Quantitative aspects of adenosine triphosphate-driven proton translocation in spinach chloroplast thylakoids

Intrinsic uncoupling in the ATP synthase of Escherichia coli

The ATP hydrolysis activity and proton pumping of the ATP synthase of Escherichia coli in isolated native membranes have been measured and compared as a function of ADP and Pi concentration. The ATP hydrolysis activity was inhibited by Pi with an half-maximal effect at 140 microM, which increased progressively up in the millimolar range when the ADP concentration was progressively decreased by increasing amounts of an ADP trap. In addition, the relative extent of this inhibition decreased with decreasing ADP. The half-maximal inhibition by ADP was found in the submicromolar range, and the extent of inhibition was enhanced by the presence of Pi. The parallel measurement of ATP hydrolysis activity and proton pumping indicated that, while the rate of ATP hydrolysis was decreased as a function of either ligand, the rate of proton pumping increased. The latter showed a biphasic response to the concentration of Pi, in which an inhibition followed the initial stimulation. Similarly as previously found for the ATP synthase from Rhodobacter caspulatus [P. Turina, D. Giovannini, F. Gubellini, B.A. Melandri, Physiological ligands ADP and Pi modulate the degree of intrinsic coupling in the ATP synthase of the photosynthetic bacterium Rhodobacter capsulatus, Biochemistry 43 (2004) 11126-11134], these data indicate that the E. coli ATP synthase can operate at different degrees of energetic coupling between hydrolysis and proton transport, which are modulated by ADP and Pi.

Physiological ligands ADP and Pi modulate the degree of intrinsic coupling in the ATP synthase of the photosynthetic bacterium Rhodobacter capsulatus

The proton-pumping and the ATP hydrolysis activities of the ATP synthase of Rhodobacter capsulatus have been compared as a function of the ADP and P(i) concentrations. The proton pumping was measured either with the transmembrane pH difference probe, 9-amino-6-chloro-2-methoxyacridine, or with the transmembrane electric potential difference probe, bis(3-propyl-5-oxoisoxazol-4-yl)pentamethine oxonol, obtaining consistent results. The comparison indicates that an intrinsic uncoupling of ATP synthase is induced when the concentration of either ligand is decreased. The half-maximal effect was found in the submicromolar range for ADP and at about 70 microM for P(i). It is proposed that a switch from a partially uncoupled state of ATP synthase to the coupled state is induced by the simultaneous binding of ADP and P(i).

Regulatory role of the C-terminus of the epsilon subunit from the chloroplast ATP synthase

The ATP synthases from chloroplasts and Escherichia coli are regulated by several factors, one of which is the epsilon subunit. This small subunit is also required for ATP synthesis. Thylakoid membranes reconstituted with CF1 lacking the epsilon subunit (CF1-epsilon) exhibit no ATP synthesis and very high ATP hydrolysis. Either native or recombinant epsilon restores ATP synthesis and inhibits ATP hydrolysis. Previously, we showed that truncated epsilon, lacking the last 45 C-terminal amino acids, restored ATP synthesis to membranes reconstituted with CF1-epsilon but was not an efficient inhibitor of ATP hydrolysis. In this paper, we show that this truncated epsilon is unable to inhibit ATP hydrolysis when Mg(2+) is the divalent cation present, both for the enzyme in solution and on the thylakoid membrane. In addition, the rate of reduction of the disulfide bond of the gamma subunit by dithiothreitol is not decreased by truncated epsilon, although full-length epsilon greatly impedes reduction. Thylakoid membranes can synthesize ATP at the expense of proton gradients generated by pH transitions in the dark. Our reconstituted membranes are able to produce a limited amount of ATP under these "acid-bath" conditions, with approximately equal amounts produced by the membranes containing wild-type epsilon and those containing truncated epsilon. However, the membranes containing truncated epsilon exhibit much higher background ATP hydrolysis under the same acid-bath conditions, leading to the conclusion that, without the C-terminus of epsilon, the CF1CFo is unable to check unwanted ATP hydrolysis.

Simultaneous measurement of deltapH and electron transport in chloroplast thylakoids by 9-aminoacridine fluorescence

Electron transport and the electrochemical proton gradient across the thylakoid membrane are two fundamental parameters of photosynthesis. A combination of the electron acceptor, ferricyanide and the DeltapH indicator, 9-aminoacridine, was used to measure simultaneously electron transport rates and DeltapH solely by changes in the fluorescence of 9-aminoacridine. This method yields values for the rate of electron transport that are comparable with those obtained by established methods. Using this method a relationship between the rate of electron transport and DeltapH at various uncoupler concentrations or light intensities was obtained. In addition, the method was used to study the effect of reducing the disulfide bridge in the gamma-subunit of the chloroplast ATP synthase on the relation of electron transport to DeltapH. When the ATP synthase is reduced and alkylated, the threshold DeltapH at which the ATP synthase becomes leaky to protons is lower compared with the oxidized enzyme. Proton flow through the enzyme at a lower DeltapH may be a key step in initiation of ATP synthesis in the reduced enzyme and may be the way by which reduction of the disulfide bridge in the gamma-subunit enables high rates of ATP synthesis at low DeltapH values.

