Coenzyme Q - Biosynthesis and functions

In addition to its role as a component of the mitochondrial respiratory chain and our only lipid-soluble antioxidant synthesized endogenously, in recent years coenzyme Q (CoQ) has been found to have an increasing number of other important functions required for normal metabolic processes. A number of genetic mutations that reduce CoQ biosynthesis are associated with serious functional disturbances that can be eliminated by dietary administration of this lipid, making CoQ deficiencies the only mitochondrial diseases which can be successfully treated at present. In connection with certain other diseases associated with excessive oxidative stress, the level of CoQ is elevated as a protective response. Aging, certain experimental conditions and several human diseases reduce this level, resulting in serious metabolic disturbances. Since dietary uptake of this lipid is limited, up-regulation of its biosynthetic pathway is of considerable clinical interest. One approach for this purpose is administration of epoxidated all-trans polyisoprenoids, which enhance both CoQ biosynthesis and levels in experimental systems.     

Coenzyme Q and vitamin E need each other as antioxidants

Summary Both vitamin E and coenzyme Q possess distinct lipoprotective antioxidant properties in biological membranes. Their combined antioxidant activity, however, is markedly synergistic when both are present together. While it is likely that vitamin E represents the initial chain-breaking antioxidant during lipid peroxidation, both fully reduced CoQH₂ (ubiquinol) and semireduced CoQH^. (ubisemiquinone) appear to efficiently recycle the resultant vitamin E phenoxyl radical back to its biologically active reduced form. We describe and support a potential kinetic mechanism whereby vitamin E and coenzyme Q interact in such a way as to usurp the prooxidant effects of O ₂ ^−. . Physical interactions of vitamin E and coenzyme Q within the environment of the membrane lipid bilayer facilitate the recycling of vitamin E by ubisemiquinone and ubiquinol. Lastly, data are linked into a catalytic cycle that serves to connect normal electron transport mechanisms within biological membranes to the maintenance of lipoprotective antioxidant mechanisms.     

Identification of Coq11, a New Coenzyme Q Biosynthetic Protein in the CoQ-Synthome in Saccharomyces cerevisiae

Coenzyme Q (Q or ubiquinone) is a redox active lipid composed of a fully substituted benzoquinone ring and a polyisoprenoid tail and is required for mitochondrial electron transport. In the yeast Saccharomyces cerevisiae, Q is synthesized by the products of 11 known genes, COQ1–COQ9, YAH1, and ARH1. The function of some of the Coq proteins remains unknown, and several steps in the Q biosynthetic pathway are not fully characterized. Several of the Coq proteins are associated in a macromolecular complex on the matrix face of the inner mitochondrial membrane, and this complex is required for efficient Q synthesis. Here, we further characterize this complex via immunoblotting and proteomic analysis of tandem affinity-purified tagged Coq proteins. We show that Coq8, a putative kinase required for the stability of the Q biosynthetic complex, is associated with a Coq6-containing complex. Additionally Q₆ and late stage Q biosynthetic intermediates were also found to co-purify with the complex. A mitochondrial protein of unknown function, encoded by the YLR290C open reading frame, is also identified as a constituent of the complex and is shown to be required for efficient de novo Q biosynthesis. Given its effect on Q synthesis and its association with the biosynthetic complex, we propose that the open reading frame YLR290C be designated COQ11.     

CoQ10 deficiencies and MNGIE: Two treatable mitochondrial disorders

Background

Although causative mutations have been identified for numerous mitochondrial disorders, few disease-modifying treatments are available. Two examples of treatable mitochondrial disorders are coenzyme Q₁₀ (CoQ₁₀ or ubiquinone) deficiency and mitochondrial neurogastrointestinal encephalomyopathy (MNGIE).

