The presence of two hydrolytic sites on beef heart mitochondrial adenosine triphosphatase

ATP regulation of calcium transport in back-inhibited sarcoplasmic reticulum vesicles

At high concentrations of ATP, ATP hydrolysis and Ca2+ transport by the (Ca2+ + MG2+)-ATPase of intact sarcoplasmic reticulum vesicles exhibit a secondary activation that varies with the extent of back-inhibition by Ca2+ accumulated within the vesicles. When the internal ionized Ca2+ is clamped at low and intermediate levels by the use of Ca-precipitating anions, the apparent Km values for activation by ATP are lower than in fully back-inhibited vesicles (high internal Ca2+). In leaky vesicles unable to accumulate Ca2+, raising Ca2+ in the assay medium from 20-30 microM to 5 mM abolishes the activation of hydrolysis by high concentrations of ATP. The level of [32P]phosphoenzyme formed during ATP hydrolysis from [32P]phosphate added to the medium also varies with the extent of back-inhibition; it is highest when Ca2+ is raised to a level that saturates the internal, low-affinity Ca2+ binding sites. In intact vesicles, increasing the ATP concentration from 10 to 400 microM competitively inhibits the reaction of inorganic phosphate with the enzyme but does not change the rate of hydrolysis. In a previous report (De Meis, L., Gomez-Puyou, M.T. and Gomez-Puyou, A. (1988) Eur. J. Biochem. 171, 343-349), it has been shown that the hydrophobic molecules trifluoperazine and iron bathophenanthroline compete for the catalytic site of the Pi-reactive form of the enzyme. Here it is shown that inhibition of ATP hydrolysis by these compounds is reduced or abolished when Ca2+ binds to the low-affinity Ca2+ binding sites of the enzyme. Since inhibition by these agents is indifferent to activation of hydrolysis by high concentrations of ATP, it is suggested that the second Km for ATP and the inhibition by hydrophobic molecules involve two different Ca-free forms of the enzyme.

Three adenine nucleotide binding sites in F1-F0 mitochondrial ATPase as revealed by presteady-state and steady-state kinetics of ATP hydrolysis. Evidence for two inhibitory ADP-specific noncatalytic sites

Preincubation of submitochondrial particles with ADP in the presence of Mg2+ results in the complete inhibition of ATPase which is slowly reactivated in the assay mixture containing ATP and the ATP regenerating system. Significantly, the rate of activation increases as the concentration of ADP in the preincubation mixture rises from 1 microM to 20 microM and reaches a constant value at higher ADP concentrations. The first-order rate constant for the activation process in the assay mixture is ATP-dependent at any level of inhibitory ADP. The data obtained strongly suggest that two ADP-specific inhibitory sites and one ATP-specific hydrolytic site are present in F1-F0 ATPase. Taking into account the (3 alpha.3 beta).gamma.delta.epsilon structure of F1, it is concluded that the synchronous discharge of ADP from two inhibitory sites during the activation occurs after ATP binds to the ATPase catalytic site.

ATP synthases--structure of the F1-moiety and its relationship to function and mechanism

A great deal of progress has been made in understanding both the structure and the mechanism of F1-ATPase. The primary structure is now fully known for at least five species. Sequence comparison between chloroplast, photobacteria, aerobic bacteria, and mitochondrial representatives allow us to infer more general functional relationships and evolutionary trends. Although the F1 moiety is the most studied segment of the H+-ATPase complex, there is not a full understanding of the mechanism and regulation of its hydrolytic activity. The beta subunit is now known to contain one and probably two nucleotide binding domains, one of which is believed to be a catalytic site. Recently, two similar models have been proposed to attempt to describe the "active" part of the beta subunits. These models are mainly an attempt to use the structure of adenylate kinase to represent a more general working model for nucleotide binding phosphotransferases. Labelling experiments seem to indicate that several critical residues outside the region described by the "adenylate kinase" part of this model are also actively involved in the ATPase activity. New models will have to be introduced to include these regions. Finally, it seems that a consensus has been reached with regard to a broad acceptance of the asymmetric structure of the F1-moiety. In addition, recent experimental evidence points toward the presence of nonequivalent subunits to describe the functional activity of the F1-ATPase. A summary diagram of the conformational and binding states of the enzyme including the nonequivalent beta subunit is presented. Additional research is essential to establish the role of the minor subunits--and of the asymmetry they introduce in F1--on the physiological function of the enzyme.

