Structure of Mammalian Respiratory Supercomplex I 1 III 2 IV 1

Elucidating the contribution of ETC complexes I and II to the respirasome formation in cardiac mitochondria

Mitochondrial electron transport chain (ETC) plays a central role in ATP synthesis, and its dysfunction is associated with human diseases. Recent studies revealed that individual ETC complexes are assembled into supercomplexes. The main supercomplex, respirasome composed of complexes I, III, and IV has been suggested to improve electron channeling and control ROS production, maintain the structural integrity of ETC complexes and prevent protein aggregation in the inner mitochondrial membrane. However, many questions related to the structural organization of the respirasome, particularly, a possible role of complexes I and II in respirasome formation remain unclear. Here, we investigated whether genetic and pharmacological inhibition of complexes I and II affect respirasome assembly in cardioblast cells and isolated cardiac mitochondria. Pharmacological inhibition of the enzymatic activity of complexes I and II stimulated disruption of the respirasome. Likewise, knockdown of the complex I subunit NDUFA11 stimulated dissociation of respirasome and reduced the activity of complexes I, III, and IV. However, silencing of the membrane-anchored SDHC subunit of complex II had no effect on the respirasome assembly but reduced the activity of complexes II and IV. Downregulation of NDUFA11 or SDHC reduced ATP production and increased mitochondrial ROS production. Overall, these studies, for the first time, provide biochemical evidence that the complex I activity, and the NDUFA11 subunit are important for assembly and stability of the respirasome. The SDHC subunit of complex II is not involved in the respirasome however the complex may play a regulatory role in respirasome formation.

Long distance electron transfer through the aqueous solution between redox partner proteins

Despite the importance of electron transfer between redox proteins in photosynthesis and respiration, the inter-protein electron transfer rate between redox partner proteins has never been measured as a function of their separation in aqueous solution. Here, we use electrochemical tunneling spectroscopy to show that the current between two protein partners decays along more than 10 nm in the solution. Molecular dynamics simulations reveal a reduced ionic density and extended electric field in the volume confined between the proteins. The distance-decay factor and the calculated local barrier for electron transfer are regulated by the electrochemical potential applied to the proteins. Redox partners could use electrochemically gated, long distance electron transfer through the solution in order to conciliate high specificity with weak binding, thus keeping high turnover rates in the crowded environment of cells.

Electron transport chains rely on interactions between redox proteins, but the distance-dependence of the electron transfer rate through the solution is unknown. Here, the authors show that the current between two redox protein partners occurs at long distances and is electrochemically gated.

Human COX7A2L Regulates Complex III Biogenesis and Promotes Supercomplex Organization Remodeling without Affecting Mitochondrial Bioenergetics

The mitochondrial respiratory chain is organized in a dynamic set of supercomplexes (SCs). The COX7A2L protein is essential for mammalian SC III2+IV assembly. However, its function in respirasome (SCs I+III2+IVn) biogenesis remains controversial. To unambiguously determine the COX7A2L role, we generated COX7A2L-knockout (COX7A2L-KO) HEK293T and U87 cells. COX7A2L-KO cells lack SC III2+IV but have enhanced complex III steady-state levels, activity, and assembly rate, normal de novo complex IV biogenesis, and delayed respirasome formation. Nonetheless, the KOs have normal respirasome steady-state levels, and only larger structures (SCs I1-2+III2+IV2-n or megacomplexes) were undetected. Functional substrate-driven competition assays showed normal mitochondrial respiration in COX7A2L-KO cells in standard and nutritional-, environmental-, and oxidative-stress-challenging conditions. We conclude that COX7A2L establishes a regulatory checkpoint for the biogenesis of CIII2 and specific SCs, but the COX7A2L-dependent MRC remodeling is essential neither to maintain mitochondrial bioenergetics nor to cope with acute cellular stresses.

Exploring the quinone/inhibitor-binding pocket in mitochondrial respiratory complex I by chemical biology approaches

NADH-quinone oxidoreductase (respiratory complex I) couples NADH-to-quinone electron transfer to the translocation of protons across the membrane. Even though the architecture of the quinone-access channel in the enzyme has been modeled by X-ray crystallography and cryo-EM, conflicting findings raise the question whether the models fully reflect physiologically relevant states present throughout the catalytic cycle. To gain further insights into the structural features of the binding pocket for quinone/inhibitor, we performed chemical biology experiments using bovine heart sub-mitochondrial particles. We synthesized ubiquinones that are oversized (SF-UQs) or lipid-like (PC-UQs) and are highly unlikely to enter and transit the predicted narrow channel. We found that SF-UQs and PC-UQs can be catalytically reduced by complex I, albeit only at moderate or low rates. Moreover, quinone-site inhibitors completely blocked the catalytic reduction and the membrane potential formation coupled to this reduction. Photoaffinity-labeling experiments revealed that amiloride-type inhibitors bind to the interfacial domain of multiple core subunits (49 kDa, ND1, and PSST) and the 39-kDa supernumerary subunit, although the latter does not make up the channel cavity in the current models. The binding of amilorides to the multiple target subunits was remarkably suppressed by other quinone-site inhibitors and SF-UQs. Taken together, the present results are difficult to reconcile with the current channel models. On the basis of comprehensive interpretations of the present results and of previous findings, we discuss the physiological relevance of these models.

MDM2 and mitochondrial function: One complex intersection

Decades of research reveal that MDM2 participates in cellular processes ranging from macro-molecular metabolism to cancer signaling mechanisms. Two recent studies uncovered a new role for MDM2 in mitochondrial bioenergetics. Through the negative regulation of NDUFS1 (NADH:ubiquinone oxidoreductase 75 kDa Fe-S protein 1) and MT-ND6 (NADH dehydrogenase 6), MDM2 decreases the function and efficiency of Complex I (CI). These observations propose several important questions: (1) Where does MDM2 affect CI activity? (2) What are the cellular consequences of MDM2-mediated regulation of CI? (3) What are the physiological implications of these interactions? Here, we will address these questions and position these observations within the MDM2 literature.

