The rate of ATP synthesis by reconstituted CF0F1 liposomes

ATP synthase FOF1 structure, function, and structure-based drug design

ATP synthases are unique rotatory molecular machines that supply biochemical reactions with adenosine triphosphate (ATP)-the universal "currency", which cells use for synthesis of vital molecules and sustaining life. ATP synthases of F-type (FOF1) are found embedded in bacterial cellular membrane, in thylakoid membranes of chloroplasts, and in mitochondrial inner membranes in eukaryotes. The main functions of ATP synthases are control of the ATP synthesis and transmembrane potential. Although the key subunits of the enzyme remain highly conserved, subunit composition and structural organization of ATP synthases and their assemblies are significantly different. In addition, there are hypotheses that the enzyme might be involved in the formation of the mitochondrial permeability transition pore and play a role in regulation of the cell death processes. Dysfunctions of this enzyme lead to numerous severe disorders with high fatality levels. In our review, we focus on FOF1-structure-based approach towards development of new therapies by using FOF1 structural features inherited by the representatives of this enzyme family from different taxonomy groups. We analyzed and systematized the most relevant information about the structural organization of FOF1 to discuss how this approach might help in the development of new therapies targeting ATP synthases and design tools for cellular bioenergetics control.

Membranes, energetics, and evolution across the prokaryote-eukaryote divide

The evolution of the eukaryotic cell marked a profound moment in Earth’s history, with most of the visible biota coming to rely on intracellular membrane-bound organelles. It has been suggested that this evolutionary transition was critically dependent on the movement of ATP synthesis from the cell surface to mitochondrial membranes and the resultant boost to the energetic capacity of eukaryotic cells. However, contrary to this hypothesis, numerous lines of evidence suggest that eukaryotes are no more bioenergetically efficient than prokaryotes. Thus, although the origin of the mitochondrion was a key event in evolutionary history, there is no reason to think membrane bioenergetics played a direct, causal role in the transition from prokaryotes to eukaryotes and the subsequent explosive diversification of cellular and organismal complexity.

Over time, life on Earth has evolved into three large groups: archaea, bacteria, and eukaryotes. The most familiar forms of life – such as fungi, plants and animals – all belong to the eukaryotes. Bacteria and archaea are simpler, single-celled organisms and are collectively referred to as prokaryotes.

The hallmark feature that distinguishes eukaryotes from prokaryotes is that eukaryotic cells contain compartments called organelles that are surrounded by membranes. Each organelle supports different activities in the cell. Mitochondria, for example, are organelles that provide eukaryotes with most of their energy by producing energy-rich molecules called ATP. Prokaryotes lack mitochondria and instead produce their ATP on their cell surface membrane.

Some researchers have suggested that mitochondria might actually be one of the reasons that eukaryotic cells are typically larger than prokaryotes and more varied in their shape and structure. The thinking is that producing ATP on dedicated membranes inside the cell, rather than on the cell surface, boosted the amount of energy available to eukaryotic cells and allowed them to diversify more. However, other researchers are not convinced by this view. Moreover, some recent evidence suggested that eukaryotes are no more efficient in producing energy than prokaryotes.

Lynch and Marinov have now used computational and comparative analysis to compare the energy efficiency of different organisms including prokaryotes and eukaryotes grown under defined conditions. To do the comparison, the results were scaled based on cell volume and the total surface area deployed in energy production.

From their findings, Lynch and Marinov concluded that mitochondria did not enhance how much energy eukaryotes could produce per unit of cell volume in any substantial way. Although the origin of mitochondria was certainly a key event in evolutionary history, it is unlikely to have been responsible for the diversity and complexity of today’s life forms.

Thermodynamics of proton transport coupled ATP synthesis

The thermodynamic H(+)/ATP ratio of the H(+)-ATP synthase from chloroplasts was measured in proteoliposomes after energization of the membrane by an acid base transition (Turina et al. 2003 [13], 418-422). The method is discussed, and all published data obtained with this system are combined and analyzed as a single dataset. This meta-analysis led to the following results. 1) At equilibrium, the transmembrane ΔpH is energetically equivalent to the transmembrane electric potential difference. 2) The standard free energy for ATP synthesis (reference reaction) is ΔG°(ref)=33.8±1.3kJ/mol. 3) The thermodynamic H(+)/ATP ratio, as obtained from the shift of the ATP synthesis equilibrium induced by changing the transmembrane ΔpH (varying either pH(in) or pH(out)) is 4.0±0.1. The structural H(+)/ATP ratio, calculated from the ratio of proton binding sites on the c-subunit-ring in F(0) to the catalytic nucleotide binding sites on the β-subunits in F(1), is c/β=14/3=4.7. We infer that the energy of 0.7 protons per ATP that flow through the enzyme, but do not contribute to shifting the ATP/(ADP·Pi) ratio, is used for additional processes within the enzyme, such as activation, and/or energy dissipation, due e.g. to internal uncoupling. The ratio between the thermodynamic and the structural H(+)/ATP values is 0.85, and we conclude that this value represents the efficiency of the chemiosmotic energy conversion within the chloroplast H(+)-ATP synthase.

