Dependence of the proton translocation stoichiometry of cyanobacterial and chloroplast H+-ATP synthase on the membrane composition

Chlorophyll a fluorescence and transcriptome reveal the toxicological effects of bisphenol A on an invasive cyanobacterium, Cylindrospermopsis raciborskii

Bisphenol A has attracted worldwide attention due to its harmful effects on humans, animals and plants. In this study, the toxicological effects of BPA on Cylindrospermopsis raciborskii were assessed based on chlorophyll a fluorescence and transcriptome analyses. The results showed that the growth of C. raciborskii was significantly inhibited when BPA exceeded 0.1 mg L^-1. A marked rise of phase J was observed at a concentration greater than 0.1 mg L^-1, while a K phase appeared at 20 mg L^-1. The chlorophyll a fluorescence parameters of RC/CS₀, F₀, φ_P0, φ_E0, and ψ₀, underwent a significant decline under all treatments of BPA, whereas a significant increase in both V_J and M₀ occurred under all concentrations of BPA. Additionally, ABS/RC and DIo/RC markedly increased at 10 mg L^-1 and 20 mg L^-1. The transcriptome analysis revealed that the genes of photosynthesis, including psbA, psbB, psbC, psbD, apcA, apcB, cpcA, and cpcB, as well as those of chlorophyll and carotenoid biosynthesis, namely hemN, acsF, chlL, chlN, chlP, crtB, pds, were all down-regulated. Moreover, BPA also inhibited the oxidative phosphorylation, glycolysis/gluconeogenesis, citrate cycle (TCA cycle), and fatty acid metabolism in C. raciborskii. Taken together, these results suggest BPA can negatively affect the expression of multiple genes and the vital energy metabolism process to arrest the growth and photosynthesis of C. raciborskii.

ATP synthase--past and future

This paper gives an overview of a lecture scheduled for the opening of the 10th European Bioenergetics Congress. In this lecture I plan to first reflect on the accomplishments of some of the individuals who were involved in research on the ATP synthase during the past 50 years. Then I will give a brief view of the present information about rotational catalysis by the ATP synthase. This will be followed by a discussion of some results from my laboratory that call for additional experimentation. Finally I will direct attention to other questions about the ATP synthase that should be addressed in future studies.

Energetic problems of extremely alkaliphilic aerobes

Over a decade of work on extremely alkaliphilic Bacillus species has clarified the extraordinary capacity that these bacteria have for regulating their cytoplasmic pH during growth at pH values well over 10. However, a variety of interesting energetic problems related to their Na(+)-dependent pH homeostatic mechanism are yet to be solved. They include: (1) the clarification of how cell surface layers play a role in a category of alkaliphiles for which this is the case; (2) identification of the putative, electrogenic Na+/H+ antiporter(s) that, in at least some alkaliphiles, may completely account for a cytoplasmic pH that is over 2 pH units lower than the external pH; (3) the determination of whether specific modules or accessory proteins are essential for the efficacy of such antiporters; (4) the mechanistic basis for the increase in the transmembrane electrical potential at the high external pH values at which the potential-consuming antiporter(s) must be most active; and (5) an explanation for the Na(+)-specificity of pH homeostasis in the extremely alkaliphilic bacilli as opposed to the almost equivalent efficacy of K+ for pH homeostasis in at least some non-alkaliphilic aerobes. The current status of such studies and future strategies will be outlined for this central area of alkaliphile energetics. Also considered, will be strategies to elucidate the basis for robust H(+)-coupled oxidative phosphorylation by alkaliphiles at pH values over 10. The maintenance of a cytoplasmic pH over 2 units below the high external pH results in a low bulk electrochemical proton gradient (delta p). To bypass this low delta p, Na(+)-coupling is used for solute uptake even by alkaliphiles that are mesophiles from environments that are not especially Na(+)-rich. This indicates that these bacteria indeed experience a low delta p, to which such coupling is an adaptation. Possible reasons and mechanisms for using a H(+)-coupled rather than a Na(+)-coupled ATP synthase under such circumstances will be discussed.

The H+/ATP coupling ratio of the ATP synthase from thiol-modulated chloroplasts and two cyanobacterial strains is four

In this paper the authors emphasise that the proton translocating ATP synthase from thiol-modulated chloroplasts and two cyanobacterial strains has a coupling ratio of 4 protons per ATP synthesised or hydrolysed. This ratio is determined by several thermodynamic studies at equilibrium between phosphate potential (Delta Gp) and proton gradient (Delta(mu)H+), and is confirmed by measurement of proton flux during ATP hydrolysis. Ratios lower than 4 H+/ATP that have been published in the past have predominantly been determined with the oxidised chloroplast enzyme. Errors in these measurements will be discussed.

