The Complexity of Making Ubiquinone

COQ4 is required for the oxidative decarboxylation of the C1 carbon of coenzyme Q in eukaryotic cells

Coenzyme Q (CoQ) is a redox lipid that fulfills critical functions in cellular bioenergetics and homeostasis. CoQ is synthesized by a multi-step pathway that involves several COQ proteins. Two steps of the eukaryotic pathway, the decarboxylation and hydroxylation of position C1, have remained uncharacterized. Here, we provide evidence that these two reactions occur in a single oxidative decarboxylation step catalyzed by COQ4. We demonstrate that COQ4 complements an Escherichia coli strain deficient for C1 decarboxylation and hydroxylation and that COQ4 displays oxidative decarboxylation activity in the non-CoQ producer Corynebacterium glutamicum. Overall, our results substantiate that COQ4 contributes to CoQ biosynthesis, not only via its previously proposed structural role but also via the oxidative decarboxylation of CoQ precursors. These findings fill a major gap in the knowledge of eukaryotic CoQ biosynthesis and shed light on the pathophysiology of human primary CoQ deficiency due to COQ4 mutations.

Activation of Coq6p, a FAD Monooxygenase Involved in Coenzyme Q Biosynthesis, by Adrenodoxin Reductase/Ferredoxin

Adrenodoxin reductase (AdxR) plays a pivotal role in electron transfer, shuttling electrons between NADPH and iron/sulfur adrenodoxin proteins in mitochondria. This electron transport system is essential for P450 enzymes involved in various endogenous biomolecules biosynthesis. Here, we present an in-depth examination of the kinetics governing the reduction of human AdxR by NADH or NADPH. Our results highlight the efficiency of human AdxR when utilizing NADPH as a flavin reducing agent. Nevertheless, akin to related flavoenzymes such as cytochrome P450 reductase, we observe that low NADPH concentrations hinder flavin reduction due to intricate equilibrium reactions between the enzyme and its substrate/product. Remarkably, the presence of MgCl2 suppresses this complex kinetic behavior by decreasing NADPH binding to oxidized AdxR, effectively transforming AdxR into a classical Michaelis-Menten enzyme. We propose that the addition of MgCl2 may be adapted for studying the reductive half-reactions of other flavoenzymes with NADPH. Furthermore, in vitro experiments provide evidence that the reduction of the yeast flavin monooxygenase Coq6p relies on an electron transfer chain comprising NADPH-AdxR-Yah1p-Coq6p, where Yah1p shuttles electrons between AdxR and Coq6p. This discovery explains the previous in vivo observation that Yah1p and the AdxR homolog, Arh1p, are required for the biosynthesis of coenzyme Q in yeast.

In vitro construction of the COQ metabolon unveils the molecular determinants of coenzyme Q biosynthesis

Metabolons are protein assemblies that perform a series of reactions in a metabolic pathway. However, the general importance and aptitude of metabolons for enzyme catalysis remain poorly understood. In animals, biosynthesis of coenzyme Q is currently attributed to ten different proteins, with COQ3, COQ4, COQ5, COQ6, COQ7 and COQ9 forming the iconic COQ metabolon. Yet several reaction steps conducted by the metabolon remain enigmatic. To elucidate the prerequisites for animal coenzyme Q biosynthesis, we sought to construct the entire metabolon in vitro. Here we show that this approach, rooted in ancestral sequence reconstruction, reveals the enzymes responsible for the uncharacterized steps and captures the biosynthetic pathway in vitro. We demonstrate that COQ8, a kinase, increases and streamlines coenzyme Q production. Our findings provide crucial insight into how biocatalytic efficiency is regulated and enhanced by these biosynthetic engines in the context of the cell.

COQ4 is required for the oxidative decarboxylation of the C1 carbon of Coenzyme Q in eukaryotic cells

Coenzyme Q (CoQ) is a redox lipid that fulfills critical functions in cellular bioenergetics and homeostasis. CoQ is synthesized by a multi-step pathway that involves several COQ proteins. Two steps of the eukaryotic pathway, the decarboxylation and hydroxylation of position C1, have remained uncharacterized. Here, we provide evidence that these two reactions occur in a single oxidative decarboxylation step catalyzed by COQ4. We demonstrate that COQ4 complements an Escherichia coli strain deficient for C1 decarboxylation and hydroxylation and that COQ4 displays oxidative decarboxylation activity in the non-CoQ producer Corynebacterium glutamicum. Overall, our results substantiate that COQ4 contributes to CoQ biosynthesis, not only via its previously proposed structural role, but also via oxidative decarboxylation of CoQ precursors. These findings fill a major gap in the knowledge of eukaryotic CoQ biosynthesis, and shed new light on the pathophysiology of human primary CoQ deficiency due to COQ4 mutations.

An oviduct glycan increases sperm lifespan by diminishing the production of ubiquinone and reactive oxygen species†

Sperm storage by females after mating for species-dependent periods is used widely among animals with internal fertilization to allow asynchrony between mating and ovulation. Many mammals store sperm in the lower oviduct where specific glycans on oviduct epithelial cells retain sperm to form a reservoir. Binding to oviduct cells suppresses sperm intracellular Ca2+ and increases sperm longevity. We investigated the mechanisms by which a specific oviduct glycan, 3-O-sulfated Lewis X trisaccharide (suLeX), prolongs the lifespan of porcine sperm. Using targeted metabolomics, we found that binding to suLeX diminishes the abundance of 4-hydroxybenzoic acid, the precursor to ubiquinone (also known as Coenzyme Q), 30 min after addition. Ubiquinone functions as an electron acceptor in the electron transport chain (ETC). 3-O-sulfated Lewis X trisaccharide also suppressed the formation of fumarate. A component of the citric acid cycle, fumarate is synthesized by succinate-coenzyme Q reductase, which employs ubiquinone and is also known as Complex II in the ETC. Consistent with the reduced activity of the ETC, the production of harmful reactive oxygen species (ROS) was diminished. The enhanced sperm lifespan in the oviduct may be because of suppressed ROS production because high ROS concentrations have toxic effects on sperm.

