Studies on the mechanism of oxidative phosphorylation. Different effects of F0 inhibitors on unisite and multisite ATP hydrolysis by bovine submitochondrial particles

Recent developments of imidazo[1,2-a]pyridine analogues as antituberculosis agents

Over the past 2000 years, tuberculosis (TB) has killed more people than any other infectious disease. In 2021, TB claimed 1.6 million lives worldwide, making it the second leading cause of death from an infectious disease after COVID-19. Unfortunately, TB drug discovery research was neglected in the last few decades of the twentieth century. Recently, the World Health Organization has taken the initiative to develop new TB drugs. Imidazopyridine, an important fused bicyclic 5,6 heterocycle has been recognized as a "drug prejudice" scaffold for its wide range of applications in medicinal chemistry. A few examples of imidazo[1,2-a]pyridine exhibit significant activity against multidrug-resistant TB (MDR-TB) and extensively drug-resistant TB (XDR-TB). Here, we critically review anti-TB compounds of the imidazo[1,2-a]pyridine class by discussing their development based on the structure-activity relationship, mode-of-action, and various scaffold hopping strategies over the last decade, which is identified as a renaissance era of TB drug discovery research.

QTL mapping suggests that both cytochrome P450-mediated detoxification and target-site resistance are involved in fenbutatin oxide resistance in Tetranychus urticae

The organotin acaricide fenbutatin oxide (FBO) - an inhibitor of mitochondrial ATP-synthase - has been one of the most extensively used acaricides for the control of spider mites, and is still in use today. Resistance against FBO has evolved in many regions around the world but only few studies have investigated the molecular and genetic mechanisms of resistance to organotin acaricides. Here, we found that FBO resistance is polygenic in two genetically distant, highly resistant strains of the spider mite Tetranychus urticae, MAR-AB and MR-VL. To identify the loci underlying FBO resistance, two independent bulked segregant analysis (BSA) based QTL mapping experiments, BSA MAR-AB and BSA MR-VL, were performed. Two QTLs on chromosome 1 were associated with FBO resistance in each mapping experiment. At the second QTL of BSA MAR-AB, several cytochrome P450 monooxygenase (CYP) genes were located, including CYP392E4, CYP392E6 and CYP392E11, the latter being overexpressed in MAR-AB. Synergism tests further implied a role for CYPs in FBO resistance. Subunit c of mitochondrial ATP-synthase was located near the first QTL of both mapping experiments and harbored a unique V89A mutation enriched in the resistant parents and selected BSA populations. Marker-assisted introgression into a susceptible strain demonstrated a moderate but significant effect of the V89A mutation on toxicity of organotin acaricides. The impact of the mutation on organotin inhibition of ATP synthase was also functionally confirmed by ATPase assays on mitochondrial preparations. To conclude, our findings suggest that FBO resistance in the spider mite T. urticae is a complex interplay between CYP-mediated detoxification and target-site resistance.

Challenging the Drug-Likeness Dogma for New Drug Discovery in Tuberculosis

The emergence of multi- and extensively drug resistant tuberculosis worldwide poses a great threat to human health and highlight the need to discover and develop new, effective and inexpensive antituberculosis agents. High-throughput screening assays against well-validated drug targets and structure based drug design have been employed to discover new lead compounds. However, the great majority fail to demonstrate any antimycobacterial activity when tested against Mycobacterium tuberculosis in whole-cell screening assays. This is mainly due to some of the intrinsic properties of the bacilli, such as the extremely low permeability of its cell wall, slow growth, drug resistance, drug tolerance, and persistence. In this sense, understanding the pathways involved in M. tuberculosis drug tolerance, persistence, and pathogenesis, may reveal new approaches for drug development. Moreover, the need for compounds presenting a novel mode of action is of utmost importance due to the emergence of resistance not only to the currently used antituberculosis agents, but also to those in the pipeline. Cheminformatics studies have shown that drugs endowed with antituberculosis activity have the peculiarity of being more lipophilic than many other antibacterials, likely because this leads to improved cell penetration through the extremely waxy mycobacterial cell wall. Moreover, the interaction of the lipophilic moiety with the membrane alters its stability and functional integrity due to the disruption of the proton motive force, resulting in cell death. When a ligand-based medicinal chemistry campaign is ongoing, it is always difficult to predict whether a chemical modification or a functional group would be suitable for improving the activity. Nevertheless, in the “instruction manual” of medicinal chemists, certain functional groups or certain physicochemical characteristics (i.e., high lipophilicity) are considered red flags to look out for in order to safeguard drug-likeness and avoid attritions in the drug discovery process. In this review, we describe how antituberculosis compounds challenge established rules such as the Lipinski's “rule of five” and how medicinal chemistry for antituberculosis compounds must be thought beyond such dogmatic schemes.

