The oxidation of ubiquinol by the isolated Rieske iron-sulfur protein in solution

Regulation of mitochondrial biosynthesis and function by dietary carbohydrate levels and lipid sources in juvenile blunt snout bream Megalobrama amblycephala

This study aimed to investigate the effects of dietary non-protein energy adjustments on the mitochondrial biosynthesis and function of juvenile Megalobrama amblycephala. Fish (average weight: 37.98 ± 0.07 g) were fed eight diets containing two dietary carbohydrate levels (30% and 43%) and four lipid sources (fish oil, soybean oil, palm oil and the mixed oil) for 11 weeks. Liver mitochondrial respiratory chain complex V activity and ATP (adenosine triphosphate) content both increased significantly with increasing dietary carbohydrate levels, whereas the opposite was true for the AMP (adenosine 5'-monophosphate)/ATP ratio, hepatic transcripts of AMP-activated protein kinase α1 (AMPKα1), AMPKα2, peroxisome proliferators γ-activated receptor coativator-1α (PGC-1α), NADH dehydrogenase 1 and cytochrome c oxidase 1 (COX1) as well as the activities of Na⁺-K⁺-ATPase, succinate dehydrogenase (SDH), citrate synthase (CS) and mitochondrial respiratory chain complex I, III and IV. Additionally, hepatic ATP content, the transcripts of AMPKα, COX1 and ATP6 and the activities of Na⁺-K⁺-ATPase, SDH, CS and mitochondrial respiratory chain complex III were all significantly affected by lipid sources. Furthermore, an interaction between dietary carbohydrate levels and lipid sources was also observed in the activities of liver mitochondrial Na⁺-K⁺-ATPase and respiratory chain complex III as well as the transcripts of ATP6 and PGC-1α. Overall, these findings suggested that dietary carbohydrate levels and lipid sources remarkably affected the mitochondrial biosynthesis and function of M. amblycephala. A diet containing 30% carbohydrate and FO could boost its mitochondrial biosynthesis, while that of 30% carbohydrate and SO could enhance the mitochondrial function.

A Comparative Proteome Profile of Female Mouse Gonads Suggests a Tight Link between the Electron Transport Chain and Meiosis Initiation

Generation of haploid gametes by meiosis is a unique property of germ cells and is critical for sexual reproduction. Leaving mitosis and entering meiosis is a key step in germ cell development. Several inducers or intrinsic genes are known to be important for meiotic initiation, but the regulation of meiotic initiation, especially at the protein level, is still not well understood. We constructed a comparative proteome profile of female mouse fetal gonads at specific time points (11.5, 12.5, and 13.5 days post coitum), spanning a critical window for initiation of meiosis in female germ cells. We identified 3666 proteins, of which 473 were differentially expressed. Further bioinformatics analysis showed that these differentially expressed proteins were enriched in the mitochondria, especially in the electron transport chain and, notably, 9 proteins in electron transport chain Complex I were differentially expressed. We disrupted the mitochondrial electron transport chain function by adding the complex I inhibitor, rotenone to 11.5 days post coitum female gonads cultured in vitro. This treatment resulted in a decreased proportion of meiotic germ cells, as assessed by staining for histone γH2AX. Rotenone treatment also caused decreased ATP levels, increased reactive oxygen species levels and failure of the germ cells to undergo premeiotic DNA replication. These effects were partially rescued by adding Coenzyme Q10. Taken together, our results suggested that a functional electron transport chain is important for meiosis initiation. Our characterization of the quantitative proteome of female gonads provides an inventory of proteins, useful for understanding the mechanisms of meiosis initiation and female fertility.

Coenzyme Q10 depletion in medical and neuropsychiatric disorders: potential repercussions and therapeutic implications

Coenzyme Q10 (CoQ10) is an antioxidant, a membrane stabilizer, and a vital cofactor in the mitochondrial electron transport chain, enabling the generation of adenosine triphosphate. It additionally regulates gene expression and apoptosis; is an essential cofactor of uncoupling proteins; and has anti-inflammatory, redox modulatory, and neuroprotective effects. This paper reviews the known physiological role of CoQ10 in cellular metabolism, cell death, differentiation and gene regulation, and examines the potential repercussions of CoQ10 depletion including its role in illnesses such as Parkinson's disease, depression, myalgic encephalomyelitis/chronic fatigue syndrome, and fibromyalgia. CoQ10 depletion may play a role in the pathophysiology of these disorders by modulating cellular processes including hydrogen peroxide formation, gene regulation, cytoprotection, bioenegetic performance, and regulation of cellular metabolism. CoQ10 treatment improves quality of life in patients with Parkinson's disease and may play a role in delaying the progression of that disorder. Administration of CoQ10 has antidepressive effects. CoQ10 treatment significantly reduces fatigue and improves ergonomic performance during exercise and thus may have potential in alleviating the exercise intolerance and exhaustion displayed by people with myalgic encepholamyletis/chronic fatigue syndrome. Administration of CoQ10 improves hyperalgesia and quality of life in patients with fibromyalgia. The evidence base for the effectiveness of treatment with CoQ10 may be explained via its ability to ameliorate oxidative stress and protect mitochondria.