Mitochondrial defects caused by PARL deficiency lead to arrested spermatogenesis and ferroptosis

Impaired spermatogenesis and male infertility are common manifestations associated with mitochondrial diseases, yet the underlying mechanisms linking these conditions remain elusive. In this study, we demonstrate that mice deficient for the mitochondrial intra-membrane rhomboid protease PARL, a recently reported model of the mitochondrial encephalopathy Leigh syndrome, develop early testicular atrophy caused by a complete arrest of spermatogenesis during meiotic prophase I, followed by degeneration and death of arrested spermatocytes. This process is independent of neurodegeneration. Interestingly, genetic modifications of PINK1, PGAM5, and TTC19 - three major substrates of PARL with important roles in mitochondrial homeostasis - fail to reproduce or modify this severe phenotype, indicating that the spermatogenic arrest arises from distinct molecular pathways. We further observed severe abnormalities in mitochondrial ultrastructure in PARL-deficient spermatocytes, along with prominent electron transfer chain defects, disrupted coenzyme Q (CoQ) biosynthesis, and metabolic rewiring. These mitochondrial defects are associated with a germ cell-specific decrease in GPX4 expression leading arrested spermatocytes to ferroptosis - a regulated cell death modality characterized by uncontrolled lipid peroxidation. Our results suggest that mitochondrial defects induced by PARL depletion act as an initiating trigger for ferroptosis in primary spermatocytes through simultaneous effects on GPX4 and CoQ - two major inhibitors of ferroptosis. These findings shed new light on the potential role of ferroptosis in the pathogenesis of mitochondrial diseases and male infertility warranting further investigation.

Biosynthesis, Deficiency, and Supplementation of Coenzyme Q

Originally identified as a key component of the mitochondrial respiratory chain, Coenzyme Q (CoQ or CoQ₁₀ for human tissues) has recently been revealed to be essential for many different redox processes, not only in the mitochondria, but elsewhere within other cellular membrane types. Cells rely on endogenous CoQ biosynthesis, and defects in this still-not-completely understood pathway result in primary CoQ deficiencies, a group of conditions biochemically characterised by decreased tissue CoQ levels, which in turn are linked to functional defects. Secondary CoQ deficiencies may result from a wide variety of cellular dysfunctions not directly linked to primary synthesis. In this article, we review the current knowledge on CoQ biosynthesis, the defects leading to diminished CoQ₁₀ levels in human tissues and their associated clinical manifestations.

An oviduct glycan increases sperm lifespan by diminishing ubiquinone and production of reactive oxygen species

Sperm storage by females after mating for species-dependent periods is used widely among animals with internal fertilization to allow asynchrony between mating and ovulation. Many mammals store sperm in the lower oviduct where specific glycans on epithelial cells retain sperm to form a reservoir. Binding to oviduct cells suppresses sperm intracellular Ca 2+ and increases sperm longevity. We investigated the mechanisms by which a specific oviduct glycan, 3-O-sulfated Lewis X trisaccharide (suLe X ), prolongs the lifespan of porcine sperm. Using targeted metabolomics, we report that binding to suLe X diminishes the abundance of the precursor to ubiquinone and suppresses formation of fumarate, a specific citric acid cycle component, diminishing the activity of the electron transport chain and reducing the production of harmful reactive oxygen species (ROS). The enhanced sperm lifespan in the oviduct may be due to suppressed ROS production as many reports have demonstrated toxic effects of high ROS concentrations on sperm.

Beneficial antioxidant effects of Coenzyme Q10 on reproduction

This chapter focuses on preclinical and clinical studies conducted in recent years that contribute to increasing knowledge on the role of Coenzyme Q10 in female reproductive health. General aspects of CoQ10, such as its role as an antioxidant and in mitochondrial bioenergetics are considered. The age-dependent decline in human female reproductive potential is associated with cellular mitochondrial dysfunction and oxidative stress, and in some cases accompanied by a decrease in CoQ10 levels. Herein, we discuss experimental and clinical evidence on CoQ10 protective effects on reproductive health. We also address the potential of supplementation with this coenzyme to rescue reprotoxicity induced by exposure to environmental xenobiotics. This review not only contributes to our general understanding of the effects of aging on female reproduction but also provides new insights into strategies promoting reproductive health. The use of CoQ10 supplementation can improve reproductive performance through the scavenging of reactive oxygen species and free radicals. This strategy can constitute a low-risk and low-cost strategy to attenuate the impact on fertility related to aging and exposure to environmental chemicals.

Immunosuppressive therapies attenuate paraquat-induced renal dysfunction by suppressing inflammatory responses and lipid peroxidation

Although paraquat (PQ) induces oxidative damage and inflammatory responses in the lungs, the mechanism underlying PQ-induced acute kidney injury in patients is unclear. Immunosuppressive therapy with glucocorticoids and the immunosuppressant cyclophosphamide (CP) has been employed to treat patients with PQ poisoning. This study examined whether PQ could concurrently cause renal injury, inflammatory responses, and oxidative damage in the kidneys, and whether CP and dexamethasone (DEX) could suppress PQ-induced alterations. Mice were assigned to eight groups: Control, PQ, DEX, PQ plus DEX, CP, PQ plus CP, DEX plus CP, and PQ plus DEX with CP. DEX, CP, and DEX plus CP reversed PQ-induced renal injury, as indicated by urinary albumin-to-creatinine ratios and urea nitrogen levels in serum. The treatments also attenuated PQ-induced renal infiltration of leukocytes and macrophages and induction of the Il6, Tnf, Icam, Cxcl2, Tlr4, and Tlr9 genes encoding the inflammatory mediators in the kidneys. However, DEX only partially suppressed the macrophage infiltration, whereas DEX plus CP provided stronger protection than DEX or CP alone for the induction of Il6 and Cxcl2. Moreover, through the detection of F₂-isoprostanes (F₂-IsoPs) and isofurans in the kidneys and lungs and F₂-IsoPs in the plasma and urine, the therapies were found to suppress PQ-induced lipid peroxidation, although DEX was less effective. Finally, PQ decreased ubiquinol-9:ubiquinone-9 ratios in the kidneys. This effect of PQ was not found under CP treatment, but the ratio was lower than that of the control group. Our findings suggest that the suppression of PQ-induced inflammatory responses by DEX and CP in the kidneys can mitigate oxidative damage and acute kidney injury.

The CoQ biosynthetic di-iron carboxylate hydroxylase COQ7 is inhibited by in vivo metalation with manganese but remains functional by metalation with cobalt

Coenzyme Q (CoQ; ubiquinone) is an obligate component of the mitochondrial electron transport chain. COQ7 is a mitochondrial hydroxylase that is required for CoQ biosynthesis. COQ7 belongs to di-iron carboxylate enzymes, a rare type of enzyme that carries out a wide range of reactions. We found that manganese exposure of mouse cells leads to decreased COQ7 activity, but that pre-treatment with cobalt interferes with the inhibition by manganese. Our findings suggest that cobalt has greater affinity for the active site of COQ7 than both iron and manganese and that replacement of iron by cobalt at the active site preserves catalytic activity.

