On the redox potentials of ubiquinone and cytochrome b in the respiratory chain

Marine Biofilm Engineered to Produce Current in Response to Small Molecules

Engineered electroactive bacteria have potential applications ranging from sensing to biosynthesis. In order to advance the use of engineered electroactive bacteria, it is important to demonstrate functional expression of electron transfer modules in chassis adapted to operationally relevant conditions, such as non-freshwater environments. Here, we use the Shewanella oneidensis electron transfer pathway to induce current production in a marine bacterium, Marinobacter atlanticus, during biofilm growth in artificial seawater. Genetically encoded sensors optimized for use in Escherichia coli were used to control protein expression in planktonic and biofilm attached cells. Significant current production required the addition of menaquinone, which M. atlanticus does not produce, for electron transfer from the inner membrane to the expressed electron transfer pathway. Current through the S. oneidensis pathway in M. atlanticus was observed when inducing molecules were present during biofilm formation. Electron transfer was also reversible, indicating that electron transfer into M. atlanticus could be controlled. These results show that an operationally relevant marine bacterium can be genetically engineered for environmental sensing and response using an electrical signal.

A Third Way of Energy Conservation in Acetogenic Bacteria

Acetogenic bacteria are a group of strictly anaerobic bacteria that make a living from acetate formation from two molecules of CO₂ via the Wood-Ljungdahl pathway (WLP). The free energy change of this reaction is very small and allows the synthesis of only a fraction of an ATP. How this pathway is coupled to energy conservation has been an enigma since its discovery ~90 years ago. Here, we describe an electron transport chain in the cytochrome- and quinone-containing acetogen Sporomusa ovata that leads from molecular hydrogen as an electron donor to an intermediate of the WLP, methylenetetrahydrofolate (methylene-tetrahydrofolate [THF]), as an electron acceptor. The catalytic site of the hydrogenase is periplasmic and likely linked cytochrome b to the membrane. We provide evidence that the MetVF-type methylenetetrahydrofolate reductase is linked proteins MvhD and HdrCBA to the cytoplasmic membrane. Membrane preparations catalyzed the H₂-dependent reduction of methylene-THF to methyl-THF. In our model, a transmembrane electrochemical H⁺ gradient is established by both scalar and vectorial protons that leads to the synthesis of 0.5 mol ATP/mol methylene-THF by a H⁺-F₁F_o ATP synthase. This H₂- and methylene-THF-dependent electron transport chain may be present in other cytochrome-containing acetogens as well and represents a third way of chemiosmotic energy conservation in acetogens, but only in addition to the well-established respiratory enzymes Rnf and Ech.

IMPORTANCE Acetogenic bacteria grow by making acetate from CO₂ and are considered the first life forms on Earth since they couple CO₂ reduction to the conservation of energy. How this is achieved has been an enigma ever since. Recently, two respiratory enzymes, a ferredoxin:NAD⁺ oxidoreductase (Rnf) and a ferredoxin:H⁺ oxidoreductase (Ech), have been found in cytochrome-free acetogenic model bacteria. However, some acetogens contain cytochromes in addition, and there has been a long-standing assumption of a cytochrome-containing electron transport chain in those acetogens. Here, we provide evidence for a respiratory chain in Sporomusa ovata that has a cytochrome-containing hydrogenase as the electron donor and a methylenetetrahydrofolate reductase as the terminal electron acceptor. This is the third way of chemiosmotic energy conservation found in acetogens.

3D Conducting Polymeric Membrane and Scaffold Saccharomyces cerevisiae Biofilms to Enhance Energy Conversion in Microbial Fuel Cells

Microbial fuel cells (MFCs) can spontaneously convert chemical energy into electricity using biocatalytic microorganisms and organic matter as fuel feedstocks. Three-dimensional cross-linked poly(vinyl alcohol)-based membranes were produced by a sol-gel method under homogeneous catalysis and used as the electrolyte to facilitate effective proton conduction. Under dry conditions, these polymeric membranes showed high water uptake (120%) and ionic conductivity (2.815 mS cm^-1). In the anode compartment, the scaffold Saccharomyces cerevisiae film biocatalysts were used to improve electron transfer to the cathode, using three major configurations to generate a higher power output. It was found that the graphene anchoring, red light (RL) stimulation, and methylene blue (MB) mediation-enhanced device performance. The electrochemically derived graphene improved the power and current density by 40% because of its high conductivity. The RL stimulation increased the power density by 80% because of a shortened electron flow path to complex III. The MB mediation also yielded a higher current density by 340% because MB can bypass the electron flow from complex II to cytochrome c and transfer electrons directly to complex III. The individual and collective increase in power output was due to more efficient electron flow from the electronic network permeating the biofilm. The generated electrons were transferred either to graphene as an energy-efficient direct transfer mode or to methylene blue as a long-range redox mediator for indirect transfer. Red light stimulation enhanced oxygen utilization efficiency and stimulated electrons in redox proteins enhancing electron flux. These processes generated higher power through the more efficient generation of electrons and faster transport to the external circuit. As society migrates from gasoline consumption to low carbon-based fuels, the MFCs become important in producing electrical energy with low net emissions.

Gas regulation of complex II reversal via electron shunting to fumarate in the mammalian ETC

The electron transport chain (ETC) is a major currency converter that exchanges the chemical energy of fuel oxidation to proton motive force and, subsequently, ATP generation, using O₂ as a terminal electron acceptor. Discussed herein, two new studies reveal that the mammalian ETC is forked. Hypoxia or H₂S exposure promotes the use of fumarate as an alternate terminal electron acceptor. The fumarate/succinate and CoQH₂/CoQ redox couples are nearly iso-potential, revealing that complex II is poised for facile reverse electron transfer, which is sensitive to CoQH₂ and fumarate concentrations. The gas regulators, H₂S and ^•NO, modulate O₂ affinity and/or inhibit the electron transfer rate at complex IV. Their induction under hypoxia suggests a mechanism for how traffic at the ETC fork can be regulated.