Mitochondrial production of oxygen radical species and the role of Coenzyme Q as an antioxidant

The mitochondrial respiratory chain is a powerful source of reactive oxygen species (ROS), which is considered as the pathogenic agent of many diseases and of aging. We have investigated the role of complex I in superoxide radical production and found by the combined use of specific inhibitors of complex I that the one-electron donor to oxygen in the complex is a redox center located prior to the sites where three different types of Coenzyme Q (CoQ) competitors bind, to be identified with an Fe-S cluster, most probably N2, or possibly an ubisemiquinone intermediate insensitive to all the above inhibitors. Short-chain Coenzyme Q analogs enhance superoxide formation, presumably by mediating electron transfer from N2 to oxygen. The clinically used CoQ analog, idebenone, is particularly effective, raising doubts on its safety as a drug. Cells counteract oxidative stress by antioxidants. CoQ is the only lipophilic antioxidant to be biosynthesized. Exogenous CoQ, however, protects cells from oxidative stress by conversion into its reduced antioxidant form by cellular reductases. The plasma membrane oxidoreductase and DT-diaphorase are two such systems, likewise, they are overexpressed under oxidative stress conditions.

New genes encoding subunits of a cytochrome bc1-analogous complex in the respiratory chain of the hyperthermoacidophilic crenarchaeon Sulfolobus acidocaldarius

The soxL gene from Sulfolobus acidocaldarius (DSM 639) encodes a Rieske iron-sulfur protein. In this study we report the identification of two open reading frames in its downstream region. The first one, named soxN, codes for a membrane protein bearing a resemblance to the b-type cytochromes of the cytochrome bc1 and b6f complexes. The protein is predicted to contain at least 10 transmembrane helices and features the two conserved histidine pairs coordinating the heme groups of these cytochromes. The second open reading frame, named odsN, encodes a soluble protein of unknown function. The genomic region displays a complex transcription pattern. Northern blot and RT-PCR analyses revealed the presence of mono- and bi-cistronic transcripts as well as a tri-cistronic transcript of soxL and cbsAB, encoding the mono-heme cytochrome b558/566. Phylogenetic analyses of the genes of the soxLN pair and of other archaeal gene pairs encoding Rieske iron-sulfur proteins and b-type cytochromes revealed an identical branching patterns for both protein families, suggesting an evolutionary link of these genes provided by the functional interaction of the proteins. On the basis of the findings of this study and the previously studied properties of the soxL and cbsA proteins, we propose the occurrence of a novel cytochrome bc1-analogous complex in the membranes of Sulfolobus, consisting of the cytochrome b homolog soxN, the Rieske protein soxL, the high potential cytochrome cbsA, as well as the non-redox-active subunits cbsB and odsN.

Role of mitochondria in oxidative stress and aging

The mitochondrial respiratory chain is a powerful source of reactive oxygen species (ROS), considered as the pathogenic agent of many diseases and of aging. We have investigated the role of Complex I in superoxide radical production and found by combined use of specific inhibitors of Complex I that the one-electron donor in the Complex to oxygen is a redox center located prior to the sites where three different types of coenzyme Q (CoQ) competitors bind, to be identified with an Fe-S cluster, most probably N2, or possibly an ubisemiquinone intermediate insensitive to all the above inhibitors. Short-chain coenzyme Q analogues enhance superoxide formation, presumably by mediating electron transfer from N2 to oxygen. The clinically used CoQ analogue idebenone is particularly effective, raising doubts about its safety as a drug. The mitochondrial theory of aging considers somatic mutations of mitochondrial DNA induced by ROS as the primary cause of energy decline; in rat liver mitochondria, Complex I appears to be most affected by aging and to become strongly rate limiting for electron transfer. Mitochondrial energetics is also deranged in human platelets upon aging, as demonstrated by the decreased Pasteur effect (enhancement of lactate production by respiratory inhibitors). Cells counteract oxidative stress by antioxidants: CoQ is the only lipophilic antioxidant to be biosynthesized. Exogenous CoQ, however, protects cells from oxidative stress by conversion into its reduced antioxidant form by cellular reductases. The plasma membrane oxidoreductase and DT-diaphorase are two such systems: likewise, they are overexpressed under oxidative stress conditions.