How plants synthesize coenzyme Q

Coenzyme Q (CoQ) is a conserved redox-active lipid that has a wide distribution across the domains of life. CoQ plays a key role in the oxidative electron transfer chain and serves as a crucial antioxidant in cellular membranes. Our understanding of CoQ biosynthesis in eukaryotes has come mostly from studies of yeast. Recently, significant advances have been made in understanding CoQ biosynthesis in plants. Unique mitochondrial flavin-dependent monooxygenase and benzenoid ring precursor biosynthetic pathways have been discovered, providing new insights into the diversity of CoQ biosynthetic pathways and the evolution of phototrophic eukaryotes. We summarize research progress on CoQ biosynthesis and regulation in plants and recent efforts to increase the CoQ content in plant foods.

Evaluation of biological mechanisms of artemisinin on bovine mammary epithelial cells by integration of network pharmacology and TMT-based quantitative proteomics

The sesquiterpene lactone, artemisinin, is a primary component of the medicinal plant Artemisia annua L., which has anti-inflammatory, antibacterial and antioxidant activities. However, the potential effects of artemisinin on the mammary gland of dairy cows and the underlying molecular mechanisms remain unclear. Here, we utilized systematic network pharmacology and proteomics to elucidate the mechanism by which artemisinin affects milk production and the proliferation of bovine mammary epithelial cells (BMECs). Nineteen bioactive compounds and 56 key targets were identified through database mining. To delineate the mechanism of artemisia’s activity, a protein-protein interaction network and integrated visual display were generated from bioinformatics assays to explore the relationships and interactions among the bioactive molecules and their targets. The gene ontology (GO) terms and kyoto encyclopedia of genes and genomes annotation suggested that the apoptotic process, cell division, p53 pathway, prolactin and PI3K-Akt pathways played vital roles in mammary gland development. Using proteomics analysis, we identified 122 up-regulated and 96 down-regulated differentially significant expressed proteins (DSEPs). The differentially significant expressed proteins had multiple biological functions associated with cell division, apoptosis, differentiation, and migration. Gene ontology enrichment analysis suggested that differentially significant expressed proteins may promote cell proliferation and regulate apoptosis in bovine mammary epithelial cells. Kyoto encyclopedia of genes and genomes pathway analysis indicated that several biological pathways, such as those involved in antigen processing and presentation, cell adhesion molecules and ribosomes, played significant roles in the effects of artemisinin on bovine mammary epithelial cells. These findings contribute to a comprehensive understanding of the mechanism by which artemisinin affects bovine mammary epithelial cells to improve mammary gland turnover by inducing cell proliferation and mammary gland development.

The efficacy of coenzyme Q10 treatment in alleviating the symptoms of primary coenzyme Q10 deficiency: A systematic review

Coenzyme Q₁₀ (CoQ₁₀) is necessary for mitochondrial electron transport. Mutations in CoQ₁₀ biosynthetic genes cause primary CoQ₁₀ deficiency (PCoQD) and manifest as mitochondrial disorders. It is often stated that PCoQD patients can be treated by oral CoQ₁₀ supplementation. To test this, we compiled all studies describing PCoQD patients up to May 2022. We excluded studies with no data on CoQ₁₀ treatment, or with insufficient description of effectiveness. Out of 303 PCoQD patients identified, we retained 89 cases, of which 24 reported improvements after CoQ₁₀ treatment (27.0%). In five cases, the patient's condition was reported to deteriorate after halting of CoQ₁₀ treatment. 12 cases reported improvement in the severity of ataxia and 5 cases in the severity of proteinuria. Only a subjective description of improvement was reported for 4 patients described as responding. All reported responses were partial improvements of only some symptoms. For PCoQD patients, CoQ₁₀ supplementation is replacement therapy. Yet, there is only very weak evidence for the efficacy of the treatment. Our findings, thus, suggest a need for caution when seeking to justify the widespread use of CoQ₁₀ for the treatment of any disease or as dietary supplement.

A novel COQ7 mutation causing primarily neuromuscular pathology and its treatment options

Coenzyme Q₁₀ (CoQ₁₀) is necessary as electron transporter in mitochondrial respiration and other cellular functions. CoQ₁₀ is synthesized by all cells and defects in the synthesis pathway result in primary CoQ₁₀ deficiency that frequently leads to severe mitochondrial disease syndrome. CoQ₁₀ is exceedingly hydrophobic, insoluble, and poorly bioavailable, with the result that dietary CoQ₁₀ supplementation produces no or only minimal relief for patients. We studied a patient from Turkey and identified and characterized a new mutation in the CoQ₁₀ biosynthetic gene COQ7 (c.161G > A; p.Arg54Gln). We find that unexpected neuromuscular pathology can accompany CoQ₁₀ deficiency caused by a COQ7 mutation. We also show that by-passing the need for COQ7 by providing the unnatural precursor 2,4-dihydroxybenzoic acid, as has been proposed, is unlikely to be an effective and safe therapeutic option. In contrast, we show for the first time in human patient cells that the respiratory defect resulting from CoQ₁₀ deficiency is rescued by providing CoQ₁₀ formulated with caspofungin (CF/CoQ). Caspofungin is a clinically approved intravenous fungicide whose surfactant properties lead to CoQ₁₀ micellization, complete water solubilization, and efficient uptake by cells and organs in animal studies. These findings reinforce the possibility of using CF/CoQ in the clinical treatment of CoQ₁₀-deficient patients.