Bi-allelic Mutations in NDUFA6 Establish Its Role in Early-Onset Isolated Mitochondrial Complex I Deficiency

Isolated complex I deficiency is a common biochemical phenotype observed in pediatric mitochondrial disease and often arises as a consequence of pathogenic variants affecting one of the ∼65 genes encoding the complex I structural subunits or assembly factors. Such genetic heterogeneity means that application of next-generation sequencing technologies to undiagnosed cohorts has been a catalyst for genetic diagnosis and gene-disease associations. We describe the clinical and molecular genetic investigations of four unrelated children who presented with neuroradiological findings and/or elevated lactate levels, highly suggestive of an underlying mitochondrial diagnosis. Next-generation sequencing identified bi-allelic variants in NDUFA6, encoding a 15 kDa LYR-motif-containing complex I subunit that forms part of the Q-module. Functional investigations using subjects’ fibroblast cell lines demonstrated complex I assembly defects, which were characterized in detail by mass-spectrometry-based complexome profiling. This confirmed a marked reduction in incorporated NDUFA6 and a concomitant reduction in other Q-module subunits, including NDUFAB1, NDUFA7, and NDUFA12. Lentiviral transduction of subjects’ fibroblasts showed normalization of complex I. These data also support supercomplex formation, whereby the ∼830 kDa complex I intermediate (consisting of the P- and Q-modules) is in complex with assembled complex III and IV holoenzymes despite lacking the N-module. Interestingly, RNA-sequencing data provided evidence that the consensus RefSeq accession number does not correspond to the predominant transcript in clinically relevant tissues, prompting revision of the NDUFA6 RefSeq transcript and highlighting not only the importance of thorough variant interpretation but also the assessment of appropriate transcripts for analysis.

YoungMito 2018: Report on the 1st International Mitochondria Meeting for Young Scientists

The 1st International Mitochondria Meeting for Young Scientists (International YoungMito 2018) was held at Hotel Co-op Inn Kyoto in Kyoto, Japan, from 20 to 22 April 2018. The meeting was attended by 130 mitochondrial researchers from 15 countries. International YoungMito 2018 was the first international mitochondria meeting held in Japan organized by and for young mitochondrial researchers. Over the 3-day period, there were 28 oral presentations including two keynote lectures, 20 presentations from invited speakers, and six short talks selected from abstract submissions. Many different topics were covered including quality control pathways acting against mitochondrial stresses, mitochondrial dynamics, protein/lipid transport, cristae organization, respiration/ATP synthesis, mtDNA maintenance, mitochondrial disease models, and pharmacological approaches. In addition, we had 64 posters, a number which represented almost half of all attendees. Thanks to the cutting-edge information and high-quality unpublished data that were presented, there were many lively discussions during oral and poster sessions that continued into the coffee breaks, lunchtime, and nighttime discussions. The 1st international YoungMito meeting was successful in promoting intellectual exchange among all participants, facilitating collaborations beyond national boundaries, and closed with great success. It was a great pleasure that many participants were looking forward to the next YoungMito meeting.

Cryo-EM structure of respiratory complex I at work

Mitochondrial complex I has a key role in cellular energy metabolism, generating a major portion of the proton motive force that drives aerobic ATP synthesis. The hydrophilic arm of the L-shaped ~1 MDa membrane protein complex transfers electrons from NADH to ubiquinone, providing the energy to drive proton pumping at distant sites in the membrane arm. The critical steps of energy conversion are associated with the redox chemistry of ubiquinone. We report the cryo-EM structure of complete mitochondrial complex I from the aerobic yeast Yarrowia lipolytica both in the deactive form and after capturing the enzyme during steady-state activity. The site of ubiquinone binding observed during turnover supports a two-state stabilization change mechanism for complex I.

Reprogramming of mitochondrial metabolism by innate immunity

The reprogramming of cellular metabolism has emerged as a major aspect of innate immune cell activation. Mitochondria, which are well known for their critical functions in cellular bioenergetics and metabolism, also serve innate immune purposes by providing specific signaling platforms. Latest advances in our understanding of innate immune receptor-mediated metabolic reprogramming have unraveled specific immune functions of mitochondrial metabolites that place mitochondrial metabolism and particularly the mitochondrial respiratory chain at the center of innate immunity. This review highlights some recent studies that support mitochondrial metabolism as major immune signaling rheostat upon microbe recognition by innate immune cells.

Cytonuclear integration and co-evolution

The partitioning of genetic material between the nucleus and cytoplasmic (mitochondrial and plastid) genomes within eukaryotic cells necessitates coordinated integration between these genomic compartments, with important evolutionary and biomedical implications. Classic questions persist about the pervasive reduction of cytoplasmic genomes via a combination of gene loss, transfer and functional replacement - and yet why they are almost always retained in some minimal form. One striking consequence of cytonuclear integration is the existence of 'chimeric' enzyme complexes composed of subunits encoded in two different genomes. Advances in structural biology and comparative genomics are yielding important insights into the evolution of such complexes, including correlated sequence changes and recruitment of novel subunits. Thus, chimeric cytonuclear complexes provide a powerful window into the mechanisms of molecular co-evolution.

Cryo-electron tomography for the structural study of mitochondrial translation

Cryo-electron tomography (cryo-ET) enables the three-dimensional (3D) structural characterization of macromolecular complexes in their physiological environment. Thus, cryo-ET is uniquely suited to study the structural basis of biomolecular processes that are extremely difficult or even impossible to reconstitute using purified components. Translation of mitochondrial genes, which occurs in the secluded interior of mitochondria, falls into this category. Here, we describe the principles of cryo-ET in the context of mitochondrial translation and outline recent developments and challenges of the method. The 3D image of a frozen-hydrated biological sample is computed from its 2D projections, which are acquired using a transmission electron microscope. In conjunction with automated detection of different copies of the molecule of interest and averaging of the corresponding subtomograms, cryo-ET enables macromolecular structure determination in the native environment (i.e. in situ) at sub-nanometer resolution. The preservation of the native environment furthermore allows the extraction of contextual information about the molecules, including the location of specific molecules with respect to membranes, their relative positioning and the spatial organization with respect to other types of macromolecules. Recent preparative developments extend the field of application of cryo-ET from isolated organelles to cultured eukaryotic cells and even tissue, making the traditional borders between molecular and cellular structural biology disappear.

