The reduction state of the Q-pool regulates the electron flux through the branched respiratory network of Paracoccus denitrificans

In this work we demonstrate how the reduction state of the Q-pool determines the distribution of electron flow over the two quinol-oxidising branches in Paracoccus denitrificans: one to quinol oxidase, the other via the cytochrome bc1 complex to the cytochrome c oxidases. The dependence of the electron-flow rate to oxygen on the fraction of quinol in the Q-pool was determined in membrane fractions and in intact cells of the wild-type strain, a bc1-negative mutant and a quinol oxidase-negative mutant. Membrane fractions of the bc1-negative mutant consumed oxygen at significant rates only at much higher extents of Q reduction than did the wild-type strain or the quinol oxidase-negative mutant. In the membrane fractions, dependence on the Q redox state was exceptionally strong corresponding to elasticity coefficients close to 2 or higher. In intact cells, the dependence was weaker. In uncoupled cells the dependence of the oxygen-consumption rates on the fractions of quinol in the Q-pool in the wild-type strain and in the two mutants came closer to that found for the membrane fractions. We also determined the dependence for membrane fractions of the wild-type in the absence and presence of antimycin A, an inhibitor of the bc1 complex. The dependence in the presence of antimycin A resembled that of the bc1-negative mutant. These results indicate that electron-flow distribution between the two quinol-oxidising branches in P. denitrificans is not only determined by regulated gene expression but also, and to a larger extent, by the reduction state of the Q-pool.

Functional expression of the plant alternative oxidase affects growth of the yeast Schizosaccharomyces pombe

We have investigated the extent to which functional expression of the plant alternative oxidase (from Sauromatum guttatum) in Schizosaccharomyces pombe affects yeast growth. When cells are cultured on glycerol, the maximum specific growth rate is decreased from 0.13 to 0.11 h-1 while growth yield is lowered by 20% (from 1. 14 x 10(8) to 9.12 x 10(7) cells ml-1). Kinetic studies suggest that the effect on growth is mitochondrial in origin. In isolated mitochondria we found that the alternative oxidase actively competes with the cytochrome pathway for reducing equivalents and contributes up to 24% to the overall respiratory activity. Metabolic control analysis reveals that the alternative oxidase exerts a considerable degree of control (22%) on total electron flux. Furthermore, the negative control exerted by the alternative oxidase on the flux ratio of electrons through the cytochrome and alternative pathways is comparable with the positive control exerted on this flux-ratio by the cytochrome pathway. To our knowledge, this is the first paper to report a phenotypic effect because of plant alternative oxidase expression. We suggest that the effect on growth is the result of high engagement of the non-protonmotive alternative oxidase in yeast respiration that, consequently, lowers the efficiency of energy conservation and hence growth.

The effect of sulfite on the ATP hydrolysis and synthesis activities in chloroplasts and cyanobacterial membrane vesicles can be explained by competition with phosphate

The effect of sulfite on ATP synthesis and hydrolysis activities is investigated in spinach chloroplasts and in membrane vesicles from the cyanobacterium Synechococcus 6716. Sulfite inhibits phenazine methosulfate-mediated cyclic photophosphorylation both in thiol-modulated chloroplasts and in cyanobacterial membranes with HSO3- (bisulfite) as the active ionic species. The observed inhibition is not due to inhibition of electron transfer or to uncoupling by sulfite. ATP synthesis in cyanobacterial membranes is more sensitive to sulfite when the inorganic phosphate concentration is decreased. This indicates competition between sulfite and phosphate for the same binding site on the ATP synthase. In cyanobacterial membranes sulfite can replace a proton gradient as activator of ATP hydrolysis in the same way as in reduced chloroplasts. By modeling, competition between sulfite and phosphate can fully explain the findings concerning both inhibition and activation.

