Effects of P 1 and ADP on ATPase activity in chloroplasts

Attenuated ADP-inhibition of FOF1 ATPase mitigates manifestations of mitochondrial dysfunction in yeast

Proton-translocating F_OF₁ ATP synthase (F-ATPase) couples ATP synthesis or hydrolysis to transmembrane proton transport in bacteria, chloroplasts, and mitochondria. The primary function of the mitochondrial F_OF₁ is ATP synthesis driven by protonmotive force (pmf) generated by the respiratory chain. However, when pmf is low or absent (e.g. during anoxia), F_OF₁ consumes ATP and functions as a proton-pumping ATPase. Several regulatory mechanisms suppress the ATPase activity of F_OF₁ at low pmf. In yeast mitochondria they include special inhibitory proteins Inh1p and Stf1p, and non-competitive inhibition of ATP hydrolysis by MgADP (ADP-inhibition). Presumably, these mechanisms help the cell to preserve the ATP pool upon membrane de-energization. However, no direct evidence was presented to support this hypothesis so far. Here we report that a point mutation Q263L in subunit beta of Saccharomyces cerevisiae ATP synthase significantly attenuated ADP-inhibition of the enzyme without major effect on the rate of ATP production by mitochondria. The mutation also decreased the sensitivity of the enzyme ATPase activity to azide. Similar effects of the corresponding mutations were observed in earlier studies in bacterial enzymes. This observation indicates that the molecular mechanism of ADP-inhibition is probably the same in mitochondrial and in bacterial F_OF₁. The mutant yeast strain had lower growth rate and had a longer lag period preceding exponential growth phase when starved cells were transferred to fresh growth medium. However, upon the loss of mitochondrial DNA (ρ⁰) the βQ263L mutation effect was reversed: the βQ263L ρ⁰ mutant grew faster than the wild-type ρ⁰ yeast. The results suggest that ADP-inhibition might play a role in prevention of wasteful ATP hydrolysis in the mitochondrial matrix.

Regulation of ATP hydrolysis by the ε subunit, ζ subunit and Mg-ADP in the ATP synthase of Paracoccus denitrificans

F₁F_O-ATP synthase is a crucial metabolic enzyme that uses the proton motive force from respiration to regenerate ATP. For maximum thermodynamic efficiency ATP synthesis should be fully reversible, but the enzyme from Paracoccus denitrificans catalyzes ATP hydrolysis at far lower rates than it catalyzes ATP synthesis, an effect often attributed to its unique ζ subunit. Recently, we showed that deleting ζ increases hydrolysis only marginally, indicating that other common inhibitory mechanisms such as inhibition by the C-terminal domain of the ε subunit (ε-CTD) or Mg-ADP may be more important. Here, we created mutants lacking the ε-CTD, and double mutants lacking both the ε-CTD and ζ subunit. No substantial activation of ATP hydrolysis was observed in any of these strains. Instead, hydrolysis in even the double mutant strains could only be activated by oxyanions, the detergent lauryldimethylamine oxide, or a proton motive force, which are all considered to release Mg-ADP inhibition. Our results establish that P. denitrificans ATP synthase is regulated by a combination of the ε and ζ subunits and Mg-ADP inhibition.

Amino Acid Residues β139, β189, and β319 Modulate ADP-Inhibition in Escherichia coli H+-FOF1-ATP Synthase

Proton-translocating F_OF₁-ATP synthase (F-type ATPase, F-ATPase or F_OF₁) performs ATP synthesis/hydrolysis coupled to proton transport across the membrane in mitochondria, chloroplasts, and most eubacteria. The ATPase activity of the enzyme is suppressed in the absence of protonmotive force by several regulatory mechanisms. The most conserved of these mechanisms is noncompetitive inhibition of ATP hydrolysis by the MgADP complex (ADP-inhibition) which has been found in all the enzymes studied. When MgADP binds without phosphate in the catalytic site, the enzyme enters an inactive state, and MgADP gets locked in the catalytic site and does not exchange with the medium. The degree of ADP-inhibition varies in F_OF₁ enzymes from different organisms. In the Escherichia coli enzyme, ADP-inhibition is relatively weak and, in contrast to other organisms, is enhanced rather than suppressed by phosphate. In this study, we used site-directed mutagenesis to investigate the role of amino acid residues β139, β158, β189, and β319 of E. coli F_OF₁-ATP synthase in the mechanism of ADP-inhibition and its modulation by the protonmotive force. The amino acid residues in these positions differ in the enzymes from beta- and gammaproteobacteria (including E. coli) and F_OF₁-ATP synthases from other eubacteria, mitochondria, and chloroplasts. The βN158L substitution produced no effect on the enzyme activity, while substitutions βF139Y, βF189L, and βV319T only slightly affected ATP (1 mM) hydrolysis. However, in a mixture of ATP and ADP, the activity of the mutants was less suppressed than that of the wild-type enzyme. In addition, mutations βF189L and βV319T weakened the ATPase activity inhibition by phosphate in the presence of ADP. We suggest that residues β139, β189, and β319 are involved in the mechanism of ADP-inhibition and its modulation by phosphate.

