Gaining mass: the structure of respiratory complex I-from bacterial towards mitochondrial versions

Mitochondrial complex-1 as a therapeutic target for cardiac diseases

Mitochondrial dysfunction is critical for the development and progression of cardiovascular diseases (CVDs). Complex-1 (CI) is an essential component of the mitochondrial electron transport chain that participates in oxidative phosphorylation and energy production. CI is the largest multisubunit complex (~ 1 Mda) and comprises 45 protein subunits encoded by seven mt-DNA genes and 38 nuclear genes. These subunits function as the enzyme nicotinamide adenine dinucleotide  hydrogen (NADH): ubiquinone oxidoreductase. CI dysregulation has been implicated in various CVDs, including heart failure, ischemic heart disease, pressure overload, hypertrophy, and cardiomyopathy. Several studies demonstrated that impaired CI function contributes to increased oxidative stress, altered calcium homeostasis, and mitochondrial DNA damage in cardiac cells, leading to cardiomyocyte dysfunction and apoptosis. CI dysfunction has been associated with endothelial dysfunction, inflammation, and vascular remodeling, critical processes in developing atherosclerosis and hypertension. Although CI is crucial in physiological and pathological conditions, no potential therapeutics targeting CI are available to treat CVDs. We believe that a lack of understanding of CI's precise mechanisms and contributions to CVDs limits the development of therapeutic strategies. In this review, we comprehensively analyze the role of CI in cardiovascular health and disease to shed light on its potential therapeutic target role in CVDs.

Structural insights into the assembly and the function of the plant oxidative phosphorylation system

One of the key functions of mitochondria is the production of ATP to support cellular metabolism and growth. The last step of mitochondrial ATP synthesis is performed by the oxidative phosphorylation (OXPHOS) system, an ensemble of protein complexes embedded in the inner mitochondrial membrane. In the last 25 yr, many structures of OXPHOS complexes and supercomplexes have been resolved in yeast, mammals, and bacteria. However, structures of plant OXPHOS enzymes only became available very recently. In this review, we highlight the plant-specific features revealed by the recent structures and discuss how they advance our understanding of the function and assembly of plant OXPHOS complexes. We also propose new hypotheses to be tested and discuss older findings to be re-evaluated. Further biochemical and structural work on the plant OXPHOS system will lead to a deeper understanding of plant respiration and its regulation, with significant agricultural, environmental, and societal implications.

The Mysterious Multitude: Structural Perspective on the Accessory Subunits of Respiratory Complex I

Complex I (CI) is the largest protein complex in the mitochondrial oxidative phosphorylation electron transport chain of the inner mitochondrial membrane and plays a key role in the transport of electrons from reduced substrates to molecular oxygen. CI is composed of 14 core subunits that are conserved across species and an increasing number of accessory subunits from bacteria to mammals. The fact that adding accessory subunits incurs costs of protein production and import suggests that these subunits play important physiological roles. Accordingly, knockout studies have demonstrated that accessory subunits are essential for CI assembly and function. Furthermore, clinical studies have shown that amino acid substitutions in accessory subunits lead to several debilitating and fatal CI deficiencies. Nevertheless, the specific roles of CI’s accessory subunits have remained mysterious. In this review, we explore the possible roles of each of mammalian CI’s 31 accessory subunits by integrating recent high-resolution CI structures with knockout, assembly, and clinical studies. Thus, we develop a framework of experimentally testable hypotheses for the function of the accessory subunits. We believe that this framework will provide inroads towards the complete understanding of mitochondrial CI physiology and help to develop strategies for the treatment of CI deficiencies.

