bims-cytox1 Biomed News
on Cytochrome oxidase subunit 1
Issue of 2020–02–23
fourteen papers selected by
Gavin McStay, Staffordshire University
  1. Three-parent babies: Mitochondrial replacement therapies.
  2. The Plant Mitochondrial TAT Pathway Is Essential for Complex III Biogenesis.
  3. COA6 facilitates cytochrome c oxidase biogenesis as thiol-reductase for copper metallochaperones in mitochondria.
  4. Parkin-mediated mitophagy and autophagy flux disruption in cellular models of MERRF syndrome.
  5. Mitochondrial Proteases: Multifaceted Regulators of Mitochondrial Plasticity.
  6. Phenotypes and genotypes of mitochondrial aminoacyl-tRNA synthetase deficiencies from a single neurometabolic clinic.
  7. Endoplasmic reticulum stress-induced complex I defect: Central role of calcium overload.
  8. "Labile" heme critically regulates mitochondrial biogenesis through the transcriptional co-activator Hap4p in Saccharomyces cerevisiae.
  9. The spatio-temporal features of chicken mitochondrial ND2 gene heteroplasmy and the effects of nutrition factors on this gene.
  10. The biogenesis of mitochondrial intermembrane space proteins.
  11. From TOM to the TIM23 complex - handing over of a precursor.
  12. Assembly of Mitochondrial Complex I Requires the Low-Complexity Protein AMC1 in Chlamydomonas reinhardtii.
  13. Mitochondrial DNA heteroplasmy in disease and targeted nuclease-based therapeutic approaches.
  14. Cytochrome c Oxidase Subunit 4 Isoform Exchange Results in Modulation of Oxygen Affinity.
  1. JBRA Assist Reprod. 2020 Feb 19.
    Three-parent babies: Mitochondrial replacement therapies.
    Hana Carolina Moreira Farnezi, Ana Carolina Xavier Goulart, Adriana Dos Santos, Mariana Gontijo Ramos, Maria Lectícia Firpe Penna.
      The mitochondria are intracellular organelles, and just like the cell nucleus they have their own genome. They are extremely important for normal body functioning and are responsible for ATP production - the main energy source for the cell. Mitochondrial diseases are associated with mutations in mitochondrial DNA and are inherited exclusively from the mother. They can affect organs that depend on energy metabolism, such as skeletal muscles, the cardiac system, the central nervous system, the endocrine system, the retina and liver, causing various incurable diseases. Mitochondrial replacement techniques provide women with mitochondrial defects a chance to have normal biological children. The goal of such treatment is to reconstruct functional oocytes and zygotes, in order to avoid the inheritance of mutated genes; for this the nuclear genome is withdrawn from an oocyte or zygotes, which carries mitochondrial mutations, and is implanted in a normal anucleated cell donor. Currently, the options of a couple to prevent the transmission of mitochondrial diseases are limited, and mitochondrial donation techniques provide women with mitochondrial defects a chance to have normal children. The nuclear genome can be transferred from oocytes or zygotes using techniques such as pronuclear transfer, spindle transfer, polar body transfer and germinal vesicle transfer. This study presents a review of developed mitochondrial substitution techniques, and its ability to prevent hereditary diseases.
    Keywords:  mitochondrial; mitochondrial donation; mitochondrial mutations; mitochondrial replacement; mtDNA; reproductive technology
    DOI:  https://doi.org/10.5935/1518-0557.20190086
  2. Curr Biol. 2020 Feb 04. pii: S0960-9822(20)30001-4. [Epub ahead of print]
    The Plant Mitochondrial TAT Pathway Is Essential for Complex III Biogenesis.
    Kerstin Schäfer, Patrick Künzler, Andreas Klingl, Holger Eubel, Chris Carrie.