ATP synthesis by the F0F1 ATP synthase from thermophilic Bacillus PS3 reconstituted into liposomes with bacteriorhodopsin. 2. Relationships between proton motive force and ATP synthesis

The correlation between the rate of ATP synthesis and light-induced proton flux was investigated in proteoliposomes reconstituted with bacteriorhodopsin and ATP synthase from thermophilic Bacillus PS3. By variation of the actinic light intensity it was found that ATP synthase activity depended in a sigmoidal manner on the amplitude of the transmembrane light-induced pH gradient. Maximal rates of ATP synthesis (up to to 200 nmol ATP x min(-1) x mg protein (-1) were obtained at saturating light intensities under a steady-state pH gradient of about pH 1.25. It was demonstrated that this was the maximal deltapH attainable at 40 degrees C in reconstituted proteoliposomes, due to the feedback inhibition of bacteriorhodopsin by the proton gradient it generates. In the absence of valinomycin, a small but significant transmembrane electrical potential could develop at 40 degrees C, contributing to an increase in the rate of ATP synthesis. The H+/ATP stoichiometry was measured at the static-head (equilibrium) conditions from the ratio of the phosphate potential to the size of the light-induced pH gradient and a value of about four was obtained under the maximal electrochemical proton gradient. Increasing the amount of bacteriorhodopsin in the proteoliposomes at a constant F0F1 concentration led to a large increase in the rate of ATP synthesis whereas the magnitude of delta pH remained the same or, at very high bacteriorhodopsin levels, decreased. Consequently the H+/ATP stoichiometry was found to increase significantly with increasing bacteriorhodopsin content. Reconstitutions with mixtures of native and impaired bacteriorhodopsin (Asp96-->Asn mutated bacteriorhodopsin) further demonstrated that this increase in the coupling efficiency could not be related to protein-protein interactions but rather to bacteriorhodopsin donating H+ to the ATP synthase. Increasing the amount of negatively charged phospholipids in the proteoliposomes also increased the coupling efficiency between bacteriorhodopsin and ATP synthase at a constant transmembrane pH gradient. Similar results were obtained with chloroplast ATP synthase. Furthermore, ATP synthase activities induced by delta pH/delta psi transitions were independent of bacteriorhodopsin or anionic lipid levels. These observations were interpreted as indicating that, in bacteriorhodopsin/ATP synthase, proteoliposomes, a localized pathway for coupling light-driven H+ transport by bacteriorhodopsin to ATP synthesis by F0F1 might exist under specific experimental conditions.

Metabolic modulation of transport coupling ratio in yeast plasma membrane H(+)-ATPase

The plasma membrane proton pump (H(+)-ATPase) of yeast energizes solute uptake by secondary transporters and regulates cytoplasmic pH. The addition of glucose to yeast cells stimulates proton efflux mediated by the H(+)- ATPase. A > 50-fold increase in proton extrusion from yeast cells is observed in vivo, whereas the ATPase activity of purified plasma membranes is increased maximally 8-fold after glucose treatment (Serrano, R. (1983) FEBS Lett. 156, 11-14). The low capacity of yeast cells for proton extrusion in the absence of glucose can be explained by the finding that, in H(+)-ATPase isolated from glucose-starved cells, ATP hydrolysis is essentially uncoupled from proton pumping. The number of protons transported per ATP hydrolyzed is significantly increased after glucose activation. We suggest that intrinsic uncoupling is an important mechanism for regulation of pump activity.

Reconstitution of energy-linked activities of the solubilized F1F0 ATP synthase from Bacillus subtilis

The F1F0 ATP synthases from wild-type Bacillus subtilis and an uncoupler-resistant mutant have comparable subunit structures. In accord with an earlier hypothesis, ATP hydrolysis and ATP-Pi exchange by the two synthases were equally stimulated and inhibited by protonophores, respectively, when reconstituted alone in either wild-type or mutant lipids.

Growth and bioenergetics of alkaliphilic Bacillus firmus OF4 in continuous culture at high pH

