Control of oxidative phosphorylation in plant mitochondria: The role of non-phosphorylating pathways

NADPH Oxidase System as a Superoxide-generating Cyanide-Resistant Pathway in the Respiratory Chain of Corynebacterium glutamicum

The respiratory chain of Corynebacterium glutamicum was investigated, especially with respect to a cyanide-resistant respiratory chain bypass oxidase. The membranes of C. glutamicum had NADH, succinate, lactate, and NADPH oxidase activities, and menaquinone, and cytochromes a 598, b 562(558), and c 550 as respiratory components. The NADH, succinate, lactate, and NADPH oxidase systems, all of which were more cyanide-resistant than N,N,N',N'-tetramethyl-p-phenylene diamine oxidase activity (cytochrome aa 3 terminal oxidase), had different sensitivities to cyanide; the cyanide sensitivity of these oxidase systems increased in the order, NADPH, lactate, NADH, and succinate. Taken together with the analysis of redox kinetics in the cytochromes and the effects of respiratory inhibitors, the results suggested that there is a cyanide-resistant bypass oxidase branching at the menaquinone site, besides cyanide-sensitive cytochrome oxidase in the respiratory chain. H(+)/O measurements with resting cells suggested that the cyanide-sensitive respiratory chain has two or three coupling sites, of which one is in NADH dehydrogenase and the others between menaquinone and cytochrome oxidase, but the cyanide-resistant bypass oxidase may not have any proton coupling site. NADPH and lactate oxidase systems were more resistant to UV irradiation than other systems and the UV insensitivity was highest in the NADPH oxidase system, suggesting that a specific quinone resistant to UV or no such a quinone works in at least NADPH oxidase system while the UV-sensitive menaquinone pool does in other oxidase systems. Furthermore, superoxide was generated in well-washed membranes, most strongly in the NADPH oxidase system. Thus, it was suggested that the cyanide-resistant bypass oxidase system of C. glutamicum is related to the NADPH oxidase system, which may be involved in generation of superoxide anions and probably functions together with superoxide dismutase and catalase.

Fungal respiration: a fusion of standard and alternative components

In animals, electron transfer from NADH to molecular oxygen proceeds via large respiratory complexes in a linear respiratory chain. In contrast, most fungi utilise branched respiratory chains. These consist of alternative NADH dehydrogenases, which catalyse rotenone insensitive oxidation of matrix NADH or enable cytoplasmic NADH to be used directly. Many also contain an alternative oxidase that probably accepts electrons directly from ubiquinol. A few fungi lack Complex I. Although the alternative components are non-energy conserving, their organisation within the fungal electron transfer chain ensures that the transfer of electrons from NADH to molecular oxygen is generally coupled to proton translocation through at least one site. The alternative oxidase enables respiration to continue in the presence of inhibitors for ubiquinol:cytochrome c oxidoreductase and cytochrome c oxidase. This may be particularly important for fungal pathogens, since host defence mechanisms often involve nitric oxide, which, whilst being a potent inhibitor of cytochrome c oxidase, has no inhibitory effect on alternative oxidase. Alternative NADH dehydrogenases may avoid the active oxygen production associated with Complex I. The expression and activity regulation of alternative components responds to factors ranging from oxidative stress to the stage of fungal development.

Control of plant mitochondrial respiration

Plant mitochondria are characterised by the presence of both phosphorylating (cytochrome) and non-phosphorylating (alternative) respiratory pathways, the relative activities of which directly affect the efficiency of mitochondrial energy conservation. Different approaches to study the regulation of the partitioning of reducing equivalents between these routes are critically reviewed. Furthermore, an updated view is provided regarding the understanding of plant mitochondrial respiration in terms of metabolic control. We emphasise the extent to which kinetic modelling and 'top-down' metabolic control analysis improve the insight in phenomena related to plant mitochondrial respiration. This is illustrated with an example regarding the affinity of the plant alternative oxidase for oxygen.

Carvedilol inhibits the exogenous NADH dehydrogenase in rat heart mitochondria

There are several reports on the oxidation of external NADH by an exogenous NADH dehydrogenase in the outer leaflet of the inner membrane of rat heart mitochondria. Until now, however, little was known about its physiological role in cellular metabolism. The present work shows that carvedilol (¿1-[carbazolyl-(4)-oxy]-3-[2-methoxyphenoxyethyl)amino]-pro - panol-(2)¿) is a specific inhibitor of an exogenous NADH dehydrogenase in rat heart mitochondria. Carvedilol does not affect oxygen consumption linked to the oxidation of succinate and internal NADH. It is also demonstrated that the inhibition of exogenous NADH dehydrogenase by carvedilol is accompanied by the inhibition of alkalinization of the external medium. In contrast to the addition of glutamate/malate or succinate, exogenous NADH does not generate a membrane potential in rat heart mitochondria, as observed with a TPP(+) electrode. It is also demonstrated that the oxygen consumption linked to NADH oxidation is not due to permeabilized mitochondria, but to actual oxidase activity in the inner membrane. The enzyme has a K(m) for NADH of 13 microM. Carvedilol is a noncompetitive inhibitor of this external NADH dehydrogenase with a K(i) of 15 microM. Carvedilol is the first inhibitor described to this organospecific enzyme. Since this enzyme was demonstrated to play a key role in the cardiotoxicity of anticancer drugs of the anthracycline family (e.g., adriamycin), we may suggest that the administration of carvedilol to tumor patients treated with adriamycin might be of great help in the prevention of the cardioselective toxicity of this antibiotic.

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.