Role of mitochondria in oxidative stress and ageing

Mitochondria are deeply involved in the production of reactive oxygen species through one-electron carriers in the respiratory chain; mitochondrial structures are also very susceptible to oxidative stress as evidenced by massive information on lipid peroxidation, protein oxidation, and mitochondrial DNA (mtDNA) mutations. Oxidative stress can induce apoptotic death, and mitochondria have a central role in this and other types of apoptosis, since cytochrome c release in the cytoplasm and opening of the permeability transition pore are important events in the apoptotic cascade. The discovery that mtDNA mutations are at the basis of a number of human pathologies has profound implications: maternal inheritance of mtDNA is the basis of hereditary mitochondrial cytopathies; accumulation of somatic mutations of mtDNA with age has represented the basis of the mitochondrial theory of ageing, by which a vicious circle is established of mtDNA damage, altered oxidative phosphorylation and overproduction of reactive oxygen species. Experimental evidence of respiratory chain defects and of accumulation of multiple mtDNA deletions with ageing is in accordance with the mitochondrial theory, although some other experimental findings are not directly ascribable to its postulates.

Mitochondria and cells produce reactive oxygen species in virtual anaerobiosis: relevance to ceramide-induced apoptosis

Observations of apoptosis in virtual anaerobiosis have raised doubts on the significance of reactive oxygen species in the cascade of events of programmed cell death. This work presents evidence that cells and mitochondrial preparations produce similar levels of hydrogen peroxide under either aerobic or virtually anaerobic conditions. These levels are relevant to the increased production of radicals induced by a ceramide analog that promotes apoptosis. This ceramide acts at center o of mitochondrial complex III.

Two different respiratory Rieske proteins are expressed in the extreme thermoacidophilic crenarchaeon Sulfolobus acidocaldarius: cloning and sequencing of their genes

We have isolated two genes encoding Rieske iron sulfur proteins from the genomic DNA of the thermoacidophilic crenarchaeon Sulfolobus acidocaldarius (DSM 639). One of the genes, named soxL, codes for the previously isolated novel Rieske-I protein. The second gene (soxF) 121 codes for the Rieske-II protein associated with the second terminal oxidase of Sulfolobus. Both proteins exhibit only 24% identical residues. The Rieske-I protein shows a number of unusual features. (i) The distance between the two cluster binding sites is significantly larger than in all known proteins. (ii) An unexpected Pro --> Asp exchange in one of the cluster binding sites. (iii) It shows some resemblance to the mitochondrial and plastidic Rieske proteins insofar as the soxL gene codes for a pre-sequence which is no longer present in the mature Rieske-I protein. Both proteins cluster together on a separate branch of the phylogenetic tree. To our knowledge this is the first proven case of two significantly different Rieske proteins in a prokaryote.

Isolation, characterisation and crystallisation of a water-soluble fragment of the Rieske iron-sulfur protein of bovine heart mitochondrial bc1 complex

A water-soluble fragment of the bc1 complex from bovine heart mitochondria was isolated containing the intact Rieske [2Fe-2S] cluster. The fragment consists of the last 129 amino acid residues of the Rieske iron-sulfur protein and has a molecular mass of 14592 Da including two iron atoms. The absorption, visible CD, and EPR spectra of the fragment are indistinguishable from those of the membrane-bound iron-sulfur protein. The redox potential as determined by EPR-monitored redox titration was + 306 mV. The far-ultraviolet CD spectrum is indicative of a protein with little regular secondary structure, while significant alpha-helix content was detected in the membrane anchor of the complete iron-sulfur protein. The fragment could be crystallized using poly(ethylene glycol) 6000 as precipitant. Needle-shaped single crystals have been grown by the hanging-drop vapor diffusion technique. These crystals belong to the space group P21 and diffract well beyond 0.2 nm resolution. Phase determination using the multiple-wavelength anomalous-scattering technique is underway.