Mutation Q259L in subunit beta in Bacillus subtilis ATP synthase attenuates ADP-inhibition and decreases fitness in mixed cultures

The ATPase activity of H⁺-F_OF₁-ATP synthase (F_OF₁) is down-regulated by several mechanisms. The most universal of them found in bacterial, chloroplast and mitochondrial enzymes is non-competitive inhibition by MgADP (ADP-inhibition). When MgADP binds in a catalytic site in the absence of phosphate, the nucleotide might be trapped instead of being released and replaced by new MgATP. In this case the enzyme becomes inactivated, and MgADP release is required for re-activation. The degree of ADP-inhibition varies between different organisms: it is strong in mitochondrial and chloroplast F_OF₁ and in enzymes of some bacteria (including Bacillus PS3 sp., and Bacillus subtilis), but in F_OF₁ of Escherichia coli it is much weaker. It was shown that mutation betaGln259Leu in Bacillus PS3 F_OF₁ noticeably relieves its strong ADP-inhibition. In this work, we introduced the same mutation in F_OF₁ from B. subtilis. ADP-inhibition in the mutant F_OF₁ was also attenuated in comparison to the wild-type enzyme. The ATPase activity in membrane preparations was 3 fold higher in the mutant. Mutant enzyme was capable of ATP-driven proton pumping, and its ATPase activity was stimulated by dissipation of the protonmotive force, implying that the coupling efficiency between ATP hydrolysis and proton transport was not impaired by the mutation. We observed no effect of mutation on the growth rate of B. subtilis in pure cultures. However, in competition growth experiments when the wild type and the mutant strains were cultivated together in mixed cultures, the wild type strain always crowded out the mutant. To our knowledge, this is the first demonstration of the negative effect of F_OF₁ ADP-inhibition attenuation in vivo.

ADP-Inhibition of H+-FOF1-ATP Synthase

H+-F_OF₁-ATP synthase (F-ATPase, F-type ATPase, F_OF₁ complex) catalyzes ATP synthesis from ADP and inorganic phosphate in eubacteria, mitochondria, chloroplasts, and some archaea. ATP synthesis is powered by the transmembrane proton transport driven by the proton motive force (PMF) generated by the respiratory or photosynthetic electron transport chains. When the PMF is decreased or absent, ATP synthase catalyzes the reverse reaction, working as an ATP-dependent proton pump. The ATPase activity of the enzyme is regulated by several mechanisms, of which the most conserved is the non-competitive inhibition by the MgADP complex (ADP-inhibition). When ADP binds to the catalytic site without phosphate, the enzyme may undergo conformational changes that lock bound ADP, resulting in enzyme inactivation. PMF can induce release of inhibitory ADP and reactivate ATP synthase; the threshold PMF value required for enzyme reactivation might exceed the PMF for ATP synthesis. Moreover, membrane energization increases the catalytic site affinity to phosphate, thereby reducing the probability of ADP binding without phosphate and preventing enzyme transition to the ADP-inhibited state. Besides phosphate, oxyanions (e.g., sulfite and bicarbonate), alcohols, lauryldimethylamine oxide, and a number of other detergents can weaken ADP-inhibition and increase ATPase activity of the enzyme. In this paper, we review the data on ADP-inhibition of ATP synthases from different organisms and discuss the in vivo role of this phenomenon and its relationship with other regulatory mechanisms, such as ATPase activity inhibition by subunit ε and nucleotide binding in the noncatalytic sites of the enzyme. It should be noted that in Escherichia coli enzyme, ADP-inhibition is relatively weak and rather enhanced than prevented by phosphate.

Residue 249 in subunit beta regulates ADP inhibition and its phosphate modulation in Escherichia coli ATP synthase

ATPase activity of proton-translocating F_OF₁-ATP synthase (F-type ATPase or F-ATPase) is suppressed in the absence of protonmotive force by several regulatory mechanisms. The most conservative of these mechanisms found in all enzymes studied so far is allosteric inhibition of ATP hydrolysis by MgADP (ADP-inhibition). When MgADP is bound without phosphate in the catalytic site, the enzyme lapses into an inactive state with MgADP trapped. In chloroplasts and mitochondria, as well as in most bacteria, phosphate prevents MgADP inhibition. However, in Escherichia coli ATP synthase ADP-inhibition is relatively weak and phosphate does not prevent it but seems to enhance it. We found that a single amino acid residue in subunit β is responsible for these features of E. coli enzyme. Mutation βL249Q significantly enhanced ADP-inhibition in E. coli ATP synthase, increased the extent of ATP hydrolysis stimulation by sulfite, and rendered the ADP-inhibition sensitive to phosphate in the same manner as observed in F_OF₁ from mitochondria, chloroplasts, and most aerobic\photosynthetic bacteria.

Structure, mechanism, and regulation of the chloroplast ATP synthase

The chloroplast adenosine triphosphate (ATP) synthase uses the electrochemical proton gradient generated by photosynthesis to produce ATP, the energy currency of all cells. Protons conducted through the membrane-embedded Fo motor drive ATP synthesis in the F1 head by rotary catalysis. We determined the high-resolution structure of the complete cF1Fo complex by cryo-electron microscopy, resolving side chains of all 26 protein subunits, the five nucleotides in the F1 head, and the proton pathway to and from the rotor ring. The flexible peripheral stalk redistributes differences in torsional energy across three unequal steps in the rotation cycle. Plant ATP synthase is autoinhibited by a β-hairpin redox switch in subunit γ that blocks rotation in the dark.

