A study on the membrane potential and pH gradient in chromatophores and intact cells of photosynthetic bacteria

Plant proton pumping pyrophosphatase: the potential for its pyrophosphate synthesis activity to modulate plant growth

Cellular pyrophosphate (PPi) homeostasis is vital for normal plant growth and development. Plant proton-pumping pyrophosphatases (H⁺ -PPases) are enzymes with different tissue-specific functions related to the regulation of PPi homeostasis. Enhanced expression of plant H⁺ -PPases increases biomass and yield in different crop species. Here, we emphasise emerging studies utilising heterologous expression in yeast and plant vacuole electrophysiology approaches, as well as phylogenetic relationships and structural analysis, to showcase that the H⁺ -PPases possess a PPi synthesis function. We postulate this synthase activity contributes to modulating and promoting plant growth both in H⁺ -PPase-engineered crops and in wild-type plants. We propose a model where the PPi synthase activity of H⁺ -PPases maintains the PPi pool when cells adopt PPi-dependent glycolysis during high energy demands and/or low oxygen environments. We conclude by proposing experiments to further investigate the H⁺ -PPase-mediated PPi synthase role in plant growth.

H+ -PPases: a tightly membrane-bound family

The earliest known H+-PPase (proton-pumping inorganic pyrophosphatase), the integrally membrane-bound H+-PPi synthase (proton-pumping inorganic pyrophosphate synthase) from Rhodospirillum rubrum, is still the only alternative to H+-ATP synthase in biological electron transport phosphorylation. Cloning of several higher plant vacuolar H+-PPase genes has led to the recognition that the corresponding proteins form a family of extremely similar proton-pumping enzymes. The bacterial H+-PPi synthase and two algal vacuolar H+-PPases are homologous with this family, as deduced from their cloned genes. The prokaryotic and algal homologues differ more than the H+-PPases from higher plants, facilitating recognition of functionally significant entities. Primary structures of H+-PPases are reviewed and compared with H+-ATPases and soluble PPases.

H+-proton-pumping inorganic pyrophosphatase: a tightly membrane-bound family

The earliest known H+-proton-pumping inorganic pyrophosphatase, the integrally membrane-bound H+-proton-pumping inorganic pyrophosphate synthase from Rhodospirillum rubrum, is still the only alternative to H+-ATP synthase in biological electron transport phosphorylation. Cloning of several higher plant vacuolar H+-proton-pumping inorganic pyrophosphatase genes has led to the recognition that the corresponding proteins form a family of extremely similar proton-pumping enzymes. The bacterial H+-proton-pumping inorganic pyrophosphate synthase and two algal vacuolar H+-proton-pumping inorganic pyrophosphatases are homologous with this family, as deduced from their cloned genes. The prokaryotic and algal homologues differ more than the H+-proton-pumping inorganic pyrophosphatases from higher plants, facilitating recognition of functionally significant entities. Primary structures of H+-proton-pumping inorganic pyrophosphatases are reviewed and compared with H+-ATPases and soluble proton-pumping inorganic pyrophosphatases.

Effect of membrane surface potential on the uptake of anionic compounds by liposomes

The effect of membrane surface potential on the uptake of several anionic compounds by liposomes (large unilamellar vesicles), which contain various amounts of dipalmitoylphosphatidylserine (DPPS), was investigated. The uptake amount of four tested anionic compounds (cefixime, benzyloxyindoleacetic acid (BOIAA), ceftibuten and S-1006) decreased with an increase in the DPPS content of liposomes, and was correlated with the membrane surface potential monitored using a fluorescent dye, 8-anilino-1-naphthalene sulfonate (ANS). Moreover, for all of the tested anionic compounds, a good correlation was observed between the ratio of the uptake value (5 min) by each of the liposomes comprising various amounts of DPPS to the uptake value by liposomes containing 10% DPPS and a relative membrane surface potential monitored by ANS. On the other hand, the uptake of zwitterionic compounds (enoxacin, cephradine and benzyloxytryptophan (BOTP)) was independent of DPPS content. These results suggest that the uptake of tested anionic compounds by large unilamellar lipid vesicles is dependent on the membrane surface potential which originates in the surface negative charge.