Ubiquinol-cytochrome c oxidoreductase. The redox reactions of the bis-heme cytochrome b in ubiquinone-sufficient and ubiquinone-deficient systems

Antimycin and myxothiazol are stoichiometric inhibitors of complex III (ubiquinol-cytochrome c oxidoreductase), exerting their highest degree of inhibition at I mol each/mol of complex III monomer. Phenomenologically, however, they each inhibit three steps in the redox reaction of the bis-heme cytochrome b in submitochondrial particles (SMP), and all three inhibitions are incomplete to various extents. (i) In SMP, reduction of hemes bH and bL by NADH or succinate is inhibited when the particles are treated with both antimycin and myxothiazol. Each inhibitor alone allows reduced bH and bL to accumulate, indicating that each inhibits the reoxidation of these hemes. (E)-Methyl-3-methoxy-2-(4')-trans-stilbenyl)acrylatc in combination with antimycin or 2-n-heptyl-4-hydroxyquinoline-N-oxide in combination with myxothiazol causes less inhibition of b reduction than the combination of antimycin and myxothiazol. (ii) Reoxidation of reduced b, is inhibited by either antimycin or myxothiazol (or 2-n-heptyl-4-hydroxyquinoline-N-oxide, (E)-methyl-3-methoxy-2-(4'-trans-stilbenyl)acrylate, or stigmatellin). (iii) Reoxidation of reduced bH is also inhibited by any one of these reagents. These inhibitions are also incomplete, and reduced bL is oxidized through the leaks allowed by these inhibitors at least 10 times faster than reduced bH. Heme bH can be reduced in SMP via cytochrome c, and the Rieske iron-sulfur protein by ascorbate and faster by ascorbate + TMPD (N,N,N',N'-tetramethyl-p-phenylenediamine). Energization of SMP by the addition of ATP affords reduction of bL as well. Reverse electron transfer to bH and bL is inhibited partially by myxothiazol, much more by antimycin. Ascorbate + TMPD also reduce bH in ubiquinone-extracted SMP in which the molar ratio of ubiquinone to cytochrome b has been reduced 200-fold from 12.5 to aproximately 0.06. Reconstitution of the extracted particles with ubiquinone-10 restores substrate oxidation but does not improve the rate or the extent of b, reduction by ascorbate + TMPD. These reagents also partially reduce cytochrome b in SMP from a ubiquinone-deficient yeast mutant. The above results are discussed in relation to the Q-cycle hypothesis.

Purification and characterization of the Rieske iron-sulfur protein from the thermoacidophilic crenarchaeon Sulfolobus acidocaldarius

The previously detected Rieske iron-sulfur protein from the membranes of the thermoacidophile Sulfolobus acidocaldarius [Anemüller, S., et al. (1993) FEBS Lett. 318, 61-64] was purified to electrophoretic homogeneity and the N-terminal amino acids determined. The apparent molecular weight was estimated to be 32 kDa. The reduced protein displays a rhombic EPR spectrum with gxyz = 1.768, 1.895, 2.035. The average g-value of 1.902 is typical for nitrogen ligand-containing clusters. EPR spin quantification and the iron content indicate the presence of one [2Fe-2S] cluster. The purified protein displays ubiquinol cytochrome c reductase activity. The pH optimum of this reaction is temperature dependent and was determined to be pH 7 at 56 degrees C. The results presented in this study clearly prove that the Sulfolobus Rieske protein belongs to the family of the true Rieske iron-sulfur proteins.

The kinetic mechanism of ubiquinol: cytochrome c reductase at steady state

The steady-state kinetics of ubiquinol: cytochrome c reductase (cytochrome bc1 complex) is analyzed in this work. The graphical pattern of the titrations is clearly indicative of a ping-pong mechanism, but the two products ubiquinone and reduced cytochrome c behave competitively with their substrate and noncompetitively with the other substrate. Hence, the mechanism of the reductase is of a ping-pong two-site type. A minimal reaction scheme for the enzymatic mechanism is proposed and approximate values of its rate constants are deduced on the assumption that each substrate is in rapid equilibrium at its catalytic site. This has been substantiated by presteady-state measurements of the reduction and oxidation of cytochrome b by a short-chain homolog of ubiquinol. Values of the rate constants of the reaction scheme have been deduced from the steady-state titrations for a series of 2,3-dimethoxy-5-methyl quinols having different hydrophobic substituents in position 6 of the ring. The results provide a quantitative estimation of the specificity of the quinol catalytic site in the transmembrane portion of the bc1 complex. In particular, a reasonable correlation is found between the rate of the second-order reaction of quinols with the enzyme and their solubility in lipids.