Modulation of coupling in the Escherichia coli ATP synthase by ADP and Pi: Role of the ε subunit C-terminal domain

The ε-subunit of ATP-synthase is an endogenous inhibitor of the hydrolysis activity of the complex and its α-helical C-terminal domain (εCTD) undergoes drastic changes among at least two different conformations. Even though this domain is not essential for ATP synthesis activity, there is evidence for its involvement in the coupling mechanism of the pump. Recently, it was proposed that coupling of the ATP synthase can vary as a function of ADP and P_i concentration. In the present work, we have explored the possible role of the εCTD in this ADP- and P_i-dependent coupling, by examining an εCTD-lacking mutant of Escherichia coli. We show that the loss of P_i-dependent coupling can be observed also in the εCTD-less mutant, but the effects of P_i on both proton pumping and ATP hydrolysis were much weaker in the mutant than in the wild-type. We also show that the εCTD strongly influences the binding of ADP to a very tight binding site (half-maximal effect≈1nM); binding at this site induces higher coupling in EF_OF₁ and increases responses to P_i. It is proposed that one physiological role of the εCTD is to regulate the kinetics and affinity of ADP/P_i binding, promoting ADP/P_i-dependent coupling.

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.

Simple mechanism whereby the F1-ATPase motor rotates with near-perfect chemomechanical energy conversion

F1-ATPase is a motor enzyme in which a central shaft γ subunit rotates 120° per ATP in the cylinder made of α3β3 subunits. During rotation, the chemical energy of ATP hydrolysis (ΔGATP) is converted almost entirely into mechanical work by an elusive mechanism. We measured the force for rotation (torque) under various ΔGATP conditions as a function of rotation angles of the γ subunit with quasi-static, single-molecule manipulation and estimated mechanical work (torque × traveled angle) from the area of the function. The torque functions show three sawtooth-like repeats of a steep jump and linear descent in one catalytic turnover, indicating a simple physical model in which the motor is driven by three springs aligned along a 120° rotation angle. Although the second spring is unaffected by ΔGATP, activation of the first spring (timing of the torque jump) delays at low [ATP] (or high [ADP]) and activation of the third spring delays at high [Pi]. These shifts decrease the size and area of the sawtooth (magnitude of the work). Thus, F1-ATPase responds to the change of ΔGATP by shifting the torque jump timing and uses ΔGATP for the mechanical work with near-perfect efficiency.

Noncatalytic nucleotide binding sites: properties and mechanism of involvement in ATP synthase activity regulation

ATP synthases (FoF1-ATPases) of chloroplasts, mitochondria, and bacteria catalyze ATP synthesis or hydrolysis coupled with the transmembrane transfer of protons or sodium ions. Their activity is regulated through their reversible inactivation resulting from a decreased transmembrane potential difference. The inactivation is believed to conserve ATP previously synthesized under conditions of sufficient energy supply against unproductive hydrolysis. This review is focused on the mechanism of nucleotide-dependent regulation of the ATP synthase activity where the so-called noncatalytic nucleotide binding sites are involved. Properties of these sites varying upon free enzyme transition to its membrane-bound form, their dependence on membrane energization, and putative mechanisms of noncatalytic site-mediated regulation of the ATP synthase activity are discussed.

pH-dependent regulation of electron transport and ATP synthesis in chloroplasts

This review is focused on pH-dependent mechanisms of regulation of photosynthetic electron transport and ATP synthesis in chloroplasts. The light-induced acidification of the thylakoid lumen is known to decelerate the plastoquinol oxidation by the cytochrome b 6 f complex, thus impeding the electron flow between photosystem II and photosystem I. Acidification of the lumen also triggers the dissipation of excess energy in the light-harvesting antenna of photosystem II, thereby protecting the photosynthetic apparatus against a solar stress. After brief description of structural and functional organization of the chloroplast electron transport chain, our attention is focused on the nature of the rate-limiting step of electron transfer between photosystem II and photosystem I. In the context of pH-dependent mechanism of photosynthetic control in chloroplasts, the mechanisms of plastoquinol oxidation by the cytochrome b 6 f complex have been considered. The light-induced alkalization of stroma is another factor of pH-dependent regulation of electron transport in chloroplasts. Alkalization of stroma induces activation of the Bassham-Benson-Calvin cycle reactions, thereby promoting efflux of electrons from photosystem I to NADP(+). The mechanisms of the light-induced activation of ATP synthase are briefly considered.

Nucleotide binding to noncatalytic sites is essential for ATP-dependent stimulation and ADP-dependent inactivation of the chloroplast ATP synthase

Light-dependent binding of labeled ADP and ATP to noncatalytic sites of chloroplast ATP synthase and the effect of light-exposed thylakoid membrane preincubation with ADP or ATP on ATPase activity were studied. ADP binding during the preincubation was shown to inactivate the chloroplast ATPase, whereas ATP binding caused its activation. The rate and equilibrium constants of ATPase inactivation and activation were close to those of ADP and ATP binding to a noncatalytic site, with K (d) values of 38 and 33 μM, respectively. It is suggested that ADP- or ATP-binding to one of the noncatalytic sites affects the ATPase activity of chloroplast ATP synthase through a mechanism that modulates tightness of ADP binding to a catalytic site.