      Twin arginine translocation (TAT) pathways have been extensively studied in bacteria and chloroplasts for their role in membrane translocation of folded proteins. However, an increasing number of organisms have been found to contain mitochondria-located TAT subunits, including plant mitochondria, which contain TAT subunits, though in an unusual arrangement with only TatB and TatC subunits. To date, no confirmed function has been attributed to mitochondrial TAT pathways in any organism. Using a truncation mutant approach, we demonstrate that the plant mitochondrial TatB (MTTATB) is required for complex III biogenesis. More specifically, MTTATB performs at a late stage in complex III biogenesis, conveying the translocation of the C terminus of the Rieske FeS subunit back across the inner membrane. This work confirms that plant mitochondria retained a functional TAT pathway for the Rieske FeS translocation, most likely from the original mitochondrial ancestor. It is hypothesized that the original mitochondria contained a bacteria-derived TAT pathway required for at least the Rieske FeS translocation. In several eukaryotic lineages, this mitochondrial TAT pathway was lost and replaced by BCS1. Interestingly, plant mitochondria appear to assemble complex III in the same subunit order as yeast and mammals but in contrast use bacteria-like assembly factors for this process.
    Keywords:  Rieske FeS protein; complex III; complex III assembly; mitochondria; mitochondrial evolution; mitochondrial protein import
    DOI:  https://doi.org/10.1016/j.cub.2020.01.001
  3. J Mol Biol. 2020 Feb 13. pii: S0022-2836(20)30102-9. [Epub ahead of print]
    COA6 facilitates cytochrome c oxidase biogenesis as thiol-reductase for copper metallochaperones in mitochondria.
    David Pacheu-Grau, Michał Wasilewski, Silke Oeljeklaus, Christine Silvia Gibhardt, Abhishek Aich, Margarita Chudenkova, Sven Dennerlein, Markus Deckers, Ivan Bogeski, Bettina Warscheid, Agnieszka Chacinska, Peter Rehling.
      The mitochondrial cytochrome c oxidase, the terminal enzyme of the respiratory chain, contains heme and copper centers for electron transfer. The conserved COX2 subunit contains the CuA site, a binuclear copper center. The copper chaperones SCO1, SCO2, and COA6 are required for CuA center formation. Loss of function of these chaperones and the concomitant cytochrome c oxidase deficiency cause severe human disorders. Here we analyzed the molecular function of COA6 and the consequences of COA6 deficiency for mitochondria. Our analyses show that loss of COA6 causes combined complex I and complex IV deficiency and impacts membrane potential driven protein transport across the inner membrane. We demonstrate that COA6 acts as a thiol-reductase to reduce disulphide bridges of critical cysteine residues in SCO1 and SCO2. Cysteines within the CX3CXNH domain of SCO2 mediate its interaction with COA6 but are dispensable for SCO2-SCO1 interaction. Our analyses define COA6 as thiol-reductase, which is essential for CuA biogenesis.
    Keywords:  COA6; Cu(A) center; copper metallochaperones; cytochrome c oxidase; mitochondria
    DOI:  https://doi.org/10.1016/j.jmb.2020.01.036
  4. Biochim Biophys Acta Mol Basis Dis. 2020 Feb 13. pii: S0925-4439(20)30071-5. [Epub ahead of print] 165726
    Parkin-mediated mitophagy and autophagy flux disruption in cellular models of MERRF syndrome.
    Marina Villanueva-Paz, Suleva Povea-Cabello, Irene Villalón-García, Mónica Álvarez-Córdoba, Juan M Suárez-Rivero, Marta Talaverón-Rey, Sandra Jackson, Rafael Falcón-Moya, Antonio Rodríguez-Moreno, José A Sánchez-Alcázar.
      Mitochondrial diseases are considered rare genetic disorders characterized by defects in oxidative phosphorylation (OXPHOS). They can be provoked by mutations in nuclear DNA (nDNA) or mitochondrial DNA (mtDNA). MERRF (Myoclonic Epilepsy with Ragged-Red Fibers) syndrome is one of the most frequent mitochondrial diseases, principally caused by the m.8344A>G mutation in mtDNA, which affects the translation of all mtDNA-encoded proteins and therefore impairs mitochondrial function. In the present work, we evaluated autophagy and mitophagy flux in transmitochondrial cybrids and fibroblasts derived from a MERRF patient, reporting that Parkin-mediated mitophagy is increased in MERRF cell cultures. Our results suggest that supplementation with coenzyme Q10 (CoQ), a component of the electron transport chain (ETC) and lipid antioxidant, prevents Parkin translocation to the mitochondria. In addition, CoQ acts as an enhancer of autophagy and mitophagy flux, which partially improves cell pathophysiology. The significance of Parkin-mediated mitophagy in cell survival was evaluated by silencing the expression of Parkin in MERRF cybrids. Our results show that mitophagy acts as a cell survival mechanism in mutant cells. To confirm these results in one of the main affected cell types in MERRF syndrome, mutant induced neurons (iNs) were generated by direct reprogramming of patients-derived skin fibroblasts. The treatment of MERRF iNs with Guttaquinon CoQ10 (GuttaQ), a water-soluble derivative of CoQ, revealed a significant improvement in cell bioenergetics. These results indicate that iNs, along with fibroblasts and cybrids, can be utilized as reliable cellular models to shed light on disease pathomechanisms as well as for drug screening.