Critical Role of NdhA in the Incorporation of the Peripheral Arm into the Membrane-Embedded Part of the Chloroplast NADH Dehydrogenase-Like Complex

The chloroplast NADH dehydrogenase-like (NDH) complex mediates ferredoxin-dependent plastoquinone reduction in the thylakoid membrane. In angiosperms, chloroplast NDH is composed of five subcomplexes and further forms a supercomplex with photosystem I (PSI). Subcomplex A (SubA) mediates the electron transport and consists of eight subunits encoded by both plastid and nuclear genomes. The assembly of SubA in the stroma has been extensively studied, but it is unclear how SubA is incorporated into the membrane-embedded part of the NDH complex. Here, we isolated a novel Arabidopsis mutant chlororespiratory reduction 16 (crr16) defective in NDH activity. CRR16 encodes a chloroplast-localized P-class pentatricopeptide repeat protein conserved in angiosperms. Transcript analysis of plastid-encoded ndh genes indicated that CRR16 was responsible for the efficient splicing of the group II intron in the ndhA transcript, which encodes a membrane-embedded subunit localized to the connecting site between SubA and the membrane subcomplex (SubM). To analyze the roles of NdhA in the assembly and stability of the NDH complex, the homoplastomic knockout plant of ndhA (ΔndhA) was generated in tobacco (Nicotiana tabacum). Biochemical analyses of crr16 and ΔndhA plants indicated that NdhA was essential for stabilizing SubA and SubE but not for the accumulation of the other three subcomplexes. Furthermore, the crr16 mutant accumulated the SubA assembly intermediates in the stroma more than that in the wild type. These results suggest that NdhA biosynthesis is essential for the incorporation of SubA into the membrane-embedded part of the NDH complex at the final assembly step of the NDH-PSI supercomplex.

Mitochondria: Their relevance during oocyte ageing

The oocyte is recognised as the largest cell in mammalian species and other multicellular organisms. Mitochondria represent a high proportion of the cytoplasm in oocytes and mitochondrial architecture is different in oocytes than in somatic cells, characterised by a rounder appearance and fragmented network. Although the number of mitochondria per oocyte is higher than in any other mammalian cell, their number and activity decrease with advancing age. Mitochondria integrate numerous processes essential for cellular function, such as metabolic processes related to energy production, biosynthesis, and waste removal, as well as Ca²⁺ signalling and reactive oxygen species (ROS) homeostasis. Further, mitochondria are responsible for the cellular adaptation to different types of stressors such as oxidative stress or DNA damage. When these stressors outstrip the adaptive capacity of mitochondria to restore homeostasis, it leads to mitochondrial dysfunction. Decades of studies indicate that mitochondrial function is multifaceted, which is reflected in the oocyte, where mitochondria support numerous processes during oocyte maturation, fertilization, and early embryonic development. Dysregulation of mitochondrial processes has been consistently reported in ageing and age-related diseases. In this review, we describe the functions of mitochondria as bioenergetic powerhouses and signal transducers in oocytes, how dysfunction of mitochondrial processes contributes to reproductive ageing, and whether mitochondria could be targeted to promote oocyte rejuvenation.

Is there a special relationship between complex I activity and nigral neuronal loss in Parkinson's disease? A critical reappraisal

Parkinson’s disease (PD) is a progressive neurodegenerative disease manifesting both motor and non-motor symptoms. The motor features are generally ascribed to the selective loss of dopamine neurons within the substantia nigra pars compacta. While the precise etiology of PD remains elusive, multiple genetic and environmental elements have emerged as contributing factors. The discovery of MPTP-induced parkinsonism directed intense inquiry towards mitochondrial pathways, with a specific focus on mitochondrial complex I. Consisting of more than 40 subunits, complex I is the first enzyme of the electron transport chain that is required for mitochondrial ATP production. In this review, we present a critical analysis of studies assessing the prevalence and specificity of mitochondrial complex I deficiency in PD. In addition, we take the novel view of incorporating the features of genetically-defined bona fide complex I disorders and the prevalence of nigral involvement in such cases. Through this innovative bi-directional view, we consider both complex I changes in a disease of the substantia nigra and nigral changes in diseases of complex I. We assess the strength of association between nigral cell loss and complex I deficits, as well as the oft under-appreciated heterogeneity of complex I deficiency disorders and the variability of the PD data.