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.

Detecting localized proton currents in photophosphorylation by procaine inhibition of the transthylakoid pH-gradient

The relationship between the transthylakoid pH-gradient, delta pH, and the velocity of photophosphorylation, Vp, in thylakoid membranes from spinach was investigated using the local anesthetic amine procaine as inhibitor of delta pH. When delta pH was driven by Photosystem (PS) II+I-dependent electron flow, passing through the cytochrome b6/f complex, inhibition by procaine was accompanied by an increase of ATP formation. It appeared that procaine allowed for values of Vp similar to those in controls (without procaine) at a significantly lower delta pH than in the controls. In contrast, when delta pH was driven by cyclic electron flow around PS I or by PS-II+I-dependent electron flow via a bypass around the cytochrome b6/f complex, or by PS II alone, procaine simultaneously caused an inhibition of delta pH and a decrease of ATP formation. Inhibition of delta pH by procaine did not induce an electrical membrane potential gradient that otherwise may have energetically compensated for the observed decline of delta pH. The electron flow capacity was unaffected by procaine. However, inhibition of delta pH did not significantly relax pH-dependent control of electron flux. Procaine accelerated ATP hydrolysis by pre-activated thylakoid ATPase to rates which were observed in the presence of uncouplers and had no direct effect on the activation state of the ATPase. The shift in the relationship between delta pH and Vp towards lower delta pH persisted in thermodynamic equilibrium between the phosphorylation potential and delta pH. The data indicated that the unconventional effect of procaine on photophosphorylation may be related to effects on proton translocation at the cytochrome b6/f complex and that a localized protonic coupling may occur between cytochrome b6/f and thylakoid-ATP-synthase complexes.

The proton channel, CF0, in thylakoid membranes. Only a low proportion of CF1-lacking CF0 is active with a high unit conductance (169 fS)

We investigated the conductance of pea thylakoid membranes and their capacity for photophosphorylation as function of the extraction of chloroplast coupling factor CF1. The degree of extraction was varied via the incubation time in EDTA-containing hypo-osmolar medium and was measured by rocket electroimmunodiffusion. The conductance of thylakoid membranes was measured by flash kinetic spectrophotometry. The time course of extraction followed the time course of thylakoid swelling. Contrary to expectation increasing loss of CF1 did not primarily increase the velocity of proton efflux from each vesicle. Instead proton-tight vesicles were converted to leaky ones, which lost phosphorylating activity. Two subpopulations occurred, although both types of vesicles, leaky and proton-tight ones, were CF1-depleted to a similar degree. This implied that only a small fraction of CF1-lacking CF0 was functional as a proton channel. Tight vesicles had no functional channels while leaky ones had at least one. We determined the proportion of tight vesicles in three independent ways: via the residual phosphorylation activity, via measurements of proton efflux and via measurements of the electric relaxation across the membrane. The results obtained were identical. A statistical evaluation of the data led us to the following conclusions. EDTA treatment produced vesicles containing approximately 10(5) chlorophyll molecules, equivalent to a total of approximately 100 CF0CF1 per vesicle. Even at the highest degree of extraction (75% of total CF1 extracted) only 2.5 out of 75 exposed CF0 per vesicle were proton-conducting. The unit conductance of one open CF0 channel was 169 +/- 18 fS at pH 7.5 and room temperature. At an electrical driving force of 100 mV this was equivalent to the passage of approximately 10(5) protons/s. The most important consequence of this relatively high unit conductance was that a single open CF0 channel was capable of dissipating the protonmotive force of one vesicle, thereby deactivating the whole remaining catalytic capacity of this vesicle.

Correlation between membrane-localized protons and flash-driven ATP formation in chloroplast thylakoids

Flash-driven ATP formation by spinach chloroplast thylakoids, using the luciferin luminescence assay to detect ATP formed in single turnover flashes, was studied under conditions where a membrane protein amine buffering pool was either protonated or deprotonated before the beginning of the flash trains. The flash number for the onset of ATP formation was delayed by about 10 flashes (from 15 to about 25) when the amine pool was deprotonated as compared to the protonated state. The delay was substantially reversed again by reprotonating the pool upon application of 20-30 single-turnover flashes and 8 min of dark before addition of ADP, Pi, and the luciferin system. In the case of deprotonation by desaspidin, the uncoupler was removed by binding to BSA before the reprotonating flashes were given. Reprotonation was carried out before addition of ADP and Pi, to avoid a possible interference by the ATP-ase, which can energize the system by pumping protons. The reprotonated state, as indicated by an onset lag of about 15 flashes rather than 25 for the deprotonated state, was stable in the dark over extended dark times. The number of protons released by 10 flashes is approximately 30 nmol H+ (mg chl)-1, an amount similar to the size of the reversibly protonated amine group buffering pool. The data are consistent with the hypothesis that the amine buffering groups must be in the protonated state before any protons proceed to the coupling complex and energize ATP formation. Other work has suggested that the amine buffering pool is sequestered within membrane proteins rather than being exposed directly to the inner aqueous bulk phase. Therefore, it is possible that the sequestered amine group array may provide localized association-dissociation sites for proton movement to the coupling complex.