    Keywords:  Autophagy; Coenzyme Q(10); Mitochondria; Mitochondrial diseases; Mitophagy
    DOI:  https://doi.org/10.1016/j.bbadis.2020.165726
  5. Annu Rev Biochem. 2020 Feb 19.
    Mitochondrial Proteases: Multifaceted Regulators of Mitochondrial Plasticity.
    Soni Deshwal, Kai Uwe Fiedler, Thomas Langer.
      Mitochondria are essential metabolic hubs that dynamically adapt to physiological demands. More than 40 proteases residing in different compartments of mitochondria, termed mitoproteases, preserve mitochondrial proteostasis and are emerging as central regulators of mitochondrial plasticity. These multifaceted enzymes limit the accumulation of short-lived, regulatory proteins within mitochondria, modulate the activity of mitochondrial proteins by protein processing, and mediate the degradation of damaged proteins. Various signaling cascades coordinate the activity of mitoproteases to preserve mitochondrial homeostasis and ensure cell survival. Loss of mitoproteases severely impairs the functional integrity of mitochondria, is associated with aging, and causes pleiotropic diseases. Understanding the dual function of mitoproteases as regulatory and quality control enzymes will help unravel the role of mitochondrial plasticity in aging and disease. Expected final online publication date for the Annual Review of Biochemistry, Volume 89 is June 22, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
    DOI:  https://doi.org/10.1146/annurev-biochem-062917-012739
  6. JIMD Rep. 2020 Jan;51(1): 3-10
    Phenotypes and genotypes of mitochondrial aminoacyl-tRNA synthetase deficiencies from a single neurometabolic clinic.
    Aaisha Al Balushi, Diana Matviychuk, Rebekah Jobling, Gajja S Salomons, Susan Blaser, Saadet Mercimek-Andrews.
      Mitochondrial aminoacyl-tRNA synthetases play a major role in protein translation, synthesis, and oxidative phosphorylation. We reviewed all patients diagnosed with mitochondrial aminoacyl-tRNA synthetase deficiencies diagnosed in a single neurometabolic clinic. We report five patients with mitochondrial aminoacyl-tRNA synthetase deficiencies including DARS2, EARS2, PARS2, and RARS2 deficiencies. Siblings with DARS2 deficiency presented with global developmental delay within the first year of life. DARS2, EARS2, PARS2, and RARS2 deficiencies were identified by whole exome sequencing. We report coagulation factor abnormalities in PARS2 deficiency for the first time. We also report symmetric increased signal intensity in globus pallidi in FLAIR images in brain MRI in EARS2 deficiency for the first time. One patient with RARS2 deficiency had compound heterozygous variants in RARS2. One of those variants was an intronic variant. We confirmed the pathogenicity by mRNA studies. Mitochondrial aminoacyl-tRNA synthetase deficiencies are diagnosed by molecular genetic investigations. Clinically available non-invasive biochemical investigations are non-specific for the diagnosis of mitochondrial aminoacyl-tRNA synthetase deficiencies. A combination of brain MRI features and molecular genetic investigations should be undertaken to confirm the diagnosis of mitochondrial aminoacyl-tRNA synthetase deficiencies.
    Keywords:  DARS2; EARS2; PARS2; RARS2; global developmental delay; leukodystrophy; mitochondrial aminoacyl‐tRNA synthetase deficiencies; movement disorders; seizures
    DOI:  https://doi.org/10.1002/jmd2.12079
  7. Arch Biochem Biophys. 2020 Feb 12. pii: S0003-9861(19)31131-2. [Epub ahead of print]683 108299
    Endoplasmic reticulum stress-induced complex I defect: Central role of calcium overload.