Scope of review

Here, we describe clinical and molecular features of CoQ₁₀ deficiencies and MNGIE and explain how understanding their pathomechanisms have led to rationale therapies. Primary CoQ₁₀ deficiencies, due to mutations in genes required for ubiquinone biosynthesis, and secondary deficiencies, caused by genetic defects not directly related to CoQ₁₀ biosynthesis, often improve with CoQ₁₀ supplementation. In vitro and in vivo studies of CoQ₁₀ deficiencies have revealed biochemical alterations that may account for phenotypic differences among patients and variable responses to therapy. In contrast to the heterogeneous CoQ₁₀ deficiencies, MNGIE is a single autosomal recessive disease due to mutations in the TYMP gene encoding thymidine phosphorylase (TP). In MNGIE, loss of TP activity causes toxic accumulations of the nucleosides thymidine and deoxyuridine that are incorporated by the mitochondrial pyrimidine salvage pathway and cause deoxynucleoside triphosphate pool imbalances, which, in turn cause mtDNA instability. Allogeneic hematopoetic stem cell transplantation to restore TP activity and eliminate toxic metabolites is a promising therapy for MNGIE.

Major conclusions

CoQ₁₀ deficiencies and MNGIE demonstrate the feasibility of treating specific mitochondrial disorders through replacement of deficient metabolites or via elimination of excessive toxic molecules.

General significance

Studies of CoQ₁₀ deficiencies and MNGIE illustrate how understanding the pathogenic mechanisms of mitochondrial diseases can lead to meaningful therapies. This article is part of a Special Issue entitled: Biochemistry of Mitochondria, Life and Intervention 2010.     

The Phosphatase Ptc7 Induces Coenzyme Q Biosynthesis by Activating the Hydroxylase Coq7 in Yeast

The study of the components of mitochondrial metabolism has potential benefits for health span and lifespan because the maintenance of efficient mitochondrial function and antioxidant capacity is associated with improved health and survival. In yeast, mitochondrial function requires the tight control of several metabolic processes such as coenzyme Q biosynthesis, assuring an appropriate energy supply and antioxidant functions. Many mitochondrial processes are regulated by phosphorylation cycles mediated by protein kinases and phosphatases. In this study, we determined that the mitochondrial phosphatase Ptc7p, a Ser/Thr phosphatase, was required to regulate coenzyme Q₆ biosynthesis, which in turn activated aerobic metabolism and enhanced oxidative stress resistance. We showed that Ptc7p phosphatase specifically activated coenzyme Q₆ biosynthesis through the dephosphorylation of the demethoxy-Q₆ hydroxylase Coq7p. The current findings revealed that Ptc7p is a regulator of mitochondrial metabolism that is essential to maintain proper function of the mitochondria by regulating energy metabolism and oxidative stress resistance.     

Saccharomyces cerevisiae Coq9 polypeptide is a subunit of the mitochondrial coenzyme Q biosynthetic complex

Coenzyme Q (Q) is a redox active lipid that is an essential component of the electron transport chain. Here, we show that steady state levels of Coq3, Coq4, Coq6, Coq7 and Coq9 polypeptides in yeast mitochondria are dependent on the expression of each of the other COQ genes. Submitochondrial localization studies indicate Coq9p is a peripheral membrane protein on the matrix side of the mitochondrial inner membrane. To investigate whether Coq9p is a component of a complex of Q-biosynthetic proteins, the native molecular mass of Coq9p was determined by Blue Native-PAGE. Coq9p was found to co-migrate with Coq3p and Coq4p at a molecular mass of approximately 1 MDa. A direct physical interaction was shown by the immunoprecipitation of HA-tagged Coq9 polypeptide with Coq4p, Coq5p, Coq6p and Coq7p. These findings, together with other work identifying Coq3p and Coq4p interactions, identify at least six Coq polypeptides in a multi-subunit Q biosynthetic complex.     

辅酶Q10生物合成与生产研究进展

微生物发酵法是生产辅酶Q10的最佳工艺.辅酶Q10的生物合成途径包括异戊二烯焦磷酸合成、聚十异戊二烯焦磷酸合成、苯环修饰等过程.1-脱氧-D-木酮糖-5-磷酸合成酶、聚十异戊二烯焦磷酸合成酶、对羟基笨甲酸聚十异戊二烯焦磷酸转移酶等是Q10合成的关键酶.生产辅酶Q10的菌种可通过诱变、基因重组和支路敲除等方法获得.氧化还原电位控制、pH控制补料分批发酵、发酵萃取耦合技术等新工艺逐浙应用于辅酶Q10生产.     