Blackout in the powerhouse: clinical phenotypes associated with defects in the assembly of OXPHOS complexes and the mitoribosome

Mitochondria produce the bulk of the energy used by almost all eukaryotic cells through oxidative phosphorylation (OXPHOS) which occurs on the four complexes of the respiratory chain and the F₁–F₀ ATPase. Mitochondrial diseases are a heterogenous group of conditions affecting OXPHOS, either directly through mutation of genes encoding subunits of OXPHOS complexes, or indirectly through mutations in genes encoding proteins supporting this process. These include proteins that promote assembly of the OXPHOS complexes, the post-translational modification of subunits, insertion of cofactors or indeed subunit synthesis. The latter is important for all 13 of the proteins encoded by human mitochondrial DNA, which are synthesised on mitochondrial ribosomes. Together the five OXPHOS complexes and the mitochondrial ribosome are comprised of more than 160 subunits and many more proteins support their biogenesis. Mutations in both nuclear and mitochondrial genes encoding these proteins have been reported to cause mitochondrial disease, many leading to defective complex assembly with the severity of the assembly defect reflecting the severity of the disease. This review aims to act as an interface between the clinical and basic research underpinning our knowledge of OXPHOS complex and ribosome assembly, and the dysfunction of this process in mitochondrial disease.

The road to the structure of the mitochondrial respiratory chain supercomplex

The four complexes of the mitochondrial respiratory chain are critical for ATP production in most eukaryotic cells. Structural characterisation of these complexes has been critical for understanding the mechanisms underpinning their function. The three proton-pumping complexes, Complexes I, III and IV associate to form stable supercomplexes or respirasomes, the most abundant form containing 80 subunits in mammals. Multiple functions have been proposed for the supercomplexes, including enhancing the diffusion of electron carriers, providing stability for the complexes and protection against reactive oxygen species. Although high-resolution structures for Complexes III and IV were determined by X-ray crystallography in the 1990s, the size of Complex I and the supercomplexes necessitated advances in sample preparation and the development of cryo-electron microscopy techniques. We now enjoy structures for these beautiful complexes isolated from multiple organisms and in multiple states and together they provide important insights into respiratory chain function and the role of the supercomplex. While we as non-structural biologists use these structures for interpreting our own functional data, we need to remind ourselves that they stand on the shoulders of a large body of previous structural studies, many of which are still appropriate for use in understanding our results. In this mini-review, we discuss the history of respiratory chain structural biology studies leading to the structures of the mammalian supercomplexes and beyond.

Atomic structure of a mitochondrial complex I intermediate from vascular plants

Respiration, an essential metabolic process, provides cells with chemical energy. In eukaryotes, respiration occurs via the mitochondrial electron transport chain (mETC) composed of several large membrane-protein complexes. Complex I (CI) is the main entry point for electrons into the mETC. For plants, limited availability of mitochondrial material has curbed detailed biochemical and structural studies of their mETC. Here, we present the cryoEM structure of the known CI assembly intermediate CI* from Vigna radiata at 3.9 Å resolution. CI* contains CI's NADH-binding and CoQ-binding modules, the proximal-pumping module and the plant-specific γ-carbonic-anhydrase domain (γCA). Our structure reveals significant differences in core and accessory subunits of the plant complex compared to yeast, mammals and bacteria, as well as the details of the γCA domain subunit composition and membrane anchoring. The structure sheds light on differences in CI assembly across lineages and suggests potential physiological roles for CI* beyond assembly.

Assembly of Mitochondrial Complex I Requires the Low-Complexity Protein AMC1 in Chlamydomonas reinhardtii

Complex I is the first enzyme involved in the mitochondrial electron transport chain. With >40 subunits of dual genetic origin, the biogenesis of complex I is highly intricate and poorly understood. We used Chlamydomonas reinhardtii as a model system to reveal factors involved in complex I biogenesis. Two insertional mutants, displaying a complex I assembly defect characterized by the accumulation of a 700 kDa subcomplex, were analyzed. Genetic analyses showed these mutations were allelic and mapped to the gene AMC1 (Cre16.g688900) encoding a low-complexity protein of unknown function. The complex I assembly and activity in the mutant was restored by complementation with the wild-type gene, confirming AMC1 is required for complex I biogenesis. The N terminus of AMC1 targets a reporter protein to yeast mitochondria, implying that AMC1 resides and functions in the Chlamydomonas mitochondria. Accordingly, in both mutants, loss of AMC1 function results in decreased abundance of the mitochondrial nd4 transcript, which encodes the ND4 membrane subunit of complex I. Loss of ND4 in a mitochondrial nd4 mutant is characterized by a membrane arm assembly defect, similar to that exhibited by loss of AMC1. These results suggest AMC1 is required for the production of mitochondrially-encoded complex I subunits, specifically ND4. We discuss the possible modes of action of AMC1 in mitochondrial gene expression and complex I biogenesis.