Monovalent cations differentially affect membrane surface properties and membrane curvature, as revealed by fluorescent probes and dynamic light scattering

The effects of monovalent cations on the interfacial electrostatic potential (psi d), hydrodynamic shear boundary distance (ds), and membrane curvature were studied in large unilamellar phospholipid and galacto/sulfolipid liposomes containing different fractions of negatively charged lipids. The differential effects of alkali metal ions on psi d could be accurately determined at physiological surface charge densities with a surface-anchored fluorescent probe. Li+ and Na+ more effectively decrease psi d and exhibit higher association constants (Kas) than K+ and Cs+. These two groups of cations display qualitatively different perturbations of the interfacial structure. Combining Kas values with the electrokinetic (zeta) potentials yielded the respective ds values. At low ionic strength ds more substantially increases with Li+ or Na+ than with K+ or Cs+. Increasing surface charge density causes increased membrane curvature in the presence of K+ or Cs+, but this is largely prevented by Li+ or Na+. Membrane binding of the amphiphilic cation acridine orange decreases surface charge and membrane curvature more extensively than H3O+, Li+, and Na+. The differential interface-perturbing behavior of monovalent cations is discussed with regard to their different hydration tendencies that will modulate the extent and stability of the hydrogen-bond network along the charged membrane surface.

Kinetic analysis of the mitochondrial quinol-oxidizing enzymes during development of thermogenesis in Arum maculatum L

The dependence of the rate of oxygen uptake upon the ubiquinone (Q)-pool reduction level in mitochondria isolated during the development of thermogenesis of Arum maculatum spadices has been investigated. At the alpha-stage of development, the respiratory rate was linearly dependent upon the reduction level of the Q-pool (Qr) both under state-3 and -4 conditions. Progression through the beta/gamma to the delta-stage resulted in a non-linear dependence of the state-4 rate on Qr. In the delta-stage of development, both state-3 and -4 respiratory rates were linearly dependent upon Qr due to a shift in the engagement of the alternative oxidase to lower levels of Qr. Western blot analysis revealed that increased alternative oxidase activity could be correlated with expression of a 35 kDa protein. Respiratory control was only observed with mitochondria in the alpha-stage of development. At the beta/gamma-stage of development, the addition of ADP resulted in a significant oxidation of the Q-pool which was accompanied by a decrease in the respiratory rate. This was due either to decreased contribution of the alternative pathway to the overall respiratory rate under state 3 or by deactivation of succinate dehydrogenase activity by ADP. Cold-storage of the spadices at the beta-stage of development led to increased activity of both the cytochrome pathway and succinate dehydrogenase, without any change in alternative oxidase activity. Results are discussed in terms of how changes in the activation level of the alternative oxidase and succinate dehydrogenase influence the activity and engagement of the quinol-oxidizing pathways during the development of thermogenesis in A. maculatum.

Structural and functional analysis of aa3-type and cbb3-type cytochrome c oxidases of Paracoccus denitrificans reveals significant differences in proton-pump design

In Paracoccus denitrificans the aa3-type cytochrome c oxidase and the bb3-type quinol oxidase have previously been characterized in detail, both biochemically and genetically. Here we report on the isolation of a genomic locus that harbours the gene cluster ccoNOOP, and demonstrate that it encodes an alternative cbb3-type cytochrome c oxidase. This oxidase has previously been shown to be specifically induced at low oxygen tensions, suggesting that its expression is controlled by an oxygen-sensing mechanism. This view is corroborated by the observation that the ccoNOOP gene cluster is preceded by a gene that encodes an FNR homologue and that its promoter region contains an FNR-binding motif. Biochemical and physiological analyses of a set of oxidase mutants revealed that, at least under the conditions tested, cytochromes aa3, bb3 and cbb3 make up the complete set of terminal oxidases in P. denitrificans. Proton-translocation measurements of these oxidase mutants indicate that all three oxidase types have the capacity to pump protons. Previously, however, we have reported decreased H+/e- coupling efficiencies of the cbb3-type oxidase under certain conditions. Sequence alignment suggests that many residues that have been proposed to constitute the chemical and pumped proton channels in cytochrome aa3 (and probably also in cytochrome bb3) are not conserved in cytochrome cbb3. It is concluded that the design of the proton pump in cytochrome cbb3 differs significantly from that in the other oxidase types.