Mitochondrial Supercomplexes Do Not Enhance Catalysis by Quinone Channeling

Mitochondrial respiratory supercomplexes, comprising complexes I, III, and IV, are the minimal functional units of the electron transport chain. Assembling the individual complexes into supercomplexes may stabilize them, provide greater spatiotemporal control of respiration, or, controversially, confer kinetic advantages through the sequestration of local quinone and cytochrome c pools (substrate channeling). Here, we have incorporated an alternative quinol oxidase (AOX) into mammalian heart mitochondrial membranes to introduce a competing pathway for quinol oxidation and test for channeling. AOX substantially increases the rate of NADH oxidation by O2 without affecting the membrane integrity, the supercomplexes, or NADH-linked oxidative phosphorylation. Therefore, the quinol generated in supercomplexes by complex I is reoxidized more rapidly outside the supercomplex by AOX than inside the supercomplex by complex III. Our results demonstrate that quinone and quinol diffuse freely in and out of supercomplexes: substrate channeling does not occur and is not required to support respiration.

Barth syndrome cells display widespread remodeling of mitochondrial complexes without affecting metabolic flux distribution

Barth syndrome (BTHS) is a rare X-linked disorder that is characterized by cardiac and skeletal myopathy, neutropenia and growth abnormalities. The disease is caused by mutations in the tafazzin (TAZ) gene encoding an enzyme involved in the acyl chain remodeling of the mitochondrial phospholipid cardiolipin (CL). Biochemically, this leads to decreased levels of mature CL and accumulation of the intermediate monolysocardiolipin (MLCL). At a cellular level, this causes mitochondrial fragmentation and reduced stability of the respiratory chain supercomplexes. However, the exact mechanism through which tafazzin deficiency leads to disease development remains unclear. We therefore aimed to elucidate the pathways affected in BTHS cells by employing proteomic and metabolic profiling assays. Complexome profiling of patient skin fibroblasts revealed significant effects for about 200 different mitochondrial proteins. Prominently, we found a specific destabilization of higher order oxidative phosphorylation (OXPHOS) supercomplexes, as well as changes in complexes involved in cristae organization and CL trafficking. Moreover, the key metabolic complexes 2-oxoglutarate dehydrogenase (OGDH) and branched-chain ketoacid dehydrogenase (BCKD) were profoundly destabilized in BTHS patient samples. Surprisingly, metabolic flux distribution assays using stable isotope tracer-based metabolomics did not show reduced flux through the TCA cycle. Overall, insights from analyzing the impact of TAZ mutations on the mitochondrial complexome provided a better understanding of the resulting functional and structural consequences and thus the pathological mechanisms leading to Barth syndrome.

Respiratory chain Complex I of unparalleled divergence in diplonemids

Mitochondrial genes of Euglenozoa (Kinetoplastida, Diplonemea, and Euglenida) are notorious for being barely recognizable, raising the question of whether such divergent genes actually code for functional proteins. Here we demonstrate the translation and identify the function of five previously unassigned y genes encoded by mitochondrial DNA (mtDNA) of diplonemids. As is the rule in diplonemid mitochondria, y genes are fragmented, with gene pieces transcribed separately and then trans-spliced to form contiguous mRNAs. Further, y transcripts undergo massive RNA editing, including uridine insertions that generate up to 16-residue-long phenylalanine tracts, a feature otherwise absent from conserved mitochondrial proteins. By protein sequence analyses, MS, and enzymatic assays in Diplonema papillatum, we show that these y genes encode the subunits Nad2, -3, -4L, -6, and -9 of the respiratory chain Complex I (CI; NADH:ubiquinone oxidoreductase). The few conserved residues of these proteins are essentially those involved in proton pumping across the inner mitochondrial membrane and in coupling ubiquinone reduction to proton pumping (Nad2, -3, -4L, and -6) and in interactions with subunits containing electron-transporting Fe-S clusters (Nad9). Thus, in diplonemids, 10 CI subunits are mtDNA-encoded. Further, MS of D. papillatum CI allowed identification of 26 conventional and 15 putative diplonemid-specific nucleus-encoded components. Most conventional accessory subunits are well-conserved but unusually long, possibly compensating for the streamlined mtDNA-encoded components and for missing, otherwise widely distributed, conventional subunits. Finally, D. papillatum CI predominantly exists as a supercomplex I:III:IV that is exceptionally stable, making this protist an organism of choice for structural studies.

The role of mitochondrial cardiolipin in heart function and its implication in cardiac disease

Mitochondria play an essential role in the energy metabolism of the heart. Many of the essential functions are associated with mitochondrial membranes and oxidative phosphorylation driven by the respiratory chain. Mitochondrial membranes are unique in the cell as they contain the phospholipid cardiolipin. The important role of cardiolipin in cardiovascular health is highlighted by several cardiac diseases, in which cardiolipin plays a fundamental role. Barth syndrome, Sengers syndrome, and Dilated cardiomyopathy with ataxia (DCMA) are genetic disorders, which affect cardiolipin biosynthesis. Other cardiovascular diseases including ischemia/reperfusion injury and heart failure are also associated with changes in the cardiolipin pool. Here, we summarize molecular functions of cardiolipin in mitochondrial biogenesis and morphology. We highlight the role of cardiolipin for the respiratory chain, metabolite carriers, and mitochondrial metabolism and describe links to apoptosis and mitochondria specific autophagy (mitophagy) with possible implications in cardiac disease.

Fucoidan Protects Dopaminergic Neurons by Enhancing the Mitochondrial Function in a Rotenone-induced Rat Model of Parkinson's Disease

The mitochondrion is susceptible to neurodegenerative disorders such as Parkinson’s disease (PD). Mitochondrial dysfunction has been considered to play an important role in the dopaminergic degeneration in PD. However, there are no effective drugs to protect mitochondria from dysfunction during the disease development. In the present study, fucoidan, a sulfated polysaccharide derived from Laminaria japonica, was investigated and characterized for its protective effect on the dopamine system and mitochondrial function of dopaminergic neurons in a rotenone-induced rat model of PD. We found that chronic treatment with fucoidan significantly reversed the loss of nigral dopaminergic neurons and striatal dopaminergic fibers and the reduction of striatal dopamine levels in PD rats. Fucoidan also alleviated rotenone-induced behavioral deficits. Moreover, the mitochondrial respiratory function as detected by the mitochondrial oxygen consumption and the expression of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) and nuclear transcription factor 2 (NRF2) were reduced in the substantia nigra of PD rats, which were markedly reversed by fucoidan. Oxidative products induced by rotenone were significantly reduced by fucoidan. Taken together, these results demonstrate that fucoidan possesses the ability to protect the dopamine system in PD rats. The neuroprotective effect of fucoidan may be mediated via reserving mitochondrial function involving the PGC-1α/NRF2 pathway. This study provides new evidence that fucoidan can be explored in PD therapy.