Mechanisms and Therapeutic Effects of Benzoquinone Ring Analogs in Primary CoQ Deficiencies

Coenzyme Q (CoQ) is a conserved polyprenylated lipid composed of a redox-active benzoquinone ring and a long polyisoprenyl tail that serves as a membrane anchor. CoQ biosynthesis involves multiple steps, including multiple modifications of the precursor ring 4-hydroxybenzoic acid. Mutations in the enzymes involved in CoQ biosynthesis pathway result in primary coenzyme Q deficiencies, mitochondrial disorders whose clinical heterogenicity reflects the multiple biological function of CoQ. Patients with these disorders do not always respond to CoQ supplementation, and CoQ analogs have not been successful as alternative approaches. Progress made in understanding the CoQ biosynthesis pathway and studies of supplementation with 4-hydroxybenzoic acid ring analogs have opened a new area in the field of primary CoQ deficiencies treatment. Here, we will review these studies, focusing on efficacy of the different 4-hydroxybenzoic acid ring analogs, models in which they have been tested, and their mechanisms of action. Understanding how these compounds ameliorate biochemical, molecular, and/or clinical phenotypes of CoQ deficiencies is important to develop the most rational treatment for CoQ deficient patients, depending on their molecular defects.

Levels of Coenzyme Q10 and Several COQ Proteins in Human Astrocytoma Tissues Are Inversely Correlated with Malignancy

In a previous study, we reported the alterations of primary antioxidant enzymes and decreased citrate synthase (CS) activities in different grades of human astrocytoma tissues. Here, we further investigated coenzyme Q₁₀ (CoQ₁₀) levels and protein levels of polyprenyl diphosphate synthase subunit (PDSS2) and several COQ proteins required for CoQ₁₀ biosynthesis in these tissues. We found that the level of endogenous CoQ₁₀, but not of exogenous α-tocopherol, was higher in nontumor controls than in all grades of astrocytoma tissues. The levels of COQ3, COQ5, COQ6, COQ7, COQ8A, and COQ9, but not of COQ4, were lower in Grade IV astrocytoma tissues than in controls or low-grade (Grades I and II) astrocytomas, but PDSS2 levels were higher in astrocytoma tissues than in controls. Correlation analysis revealed that the levels of CoQ₁₀ and COQ proteins were negatively correlated with malignancy degree and positively correlated with CS activity, whereas PDSS2 level was positively correlated with malignancy. Moreover, lower level of mitochondrial DNA-encoded cytochrome c oxidase subunit 2 was not only associated with a higher malignancy degree but also with lower level of all COQ proteins detected. The results revealed that mitochondrial abnormalities are associated with impaired CoQ₁₀ maintenance in human astrocytoma progression.

Recent advances in the metabolic pathways and microbial production of coenzyme Q

Coenzyme Q (CoQ) serves as an electron carrier in aerobic respiration and has become an interesting target for biotechnological production due to its antioxidative effect and benefits in supplementation to patients with various diseases. Here, we review discovery of the pathway with a particular focus on its superstructuration and regulation, and we summarize the metabolic engineering strategies for overproduction of CoQ by microorganisms. Studies in model microorganisms elucidated the details of CoQ biosynthesis and revealed the existence of multiprotein complexes composed of several enzymes that catalyze consecutive reactions in the CoQ pathways of Saccharomyces cerevisiae and Escherichia coli. Recent findings indicate that the identity and the total number of proteins involved in CoQ biosynthesis vary between species, which raises interesting questions about the evolution of the pathway and could provide opportunities for easier engineering of CoQ production. For the biotechnological production, so far only microorganisms have been used that naturally synthesize CoQ₁₀ or a related CoQ species. CoQ biosynthesis requires the aromatic precursor 4-hydroxybenzoic acid and the prenyl side chain that defines the CoQ species. Up to now, metabolic engineering strategies concentrated on the overproduction of the prenyl side chain as well as fine-tuning the expression of ubi genes from the ubiquinone modification pathway, resulting in high CoQ yields. With expanding knowledge about CoQ biosynthesis and exploration of new strategies for strain engineering, microbial CoQ production is expected to improve.

Insights into the post-translational modification and its emerging role in shaping the tumor microenvironment

More and more in-depth studies have revealed that the occurrence and development of tumors depend on gene mutation and tumor heterogeneity. The most important manifestation of tumor heterogeneity is the dynamic change of tumor microenvironment (TME) heterogeneity. This depends not only on the tumor cells themselves in the microenvironment where the infiltrating immune cells and matrix together forming an antitumor and/or pro-tumor network. TME has resulted in novel therapeutic interventions as a place beyond tumor beds. The malignant cancer cells, tumor infiltrate immune cells, angiogenic vascular cells, lymphatic endothelial cells, cancer-associated fibroblastic cells, and the released factors including intracellular metabolites, hormonal signals and inflammatory mediators all contribute actively to cancer progression. Protein post-translational modification (PTM) is often regarded as a degradative mechanism in protein destruction or turnover to maintain physiological homeostasis. Advances in quantitative transcriptomics, proteomics, and nuclease-based gene editing are now paving the global ways for exploring PTMs. In this review, we focus on recent developments in the PTM area and speculate on their importance as a critical functional readout for the regulation of TME. A wealth of information has been emerging to prove useful in the search for conventional therapies and the development of global therapeutic strategies.

A unique flavoenzyme operates in ubiquinone biosynthesis in photosynthesis-related eukaryotes

Coenzyme Q (CoQ) is an electron transporter in the mitochondrial respiratory chain, yet the biosynthetic pathway in eukaryotes remains only partially resolved. C6-hydroxylation completes the benzoquinone ring full substitution, a hallmark of CoQ. Here, we show that plants use a unique flavin-dependent monooxygenase (CoqF), instead of di-iron enzyme (Coq7) operating in animals and fungi, as a C6-hydroxylase. CoqF evolved early in eukaryotes and became widely distributed in photosynthetic and related organisms ranging from plants, algae, apicomplexans, and euglenids. Independent alternative gene losses in different groups and lateral gene transfer have ramified CoqF across the eukaryotic tree with predominance in green lineages. The exclusive presence of CoqF in Streptophyta hints at an association of the flavoenzyme with photoautotrophy in terrestrial environments. CoqF provides a phylogenetic marker distinguishing eukaryotes and represents a previously unknown target for drug design against parasitic protists.

Evolutionary assembly of cooperating cell types in an animal chemical defense system

How the functions of multicellular organs emerge from the underlying evolution of cell types is poorly understood. We deconstructed evolution of an organ novelty: a rove beetle gland that secretes a defensive cocktail. We show how gland function arose via assembly of two cell types that manufacture distinct compounds. One cell type, comprising a chemical reservoir within the abdomen, produces alkane and ester compounds. We demonstrate that this cell type is a hybrid of cuticle cells and ancient pheromone and adipocyte-like cells, executing its function via a mosaic of enzymes from each parental cell type. The second cell type synthesizes benzoquinones using a chimera of conserved cellular energy and cuticle formation pathways. We show that evolution of each cell type was shaped by coevolution between the two cell types, yielding a potent secretion that confers adaptive value. Our findings illustrate how cooperation between cell types arises, generating new, organ-level behaviors.