Comparison of the contribution from different energy-linked reactions to the function of a membrane potential in photosynthetic bacteria

The steady-state membrane potentials generated by light, PP(i), ATP or the reverse transhydrogenase reaction were studied in chromatophores from two different phototrophic bacteria, Rhodospirillum rubrum and Rhodopseudomonas viridis. The membrane potentials generated by the different energy-linked reactions were evaluated by a tetraphenylboron(TPB(-)) ion-selective electrode. The generated by light was estimated to be 110 mV and 50 mV in R. rubrum and Rps. viridis chromatophores, respectively. In the dark, PP (i), ATP and reversed transhydrogenase generated membrane potentials in R. rubrum and Rps. viridis chromatophores 50, 60 and 35 mV, and 14, 35 and 25 mV,respectively. The effect of magnesium ion on the membrane potential generated by different energy-linked reactions was also studied. The induced by different energy-generating reactions in R. rubrum and Rps. viridis chromatophores and the possible relationship to the chromatophore structures are discussed.

Orthophosphate-pyrophosphate exchange catalyzed by soluble and membrane-bound inorganic pyrophosphatases. Role of H+ gradient

A comparative study of the orthophosphate-pyrophosphate exchange reaction catalyzed by the soluble pyrophosphatase from baker's yeast and by the membrane-bound pyrophosphatase of Rhodospirillum rubrum chromatophores was performed. In both systems the rate of exchange increased when the pH of the medium was raised from 6.0 to 7.8 and when the MgCl2 concentration was raised from 0.1 mM to 20 mM. For the yeast pyrophosphatase the exchange rates measured at different pH values and in the presence of 6.7 to 8.8 mM free Mg2+ superimposed as a single curve when plotted as a function of the concentrations of either HPO4(2-) or MgHPO4. This was not observed with the use of R. rubrum chromatophores. With yeast pyrophosphatase, the Km for Pi was higher than 10 mM and could not be measured when the free Mg2+ concentration in the medium was lower than 0.5 mM. There was a decrease in the Km for Pi when the free Mg2+ concentration was raised to 6.7-8.8 mM or when, in the presence of low free Mg2+, the organic solvents dimethylsulfoxide (20% v/v) or ethyleneglycol (40% v/v) were included in the assay medium. In the presence of 6.7-8.8 mM free Mg2+ the Km for total Pi was 7 mM at pH 7.0 and 12 mM at pH 7.8. For the ionic species HPO4(2-) and MgHPO4, the Km values were 5.8 mM and 4.2 mM respectively. In the presence of 0.24-0.42 mM free Mg2+ and either 20% (v/v) dimethylsulfoxide or 40% (v/v) ethyleneglycol the Km values for total Pi, HPO4(2-) and MgHPO4 were 7.6, 3.5 and 0.5 mM respectively. With R. rubrum chromatophores, the Km for Pi in the presence of 5.5-7.5 mM free Mg2+ was very high and could not be measured. In the presence of 0.24-0.45 mM free Mg2+ the ratio between the velocities of hydrolysis and synthesis of pyrophosphate measured at pH 7.8 with yeast pyrophosphatase and chromatophores of R. rubrum were practically the same. When the free Mg2+ concentration was raised to 5.5-8.8 mM this ratio decreased from 1028 to 540 when the yeast pyrophosphatase was used and from 754 to 46 when chromatophores were used.

Synthesis of pyrophosphate by chromatophores of Rhodospirillum rubrum in the light and by soluble yeast inorganic pyrophosphatase in water-organic solvent mixtures

Chromatophores of Rhodospirillum rubrum contain a membrane-bound pyrophosphatase that synthesizes pyrophosphate when an electrochemical H+ gradient is formed across the chromatophore membrane upon illumination. In this report it is shown that MgCl2 and Pi have different effects on the synthesis of pyrophosphate in the light depending on whether initial velocities or steady-state levels are examined. When the water activity of the medium is reduced by the addition of organic solvents, soluble yeast inorganic pyrophosphatase (no H+ gradient present) synthesizes pyrophosphate in amounts similar to those synthesized by the chromatophores in totally aqueous medium during illumination, (H+ gradient present). The pH, MgCl2 and Pi dependence for the synthesis of pyrophosphate by the chromatophores at steady-state is similar to that observed at equilibrium with the soluble enzyme in the presence of organic solvents. The possibility is raised that a decrease in water activity may play a role in the mechanism by which the energy derived from the electrochemical H+ gradient is used for the synthesis of pyrophosphate in chromatophores of R. rubrum.

