new supercomplex IV1Va2, along with assemblies I1III2 and I1-2III2-4IVn, prompted us to suggest the occurrence of in vivo oxphosomes comprising complexes I, III2, IV, and V. The putative oxphosome's stoichiometry, historical background, assumed functional significance, and subcompartmental location are discussed herein.

The Human Cytochrome c Domain-Swapped Dimer Facilitates Tight Regulation of Intrinsic Apoptosis

Oxidation of cardiolipin (CL) by cytochrome c (cytc) has been proposed to initiate the intrinsic pathway of apoptosis. Domain-swapped dimer (DSD) conformations of cytc have been reported both by our laboratory and by others. The DSD is an alternate conformer of cytc that could oxygenate CL early in apoptosis. We demonstrate here that the cytc DSD has a set of properties that would provide tighter regulation of the intrinsic pathway. We show that the human DSD is kinetically more stable than horse and yeast DSDs. Circular dichroism data indicate that the DSD has a less asymmetric heme environment, similar to that seen when the monomeric protein binds to CL vesicles at high lipid:protein ratios. The dimer undergoes the alkaline conformational transition near pH 7.0, 2.5 pH units lower than that of the monomer. Data from fluorescence correlation spectroscopy and fluorescence anisotropy suggest that the alkaline transition of the DSD may act as a switch from a high affinity for CL nanodiscs at pH 7.4 to a much lower affinity at pH 8.0. Additionally, the peroxidase activity of the human DSD increases 7-fold compared to that of the monomer at pH 7 and 8, but by 14-fold at pH 6 when mixed Met80/H2O ligation replaces the lysine ligation of the alkaline state. We also present data that indicate that cytc binding shows a cooperative effect as the concentration of cytc is increased. The DSD appears to have evolved into a pH-inducible switch that provides a means to control activation of apoptosis near pH 7.0.

Mitochondrial OXPHOS Biogenesis: Co-Regulation of Protein Synthesis, Import, and Assembly Pathways

The assembly of mitochondrial oxidative phosphorylation (OXPHOS) complexes is an intricate process, which—given their dual-genetic control—requires tight co-regulation of two evolutionarily distinct gene expression machineries. Moreover, fine-tuning protein synthesis to the nascent assembly of OXPHOS complexes requires regulatory mechanisms such as translational plasticity and translational activators that can coordinate mitochondrial translation with the import of nuclear-encoded mitochondrial proteins. The intricacy of OXPHOS complex biogenesis is further evidenced by the requirement of many tightly orchestrated steps and ancillary factors. Early-stage ancillary chaperones have essential roles in coordinating OXPHOS assembly, whilst late-stage assembly factors—also known as the LYRM (leucine–tyrosine–arginine motif) proteins—together with the mitochondrial acyl carrier protein (ACP)—regulate the incorporation and activation of late-incorporating OXPHOS subunits and/or co-factors. In this review, we describe recent discoveries providing insights into the mechanisms required for optimal OXPHOS biogenesis, including the coordination of mitochondrial gene expression with the availability of nuclear-encoded factors entering via mitochondrial protein import systems.

Kinetic advantage of forming respiratory supercomplexes

Components of respiratory chains in mitochondria and some aerobic bacteria assemble into larger, multiprotein membrane-bound supercomplexes. Here, we address the functional significance of supercomplexes composed of respiratory-chain complexes III and IV. Complex III catalyzes oxidation of quinol and reduction of water-soluble cytochrome c (cyt c), while complex IV catalyzes oxidation of the reduced cyt c and reduction of dioxygen to water. We focus on two questions: (i) under which conditions does diffusion of cyt c become rate limiting for electron transfer between these two complexes? (ii) is there a kinetic advantage of forming a supercomplex composed of complexes III and IV? To answer these questions, we use a theoretical approach and assume that cyt c diffuses in the water phase while complexes III and IV either diffuse independently in the two dimensions of the membrane or form supercomplexes. The analysis shows that the electron flux between complexes III and IV is determined by the equilibration time of cyt c within the volume of the intermembrane space, rather than the cyt c diffusion time constant. Assuming realistic relative concentrations of membrane-bound components and cyt c and that all components diffuse independently, the data indicate that electron transfer between complexes III and IV can become rate limiting. Hence, there is a kinetic advantage of bringing complexes III and IV together in the membrane to form supercomplexes.

Cardiac Function is not Susceptible to Moderate Disassembly of Mitochondrial Respiratory Supercomplexes