ATP hydrolysis in ATP synthases can be differently coupled to proton transport and modulated by ADP and phosphate: a structure based model of the mechanism

In the ATP synthases of Escherichia coli ADP and phosphate exert an apparent regulatory role on the efficiency of proton transport coupled to the hydrolysis of ATP. Both molecules induce clearly biphasic effects on hydrolysis and proton transfer. At intermediate concentrations (approximately 0.5-1 microM and higher) ADP inhibits hydrolysis and proton transfer; a quantitative analysis of the fluxes however proves that the coupling efficiency remains constant in this concentration range. On the other hand at nanomolar concentrations of ADP (a level obtainable only using an enzymatic ATP regenerating system) the efficiency of proton transport drops progressively, while the rate of hydrolysis remains high. Phosphate, at concentrations>or=0.1 mM, inhibits hydrolysis only if ADP is present at sufficiently high concentrations, keeping the coupling efficiency constant. At lower ADP levels phosphate is, however, necessary for an efficiently coupled catalytic cycle. We present a model for a catalytic cycle of ATP hydrolysis uncoupled from the transport of protons. The model is based on the available structures of bovine and yeast F1 and on the known binding affinities for ADP and Pi of the catalytic sites in their different functional states. The binding site related to the inhibitory effects of Pi (in association with ADP) is identified as the alphaHCbetaHC site, the pre-release site for the hydrolysis products. We suggest, moreover, that the high affinity site, associated with the operation of an efficient proton transport, could coincide with a conformational state intermediate between the alphaTPbetaTP and the alphaDPbetaDP (similar to the transition state of the hydrolysis/synthesis reaction) that does not strongly bind the ligands and can exchange them rather freely with the external medium. The emptying of this site can lead to an unproductive hydrolysis cycle that occurs without a net rotation of the central stalk and, consequently, does not translocate protons.

Activation and stiffness of the inhibited states of F1-ATPase probed by single-molecule manipulation

F(1)-ATPase (F(1)), a soluble portion of F(o)F(1)-ATP synthase (F(o)F(1)), is an ATP-driven motor in which gammaepsilon subunits rotate in the alpha(3)beta(3) cylinder. Activity of F(1) and F(o)F(1) from Bacillus PS3 is attenuated by the epsilon subunit in an inhibitory extended form. In this study we observed ATP-dependent transition of epsilon in single F(1) molecules from extended form to hairpin form by fluorescence resonance energy transfer. The results justify the previous bulk experiments and ensure that fraction of F(1) with hairpin epsilon directly determines the fraction of active F(1) at any ATP concentration. Next, mechanical activation and stiffness of epsilon-inhibited F(1) were examined by the forced rotation of magnetic beads attached to gamma. Compared with ADP inhibition, which is another manner of inhibition, rotation by a larger angle was required for the activation from epsilon inhibition when the beads were forced to rotate to ATP hydrolysis direction, and more torque was required to reach the same rotation angle when beads were forced to rotate to ATP synthesis direction. The results imply that if F(o)F(1) is resting in the epsilon-inhibited state, F(o) motor must transmit to gamma a torque larger than expected from thermodynamic equilibrium to initiate ATP synthesis.

Iron transport by proteoliposomes containing mitochondrial F(1)F(0) ATP synthase isolated from rat heart

In this work, we present evidence of Fe(2+) transport by rat heart mitochondrial F(1)F(0) ATP synthase. Iron uptake by the vesicles containing the enzyme was concentration- and temperature-dependent, with an optimum temperature of 37 degrees C. Both ATP and ADP stimulated iron uptake in a concentration-dependent manner, whereas AMP, AMPPCP, and mADP did not. Inhibitors of the enzyme, oligomycin, and resveratrol similarly blocked iron transport. The iron uptake was confirmed by inhibition using specific antibodies against the alpha, beta, and c subunits of the enzyme. Interestingly, slight transport of common divalent and trivalent metal ions such as Mg(+2), Ca(+2), Mn(+2), Zn(+2), Cu(+2), Fe(+3), and Al(+3) was observed. Moreover, Cu(+2), even in the nM range, inhibited iron uptake and attained maximum inhibition of approximately 56%. Inorganic phosphate (Pi) in the medium exerted an opposite effect depending on the type of adenosine nucleotide, which was suppressed with ATP, but enhanced with ADP. A similarly stimulating effect of ATP and ADP with an inverse effect of Pi suggests that the activity of ATPase and ATP synthase may be associated with iron uptake in a different manner, probably via antiport of H(+).