Electron bifurcation reactions in dark fermentation: An overview for better understanding and improvement

Electron bifurcation (EB) is the most recently found mode of energy conservation, which involves both exergonic and endergonic electron transfer reactions to minimize energy loss. Several works have been devoted on EB reactions (EBRs) in anaerobic digestion but limited in dark fermentative hydrogen production (DF). Two main electron carriers in DF are ferredoxin (Fd) and reduced nicotinamide adenine dinucleotide (NADH), complicatedly involved in EB. Here, i) the importance of EB involvement in DF, ii) all EBRs possible to present in DF, as well as iii) the limitation of previous studies that tried incorporating any of EBRs in DF metabolic model, were highlighted. In addition, the concept of using metagenomic analysis for estimating the share of each EB reaction in the metabolic model, was proposed. This review is expected to initiate a new wave for studying EB, as a tool for explaining and predicting DF products.

A redox cycle with complex II prioritizes sulfide quinone oxidoreductase-dependent H2S oxidation

The dual roles of H₂S as an endogenously synthesized respiratory substrate and as a toxin raise questions as to how it is cleared when the electron transport chain is inhibited. Sulfide quinone oxidoreductase (SQOR) catalyzes the first step in the mitochondrial H₂S oxidation pathway, using CoQ as an electron acceptor, and connects to the electron transport chain at the level of complex III. We have discovered that at high H₂S concentrations, which are known to inhibit complex IV, a new redox cycle is established between SQOR and complex II, operating in reverse. Under these conditions, the purine nucleotide cycle and the malate aspartate shuttle furnish fumarate, which supports complex II reversal and leads to succinate accumulation. Complex II knockdown in colonocytes decreases the efficiency of H₂S clearance while targeted knockout of complex II in intestinal epithelial cells significantly decreases the levels of thiosulfate, a biomarker of H₂S oxidation, to approximately one-third of the values seen in serum and urine samples from control mice. These data establish the physiological relevance of this newly discovered redox circuitry between SQOR and complex II for prioritizing H₂S oxidation and reveal the quantitatively significant contribution of intestinal epithelial cells to systemic H₂S metabolism.

Catalytic Reactions and Energy Conservation in the Cytochrome bc1 and b6f Complexes of Energy-Transducing Membranes

This review focuses on key components of respiratory and photosynthetic energy-transduction systems: the cytochrome bc1 and b6f (Cytbc1/b6f) membranous multisubunit homodimeric complexes. These remarkable molecular machines catalyze electron transfer from membranous quinones to water-soluble electron carriers (such as cytochromes c or plastocyanin), coupling electron flow to proton translocation across the energy-transducing membrane and contributing to the generation of a transmembrane electrochemical potential gradient, which powers cellular metabolism in the majority of living organisms. Cytsbc1/b6f share many similarities but also have significant differences. While decades of research have provided extensive knowledge on these enzymes, several important aspects of their molecular mechanisms remain to be elucidated. We summarize a broad range of structural, mechanistic, and physiological aspects required for function of Cytbc1/b6f, combining textbook fundamentals with new intriguing concepts that have emerged from more recent studies. The discussion covers but is not limited to (i) mechanisms of energy-conserving bifurcation of electron pathway and energy-wasting superoxide generation at the quinol oxidation site, (ii) the mechanism by which semiquinone is stabilized at the quinone reduction site, (iii) interactions with substrates and specific inhibitors, (iv) intermonomer electron transfer and the role of a dimeric complex, and (v) higher levels of organization and regulation that involve Cytsbc1/b6f. In addressing these topics, we point out existing uncertainties and controversies, which, as suggested, will drive further research in this field.

Mitochondrial Structure and Bioenergetics in Normal and Disease Conditions

Mitochondria are ubiquitous intracellular organelles found in almost all eukaryotes and involved in various aspects of cellular life, with a primary role in energy production. The interest in this organelle has grown stronger with the discovery of their link to various pathologies, including cancer, aging and neurodegenerative diseases. Indeed, dysfunctional mitochondria cannot provide the required energy to tissues with a high-energy demand, such as heart, brain and muscles, leading to a large spectrum of clinical phenotypes. Mitochondrial defects are at the origin of a group of clinically heterogeneous pathologies, called mitochondrial diseases, with an incidence of 1 in 5000 live births. Primary mitochondrial diseases are associated with genetic mutations both in nuclear and mitochondrial DNA (mtDNA), affecting genes involved in every aspect of the organelle function. As a consequence, it is difficult to find a common cause for mitochondrial diseases and, subsequently, to offer a precise clinical definition of the pathology. Moreover, the complexity of this condition makes it challenging to identify possible therapies or drug targets.

The oxidation-reduction and electrocatalytic properties of CO dehydrogenase from Oligotropha carboxidovorans

CO dehydrogenase (CODH) from the Gram-negative bacterium Oligotropha carboxidovorans is a complex metalloenzyme from the xanthine oxidase family of molybdenum-containing enzymes, bearing a unique binuclear Mo-S-Cu active site in addition to two [2Fe-2S] clusters (FeSI and FeSII) and one equivalent of FAD. CODH catalyzes the oxidation of CO to CO₂ with the concomitant introduction of reducing equivalents into the quinone pool, thus enabling the organism to utilize CO as sole source of both carbon and energy. Using a variety of EPR monitored redox titrations and spectroelectrochemistry, we report the redox potentials of CO dehydrogenase at pH 7.2 namely Mo^VI/V, Mo^V/IV, FeSI^2+/+, FeSII^2+/+, FAD/FADH and FADH/FADH^-. These potentials are systematically higher than the corresponding potentials seen for other members of the xanthine oxidase family of Mo enzymes, and are in line with CODH utilising the higher potential quinone pool as an electron acceptor instead of pyridine nucleotides. CODH is also active when immobilised on a modified Au working electrode as demonstrated by cyclic voltammetry in the presence of CO.