Comparison of the H+/ATP ratios of the H+-ATP synthases from yeast and from chloroplast

F(0)F(1)-ATP synthases use the free energy derived from a transmembrane proton transport to synthesize ATP from ADP and inorganic phosphate. The number of protons translocated per ATP (H(+)/ATP ratio) is an important parameter for the mechanism of the enzyme and for energy transduction in cells. Current models of rotational catalysis predict that the H(+)/ATP ratio is identical to the stoichiometric ratio of c-subunits to β-subunits. We measured in parallel the H(+)/ATP ratios at equilibrium of purified F(0)F(1)s from yeast mitochondria (c/β = 3.3) and from spinach chloroplasts (c/β = 4.7). The isolated enzymes were reconstituted into liposomes and, after energization of the proteoliposomes with acid-base transitions, the initial rates of ATP synthesis and hydrolysis were measured as a function of ΔpH. The equilibrium ΔpH was obtained by interpolation, and from its dependency on the stoichiometric ratio, [ATP]/([ADP]·[P(i)]), finally the thermodynamic H(+)/ATP ratios were obtained: 2.9 ± 0.2 for the mitochondrial enzyme and 3.9 ± 0.3 for the chloroplast enzyme. The data show that the thermodynamic H(+)/ATP ratio depends on the stoichiometry of the c-subunit, although it is not identical to the c/β ratio.

Proton transport coupled ATP synthesis by the purified yeast H+ -ATP synthase in proteoliposomes

The H(+)/ATP synthase from yeast mitochondria, MF₀F₁, was purified and reconstituted into liposomes prepared from phosphatidylcholine and phosphatidic acid. Analysis by mass spectrometry revealed the presence of all subunits of the yeast enzyme with the exception of the K-subunit. The MF₀F₁ liposomes were energized by acid-base transitions (DeltapH) and a K(+)/valinomycin diffusion potential (Deltaphi). ATP synthesis was completely abolished by the addition of uncouplers as well as by the inhibitor oligomycin. The rate of ATP synthesis was optimized as a function of various parameters and reached a maximum value (turnover number) of 120s⁻¹ at a transmembrane pH difference of 3.2 units (at pH(in)=4.8 and pH(out)=8.0) and a Deltaphi of 133mV (Nernst potential). Functional studies showed that the monomeric MF₀F₁, was fully active in ATP synthesis. The turnover increased in a sigmoidal way with increasing internal and decreasing external proton concentration. The dependence of the turnover on the phosphate concentration and the dependence of K(M) on pH(out) indicated that the substrate for ATP synthesis is the monoanionic phosphate species H₂PO⁻₄.

Characterization of the nature of photosynthetic recovery of wheat seedlings from short-term dark heat exposures and analysis of the mode of acclimation to different light intensities

The nature of photosynthetic recovery was investigated in 10-d-old wheat (Triticum aestivum L., cv. Moskovskaya-35) seedlings exposed to temperatures of 40 and 42 degrees C for 20 min and to temperature 42 degrees C for 40 min in the dark. The aftereffect of heat treatment was monitored by growing the heat-treated plants in low/moderate/high light at 20 degrees C for 72h. The net photosynthetic rates (P(N)) and the fluorescence ratios F(v)/F(m) were evaluated in intact primary leaves and the rates of cyclic and non-cyclic photophosphorylation were measured in the isolated thylakoids. At least two temporally separated steps were identified in the path of recovery from heat stress at 40 and 42 degrees C in the plants growing in high and moderate/high light, respectively. Both photochemical activity of the photosystem II (PSII) and the activity of CO(2) assimilation system were lowered during the first step in comparison with the corresponding activities immediately after heat treatment. During the second step, the photosynthetic activities completely or partly recovered. Recovery from heat stress at 40 degrees C was accompanied by an appreciably higher rate of cyclic photophosphorylation in comparison with control non-heated seedlings. In pre-heated seedlings, the tolerance of the PSII to photoinhibition was higher than in non-treated ones. The mode of acclimation to different light intensities after heat exposures is analyzed.

