High-affinity binding of ADP and of ADP analogues to mitochondrial F1-ATPase

Subunit stoichiometry and juxtaposition of the photosynthetic coupling factor 1: Immunoelectron microscopy using monoclonal antibodies

Monoclonal antibodies specific to the alpha subunits of the photosynthetic coupling factor 1 (CF(1)) were used as marker molecules in an electron microscopic analysis of the subunit organization of this enzyme. Immune complexes were obtained by incubation of CF(1) with saturating amounts of anti-alpha-subunit IgG, isolated by gel filtration, and visualized by electron microscopy. The maximum number of antibodies bound to a CF(1) molecule was three, the angle defined by a neighboring pair of antibodies characteristically being 120 degrees . These results are interpreted as direct evidence for the presence of three alpha subunits in the CF(1) complex, the relative orientation of them being described by 3-fold rotary symmetry. Our observations thus favor an overall subunit stoichiometry of alpha(3)beta(3)gammadeltaepsilon.

ATP synthase and the actions of inhibitors utilized to study its roles in human health, disease, and other scientific areas

ATP synthase, a double-motor enzyme, plays various roles in the cell, participating not only in ATP synthesis but in ATP hydrolysis-dependent processes and in the regulation of a proton gradient across some membrane-dependent systems. Recent studies of ATP synthase as a potential molecular target for the treatment of some human diseases have displayed promising results, and this enzyme is now emerging as an attractive molecular target for the development of new therapies for a variety of diseases. Significantly, ATP synthase, because of its complex structure, is inhibited by a number of different inhibitors and provides diverse possibilities in the development of new ATP synthase-directed agents. In this review, we classify over 250 natural and synthetic inhibitors of ATP synthase reported to date and present their inhibitory sites and their known or proposed modes of action. The rich source of ATP synthase inhibitors and their known or purported sites of action presented in this review should provide valuable insights into their applications as potential scaffolds for new therapeutics for human and animal diseases as well as for the discovery of new pesticides and herbicides to help protect the world's food supply. Finally, as ATP synthase is now known to consist of two unique nanomotors involved in making ATP from ADP and P(i), the information provided in this review may greatly assist those investigators entering the emerging field of nanotechnology.

Differential nucleotide binding to catalytic and noncatalytic sites and related conformational changes involving alpha/beta-subunit interactions as monitored by sensitive intrinsic fluorescence in Schizosaccharomyces pombe mitochondrial F1

Mitochondrial F1 from the yeast Schizosaccharomyces pombe exhibits an intrinsic tryptophan fluorescence sensitive to adenine nucleotides and inorganic phosphate [Divita, G., Di Pietro, A., Deléage, G., Roux, B., & Gautheron, D.C. (1991) Biochemistry 30, 3256-3262]. The present results indicate that the intrinsic fluorescence is differentially modified by nucleotide binding to either catalytic or noncatalytic sites. Guanine or hypoxanthine nucleotides, which selectively bind to the catalytic site, produce a hyperbolic saturation monitored by fluorescence quenching at 332 nm, the maximal emission wavelength. On the contrary, adenine nucleotides, which bind to both catalytic and noncatalytic sites, exhibit a biphasic saturation. High-affinity ATP binding produces a marked quenching as opposed to the lower-affinity one. In contrast, ADP exhibits a sigmoidal saturation, with high-affinity binding producing no quenching but responsible for positive cooperativity of binding to the lower-affinity site. The catalytic-site affinity for GDP is almost 20-fold higher at pH 5.0 as compared to pH 9.0, and the high sensitivity of the method allows detection of the 10-fold lower-affinity GMP binding. In contrast, high-affinity binding of ADP, or AMP, is not pH-dependent. The selective catalytic-site saturation induces a F1 conformational change decreasing the Stern-Volmer constant for acrylamide and the tryptophan fraction accessible to iodide. ATP saturation of both catalytic and noncatalytic sites produces an additional reduction of the accessible fraction to acrylamide.