Interaction of pyrophosphate with catalytic and noncatalytic sites of chloroplast ATP synthase

The effect of pyrophosphate (PP(i)) on labeled nucleotide incorporation into noncatalytic sites of chloroplast ATP synthase was studied. In illuminated thylakoid membranes, PP(i) competed with nucleotides for binding to noncatalytic sites. In the dark, PP(i) was capable of tight binding to noncatalytic sites previously vacated by endogenous nucleotides, thereby preventing their subsequent interaction with ADP and ATP. The effect of PP(i) on ATP hydrolysis kinetics was also elucidated. In the dark at micromolar ATP concentrations, PP(i) inhibited ATPase activity of ATP synthase. Addition of PP(i) to the reaction mixture at the step of preliminary illumination inhibited high initial activity of the enzyme, but stimulated its activity during prolonged incubation. These results indicate that the stimulating effect of PP(i) light preincubation with thylakoid membranes on ATPase activity is caused by its binding to ATP synthase noncatalytic sites. The inhibition of ATP synthase results from competition between PP(i) and ATP for binding to catalytic sites.

Quantification of the electrochemical proton gradient and activation of ATP synthase in leaves

We have developed a new method to quantify the transmembrane electrochemical proton gradient present in chloroplasts of dark-adapted leaves. When a leaf is illuminated by a short pulse of intense light, we observed that the light-induced membrane potential changes, measured by the difference of absorption (520 nm-546 nm), reach a maximum value (approximately 190 mV) determined by ion leaks that occur above a threshold level of the electrochemical proton gradient. After the light-pulse, the decay of the membrane potential follows a multiphasic kinetics. A marked slowdown of the rate of membrane potential decay occurs approximately 100 ms after the light-pulse, which has been previously interpreted as reflecting the switch from an activated to an inactivated state of the ATP synthase (Junge, W., Rumberg, B. and Schröder, H., Eur. J. Biochem. 14 (1970) 575-581). This transition occurs at approximately 110 mV, thereby providing a second reference level. On this basis, we have estimated the Delta micro (H(+)) level that pre-exists in the dark. Depending upon the physiological state of the leaf, this level varies from 40 to 70 mV. In the dark, the Delta micro (H(+)) collapses upon addition of inhibitors of the respiratory chain, thus showing that it results from the hydrolysis of ATP of mitochondrial origin. Illumination of the leaf for a period longer than several seconds induces a long-lived Delta micro (H(+)) increase (up to approximately 150 mV) that reflects the light-induced increase in ATP concentration. Following the illumination, Delta micro (H(+)) relaxes to its dark-adapted value according a multiphasic kinetics that is completed in more than 1 h. In mature leaf, the deactivation of the Benson-Calvin cycle follows similar kinetics as Delta micro (H(+)) decay, showing that its state of activation is mainly controlled by ATP concentration.

Regulatory mechanisms of proton-translocating F(O)F (1)-ATP synthase

H(+)-F(O)F(1)-ATP synthase catalyzes synthesis of ATP from ADP and inorganic phosphate using the energy of transmembrane electrochemical potential difference of proton (deltamu(H)(+). The enzyme can also generate this potential difference by working as an ATP-driven proton pump. Several regulatory mechanisms are known to suppress the ATPase activity of F(O)F(1): 1. Non-competitive inhibition by MgADP, a feature shared by F(O)F(1) from bacteria, chloroplasts and mitochondria 2. Inhibition by subunit epsilon in chloroplast and bacterial enzyme 3. Inhibition upon oxidation of two cysteines in subunit gamma in chloroplast F(O)F(1) 4. Inhibition by an additional regulatory protein (IF(1)) in mitochondrial enzyme In this review we summarize the information available on these regulatory mechanisms and discuss possible interplay between them.

Met23Lys mutation in subunit gamma of F(O)F(1)-ATP synthase from Rhodobacter capsulatus impairs the activation of ATP hydrolysis by protonmotive force

H(+)-F(O)F(1)-ATP synthase couples proton flow through its membrane portion, F(O), to the synthesis of ATP in its headpiece, F(1). Upon reversal of the reaction the enzyme functions as a proton pumping ATPase. Even in the simplest bacterial enzyme the ATPase activity is regulated by several mechanisms, involving inhibition by MgADP, conformational transitions of the epsilon subunit, and activation by protonmotive force. Here we report that the Met23Lys mutation in the gamma subunit of the Rhodobacter capsulatus ATP synthase significantly impaired the activation of ATP hydrolysis by protonmotive force. The impairment in the mutant was due to faster enzyme deactivation that was particularly evident at low ATP/ADP ratio. We suggest that the electrostatic interaction of the introduced gammaLys23 with the DELSEED region of subunit beta stabilized the ADP-inhibited state of the enzyme by hindering the rotation of subunit gamma rotation which is necessary for the activation.

Interaction of ADP and ATP with noncatalytic sites of isolated and membrane-bound chloroplast coupling factor CF1

This study of ATP and ADP binding to noncatalytic sites of membrane-bound CF1 (ATP synthase) revealed two noncatalytic sites with different specificities and affinities for nucleotides. One of these is characterized by a high affinity and specificity to ADP (Kd=2.6+/-0.3 microM). However, a certain increase in ADP apparent dissociation constant at high ATP/ADP ratio in the medium allows a possibility that ATP binds to this site as well. The other site displays high specificity to ATP. When the ADP-binding site is vacant, it shows a comparatively low affinity for ATP, which greatly increases with increasing ADP concentration accompanied by filling of the ADP-binding site. The reported specificities of these two sites are independent of thylakoid membrane energization, since both in the dark and in the light the ratios of ATP/ADP tightly bound to the noncatalytic sites were very close. The difference in noncatalytic site affinity for ATP and ADP is shown to depend on the amount of delta subunit in a particular sample. Thylakoid membrane ATP synthase, with stoichiometric content of delta-subunit (one delta-subunit per CF1 molecule), showed the maximal difference in ADP and ATP affinities for the noncatalytic sites. For CF1, with substoichiometric delta subunit values, this difference was less, and after delta subunit removal it decreased still more.