The H+-ATPase from chloroplasts: effect of different reconstitution procedures on ATP synthesis activity and on phosphate dependence of ATP synthesis

The H+-ATP synthase from chloroplasts, CF0F1, was isolated, reconstituted into liposomes and ATP synthesis activity was measured after energization of the proteoliposomes with an acid-base transition. The ATP yield was measured as a function of the reaction time after energization, the data were fitted by an exponential function and the initial rate was calculated from the fit parameters. CF0F1 was reconstituted by detergent dialysis in asolectin liposomes and phosphatidylcholine/phosphatidic acid (PtdCho/PtdAc from egg yolk) liposomes. In asolectin liposomes, high initial rates of ATP synthesis (up to 400 s(-1)) were observed with a rapid decline of the rate; in PtdCho/PtdAc liposomes the initial rate is smaller (up to 200 s(-1)), but the decline of the activity is slower. CF0F1 was reconstituted into PtdCho/PtdAc liposomes either by detergent dialysis or into reverse phase liposomes. The dependence of the rate of ATP synthesis on the phosphate concentration was measured with both types of proteoliposomes. The data can be described by Michaelis-Menten kinetics with a K(M) value of 350 microM for reverse phase liposomes and a K(M) value of 970 microM for dialysis liposomes. Both K(M) values depend neither on the magnitude of DeltapH nor on the electric potential difference, whereas V(max) decreases strongly with decreasing energization. At low phosphate concentration, there are small deviations from Michaelis-Menten kinetics. The measured rates are higher than those calculated from the fitted Michaelis-Menten parameters. This effect is interpreted as evidence that more than one phosphate binding site is involved in ATP synthesis.

Covalent modification of the non-catalytic sites of the H(+)-ATPase from chloroplasts with 2-azido-[alpha-(32)P]ATP and its effect on ATP synthesis and ATP hydrolysis

Incubation of the isolated H(+)-ATPase from chloroplasts, CF(0)F(1), with 2-azido-[alpha-(32)P]ATP leads to the binding of this nucleotide to different sites. These sites were identified after removal of free nucleotides, UV-irradiation and trypsin treatment by separation of the tryptic peptides by ion exchange chromatography. The nitreno-AMP, nitreno-ADP and nitreno-ATP peptides were further separated on a reversed phase column, the main fractions were subjected to amino acid sequence analysis and the derivatized tyrosines were used to distinguish between catalytic (beta-Tyr362) and non-catalytic (beta-Tyr385) sites. Several incubation procedures were developed which allow a selective occupation of each of the three non-catalytic sites. The non-catalytic site with the highest dissociation constant (site 6) becomes half maximally filled at 50 microM 2-azido-[alpha-(32)P]ATP, that with the intermediate dissociation constant (site 5) at 2 microM. The ATP at the site with the lowest dissociation constant had to be hydrolyzed first to ADP before a replacement by 2-azido-[alpha-(32)P]ATP was possible. CF(0)F(1) with non-covalently bound 2-azido-[alpha-(32)P]ATP and after covalent derivatization was reconstituted into liposomes and the rates of ATP synthesis as well as ATP hydrolysis were measured after energization of the proteoliposomes by Delta pH/Delta phi. Non-covalent binding of 2-azido-ATP to any of the three non-catalytic sites does not influence ATP synthesis and ATP hydrolysis, whereas covalent derivatization of any of the three sites inhibits both, the degree being proportional to the degree of derivatization. Extrapolation to complete inhibition indicates that derivatization of one site (either 4 or 5 or 6) is sufficient to block completely multi-site catalysis. The rates of ATP synthesis and ATP hydrolysis were measured as a function of the ADP and ATP concentration from uni-site to multi-site conditions with covalently derivatized and non-derivatized CF(0)F(1). Uni-site ATP synthesis and ATP hydrolysis were not inhibited by covalent derivatization of any of the non-catalytic sites, whereas multi-site catalysis is inhibited. These results indicate that multi-site catalysis requires some flexibility between beta- and alpha-subunits which is abolished by covalent derivatization of beta-Tyr385 with a 2-nitreno-adenine nucleotide. Conformational changes connected with energy transduction between the F(0)-part and the F(1)-part are either not required for uni-site ATP synthesis or they are not impaired by the derivatization of any of the three beta-Tyr385.