Structures of Respiratory Supercomplex I+III2 Reveal Functional and Conformational Crosstalk

The mitochondrial electron transport chain complexes are organized into supercomplexes (SCs) of defined stoichiometry, which have been proposed to regulate electron flux via substrate channeling. We demonstrate that CoQ trapping in the isolated SC I+III₂ limits complex (C)I turnover, arguing against channeling. The SC structure, resolved at up to 3.8 Å in four distinct states, suggests that CoQ oxidation may be rate limiting because of unequal access of CoQ to the active sites of CIII₂. CI shows a transition between "closed" and "open" conformations, accompanied by the striking rotation of a key transmembrane helix. Furthermore, the state of CI affects the conformational flexibility within CIII₂, demonstrating crosstalk between the enzymes. CoQ was identified at only three of the four binding sites in CIII₂, suggesting that interaction with CI disrupts CIII₂ symmetry in a functionally relevant manner. Together, these observations indicate a more nuanced functional role for the SCs.

Low VDAC1 Expression Is Associated with an Aggressive Phenotype and Reduced Overall Patient Survival in Cholangiocellular Carcinoma

Cancer cells frequently exhibit dysfunctional oxidative phosphorylation (OXPHOS) and a concomitant increase in glycolytic flux. We investigated the expression of OXPHOS complex subunits and mitochondrial mass in 34 human cholangiocellular carcinomas (CCCs) and adjacent normal tissue by using tissue microarrays. In the tumor periphery, all OXPHOS complexes were reduced except complex I. In addition, significantly lower levels of complex IV were found at the tumor center (p < 0.0001). Mitochondrial mass, as indicated by VDAC1 expression, was significantly increased in CCCs compared to corresponding normal tissue (p < 0.0001). VDAC1 levels were inversely correlated with UICC (Union Internationale Contre le Cancer) cancer stage classification (p = 0.0065). Furthermore, significantly lower VDAC1 was present in patients with lymph node involvement (p = 0.02). Consistent with this, patients whose carcinomas expressed VDAC1 at low to moderate levels had significantly reduced survival compared to high expressors (p < 0.05). Therefore, low mitochondrial mass is associated with more aggressive CCC. These metabolic features are indicative of a Warburg phenotype in CCCs. This metabolic signature has potential therapeutic implications because tumors with low mitochondrial function may be targeted by metabolic therapies such as a high-fat, low-carbohydrate ketogenic diet.

Animal Model for Leigh Syndrome

Leigh syndrome (LS) is a common neurodegenerative disease affecting neonates with devastating sequences. One of the characteristic features for LS is the phenotypic polymorphism, which-in part-can be dedicated to variety of genetic causes. A strong correlation with mitochondrial dysfunction has been assumed as the main cause of LS. This was based on the fact that most genetic causes are related to mitochondrial complex I genome. The first animal LS model was designed based on NDUFS4 knockdown. Interestingly, however, this one or others could not recapitulate the whole spectrum of manifestations encountered in different cases of LS. We show in this chapter a new animal model for LS based on silencing of one gene that is reported previously in clinical cases, FOXRED1. The new model carries some differences from previous models in the fact that more histopathological degeneration in dopaminergic system is seen and more behavioral changes can be recognized. FOXRED1 is an interesting gene that is related to complex I assembly, hence, plays important role in different neurodegenerative disorders leading to different clinical manifestations.