Mammalian Mitochondrial Complex I Structure and Disease-Causing Mutations

Complex I has an essential role in ATP production by coupling electron transfer from NADH to quinone with translocation of protons across the inner mitochondrial membrane. Isolated complex I deficiency is a frequent cause of mitochondrial inherited diseases. Complex I has also been implicated in cancer, ageing, and neurodegenerative conditions. Until recently, the understanding of complex I deficiency on the molecular level was limited due to the lack of high-resolution structures of the enzyme. However, due to developments in single particle cryo-electron microscopy (cryo-EM), recent studies have reported nearly atomic resolution maps and models of mitochondrial complex I. These structures significantly add to our understanding of complex I mechanism and assembly. The disease-causing mutations are discussed here in their structural context.

Structure of the intact 14-subunit human cytochrome c oxidase

Respiration is one of the most basic features of living organisms, and the electron transport chain complexes are probably the most complicated protein system in mitochondria. Complex-IV is the terminal enzyme of the electron transport chain, existing either as randomly scattered complexes or as a component of supercomplexes. NDUFA4 was previously assumed as a subunit of complex-I, but recent biochemical data suggested it may be a subunit of complex-IV. However, no structural evidence supporting this notion was available till now. Here we obtained the 3.3 Å resolution structure of complex-IV derived from the human supercomplex I₁III₂IV₁ and assigned the NDUFA4 subunit into complex-IV. Intriguingly, NDUFA4 lies exactly at the dimeric interface observed in previously reported crystal structures of complex-IV homodimer which would preclude complex-IV dimerization. Combining previous structural and biochemical data shown by us and other groups, we propose that the intact complex-IV is a monomer containing 14 subunits.

Assembly of mammalian oxidative phosphorylation complexes I-V and supercomplexes

The assembly of the five oxidative phosphorylation system (OXPHOS) complexes in the inner mitochondrial membrane is an intricate process. The human enzymes comprise core proteins, performing the catalytic activities, and a large number of 'supernumerary' subunits that play essential roles in assembly, regulation and stability. The correct addition of prosthetic groups as well as chaperoning and incorporation of the structural components require a large number of factors, many of which have been found mutated in cases of mitochondrial disease. Nowadays, the mechanisms of assembly for each of the individual complexes are almost completely understood and the knowledge about the assembly factors involved is constantly increasing. On the other hand, it is now well established that complexes I, III and IV interact with each other, forming the so-called respiratory supercomplexes or 'respirasomes', although the pathways that lead to their formation are still not completely clear. This review is a summary of our current knowledge concerning the assembly of complexes I-V and of the supercomplexes.

Mitochondrial dysfunction in the neuro-degenerative and cardio-degenerative disease, Friedreich's ataxia

Mitochondrial homeostasis is essential for maintaining healthy cellular function and survival. The detrimental involvement of mitochondrial dysfunction in neuro-degenerative diseases has recently been highlighted in human conditions, such as Parkinson's, Alzheimer's and Huntington's disease. Friedreich's ataxia (FA) is another neuro-degenerative, but also cardio-degenerative condition, where mitochondrial dysfunction plays a crucial role in disease progression. Deficient expression of the mitochondrial protein, frataxin, is the primary cause of FA, which leads to adverse alterations in whole cell and mitochondrial iron metabolism. Dys-regulation of iron metabolism in these compartments, results in the accumulation of inorganic iron deposits in the mitochondrial matrix that is thought to potentiate oxidative damage observed in FA. Therefore, the maintenance of mitochondrial homeostasis is crucial in the progression of neuro-degenerative conditions, particularly in FA. In this review, vital mitochondrial homeostatic processes and their roles in FA pathogenesis will be discussed. These include mitochondrial iron processing, mitochondrial dynamics (fusion and fission processes), mitophagy, mitochondrial biogenesis, mitochondrial energy production and calcium metabolism.

Cryo-EM structures of complex I from mouse heart mitochondria in two biochemically defined states

Complex I (NADH:ubiquinone oxidoreductase) uses the reducing potential of NADH to drive protons across the energy-transducing inner membrane and power oxidative phosphorylation in mammalian mitochondria. Recent cryo-EM analyses have produced near-complete models of all 45 subunits in the bovine, ovine and porcine complexes and have identified two states relevant to complex I in ischemia-reperfusion injury. Here, we describe the 3.3-Å structure of complex I from mouse heart mitochondria, a biomedically relevant model system, in the 'active' state. We reveal a nucleotide bound in subunit NDUFA10, a nucleoside kinase homolog, and define mechanistically critical elements in the mammalian enzyme. By comparisons with a 3.9-Å structure of the 'deactive' state and with known bacterial structures, we identify differences in helical geometry in the membrane domain that occur upon activation or that alter the positions of catalytically important charged residues. Our results demonstrate the capability of cryo-EM analyses to challenge and develop mechanistic models for mammalian complex I.

Metabolism-Associated Markers and Childhood Autism Rating Scales (CARS) as a Measure of Autism Severity

Autism spectrum disorder (ASD) is a neuro-behavioral syndrome with a broad spectrum of different mechanisms and etiologies that are caused by abnormal brain development. To date, no highly reliable and effective diagnostic biomarker to assess ASD is available so far. The present study investigated the predictivity potential of some suggested markers in ASD diagnosis focusing onto the relative ratios of several plasma biomarkers of electron transport chain function, and mitochondrial metabolism in 41 patients with ASD evaluated for behavior deficits measured using Childhood Autism Rating Scales (CARS). The control matched for further 41 healthy subjects. The relation of these relative ratios to ASD severity was also examined, as well as their ability to distinguish ASD children from neurotypical children. All predictive ratios were found to be markedly altered and correlated in ASD patients. However, no ratio was connected with autism severity. Interestingly, MRCC-I/caspase-7, GSH/GST, and MRCC-I/COQ10 were the most distinctive relative ratios between neurotypical controls and ASD patients and may thereby be useful biomarkers for early diagnosis of ASD. Overall, this investigation proves that relative ratios of numerous mitochondrial biomarkers might be predictive and efficient to differentiate between neurotypical children and ASD.