Covalent modification of the catalytic sites of the H(+)-ATPase from chloroplasts, CF(0)F(1), with 2-azido-[alpha-(32)P]ADP: modification of the catalytic site 2 (loose) and the catalytic site 3 (open) impairs multi-site, but not uni-site catalysis of both ATP synthesis and ATP hydrolysis

The H(+)-ATPase from chloroplasts, CF(0)F(1), was isolated and purified. The enzyme contained one endogenous ADP at a catalytic site, and two endogenous ATP at non-catalytic sites. Incubation with 2-azido-[alpha-(32)P]AD(T)P leads to a tight binding of the azido-nucleotides. Free nucleotides were removed by three consecutive passages through centrifugation columns, and after UV-irradiation, the label was covalently bound. The labelled enzyme was digested by trypsin, the peptides were separated by ion exchange chromatography into nitreno-AMP, nitreno-ADP and nitreno-ATP labelled peptides, and these were then separated by reversed phase chromatography. Amino acid sequence analysis was used to identify the type of the nucleotide binding site. After incubation with 2-azido-[alpha-(32)P]ADP, the covalently bound label was found exclusively at beta-Tyr-362, i.e. binding occurs only to catalytic sites. Incubation conditions with 2-azido-[alpha-(32)P]ADP were varied, and conditions were found which allow selective binding of the label to different catalytic sites, either to catalytic site 2 or to catalytic site 3. For measurements of the degree of inhibition by covalent modification, CF(0)F(1) was reconstituted into phosphatidylcholine liposomes, and the membranes were energised by an acid-base transition in the presence of a K(+)/valinomycin diffusion potential. The rate of ATP synthesis was 120 s(-1), and the rate of ATP hydrolysis was 20 s(-1), both measured under multi-site conditions. Covalent modification of either catalytic site 2 or catalytic site 3 inhibited both ATP synthesis and ATP hydrolysis, the degree of inhibition being proportional to the degree of modification. Extrapolation to complete inhibition indicates that modification of one catalytic site, either site 2 or site 3, is sufficient to completely block multi-site ATP synthesis and ATP hydrolysis. The rate of ATP synthesis and the rate of ATP hydrolysis were measured as a function of the substrate concentration from multi-site to uni-site conditions with covalently modified CF(0)F(1) and with non-modified CF(0)F(1). The result was that uni-site ATP synthesis and ATP hydrolysis were not inhibited by covalent modification of either catalytic site 2 or site 3. The results indicate cooperative interactions between catalytic nucleotide binding sites during multi-site catalysis, whereas neither uni-site ATP synthesis nor uni-site ATP hydrolysis require interaction with other sites.

Detergent effect on anion exchange perfusion chromatography and gel filtration of intact chloroplast H(+)-ATP synthase

To gain a pure enzyme preparation for functional and crystallization studies, an additional purification step in the isolation of the chloroplast ATP synthase (CF(0)F(1)) has been introduced. By applying gel filtration or anion exchange perfusion chromatography in presence of the detergents CHAPS and n-dodecyl-beta-d-maltoside, respectively, Rubisco and other contaminants were separated from CF(0)F(1). The purity and activity depended on the chromatographic method and the detergent employed. The highest purity and activity were achieved by anion exchange chromatography for the detergent dodecyl-maltoside and by gel filtration for the detergent CHAPS. The detergent Triton X-100, which is frequently used to solubilize CF(0)F(1), was found to be inadequate to stabilize the ATP synthase during chromatography.

Direct indication for the existence of a double stalk in CF0F1

The H+-ATPase from chloroplasts (CF0F1) was investigated by electron microscopy of negatively stained single molecules followed by image processing. The analysis of about 4700 particles from 72 micrographs gave clear evidence that the membrane-integrated F0 part is connected by at least two stalks to the F1 part. One of the two stalks is more prominent and connects a central part of F1 with a slightly peripheral part of F0. The other stalk connects a peripheral part of F1 to a peripheral part of F0.

Light-driven production of ATP catalysed by F0F1-ATP synthase in an artificial photosynthetic membrane

Energy-transducing membranes of living organisms couple spontaneous to non-spontaneous processes through the intermediacy of protonmotive force (p.m.f.)--an imbalance in electrochemical potential of protons across the membrane. In most organisms, p.m.f. is generated by redox reactions that are either photochemically driven, such as those in photosynthetic reaction centres, or intrinsically spontaneous, such as those of oxidative phosphorylation in mitochondria. Transmembrane proteins (such as the cytochromes and complexes I, III and IV in the electron-transport chain in the inner mitochondrial membrane) couple the redox reactions to proton translocation, thereby conserving a fraction of the redox chemical potential as p.m.f. Many transducer proteins couple p.m.f. to the performance of biochemical work, such as biochemical synthesis and mechanical and transport processes. Recently, an artificial photosynthetic membrane was reported in which a photocyclic process was used to transport protons across a liposomal membrane, resulting in acidification of the liposome's internal volume. If significant p.m.f. is generated in this system, then incorporating an appropriate transducer into the liposomal bilayer should make it possible to drive a non-spontaneous chemical process. Here we report the incorporation of F0F1-ATP synthase into liposomes containing the components of the proton-pumping photocycle. Irradiation of this artificial membrane with visible light results in the uncoupler- and inhibitor-sensitive synthesis of adenosine triphosphate (ATP) against an ATP chemical potential of approximately 12 kcal mol(-1), with a quantum yield of more than 7%. This system mimics the process by which photosynthetic bacteria convert light energy into ATP chemical potential.