Intrinsic tryptophan fluorescence of Schizosaccharomyces pombe mitochondrial F1-ATPase. A powerful probe for phosphate and nucleotide interactions

Mitochondrial F1 from the yeast Schizosaccharomyces pombe, in contrast to the mammalian enzyme, exhibits a characteristic intrinsic tryptophan fluorescence with a maximal excitation at 291 nm and a maximal emission at 332 nm. Low values of Stern-Volmer quenching constants, 4.0 M-1 or 1.8 M-1, respectively, in the presence of either acrylamide or iodide, indicate that tryptophans are mainly buried inside the native enzyme. Upon subunit dissociation and unfolding by 6 M guanidine hydrochloride (Gdn.HCl), the maximal emission is shifted to 354 nm, a value very similar to that obtained with N-acetyltryptophanamide, a solute-tryptophan model compound. The tryptophan content of each isolated subunit has been estimated by fluorescence titration in the presence of Gdn.HCl with free tryptophan as a standard. Two tryptophans and one tryptophan are found respectively in the alpha and epsilon subunits, whereas none is detected in the beta, gamma, and delta subunits. These subunit contents are consistent with the total of seven tryptophans estimated for native F1 with alpha 3 beta 3 gamma 1 delta 1 epsilon 1 stoichiometry. The maximal emission of the isolated epsilon subunit is markedly blue-shifted to 310-312 nm by interaction with the isolated delta subunit, which suggests that the epsilon subunit tryptophan might be a very minor contributor to the native F1 fluorescence measured at 332 nm. This fluorescence is very sensitive to phosphate, which produces a marked blue shift indicative of tryptophans in a more hydrophobic environment. On the other hand, ADP and ATP quench the maximal emission at 332 nm, lower tryptophan accessibility to acrylamide, and reveal tryptophan heterogeneity.

Interaction between delta and epsilon subunits of F1-ATPase from pig heart mitochondria. Circular dichroism and intrinsic fluorescence of purified and reconstituted delta epsilon complex

A delta epsilon complex has been purified as a molecular entity from pig heart mitochondrial F1-ATPase. This delta epsilon complex has also been reconstituted from purified delta and epsilon subunits. Both isolated and reconstituted delta epsilon complexes have delta 1 epsilon 1 stoichiometry and are indistinguishable by their chromatographic behavior, their circular dichroism spectra (CD spectra), and their intrinsic fluorescence features. The content of secondary structures deduced from CD spectra of the delta epsilon complex appears to be the sum of the respective contributions of purified delta and epsilon subunits. All intrinsic fluorescence studies carried out on isolated epsilon subunit and delta epsilon complex show that the single tryptophan residue located on epsilon is involved in the interaction between delta and epsilon subunits. Results obtained with F1-ATPase are in favor of the same delta epsilon interaction in the entire enzyme.

Temperature-induced states of isolated F1-ATPase affect catalysis, enzyme conformation and high-affinity nucleotide binding sites

Isolated, nucleotide-depleted bovine-heart F1-ATPase exhibits a break in Arrhenius plot with a 2.7-fold increase in activation energy of ATP hydrolysis below 18-19 degrees C. Analysis of intrinsic tyrosine fluorescence and of the circular dichroism of F1-ATPase showed an abrupt and reversible conformational change occurring at the break temperature, characteristic of a structural tightening at low temperature. Analysis of catalytic nucleotide binding sites using fluorescent ADP analog, 3'-O-(1-naphthoyl)adenosine diphosphate did not show any significant change in affinity of nucleotide binding around the transition temperature but the bound fluorophore exerted a more restricted motion and slower rotation at temperature below the break, indicating a change in the mobility of groups in the close neighbourhood. It is concluded that, as a result of temperature, two kinetically distinct states of F1-ATPase are induced, due to a change in enzyme conformation, which influences directly the properties of catalytic nucleotide binding sites.

Synthesis of a photoaffinity-spin-labeled derivative of ATP and its first application to F1-ATPase

The synthesis of an ATP derivative is described, in which a spin-label is attached to the 3'-position of the ribose moiety and an azido group to C8 of the adenine ring (SL-N3-ATP). Irradiation of this compound at 350 nm generates a nitrene, which will react with any functional group in its vicinity. SL-N3-ATP exhibits a strongly immobilized ESR spectrum in a complex with F1-ATPase from beef heart mitochondria. It was covalently incorporated into this enzyme. SL-N3-ATP may be employed in ESR investigations under conditions in which non-covalent interactions are too weak for motionally restricted species to be easily observed.