The participation of coenzyme Q in free radical production and antioxidation

Published experimental data pertaining to the participation of coenzyme Q as a site of free radical formation in the mitochondrial electron transfer chain and the conditions required for free radical production have been reviewed critically. The evidence suggests that a component from each of the mitochondrial NADH-coenzyme Q, succinate-coenzyme Q, and coenzyme QH₂-cytochrome c reductases (complexes I, II, and III, most likely a nonheme iron-sulfur protein of each complex, is involved in free radical formation. Although the semiquinone form of coenzyme Q may be formed during electron transport, its unpaired electron most likely serves to aid in the dismutation of superoxide radicals instead of participating in free radical formation. Results of studies with electron transfer chain inhibitors make the conclusion dubious that coenzyme Q is a major free radical generator under normal physiological conditions but may be involved in superoxide radical formation during ischemia and subsequent reperfusion. Experiments at various levels of organization including subcellular systems, intact animals, and human subjects in theclinical setting, support the view that coenzyme Q, mainly in its reduced state, may act as an antioxidant protecting a number of cellular membranes from free radical damage.     

para-Aminobenzoic Acid Is a Precursor in Coenzyme Q6 Biosynthesis in Saccharomyces cerevisiae

Coenzyme Q (ubiquinone or Q) is a crucial mitochondrial lipid required for respiratory electron transport in eukaryotes. 4-Hydroxybenozoate (4HB) is an aromatic ring precursor that forms the benzoquinone ring of Q and is used extensively to examine Q biosynthesis. However, the direct precursor compounds and enzymatic steps for synthesis of 4HB in yeast are unknown. Here we show that para-aminobenzoic acid (pABA), a well known precursor of folate, also functions as a precursor for Q biosynthesis. A hexaprenylated form of pABA (prenyl-pABA) is normally present in wild-type yeast crude lipid extracts but is absent in yeast abz1 mutants starved for pABA. A stable ¹³C₆-isotope of pABA (p- amino[aromatic-¹³C₆]benzoic acid ([¹³C₆]pABA)), is prenylated in either wild-type or abz1 mutant yeast to form prenyl-[¹³C₆]pABA. We demonstrate by HPLC and mass spectrometry that yeast incubated with either [¹³C₆]pABA or [¹³C₆]4HB generate both ¹³C₆-demethoxy-Q (DMQ), a late stage Q biosynthetic intermediate, as well as the final product ¹³C₆-coenzyme Q. Pulse-labeling analyses show that formation of prenyl-pABA occurs within minutes and precedes the synthesis of Q. Yeast utilizing pABA as a ring precursor produce another nitrogen containing intermediate, 4-imino-DMQ₆. This intermediate is produced in small quantities in wild-type yeast cultured in standard media and in abz1 mutants supplemented with pABA. We suggest a mechanism where Schiff base-mediated deimination forms DMQ₆ quinone, thereby eliminating the nitrogen contributed by pABA. This scheme results in the convergence of the 4HB and pABA pathways in eukaryotic Q biosynthesis and has implications regarding the action of pABA-based antifolates.     

辅酶Q10 对子痫前期中氧化应激的防治作用的研究进展

子痫前期是导致全球孕产妇和围生儿发病和死亡的主要原因之一.子痫前期的病因至今尚未明确,但是大量研究已证实多系统的氧化应激与子痫前期发病机制有关.辅酶Q10是目前受到广泛关注的一种抗氧化剂,并且已有辅酶Q10药品制剂问世.本文从细胞水平简要总结了氧化应激与子痫前期发病机制的关系,并讨论了辅酶Q10对予痫前期中氧化应激的防治作用.希望为子痫前期的早期治疗及改善预后提供新的思路.     