Turnover number of Escherichia coli F0F1 ATP synthase for ATP synthesis in membrane vesicles

The rate of ATP synthesized by the ATP synthase (F0F1-ATPase) is limited by the rate of energy production via the respiratory chain, when measured in everted membrane vesicles of an Escherichia coli atp wild-type strain. After energization of the membranes with NADH, fractional inactivation of F0F1 by the covalent inhibitor N,N'-dicyclohexylcarbodiimide allowed the rate of ATP synthesis/mol remaining active ATP synthase complexes to increase; the active ATP synthase complexes were calculated using ATP hydrolysis rates as the defining parameter. In addition, variation of the assay temperature revealed an increase of the ATP synthesis rate up to a temperature of 37 degrees C, the optimal growth temperature of E. coli. In parallel, the amount of F0F1 complexes present in membrane vesicles was determined by immunoquantitation to be 3.3 +/- 0.3% of the membrane protein for cells grown in rich medium and 6.6 +/- 0.3% for cells grown in minimal medium with glycerol as sole carbon and energy source. Based on these data, a turnover number for ATP synthesis of 270 +/- 40 s(-1) could be determined in the presence of 5% active F0F1 complexes. Therefore, these studies demonstrate that the ATP synthase complex of E. coli has, with respect to maximum rates, the same capacity as the corresponding enzymes of eukaryotic organells.

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.

ATP synthesis catalyzed by the ATP synthase of Escherichia coli reconstituted into liposomes

The H(+)-translocating F0F1-ATPase from Escherichia coli (EF0F1) was purified and reconstituted into preformed reverse-phase liposomes prepared from egg yolk phosphatidylcholine/phosphatidic acid. The EF0F1 liposomes were energized by an acid/base transition (pHout = 8.3; pHin = 5.0) and a superimposed K+/valinomycin diffusion potential ([K+]out = 100 mM; [K+]in = 0.6 mM) yielding a maximum rate (turnover number) of ATP synthesis of 27 +/- 8 mol ATP . mol EF0F1(-1) . s-1), i.e. 27 +/- 8 s-1. This reaction was inhibited by NH4Cl or by addition of the F0F1 inhibitor N,N'-dicyclohexylcarbodiimide. The rate of ATP synthesis measured as a function of the phosphate and ADP concentrations, can be described by Michaelis-Menten kinetics with a Km of 0.7 +/- 0.2 mM for phosphate ([ADP] = 200 microM) and a Km of 27 +/- 7 microM for ADP ([phosphate] = 5 mM), respectively.

ADP binding and ATP synthesis by reconstituted H(+)-ATPase from chloroplasts

The H(+)-ATPase from chloroplasts, CF0F1, was isolated, purified and reconstituted into asolectin liposomes. The enzyme was brought either into the oxidized state or into the reduced state, and the rate of ATP synthesis was measured after energisation of the proteoliposomes with an acid-base transition delta pH (pHin = 5.0, pHout = 8.5) and a K+/valinomycin diffusion potential, delta phi (Kin+ = 0.6 mM, Kout+ = 60 mM). A rate of 250 s-1 was observed with the reduced enzyme (85 s-1 in the absence of delta phi). A rate of 50 s-1 was observed with the oxidized enzyme under the same conditions (15 s-1 in the absence of delta phi). The reconstituted enzyme contained 2 ATPbound per CF0F1 and 1 ADPbound per CF0F1. Upon energisation the enzyme was activated and 0.9 ADP per CF0F1 was released. Binding of ADP to the active reduced enzyme was observed under different conditions. In the absence of phosphate the rate constant for ADP binding was 10(5) M-1.s-1 under energized and de-energized conditions. In the presence of phosphate the rate of ADP binding drastically increased under energized conditions, and strongly decreased under de-energized conditions.