Bioenergetics of Mycobacterium: An Emerging Landscape for Drug Discovery

Mycobacterium tuberculosis (Mtb) exhibits remarkable metabolic flexibility that enables it to survive a plethora of host environments during its life cycle. With the advent of bedaquiline for treatment of multidrug-resistant tuberculosis, oxidative phosphorylation has been validated as an important target and a vulnerable component of mycobacterial metabolism. Exploiting the dependence of Mtb on oxidative phosphorylation for energy production, several components of this pathway have been targeted for the development of new antimycobacterial agents. This includes targeting NADH dehydrogenase by phenothiazine derivatives, menaquinone biosynthesis by DG70 and other compounds, terminal oxidase by imidazopyridine amides and ATP synthase by diarylquinolines. Importantly, oxidative phosphorylation also plays a critical role in the survival of persisters. Thus, inhibitors of oxidative phosphorylation can synergize with frontline TB drugs to shorten the course of treatment. In this review, we discuss the oxidative phosphorylation pathway and development of its inhibitors in detail.

Discovery of Imidazo[1,2-a]pyridine Ethers and Squaramides as Selective and Potent Inhibitors of Mycobacterial Adenosine Triphosphate (ATP) Synthesis

The approval of bedaquiline to treat tuberculosis has validated adenosine triphosphate (ATP) synthase as an attractive target to kill Mycobacterium tuberculosis (Mtb). Herein, we report the discovery of two diverse lead series imidazo[1,2-a]pyridine ethers (IPE) and squaramides (SQA) as inhibitors of mycobacterial ATP synthesis. Through medicinal chemistry exploration, we established a robust structure-activity relationship of these two scaffolds, resulting in nanomolar potencies in an ATP synthesis inhibition assay. A biochemical deconvolution cascade suggested cytochrome c oxidase as the potential target of IPE class of molecules, whereas characterization of spontaneous resistant mutants of SQAs unambiguously identified ATP synthase as its molecular target. Absence of cross resistance against bedaquiline resistant mutants suggested a different binding site for SQAs on ATP synthase. Furthermore, SQAs were found to be noncytotoxic and demonstrated efficacy in a mouse model of tuberculosis infection.

The compound BTB06584 is an IF1 -dependent selective inhibitor of the mitochondrial F1 Fo-ATPase

BACKGROUND AND PURPOSE

Ischaemia compromises mitochondrial respiration. Consequently, the mitochondrial F1 Fo-ATPsynthase reverses and acts as a proton-pumping ATPase, so maintaining the mitochondrial membrane potential (ΔΨm ), while accelerating ATP depletion and cell death. Here we have looked for a molecule that can selectively inhibit this activity without affecting ATP synthesis, preserve ATP and delay ischaemic cell death.