Post-Translational Oxidative Modifications of Mitochondrial Complex I (NADH: Ubiquinone Oxidoreductase): Implications for Pathogenesis and Therapeutics in Human Diseases

Mitochondrial complex I (NADH: ubiquinone oxidoreductase; CI) is central to the electron transport chain (ETC), oxidative phosphorylation, and ATP production in eukaryotes. CI is a multi-subunit complex with a complicated yet organized structure that optimally connects electron transfer with proton translocation and forms higher-order supercomplexes with other ETC complexes. Efforts to understand the molecular genetics, expression profile of subunits, and structure-function relationship of CI have increased over the years due to the direct role of the complex in human diseases. Although mutations in the nuclear and mitochondrial genes of CI and altered expression of subunits could potentially lower CI activity leading to mitochondrial dysfunction in many diseases, oxidative post-translational modifications (PTMs) have emerged as an important mechanism contributing to altered CI activity. These mainly include reversible and irreversible cysteine modifications, tyrosine nitration, carbonylation, and tryptophan oxidation that are generated following exposure to reactive oxygen species/reactive nitrogen species. Interestingly, oxidative PTMs could contribute either to CI damage, mitochondrial dysfunction, and ensuing cell death or a response mechanism with potential cytoprotective effects. This has also emerged as a promising field for structural biologists since analysis of PTMs could assist in understanding the structure-function relationship of the complex and correlate electron transfer mechanism with energy production. However, analysis of PTMs of CI and their contribution to CI function are incomplete in many physiological and pathological conditions. This review aims to highlight the role of oxidative PTMs in modulating CI activity with implications toward pathobiology of CNS diseases and novel therapeutics.

In vivo chlorophyll fluorescence screening allows the isolation of a Chlamydomonas mutant defective for NDUFAF3, an assembly factor involved in mitochondrial complex I assembly

The qualitative screening method used to select complex I mutants in the microalga Chlamydomonas, based on reduced growth under heterotrophic conditions, is not suitable for high-throughput screening. In order to develop a fast screening method based on measurements of chlorophyll fluorescence, we first demonstrated that complex I mutants displayed decreased photosystem II efficiency in the genetic background of a photosynthetic mutation leading to reduced formation of the electrochemical proton gradient in the chloroplast (pgrl1 mutation). In contrast, single mutants (complex I and pgrl1 mutants) could not be distinguished from the wild type by their photosystem II efficiency under the conditions tested. We next performed insertional mutagenesis on the pgrl1 mutant. Out of about 3000 hygromycin-resistant insertional transformants, 46 had decreased photosystem II efficiency and three were complex I mutants. One of the mutants was tagged and whole genome sequencing identified the resistance cassette in NDUFAF3, a homolog of the human NDUFAF3 gene, encoding for an assembly factor involved in complex I assembly. Complemented strains showed restored complex I activity and assembly. Overall, we describe here a screening method which is fast and particularly suited for the identification of Chlamydomonas complex I mutants.

The Mitochondrial Complex(I)ty of Cancer

Recent evidence highlights that the cancer cell energy requirements vary greatly from normal cells and that cancer cells exhibit different metabolic phenotypes with variable participation of both glycolysis and oxidative phosphorylation. NADH-ubiquinone oxidoreductase (Complex I) is the largest complex of the mitochondrial electron transport chain and contributes about 40% of the proton motive force required for mitochondrial ATP synthesis. In addition, Complex I plays an essential role in biosynthesis and redox control during proliferation, resistance to cell death, and metastasis of cancer cells. Although knowledge about the structure and assembly of Complex I is increasing, information about the role of Complex I subunits in tumorigenesis is scarce and contradictory. Several small molecule inhibitors of Complex I have been described as selective anticancer agents; however, pharmacologic and genetic interventions on Complex I have also shown pro-tumorigenic actions, involving different cellular signaling. Here, we discuss the role of Complex I in tumorigenesis, focusing on the specific participation of Complex I subunits in proliferation and metastasis of cancer cells.