Electron partitioning between the two branching quinol-oxidizing pathways in Acanthamoeba castellanii mitochondria during steady-state state 3 respiration

Amoeba mitochondria possess a respiratory chain with two quinol-oxidizing pathways: the cytochrome pathway and the cyanide-resistant alternative oxidase pathway. The ADP/O method, based on the non-phosphorylating property of alternative oxidase, was used to determine contributions of both pathways in overall state 3 respiration in the presence of GMP (an activator of the alternative oxidase in amoeba) and succinate as oxidizable substrate. This method involves pair measurements of ADP/O ratios plus and minus benzohydroxamate (an inhibitor of the alternative oxidase). The requirements of the method are listed and verified. When overall state 3 respiration was decreased by increasing concentrations of n-butyl malonate (a non-penetrating inhibitor of succinate uptake), the quinone reduction level declined. At the same time, the alternative pathway contribution decreased sharply and became negligible when quinone redox state was lower than 50%, whereas the cytochrome pathway contribution first increased and then passed through a maximum at a quinone redox state of 58% and sharply decreased at a lower level of quinone reduction. This study is the first attempt to examine the steady-state kinetics of the two quinol-oxidizing pathways when both are active and to describe electron partitioning between them when the steady-state rate of the quinone-reducing pathway is varied.

ALTERNATIVE OXIDASE: From Gene to Function

Plants, some fungi, and protists contain a cyanide-resistant, alternative mitochondrial respiratory pathway. This pathway branches at the ubiquinone pool and consists of an alternative oxidase encoded by the nuclear gene Aox1. Alternative pathway respiration is only linked to proton translocation at Complex 1 (NADH dehydrogenase). Alternative oxidase expression is influenced by stress stimuli-cold, oxidative stress, pathogen attack-and by factors constricting electron flow through the cytochrome pathway of respiration. Control is exerted at the levels of gene expression and in response to the availability of carbon and reducing potential. Posttranslational control involves reversible covalent modification of the alternative oxidase and activation by specific carbon metabolites. This dynamic system of coarse and fine control may function to balance upstream respiratory carbon metabolism and downstream electron transport when these coupled processes become imbalanced as a result of changes in the supply of, or demand for, carbon, reducing power, and ATP.

Evidence for anion-translocating plant uncoupling mitochondrial protein in potato mitochondria

Transport properties of plant mitochondria from potato tubers were investigated using the swelling technique and membrane potential measurements. Proton-dependent swelling of fatty acid-depleted mitochondria in potassium acetate with valinomycin was possible only in the presence of fatty acids (linoleic acid and 12-(4-azido-2-nitrophenylamino)dodecanoic acid) and was inhibited by various purine nucleotides including ATP, GDP, and GTP. Swelling representing uptake of hexanesulfonate was also inhibited by purine nucleotides. Also, the membrane potential of fatty acid-depleted potato mitochondria energized by succinate declined upon the addition of linoleic acid or 12-(4-azido-2-nitrophenylamino)dodecanoic acid, and this decrease was prevented by ATP and other purine nucleotides. These transport activities are identical to those reported for brown adipose tissue mitochondria and related to the uncoupling protein; therefore, we ascribed them to the plant mitochondrial uncoupling protein (PUMP). A major difference between plant and mammalian uncoupling protein is that PUMP transports small hydrophilic anions such as Cl- very slowly, if at all. We suggest that PUMP may play an important role in plant physiology, where a regulated uncoupling and thermogenesis can proceed during fruit and seed development.

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.

Top-down control analysis of systems with more than one common intermediate

The analysis of the control of complex metabolic systems can be greatly simplified by application of the top-down approach of metabolic control analysis, in which the reactions of the system are grouped together into a small number of blocks connected by a common intermediate. The experimental application of the top-down approach has so far been limited to systems that have only a single intermediate. In this study, we demonstrate that the connectivity and summation theorems of metabolic control analysis hold with any number of intermediates between the metabolic blocks, and in doing so show that top-down analysis is valid for systems with multiple intermediates and so can be applied to most metabolic systems regardless of their complexity; an example of such an application is provided. Top-down control analysis has successfully described the control of mitochondrial respiration by dividing the system into three blocks, the respiratory chain, phosphorylation system and proton leak, all linked by a single intermediate, proton motive force. Here, we subdivide the respiratory chain into succinate consumers and cytochrome oxidase so that a second intermediate, cytochrome c redox state, is generated. Despite the fact that the redox state of cytochrome c is not measured, we solve the control over the system fluxes. In common with previous studies, we find that under conditions where there is no ATP turnover (state 4), respiration is largely controlled by proton leak, while at maximal ATP turnover (state 3) respiration is controlled by the respiratory chain and the phosphorylating system. In state 4,85% of the control by the respiratory chain resides with cytochrome oxidase. As ATP turnover increases, the respiration rate increases, and the control by the respiratory chain shifts from cytochrome oxidase to the succinate consumers, so that in state 3 83% of the control by the respiratory chain lies in the reactions between succinate and cytochrome c and only 17% resides with cytochrome oxidase.

Structure-function relationships of the alternative oxidase of plant mitochondria: a model of the active site

A major characteristic of plant mitochondria is the presence of a cyanide-insensitive alternative oxidase which catalyzes the reduction of oxygen to water. Current information on the properties of the oxidase is reviewed. Conserved amino acid motifs have been identified which suggest the presence of a hydroxo-bridged di-iron center in the active site of the alternative oxidase. On the basis of sequence comparison with other di-iron center proteins, a structural model for the active site of the alternative oxidase has been developed that has strong similarity to that of methane monoxygenase. Evidence is presented to suggest that the alternative oxidase of plant mitochondria is the newest member of the class II group of di-iron center proteins.