Calcium regulation by lens plasma membrane vesicles

The role of the plasma membrane in the regulation of lens fiber cell cytosolic Ca2+ concentration has been examined using a vesicular preparation derived from calf lenses. Calcium accumulation by these vesicles was ATP dependent, and was releasable by the ionophore A23187, indicating that calcium was transported into a vesicular space. Calcium accumulation was stimulated by Ca2+ (K1/2 = 0.08 microM Ca2+) potassium (maximally at 50 mM K+), and cAMP-dependent protein kinase; it was inhibited by both vanadate (IC50 = 5 microM) and the calmodulin inhibitor R24571 (IC50 = 5 microM), indicating that this pump was plasma-membrane derived and likely calmodulin dependent. Valinomycin, in the presence of K+, stimulated calcium uptake, suggesting that the calcium pump either countertransports K+, or is regulated in an electrogenic fashion. Inhibition of calcium uptake by selenite and p-chloromercuribenzoate demonstrates the presence of an essential -SH group(s) in this enzyme. Calcium release from calcium-filled lens vesicles was enhanced by Na+, demonstrating that these vesicles also contain a Na:Ca exchange carrier. p-Chloromercuribenzoate and p-chloromercuribenzoate sulfonic acid also promoted calcium release from calcium-filled vesicles, suggesting that this release, like calcium uptake, is in part mediated by a cysteine-containing protein. We conclude that lens fiber cell cytosolic Ca2+ concentration could be regulated by a number of plasma membrane processes. The sensitivity of both calcium uptake and release to -SH reagents has implications in lens cataract formation, where oxidation of lens proteins has been proposed to account for the elevated cytosolic Ca2+ in this condition.

The number of functional catalytic sites on F1-ATPases and the effects of quaternary structural asymmetry on their properties

Recent structural and kinetic studies of F1 and F0F1 are reviewed with regard to their implications for the binding change mechanism for ATP synthesis by oxidative phosphorylation and photophosphorylation. It is concluded that at least two and probably all three of the catalytic sites on F1 are functionally equivalent despite permanent structural asymmetry in the soluble enzyme. A rotary mechanism in which all three catalytic subunits experience all possible interactions with the single-copy subunits during turnover is thought not to apply to soluble F1 but remains an attractive model for the membrane bound enzyme.

Role of water activity on the rates of acetyl phosphate and ATP hydrolysis

The rates of hydrolysis of acetyl phosphate in the presence of 0.1 M NaOH and of ATP in the presence of either 1 M HCl or 1 M NaOH were measured at different temperatures and in the presence of different concentrations of the organic solvents dimethyl sulfoxide or ethylene glycol. Under all conditions tested, there was a progressive increase in the rate constant of hydrolysis of both phosphate compounds as the water activity of the medium was decreased by the addition of organic solvents. At 25 degrees C, substitution of 70% of the water of the medium by dimethyl sulfoxide promoted an increase of two orders of magnitude in the rate constant of acetyl phosphate hydrolysis. In the presence of 80% and 90% dimethyl sulfoxide the rate of acetyl phosphate hydrolysis increased by more than two orders of magnitude and was so fast that it could not be measured with the method used. The effect of organic solvents on the rate of ATP hydrolysis was less pronounced than that observed for acetyl phosphate hydrolysis. At 30 degrees C, substitution of 90% of water by an organic solvent promoted a 4-6-fold increase of the rate of ATP hydrolysis. Acceleration of either acetyl phosphate or ATP hydrolysis rates was promoted by a decrease in both activation energies (Ea) and in entropies of activation delta S. The data obtained are discussed with reference to the mechanism of catalysis of enzymes involved in energy transduction such as the Ca2+-ATPase of sarcoplasmic reticulum and the F1-ATPase of mitochondria.

Exchange and hydrolysis of tightly bound nucleotides in normal and photolabelled bovine heart mitochondrial F1-ATPase

Treatment of F1 by threefold fast-column centrifugation or by single ammonium sulphate precipitation followed by fast-column centrifugation resulted in enzyme preparations containing 2.5-2.8 mol of bound nucleotides per mol of F1. Short incubations of such F1 preparations in the presence of relatively low concentrations of [14C]ATP and 2-azido[alpha-32P]ATP (100-250 microM), followed by ammonium sulphate precipitation and fast-column centrifugation, resulted in exchange of about 1 mol of the bound nucleotide per mol of F1 not affecting the total amount of bound nucleotides. Exchange of bound nucleotides with 2-azidoATP, followed by ultraviolet irradiation, results in inhibition of the enzyme activity, full inhibition being obtained (via extrapolation) when 1 mol of 2-nitreno-adenosine 5'-tri- or diphosphate (2-N-AT(D)P) is covalently bound to the presumably catalytic site on the enzyme (Van Dongen, M.B.M., De Geus, J.P., Korver, T., Harton, A.F. and Berden, J.A. (1986) Biochim. Biophys. Acta 850, 359-368). In agreement with this, it was found that incorporated [gamma-32P]ATP was hydrolysed by more than 80%. Newly incorporated, not covalently bound radioactive nucleotides could be rapidly exchanged again by the addition of non-radioactive nucleotides, but a higher concentration of nucleotides was needed to fully exchange the incorporated nucleotide. Also, when F1 was depleted of most of its bound nucleotides by repeated ammonium sulphate precipitation, part of the residual nucleotides was still rapidly exchangeable. The ability of F1 to exchange (and hydrolyse) one of the bound nucleotides was not lost when one catalytic and one non-catalytic binding site were occupied by covalently bound 8-N-ATP. Similar results were obtained with F1 containing 2-nitrenoATP covalently bound to one of the catalytic sites. Also, after photolabelling of up to four binding sites with 8-N[( 2-3H]AT(D)P, part of the two remaining non-covalently bound nucleotides could still be rapidly exchanged. In this case the exchanged nucleotide was also hydrolysed. It is concluded that one of the two bound nucleotides became exchangeable when all four other sites (i.e., two catalytic and two non-catalytic) were occupied with covalently bound nucleotides. The site involved showed catalytic properties suggestive of localisation on a beta-subunit.