CHLORORESPIRATORY REDUCTION 9 is a Novel Factor Required for Formation of Subcomplex A of the Chloroplast NADH Dehydrogenase-Like Complex

In vascular plants, the chloroplast NADH dehydrogenase-like (NDH) complex, a homolog of respiratory NADH:quinone oxidoreductase (Complex I), mediates plastoquinone reduction using ferredoxin as an electron donor in cyclic electron transport around PSI in the thylakoid membrane. In angiosperms, chloroplast NDH is composed of five subcomplexes and forms a supercomplex with PSI. The modular assembly of stroma-protruded subcomplex A, which corresponds to the Q module of Complex I, was recently reported. However, the factors involved in the specific assembly steps have not been completely identified. Here, we isolated an Arabidopsis mutant, chlororespiratory reduction 9 (crr9), defective in NDH activity. The CRR9 gene encodes a novel stromal protein without any known functional domains or motifs. CRR9 is highly conserved in cyanobacteria and land plants but not in green algae, which do not have chloroplast NDH. Blue native-PAGE and immunoblot analyses of thylakoid proteins indicated that formation of subcomplex A was impaired in crr9 CRR9 was specifically required for the accumulation of NdhK, a subcomplex A subunit, in NDH assembly intermediates in the stroma. Furthermore, two-dimensional clear native/SDS-PAGE analysis of the stroma fraction indicated that incorporation of NdhM into NDH assembly intermediate complex 400 was impaired in crr9 These results suggest that CRR9 is a novel factor required for the formation of NDH subcomplex A.

Purification of Ovine Respiratory Complex I Results in a Highly Active and Stable Preparation

NADH-ubiquinone oxidoreductase (complex I) is the largest (∼1 MDa) and the least characterized complex of the mitochondrial electron transport chain. Because of the ease of sample availability, previous work has focused almost exclusively on bovine complex I. However, only medium resolution structural analyses of this complex have been reported. Working with other mammalian complex I homologues is a potential approach for overcoming these limitations. Due to the inherent difficulty of expressing large membrane protein complexes, screening of complex I homologues is limited to large mammals reared for human consumption. The high sequence identity among these available sources may preclude the benefits of screening. Here, we report the characterization of complex I purified from Ovis aries (ovine) heart mitochondria. All 44 unique subunits of the intact complex were identified by mass spectrometry. We identified differences in the subunit composition of subcomplexes of ovine complex I as compared with bovine, suggesting differential stability of inter-subunit interactions within the complex. Furthermore, the 42-kDa subunit, which is easily lost from the bovine enzyme, remains tightly bound to ovine complex I. Additionally, we developed a novel purification protocol for highly active and stable mitochondrial complex I using the branched-chain detergent lauryl maltose neopentyl glycol. Our data demonstrate that, although closely related, significant differences exist between the biochemical properties of complex I prepared from ovine and bovine mitochondria and that ovine complex I represents a suitable alternative target for further structural studies.

The architecture of respiratory supercomplexes

Mitochondrial electron transport chain complexes are organized into supercomplexes responsible for carrying out cellular respiration. Here we present three architectures of mammalian (ovine) supercomplexes determined by cryo-electron microscopy. We identify two distinct arrangements of supercomplex CICIII₂CIV (the respirasome)-a major 'tight' form and a minor 'loose' form (resolved at the resolution of 5.8 Å and 6.7 Å, respectively), which may represent different stages in supercomplex assembly or disassembly. We have also determined an architecture of supercomplex CICIII₂ at 7.8 Å resolution. All observed density can be attributed to the known 80 subunits of the individual complexes, including 132 transmembrane helices. The individual complexes form tight interactions that vary between the architectures, with complex IV subunit COX7a switching contact from complex III to complex I. The arrangement of active sites within the supercomplex may help control reactive oxygen species production. To our knowledge, these are the first complete architectures of the dominant, physiologically relevant state of the electron transport chain.