Allosteric Cooperativity in Proton Energy Conversion in A1-Type Cytochrome c Oxidase

Cytochrome c oxidase (CcO), the Cu_A, heme a, heme a₃, Cu_B enzyme of respiratory chain, converts the free energy released by aerobic cytochrome c oxidation into a membrane electrochemical proton gradient (ΔμH⁺). ΔμH⁺ derives from the membrane anisotropic arrangement of dioxygen reduction to two water molecules and transmembrane proton pumping from a negative (N) space to a positive (P) space separated by the membrane. Spectroscopic, potentiometric, and X-ray crystallographic analyses characterize allosteric cooperativity of dioxygen binding and reduction with protonmotive conformational states of CcO. These studies show that allosteric cooperativity stabilizes the favorable conformational state for conversion of redox energy into a transmembrane ΔμH⁺.

Chemiosmotic energy for primitive cellular life: Proton gradients are generated across lipid membranes by redox reactions coupled to meteoritic quinones

Transmembrane proton gradients coupled to, and maintained by, electron transport are ubiquitous sources of chemiosmotic energy in all life today, but how this system first emerged is uncertain. Here we report a model liposome system in which internal ferricyanide serves as an oxidant and external ascorbate or dithionite provide a source of electrons to electron carriers embedded in liposome membranes. Quinones linked the donor to the acceptor in a coupled redox reaction that released protons into the vesicle internal volume as electrons were transported across the membranes, thereby producing substantial pH gradients. Using this system, we found that one or more quinones in extracts from carbonaceous meteorites could serve as coupling agents and that substantial pH gradients developed in the acidic interior of liposomes. If amphiphilic compounds present on the prebiotic Earth assembled into membranous compartments that separate reduced solutes in the external medium from an encapsulated acceptor, quinones can mediate electron and proton transport across the membranes, thereby providing a source of chemiosmotic energy for primitive metabolic reactions.

Succinate in ischemia: Where does it come from?

During tissue ischemia succinate accumulates. Herein, literature spanning the past nine decades is reviewed leaning towards the far greater role of Krebs cycle's canonical activity yielding succinate through α-ketoglutarate -> succinyl-CoA -> succinate even in hypoxia, as opposed to reversal of succinate dehydrogenase. Furthermore, the concepts of i) a diode-like property of succinate dehydrogenase rendering it difficult to reverse, and ii) the absence of mammalian mitochondrial quinones exhibiting redox potentials in the [-60, -80] mV range needed for fumarate reduction, are discussed. Finally, it is emphasized that a "fumarate reductase" enzyme entity reducing fumarate to succinate found in some bacteria and lower eukaryotes remains to be discovered in mammalian mitochondria.

C-C bond forming radical SAM enzymes involved in the construction of carbon skeletons of cofactors and natural products

Covering: up to the end of 2017 C-C bond formations are frequently the key steps in cofactor and natural product biosynthesis. Historically, C-C bond formations were thought to proceed by two electron mechanisms, represented by Claisen condensation in fatty acids and polyketide biosynthesis. These types of mechanisms require activated substrates to create a nucleophile and an electrophile. More recently, increasing number of C-C bond formations catalyzed by radical SAM enzymes are being identified. These free radical mediated reactions can proceed between almost any sp3 and sp2 carbon centers, allowing introduction of C-C bonds at unconventional positions in metabolites. Therefore, free radical mediated C-C bond formations are frequently found in the construction of structurally unique and complex metabolites. This review discusses our current understanding of the functions and mechanisms of C-C bond forming radical SAM enzymes and highlights their important roles in the biosynthesis of structurally complex, naturally occurring organic molecules. Mechanistic consideration of C-C bond formation by radical SAM enzymes identifies the significance of three key mechanistic factors: radical initiation, acceptor substrate activation and radical quenching. Understanding the functions and mechanisms of these characteristic enzymes will be important not only in promoting our understanding of radical SAM enzymes, but also for understanding natural product and cofactor biosynthesis.

Analysis of a Functional Dimer Model of Ubiquinol Cytochrome c Oxidoreductase

Ubiquinol cytochrome c oxidoreductase (bc₁ complex) serves as an important electron junction in many respiratory systems. It funnels electrons coming from NADH and ubiquinol to cytochrome c, but it is also capable of producing significant amounts of the free radical superoxide. In situ and in other experimental systems, the enzyme exists as a dimer. But until recently, it was believed to operate as a functional monomer. Here we show that a functional dimer model is capable of explaining both kinetic and superoxide production rate data. The model consists of six electronic states characterized by the number of electrons deposited on the complex. It is fully reversible and strictly adheres to the thermodynamics governing the reactions. A total of nine independent data sets were used to parameterize the model. To explain the data with a consistent set of parameters, it was necessary to incorporate intramonomer Coulombic effects between hemes b_L and b_H and intermonomer Coulombic effects between b_L hemes. The fitted repulsion energies fall within the theoretical range of electrostatic calculations. In addition, model analysis demonstrates that the Q pool is mostly oxidized under normal physiological operation but can switch to a more reduced state when reverse electron transport conditions are in place.

Substrate-Protein Interactions of Type II NADH:Quinone Oxidoreductase from Escherichia coli

Type II NADH:quinone oxidoreductases (NDH-2s) are membrane proteins involved in respiratory chains and responsible for the maintenance of NADH/NAD(+) balance in cells. NDH-2s are the only enzymes with NADH dehydrogenase activity present in the respiratory chain of many pathogens, and thus, they were proposed as suitable targets for antimicrobial therapies. In addition, NDH-2s were also considered key players for the treatment of complex I-related neurodegenerative disorders. In this work, we explored substrate-protein interaction in NDH-2 from Escherichia coli (EcNDH-2) combining surface-enhanced infrared absorption spectroscopic studies with electrochemical experiments, fluorescence spectroscopy assays, and quantum chemical calculations. Because of the specific stabilization of substrate complexes of EcNDH-2 immobilized on electrodes, it was possible to demonstrate the presence of two distinct substrate binding sites for NADH and the quinone and to identify a bound semiprotonated quinol as a catalytic intermediate.