High-intensity training induces non-stoichiometric changes in the mitochondrial proteome of human skeletal muscle without reorganisation of respiratory chain content

Mitochondrial defects are implicated in multiple diseases and aging. Exercise training is an accessible, inexpensive therapeutic intervention that can improve mitochondrial bioenergetics and quality of life. By combining multiple omics techniques with biochemical and in silico normalisation, we removed the bias arising from the training-induced increase in mitochondrial content to unearth an intricate and previously undemonstrated network of differentially prioritised mitochondrial adaptations. We show that changes in hundreds of transcripts, proteins, and lipids are not stoichiometrically linked to the overall increase in mitochondrial content. Our findings suggest enhancing electron flow to oxidative phosphorylation (OXPHOS) is more important to improve ATP generation than increasing the abundance of the OXPHOS machinery, and do not support the hypothesis that training-induced supercomplex formation enhances mitochondrial bioenergetics. Our study provides an analytical approach allowing unbiased and in-depth investigations of training-induced mitochondrial adaptations, challenging our current understanding, and calling for careful reinterpretation of previous findings.

Coenzyme Q at the Hinge of Health and Metabolic Diseases

Coenzyme Q is a unique lipidic molecule highly conserved in evolution and essential to maintaining aerobic metabolism. It is endogenously synthesized in all cells by a very complex pathway involving a group of nuclear genes that share high homology among species. This pathway is tightly regulated at transcription and translation, but also by environment and energy requirements. Here, we review how coenzyme Q reacts within mitochondria to promote ATP synthesis and also integrates a plethora of metabolic pathways and regulates mitochondrial oxidative stress. Coenzyme Q is also located in all cellular membranes and plasma lipoproteins in which it exerts antioxidant function, and its reaction with different extramitochondrial oxidoreductases contributes to regulate the cellular redox homeostasis and cytosolic oxidative stress, providing a key factor in controlling various apoptosis mechanisms. Coenzyme Q levels can be decreased in humans by defects in the biosynthesis pathway or by mitochondrial or cytosolic dysfunctions, leading to a highly heterogeneous group of mitochondrial diseases included in the coenzyme Q deficiency syndrome. We also review the importance of coenzyme Q levels and its reactions involved in aging and age-associated metabolic disorders, and how the strategy of its supplementation has had benefits for combating these diseases and for physical performance in aging.

Animal Models of Coenzyme Q Deficiency: Mechanistic and Translational Learnings

Coenzyme Q (CoQ) is a vital lipophilic molecule that is endogenously synthesized in the mitochondria of each cell. The CoQ biosynthetic pathway is complex and not completely characterized, and it involves at least thirteen catalytic and regulatory proteins. Once it is synthesized, CoQ exerts a wide variety of mitochondrial and extramitochondrial functions thank to its redox capacity and its lipophilicity. Thus, low levels of CoQ cause diseases with heterogeneous clinical symptoms, which are not always understood. The decreased levels of CoQ may be primary caused by defects in the CoQ biosynthetic pathway or secondarily associated with other diseases. In both cases, the pathomechanisms are related to the CoQ functions, although further experimental evidence is required to establish this association. The conventional treatment for CoQ deficiencies is the high doses of oral CoQ₁₀ supplementation, but this therapy is not effective for some specific clinical presentations, especially in those involving the nervous system. To better understand the CoQ biosynthetic pathway, the biological functions linked to CoQ and the pathomechanisms of CoQ deficiencies, and to improve the therapeutic outcomes of this syndrome, a variety of animal models have been generated and characterized in the last decade. In this review, we show all the animal models available, remarking on the most important outcomes that each model has provided. Finally, we also comment some gaps and future research directions related to CoQ metabolism and how the current and novel animal models may help in the development of future research studies.

Minimal mitochondrial respiration is required to prevent cell death by inhibition of mTOR signaling in CoQ-deficient cells

Coenzyme Q (CoQ) is a lipid-like mobile electron transporter of the mitochondrial respiratory chain. Patients with partial loss-of-function mutations in the CoQ biosynthesis pathway suffer from partial primary CoQ deficiency (MIM 607426). This leads to mitochondrial dysfunction, which presents like mitochondrial disease syndrome (MDS). In addition, many other conditions, including MDS itself, lead to secondary CoQ deficiency. We sought to identify drugs that can alleviate the consequences of the mitochondrial dysfunction that is associated with CoQ deficiency. Loss of the CoQ-biosynthetic enzyme COQ7 prevents CoQ synthesis but leads to the accumulation of the biosynthetic intermediate demethoxyubiquinone (DMQ). Coq7-knockout mouse embryonic fibroblasts (MEFs) die when rapid ATP generation from glycolysis is prevented. We screened for drugs that could rescue cell death under these conditions. All compounds that were identified inhibit mTOR signaling. In the CoQ-deficient cells, the beneficial action mTOR inhibition appears to be mediated by inhibition of protein translation rather than by stimulation of autophagy. We further studied the Coq7-knockout cells to better determine under which conditions mTOR inhibition could be beneficial. We established that Coq7-knockout cells remain capable of a low level of mitochondrial respiration mediated by DMQ. To obtain more profound mitochondrial dysfunction, we created double-knockout mutant MEFs lacking both Coq7, as well as Pdss2, which is required for sidechain synthesis. These cells make neither CoQ nor DMQ, and their extremely small residual respiration depends on uptake of CoQ from the culture medium. Although these cells are healthy in the presence of sufficient glucose for glycolysis and do not require uridine or pyruvate supplementation, mTOR inhibitors were unable to prevent their death in the absence of sufficient glycolysis. We conclude that, for reasons that remain to be elucidated, the energy-sparing benefits of the inhibition of mTOR signaling require a minimally functional respiratory chain.