Kinetic and regulatory aspects of the function of the alternative oxidase in plant respiration

The kinetic modelling of the respiratory network in plant mitochondria is discussed, with emphasis on the importance of the choice of boundary conditions, and of modelling of both quinol-oxidising and quinone-reducing pathways. This allows quantitative understanding of the interplay between the different pathways, and of the functioning of the plant respiratory network in terms of the kinetic properties of its component parts. The effects of activation of especially succinate dehydrogenase and the cyanide-insensitive alternative oxidase are discussed. Phenomena, such as respiratory control ratios depending on the substrate, shortcomings of the Bahr and Bonner model for electron distribution between the oxidases and reversed respiratory control, are explained. The relation to metabolic control analysis of the respiratory network is discussed in terms of top-down analysis.

The relationship between electron flux and the redox poise of the quinone pool in plant mitochondria. Interplay between quinol-oxidizing and quinone-reducing pathways

The dependence of electron flux through quinone-reducing and quinol-oxidizing pathways on the redox state of the ubiquinone (Q) pool was investigated in plant mitochondria isolated from potato (Solanum tuberosum cv. Bintje, fresh tissue and callus), sweet potato (Ipomoea batatas) and Arum italicum. We have determined the redox state of the Q pool with two different methods, the Q-electrode and Q-extraction techniques. Although results from the two techniques agree well, in all tissues tested (with the exception of fresh potato) an inactive pool of QH2 was detected by the extraction technique that was not observed with the electrode. In potato callus mitochondria, an inactive Q pool was also found. An advantage of the extraction method is that it permits determination of the Q redox state in the presence of substances that interfere with the Q-electrode, such as benzohydroxamate and NADH. We have studied the relation between rate and Q redox state for both quinol-oxidizing and quinone-reducing pathways under a variety of metabolic conditions including state 3, state 4, in the presence of myxothiazol, and benzohydroxamate. Under state 4 conditions or in the presence of myxothiazol, a non-linear dependence of the rate of respiration on the Q-redox state was observed in potato callus mitochondria and in sweet potato mitochondria. The addition of benzohydroxamate, under state 4 conditions, removed this non-linearity confirming that it is due to activity of the cyanide-resistant pathway. The relation between rate and Q redox state for the external NADH dehydrogenase in potato callus mitochondria was found to differ from that of succinate dehydrogenase. It is suggested that the oxidation of cytoplasmic NADH in vivo uses the cyanide-resistant pathway more than the pathway involving the oxidation of succinate. A model is used to predict the kinetic behaviour of the respiratory network. It is shown that titrations with inhibitors of the alternative oxidase cannot be used to demonstrate a pure overflow function of the alternative oxidase.

The effect of sulfite on the ATP hydrolysis and synthesis activity of membrane-bound H(+)-ATP synthase from various species

The action of sulfite on ATP hydrolysis and synthesis activities is investigated in membrane vesicles prepared from the cyanobacterium Synechococcus 6716, chromatophores from the photosynthetic purple bacterium Rhodospirillum rubrum, membrane vesicles from the related non-photosynthetic bacterium Paracoccus denitrificans, and bovine heart submitochondrial particles. Without any further pretreatment ATP hydrolysis is stimulated by sulfite in all four membrane preparations. Typically ATP synthesis in the cyanobacterial membrane vesicles is inhibited by sulfite, whereas ATP synthesis in chromatophores and the submitochondrial particles is not. These differences in sensitivity of ATP synthesis to sulfite, however, correspond well with the distribution of (photosynthetic) sulfur oxidizing pathways in the remaining three organisms/organelles compared in this study.

On the activation mechanism of the H(+)-ATP synthase and unusual thermodynamic properties in the alkalophilic cyanobacterium Spirulina platensis