Inorganic pyrophosphate-driven ATP-synthesis in liposomes containing membrane-bound inorganic pyrophosphatase and F0-F1 complex from Rhodospirillum rubrum

PPi driven ATP synthesis has been reconstituted in a liposomal system containing the membrane-bound energy-linked PPiase and coupling factor complex, both highly purified from Rhodospirillum rubrum. This energy converting model system was made by mixing both enzyme preparations with an aqueous suspension of sonicated soybean phospholipids and subjecting to a freeze-thaw procedure. In the presence of ADP, Mg2+, Pi and PPi the system catalyzed phosphorylation by up to 25 nmol ATP formed X mg protein-1 X min-1, at 20 degrees C, which was sensitive to uncouplers and inhibitors of phosphorylation such as oligomycin, efrapeptin and N,N'-dicyclohexylcarbodiimide.

Mg(2+)-Dependent, cation-stimulated inorganic pyrophosphatase associated with vacuoles isolated from storage roots of red beet (Beta vulgaris L.)

Vacuoles isolated from storage roots of red beet (Beta vulgaris L.) posess a Mg(2+)-dependent, alkaline pyrophosphatase (PPase) activity which is further stimulated by salts of monovalent cations. The requirement for Mg(2+) is specific. Mn(2+) and Zn(2+) permitted only 20% and 12%, respectively, of the PPase activity obtained in the presence of Mg(2+) while Ca(2+), Co(2+) and Cu(2+) were ineffective. Stimulation of Mg(2+)-PPase activity by salts of certain monovalent cations was due to the cation and the order of effectiveness of the cations tested was K(+)=Rb(+)=NH 4 (+) >Cs(+). Salts of Li(+) and Na(+) inhibited Mg(2+)-PPase activity by 44% and 24%, respectively. KCl-stimulation of Mg(2+)-PPase activity was maximal with 60-100 mM KCl. There was a sigmoidal relationship between PPase activity and Mg(2+) concentrations which resulted in markedly non-linear Lineweaver-Burk plots. At pH 8.0, the optimal [Mg(2+)]:[PPi] ratio for both Mg(2+)-PPase and (Mg(2+)+KCl)-PPase activities was approximately 1:1, which probably indicates MgP2O7 (2-) is the true substrate.

Fast stages of photoelectric processes in biological membranes. III. Bacterial photosynthetic redox system

Chromatophores of photosynthetic bacteria Rhodospirillum rubrum, Rhodopseudomonas sphaeroides and Chromatium minutissimum were associated with a collodion film impregnated with a decane solution of asolectin. A very short light flash inducing a single turnover of the chromatophore photosynthetic redox system was found to induce the formation of an electrical potential difference amounting to 60 mV, directed across the film as measured with an orthodox electrometer technique. The main phase of the photoelectric response had a tau value of less than 200 ns. Addition of menadione and some other redox mediators increases the main phase amplitude and induces a slower phase (tau = 200 microseconds). In Ch. minutissimum chromatophores that retained their endogenous cytochrome c pool, one more electrogenic phase was revealed (tau = 20 microseconds). Redox titrations of the electric response and bacteriochlorophyll absorption at 430 nm as well as measurements of the kinetics of cytochrome c oxidation have indicated that the fastest electrogenic phase is due to electron transfer from bacteriochlorophyll to Fe-ubiquinone, the 20-microseconds phase to cytochrome c2+ - bacteriochlorophyll+ oxidoreduction, and the 200-microseconds phase to Fe-ubiquinone- oxidation by a secondary quinone. In the decay of the photoelectric response, a 30-ms phase was identified which was explained by a reverse electron transfer from reduced Fe-ubiquinone to oxidated bacteriochlorophyll. The difference in the fast kinetics of photoelectric generation by the bacteriochlorophyll system from those by bacterial and animal rhodopsins has been discussed.

The carotenoid shift in Rhodopseudomonas sphaeroides. The flash induced change

A mutant, Rhodopseudomonas sphaeroides GIC, having only one major carotenoid, neurosporene, is described. The spectrum of the carotenoid shift in this mutant is analysed and it is concluded that only 7-11% of the pigment is involved under conditions of steady-state illumination and that this pigment undergoes a shift of 7 nm. The spectrum of the carotenoid shift under conditions of multi-flash illumination is examined for changes in shape concordant with a progressive red shift of the pigment with increasing membrane potential; the spectra of the fast change after each of three flashes does not agree well with predictions from a model involving a progressive shift of the pigment, the slow change shows qualitative agreement with such a model but the small size of the signal and the presence of more than one phase makes analysis of this phase more difficult. No separate pool of carotenoid, that might correspond to that postulated to participate in the carotenoid shift, could be identified by fourth derivative analysis of, or curve fitting to, the spectrum of the neurosporene.