The effect of external pH on growth of alkaliphilic Bacillus firmus OF4 was studied in steady-state, pH-controlled cultures at various pH values. Generation times of 54 and 38 min were observed at external pH values of 7.5 and 10.6, respectively. At more alkaline pH values, generation times increased, reaching 690 min at pH 11.4; this was approximately the upper limit of pH for growth with doubling times below 12 h. Decreasing growth rates above pH 11 correlated with an apparent decrease in the ability to tightly regulate cytoplasmic pH and with the appearance of chains of cells. Whereas the cytoplasmic pH was maintained at pH 8.3 or below up to external pH values of 10.8, there was an increase up to pH 8.9 and 9.6 as the growth pH was increased to 11.2 and 11.4, respectively. Both the transmembrane electrical potential and the phosphorylation potential (delta Gp) generally increased over the total pH range, except for a modest fall-off in the delta Gp at pH 11.4. The capacity for pH homeostasis rather than that for oxidative phosphorylation first appeared to become limiting for growth at the high edge of the pH range. No cytoplasmic or membrane-associated organelles were observed at any growth pH, confirming earlier conclusions that structural sequestration of oxidative phosphorylation was not used to resolve the discordance between the total electrochemical proton gradient (delta p) and the delta Gp as the external pH is raised. Were a strictly bulk chemiosmotic coupling mechanism to account for oxidative phosphorylation over the entire range, the deltaGp/deltap ration (which would equal the H+/ATP ratio) would rise from about 3 at pH 7.5 to 13 at pH 11.2, dropping to 7 at pH 11.4 only because of the rise in cytoplasmic pH relative to other parameters. Moreover, the molar growth yields on malate were higher at pH 10.5 than at pH 7.5, indicating greater rather than lesser efficiency in the use of substrate at the more alkaline pH.

The proton pumping activity of H(+)-ATPases: an improved fluorescence assay

A new method for the estimation of steady-state delta pH, and the rate of acidification, by H(+)-ATPases (and other proton transporters) in inverted membrane vesicles is described. The method is based on a combination of two widely used fluorescent delta pH probes, 9-aminoacridine and 9-amino-6-chloro-2-methoxyacridine. It is demonstrated that 9-amino-6-chloro-2-methoxyacridine fluorescence quenching, which is very sensitive to small pH gradients, is not sensitive to the magnitude of large pH gradient, while 9-aminoacridine, which does not sense small gradients, is very sensitive to large pH gradients. A proper mixture of the two probes provides a method which is equally sensitive to pH gradients from very small values up to 3.5 pH units. The probe response was evaluated by titrations of the fluorescence signal with nigericin and adjusted by changing the concentration ratio and the emission wavelength. In liposomes, submitochondrial particles and bacterial vesicles an almost linear dependence of quenching on delta pH over the entire range can be obtained with this method. It is demonstrated that the new method can be used to obtain more reliable estimates of the rate of acidification as well as the magnitude of delta pH, whereas each of these and similar probes, by themselves are not as reliable. A determination of the ratio delta Gp/delta muH over a wide range of values reveal that this ratio is not constant but decreases with delta Gp. This finding should be taken into consideration when attempting to estimate the H+/ATP ratio form the measurement of delta Gp/delta muH.

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.

Evidence that localized energy coupling in thylakoids can continue beyond the energetic threshold onset into steady illumination

Energy transduction from proton gradients into ATP formation in chloroplast thylakoids has been hypothesized to be driven equally efficiently by localized domain delta mu H+ or by a delocalized delta mu H+ (Beard, W. A. and Dilley, R. A. (1988) J. Bioenerg. Biomembr. 20, 129-154). An important question is whether the apparent localized protonmotive force energy coupling mode can be observed only in the dark-to-light transient in the flash excitation protocol commonly used, or whether the localized energy coupling gradient can be maintained under conditions of continuous illumination ATP formation. The assay in the previous work was to use permeable amines, added to thylakoids in the dark, and observe the effect of the amine on the length of the energization lag (number of single-turnover flashes) required to initiate ATP formation in the dark-to-light transition. Amine buffers delayed the ATP onset in high-salt-stored membranes but did not delay the onset with low salt-stored membranes. This work tested whether permeable amines show the different effects in low- or high-salt-stored thylakoids which had attained a steady-state ATP formation rate (in continuous light) for 20-40 s prior to adding the amine. Hydroxyethylmorpholine was the preferred amine for such experiments, a suitable choice inasmuch as it behaves similarly to pyridine in the flash-induced ATP formation onset experiments, but it permeates more rapidly than pyridine and it has a higher pKa, which enhances its buffering effects. With high-salt-stored thylakoids, 0.5 or 1.0 mM hydroxyethylmorpholine added after 40 s of continuous illumination caused a marked, but transient, slowing of the ATP formation rate, but little or no slowing of the rate was observed with low-salt-stored thylakoids (at similar phosphorylation rates for the two thylakoid samples). Those data indicate that in continuous illumination conditions the proton gradient driving ATP formation in thylakoids from the low-salt-stored treatment did not equilibrate with the lumen, but in thylakoids stored in high-salt the delta mu H+ freely equilibrated with the lumen. That suggestion was supported by measurement of the luminal pH under coupling conditions by the [14C]methylamine distribution method using low- or high-salt-stored thylakoids. Further supportive evidence was obtained from measuring the effect of permeable amine buffers on H+ uptake under coupled and basal conditions with both types of thylakoid.(ABSTRACT TRUNCATED AT 400 WORDS)