Coenzyme Q nanodisks counteract the effect of statins on C2C12 myotubes

Depletion of coenzyme Q (CoQ) is associated with disease, ranging from myopathy to heart failure. To induce a CoQ deficit, C2C12 myotubes were incubated with high dose simvastatin. This resulted in a concentration-dependent inhibition of cell viability. Simvastatin-induced effects were prevented by co-incubation with mevalonic acid. When myotubes were incubated with 60 μM simvastatin, mitochondrial CoQ content decreased while co-incubation with CoQ nanodisks (ND) increased mitochondrial CoQ levels and improved cell viability. Incubation of myotubes with simvastatin also led to a reduction in oxygen consumption rate (OCR). When myotubes were co-incubated with simvastatin and CoQ ND, the decline in OCR was ameliorated. The data indicate that CoQ ND represent a water soluble vehicle capable of delivering CoQ to cultured myotubes. Thus, these biocompatible nanoparticles have the potential to bypass poor CoQ oral bioavailability as a treatment option for individuals with severe CoQ deficiency syndromes and/or aging-related CoQ depletion.

Regulation of hepatic coenzyme Q biosynthesis by dietary omega-3 polyunsaturated fatty acids

Dietary fats are important for human health, yet it is not fully understood how different fats affect various health problems. Although polyunsaturated fatty acids (PUFAs) are generally considered as highly oxidizable, those of the n-3 series can ameliorate the risk of many age-related disorders. Coenzyme Q (CoQ) is both an essential component of the mitochondrial electron transport chain and the only lipid-soluble antioxidant that animal cells can synthesize. Previous work has documented the protective antioxidant properties of CoQ against the autoxidation products of PUFAs. Here, we have explored in vitro and in vivo models to better understand the regulation of CoQ biosynthesis by dietary fats. In mouse liver, PUFAs increased CoQ content, and PUFAs of the n-3 series increased preferentially CoQ₁₀. This response was recapitulated in hepatic cells cultured in the presence of lipid emulsions, where we additionally demonstrated a role for n-3 PUFAs as regulators of CoQ biosynthesis via the upregulation of several COQ proteins and farnesyl pyrophosphate levels. In both models, n-3 PUFAs altered the mitochondrial network without changing the overall mitochondrial mass. Furthermore, in cellular systems, n-3 PUFAs favored the synthesis of CoQ₁₀ over CoQ₉, thus altering the ratio between CoQ isoforms through a mechanism that involved downregulation of farnesyl diphosphate synthase activity. This effect was recapitulated by both siRNA silencing and by pharmacological inhibition of farnesyl diphosphate synthase with zoledronic acid. We highlight here the ability of n-3 PUFAs to regulate CoQ biosynthesis, CoQ content, and the ratio between its isoforms, which might be relevant to better understand the health benefits associated with this type of fat. Additionally, we identify for the first time zoledronic acid as a drug that inhibits CoQ biosynthesis, which must be also considered with respect to its biological effects on patients.

Secondary CoQ10 deficiency, bioenergetics unbalance in disease and aging

Coenzyme Q₁₀ (CoQ₁₀ ) deficiency is a rare disease characterized by a decreased accumulation of CoQ₁₀ in cell membranes. Considering that CoQ₁₀ synthesis and most of its functions are carried out in mitochondria, CoQ₁₀ deficiency cases are usually considered a mitochondrial disease. A relevant feature of CoQ₁₀ deficiency is that it is the only mitochondrial disease with a successful therapy available, the CoQ₁₀ supplementation. Defects in components of the synthesis machinery caused by mutations in COQ genes generate the primary deficiency of CoQ₁₀ . Mutations in genes that are not directly related to the synthesis machinery cause secondary deficiency. Cases of CoQ₁₀ deficiency without genetic origin are also considered a secondary deficiency. Both types of deficiency can lead to similar clinical manifestations, but the knowledge about primary deficiency is deeper than secondary. However, secondary deficiency cases may be underestimated since many of their clinical manifestations are shared with other pathologies. This review shows the current state of secondary CoQ₁₀ deficiency, which could be even more relevant than primary deficiency for clinical activity. The analysis covers the fundamental features of CoQ₁₀ deficiency, which are necessary to understand the biological and clinical differences between primary and secondary CoQ₁₀ deficiencies. Further, a more in-depth analysis of CoQ₁₀ secondary deficiency was undertaken to consider its origins, introduce a new way of classification, and include aging as a form of secondary deficiency.

Coenzyme Q Biosynthesis: An Update on the Origins of the Benzenoid Ring and Discovery of New Ring Precursors

Coenzyme Q (ubiquinone or CoQ) is a conserved polyprenylated lipid essential for mitochondrial respiration. CoQ is composed of a redox-active benzoquinone ring and a long polyisoprenyl tail that serves as a membrane anchor. A classic pathway leading to CoQ biosynthesis employs 4-hydroxybenzoic acid (4HB). Recent studies with stable isotopes in E. coli, yeast, and plant and animal cells have identified CoQ intermediates and new metabolic pathways that produce 4HB. Stable isotope labeling has identified para-aminobenzoic acid as an alternate ring precursor of yeast CoQ biosynthesis, as well as other natural products, such as kaempferol, that provide ring precursors for CoQ biosynthesis in plants and mammals. In this review, we highlight how stable isotopes can be used to delineate the biosynthetic pathways leading to CoQ.

Primary Coenzyme Q deficiencies: A literature review and online platform of clinical features to uncover genotype-phenotype correlations

Primary Coenzyme Q (CoQ) deficiencies are clinically heterogeneous conditions and lack clear genotype-phenotype correlations, complicating diagnosis and prognostic assessment. Here we present a compilation of all the symptoms and patients with primary CoQ deficiency described in the literature so far and analyse the most common clinical manifestations associated with pathogenic variants identified in the different COQ genes. In addition, we identified new associations between the age of onset of symptoms and different pathogenic variants, which could help to a better diagnosis and guided treatment. To make these results useable for clinicians, we created an online platform (https://coenzymeQbiology.github.io/clinic-CoQ-deficiency) about clinical manifestations of primary CoQ deficiency that will be periodically updated to incorporate new information published in the literature. Since CoQ primary deficiency is a rare disease, the available data are still limited, but as new patients are added over time, this tool could become a key resource for a more efficient diagnosis of this pathology.

Metabolic Targets of Coenzyme Q10 in Mitochondria

Coenzyme Q10 (CoQ10) is classically viewed as an important endogenous antioxidant and key component of the mitochondrial respiratory chain. For this second function, CoQ molecules seem to be dynamically segmented in a pool attached and engulfed by the super-complexes I + III, and a free pool available for complex II or any other mitochondrial enzyme that uses CoQ as a cofactor. This CoQ-free pool is, therefore, used by enzymes that link the mitochondrial respiratory chain to other pathways, such as the pyrimidine de novo biosynthesis, fatty acid β-oxidation and amino acid catabolism, glycine metabolism, proline, glyoxylate and arginine metabolism, and sulfide oxidation metabolism. Some of these mitochondrial pathways are also connected to metabolic pathways in other compartments of the cell and, consequently, CoQ could indirectly modulate metabolic pathways located outside the mitochondria. Thus, we review the most relevant findings in all these metabolic functions of CoQ and their relations with the pathomechanisms of some metabolic diseases, highlighting some future perspectives and potential therapeutic implications.