    Ahmed A Mohsin, Jeremy Thompson, Ying Hu, John Hollander, Edward J Lesnefsky, Qun Chen.
       BACKGROUND: ER (endoplasmic reticulum) stress leads to decreased complex I activity in cardiac mitochondria. The aim of the current study is to explore the potential mechanisms by which ER stress leads to the complex I defect. ER stress contributes to intracellular calcium overload and oxidative stress that are two key factors to induce mitochondrial dysfunction. Since oxidative stress is often accompanied by intracellular calcium overload during ER stress in vivo, the role of oxidative stress and calcium overload in mitochondrial dysfunction was studied using in vitro models. ER stress results in intracellular calcium overload that favors activation of calcium-dependent calpains. The contribution of mitochondrial calpain activation in ER stress-mediated complex I damage was studied.
    METHODS: Thapsigargin (THAP) was used to induce acute ER stress in H9c2 cells and C57BL/6 mice. Exogenous calcium (25 μM) and H2O2 (100 μM) were used to induce modest calcium overload and oxidative stress in isolated mitochondria. Calpain small subunit 1 (CAPNS1) is essential to maintain calpain 1 and calpain 2 (CPN1/2) activities. Deletion of CAPNS1 eliminates the activities of CPN1/2. Wild type and cardiac-specific CAPNS1 deletion mice were used to explore the role of CPN1/2 activation in calcium-induced mitochondrial damage.
    RESULTS: In isolated mitochondria, exogenous calcium but not H2O2 treatment led to decreased oxidative phosphorylation, supporting that calcium overload contributes a key role in the mitochondrial damage. THAP treatment of H9c2 cells decreased respiration selectively with complex I substrates. THAP treatment activated cytosolic and mitochondrial CPN1/2 in C57BL/6 mice and led to degradation of complex I subunits including NDUFS7. Calcium treatment decreased NDUFS7 content in wild type but not in CAPNS1 knockout mice.
    CONCLUSION: ER stress-mediated activation of mitochondria-localized CPN1/2 contributes to complex I damage by cleaving component subunits.
    Keywords:  CPNS1 knockout mice; Calpain; Electron transport chain
    DOI:  https://doi.org/10.1016/j.abb.2020.108299
  8. J Biol Chem. 2020 Feb 18. pii: jbc.RA120.012739. [Epub ahead of print]
    "Labile" heme critically regulates mitochondrial biogenesis through the transcriptional co-activator Hap4p in Saccharomyces cerevisiae.
    Cyrielle L Bouchez, Edgar D Yoboue, Livier E de la Rosa Vargas, Bénédicte Salin, Sylvain Cuvellier, Michel Rigoulet, Stéphane Duvezin-Caubet, Anne Devin.
      Heme (iron protoporphyrin IX) is a well-known prosthetic group for enzymes involved in metabolic pathways such as oxygen transport and electron transfer through the mitochondrial respiratory chain. However, heme has also been shown to be an important regulatory molecule (as "labile" heme) for diverse processes such as translation, kinase activity, and transcription in mammals, yeast, and bacteria. Taking advantage of a yeast strain deficient for heme production that enabled controlled modulation and monitoring of "labile" heme levels, here we investigated the role of "labile" heme in the regulation of mitochondrial biogenesis. This process is regulated by the HAP complex in yeast. Using several biochemical assays along with EM and epifluorescence microscopy, to the best of our knowledge, we show for the first time that cellular "labile" heme is critical for the post-translational regulation of HAP complex activity, most likely through the stability of the transcriptional co-activator Hap4p. Consequently, we found that "labile" heme regulates mitochondrial biogenesis and cell growth. The findings of our work highlight a new mechanism in the regulation of mitochondrial biogenesis by cellular metabolites.
    Keywords:  Hap4p; bioenergetics; heme; labile heme; mitochondria; mitochondrial biogenesis; oxidation-reduction (redox); oxidative phosphorylation (OXPHOS); transcription factor; yeast
    DOI:  https://doi.org/10.1074/jbc.RA120.012739
  9. Sci Rep. 2020 Feb 19. 10(1): 2972
    The spatio-temporal features of chicken mitochondrial ND2 gene heteroplasmy and the effects of nutrition factors on this gene.