Atomic structure of the entire mammalian mitochondrial complex I

Mitochondrial complex I (also known as NADH:ubiquinone oxidoreductase) contributes to cellular energy production by transferring electrons from NADH to ubiquinone coupled to proton translocation across the membrane. It is the largest protein assembly of the respiratory chain with a total mass of 970 kilodaltons. Here we present a nearly complete atomic structure of ovine (Ovis aries) mitochondrial complex I at 3.9 Å resolution, solved by cryo-electron microscopy with cross-linking and mass-spectrometry mapping experiments. All 14 conserved core subunits and 31 mitochondria-specific supernumerary subunits are resolved within the L-shaped molecule. The hydrophilic matrix arm comprises flavin mononucleotide and 8 iron-sulfur clusters involved in electron transfer, and the membrane arm contains 78 transmembrane helices, mostly contributed by antiporter-like subunits involved in proton translocation. Supernumerary subunits form an interlinked, stabilizing shell around the conserved core. Tightly bound lipids (including cardiolipins) further stabilize interactions between the hydrophobic subunits. Subunits with possible regulatory roles contain additional cofactors, NADPH and two phosphopantetheine molecules, which are shown to be involved in inter-subunit interactions. We observe two different conformations of the complex, which may be related to the conformationally driven coupling mechanism and to the active-deactive transition of the enzyme. Our structure provides insight into the mechanism, assembly, maturation and dysfunction of mitochondrial complex I, and allows detailed molecular analysis of disease-causing mutations.

Dynamics of Human Mitochondrial Complex I Assembly: Implications for Neurodegenerative Diseases

Neurons are extremely energy demanding cells and highly dependent on the mitochondrial oxidative phosphorylation (OXPHOS) system. Mitochondria generate the energetic potential via the respiratory complexes I to IV, which constitute the electron transport chain (ETC), together with complex V. These redox reactions release energy in the form of ATP and also generate reactive oxygen species (ROS) that are involved in cell signaling but can eventually lead to oxidative stress. Complex I (CI or NADH:ubiquinone oxidoreductase) is the largest ETC enzyme, containing 44 subunits and the main contributor to ROS production. In recent years, the structure of the CI has become available and has provided new insights into CI assembly. A number of chaperones have been identified in the assembly and stability of the mature holo-CI, although they are not part of its final structure. Interestingly, CI dysfunction is the most common OXPHOS disorder in humans and defects in the CI assembly process are often observed. However, the dynamics of the events leading to CI biogenesis remain elusive, which precludes our understanding of how ETC malfunctioning affects neuronal integrity. Here, we review the current knowledge of the structural features of CI and its assembly factors and the potential role of CI misassembly in human disorders such as Complex I Deficiencies or Alzheimer's and Parkinson's diseases.

Mitochondrial gamma carbonic anhydrases are required for complex I assembly and plant reproductive development

Complex I of the mitochondrial electron transport chain (mETC) in plants contains an extra domain that is made up from proteins homologous to prokaryotic gamma-carbonic anhydrases (γCA). This domain has been suggested to participate in complex I assembly or to support transport of mitochondrial CO2 to the chloroplast. Here, we generated mutants lacking CA1 and CA2 - two out of three CA proteins in Arabidopsis thaliana. Double mutants were characterized at the developmental and physiological levels. Furthermore, the composition and activity of the mETC were determined, and mutated CA versions were used for complementation assays. Embryo development of double mutants was strongly delayed and seed development stopped before maturation. Mutant plants could only be rescued on sucrose media, showed severe stress symptoms and never produced viable seeds. By contrast, callus cultures were only slightly affected in growth. Complex I was undetectable in the double mutants, but complex II and complex IV were upregulated concomitant with increased oxygen consumption in mitochondrial respiration. Ectopic expression of inactive CA variants was sufficient to complement the mutant phenotype. Data indicate that CA proteins are structurally required for complex I assembly and that reproductive development is dependent on the presence of complex I.