Secondary coenzyme Q deficiency in neurological disorders

Coenzyme Q (CoQ) is a ubiquitous lipid serving essential cellular functions. It is the only component of the mitochondrial respiratory chain that can be exogenously absorbed. Here, we provide an overview of current knowledge, controversies, and open questions about CoQ intracellular and tissue distribution, in particular in brain and skeletal muscle. We discuss human neurological diseases and mouse models associated with secondary CoQ deficiency in these tissues and highlight pharmacokinetic and anatomical challenges in exogenous CoQ biodistribution, recent improvements in CoQ formulations and imaging, as well as alternative therapeutical strategies to CoQ supplementation. The last section proposes possible mechanisms underlying secondary CoQ deficiency in human diseases with emphasis on neurological and neuromuscular disorders.

Hypoglycemia-Sensing Neurons of the Ventromedial Hypothalamus Require AMPK-Induced Txn2 Expression but Are Dispensable for Physiological Counterregulation

The ventromedial nucleus of the hypothalamus (VMN) is involved in the counterregulatory response to hypoglycemia. VMN neurons activated by hypoglycemia (glucose-inhibited [GI] neurons) have been assumed to play a critical although untested role in this response. Here, we show that expression of a dominant negative form of AMPK or inactivation of AMPK α1 and α2 subunit genes in Sf1 neurons of the VMN selectively suppressed GI neuron activity. We found that Txn2, encoding a mitochondrial redox enzyme, was strongly downregulated in the absence of AMPK activity and that reexpression of Txn2 in Sf1 neurons restored GI neuron activity. In cell lines, Txn2 was required to limit glucopenia-induced reactive oxygen species production. In physiological studies, absence of GI neuron activity after AMPK suppression in the VMN had no impact on the counterregulatory hormone response to hypoglycemia or on feeding. Thus, AMPK is required for GI neuron activity by controlling the expression of the antioxidant enzyme Txn2. However, the glucose-sensing capacity of VMN GI neurons is not required for the normal counterregulatory response to hypoglycemia. Instead, it may represent a fail-safe system in case of impaired hypoglycemia sensing by peripherally located glucose detection systems that are connected to the VMN.

Micellization of coenzyme Q by the fungicide caspofungin allows for safe intravenous administration to reach extreme supraphysiological concentrations

Coenzyme Q₁₀ (CoQ₁₀; also known as ubiquinone) is a vital, redox-active membrane component that functions as obligate electron transporter in the mitochondrial respiratory chain, as cofactor in other enzymatic processes and as antioxidant. CoQ₁₀ supplementation has been widely investigated for treating a variety of acute and chronic conditions in which mitochondrial function or oxidative stress play a role. In addition, it is used as replacement therapy in patients with CoQ deficiency including inborn primary CoQ₁₀ deficiency due to mutations in CoQ₁₀-biosynthetic genes as well as secondary CoQ₁₀ deficiency, which is frequently observed in patients with mitochondrial disease syndrome and in other conditions. However, despite many tests and some promising results, whether CoQ₁₀ treatment is beneficial in any indication has remained inconclusive. Because CoQ₁₀ is highly insoluble, it is only available in oral formulations, despite its very poor oral bioavailability. Using a novel model of CoQ-deficient cells, we screened a library of FDA-approved drugs for an activity that could increase the uptake of exogenous CoQ₁₀ by the cell. We identified the fungicide caspofungin as capable of increasing the aqueous solubility of CoQ₁₀ by several orders of magnitude. Caspofungin is a mild surfactant that solubilizes CoQ₁₀ by forming nano-micelles with unique properties favoring stability and cellular uptake. Intravenous administration of the formulation in mice achieves unprecedented increases in CoQ₁₀ plasma levels and in tissue uptake, with no observable toxicity. As it contains only two safe components (caspofungin and CoQ₁₀), this injectable formulation presents a high potential for clinical safety and efficacy.

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Coenzyme Q₁₀ (CoQ₁₀) can be solubilized by the antifungal drug caspofungin (CF).

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CF is a mild surfactant and solubilizes CoQ₁₀ in water by forming micellar structures with a high CoQ₁₀ content.

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CF/CoQ₁₀ micelles have unique properties favoring rapid and efficient uptake into cells and mitochondria.

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CF/CoQ₁₀ micelles can be intravenously administrated without signs of toxicity.

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Intravenous administration of CF/CoQ₁₀ in mice achieves unprecedented elevation of CoQ₁₀ plasma levels and tissue uptake.

Advances in bacterial pathways for the biosynthesis of ubiquinone

Ubiquinone is an important component of the electron transfer chains in proteobacteria and eukaryotes. The biosynthesis of ubiquinone requires multiple steps, most of which are common to bacteria and eukaryotes. Whereas the enzymes of the mitochondrial pathway that produces ubiquinone are highly similar across eukaryotes, recent results point to a rather high diversity of pathways in bacteria. This review focuses on ubiquinone in bacteria, highlighting newly discovered functions and detailing the proteins that are known to participate to its biosynthetic pathways. Novel results showing that ubiquinone can be produced by a pathway independent of dioxygen suggest that ubiquinone may participate to anaerobiosis, in addition to its well-established role for aerobiosis. We also discuss the supramolecular organization of ubiquinone biosynthesis proteins and we summarize the current understanding of the evolution of the ubiquinone pathways relative to those of other isoprenoid quinones like menaquinone and plastoquinone.

Potential use of ubiquinol and d-ribose in patients with heart failure with preserved ejection fraction

•Manuscript Highlights.•HFpEF is associated with reduced ATP production in the myocardium.•Ubiquinol and d-ribose both contribute to the generation of myocardial ATP.•Both ubiquinol and d-ribose are being studied as supplemental treatments for patients with HFpEF.