The plasma membrane ATPase of the thermoacidophilic archaebacterium Sulfolobus acidocaldarius. Purification and immunological relationships to F1-ATPases

The plasma-membrane-associated ATPase of the thermoacidophilic archaebacterium Sulfolobus acidocaldarius characterized in a previous work [M. Lübben & G. Schäfer (1987) Eur. J. Biochem. 164, 533-540] has been solubilized. It can be easily removed from the membrane by mild treatment with zwitterionic detergents, therefore it appears to be a peripheral membrane protein analogous to the soluble F1-ATPase of eubacteria and eukaryotes. Further purification has been achieved by subsequent gel permeation and ion-exchange chromatography. The final purity is greater than 70% as judged by staining intensities after SDS/polyacrylamide gel electrophoresis. The ATPase consists of two major polypeptides of 65 kDa (alpha) and 51 kDa (beta) in comparable quantities; a minor band (20 kDa) is assumed to be a contaminant or a constitutive part of the enzyme, possibly copurified in substoichiometric amount. The native molecular mass of the solubilized ATPase determined by gel permeation is 430 kDa. Considering the precision of these methods, it remains open whether a 3:3 stoichiometry reflects the contribution of alpha and beta subunits to the quaternary structure, in analogy to known F1-ATPases. The catalytic properties resemble those of the membrane-bound state. There are two pH optima at 5.3 and 8.0 in the absence and only one optimum at 6.5 in the presence of the activating anion sulfite. Activity is strictly dependent on the divalent cations Mg2+ or Mn2+. ATP and dATP are hydrolyzed with highest rates; also other purine and pyrimidine nucleotides are cleaved significantly, but not ADP, pyrophosphate and p-nitrophenyl phosphate. The ATPase is insensitive to azide or vanadate but is inhibited by relatively low concentrations of nitrate. Polyclonal antisera have been raised against the beta subunit of the Sulfolobus ATPase. Cross-reactivities with cellular or membrane extracts of a number of archaebacteria, eubacteria and chloroplasts have been analyzed by means of Western blotting and immunodecoration. A strong cross-reactivity with other genera of the Sulfolobales is observed, also with Methanobacterium, Methanosarcina, Methanolobus and Halobacterium. Even membranes of the eubacterium Escherichia coli and of eukaryotic chloroplast react with the antibodies. With one exception, in all cases the molecular mass of the cross-reacting polypeptide falls in the range of 51-56 kDa. Only in Halobacterium halobium, bands at 66 and 68 kDa have been detected. In order to identify the cross-reacting polypeptides, the purified F1-ATPases of E. coli, chloroplasts and beef heart mitochondria have been tested.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Interaction between the soluble F1 ATPase and its naturally occurring inhibitor protein. Studies using hydrophilic high-performance liquid chromatography and immunoelectron microscopy

Binding of the isolated ATPase (F1) to its naturally occurring inhibitor protein was studied by two novel, independent techniques. High-pressure gel permeation chromatography revealed one tight binding site (Kd = 0.46 microM) for the inhibitor on F1, and a number of weak, non-specific sites. Use of an antibody directed against a non-binding region of the inhibitor protein demonstrated the formation of inhibitor/F1/immunoglobulin G complexes of 1:1:1 and 2:2:1 stoichiometry, but not of the putatively more stable cyclic 4:2:2 complexes. It was concluded that, despite the presence of three beta-subunits, only one site per F1 molecule is available for binding its inhibitor protein.

Total number and differentiation of nucleotide binding sites on mitochondrial F1-ATPase. An approach by photolabeling and equilibrium binding studies

In this study 3'-O-[3-(4-azido-2-nitrophenyl)propionyl]-ADP was used as a photoaffinity analog for nucleotide binding sites on nucleotide-depleted F1-ATPase. Catalytic and binding properties of the labeled enzyme were investigated. The analog behaves as a competitive inhibitor in the dark (Ki = 50 microM). Photoirradiation of F1 in the presence of the analog leads to inactivation depending linearly on the incorporation of label. Complete inactivation is achieved at a stoichiometry of 3 mol/mol F1. The label is distributed between alpha and beta subunits in a ratio of 30%:70%. Although three sites were blocked covalently by photolabeling, three reversible sites of much higher affinity than the labeled sites were preserved. Mild alkaline treatment of photoinactivated enzyme leads to almost complete reactivation which is due to hydrolysis of the 3'-ester bond and release of the ADP moiety from the covalently bound analog. The conclusions drawn are as follows. The total number of sites which can be simultaneously occupied by nucleotides on F1 is six. Adopting the finding [Grubmeyer, C. & Penefsky, H. S. (1981) J. Biol. Chem. 256, 3718-3727] that the high-affinity sites are the catalytic ones which can be covalently labeled by 3'-O-[5-azidonaphthoyl(1)]-ADP [Lübben, M., Lücken, U., Weber, J. & Schäfer, G. (1984) Eur. J. Biochem. 143, 483-490], it appears likely that azidonitrophenylpropionyl-ADP is a specific photolabel for the lower-affinity sites on nucleotide-depleted F1. This means that both types of sites can be differentiated by specific photoaffinity analogs. The labeled low-affinity sites interact with the catalytic sites, abolishing enzyme turnover, when steadily occupied by ADP kept in place by the covalently linking residue, which by itself has no inhibitory effect on the enzyme.

