Heterogeneous hydrolysis of substoichiometric ATP by the F1-ATPase from Escherichia coli

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.

ATP binding and hydrolysis steps of the uni-site catalysis by the mitochondrial F(1)-ATPase are affected by inorganic phosphate

The effect of inorganic phosphate (P(i)) on uni-site ATP binding and hydrolysis by the nucleotide-depleted F(1)-ATPase from beef heart mitochondria (ndMF(1)) has been investigated. It is shown for the first time that P(i) decreases the apparent rate constant of uni-site ATP binding by ndMF(1) 3-fold with the K(d) of 0.38+/-0.14mM. During uni-site ATP hydrolysis, P(i) also shifts equilibrium between bound ATP and ADP+P(i) in the direction of ATP synthesis with the K(d) of 0.17+/-0.03mM. However, 10mM P(i) does not significantly affect ATP binding during multi-site catalysis.

Intrinsic uncoupling in the ATP synthase of Escherichia coli

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

The presence of phosphate at a catalytic site suppresses the formation of the MgADP-inhibited form of F(1)-ATPase

F1-ATPase is inactivated by entrapment of MgADP in catalytic sites and reactivated by MgATP or P(i). Here, using a mutant alpha(3)beta(3)gamma complex of thermophilic F(1)-ATPase (alpha W463F/beta Y341W) and monitoring nucleotide binding by fluorescence quenching of an introduced tryptophan, we found that P(i) interfered with the binding of MgATP to F(1)-ATPase, but binding of MgADP was interfered with to a lesser extent. Hydrolysis of MgATP by F(1)-ATPase during the experiments did not obscure the interpretation because another mutant, which was able to bind nucleotide but not hydrolyse ATP (alpha W463F/beta E190Q/beta Y341W), also gave the same results. The half-maximal concentrations of P(i) that suppressed the MgADP-inhibited form and interfered with MgATP binding were both approximately 20 mm. It is likely that the presence of P(i) at a catalytic site shifts the equilibrium from the MgADP-inhibited form to the enzyme-MgADP-P(i) complex, an active intermediate in the catalytic cycle.

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.

Unisite catalysis without rotation of the gamma-epsilon domain in Escherichia coli F1-ATPase

Unisite [gamma-32P]ATP hydrolysis was studied in ECF1 from the mutant betaE381C after generating a single disulfide bond between beta and gamma subunits to prevent the rotation of the gamma/epsilon domain. The single beta-gamma cross-link was obtained by removal of the delta subunit from F1 and then treating with CuCl2 as described previously (Aggeler, R., Haughton, M. A., and Capaldi, R. A. (1996) J. Biol. Chem. 270, 9185-9191). The mutant enzyme, betaE381C, had an increased overall rate of unisite hydrolysis of [gamma-32P]ATP compared with the wild type ECF1 due to increases in the rate of ATP binding (k+1), Pi release (k+3), and ADP release (k+4). Release of bound substrate ([gamma-32P]ATP) was also increased in the betaE381C mutant. Cross-linking between Cys-381 and the intrinsic Cys-87 of gamma caused a further increase in the rate of unisite catalysis, mainly by additional effects on nucleotide binding in the high affinity catalytic site (k+1 and k+4). In delta-subunit-free ECF1 from wild type or betaE381C F1, addition of an excess of ATP accelerated unisite catalysis. After cross-linking, unisite catalysis of betaE381C was not enhanced by the cold chase. The covalent linkage of gamma to beta increased the rate of unisite catalysis to that obtained by cold chase of ATP of the noncross-linked enzyme. It is concluded that the conversion of Glu-381 of beta to Cys induces an activated conformation of the high affinity catalytic site with low affinity for substrate and products. This state is stabilized by cross-linking the Cys at beta381 to Cys-87 of gamma. We infer from the data that rotation of the gamma/epsilon rotor in ECF1 is not linked to unisite hydrolysis of ATP at the high affinity catalytic site but to ATP binding to a second or third catalytic site on the enzyme.

Unisite ATP hydrolysis by soluble Rhodospirillum rubrum F1-ATPase is accelerated by Ca2+

At saturating concentrations of ATP, soluble F1 from the Rhodospirillum rubrum (RF1) exhibits a higher rate of hydrolysis with Ca2+ than with Mg2+. The mechanisms involved in the expression of a higher catalytic activity with Ca2+ were explored by measuring the ATPase activity of RF1 at substiochiometric concentrations of ATP (unisite conditions). At a ratio of 0.25 [gamma-32P]ATP per RF1, the enzyme exhibited a 50 times higher hydrolytic rate with Ca2+ than with Mg2+. The rate of [gamma-32P]ATP binding to RF1 was in the same range with the two divalent metal ions. Centrifugation-filtration of RF1 exposed to substoichiometric [gamma-32P]ATP concentrations and Mg2+ through Sephadex columns yielded an enzyme that contained [gamma-32P]ATP and [32P]phosphate in a stoichiometry that was close to one. In the presence of Ca2+, the eluted enzyme did not contain [gamma-32P]ATP nor [32P]phosphate. This indicated that the rate of product release was faster with Ca2+ than with Mg2+. It was also observed that the ratio of multisite to unisite hydrolysis rates was of similar magnitude with both divalent cations. This suggests that they do not affect differently the cooperative mechanisms that may exist between catalytic sites. In consequence, the higher ATPase activity of RF1 in presence of Ca2+ strongly suggests that the retention time of products is decreased in the presence of this cation. Copyright 1998 Elsevier Science B.V.