The role of geochemistry and energetics in the evolution of modern respiratory complexes from a proton-reducing ancestor

Complex I or NADH quinone oxidoreductase (NUO) is an integral component of modern day respiratory chains and has a close evolutionary relationship with energy-conserving [NiFe]-hydrogenases of anaerobic microorganisms. Specifically, in all of biology, the quinone-binding subunit of Complex I, NuoD, is most closely related to the proton-reducing, H2-evolving [NiFe]-containing catalytic subunit, MbhL, of membrane-bound hydrogenase (MBH), to the methanophenzine-reducing subunit of a methanogenic respiratory complex (FPO) and to the catalytic subunit of an archaeal respiratory complex (MBX) involved in reducing elemental sulfur (S°). These complexes also pump ions and have at least 10 homologous subunits in common. As electron donors, MBH and MBX use ferredoxin (Fd), FPO uses either Fd or cofactor F420, and NUO uses either Fd or NADH. In this review, we examine the evolutionary trajectory of these oxidoreductases from a proton-reducing ancestral respiratory complex (ARC). We hypothesize that the diversification of ARC to MBH, MBX, FPO and eventually NUO was driven by the larger energy yields associated with coupling Fd oxidation to the reduction of oxidants with increasing electrochemical potential, including protons, S° and membrane soluble organic compounds such as phenazines and quinone derivatives. Importantly, throughout Earth's history, the availability of these oxidants increased as the redox state of the atmosphere and oceans became progressively more oxidized as a result of the origin and ecological expansion of oxygenic photosynthesis. ARC-derived complexes are therefore remarkably stable respiratory systems with little diversity in core structure but whose general function appears to have co-evolved with the redox state of the biosphere. This article is part of a Special Issue entitled Respiratory Complex I, edited by Volker Zickermann and Ulrich Brandt.

Plant mitochondrial Complex I composition and assembly: A review

In the mitochondrial inner membrane, oxidative phosphorylation generates ATP via the operation of several multimeric enzymes. The proton-pumping Complex I (NADH:ubiquinone oxidoreductase) is the first and most complicated enzyme required in this process. Complex I is an L-shaped enzyme consisting of more than 40 subunits, one FMN molecule and eight Fe-S clusters. In recent years, genetic and proteomic analyses of Complex I mutants in various model systems, including plants, have provided valuable insights into the assembly of this multimeric enzyme. Assisted by a number of key players, referred to as "assembly factors", the assembly of Complex I takes place in a sequential and modular manner. Although a number of factors have been identified, their precise function in mediating Complex I assembly still remains to be elucidated. This review summarizes our current knowledge of plant Complex I composition and assembly derived from studies in plant model systems such as Arabidopsis thaliana and Chlamydomonas reinhardtii. Plant Complex I is highly conserved and comprises a significant number of subunits also present in mammalian and fungal Complexes I. Plant Complex I also contains additional subunits absent from the mammalian and fungal counterpart, whose function in enzyme activity and assembly is not clearly understood. While 14 assembly factors have been identified for human Complex I, only two proteins, namely GLDH and INDH, have been established as bona fide assembly factors for plant Complex I. This article is part of a Special Issue entitled Respiratory complex I, edited by Volker Zickermann and Ulrich Brandt.

Assembly of the Escherichia coli NADH:ubiquinone oxidoreductase (respiratory complex I)

Energy-converting NADH:ubiquinone oxidoreductase, respiratory complex I, couples the electron transfer from NADH to ubiquinone with the translocation of four protons across the membrane. The Escherichia coli complex I is made up of 13 different subunits encoded by the so-called nuo-genes. The electron transfer is catalyzed by nine cofactors, a flavin mononucleotide and eight iron-sulfur (Fe/S)-clusters. The individual subunits and the cofactors have to be assembled together in a coordinated way to guarantee the biogenesis of the active holoenzyme. Only little is known about the assembly of the bacterial complex compared to the mitochondrial one. Due to the presence of so many Fe/S-clusters the assembly of complex I is intimately connected with the systems responsible for the biogenesis of these clusters. In addition, a few other proteins have been reported to be required for an effective assembly of the complex in other bacteria. The proposed role of known bacterial assembly factors is discussed and the information from other bacterial species is used in this review to draw an as complete as possible model of bacterial complex I assembly. In addition, the supramolecular organization of the complex in E. coli is briefly described. This article is part of a Special Issue entitled Organization and dynamics of bioenergetic systems in bacteria, edited by Prof. Conrad Mullineaux.