Cardiomyopathy-associated mutation in the ADP/ATP carrier reveals translation-dependent regulation of cytochrome c oxidase activity

How the absence of the major mitochondrial ADP/ATP carrier in yeast, Aac2p, results in a specific defect in cytochrome c oxidase (COX; complex IV) activity is a long-standing mystery. Aac2p physically associates with respiratory supercomplexes, which include complex IV, raising the possibility that its activity is dependent on its association with Aac2p. Here, we have leveraged a transport-dead pathogenic AAC2 point mutant to determine the basis for the reduced COX activity in the absence of Aac2p. The steady-state levels of complex IV subunits encoded by the mitochondrial genome are significantly reduced in the absence of Aac2p function, whether its association with respiratory supercomplexes is preserved or not. This diminution in COX amounts is not caused by a reduction in the mitochondrial genome copy number or the steady-state level of its transcripts, and does not reflect a defect in complex IV assembly. Instead, the absence of Aac2p activity, genetically or pharmacologically, results in an aberrant pattern of mitochondrial translation. Interestingly, compared with the complete absence of Aac2p, the complex IV-related defects are greater in mitochondria expressing the transport-inactive Aac2p mutant. Our results highlight a critical role for Aac2p transport in mitochondrial translation whose disturbance uniquely impacts cytochrome c oxidase.

Ethanolamine ameliorates mitochondrial dysfunction in cardiolipin-deficient yeast cells

Cardiolipin (CL) is a signature phospholipid of the mitochondria required for the formation of mitochondrial respiratory chain (MRC) supercomplexes. The destabilization of MRC supercomplexes is the proximal cause of the pathology associated with the depletion of CL in patients with Barth syndrome. Thus, promoting supercomplex formation could ameliorate mitochondrial dysfunction associated with CL depletion. However, to date, physiologically relevant small-molecule regulators of supercomplex formation have not been identified. Here, we report that ethanolamine (Etn) supplementation rescues the MRC defects by promoting supercomplex assembly in a yeast model of Barth syndrome. We discovered this novel role of Etn while testing the hypothesis that elevating mitochondrial phosphatidylethanolamine (PE), a phospholipid suggested to overlap in function with CL, could compensate for CL deficiency. We found that the Etn supplementation rescues the respiratory growth of CL-deficient Saccharomyces cerevisiae cells in a dose-dependent manner but independently of its incorporation into PE. The rescue was specifically dependent on Etn but not choline or serine, the other phospholipid precursors. Etn improved mitochondrial function by restoring the expression of MRC proteins and promoting supercomplex assembly in CL-deficient cells. Consistent with this mechanism, overexpression of Cox4, the MRC complex IV subunit, was sufficient to promote supercomplex formation in CL-deficient cells. Taken together, our work identifies a novel role of a ubiquitous metabolite, Etn, in attenuating mitochondrial dysfunction caused by CL deficiency.

A New Perspective: Mitochondrial Stat3 as a Regulator for Lymphocyte Function

Stat3 as a transcription factor regulating gene expression in lymphocytes during the immune response is well known. However, since the pioneering studies discovering the presence of Stat3 in mitochondria and its role in regulating mitochondrial metabolism, only a few studies have investigated this non-conventional function of Stat3 in lymphocytes. From this perspective, we review what is known about Stat3 as a transcription factor and what is known and unknown about mitochondrial Stat3 (mitoStat3) in lymphocytes. We also provide a framework to consider how some of the functions previously assigned to Stat3 as regulator of gene transcription could be mediated by mitoStat3 in lymphocytes. The goal of this review is to stimulate interest for future studies investigating mitoStat3 in the immune response that could lead to the generation of alternative pharmacological inhibitors of mitoStat3 for the treatment of chronic inflammatory diseases.

The Organization of Mitochondrial Supercomplexes is Modulated by Oxidative Stress In Vivo in Mouse Models of Mitochondrial Encephalopathy

We examine the effect of oxidative stress on the stability of mitochondrial respiratory complexes and their association into supercomplexes (SCs) in the neuron-specific Rieske iron sulfur protein (RISP) and COX10 knockout (KO) mice. Previously we reported that these two models display different grades of oxidative stress in distinct brain regions. Using blue native gel electrophoresis, we observed a redistribution of the architecture of SCs in KO mice. Brain regions with moderate levels of oxidative stress (cingulate cortex of both COX10 and RISP KO and hippocampus of the RISP KO) showed a significant increase in the levels of high molecular weight (HMW) SCs. High levels of oxidative stress in the piriform cortex of the RISP KO negatively impacted the stability of CI, CIII and SCs. Treatment of the RISP KO with the mitochondrial targeted antioxidant mitoTEMPO preserved the stability of respiratory complexes and formation of SCs in the piriform cortex and increased the levels of glutathione peroxidase. These results suggest that mild to moderate levels of oxidative stress can modulate SCs into a more favorable architecture of HMW SCs to cope with rising levels of free radicals and cover the energetic needs.

Absence of Complex I Implicates Rearrangement of the Respiratory Chain in European Mistletoe

The mitochondrial oxidative phosphorylation (OXPHOS) system, which is based on the presence of five protein complexes, is in the very center of cellular ATP production. Complexes I to IV are components of the respiratory electron transport chain that drives proton translocation across the inner mitochondrial membrane. The resulting proton gradient is used by complex V (the ATP synthase complex) for the phosphorylation of ADP. Occurrence of complexes I to V is highly conserved in eukaryotes, with exceptions being restricted to unicellular parasites that take up energy-rich compounds from their hosts. Here we present biochemical evidence that the European mistletoe (Viscum album), an obligate semi-parasite living on branches of trees, has a highly unusual OXPHOS system. V. album mitochondria completely lack complex I and have greatly reduced amounts of complexes II and V. At the same time, the complexes III and IV form remarkably stable respiratory supercomplexes. Furthermore, complexome profiling revealed the presence of 150 kDa complexes that include type II NAD(P)H dehydrogenases and an alternative oxidase. Although the absence of complex I genes in mitochondrial genomes of mistletoe species has recently been reported, this is the first biochemical proof that these genes have not been transferred to the nuclear genome and that this respiratory complex indeed is not assembled. As a consequence, the whole respiratory chain is remodeled. Our results demonstrate that, in the context of parasitism, multicellular life can cope with lack of one of the OXPHOS complexes and give new insights into the life strategy of mistletoe species.