Genes and lipids that impact uptake and assimilation of exogenous coenzyme Q in Saccharomyces cerevisiae

Coenzyme Q (CoQ) is an essential player in the respiratory electron transport chain and is the only lipid-soluble antioxidant synthesized endogenously in mammalian and yeast cells. In humans, genetic mutations, pathologies, certain medical treatments, and aging, result in CoQ deficiencies, which are linked to mitochondrial, cardiovascular, and neurodegenerative diseases. The only strategy available for these patients is CoQ supplementation. CoQ supplements benefit a small subset of patients, but the poor solubility of CoQ greatly limits treatment efficacy. Consequently, the efficient delivery of CoQ to the mitochondria and restoration of respiratory function remains a major challenge. A better understanding of CoQ uptake and mitochondrial delivery is crucial to make this molecule a more efficient and effective therapeutic tool. In this study, we investigated the mechanism of CoQ uptake and distribution using the yeast Saccharomyces cerevisiae as a model organism. The addition of exogenous CoQ was tested for the ability to restore growth on non-fermentable medium in several strains that lack CoQ synthesis (coq mutants). Surprisingly, we discovered that the presence of CoQ biosynthetic intermediates impairs assimilation of CoQ into a functional respiratory chain in yeast cells. Moreover, a screen of 40 gene deletions considered to be candidates to prevent exogenous CoQ from rescuing growth of the CoQ-less coq2Δ mutant, identified six novel genes (CDC10, RTS1, RVS161, RVS167, VPS1, and NAT3) as necessary for efficient trafficking of CoQ to mitochondria. The proteins encoded by these genes represent essential steps in the pathways responsible for transport of exogenously supplied CoQ to its functional sites in the cell, and definitively associate CoQ distribution with endocytosis and intracellular vesicular trafficking pathways conserved from yeast to human cells.

Metabolism of the Flavonol Kaempferol in Kidney Cells Liberates the B-ring to Enter Coenzyme Q Biosynthesis

Coenzyme Q (CoQ) is an essential component of the mitochondrial electron transport chain and an important antioxidant present in all cellular membranes. CoQ deficiencies are frequent in aging and in age-related diseases, and current treatments are limited to CoQ supplementation. Strategies that rely on CoQ supplementation suffer from poor uptake and trafficking of this very hydrophobic molecule. In a previous study, the dietary flavonol kaempferol was reported to serve as a CoQ ring precursor and to increase the CoQ content in kidney cells, but neither the part of the molecule entering CoQ biosynthesis nor the mechanism were described. In this study, kaempferol labeled specifically in the B-ring was isolated from Arabidopsis plants. Kidney cells treated with this compound incorporated the B-ring of kaempferol into newly synthesized CoQ, suggesting that the B-ring is metabolized via a mechanism described in plant cells. Kaempferol is a natural flavonoid present in fruits and vegetables and possesses antioxidant, anticancer, and anti-inflammatory therapeutic properties. A better understanding of the role of kaempferol as a CoQ ring precursor makes this bioactive compound a potential candidate for the design of interventions aiming to increase endogenous CoQ biosynthesis and may improve CoQ deficient phenotypes in aging and disease.

Intragenic suppressor mutations of the COQ8 protein kinase homolog restore coenzyme Q biosynthesis and function in Saccharomyces cerevisiae

Saccharomyces cerevisiae Coq8 is a member of the ancient UbiB atypical protein kinase family. Coq8, and its orthologs UbiB, ABC1, ADCK3, and ADCK4, are required for the biosynthesis of coenzyme Q in yeast, E. coli, A. thaliana, and humans. Each Coq8 ortholog retains nine highly conserved protein kinase-like motifs, yet its functional role in coenzyme Q biosynthesis remains mysterious. Coq8 may function as an ATPase whose activity is stimulated by coenzyme Q intermediates and phospholipids. A key yeast point mutant expressing Coq8-A197V was previously shown to result in a coenzyme Q-less, respiratory deficient phenotype. The A197V substitution occurs in the crucial Ala-rich protein kinase-like motif I of yeast Coq8. Here we show that long-term cultures of mutants expressing Coq8-A197V produce spontaneous revertants with the ability to grow on medium containing a non-fermentable carbon source. Each revertant is shown to harbor a secondary intragenic suppressor mutation within the COQ8 gene. The intragenic suppressors restore the synthesis of coenzyme Q. One class of the suppressors fully restores the levels of coenzyme Q and key Coq polypeptides necessary for the maintenance and integrity of the high-molecular mass CoQ synthome (also termed complex Q), while the other class provides only a partial rescue. Mutants harboring the first class of suppressors grow robustly under respiratory conditions, while mutants containing the second class grow more slowly under these conditions. Our work provides insight into the function of this important yet still enigmatic Coq8 family.

COQ11 deletion mitigates respiratory deficiency caused by mutations in the gene encoding the coenzyme Q chaperone protein Coq10

Coenzyme Q (Q _n ) is a vital lipid component of the electron transport chain that functions in cellular energy metabolism and as a membrane antioxidant. In the yeast Saccharomyces cerevisiae, coq1-coq9 deletion mutants are respiratory-incompetent, sensitive to lipid peroxidation stress, and unable to synthesize Q₆ The yeast coq10 deletion mutant is also respiratory-deficient and sensitive to lipid peroxidation, yet it continues to produce Q₆ at an impaired rate. Thus, Coq10 is required for the function of Q₆ in respiration and as an antioxidant and is believed to chaperone Q₆ from its site of synthesis to the respiratory complexes. In several fungi, Coq10 is encoded as a fusion polypeptide with Coq11, a recently identified protein of unknown function required for efficient Q₆ biosynthesis. Because "fused" proteins are often involved in similar biochemical pathways, here we examined the putative functional relationship between Coq10 and Coq11 in yeast. We used plate growth and Seahorse assays and LC-MS/MS analysis to show that COQ11 deletion rescues respiratory deficiency, sensitivity to lipid peroxidation, and decreased Q₆ biosynthesis of the coq10Δ mutant. Additionally, immunoblotting indicated that yeast coq11Δ mutants accumulate increased amounts of certain Coq polypeptides and display a stabilized CoQ synthome. These effects suggest that Coq11 modulates Q₆ biosynthesis and that its absence increases mitochondrial Q₆ content in the coq10Δcoq11Δ double mutant. This augmented mitochondrial Q₆ content counteracts the respiratory deficiency and lipid peroxidation sensitivity phenotypes of the coq10Δ mutant. This study further clarifies the intricate connection between Q₆ biosynthesis, trafficking, and function in mitochondrial metabolism.