The activation requirements and thermodynamic characteristics of ATP synthase from the alkalophilic cyanobacterium Spirulina platensis were studied in coupled membrane vesicles. Activation by methanol increased the Vmax, while the Km for MgATP was unaffected (0.7 mM). We propose that in Sp. platensis, as in chloroplasts, the activating effect of methanol is based on perturbation of the gamma-epsilon subunit interaction. Light-driven ATP synthesis by membrane vesicles of Sp. platensis was stimulated by dithiothreitol. The characteristics of the activation of the ATP synthase by the proton electrochemical potential difference (delta mu H+) were analyzed on the basis of the uncoupled rates of ATP hydrolysis as a function of a previously applied proton gradient. Two values of delta mu H+, at which 50% of the enzyme is active, were found; 13-14 kJ.mol-1 for untreated membrane vesicles, and 4-8 kJ.mol-1 for light-treated and dithiothreitol-treated membrane vesicles. These values are lower than the corresponding values for the oxidized and reduced forms, respectively, of the chloroplast enzyme. Although no bulk proton gradient could be observed, membrane vesicles of Sp. platensis were able to maintain an equilibrium phosphate potential (delta Gp) of 40-43.5 kJ.mol-1, comparable to values found for Synechococcus 6716 and Anabaena 7120 membrane vesicles. Acid/base-transition experiments showed that the thermodynamic threshold, delta mu H+, for ATP synthesis, catalyzed by light-treated and dithiothreitol-treated Spirulina membrane vesicles, was less than 5 kJ.mol-1. The activation characteristics and the low thermodynamic threshold allow ATP synthesis to occur at low delta mu H+ values. The findings are discussed, both with respect to differences and similarities with the enzymes from chloroplasts and other cyanobacteria, and with respect to the alkalophilic properties of Sp. platensis.

Modulation of the Access of Exogenous NAD(P)H to the Alternative Pathway in Potato Tuber Callus Mitochondria with Triton X-100

Alternative oxidase activity in potato tuber (Solanum tuberosum L. cv Bintje) callus mitochondria with exogenous NAD(P)H as substrate is inhibited by low concentrations of the detergent Triton X-100. Alternative oxidase activity with succinate or malate as substrate is not affected by these low concentrations of Triton X-100. Cytochrome pathway activity was not influenced under these conditions, neither with endogenous nor with exogenous substrate. Washing of Triton X-100-treated mitochondria did partially restore both uninhibited and CN-resistant NADH oxidation, indicating that under these conditions Triton X-100 does not permanently remove major components from the mitochondrial membrane. Apparently, it is possible to manipulate mitochondria in such a way that the access of exogenous NADH to the alternative pathway is blocked while access to the cytochrome pathway is uninhibited. It is suggested that membrane conditions have a regulatory function (possibly via influencing the diffusion path) in the oxidation of exogenous NADH via the alternative pathway.

Principles of coupling between electron transfer and proton translocation with special reference to proton-translocation mechanisms in cytochrome oxidase

The recent general acceptance of the proton-pumping function of cytochrome oxidase has stimulated discussion and experiment on possible underlying molecular mechanisms. Adequate experimental design requires clear understanding of the theoretical principles governing such a linked function. The increasing structural knowledge of cytochrome oxidase also contributes to a present-day requirement of more precise chemical and physical description of redox-linked proton translocation, which is the fundamental process underlying conservation of energy from aerobic metabolism in all eukaryotes and many bacteria. This essay is based on our original theoretical treatment of this problem, which is expanded here to include discussion of more recent analyses by others, classification of different types of coupling principles, as well as some concrete proposed molecular mechanisms. The latter will be analysed qualitatively, and in some cases quantitatively where this is possible, using a common theoretical framework to help comparison between models. Experimental findings relevant to this problem will be critically reviewed, and some suggestions will be made to stimulate further experiments dedicated to clarify the problem.

Determination of the stoichiometry of redox-linked proton translocation from the kinetics of pulse experiments. A simulation study

We have previously published a simple kinetic model to analyse possible pitfalls in kinetic measurements of H+/O ratios in mitochondria [(1984) FEBS Lett. 178, 187-192]. While this model demonstrated how relative electrode response times may affect the results, it did not adequately describe the kinetics of proton back-diffusion across the membrane. Here this model is further developed and improved, and shown to give a good quantitative description of both oxygen-pulse type experiments as well as of experiments where the reaction is started by photolysis of the cytochrome c oxidase-CO complex. Simulations based on this model reveal that the extrapolation procedure used by Lehninger et al. [e.g. (1984) J. Biol. Chem. 259, 4802-4811] to estimate the H+/O ratio will tend to yield overestimated values. This is mainly due to the back-diffusion of protons into the mitochondria, which is not correctly accounted for by this extrapolation.