Inhibition of chloroplast ATPase by the K+ channel blocker alpha-dendrotoxin

Possible structural and functional similarities between the channel part, CF0, of chloroplast ATPase (CF0CF1) and ion channels permeable to monovalent cations were investigated using high-affinity toxins mainly targeted against voltage-sensitive K+ channels. In particular, the effect of the K(+)-channel blocker alpha-dendrotoxin and the crude scorpion venom of Leiurus quinquestriatus hebraeus (LQ venom) on ATP synthesis in thylakoid membranes and in CF0CF1-containing liposomes was characterised. Alpha-dendrotoxin (K(i) approximately 5.05 microM) and the LQ venom (K(i) approximately 1.55 micrograms/ml) specifically inhibited ATP synthesis in thylakoid membranes and in CF0CF1-containing liposomes. Our results show that alpha-dendrotoxin and peptides of the LQ venom with an apparent molecular mass of about 4.0 kDa, probably isoforms of charybdotoxin, specifically bind to CF0CF1. This binding was reversible and induced a high leak conductance for H+ through CF0. The Ca(2+)-dependent ATPase activity of the isolated soluble part of CF0CF1 (CF1) was completely inhibited by 1 microM alpha-dendrotoxin, while the crude LQ venom, at concentrations up to 10 micrograms/ml, had no affect on ATPase activity. The concentration dependence of the inhibition by alpha-dendrotoxin indicates that approximately 2 mol alpha-dendrotoxin bind/mol CF0CF1 and 1 mol alpha-dendrotoxin/mol CF1. Known inhibitors of H(+)-flow-through CF0 acted in the presence of alpha-dendrotoxin synergistically. Dicyclohexylcarbodiimide and venturicidin, in contrast to their known effect of blocking H(+)-flow-through CF0, increased the leak conductance through CF0 in the presence of alpha-dendrotoxin drastically. This uncoupling effect indicates that their normal mode of blocking is a secondary effect. Binding of the inhibitors to their respective sites apparently does not affect the proton pathway in CF0, but induces a conformation which closes the channel part for H+. Protein sequence comparison between the known binding site of charybdotoxin in the shaker K+ channel from Drosophila [MacKinnon, R. & Heginbotham, L. (1990) Neuron 5, 767-771] and the choroplast ATPase showed that subunit III reveals a significant similarity (64%) in parts of its sequence (Gln28-Leu53) to the helix 5 and helix 6 (S5-S6) linker region (Ala413-Cys462; the charybdotoxin-binding site) of the shaker K+ channel. According to secondary-structure predictions, the homologous sequences in subunit III and the shaker K+ channel represent putative hydrophilic loops connecting two transmembrane alpha-helices. Apparently the shaker K+ channel and subunit III share significant topological features in these hydrophilic loops which may be part of the respective channel entrance.

Kinetics of ATP synthesis catalyzed by the H(+)-ATPase from chloroplasts (CF0F1) reconstituted into liposomes and coreconstituted with bacteriorhodopsin

The H(+)-ATPase from chloroplasts (CF0F1) was isolated, purified and reconstituted into liposomes from phosphatidylcholine/phosphatidic acid. A transmembrane pH difference, delta pH, and a transmembrane electric potential difference, delta psi, were generated by an acid/base transition. The rate of ATP synthesis was measured at constant delta pH and constant delta psi as a function of temperature between 5 degrees C and 45 degrees C. The activation energy was 55 kJ mol-1. CF0F1 was coreconstituted with bacteriorhodopsin at a molar ratio of approximately 1:170 in the same type of liposomes. Illumination of the proteoliposomes leads to proton transport into the vesicles generating a constant delta pH = 1.8. The dependence of the rate of ATP synthesis on ADP concentration was measured with CF0F1 in the oxidized state, E(ox), and in the reduced state, E(red). The results can be described by Michaelis-Menten kinetics with the following parameters: Vmax = 0.5 s-1, Km = 8 microM for E(ox) and Vmax = 2.0 s-1, Km = 8 microM for E(red).

Calcium-dependent interaction of chlorpromazine with the chloroplast 8-kilodalton CF0 protein and calcium gating of H+ fluxes between thylakoid membrane domains and the lumen

Earlier work suggested that Ca2+ ions in the chloroplast thylakoid lumen interact with thylakoid membrane proteins to produce a proton flux gating structure which functions to regulate the expression of the energy-coupling H+ gradient between localized and delocalized modes [Chiang, G., & Dilley, R. A. (1987) Biochemistry 26, 4911-4916]. In this work, one of the phenothiazine Ca2+ antagonists, chlorpromazine, was used as a photoaffinity probe to test for Ca(2+)-dependent binding of the probe to thylakoid proteins. [3H]Chlorpromazine photoaffinity-labels thylakoid polypeptides of Mr 8K and 6K, with generally much less label occurring in other proteins (some experiments showed labeled proteins at Mr 13K-15K). More label was incorporated in circumstances where it is expected that Ca2+ occupies the putative H+ flux gating site, compared to when the gating site is not occupied by calcium. The photoaffinity labeling of the 8-kDa protein was also influenced by the energization level of the thylakoids (less labeling under H+ uptake energization). The 8-kDa protein was identified by partial amino acid sequence data as subunit III of the thylakoid CF0 H+ channel complex. The partial amino acid sequence of the 6-kDa protein (19 residues were determined with some uncertainties) was compared to data in the GCG sequence analysis data base, and no clear identity to a known sequence was revealed. Neither the exact site of putative Ca2+ binding to the CF0 proteolipid nor the site of covalent attachment of the chlorpromazine to the CF0 component has been identified. Evidence for gating of energy-linked H+ fluxes by the hypothesized Ca(2+)-CF0 gating site came from the correlation between Ca(2+)-dependent binding of chlorpromazine to the CF0 8-kDa protein with inhibition of light-driven H+ uptake into the lumen but no inhibition of H+ uptake into sequestered membrane domains. When conditions favored a delocalized delta mu H+ coupling mode, less chlorpromazine was bound to the CF0 structure, and much larger amounts of H+ ions were accumulated in the lumen. The data support the hypothesis that Ca2+ ions act in concert with the 8-kDa CF0 protein (and perhaps another protein, the 6-kDa polypeptide?) in a gating mechanism for regulating the expression of the energy-coupling H+ gradient between localized or delocalized coupling modes.