Novel approaches towards characterization of the high-affinity nucleotide binding sites on mitochondrial F1-ATPase by the fluorescence probes 3'-O-(1-naphthoyl)adenosine di- and triphosphate

The fluorescence properties of 3'-O-(1-naphthoyl)adenosine di- and triphosphates (termed N-ADP and N-ATP, respectively) were investigated in detail. Of special importance for the use of these analogues as environmental probes is their high quantum yield (0.58 in water) and the polarity dependence of shape and wavelength position of the emission spectrum. Upon binding of N-ADP and N-ATP to mitochondrial F1-ATPase, the fluorescence intensity is markedly decreased, due to polarity changes and 'ground-state' quenching. Using this signal for equilibrium binding studies, three (at least a priori) equivalent nucleotide-binding sites were detected on the enzyme. The perspective intrinsic dissociation constants are as follows: N-ADP/Mg2+ 120 nM; N-ATP/Mg2+ 160 nM; N-ADP/EDTA 560 nM; N-ATP/EDTA 3500 nM. For bound ligand the environment was found to be rather unipolar; the rotational mobility of the fluorophore is restricted, its accessibility for iodide anions (as a quencher) is hindered. These facts show a location of the binding sites quite deeply embedded in the protein. The conformation of the binding domains is strongly dependent on the absence or presence of Mg2+, as can be seen from the relative efficiencies of the singlet-singlet energy transfer from tyrosine residues in the protein to bound naphthoyl moieties. Investigation of the binding kinetics revealed this process as biphasic (in presence of Mg2+). After the first fast step (kon greater than 1 X 10(6) M-1 X s-1), in which the analogue is bound to the enzyme, a slow local conformational rearrangement occurs.

The tonoplast proton-translocating ATPase of higher plants as a third class of proton-pumps

Taken together, all the data reported recently in the literature suggest that tonoplast ATPase belongs to a new class of proton pumps. To date, the most studied system is the proton-pumping ATPase from the tonoplast of Hevea latex. Its main characteristics are presented. It resembles the mitochondrial ATPase in its specificity, its substrate affinity, and its sensitivity to different inhibitors. However, for some aspects, it resembles the plasma membrane system in its response to other inhibitors tested (quercetin for example). It differs from both ATPases in its sensitivity to nitrate as well as by its molecular structure, i.e. a complex exhibiting a least 4 or 5 polypeptides. These results favor the existence of a third class of proton pumps, intermediate between the F1F0-class and the E1E2-class.

Fluorometric evidence for control of the activity of F1-adenosinetriphosphatase by ligand-induced conformation change

The effect of ATP on the fluorescence intensity of bovine heart F1-adenosinetriphosphatase labeled at its essential Lys with 7-chloro-4-nitro-2,1,3-benzoxadiazole (N-NBD-F1) has been examined in solutions containing different concentrations of ADP. The fluorescence of N-NBD-F1 is unaffected by ATP in the absence of ADP. But when increasing amounts of ATP are added to a solution of N-NBD-F1 containing 0.37 or 1.0mM ADP, the fluorescence of N-NBD-F1 first decreases and then increases continually as the concentration of ATP is further raised. Parallel measurements of the suppression of the fluorescence of N-NBD-F1 and the inhibition of the ATPase activity of the unlabeled enzyme by ADP in the presence of ATP show a quantitative correlation between the changes in fluorescence and in ATPase activity. The data are consistent with the model for F1-ATPase with one principal catalytic beta' subunit for ATP hydrolysis and synthesis, and two auxiliary beta" subunits which control the conformation and hence the catalytic activity of beta' through interaction between all the subunits.

[Use of chemical probes in the study of F1-ATPases]

The purpose of the present review is to discuss in brief the use of chemical probes for the study of the structure and the function of F1-ATPases. Special focus is brought on probes that bind covalently to the proteins.