Transient Kinetic Analysis of Hydrogen Sulfide Oxidation Catalyzed by Human Sulfide Quinone Oxidoreductase

The first step in the mitochondrial sulfide oxidation pathway is catalyzed by sulfide quinone oxidoreductase (SQR), which belongs to the family of flavoprotein disulfide oxidoreductases. During the catalytic cycle, the flavin cofactor is intermittently reduced by sulfide and oxidized by ubiquinone, linking H2S oxidation to the electron transfer chain and to energy metabolism. Human SQR can use multiple thiophilic acceptors, including sulfide, sulfite, and glutathione, to form as products, hydrodisulfide, thiosulfate, and glutathione persulfide, respectively. In this study, we have used transient kinetics to examine the mechanism of the flavin reductive half-reaction and have determined the redox potential of the bound flavin to be -123 ± 7 mV. We observe formation of an unusually intense charge-transfer (CT) complex when the enzyme is exposed to sulfide and unexpectedly, when it is exposed to sulfite. In the canonical reaction, sulfide serves as the sulfur donor and sulfite serves as the acceptor, forming thiosulfate. We show that thiosulfate is also formed when sulfide is added to the sulfite-induced CT intermediate, representing a new mechanism for thiosulfate formation. The CT complex is formed at a kinetically competent rate by reaction with sulfide but not with sulfite. Our study indicates that sulfide addition to the active site disulfide is preferred under normal turnover conditions. However, under pathological conditions when sulfite concentrations are high, sulfite could compete with sulfide for addition to the active site disulfide, leading to attenuation of SQR activity and to an alternate route for thiosulfate formation.

Electronic connection between the quinone and cytochrome C redox pools and its role in regulation of mitochondrial electron transport and redox signaling

Mitochondrial respiration, an important bioenergetic process, relies on operation of four membranous enzymatic complexes linked functionally by mobile, freely diffusible elements: quinone molecules in the membrane and water-soluble cytochromes c in the intermembrane space. One of the mitochondrial complexes, complex III (cytochrome bc1 or ubiquinol:cytochrome c oxidoreductase), provides an electronic connection between these two diffusible redox pools linking in a fully reversible manner two-electron quinone oxidation/reduction with one-electron cytochrome c reduction/oxidation. Several features of this homodimeric enzyme implicate that in addition to its well-defined function of contributing to generation of proton-motive force, cytochrome bc1 may be a physiologically important point of regulation of electron flow acting as a sensor of the redox state of mitochondria that actively responds to changes in bioenergetic conditions. These features include the following: the opposing redox reactions at quinone catalytic sites located on the opposite sides of the membrane, the inter-monomer electronic connection that functionally links four quinone binding sites of a dimer into an H-shaped electron transfer system, as well as the potential to generate superoxide and release it to the intermembrane space where it can be engaged in redox signaling pathways. Here we highlight recent advances in understanding how cytochrome bc1 may accomplish this regulatory physiological function, what is known and remains unknown about catalytic and side reactions within the quinone binding sites and electron transfers through the cofactor chains connecting those sites with the substrate redox pools. We also discuss the developed molecular mechanisms in the context of physiology of mitochondria.

Increasing mitochondrial membrane phospholipid content lowers the enzymatic activity of electron transport complexes

Activities of the enzymes involved in cellular respiration are markedly influenced by the composition of the phospholipid environment of the inner mitochondrial membrane. Contrary to previous suppositions, we show that fusion of mitochondria isolated from healthy cardiac muscle with cardiolipin or dioleoylphosphatidylcholine results in a 2-6-fold reduction in the activity of complexes I, II, and IV. The activity of complex III was unaffected by increased phospholipid levels. Phospholipid content had an indiscriminate yet detrimental effect on the combined activities of complexes I+III and II+III. These results have strong implications for therapeutic lipid replacement strategies, in which phospholipid modification of the mitochondria is proposed to enhance mitochondrial function.

A refined analysis of superoxide production by mitochondrial sn-glycerol 3-phosphate dehydrogenase

The oxidation of sn-glycerol 3-phosphate by mitochondrial sn-glycerol 3-phosphate dehydrogenase (mGPDH) is a major pathway for transfer of cytosolic reducing equivalents to the mitochondrial electron transport chain. It is known to generate H(2)O(2) at a range of rates and from multiple sites within the chain. The rates and sites depend upon tissue source, concentrations of glycerol 3-phosphate and calcium, and the presence of different electron transport chain inhibitors. We report a detailed examination of H(2)O(2) production during glycerol 3-phosphate oxidation by skeletal muscle, brown fat, brain, and heart mitochondria with an emphasis on conditions under which mGPDH itself is the source of superoxide and H(2)O(2). Importantly, we demonstrate that a substantial portion of H(2)O(2) production commonly attributed to mGPDH originates instead from electron flow through the ubiquinone pool into complex II. When complex II is inhibited and mGPDH is the sole superoxide producer, the rate of superoxide production depends on the concentrations of glycerol 3-phosphate and calcium and correlates positively with the predicted reduction state of the ubiquinone pool. mGPDH-specific superoxide production plateaus at a rate comparable with the other major sites of superoxide production in mitochondria, the superoxide-producing center shows no sign of being overreducible, and the maximum superoxide production rate correlates with mGPDH activity in four different tissues. mGPDH produces superoxide approximately equally toward each side of the mitochondrial inner membrane, suggesting that the Q-binding pocket of mGPDH is the major site of superoxide generation. These results clarify the maximum rate and mechanism of superoxide production by mGPDH.