Coenzyme Q Cytoprotective Mechanisms for Mitochondrial Complex I Cytopathies Involves NAD(P)H: Quinone Oxidoreductase 1(NQO1)

The commonest mitochondrial diseases are probably those impairing the function of complex I of the respiratory electron transport chain. Such complex I impairment may contribute to various neurodegenerative disorders e.g. Parkinson's disease. In the following, using hepatocytes as a model cell, we have shown for the first time that the cytotoxicity caused by complex I inhibition by rotenone but not that caused by complex III inhibition by antimycin can be prevented by coenzyme Q (CoQ 1 ) or menadione. Furthermore, complex I inhibitor cytotoxicity was associated with the collapse of the mitochondrial membrane potential and reactive oxygen species (ROS) formation. ROS scavengers or inhibitors of the mitochondrial permeability transition prevented cytotoxicity. The CoQ 1 cytoprotective mechanism required CoQ 1 reduction by DT-diaphorase (NQO 1 ). Furthermore, the mitochondrial membrane potential and ATP levels were restored at low CoQ 1 concentrations (5 &#119 M). This suggests that the CoQ 1 H 2 formed by NQO 1 reduced complex III and acted as an electron bypass of the rotenone block. However cytoprotection still occurred at higher CoQ 1 concentrations (>10 &#119 M), which were less effective at restoring ATP levels but readily restored the cellular cytosolic redox potential (i.e. lactate: pyruvate ratio) and prevented ROS formation. This suggests that CoQ 1 or menadione cytoprotection also involves the NQO 1 catalysed reoxidation of NADH that accumulates as a result of complex I inhibition. The CoQ 1 H 2 formed would then also act as a ROS scavenger.     

Bioenergetic and Antioxidant Properties of Coenzyme Q<Subscript>10</Subscript>: Recent Developments

For a number of years, coenzyme Q (CoQ₁₀ in humans) was known for its key role in mitochondrial bioenergetics; later studies demonstrated its presence in other subcellular fractions and in plasma, and extensively investigated its antioxidant role. These two functions constitute the basis on which research supporting the clinical use of CoQ₁₀ is founded. Also at the inner mitochondrial membrane level, coenzyme Q is recognized as an obligatory co-factor for the function of uncoupling proteins and a modulator of the transition pore. Furthermore, recent data reveal that CoQ₁₀ affects expression of genes involved in human cell signalling, metabolism, and transport and some of the effects of exogenously administered CoQ₁₀ may be due to this property. Coenzyme Q is the only lipid soluble antioxidant synthesized endogenously. In its reduced form, CoQH₂, ubiquinol, inhibits protein and DNA oxidation but it is the effect on lipid peroxidation that has been most deeply studied. Ubiquinol inhibits the peroxidation of cell membrane lipids and also that of lipoprotein lipids present in the circulation. Dietary supplementation with CoQ₁₀ results in increased levels of ubiquinol-10 within circulating lipoproteins and increased resistance of human low-density lipoproteins to the initiation of lipid peroxidation. Moreover, CoQ₁₀ has a direct anti-atherogenic effect, which has been demonstrated in apolipoprotein E-deficient mice fed with a high-fat diet. In this model, supplementation with CoQ₁₀ at pharmacological doses was capable of decreasing the absolute concentration of lipid hydroperoxides in atherosclerotic lesions and of minimizing the size of atherosclerotic lesions in the whole aorta. Whether these protective effects are only due to the antioxidant properties of coenzyme Q remains to be established; recent data point out that CoQ₁₀ could have a direct effect on endothelial function. In patients with stable moderate CHF, oral CoQ₁₀ supplementation was shown to ameliorate cardiac contractility and endothelial dysfunction. Recent data from our laboratory showed a strong correlation between endothelium bound extra cellular SOD (ecSOD) and flow-dependent endothelial-mediated dilation, a functional parameter commonly used as a biomarker of vascular function. The study also highlighted that supplementation with CoQ₁₀ that significantly affects endothelium-bound ecSOD activity. Furthermore, we showed a significant correlation between increase in endothelial bound ecSOD activity and improvement in FMD after CoQ₁₀ supplementation. The effect was more pronounced in patients with low basal values of ecSOD. Finally, we summarize the findings, also from our laboratory, on the implications of CoQ₁₀ in seminal fluid integrity and sperm cell motility.     