Regulatory interplay between proton motive force, ADP, phosphate, and subunit epsilon in bacterial ATP synthase

ATP synthase couples transmembrane proton transport, driven by the proton motive force (pmf), to the synthesis of ATP from ADP and inorganic phosphate (P(i)). In certain bacteria, the reaction is reversed and the enzyme generates pmf, working as a proton-pumping ATPase. The ATPase activity of bacterial enzymes is prone to inhibition by both ADP and the C-terminal domain of subunit epsilon. We studied the effects of ADP, P(i), pmf, and the C-terminal domain of subunit epsilon on the ATPase activity of thermophilic Bacillus PS3 and Escherichia coli ATP synthases. We found that pmf relieved ADP inhibition during steady-state ATP hydrolysis, but only in the presence of P(i). The C-terminal domain of subunit epsilon in the Bacillus PS3 enzyme enhanced ADP inhibition by counteracting the effects of pmf. It appears that these features allow the enzyme to promptly respond to changes in the ATP:ADP ratio and in pmf levels in order to avoid potentially wasteful ATP hydrolysis in vivo.

The decay of the ATPase activity of light plus thiol-activated thylakoid membranes in the dark

Oxidized ATP synthase of spinach thylakoid membranes catalyzes high rates of ATP synthesis in the light, but very low rates of ATP hydrolysis in the dark. Reduction of the disulfide bond in the gamma subunit of the ATP synthase in the light enhances the rate of Mg2+-ATP hydrolysis in the dark. The light plus thiol-activated state decays in a few minutes in the dark after illumination in Tris buffer, but not when Tricine was used in place of Tris. In this paper, it is shown that Tris in the assay mixture is an inhibitor of the light plus thiol-activated ATPase activity of thylakoids, but only after the activated membranes had incubated in the dark. Aminopropanediols and diethanolamine, also selectively inhibited ATPase activity of activated membranes after storage in the dark, whereas NH4Cl and imidazole inhibit the ATPase activity of activated thylakoids almost equally whether they are added directly after the illumination or several minutes later. The fluorescence of 9-amino-6-chloro-2-methoxyacridine (ACMA) is quenched by the establishment of proton gradients by ATP-dependent proton uptake. Addition of ATP to activated membranes results in rapid quenching of ACMA fluorescence. If the activated membranes were incubated in the dark prior to ATP addition, a lag in the ATP-dependent ACMA fluorescence quenching as well as a similar lag in the rate ATP hydrolysis were seen. It is concluded that ADP rebinds to CF1 in the dark following illumination and inhibits the activity of the ATP synthase. Reactivation of the ATP synthase in the dark can occur by the slow generation of proton gradients by ATP hydrolysis in the dark. This reactivation takes place in Tricine buffer, but not in Tris because of its uncoupling action. Whether ADP binding plays a role in the regulation of the activity of the ATP synthase in situ remains to be established.

ATP synthase of chloroplast thylakoid membranes: a more in depth characterization of its ATPase activity

In contrast to everted mitochondrial inner membrane vesicles and eubacterial plasma membrane vesicles, the ATPase activity of chloroplast ATP synthase in thylakoid membranes is extremely low. Several treatments of thylakoids that unmask ATPase activity are known. Illumination of thylakoids that contain reduced ATP synthase (reduced thylakoids) promotes the hydrolysis of ATP in the dark. Incubation of thylakoids with trypsin can also elicit higher rates of ATPase activity. In this paper the properties of the ATPase activity of the ATP synthase in thylakoids treated with trypsin are compared with those of the ATPase activity in reduced thylakoids. The trypsin-treated membranes have significant ATPase activity in the presence of Ca2+, whereas the Ca2+-ATPase activity of reduced thylakoids is very low. The Mg2+-ATPase activity of the trypsinized thylakoids was only partially inhibited by the uncouplers, at concentrations that fully inhibit the ATPase activity of reduced membranes. Incubation of reduced thylakoids with ADP in Tris buffer prior to assay abolishes Mg2+-ATPase activity. The Mg2+-ATPase activity of trypsin-treated thylakoids was unaffected by incubation with ADP. Trypsin-treated membranes can make ATP at rates that are 75-80% of those of untreated thylakoids. The Mg2+-ATPase activity of trypsin-treated thylakoids is coupled to inward proton translocation and 10 mM sulfite stimulates both proton uptake and ATP hydrolysis. It is concluded that cleavage of the gamma subunit of the ATP synthase by trypsin prevents inhibition of ATPase activity by the epsilon subunit, but only partially overcomes inhibition by Mg2+ and ADP during assay.