    Suliang Yang, Yangyang Huo, Huanjie Wang, Jiefei Ji, Wen Chen, Yanqun Huang.
      Mitochondrial heterogeneity is the presence of two or more types of mitochondrial (mt)DNA in the same individual/tissue/cell. It is closely related to animal health and disease. ND2 is a protein-coding gene in mtDNA, which participates in mitochondrial respiratory chain and oxidative phosphorylation. In previous studies, we observed that the mt.A5703T and mt.T5727G sites in the ND2 gene were the heteroplasmic variation sites. We used pyrophosphate sequencing technology to examine chicken mt.A5703T and mt.T5727G heteroplasmic sites in the ND2 gene, in different tissues and at different development stages in chickens. We also investigated whether nutritional factors could affect the mt.A5703T and mt.T5727G heteroplasmy. Our results showed that chicken mt.A5703T and mt.T5727G heteroplasmy had clear spatio-temporal specificities, which varied between tissues/development stages. The mtDNA heterogeneity was relatively stable upon nutrition intervention, 30% dietary energy restriction (from 18 to 48 days old) and different types of dietary fats (at 5% concentration, from 1 to 42 days old) did not change the breast muscle heteroplasmy of broilers at the mt.A5703T and mt.T5727G sites. In addition, multiple potential heteroplasmic sites were detected by clone sequencing in the ND2 region, which potentially reflected abundant heteroplasmy in the chicken mitochondrial genome. These results provide an important reference for further research on heteroplasmy in chicken mitochondria.
    DOI:  https://doi.org/10.1038/s41598-020-59703-y
  10. Biol Chem. 2020 Feb 01. pii: /j/bchm.just-accepted/hsz-2020-0114/hsz-2020-0114.xml. [Epub ahead of print]
    The biogenesis of mitochondrial intermembrane space proteins.
    Ruairidh Edwards, Sarah Gerlich, Kostas Tokatlidis.
      The mitochondrial intermembrane space (IMS) houses a large spectrum of proteins with distinct and critical functions. Protein import into this mitochondrial sub-compartment is underpinned by an intriguing variety of pathways, many of which are still poorly understood. The constricted volume of the IMS and the topological segregation by the inner membrane cristae into a bulk area surrounded by the boundary inner membrane and the lumen within the cristae is an important factor that adds to the complexity of the protein import, folding and assembly processes. We discuss the main import pathways into the IMS, but also how IMS proteins are degraded or even retro-translocated to the cytosol in an integrated network of interactions that is necessary to maintain a healthy balance of IMS proteins under physiological and cellular stress conditions. We conclude this review by highlighting new and exciting perspectives in this area with a view to develop a better understanding of yet unknown, likely unconventional import pathways, how presequence-less proteins can be targeted and the basis for dual localisation in the IMS and the cytosol. Such knowledge is critical to understand the dynamic changes of the IMS proteome in response to stress, and particularly important for maintaining optimal mitochondrial fitness.
    Keywords:  intermembrane space; mitochondria; oxidative folding; protein folding; protein import
    DOI:  https://doi.org/10.1515/hsz-2020-0114
  11. Biol Chem. 2020 Feb 01. pii: /j/bchm.just-accepted/hsz-2020-0101/hsz-2020-0101.xml. [Epub ahead of print]
    From TOM to the TIM23 complex - handing over of a precursor.
    Sylvie Callegari, Luis Daniel Cruz-Zaragoza, Peter Rehling.
      Mitochondrial precursor proteins with amino-terminal presequences are imported via the presequence pathway, utilizing the TIM23 complex for inner membrane translocation. Initially, the precursors pass the outer membrane through the TOM complex and are handed over to the TIM23 complex where they are sorted into the inner membrane or translocated into the matrix. This handover process depends on the receptor proteins at the inner membrane, Tim50 and Tim23, which are critical for efficient import. In this review, we summarize key findings that shaped the current concepts of protein translocation along the presequence import pathway, with a particular focus on the precursor handover process from TOM to the TIM23 complex. In addition, we discuss functions of the human TIM23 pathway and the recently uncovered pathogenic mutations in TIM50.