A single mutation at the catalytic site of TF1-alpha3beta3gamma complex switches the kinetics of ATP hydrolysis from negative to positive cooperativity

Previously, we reported the substitution of Tyr341 of the F1-ATPase beta subunit from a thermophilic Bacillus strain PS3 with leucine, cysteine, or alanine (M. Odaka et al. J. Biochem., 115 (1994) 789-796). These mutations resulted in a great decrease in the affinity of the isolated beta subunit for ATP-Mg and an increase in the apparent Km of the alpha3beta3gamma complex in ATP hydrolysis when examined above 0.1 mM ATP. Here, we examined the ATPase activity of the mutant complexes in a wide range of ATP concentration and found that the mutants exhibited apparent positive cooperativity in ATP hydrolysis. This is the first clear demonstration that a single mutation in the catalytic sites converts the kinetics from apparent negative cooperativity in the wild-type alpha3beta3gamma complex to apparent positive cooperativity. The conversion of apparent cooperativity could be explained in terms of a simple kinetic scheme based on the binding change model proposed by Boyer.

The ATP synthase--a splendid molecular machine

An X-ray structure of the F1 portion of the mitochondrial ATP synthase shows asymmetry and differences in nucleotide binding of the catalytic beta subunits that support the binding change mechanism with an internal rotation of the gamma subunit. Other structural and mutational probes of the F1 and F0 portions of the ATP synthase are reviewed, together with kinetic and other evaluations of catalytic site occupancy and behavior during hydrolysis or synthesis of ATP. Subunit function as related to proton translocation and rotational catalysis is considered. Physical demonstrations of the gamma subunit rotation have been achieved. The findings have implications for other enzymatic catalyses.

Analysis of time-dependent change of Escherichia coli F1-ATPase activity and its relationship with apparent negative cooperativity

Except for the case of gradual activation of EF1 (F1-ATPase from Escherichia coli) caused by the dissociation of the epsilon subunit [Laget, P. P. and Smith, J. B. (1979) Arch. Biochem. Biophys. 197, 83-89], EF1 has long been thought not to show a time-dependent change in activity [Senior, A.E. et al. (1992) Arch. Biochem. Biophys. 297, 340-344]. Here, we report the time-dependent inactivation and activation of EF1, which are apparently similar to those of mitochondrial F1-ATPases [Vasilyeva, E.A. et al. (1982) Biochem. J. 202, 15-23]. Analysis of these changes as a function of ATP concentrations in relation to negative cooperativity revealed that the initial inactivation phase was attributable to the decrease in the Vmax associated with the low Km (around 10 microM), and the following activation, probably due to the dissociation of the epsilon subunit, corresponded to the increase in the Vmax associated with the high Km (in the order of 100 microM). Thus, the time-dependent change in EF1 activity is closely related to the apparent negative cooperativity (multiple Km values) of ATP hydrolysis.

From uni-site to multi-site ATP synthesis in thylakoid membranes

The membrane-bound H(+)-ATPase from chloroplasts, CF0F1, was brought into the active, reduced state by illumination in the presence of thioredoxin and dithiothreitol. The endogenous nucleotides were removed by a washing procedure so that the active, reduced enzyme contained one tightly bound ATP per CF0F1. When [14C]ADP was added in substoichiometric amounts during continuous illumination, ADP was bound to the enzyme, phosphorylated and released as [14C]ATP, i.e., the tightly bound ATP was not involved in the catalytic turnover ('uni-site ATP-synthesis'). The rate constant for ADP binding was k = (2.0 +/- 0.5) x 10(6) M-1 s-1. The rate of ATP synthesis was measured as a function of the ADP concentration from 8 nM up to 1 mM in the presence of 2 mM phosphate during continuous illumination. A linear increase of the rate was observed up to 100 nM. Above this concentration a supralinear increase was found, indicating the occupation of a second ADP-binding site. A plateau was reached between 1.5 microM and 2.3 microM ADP with a rate of vpl = 3.7 s-1. The half-maximal rate from this plateau was observed at 780 nM. Above 2.3 microM ADP up to 1 mM ADP the data were described by Michaelis-Menten kinetics (vmax = 80 s-1; apparent KM = 32 microM). These results indicated the participation of at least two different ADP binding sites in ATP synthesis catalyzed by the membrane-bound CF0F1.

Inhibitory effect of NaN3 on the F0F1 ATPase of submitochondrial particles as related to nucleotide binding

The inhibitory effects of NaN3 on the F0F1 ATPase of beef heart submitochondrial particles were investigated. It was shown that NaN3 inhibited the ATPase activity only in the presence of ATP or ADP and the inhibition proceeded slowly. Analysis of the time-course of the inhibition process lead to a conclusion that an ATP binding site which has an apparent Kd of 14.0 +/- 8.7 microM is responsible for the increase of NaN3 sensitivity. This value agreed well with the low Km of ATP hydrolysis characterized before (Muneyuki, E., and Hirata, H. (1988) FEBS Lett. 234, 455-458) and in the range of so-called bi-site catalysis. The same conclusion was derived as for isolated F1 ATPase. From similar analysis, the Kd of this site for ADP was deduced to be 1.34 +/- 0.45 microM, which also agreed with that reported by Pedersen (Pedersen, P.L. (1975) Biochem. Biophys. Res. Commun. 64, 610-616) and also in the same range as reported for the low Km of ATP synthesis by activated submitochondrial particles. These results suggest that hydrolysis through the low Km mode of ATPase reaction leads the enzyme NaN3 sensitive form and this reaction cycle corresponds to the low Km mode of ATP synthesis.