Mitochondrial respiratory chain supercomplexes (RCS), particularly, the respirasome, which contains complexes I, III, and IV, have been suggested to participate in facilitating electron transport, reducing the production of reactive oxygen species (ROS), and maintaining the structural integrity of individual electron transport chain (ETC) complexes. Disassembly of the RCS has been observed in Barth syndrome, neurodegenerative and cardiovascular diseases, diabetes mellitus, and aging. However, the physiological role of RCS in high energy-demanding tissues such as the heart remains unknown. This study elucidates the relationship between RCS assembly and cardiac function. Adult male Sprague Dawley rats underwent Langendorff retrograde perfusion in the presence and absence of ethanol, isopropanol, or rotenone (an ETC complex I inhibitor). We found that ethanol had no effects on cardiac function, whereas rotenone reduced heart contractility, which was not recovered when rotenone was excluded from the perfusion medium. Blue native polyacrylamide gel electrophoresis revealed significant reductions of respirasome levels in ethanol- or rotenone-treated groups compared to the control group. In addition, rotenone significantly increased while ethanol had no effect on mitochondrial ROS production. In isolated intact mitochondria in vitro, ethanol did not affect respirasome assembly; however, acetaldehyde, a byproduct of ethanol metabolism, induced dissociation of respirasome. Isopropanol, a secondary alcohol which was used as an alternative compound, had effects similar to ethanol on heart function, respirasome levels, and ROS production. In conclusion, ethanol and isopropanol reduced respirasome levels without any noticeable effect on cardiac parameters, and cardiac function is not susceptible to moderate reductions of RCS.

Mitochondrial Supercomplexes: Physiological Organization and Dysregulation in Age-Related Neurodegenerative Disorders

Several studies suggest that the assembly of mitochondrial respiratory complexes into structures known as supercomplexes (SCs) may increase the efficiency of the electron transport chain, reducing the rate of production of reactive oxygen species. Therefore, the study of the (dis)assembly of SCs may be relevant for the understanding of mitochondrial dysfunction reported in brain aging and major neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD). Here we briefly reviewed the biogenesis and structural properties of SCs, the impact of mtDNA mutations and mitochondrial dynamics on SCs assembly, the role of lipids on stabilization of SCs and the methodological limitations for the study of SCs. More specifically, we summarized what is known about mitochondrial dysfunction and SCs organization and activity in aging, AD and PD. We focused on the critical variables to take into account when postmortem tissues are used to study the (dis)assembly of SCs. Since few works have been performed to study SCs in AD and PD, the impact of SCs dysfunction on the alteration of brain energetics in these diseases remains poorly understood. The convergence of future progress in the study of SCs structure at high resolution and the refinement of animal models of AD and PD, as well as the use of iPSC-based and somatic cell-derived neurons, will be critical in understanding the biological relevance of the structural remodeling of SCs.

Aging membranes: Unexplored functions for lipids in the lifespan of the central nervous system

Lipids constitute a significant group of biological metabolites and the building blocks of all cell membranes. The abundance and stoichiometries of different lipid species are known to vary across the lifespan and metabolic state, yet the functional effects of these changes have been challenging to understand. Here we review the potentially powerful intersection of lipid metabolism, which determines membrane composition, and aging. We first introduce several key lipid classes that are associated with aging and aging-related disease, where they are found in organisms, and how they act on membrane structure and function. Instead of neutral lipids, which have primary roles in energy storage and homeostasis, we review known functions for polar lipids that control the physicochemical properties of cell membranes. We then focus on aging processes in the central nervous system (CNS), which is enriched in lipids and is highly dependent on membrane structure for function. Recent studies show how lipids act not just as biomarkers of aging and associated changes in the CNS, but as direct mediators of these processes. As a model system, we explore how fatty acid composition in the retina impact aging and aging-related disease. We propose that the biophysical effects of membrane structure on fundamental eukaryotic processes - mitochondrial respiration and autophagy - provide avenues by which lipid dysregulation can accelerate aging processes. Finally, we lay out ways in which an increased understanding of lipid membrane biology can be applied to studies of aging and lifespan.

Cultivation at high osmotic pressure confers ubiquinone 8-independent protection of respiration on Escherichia coli

Ubiquinone 8 (coenzyme Q8 or Q8) mediates electron transfer within the aerobic respiratory chain, mitigates oxidative stress, and contributes to gene expression in Escherichia coli In addition, Q8 was proposed to confer bacterial osmotolerance by accumulating during growth at high osmotic pressure and altering membrane stability. The osmolyte trehalose and membrane lipid cardiolipin accumulate in E. coli cells cultivated at high osmotic pressure. Here, Q8 deficiency impaired E. coli growth at low osmotic pressure and rendered growth osmotically sensitive. The Q8 deficiency impeded cellular O₂ uptake and also inhibited the activities of two proton symporters, the osmosensing transporter ProP and the lactose transporter LacY. Q8 supplementation decreased membrane fluidity in liposomes, but did not affect ProP activity in proteoliposomes, which is respiration-independent. Liposomes and proteoliposomes prepared with E. coli lipids were used for these experiments. Similar oxygen uptake rates were observed for bacteria cultivated at low and high osmotic pressures. In contrast, respiration was dramatically inhibited when bacteria grown at the same low osmotic pressure were shifted to high osmotic pressure. Thus, respiration was restored during prolonged growth of E. coli at high osmotic pressure. Of note, bacteria cultivated at low and high osmotic pressures had similar Q8 concentrations. The protection of respiration was neither diminished by cardiolipin deficiency nor conferred by trehalose overproduction during growth at low osmotic pressure, but rather might be achieved by Q8-independent respiratory chain remodeling. We conclude that osmotolerance is conferred through Q8-independent protection of respiration, not by altering physical properties of the membrane.