    Keywords:  TIM23 complex; TIM50; membrane translocation; mitochondria; presequence pathway; protein translocation
    DOI:  https://doi.org/10.1515/hsz-2020-0101
  12. Genetics. 2020 Feb 19. pii: genetics.303029.2020. [Epub ahead of print]
    Assembly of Mitochondrial Complex I Requires the Low-Complexity Protein AMC1 in Chlamydomonas reinhardtii.
    Nitya Subrahmanian, Andrew David Castonguay, Claire Remacle, Patrice Paul Hamel.
      Complex I is the first enzyme involved in the mitochondrial electron transport chain. With more than 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, with insertional mutations in 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.
    Keywords:  Chlamydomonas reinhardtii; energy transducing membranes; low-complexity protein; mitochondrial complex I assembly; mitochondrial gene expression; respiration
    DOI:  https://doi.org/10.1534/genetics.120.303029
  13. EMBO Rep. 2020 Feb 19. e49612
    Mitochondrial DNA heteroplasmy in disease and targeted nuclease-based therapeutic approaches.
    Nadee Nissanka, Carlos T Moraes.
      Mitochondrial DNA (mtDNA) encodes a subset of the genes which are responsible for oxidative phosphorylation. Pathogenic mutations in the human mtDNA are often heteroplasmic, where wild-type mtDNA species co-exist with the pathogenic mtDNA and a bioenergetic defect is only seen when the pathogenic mtDNA percentage surpasses a threshold for biochemical manifestations. mtDNA segregation during germline development can explain some of the extreme variation in heteroplasmy from one generation to the next. Patients with high heteroplasmy for deleterious mtDNA species will likely suffer from bona-fide mitochondrial diseases, which currently have no cure. Shifting mtDNA heteroplasmy toward the wild-type mtDNA species could provide a therapeutic option to patients. Mitochondrially targeted engineered nucleases, such as mitoTALENs and mitoZFNs, have been used in vitro in human cells harboring pathogenic patient-derived mtDNA mutations and more recently in vivo in a mouse model of a pathogenic mtDNA point mutation. These gene therapy tools for shifting mtDNA heteroplasmy can also be used in conjunction with other therapies aimed at eliminating and/or preventing the transfer of pathogenic mtDNA from mother to child.
    Keywords:  gene editing; heteroplasmy; mitochondrial DNA
    DOI:  https://doi.org/10.15252/embr.201949612
  14. Cells. 2020 Feb 14. pii: E443. [Epub ahead of print]9(2):
    Cytochrome c Oxidase Subunit 4 Isoform Exchange Results in Modulation of Oxygen Affinity.
    David Pajuelo Reguera, Kristýna Čunátová, Marek Vrbacký, Alena Pecinová, Josef Houštěk, Tomáš Mráček, Petr Pecina.
      Cytochrome c oxidase (COX) is regulated through tissue-, development- or environment-controlled expression of subunit isoforms. The COX4 subunit is thought to optimize respiratory chain function according to oxygen-controlled expression of its isoforms COX4i1 and COX4i2. However, biochemical mechanisms of regulation by the two variants are only partly understood. We created an HEK293-based knock-out cellular model devoid of both isoforms (COX4i1/2 KO). Subsequent knock-in of COX4i1 or COX4i2 generated cells with exclusive expression of respective isoform. Both isoforms complemented the respiratory defect of COX4i1/2 KO. The content, composition, and incorporation of COX into supercomplexes were comparable in COX4i1- and COX4i2-expressing cells. Also, COX activity, cytochrome c affinity, and respiratory rates were undistinguishable in cells expressing either isoform. Analysis of energy metabolism and the redox state in intact cells uncovered modestly increased preference for mitochondrial ATP production, consistent with the increased NADH pool oxidation and lower ROS in COX4i2-expressing cells in normoxia. Most remarkable changes were uncovered in COX oxygen kinetics. The p50 (partial pressure of oxygen at half-maximal respiration) was increased twofold in COX4i2 versus COX4i1 cells, indicating decreased oxygen affinity of the COX4i2-containing enzyme. Our finding supports the key role of the COX4i2-containing enzyme in hypoxia-sensing pathways of energy metabolism.
    Keywords:  mitochondria, OXPHOS, respiratory chain, cytochrome c oxidase, COX, COX4 isoforms, COX4i2, oxygen affinity, p50, oxygen sensing
    DOI:  https://doi.org/10.3390/cells9020443
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