EXPERIMENTAL APPROACH

We developed a chemoinformatic screen based on the structure of BMS199264, which is reported to selectively inhibit F1 Fo-ATPase activity and which is cardioprotective. Results suggested the molecule BTB06584 (hereafter referred to as BTB). Fluorescence microscopy was used to study its effects on ΔΨm and on the rate of ATP consumption following inhibition of respiration in several cell types. The effect of BTB on oxygen (O2 ) consumption was explored and protective potential determined using ischaemia/reperfusion assays. We also investigated a potential mechanism of action through its interaction with inhibitor protein of F1 subunit (IF1 ), the endogenous inhibitor of the F1 Fo-ATPase.

KEY RESULTS

BTB inhibited F1 Fo-ATPase activity with no effect on ΔΨm or O2 consumption. ATP consumption was decreased following inhibition of respiration, and ischaemic cell death was reduced. BTB efficiency was increased by IF1 overexpression and reduced by silencing the protein. In addition, BTB rescued defective haemoglobin synthesis in zebrafish pinotage (pnt) mutants in which expression of the Atpif1a gene is lost.

CONCLUSIONS AND IMPLICATIONS

BTB may represent a valuable tool to selectively inhibit mitochondrial F1 Fo-ATPase activity without compromising ATP synthesis and to limit ischaemia-induced injury caused by reversal of the mitochondrial F1 Fo-ATPsynthase.

Nanoparticles of novel organotin(IV) complexes bearing phosphoric triamide ligands

Four novel organotin(IV) complexes containing phosphoric triamide ligands were synthesized and characterized by multinuclear ((1)H, (31)P, (13)C) NMR, infrared, ultraviolet and fluorescence spectroscopy as well as elemental analysis. The (1)H NMR spectra of complexes 1-4 proved that the Sn atoms adopt octahedral configurations. The nanoparticles of the complexes were also prepared by ultrasonication, and their SEM micrographs indicated identical spherical morphologies with particles sizes about 20-25 nm. The fluorescence spectra exhibited blue shifts for the maximum wavelength of emission upon complexation.

Respiratory ATP synthesis: the new generation of mycobacterial drug targets?

Mycobacterium tuberculosis, the causative agent of tuberculosis, poses a global health challenge due to the emergence of drug-resistant strains. Recently, bacterial energy metabolism has come into focus as a promising new target pathway for the development of antimycobacterial drugs. This review summarizes our current knowledge on mycobacterial respiratory energy conversion, in particular, during the physiologically dormant state that is associated with latent or persistent tuberculosis infections. Targeting components of respiratory ATP production, such as type-2 NADH dehydrogenase or ATP synthase, is illustrated as an emerging strategy in the development of novel drugs.

Mitochondrial involvement in drug-induced liver injury

Mitochondrial dysfunction is a major mechanism of liver injury. A parent drug or its reactive metabolite can trigger outer mitochondrial membrane permeabilization or rupture due to mitochondrial permeability transition. The latter can severely deplete ATP and cause liver cell necrosis, or it can instead lead to apoptosis by releasing cytochrome c, which activates caspases in the cytosol. Necrosis and apoptosis can trigger cytolytic hepatitis resulting in lethal fulminant hepatitis in some patients. Other drugs severely inhibit mitochondrial function and trigger extensive microvesicular steatosis, hypoglycaemia, coma, and death. Milder and more prolonged forms of drug-induced mitochondrial dysfunction can also cause macrovacuolar steatosis. Although this is a benign liver lesion in the short-term, it can progress to steatohepatitis and then to cirrhosis. Patient susceptibility to drug-induced mitochondrial dysfunction and liver injury can sometimes be explained by genetic or acquired variations in drug metabolism and/or elimination that increase the concentration of the toxic species (parent drug or metabolite). Susceptibility may also be increased by the presence of another condition, which also impairs mitochondrial function, such as an inborn mitochondrial cytopathy, beta-oxidation defect, certain viral infections, pregnancy, or the obesity-associated metabolic syndrome. Liver injury due to mitochondrial dysfunction can have important consequences for pharmaceutical companies. It has led to the interruption of clinical trials, the recall of several drugs after marketing, or the introduction of severe black box warnings by drug agencies. Pharmaceutical companies should systematically investigate mitochondrial effects during lead selection or preclinical safety studies.