Genetic Evidence for Coenzyme Q Requirement in Plasma Membrane Electron Transport

Plasma membranes isolated from wild-type Saccharomyces cerevisiae crude membrane fractions catalyzed NADH oxidation using a variety of electron acceptors, such as ferricyanide, cytochrome c, and ascorbate free radical. Plasma membranes from the deletion mutant strain coq3, defective in coenzyme Q (ubiquinone) biosynthesis, were completely devoid of coenzyme Q₆ and contained greatly diminished levels of NADH–ascorbate free radical reductase activity (about 10% of wild-type yeasts). In contrast, the lack of coenzyme Q₆ in these membranes resulted in only a partial inhibition of either the ferricyanide or cytochrome-c reductase. Coenzyme Q dependence of ferricyanide and cytochrome-c reductases was based mainly on superoxide generation by one-electron reduction of quinones to semiquinones. Ascorbate free radical reductase was unique because it was highly dependent on coenzyme Q and did not involve superoxide since it was not affected by superoxide dismutase (SOD). Both coenzyme Q₆ and NADH–ascorbate free radical reductase were rescued in plasma membranes derived from a strain obtained by transformation of the coq3 strain with a single-copy plasmid bearing the wild type COQ3 gene and in plasma membranes isolated form the coq3 strain grown in the presence of coenzyme Q₆. The enzyme activity was inhibited by the quinone antagonists chloroquine and dicumarol, and after membrane solubilization with the nondenaturing detergent Zwittergent 3–14. The various inhibitors used did not affect residual ascorbate free radical reductase of the coq3 strain. Ascorbate free radical reductase was not altered significantly in mutants atp2 and cor1 which are also respiration-deficient but not defective in ubiquinone biosynthesis, demonstrating that the lack of ascorbate free radical reductase in coq3 mutants is related solely to the inability to synthesize ubiquinone and not to the respiratory-defective phenotype. For the first time, our results provide genetic evidence for the participation of ubiquinone in NADH–ascorbate free radical reductase, as a source of electrons for transmembrane ascorbate stabilization.     

Distribution of Coenzyme Q Homologues in Brain

Ubiquinone (coenzyme Q₁₀), in addition to its function as an electron and proton carrier in mitochondrial electron transport coupled to ATP synthesis, acts in its reduced form (ubiquinol) as an antioxidant, inhibiting lipid peroxidation in biological membranes and protecting mitochondrial inner-membrane proteins and DNA against oxidative damage accompanying lipid peroxidation. Tissue ubiquinone levels are subject to regulation by physiological factors that are related to the oxidative activity of the organism: they increase under the influence of oxidative stress, e.g. physical exercise, cold adaptation, thyroid hormone treatment, and decrease during aging. In the present study, coenzyme Q homologues were separated and quantified in the brains of mice, rats, rabbits, and chickens using high-performance liquid chromatography. In addition, the coenzyme Q homologues were measured in cells such as NG-108, PC-12, rat fetal brain cells and human SHSY-5Y and monocytes. In general, Q₁ content was the lowest among the coenzyme homologues quantified in the brain. Q₉ was not detectable in the brains of chickens and rabbits, but was present in the brains of rats and mice. Q₉ was also not detected in human cell lines SHSY-5Y and monocytes. Q₁₀ was detected in the brains of mice, rats, rabbits, and chickens and in cell lines. Since both coenzyme Q and vitamin E are antioxidants, and coenzyme Q recycles vitamins E and C, vitamin E was also quantified in mice brain using HPLC-electrochemical detector (ECD). The quantity of vitamin E was lowest in the substantia nigra compared with the other brain regions. This finding is crucial in elucidating ubiquinone function in bioenergetics; in preventing free radical generation, lipid peroxidation, and apoptosis in the brain; and as a potential compound in treating various neurodegenerative disorders.     