Determination of coenzyme Q10, coenzyme Q9, and melatonin contents in virgin argan oils: comparison with other edible vegetable oils

Virgin argan oil possesses high antioxidant capacity (AC), which may be partially explained by its high content in antioxidant molecules such as polyphenols and tocopherols. However, the content in other antioxidant molecules, for example, coenzyme Q10 (CoQ(10)), coenzyme Q9 (CoQ(9)), and melatonin (Mel), which have been identified in other edible vegetable oils, have not been evaluated in virgin argan oil. Consequently, it was decided to evaluate the contents of CoQ(10), CoQ(9), and Mel in virgin argan oils and compare the results to those obtained in extra virgin olive oils and some varieties of seed oils. By the use of sensitive HPLC-EC/F methods, the results showed that virgin argan oil is a rich source of CoQ(10) and Mel, but no CoQ(9) was detected. Extra virgin olive oil showed higher levels of CoQ(10) and lower levels of Mel than virgin argan oil. Between the seed oil samples, only virgin soybean oil showed higher CoQ(10) and Mel levels than virgin argan oil. The results may be relevant for the contribution of CoQ(10) and Mel to the biological activities of virgin argan oil.

The mechanism of superoxide production by the antimycin-inhibited mitochondrial Q-cycle

Superoxide production from antimycin-inhibited complex III in isolated mitochondria first increased to a maximum then decreased as substrate supply was modulated in three different ways. In each case, superoxide production had a similar bell-shaped relationship to the reduction state of cytochrome b(566), suggesting that superoxide production peaks at intermediate Q-reduction state because it comes from a semiquinone in the outer quinone-binding site in complex III (Q(o)). Imposition of a membrane potential changed the relationships between superoxide production and b(566) reduction and between b(562) and b(566) redox states, suggesting that b(562) reduction also affects semiquinone concentration and superoxide production. To assess whether this behavior was consistent with the Q-cycle mechanism of complex III, we generated a kinetic model of the antimycin-inhibited Q(o) site. Using published rate constants (determined without antimycin), with unknown rate constants allowed to vary, the model failed to fit the data. However, when we allowed the rate constant for quinol oxidation to decrease 1000-fold and the rate constant for semiquinone oxidation by b(566) to depend on the b(562) redox state, the model fit the energized and de-energized data well. In such fits, quinol oxidation was much slower than literature values and slowed further when b(566) was reduced, and reduction of b(562) stabilized the semiquinone when b(566) was oxidized. Thus, superoxide production at Q(o) depends on the reduction states of b(566) and b(562) and fits the Q-cycle only if particular rate constants are altered when b oxidation is prevented by antimycin. These mechanisms limit superoxide production and short circuiting of the Q-cycle when electron transfer slows.

Evidence for two sites of superoxide production by mitochondrial NADH-ubiquinone oxidoreductase (complex I)

Complex I (NADH-ubiquinone oxidoreductase) can form superoxide during forward electron flow (NADH-oxidizing) or, at sufficiently high protonmotive force, during reverse electron transport from the ubiquinone (Q) pool (NAD(+)-reducing). We designed an assay system to allow titration of the redox state of the superoxide-generating site during reverse electron transport in rat skeletal muscle mitochondria: a protonmotive force generated by ATP hydrolysis, succinate:malonate to alter electron supply and modulate the redox state of the Q pool, and inhibition of complex III to prevent QH(2) oxidation via the Q cycle. Stepwise oxidation of the QH(2)/Q pool by increasing malonate concentration slowed the rates of both reverse electron transport and rotenone-sensitive superoxide production by complex I. However, the superoxide production rate was not uniquely related to the resultant potential of the NADH/NAD(+) redox couple. Thus, there is a superoxide producer during reverse electron transport at complex I that responds to Q pool redox state and is not in equilibrium with the NAD reduction state. In contrast, superoxide production during forward electron transport in the presence of rotenone was uniquely related to NAD redox state. These results support a two-site model of complex I superoxide production; one site in equilibrium with the NAD pool, presumably the flavin of the FMN moiety (site I(F)) and the other dependent not only on NAD redox state, but also on protonmotive force and the reduction state of the Q pool, presumably a semiquinone in the Q-binding site (site I(Q)).

The effect of pH and free Mg2+ on ATP linked enzymes and the calculation of Gibbs free energy of ATP hydrolysis

The apparent equilibrium constants, K′, of biochemical reactions containing substrates which bind [Mg2+] unequally can be significantly altered by changes in free intracellular [Mg2+]. Intracellular free [Mg2+] can be estimated by measurements of [citrate]/[isocitrate], a ratio known to vary with tissue free [Mg2+]. The combined equilibrium constant for glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase, and triose phosphate isomerase for the three reactions (K(GG-TPI)′) was corrected using new binding constants for dihydroxyacetone-phosphate and 3-phosphoglycerate. The result of this calculation is demonstrated in the calculation of the free energy of ATP hydrolysis. In addition, the dependence of the equilibrium constant for the glutamine synthetase reaction on pH and free [Mg2+] was demonstrated. Furthermore, a theory linking the ΔG′ value of mitochondrial complex I−II and the cytosolic ΔG′ value of ATP hydrolysis is discussed with evidence from previous publications.

Cytochrome b-559 and proton conductance in oxygenic photosynthesis

Although cytochrome b-559 has long been known as a membrane-bound redox component closely linked to the reaction center of the oxygen-generating photosystem (PSII), its role in photosynthesis has remained obscure. This paper reports evidence and outlines a hypothesis in support of a "b-559 cycle"-i.e., a light-induced, cytochrome b-559-dependent, cyclic electron transport pathway around PSII that promotes translocation of protons from plastoquinol into the aqueous domain (lumen) of photosynthetic membranes (thylakoids). Light-induced proton transport coupled to light-induced electron transport is an essential aspect of energy transduction in photosynthesis because it generates an electrochemical proton gradient that drives ATP synthesis by the process of photosynthetic phosphorylation. The principal carrier of electrons and protons in thylakoids is the plastoquinone/plastoquinol couple. We propose that the b-559 cycle functions as a redox-linked proton pump that may operate jointly with the Rieske iron-sulfur pathway in oxidizing plastoquinol. The overall effect of such concerted oxidation of plastoquinol would be the translocation into the thylakoid lumen of two protons for each electron transferred from water to plastocyanin via plastoquinone.