Evidence for ATP-ase activity of arrestin from bovine photoreceptors

In vertebrate photoreceptors the soluble protein arrestin (45 kDa) is involved in controlling the light dependent activity of receptor proteins such as transducin or the cGMP-phosphodiesterase. Arrestin has further been identified as the retinal-S-antigen which is assumed to cause the autoimmune disease uveitis. In a first communication a binding of the nucleotide ATP to arrestin was described. In this subsequent study it is shown that arrestin is also able to hydrolyse ATP at a rate of (5.1 +/- 0.3) x 10(-3) U/mg.min with C1/2 = 93 +/- 5 nM and a Hill coefficient n = 1.8 +/- 0.1 at pH 7.2 and 20 degrees C. These findings suggest a new insight into the process of regulating photoreceptor activity.

Reconstitution of CF0F1 into liposomes using a new reconstitution procedure

The H(+)-ATPase (ATP synthase) from chloroplasts was isolated, purified and reconstituted into phosphatidylcholine/phosphatidic-acid liposomes. Liposomes prepared by reverse-phase evaporation were treated with various amounts of Triton X-100 and protein incorporation was studied at each step of the solubilization process. After detergent removal by SM2-Biobeads, the activities of the resulting proteoliposomes were measured indicating that the most efficient reconstitution was obtained by insertion of the protein into preformed, detergent-saturated liposomes. The conditions for the reconstitution were optimized with regard to ATP synthesis driven by an artificially generated delta pH/delta psi. An important benefit of the new reconstituted CF0F1 liposomes is the finding that the rate of ATP synthesis remains constant up to 10 s, indicating a low basal membrane permeability.

The H(+)-ATPase of the plasma membrane from yeast. Kinetics of ATP hydrolysis in native membranes, isolated and reconstituted enzymes

The H(+)-ATPase of the plasma membrane from Saccharomyces cerevisiae has been isolated, purified and reconstituted into asolectin liposomes. The kinetics of ATP hydrolysis have been compared for the H(+)-ATPase in the plasma membrane, in a protein/lipid/detergent micelle (isolated enzyme) and in asolectin proteoliposomes (reconstituted enzyme). In all three cases the kinetics of ATP hydrolysis can be described by Michaelis-Menten kinetics with Km = 0.2 mM MgATP (plasma membranes), Km = 2.4 mM MgATP (isolated enzyme) and Km = 0.2 mM MgATP (reconstituted enzyme). However, the maximal turnover decreases only by a factor of two during isolation of the enzyme and does not change during reconstitution; the activation of the H(+)-ATPase by free Mg2+ is also only slightly influenced by the detergent. The dissociation constant of the enzyme-Mg2+ complex Ka, does not alter during isolation and the dissociation constant of the enzyme-substrate complex, Ks, increases from Ks = 30 microM (plasma membranes) to Ks = 90 microM (isolated enzyme). ATP binding to the H(+)-ATPase ('single turnover' conditions) for the isolated and the reconstituted enzyme resulted in both cases in a second-order rate constant k1 = 2.6 x 10(4) M-1.s-1. From these observations it is concluded that the detergent used (Zwittergent TM 3-14) interacts reversibly with the H(+)-ATPase and that practically all H(+)-ATPase molecules are reconstituted into the liposomes with the ATP-binding site being directed to the outside of the vesicle.