The activity of the ATP synthase from Escherichia coli is regulated by the transmembrane proton motive force

The ATP synthase from Escherichia coli was reconstituted into liposomes from phosphatidylcholine/phosphatidic acid. The proteoliposomes were energized by an acid-base transition and a K(+)/valinomycin diffusion potential, and one second after energization, the electrochemical proton gradient was dissipated by uncouplers, and the ATP hydrolysis measurement was started. In the presence of ADP and P(i), the initial rate of ATP hydrolysis was up to 9-fold higher with pre-energized proteoliposomes than with proteoliposomes that had not seen an electrochemical proton gradient. After dissipating the electrochemical proton gradient, the high rate of ATP hydrolysis decayed to the rate without pre-energization within about 15 s. During this decay the enzyme carried out approximately 100 turnovers. In the absence of ADP and P(i), the rate of ATP hydrolysis was already high and could not be significantly increased by pre-energization. It is concluded that ATP hydrolysis is inhibited when ADP and P(i) are bound to the enzyme and that a high Delta mu(H(+)) is required to release ADP and P(i) and to convert the enzyme into a high activity state. This high activity state is metastable and decays slowly when Delta mu(H(+)) is abolished. Thus, the proton motive force does not only supply energy for ATP synthesis but also regulates the fraction of active enzymes.

Mechanism of activation of the chloroplast ATP synthase. A kinetic study of the thiol modulation of isolated ATPase and membrane-bound ATP synthase from spinach by Eschericia coli thioredoxin

The mechanism of thiol modulation of the chloroplast ATP synthase by Escherichia coli thioredoxin was investigated in the isolated ATPase subcomplex and in the ATP synthase complex reconstituted in bacteriorhodopsin proteoliposomes. Thiol modulation was resolved kinetically by continuously monitoring ATP hydrolysis by the isolated subcomplex and ATP synthesis by proteoliposomes. The binding rate constant of reduced thioredoxin to the oxidized ATPase subcomplex devoid of its epsilon subunit could be determined. It did not depend on the catalytic turnover. Reciprocically, the catalytic turnover did not seem to depend on thioredoxin binding. Thiol modulation by Trx of the epsilon-bearing ATPase subcomplex was slow and favored the release of epsilon. The rate constant of thioredoxin binding to the membrane-bound ATP synthase increased with the protonmotive force. It was lower in the presence of ADP than in its absence, revealing a specific effect of the ATP synthase turnover on thioredoxin-gamma subunit interaction. These findings, and more especially the comparisons between the isolated ATPase subcomplex and the ATP synthase complex, can be interpreted in the frame of the rotational catalysis hypothesis. Finally, thiol modulation changed the catalytic properties of the ATP synthase, the kinetics of which became non-Michaelian. This questions the common view about the nature of changes induced by ATP synthase thiol modulation.

Proton gradient-induced changes of the interaction between CF0 and CF1 related to activation of the chloroplast ATP synthase

Thylakoid energization by light causes destabilization of CF0CF1 so that the peripheral CF1 sector is more readily detached from the membrane by intermediate concentrations of the chaotropic salt NaSCN. Here we have investigated the correlation between the proton gradient-induced change of CF0CF1 interaction and CF0CF1 activation. The results indicate a close relationship between the two phenomena. The effect is most probably due to reduction of the electrostatic interaction between the two subcomplexes CF0 and CF1 as a consequence of protonations in the interface region.

Sulfite stimulates the ATP hydrolysis activity of but not proton translocation by the ATP synthase of Rhodobacter capsulatus and interferes with its activation by delta muH+

Sulfite stimulates the rate of ATP hydrolysis by the ATP synthase in chromatophores of Rhodobacter capsulatus. The stimulated activity is inhibited by oligomycin. The activation takes place also in uncoupled chromatophores. The activation consists in an increase of about 12-15-fold of the Vmax for the ATP hydrolysis reaction, while the Km for MgATP is unaffected at 0.16+/-0.03 mM. The dependence of Vmax on the sulfite concentration follows a hyperbolic pattern with half maximum effect at 12 mM. Sulfite affects the ability of the enzyme in translocating protons. Concomitant measurements of the rate of ATP hydrolysis and of ATP-induced protonic flows demonstrate that at sulfite concentrations of greater than 10 mM the hydrolytic reaction becomes progressively uncoupled from the process of proton translocation. This is accompanied by an inhibition of ATP synthesis, either driven by light or by artificially induced ionic gradients. ATP synthesis is totally inhibited at concentrations of at least 80 mM. Sulfite interferes with the mechanism of activation by delta muH+. Low concentrations of this anion (< or = 2 mM) prevent the activation by delta muH+. At higher concentrations a marked stimulation of the activity prevails, regardless of the occurrence of a delta muH+ across the membrane. Phosphate at millimolar concentrations can reverse the inhibition by sulfite. These experimental results can be simulated by a model assuming multiple and competitive equilibria for phosphate or sulfite binding with two binding sites for the two ligands (for sulfite K1S = 0.26 and K2S = 37 mM, and for phosphate K1P = 0.06 and K2P = 4.22 mM), and in which the state bound only to one sulfite molecule is totally inactive in hydrolysis. The competition between phosphate and sulfite is consistent with the molecular structures of the two ligands and of the enzyme.