Suppression of reactive oxygen species generation in heart mitochondria from anoxic turtles: the role of complex I S-nitrosation

Freshwater turtles (Trachemys scripta) are among the very few vertebrates capable of tolerating severe hypoxia and re-oxygenation without suffering from damage to the heart. As myocardial ischemia and reperfusion causes a burst of mitochondrial reactive oxygen species (ROS) in mammals, the question arises as to whether, and if so how, this ROS burst is prevented in the turtle heart. We find that heart mitochondria isolated from turtles acclimated to anoxia produce less ROS than mitochondria from normoxic turtles when consuming succinate. As succinate accumulates in the hypoxic heart and is oxidized when oxygen returns, this suggests an adaptation to lessen ROS production. Specific S-nitrosation of complex I can lower ROS in mammals and here we show that turtle complex I activity and ROS production can also be strongly depressed in vitro by S-nitrosation. We detect in vivo endogenous S-nitrosated complex I in turtle heart mitochondria, but these levels are unaffected upon anoxia acclimation. Thus, while heart mitochondria from anoxia-acclimated turtles generate less ROS and have a lower aerobic capacity than those from normoxic turtles, this is not due to decreases in complex I activity or expression levels. Interestingly, in-gel activity staining reveals that most complex I of heart mitochondria from normoxic and anoxic turtles forms stable super-complexes with other respiratory enzymes and, in contrast to mammals, these are not disrupted by dodecyl maltoside. Taken together, these results show that although S-nitrosation of complex I is a potent mechanism to prevent ROS formation upon re-oxygenation after anoxia in vitro, this is not a major cause of the suppression of ROS production by anoxic turtle heart mitochondria.

Respiratory chain supercomplexes: Structures, function and biogenesis

Over the past sixty years, researchers have made outmost efforts to clarify the structural organization and functional regulation of the complexes that configure the mitochondrial respiratory chain. As a result, the entire composition of each individual complex is practically known and, aided by notable structural advances in mammals, it is now widely accepted that these complexes stablish interactions to form higher-order supramolecular structures called supercomplexes and respirasomes. The mechanistic models and players that regulate the function and biogenesis of such superstructures are still under intense debate, and represent one of the hottest topics of the mitochondrial research field at present. Noteworthy, understanding the pathways involved in the assembly and organization of respiratory chain complexes and supercomplexes is of high biomedical relevance because molecular alterations in these pathways frequently result in severe mitochondrial disorders. The purpose of this review is to update the structural, biogenetic and functional knowledge about the respiratory chain supercomplexes and assembly factors involved in their formation, with special emphasis on their implications in mitochondrial disease. Thanks to the integrated data resulting from recent structural, biochemical and genetic approaches in diverse biological systems, the regulation of the respiratory chain function arises at multiple levels of complexity.

Structure and mechanism of mitochondrial electron transport chain

Respiration is one of the most vital and basic features of living organisms. In mammals, respiration is accomplished by respiratory chain complexes located on the mitochondrial inner membrane. In the past century, scientists put tremendous efforts in understanding these complexes, but failed to solve the high resolution structure until recently. In 2016, three research groups reported the structure of respiratory chain supercomplex from different species, and fortunately the structure solved by our group has the highest resolution. In this review, we will compare the recently solved structures of respirasome, probe into the relationship between cristae shape and respiratory chain organization, and discuss the highly disputed issues afterwards. Besides, our group reported the first high resolution structure of respirasome and medium resolution structure of megacomplex from cultured human cells this year. Definitely, these supercomplex structures will provide precious information for conquering the mitochondrial malfunction diseases.

Adaptive remodeling of skeletal muscle energy metabolism in high-altitude hypoxia: Lessons from AltitudeOmics

Metabolic responses to hypoxia play important roles in cell survival strategies and disease pathogenesis in humans. However, the homeostatic adjustments that balance changes in energy supply and demand to maintain organismal function under chronic low oxygen conditions remain incompletely understood, making it difficult to distinguish adaptive from maladaptive responses in hypoxia-related pathologies. We integrated metabolomic and proteomic profiling with mitochondrial respirometry and blood gas analyses to comprehensively define the physiological responses of skeletal muscle energy metabolism to 16 days of high-altitude hypoxia (5260 m) in healthy volunteers from the AltitudeOmics project. In contrast to the view that hypoxia down-regulates aerobic metabolism, results show that mitochondria play a central role in muscle hypoxia adaptation by supporting higher resting phosphorylation potential and enhancing the efficiency of long-chain acylcarnitine oxidation. This directs increases in muscle glucose toward pentose phosphate and one-carbon metabolism pathways that support cytosolic redox balance and help mitigate the effects of increased protein and purine nucleotide catabolism in hypoxia. Muscle accumulation of free amino acids favor these adjustments by coordinating cytosolic and mitochondrial pathways to rid the cell of excess nitrogen, but might ultimately limit muscle oxidative capacity in vivo Collectively, these studies illustrate how an integration of aerobic and anaerobic metabolism is required for physiological hypoxia adaptation in skeletal muscle, and highlight protein catabolism and allosteric regulation as unexpected orchestrators of metabolic remodeling in this context. These findings have important implications for the management of hypoxia-related diseases and other conditions associated with chronic catabolic stress.

Oxidative phosphorylation supercomplexes and respirasome reconstitution of the colorless alga Polytomella sp

The proposal that the respiratory complexes can associate with each other in larger structures named supercomplexes (SC) is generally accepted. In the last decades most of the data about this association came from studies in yeasts, mammals and plants, and information is scarce in other lineages. Here we studied the supramolecular association of the F₁F_O-ATP synthase (complex V) and the respiratory complexes I, III and IV of the colorless alga Polytomella sp. with an approach that involves solubilization using mild detergents, n-dodecyl-β-D-maltoside (DDM) or digitonin, followed by separation of native protein complexes by electrophoresis (BN-PAGE), after which we identified oligomeric forms of complex V (mainly V₂ and V₄) and different respiratory supercomplexes (I/IV₆, I/III₄, I/IV). In addition, purification/reconstitution of the supercomplexes by anion exchange chromatography was also performed. The data show that these complexes have the ability to strongly associate with each other and form DDM-stable macromolecular structures. The stable V₄ ATPase oligomer was observed by electron-microscopy and the association of the respiratory complexes in the so-called "respirasome" was able to perform in-vitro oxygen consumption.