Coenzyme Q10: absorption, antioxidative properties, determinants, and plasma levels

The purpose of this article is to summarise our studies, in which the main determinants and absorption of plasma coenzyme Q10 (Q10, ubiquinone) have been assessed, and the effects of moderate dose oral Q10 supplementation on plasma antioxidative capacity, lipoprotein oxidation resistance and on plasma lipid peroxidation investigated. All the supplementation trials carried out have been blinded and placebo-controlled clinical studies. Of the determinants of Q10, serum cholesterol, serum triglycerides, male gender, alcohol consumption and age were found to be associated positively with plasma Q10 concentration. A single dose of 30 mg of Q10, which is the maximum daily dose recommended by Q10 producers, had only a marginal elevating effect on plasma Q10 levels in non-Q10-deficient subjects. Following supplementation, a dose-dependent increase in plasma Q10 levels was observed up to a daily dose of 200 mg, which resulted in a 6.1-fold increase in plasma Q10 levels. However, simultaneous supplementation with vitamin E resulted in lower plasma Q10 levels. Of the lipid peroxidation measurements, Q10 supplementation did not increase LDL TRAP, plasma TRAP, VLDL+LDL oxidation resistance nor did it decrease LDL oxidation susceptibility ex vivo. Q10 with minor vitamin E dose neither decreased exercise-induced lipid peroxidation ex vivo nor muscular damage. Q10 supplementation might, however, decrease plasma lipid peroxidation in vivo , as assessed by the increased proportion of plasma ubiquinol (reduced form, Q10H 2 ) of total Q10. High dose vitamin E supplementation decreased this proportion, which suggests in vivo regeneration of tocopheryl radicals by ubiquinol.     

Effect of Oral Coenzyme Q10 Supplementation on the Oxidation Resistance of Human VLDL+LDL Fraction: Absorption and Antioxidative Properties of Oil and Granule-Based Preparations

Coenzyme Q10 (Q10) is supposed to be an important endogenous lipid-soluble antioxidant. We studied 60 healthy 46 ± 7 (mean ± SD)-year-old smoking men. They were randomized into three groups to receive oil-based or granular Q10 (90 mg/d) or placebo for 2 months. Oil-based capsule elevated Q10 in plasma by 178% and in VLDL+LDL fraction by 160%. The granular preparation increased Q10 in plasma by 168% and in VLDL+LDL by 127%. However, the 2-month Q10 supplementation did not increase the oxidation resistance of VLDL+LDL fraction, as assessed by copper induced VLDL+LDL oxidation, haemin+H₂O₂-induced VLDL+LDL oxidation, total antioxidative capacity of LDL, and plasma malondialdehyde measurements. The first and the last dose was used to carry out a 12 h pharmacokinetic study (five subjects per group), which indicated that only a small part of supplemented Q10 was absorbed to the circulation in 12 h and that the absorption varied extensively between subjects. Our results suggest that at least among smoking men, 90 mg of orally supplemented Q10 daily does not increase the oxidation resistance of VLDL+LDL. Bioavailability of both the granular and the oil-based Q10 preparation was similar during the long-term supplementation, but one dose of 30 mg had only a marginal effect on the plasma levels of Q10. © 1997 Elsevier Science Inc.     

Inhibition by coenzyme Q of ethanol- and carbon tetrachloride-stimulated lipid peroxidation in vivo and catalyzed by microsomal and mitochondrial systems

The ability of coenzyme Q to inhibit lipid peroxidation in intact animals as well as in mitochondrial, submitochondrial, and microsomal systems has been tested. Rats fed coenzyme Q prior to being treated with carbon tetrachloride or while being treated with ethanol excrete less thiobarbituric acid-reacting material in the urine than such rats not fed coenzyme Q. Liver homogenates, mitochondria, and microsomes isolated from rats treated with carbon tetrachloride and ethanol catalyze lipid peroxidation at rates which exceed those from animals also fed coenzyme Q. The rate of lipid peroxidation catalyzed by submitochondrial particles isolated from hearts of young, old, and endurance trained elderly rats was inversely proportional to the coenzyme Q content of the submitochondrial preparation in assays in which succinate was employed to reduce the endogenous coenzyme Q. Reduced, but not oxidized, coenzyme Q inhibited lipid peroxidation catalyzed by rat liver microsomal preparations. These results provide additional evidence in support of an antioxidant role for coenzyme Q.