Wanderings in bioenergetics and biomembranes

Having worked for 55 years in the center and at the fringe of bioenergetics, my major research stations are reviewed in the following wanderings: from microsomes to mitochondria, from NAD to CoQ, from reversed electron transport to reversed oxidative phosphorylation, from mitochondrial hydrogen transfer to phosphate transfer pathways, from endogenous nucleotides to mitochondrial compartmentation, from transport to mechanism, from carrier to structure, from coupling by AAC to uncoupling by UCP, and from specific to general transport laws. These wanderings are recalled with varying emphasis paid to the covered science stations.

Decolorization of malachite green by cytochrome c in the mitochondria of the fungus Cunninghamella elegans

We studied the decolorization of malachite green (MG) by the fungus Cunninghamella elegans. The mitochondrial activity for MG reduction was increased with a simultaneous increase of a 9-kDa protein, called CeCyt. The presence of cytochrome c in CeCyt protein was determined by optical absorbance spectroscopy with an extinction coefficient (E(550-535)) of 19.7+/-6.3 mM(-1) cm(-1) and reduction potential of + 261 mV. When purified CeCyt was added into the mitochondria, the specific activity of CeCyt reached 440 +/- 122 micromol min(-1) mg(-1) protein. The inhibition of MG reduction by stigmatellin, but not by antimycin A, indicated a possible linkage of CeCyt activity to the Qo site of the bc1 complex. The RT-PCR results showed tight regulation of the cecyt gene expression by reactive oxygen species. We suggest that CeCyt acts as a protein reductant for MG under oxidative stress in a stationary or secondary growth stage of this fungus.

Energy transfer in mitochondrial synthesis of ATP; a survey

The energy transduction in mitochondria, with its principal agent ATP, still represents a major challenge for biological research. In general, the energy transduction process is divided into three sections: (1) the redox processes; (2) a conservation of intermediary energy forms; (3) synthesis of ATP. All three processes are linked to the membrane and are, therefore, as difficult to resolve as are processes linked to other biomembranes. It is probable that the electron transport system is constructed in such a way as to provide energy for synthesis of ATP and related processes. Important for this function is the transversal distribution of these components across the membrane, facilitating generation of membrane potential by electron or proton transfer. The exact composition of the respiratory chain is not yet known, in particular with respect to iron-sulphur proteins. Progress is achieved by defining single species of the respiratory chain, subunit composition, amino acid sequences and genetic derivation from intra- or extra-mitochondrial translation. Energy generated by oxidation can be trapped before ATP is formed by a number of reactions, in particular reversed electron transport, energy-dependent transhydrogenation and uptake of anions or cations into the mitochondria. The latter reaction is of major importance for understanding the intermediate energy form, as it appears to use energy most directly and be driven mainly by membrane potential or proton gradient across the membrane. The formation of ATP is a major problem hindering elucidation of the mechanism of oxidative phosphorylation. The mechanism of this enzymic process is not yet understood although the enzymes have been isolated and the subunits have been defined. Most probably, a concerted reaction between ADP and phosphate, driven by some conformational transition of the complex, leads to the formation of ATP. Release of ATP from a hydrophobic to hydrophilic environment may consume most of the energy.

Kinetic characterization of the disulfide bond-forming enzyme DsbB

DsbB is an integral membrane protein responsible for the de novo synthesis of disulfide bonds in Escherichia coli and many other prokaryotes. In the process of transferring electrons from DsbA to a tightly bound ubiquinone cofactor, DsbB undergoes an unusual spectral transition at approximately 510 nm. We have utilized this spectral transition to study the kinetic cycle of DsbB in detail using stopped flow methods. We show that upon mixing of Dsb-B(ox) and DsbA(red), there is a rapid increase in absorbance at 510 nm (giving rise to a purple solution), followed by two slower decay phases. The rate of the initial phase is highly dependent upon DsbA concentration (k(1) approximately 5 x 10(5) M(-1) s(-1)), suggesting this phase reflects the rate of DsbA binding. The rates of the subsequent decay phases are independent of DsbA concentration (k(2) approximately 2 s(-1); k(3) approximately 0.3 s(-1)), indicative of intramolecular reaction steps. Absorbance measurements at 275 nm suggest that k(2) and k(3) are associated with steps of quinone reduction. The rate of DsbA oxidation was found to be the same as the rate of quinone reduction, suggestive of a highly concerted reaction. The concerted nature of the reaction may explain why previous efforts to dissect the reaction mechanism of DsbB by examining individual pairs of cysteines yielded seemingly paradoxical results. Order of mixing experiments showed that the quinone must be pre-bound to DsbB to observe the purple intermediate as well as for efficient quinone reduction. These results are consistent with a kinetic model for DsbB action in which DsbA binding is followed by a rapid disulfide exchange event. This is followed by quinone reduction, which is rate-limiting in the overall reaction cycle.