Conserved in situ arrangement of complex I and III2 in mitochondrial respiratory chain supercomplexes of mammals, yeast, and plants

We used electron cryo-tomography and subtomogram averaging to investigate the structure of complex I and its supramolecular assemblies in the inner mitochondrial membrane of mammals, fungi, and plants. Tomographic volumes containing complex I were averaged at ∼4 nm resolution. Principal component analysis indicated that ∼60% of complex I formed a supercomplex with dimeric complex III, while ∼40% were not associated with other respiratory chain complexes. The mutual arrangement of complex I and III₂ was essentially conserved in all supercomplexes investigated. In addition, up to two copies of monomeric complex IV were associated with the complex I₁III₂ assembly in bovine heart and the yeast Yarrowia lipolytica, but their positions varied. No complex IV was detected in the respiratory supercomplex of the plant Asparagus officinalis Instead, an ∼4.5-nm globular protein density was observed on the matrix side of the complex I membrane arm, which we assign to γ-carbonic anhydrase. Our results demonstrate that respiratory chain supercomplexes in situ have a conserved core of complex I and III₂, but otherwise their stoichiometry and structure varies. The conserved features of supercomplex assemblies indicate an important role in respiratory electron transfer.

Unravelling the Effects of the Mutation m.3571insC/MT-ND1 on Respiratory Complexes Structural Organization

Mammalian respiratory complex I (CI) biogenesis requires both nuclear and mitochondria-encoded proteins and is mostly organized in respiratory supercomplexes. Among the CI proteins encoded by the mitochondrial DNA, NADH-ubiquinone oxidoreductase chain 1 (ND1) is a core subunit, evolutionary conserved from bacteria to mammals. Recently, ND1 has been recognized as a pivotal subunit in maintaining the structural and functional interaction among the hydrophilic and hydrophobic CI arms. A critical role of human ND1 both in CI biogenesis and in the dynamic organization of supercomplexes has been depicted, although the proof of concept is still missing and the critical amount of ND1 protein necessary for a proper assembly of both CI and supercomplexes is not defined. By exploiting a unique model in which human ND1 is allotopically re-expressed in cells lacking the endogenous protein, we demonstrated that the lack of this protein induces a stall in the multi-step process of CI biogenesis, as well as the alteration of supramolecular organization of respiratory complexes. We also defined a mutation threshold for the m.3571insC truncative mutation in mitochondrially encoded NADH:ubiquinone oxidoreductase core subunit 1 (MT-ND1), below which CI and its supramolecular organization is recovered, strengthening the notion that a certain amount of human ND1 is required for CI and supercomplexes biogenesis.

NDUFB8 Mutations Cause Mitochondrial Complex I Deficiency in Individuals with Leigh-like Encephalomyopathy

Respiratory chain complex I deficiency is the most frequently identified biochemical defect in childhood mitochondrial diseases. Clinical symptoms range from fatal infantile lactic acidosis to Leigh syndrome and other encephalomyopathies or cardiomyopathies. To date, disease-causing variants in genes coding for 27 complex I subunits, including 7 mitochondrial DNA genes, and in 11 genes encoding complex I assembly factors have been reported. Here, we describe rare biallelic variants in NDUFB8 encoding a complex I accessory subunit revealed by whole-exome sequencing in two individuals from two families. Both presented with a progressive course of disease with encephalo(cardio)myopathic features including muscular hypotonia, cardiac hypertrophy, respiratory failure, failure to thrive, and developmental delay. Blood lactate was elevated. Neuroimaging disclosed progressive changes in the basal ganglia and either brain stem or internal capsule. Biochemical analyses showed an isolated decrease in complex I enzymatic activity in muscle and fibroblasts. Complementation studies by expression of wild-type NDUFB8 in cells from affected individuals restored mitochondrial function, confirming NDUFB8 variants as the cause of complex I deficiency. Hereby we establish NDUFB8 as a relevant gene in childhood-onset mitochondrial disease.

Mitophagy in Cardiomyocytes and in Platelets: A Major Mechanism of Cardioprotection Against Ischemia/Reperfusion Injury

Mitophagy, a process that selectively removes damaged organelles by autolysosomal degradation, is an early cellular response to ischemia. Mitophagy is activated in both cardiomyocytes and platelets during ischemia/reperfusion (I/R) and heart disease conditions. We focus on the molecular regulation of mitophagy and highlight the role of mitophagy in cardioprotection.

X-ray and cryo-EM structures of inhibitor-bound cytochrome bc1 complexes for structure-based drug discovery

Cytochrome bc1, a dimeric multi-subunit electron-transport protein embedded in the inner mitochondrial membrane, is a major drug target for the treatment and prevention of malaria and toxoplasmosis. Structural studies of cytochrome bc1 from mammalian homologues co-crystallized with lead compounds have underpinned structure-based drug design to develop compounds with higher potency and selectivity. However, owing to the limited amount of cytochrome bc1 that may be available from parasites, all efforts have been focused on homologous cytochrome bc1 complexes from mammalian species, which has resulted in the failure of some drug candidates owing to toxicity in the host. Crystallographic studies of the native parasite proteins are not feasible owing to limited availability of the proteins. Here, it is demonstrated that cytochrome bc1 is highly amenable to single-particle cryo-EM (which uses significantly less protein) by solving the apo and two inhibitor-bound structures to ∼4.1 Å resolution, revealing clear inhibitor density at the binding site. Therefore, cryo-EM is proposed as a viable alternative method for structure-based drug discovery using both host and parasite enzymes.

The Complexity of Mitochondrial Complex IV: An Update of Cytochrome c Oxidase Biogenesis in Plants

Mitochondrial respiration is an energy producing process that involves the coordinated action of several protein complexes embedded in the inner membrane to finally produce ATP. Complex IV or Cytochrome c Oxidase (COX) is the last electron acceptor of the respiratory chain, involved in the reduction of O₂ to H₂O. COX is a multimeric complex formed by multiple structural subunits encoded in two different genomes, prosthetic groups (heme a and heme a₃), and metallic centers (Cu_A and Cu_B). Tens of accessory proteins are required for mitochondrial RNA processing, synthesis and delivery of prosthetic groups and metallic centers, and for the final assembly of subunits to build a functional complex. In this review, we perform a comparative analysis of COX composition and biogenesis factors in yeast, mammals and plants. We also describe possible external and internal factors controlling the expression of structural proteins and assembly factors at the transcriptional and post-translational levels, and the effect of deficiencies in different steps of COX biogenesis to infer the role of COX in different aspects of plant development. We conclude that COX assembly in plants has conserved and specific features, probably due to the incorporation of a different set of subunits during evolution.