A time lag in the onset of ATP-Pi exchange catalyzed by purified ATP-synthase (CF0-CF1) proteoliposomes and by chloroplasts

The time course of ATP-Pi exchange which is catalyzed by the isolated chloroplast ATP synthase in phospholipid vesicles was studied. The following observations were made. (i) The onset of 32Pi incorporation into ATP lags behind ATP hydrolysis. The lag lasts for about 2 min at 37 degrees C and is followed by a steady-state rate which is constant for more than 30 min. Under the same experimental conditions, ATP hydrolysis shows an initial burst followed by a constant, slower rate. (ii) The initial lag is independent of Mg-ATP concentration in the range 0.2-5 mM and of the presence of ADP. In contrast, the steady-state rate of ATP-Pi exchange has an apparent Km of 0.3 mM for Mg-ATP and is stimulated by ADP. (iii) Increasing the temperature from 30 to 45 degrees C decreases the lag from 6 min to zero. The steady-state rate of ATP-Pi exchange is affected to a much smaller extent by the temperature in this range. (iv) The lag is insensitive to valinomycin or tetraphenylboron, while the steady-state rate is partially inhibited. Nigericin and protonophores affect both the lag and steady-state rate. (v) ATP-induced membrane potential formation, as followed by oxonol VI, does not correlate with the lag in its kinetics and temperature dependence. ATP-induced pH gradient formation could not be detected in the proteoliposome system. (vi) Light-triggered ATP-Pi exchange in chloroplasts shows essentially the same time course as the proteoliposome system, but the lag lasts for only about 20 s at room temperature and is unaffected by a preexisting proton gradient. These results suggest that the initial lag in ATP-Pi exchange does not reflect the time required for the buildup of a protomotive force (delta - mu H+) nor the time required to produce ADP. It is suggested, therefore, that the lag reflects an internal autocatalytic conformational change in the ATP-synthase complex which is initiated by ATP hydrolysis and which converts the enzyme from an "exclusive ATPase state" to a "reversible ATP-synthase state". This slow transition is not directly coupled to a trans-membrane pH or potential gradient.

Relationship between phosphorylation potential and electrochemical H+ gradient during glycolysis in Streptococcus lactis

Assays of intracellular ATP, ADP, and inorganic phosphate allowed calculation of the phosphorylation potential (delta G'ATP/F) maintained during glycolysis by Streptococcus lactis. At the same time, the electrochemical H+ gradient (delta mu-H+/F) was evaluated by distribution methods, using radioactive tetraphenylphosphonium bromide as a probe for the membrane potential and salicylic acid as a probe for the pH gradient. Detailed comparisons were made at pH 5, when the reaction mediated by the proton-translocating ATPase (BF0F1) was likely to have been poised near equilibrium; for those conditions, the ratio delta G'ATP/delta mu-H+ was used to estimate stoichiometry for BF0F1 during ATP hydrolysis. At an external pH of 5, in the presence or absence of valinomycin, this ratio was close to 3, over a range of 370 to 510 mV (8.5 to 11.7 kcal/mol) for delta G'ATP/F and a range of 128 to 167 mV for delta mu-H+/F. Other work suggested that delta G'ATP/delta mu-H+ increased from its minimum value of 3 to 4.3 as the external pH changed from pH 5 to 7.

Modulation of the chloroplast ATPase by tight ADP binding. Effect of uncouplers and ATP

Inactivation of the membrane-bound ATPase by tight ADP binding was studied under nonenergized conditions. The energy state of the system was controlled either by omitting MgCl2, preventing ATP hydrolysis, or by addition of an uncoupler which dissipates the delta mu H+. In the absence of Mg2+, ATP prevents the inactivation of the enzyme by ADP, in a competitive manner. This effect of ATP resembles that of GDP with Mg2+ present. In the presence of nigericin, Mg2+, and ATP, inactivation occurs after a 10-15-sec interval, during which the enzyme is able to hydrolyze ATP at a relatively rapid rate. The degree of inactivation is proportional to the level of bound ADP detected. This behavior is different from that of the coupled ATPase (no uncoupler added), where inactivation is attained only upon exhaustion of the ATP by its hydrolysis, despite the finding that ADP binds tightly to the active ATPase at all stages of the reaction. Higher levels of tightly bound ADP were detected in the presence of an uncoupler. We suggest that the interval during which the enzyme becomes inactive is that required for the enzyme to generate and bind ADP, and to change from the active to the inactive conformation. These results support the mechanism suggested previously for the modulation of the ATPase by tight nucleotide binding.