Kinetics of electron and proton transfer to ubiquinone-10 and from ubiquinol-10 in a self-assembled phosphatidylcholine monolayer

Upon incorporating from 0.5 to 2 mol% ubiquinone-10 (UQ) in a self-assembled monolayer of dioleoylphosphatidylcholine (DOPC) supported by mercury, the kinetics of UQ reduction to ubiquinol-10 (UQH2) as well as that of UQH2 oxidation to UQ were investigated in borate buffers over the pH range from 8 to 9.5 by cyclic voltammetry. A general kinetic approach was adopted to interpret the dependence of the applied potential upon the scan rate at constant pH and upon pH at constant scan rate, while keeping the initial reactant concentration and the faradaic charge constant. The oxidation of UQH2 to UQ in DOPC monolayers occurs via the reversible release of one electron with formation of the semiubiquinone radical cation UQH2.+, followed by its rate-determining deprotonation by hydroxyl ions with formation of the UQH. neutral radical; the latter is then instantaneously oxidized to UQ. Analogously, the rate-determining step in UQ reduction to UQH2 consists in the protonation by hydrogen ions of the semiubiquinone radical anion UQ.- resulting from the reversible uptake of one electron by UQ. However, a non-negligible fraction of UQ.- uptakes protons very slowly, and hence, retains its intermediate oxidation state during the recording of the cyclic voltammetric peak for UQ reduction.

Endogenous and exogenous regulation of redox-properties of coenzyme Q

Ubiquinol (QH2, reduced coenzyme Q) is increasingly reported to exert antioxidant functions besides its implication in mitochondrial energy metabolism. On the other hand ubisemiquinones (SQ-.) of the respiratory chain are considered to account for the production of superoxide radicals as a byproduct of cellular respiration. Since the formation of potentially prooxidative ubisemiquinones can be expected to result from the antioxidant activity of ubiquinol, the evaluation whether or not QH2 exerts antioxidant activities depends on the fate of antioxidant-derived metabolites and the existence of a natural recycling system for oxidized QH2. We have recently shown that SQ increasingly undergo autoxidation when approaching the external more polar phase of the membrane. In contrast to mitochondria where the QH2/ SQ-./Q pools are dynamically kept in relatively stable relationships the fate of semi and fully oxidized QH2 is not at all clear in LDL particles where QH2 is suggested to exert important antioxidant functions. Therefore, the antioxidant-derived metabolites of QH2 in liposomes following lipid peroxidation were studied with respect to their localization in the bilayer and the possibility to recycle oxidized QH2 via dihydrolipoic acid (DHLA). The results revealed a considerable fraction of QH2 existing in the outer membrane section where protons from the aqueous phase have access to allow autoxidation. DHLA was found to recycle oxidized QH2 although due to slow partition equilibration the reduction velocity appears to be not sufficient for therapeutic application.

The protonmotive Q cycle in mitochondria and bacteria

The cytochrome bc1 complex is an oligomeric electron transfer enzyme located in the inner membrane of mitochondria and the plasma membrane of bacteria. The cytochrome bc1 complex participates in respiration in eukaryotic cells and also participates in respiration, cyclic photosynthetic electron transfer, denitrification, and nitrogen fixation in a phylogenetically diverse collection of bacteria. In all of these organisms, the cytochrome bc1 complex transfers electrons from ubiquinol to cytochrome c and links this electron transfer to translocation of protons across the membrane in which it resides, thus converting the available free energy of the oxidation-reduction reaction into an electrochemical proton gradient. The mechanism by which the cytochrome bc1 complex achieves this energy transduction is the protonmotive Q cycle. The Q cycle mechanism has been documented by extensive experimentation, and recent investigations have focused on structural features of the three redox subunits of the bc1 complex essential to the protonmotive and electrogenic activities of this membranous enzyme.

Sulfide quinone reductase (SQR) activity in Chlorobium

Membranes of the green sulfur bacterium, Chlorobium limicola f. thiosulfatophilum, catalyze the reduction of externally added isoprenoid quinones by sulfide. This activity is highly sensitive to stigmatellin and aurachins. It is also inhibited by 2-n-nonyl-4-hydroxyquinoline-N-oxide, antimycin, myxothiazol and cyanide. It is concluded that in sulfide oxidizing bacteria like Chlorobium, sulfide oxidation involves a sulfide-quinone reductase (SQR) similar to the one found in Oscilatoria limnetica [Arieli, B., Padan, E. and Shahak, Y. (1991) J. Biol. Chem. 266, 104-111].

Mechanistic stoichiometry of mitochondrial oxidative phosphorylation

P/O ratios of rat liver mitochondria were measured with particular attention to systematic errors. Corrections for energy loss during oxidative phosphorylation were made by measurement of respiration as a function of mitochondrial membrane potential. The corrected values were close to 1, 0.5, and 1 at the three coupling sites, respectively. These values are consistent with recent measurements of mitochondrial proton transport.

Inhibitor effects on redox-linked protonations of the b haems of the mitochondrial bc1 complex

The effects of pH and inhibitors on the spectra and redox properties of the haems b of the bc1 complex of beef heart submitochondrial particles were investigated. The major findings were: (1) both haems have a weakly redox-linked protonatable group with pKox and pKred of around 6 and 8; (2) at pH values above 7, haem bH becomes heterogeneous in its redox behaviour. This heterogeneity is removed by the Qi site inhibitors antimycin A, funiculosin and HQNO, but not by the Qo site inhibitors myxothiazol or stigmatellin; (3) of all inhibitors tested only funiculosin had a large effect on the Em/pH profile of either haem b. In all cases where definite effects were found, the haem most affected was that thought to be closest to the site of inhibitor binding; (4) spectral shifts of haem groups caused by inhibitor binding were usually, but not always, of the haem group closest to the binding site; (5) titrations with succinate/fumarate were in reasonable agreement with redox-mediated data provided that strict anaerobiosis was maintained. Apparent large shifts of haem midpoint potentials with antimycin A and myxothiazol could be produced in aerobic succinate/fumarate titrations in the presence of cyanide, as already reported in the literature, but these were artefactual; (6) the heterogeneous haem bH titration behaviour can be simulated with a model similar to that proposed by Salerno et al. (J. Biol. Chem. (1989) 264, 15398-15403) in which there is redox interaction between haem bH and ubiquinone species bound at the Qi site. Simulations closely fit both the haem bH data and known semiquinone data only if it is assumed that semiquinone bound to oxidised haem bH is EPR-silent.