Activation/inactivation and uni-site catalysis by the reconstituted ATP-synthase from chloroplasts

The proton-translocating ATP-synthase of chloroplasts, CF0F1, was isolated and reconstituted into asolectin liposomes. CF0F1 can exist in at least four different states, oxidized or reduced, either inactive or active. These states are characterized by different kinetics of ADP binding: There is no binding of ADP to the inactive, oxidized state, the rate constant for ADP binding to the inactive, reduced states is 7.10(2) M-1.s-1. ADP binding to the active, reduced state occurs under deenergized conditions with 10(5) M-1.s-1 and transforms the enzyme into the inactive, reduced state. Parallel to the ADP-dependent inactivation, the enzyme can also inactivate without ADP binding with a first-order rate constant of 7.10(-3) M-1.s-1. With the active, reduced enzyme ATP-hydrolysis was measured under uni-site conditions as has been carried out with MF1 (Grubmeyer, C., Cross, R.C. and Penefsky, H.S. (1982) J. Biol. Chem. 257, 12092-12100). The rate constant for ATP binding is 10(6) M-1.s-1, the 'equilibrium constant' on the enzyme EADPPi/EATP is 0.4. The rate constants for Pi release and ADP release are 0.2 s-1 and o.1 s-1, respectively. This indicates that the enzyme carries out a complete turnover under uni-site conditions with rates much higher than that reported for MF1.

ATP-synthesis by proteoliposomes incorporating Rhodospirillum rubrum F0F1 as measured with firefly luciferase: dependence on delta psi and delta pH

ATP-synthesis catalyzed by proteoliposomes incorporating Rhodospirillum rubrum F0F1 was driven by artificially applied electrochemical proton gradients. The time-course of ATP-synthesis was followed continuously by means of firefly luciferase. Correction methods were developed which allow one to calculate the initial rate of ATP-synthesis from the observed luminescence kinetics. The following results were obtained: (1) ATP-synthesis occurred above a threshold delta mu H+ of 90 mV; this threshold is not imposed by the activation requirement of the enzyme; (2) delta psi and delta pH appear to be equivalent as driving forces for ATP-synthesis if allowance is made for the effect of the electrical capacitance of the liposome membrane on the distribution of K+ at equilibrium; and (3) the highest rate observed so far is 200 mol ATP per mol F0F1 per s.

The binding and release of the inhibitor protein are governed independently by ATP and membrane potential in ox-heart submitochondrial vesicles

(1) The effects of membrane potential (delta psi) and nucleotides on the interaction between the F1-ATP synthase and its natural inhibitor protein (IF1) are studied in ox-heart submitochondrial vesicles. (2) Membrane potential causes displacement of IF1 from submitochondrial vesicles, as shown by measuring both delta psi-dependent stimulation of ATPase capacity and release of 125I-labelled IF1 from the vesicles. These effects are abolished if ATP is included in the incubation. (3) There is a linear increase in the steady-state ATPase capacity of oxidising vesicles as delta psi is increased from 100 mV to 135 mV. Increasing delta psi above 140 mV leads to no further change. (4) At a constant membrane potential, ATP suppresses the increase in ATPase capacity, with a concentration for half maximal effect of 140 microM. This value is close to the Km for ATP hydrolysis by membrane-bound F1. This suppression is related to ATP concentration rather than to delta Gp or ATP/ADP ratio. (5) The unidirectional on- and off-rates of IF1 were measured separately. The off-rate of IF1 is increased by membrane potential but unaffected by ATP. The on-rate, conversely, is increased by ATP. Thus, the suppression of the potential-dependent net release of IF1 from submitochondrial vesicles by ATP results from an increase of the IF1 on-rate above the off-rate.

Correlation of the turnover number of the ATP synthase in liposomes with the proton flux and the proton potential across the membrane

The fluorescent indicator pyranine was used for recording the internal pH of liposomes. The proton permeability was deduced from the velocity of the internal pH increase which was caused by shifting the external pH from 7 to 9. From valinomycin titration of the proton permeability in the presence of internal and external KCl (0.1 M), the permeability coefficient of H+ (PH) was obtained as 10(-4) cm/s at 22 degrees C. The coefficient was twice this value with the ATP synthase isolated from Wolinella succinogenes present in the liposomal membrane (10 mg protein/g phospholipid). ADP and phosphate had no effect on the latter PH. The protonophore TTFB (5 mumol/g phospholipid) increased the PH by 3 orders of magnitude. The permeability coefficients of H+ and K+ were used for calculating the delta uH and the proton flux associated with the phosphorylation which was driven by gradients of H+ and K+. For the conditions of limiting permeability of K+, the following conclusions were drawn. (1) In the steady state of rapid ion flux, the electrical potential across the liposomal membrane as calculated according to the Goldman equation, is directed opposite to the corresponding Nernst potential which is calculated from the K+ gradient. (2) The maximum turnover numbers of phosphorylation require a delta uH of 200-220 mV across the liposomal membrane. These values of delta uH and the corresponding turnover numbers are close to those brought about by the bacterial electron transport and the coupled phosphorylation. (3) The velocity of phosphorylation is linearly related to the proton flux. The slope of the line can be explained on the basis of an H+/ATP ratio of approx. 3.

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.