ATP-dependent proton translocation and quenching of 9-aminoacridine fluorescence in inside-out membrane vesicles of a cytochrome-deficient mutant of Escherichia coli,

1. ATP-dependent proton translocation and ATP-dependent quenching of the fluorescence of 9-aminoacridine were measured in inside-out vesicles derived from a cytochrome-deficient mutant of Escherichia coli. 2. ATP-dependent quenching of fluorescence was inhibited by nigericin gramicidin, NH4Cl, and carbonylcyanide-m-chlorophenylhydrazone. Inhibition was also produced by the ATPase inhibitors N,N'-dicyclohexylcarbodimide (DCCD) and diphenyl phosphorazidate (DPA), and by the respiratory chain inhibitors piericidin A, 2-heptyl-4-hydroxyquinoline N-oxide, and An2+. The inhibition of ATP-dependent fluorescence quenching by the ionophores, uncouplers, and respiratory chain inhibitors was not due to an effect on ATPase activity which was insensitive to these agents. 3. By use of the ATPase inhibitors DCCD and DPA, or by replacing ATP with GTP, ITP and CTP, a correlation between the ATPase activity and the rate of ATP-dependent membrane energization, as measured by fluorescence quenching, was obtained.

Effect of inhibitors on the substrate-dependent quenching of 9-aminoacridine fluorescence in inside-out membrane vesicles of Escherichia coli

The effect of various inhibitors on the substrate-dependent quenching of the fluorescence of 9-aminoacridine was measured in inside-out membrane vesicles of Escherichia coli. The rate of fluorescence quenching in the presence of inhibitors was dependent on the rate of electron transfer through the respiratory chain with NADH, succinate, D-lactate or DL-glycerol 3-phosphate as substrates. Several patterns of response were given by the inhibitors. Inhibitors competitive with substrate, or those acting only on the dehydrogenases, gave a direct relationship between the extent of inhibition of oxidase activity and the rate of quenching. A biphasic relationship was given by 2-heptyl-4-hydroxyquinoline N-oxide and piericidin A which was due to these compounds acting both as inhibitors of the respiratory chain and, at higher concentrations, as uncoupling agents. Uncouplers inhibited fluorescence quenching with minimal inhibition of oxidase activity. The transmembrane pH difference was calculated from the extent of fluorescence quenching and the intravesicular volume. The maximum pH difference of 3.3--3.7 units was generated by each of the substrates tested.

Measurement of transmembrane potentials in Rhodospirillum rubrum chromatophores with an oxacarbocyanine dye

The fluorescent dye 3,3-dipentyloxacarbocyanine (OCC) can be used as a fluorescence probe to measure transmembrane potentials across Rhodospirillum rubrum chromatophore membranes. A reversible fluorescence increase is observed in the light which is sensitive to inhibitors, permeable ions and uncouplers. Partial interchangeability between the electrical potential and the proton concentration gradient has been demonstrated by measurement of the fluorescence increase with OCC and the fluorescence quenching with 9-aminoacridine. OCC fluorescence changes can be induced also in the dark by injection of permeable salts and by rapid pH changes presumably indicating diffusion potentials. Using salt-induced diffusion potentials for calibrating the light signals and with several assumptions, the light-induced potentials were estimated as 170 mV for the maximal signal and 90-110 mV at the steady state. OCC has been shown to apparently increase the electrical conductivity of the chromatophore membrane, a fact which may be relevant to the mechanism of action of this probe. A red shift in the OCC absorption spectrum occurs when mixed with chromatophores, with a difference spectrum maximum at 495 nm. The absorption changes at 495 nm taking place in the light are similar in kinetics to the fluorescence changes. The absorbance spectrum of OCC in organic solvents is red shifted and the extent of the shift depends on the hydrophobicity of the medium. The difference spectrum compared to water in sec-butyl acetate/n-hexane (3 : 1, v/v) with a dipole moment of 5 was nearly identical to that of chromatophore-associated dye. The uncoupling properties of OCC at high concentrations and some difficulties in calibration limit the usefulness of this probe for quantitative measurements of transmembrane potentials.