Triphasic reduction of bH and the absence of equilibration at the i-site of bc1 complex

The concept of a stabilized semiquinone radical forming the basis of a two-electron gate has long been familiar in the context of the quinone-reducing site in photosynthetic systems and has already been suggested to play a role at the i-site of the bc1-type complexes. It is here pointed out that this concept is sufficient to explain the so-called triphasic reduction kinetics of cytochrome bH, in which cytochrome bH goes partially reduced, is reoxidized and then goes fully reduced. The rate constants for binding and unbinding of quinone, quinol and semiquinone at the i-site are discussed, and a kinetic model featuring slow release of the i-site semiquinone is shown to display many features of the kinetics of electron transfer at the i-site.

Leukocytic oxygen activation and microbicidal oxidative toxins

Following a brief introduction of cellular response to stimulation comprising leukocyte activation, three major areas are discussed: (1) the neutrophil oxidase; (2) myeloperoxidase (MPO)-dependent oxidative microbicidal reactions; and (3) MPO-independent oxidative reactions. Topics included in section (A) are current views on the activation mechanism, redox composition, structural and topographic organization of the oxidase, and its respiratory products. In section (B), emphasis is placed on recent research on cidal mechanisms of HOCl, including the oxidative biochemistry of active chlorine compounds, identification of sites of lesions in bacteria, and attendant metabolic consequences. In section (C), we review the (bio)chemistry of H2O2 and .OH microbicidal reactions, with particular attention being given to addressing the controversial issue of probe methods to identify .OH radical and critical assessment of the recent proposal that MPO-independent killing arises from site-specific metal-catalyzed Fenton-type chemistry.

Proton/electron stoichiometry of mitochondrial complex I estimated from the equilibrium thermodynamic force ratio

The thermodynamic forces on electrons (delta Eh) and protons (delta p) across mitochondrial complexes I, III and IV were measured in isolated mitochondria respiring on succinate. The force ratio (delta Eh/delta p) across complex I close to equilibrium was found to be about 2. The equilibrium force ratio across complex I was measured during sulphite oxidation and was again close to 2. These results indicate that the proton/electron stoichiometry of complex I is 2, in conditions of high protonmotive force.

Rapid redox equilibrium between the mitochondrial Q pool and cytochrome b during triphasic reduction of cytochrome b by succinate

The reliability of monitoring the redox reactions of cytochrome b using the different wavelengths employed by different authors has been reexamined. It was found that 562-575 nm is suitable in succinate: cytochrome c reductase but not in mitochondria, in which case 562-540 nm is a better pair. Direct optical measurements of the redox reaction kinetics of the mitochondrial Q pool using a commercial dual-wavelength spectrophotometer are possible when succinate is used as the electron donor. Using the correct wavelength pair, and with malonate to slow down the electron input, the reduction course of cytochrome b was still triphasic but a plateau or a turn replaced the oxidation phase previously reported by several authors. At the same time, the reduction course of the Q pool was also triphasic, and in perfect match with that of cytochrome b. Destruction of the Rieske iron-sulfur cluster by British anti-Lewisite (BAL) + O2 treatment or prereduction of the high-potential components made the reduction of both Q and b monophasic. The plot of log (Q/QH2) against log (b3+/b2+) gave a straight line with an n value of 1.7 for cytochrome b at pH 7.4. This n value rose to 2.0 at pH 6.5 and dropped to 1.4 at pH 8.5. On the other hand, the mid-point potential of cytochrome b relative to that of the Q pool remained essentially unchanged between pH 6.5 and 8.4. BAL treatment had a small effect on the midpoint potential of cytochrome b relative to that of the Q pool and had no effect on the n value. Addition of quinone homologues and analogues extended the plateau phase in the reduction of cytochrome b, but exogenous quinones did not equilibrate rapidly with cytochrome b. It was concluded that the appearance of the plateau between the two reduction phases of Q and b is caused by the rapid delivery of electrons to the high-potential components of the respiratory chain as envisaged in the Q cycle; the unexpected n value for cytochrome b suggests a concerted reduction by QH2 of two species of cytochromes b-562.

Organization and function of cytochrome b and ubiquinone in the cristae membrane of beef heart mitochondria

The arrangement and function of the redox centers of the mammalian bc1 complex is described on the basis of structural data derived from amino acid sequence studies and secondary structure predictions and on the basis of functional studies (i.e., EPR data, inhibitor studies, and kinetic experiments). Two ubiquinone reaction centers do exist--a QH2 oxidation center situated at the outer, cytosolic surface of the cristae membrane (Q0 center), and a Q reduction center (Qi center) situated more to the inner surface of the cristae membrane. The Q0 center is formed by the b-566 domain of cytochrome b, the FeS protein, and maybe an additional small subunit, whereas the Qi center is formed by the b-562 domain of cytochrome b and presumably the 13.4 kDa protein ("QP-C"). The "Q binding proteins" are proposed to be protein subunits of the Q reaction centers of various multiprotein complexes. The path of electron flow branches at the Q0 center, half of the electrons flowing via the high-potential cytochrome chain to oxygen and half of the electrons cycling back into the Q pool via the cytochrome b path connecting the two Q reaction centers. During oxidation of QH2, 2H+ are released to the cytosolic space and during reduction of Q, 2H+ are taken up from the matrix side, resulting in a net transport across the membrane of 2H+ per e- flown from QH2 to cytochrome c, the H+ being transported across the membrane as H (H+ + e-) by the mobile carrier Q. The authors correct their earlier view of cytochrome b functioning as a H+ pump, proposing that the redox-linked pK changes of the acidic groups of cytochrome b are involved in the protonation/deprotonation processes taking place during the reduction and oxidation of Q. The reviewers stress that cytochrome b is in equilibrium with the Q pool via the Qi center, but not via the Q0 center. Their view of the mechanisms taking place at the reductase is a Q cycle linked to a Q-pool where cytochrome b is acting as an electron pump.