BioSITe: A Method for Direct Detection and Quantitation of Site-Specific Biotinylation

Biotin-based labeling strategies are widely employed to study protein-protein interactions, subcellular proteomes and post-translational modifications, as well as, used in drug discovery. While the high affinity of streptavidin for biotin greatly facilitates the capture of biotinylated proteins, it still presents a challenge, as currently employed, for the recovery of biotinylated peptides. Here we describe a strategy designated Biotinylation Site Identification Technology (BioSITe) for the capture of biotinylated peptides for LC-MS/MS analyses. We demonstrate the utility of BioSITe when applied to proximity-dependent labeling methods, APEX and BioID, as well as biotin-based click chemistry strategies for identifying O-GlcNAc-modified sites. We demonstrate the use of isotopically labeled biotin for quantitative BioSITe experiments that simplify differential interactome analysis and obviate the need for metabolic labeling strategies such as SILAC. Our data also highlight the potential value of site-specific biotinylation in providing spatial and topological information about proteins and protein complexes. Overall, we anticipate that BioSITe will replace the conventional methods in studies where detection of biotinylation sites is important.

The interactome of intact mitochondria by cross-linking mass spectrometry provides evidence for coexisting respiratory supercomplexes

Mitochondria exert an immense amount of cytophysiological functions, but the structural basis of most of these processes is still poorly understood. Here we use cross-linking mass spectrometry to probe the organization of proteins in native mouse heart mitochondria. Our approach provides the largest survey of mitochondrial protein interactions reported so far. In total, we identify 3,322 unique residue-to-residue contacts involving half of the mitochondrial proteome detected by bottom-up proteomics. The obtained mitochondrial protein interactome gives insights in the architecture and submitochondrial localization of defined protein assemblies, and reveals the mitochondrial localization of four proteins not yet included in the MitoCarta database. As one of the highlights, we show that the oxidative phosphorylation complexes I-V exist in close spatial proximity, providing direct evidence for supercomplex assembly in intact mitochondria. The specificity of these contacts is demonstrated by comparative analysis of mitochondria after high salt treatment, which disrupts the native supercomplexes and substantially changes the mitochondrial interactome.

Structure of the Deactive State of Mammalian Respiratory Complex I

Complex I (NADH:ubiquinone oxidoreductase) is central to energy metabolism in mammalian mitochondria. It couples NADH oxidation by ubiquinone to proton transport across the energy-conserving inner membrane, catalyzing respiration and driving ATP synthesis. In the absence of substrates, active complex I gradually enters a pronounced resting or deactive state. The active-deactive transition occurs during ischemia and is crucial for controlling how respiration recovers upon reperfusion. Here, we set a highly active preparation of Bos taurus complex I into the biochemically defined deactive state, and used single-particle electron cryomicroscopy to determine its structure to 4.1 Å resolution. We show that the deactive state arises when critical structural elements that form the ubiquinone-binding site become disordered, and we propose reactivation is induced when substrate binding to the NADH-reduced enzyme templates their reordering. Our structure both rationalizes biochemical data on the deactive state and offers new insights into its physiological and cellular roles.

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Preparation of mammalian complex I in the deactive state that forms during ischemia

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The structure of the deactive state determined using electron cryomicroscopy

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Improved particle densities and orientations obtained using PEGylated gold grids

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Localized unfolding around the quinone-binding site in the deactive state

Mitochondria are physiologically maintained at close to 50 °C

In endothermic species, heat released as a product of metabolism ensures stable internal temperature throughout the organism, despite varying environmental conditions. Mitochondria are major actors in this thermogenic process. Part of the energy released by the oxidation of respiratory substrates drives ATP synthesis and metabolite transport, but a substantial proportion is released as heat. Using a temperature-sensitive fluorescent probe targeted to mitochondria, we measured mitochondrial temperature in situ under different physiological conditions. At a constant external temperature of 38 °C, mitochondria were more than 10 °C warmer when the respiratory chain (RC) was fully functional, both in human embryonic kidney (HEK) 293 cells and primary skin fibroblasts. This differential was abolished in cells depleted of mitochondrial DNA or treated with respiratory inhibitors but preserved or enhanced by expressing thermogenic enzymes, such as the alternative oxidase or the uncoupling protein 1. The activity of various RC enzymes was maximal at or slightly above 50 °C. In view of their potential consequences, these observations need to be further validated and explored by independent methods. Our study prompts a critical re-examination of the literature on mitochondria.

To ensure a stable internal temperature, endothermic species make use of the heat released during the final steps of food burning by the mitochondria present in all cells of the organism. Indeed, only a fraction of the energy released by the oxidation of respiratory substrates is used to generate ATP, while a substantial proportion is released as heat. Using a temperature-sensitive fluorescent probe targeted to mitochondria, we measured the temperature of active mitochondria in cultured intact human cells. Mitochondria were found to be more than 10 °C warmer when the respiratory chain was functional. This differential was abolished in cells depleted of mitochondrial DNA or by respiratory inhibitors but preserved or enhanced by the expression of thermogenic enzymes such as Ciona alternative oxidase or by uncoupling protein 1. The activity of various respiratory chain enzymes was found to be maximal near 50 °C. Note that in view of their potential consequences, the observations reported here need to be validated and explored further by independent methods.

Chemical Crosslinking Mass Spectrometry Analysis of Protein Conformations and Supercomplexes in Heart Tissue

While modern structural biology technologies have greatly expanded the size and type of protein complexes that can now be studied, the ability to derive large-scale structural information on proteins and complexes as they exist within tissues is practically nonexistent. Here, we demonstrate the application of crosslinking mass spectrometry to identify protein structural features and interactions in tissue samples, providing systems structural biology insight into protein complexes as they exist in the mouse heart. This includes insights into multiple conformational states of sarcomere proteins, as well as interactions among OXPHOS complexes indicative of supercomplex assembly. The extension of crosslinking mass spectrometry analysis into the realm of tissues opens the door to increasing our understanding of protein structures and interactions within the context of the greater biological system.

Passing the Baton: Substrate Channelling in Respiratory Metabolism

Despite species-specific differences in the pathways of respiratory metabolism are remarkably conserved across the kingdoms of life with glycolysis, the tricarboxylic acid cycle, and mitochondrial electron transport chain representing the major components of the process in the vast majority of organisms. In addition to being of critical importance in fueling life itself these pathways serve as interesting case studies for substrate channelling with research on this theme having been carried out for over 40 years. Here we provide a cross-kingdom review of the ample evidence for protein-protein interaction and enzyme assemblies within the three component pathways as well as describing the scarcer available evidence for substrate channelling itself.