Kinetics and thioredoxin specificity of thiol modulation of the chloroplast H+-ATPase

The kinetics of thiol modulation of the chloroplast H+-ATPase (CF0CF1) in membrana were analyzed by employing thioredoxins that were kept reduced by 0.1 mM dithiothreitol. The kinetics of thiol modulation depend on the extent of the proton gradient. The process is an exponential function of the thioredoxin concentration and reaction time and can be described by an irreversible second order reaction. The results indicate that the formation of the complex between thioredoxin and CF0CF1 is slow compared with the subsequent reduction step. Furthermore we have compared the efficiencies of the Escherichia coli thioredoxin Trx and the two chloroplast thioredoxins Tr-m and Tr-f. The second order rate constants are 0.057 (Tr-f), 0.024 (Trx), and 0.010 s-1 microM-1 (Tr-m) suggesting that Tr-f rather than Tr-m is the physiological reductant for the chloroplast ATPase. The often employed artificial reductant dithiothreitol exhibits a second order rate constant in thiol modulation of 1.02.10(-6) s-1 microM-1.

Energy-dependent changes in the conformation of the chloroplast ATP synthase and its catalytic activity

Chloroplast ATP synthase changes its conformation depending on the transmembrane electrochemical potential difference of protons (delta mu H+). This conformational change is observable by measuring the change in the reactivity of Lys109 of the epsilon subunit of chloroplast-coupling-factor 1. Illumination of thylakoids increased the epsilon-Lys109 reactivity by a factor of 3-4 within 1 s. In the presence of ADP plus Pi, illumination of thylakoids increased the epsilon-Lys109 reactivity by a factor of only 2. Addition of ATP in the post-illumination dark or in the light after prior illumination increased the epsilon-Lys109 reactivity depending on the concentration of coexisting NH4Cl. ATP hydrolysis at high level was observed irrespective of the epsilon-Lys109 reactivity.

Tissue specificity of the regulation of ATP hydrolysis by isolated plant mitochondria

Pea leaf mitochondria had a high ATP hydrolase activity following the collapse of the membrane potential by addition of valinomycin in state 4. In mitochondria isolated from potato tubers such ATP hydrolase activity was not observed. Pea leaf mitochondria also had a delta pH, in contrast to what was previously found for potato tuber mitochondria. This delta pH could, however, not explain the different results on ATP hydrolysis since this activity was also observed in the presence of nigericin. The results suggest a tissue-specific regulation of ATP hydrolysis in resting organs (potato tubers) as compared to active organs (leaves).

Inhibition of chloroplast CF1-ATPase by vanadate

Inhibition of ATPase activity by vanadate, having K1/2 of 0.5 mM, was demonstrated in the CF1-ATPase. The Ca(2+)-dependent ATPase activity of the isolated enzyme was inhibited in an allosteric manner by vanadate with a Hill coefficient of 3.19 +/- 0.6. Vanadate also inhibited ATPase and Pi-ATP exchange activities of the chloroplast membrane-bound enzyme. Using 51V NMR it was demonstrated that ATP caused partial release of about 1.87 equivalents while ADP caused additional binding of approximately 1.46 equivalents of vanadate, when added to a solution containing CF1 equilibrated with vanadate. The relevance of these results to a possible involvement of a pentacovalent phosphate as transition state intermediate in the hydrolysis of ATP by CF1-ATPase is discussed.

Interaction of Mg2+ with F0.F1 mitochondrial ATPase as related to its slow active/inactive transition

Bovine heart submitochondrial particles incubated with a low concentration of ADP in the presence of Mg2+ and passed through a Sephadex column equilibrated with EDTA exhibit sensitivity of their initial ATPase activity to preincubation with Mg2+. By using particles thus prepared, several characteristics of a Mg(2+)-specific inhibitory site on F0.F1 ATPase were studied. The inhibition was shown to be both time- and Mg(2+)-concentration-dependent, with an equilibrium constant (at infinite time) of 2 x 10(-6) M (25 degrees C, pH 7.5). The dependence of the pseudo-first-order rate constant for the inhibition process on Mg2+ concentration suggests the presence of a single Mg(2+)-binding site with K8 = 1.1 x 10(-4) M. The data obtained are consistent with a two-step mechanism of Mg(2+)-F0.F1 interaction which results in a loss of the ATPase activity; it includes rapid pH-dependent binding of Mg2+ at the site with K8 = 1.1 x 10(-4) M, followed by a slow interconversion of the Mg(2+)-F1 complex into inactive ATPase (kin. = 0.65 min-1, kact. = 0.01 min-1). The Mg(2+)-inhibited ATPase is very slowly (t1/2 approximately 90 min) re-activated in the presence of EDTA. The rate of EDTA-induced re-activation is pH-independent and can be dramatically increased by added ATP, Pi and sulphite. The dissociation constants for free ATP and P1 (5 x 10(-7) M and 1 x 10(-3) M respectively) and the maximal activation rates were determined by measuring the hyperbolic dependencies of the EDTA-induced re-activation of Mg(2+)-de-activated ATPase on the concentrations of the accelerating ligands. Taken together, the data obtained show two functionally detectable free nucleotide-specific binding sites, one site for Pi and one Mg(2+)-specific ATPase-inhibitory site on the F0.F1 mitochondrial ATP synthase complex.

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.