Structural studies of mitochondrial coupling factor 1 using tyrosine fluorescence

The absorbance and fluorescence spectral properties of mitochondrial F1-ATPase confirm that this protein does not contain tryptophan residues and therefore its fluorescence is due to tyrosines. The 36% increase in the fluorescence and the almost 100% increase in quantum yield upon denaturation of the protein suggest that a considerable number of tyrosyl residues have a very low quantum yield in the native enzyme. Quenching experiments using iodide indicate that all of the fluorophores are quenched and also all of them with the same quenching constant. These observations are interpreted as confirmatory of what has been found with several other proteins whose fluorescence originates from tyrosyl residues, where the buried tyrosines fluoresce with a much lower quantum yield than those which are exposed. ATP added to F1 previously depleted of loosely bound nucleotides changes the quenching constant of iodide and the quantum yield and this is interpreted to be due to a conformational change induced by the binding of the nucleotide to the enzyme. Addition of 2-mercaptoethanol decreases, although slightly, the polarization of the fluorescence. However, SDS addition gives a much bigger decrease. Hence disulphide bridges are less important for the tertiary structure of the protein than hydrophobic interactions, hydrogen bonding or other forces. Nevertheless the conformational change induced by reduction of disulphide bridges is detected in iodide quenching experiments and the change of the quantum yield of the enzyme.

Azidonaphthoyl-ADP: a specific photolabel for the high-affinity nucleotide-binding sites of F1-ATPase

3'-O-[5-azidonaphthoyl]-ADP has been synthesized as a photoreactive analog to 3'-O-naphthoyl(1)-ADP which is known to bind to the high-affinity nucleotide sites of mitochondrial F1-ATPase, considered to be the catalytic sites. The photolabel in the dark acts as a ligand to F1-ATPase and as a competitive inhibitor with Ki = 11 microM. Binding to the enzyme is accompanied by a quench of endogenous protein fluorescence leveling off at an occupancy of 1 mol/mol F1, whereas the total number of reversible sites accessible to the analog is 3 mol/mol F1 as measured by isotope studies. Covalent insertion by near ultraviolet activation of the probe yields labeling of both alpha and beta polypeptides of F1; it is accompanied by corresponding removal of reversible high-affinity sites for ADP or naphthoyl-ADP and by an inhibition of the enzyme; total inactivation occurs at a covalent occupancy of 2 mol/mol F1. This is the maximum number of sites accessible to covalent modification by the label; one reversible site is still available in the totally inactivated enzyme. This observation is discussed in terms of a stochastic model requiring a minimum of two interacting catalytic domains out of three in order to commence catalysis.

An electron microscopic approach to the quaternary structure of mitochondrial F1-ATPase

The three-dimensional structure of F1-ATPase from beef heart mitochondria was investigated by electron microscopic techniques. The presence of high concentrations of nucleotides is essential for preservation of the quaternary structure. When investigated under such conditions, monodisperse F1-ATPase could not be distinguished from the membrane-bound enzyme. At low resolution, the particle shape resembles an oblate ellipsoid of revolution with an axial ratio of about 2:1. From several lines of evidence (including field micrographs at higher magnifications, Markham rotational analysis, and tilting experiments), two conclusions may be drawn concerning the three-dimensional fine structure of F1-ATPase. 1. At the periphery of the molecule, six globular protein masses are orientated in a way similar to the chair conformation of cyclohexane. This array is interpreted to be made up of an alternating sequence of alpha and beta subunits. 2. Part of the central space is occupied by a seventh protein mass, protrusions of which are likely to be in contact with some of the outer subunits. A gamma subunit is supposed to be constituent part of this central protein mass. As a consequence, this model favours a stoichiometry of alpha 3 beta 3 gamma for the large subunits of beef heart F1-ATPase.