Selectivity of TMC207 towards mycobacterial ATP synthase compared with that towards the eukaryotic homologue

The diarylquinoline TMC207 kills Mycobacterium tuberculosis by specifically inhibiting ATP synthase. We show here that human mitochondrial ATP synthase (50% inhibitory concentration [IC(50)] of >200 microM) displayed more than 20,000-fold lower sensitivity for TMC207 compared to that of mycobacterial ATP synthase (IC(50) of 10 nM). Also, oxygen consumption in mouse liver and bovine heart mitochondria showed very low sensitivity for TMC207. These results suggest that TMC207 may not elicit ATP synthesis-related toxicity in mammalian cells. ATP synthase, although highly conserved between prokaryotes and eukaryotes, may still qualify as an attractive antibiotic target.

Chloroacetaldehyde: mode of antitumor action of the ifosfamide metabolite

BACKGROUND

The ifosfamide metabolite chloroacetaldehyde had been made responsible for side effects only. We found in previous studies a strong cytotoxicity on human MX-1 tumor cells and xenografts in nude mice. Chloroacetaldehyde is supposed to act via alkylation or by inhibition of mitochondrial oxidative phosphorylation with decrease of ATP. The aim of this study was to further elucidate chloroacetaldehyde's mode of action.

METHODS

MX-1 breast carcinoma cells were measured for ATP-content after exposure to chloroacetaldehyde. Further, the effect of chloroacetaldehyde on DNA-synthesis and its potency of causing strand-breaks or cross-links were investigated by bromodeoxyuridine-incorporation, comet-assay and a DNA interstrand cross-linking-assay.

RESULTS

Chloroacetaldehyde in high concentrations induces a reduction of ATP-levels when anaerobic glycolysis is blocked by oxamate and reduces the bromodeoxyuridine-incorporation to 46.3% after 4 h when used in IC(50) concentrations (7.49 mumol/l). In addition we observed DNA single strand-breaks in MX-1 cells treated with chloroacetaldehyde visible in the Comet assay, but no DNA-cross-linking by comet assay and cross-linking assay.

CONCLUSION

In summary, our results show that chloroacetaldehyde influences the oxidative phosphorylation in mitochondria, however, this is observed only in high concentrations and is not of clinical relevance because the tumor cells regenerate ATP by anaerobic glycolysis. Nevertheless, chloroacetaldehyde causes DNA-strand-breaks and strong inhibition of DNA-synthesis.

Observation of calcium-dependent unidirectional rotational motion in recombinant photosynthetic F1-ATPase molecules

ATP hydrolysis and synthesis by the F(0)F(1)-ATP synthase are coupled to proton translocation across the membrane in the presence of magnesium. Calcium is known, however, to disrupt this coupling in the photosynthetic enzyme in a unique way: it does not support ATP synthesis, and CaATP hydrolysis is decoupled from any proton translocation, but the membrane does not become leaky to protons. Understanding the molecular basis of these calcium-dependent effects can shed light on the as yet unclear mechanism of coupling between proton transport and rotational catalysis. We show here, using an actin filament gamma-rotation assay, that CaATP is capable of sustaining rotational motion in a highly active hybrid photosynthetic F(1)-ATPase consisting of alpha and beta subunits from Rhodospirillum rubrum and gamma subunit from spinach chloroplasts (alpha(R)(3)beta(R)(3)gamma(C)). The rotation was found to be similar to that induced by MgATP in Escherichia coli F(1)-ATPase molecules. Our results suggest a possible long range pathway that enables the bound CaATP to induce full rotational motion of gamma but might block transmission of this rotational motion into proton translocation by the F(0) part of the ATP synthase.

Conformational change of the chloroplast ATP synthase on the enzyme activation process detected by the trypsin sensitivity of the gamma subunit

Delta mu H(+) is known to stimulate the enzyme activity of chloroplast ATP synthase in addition to its important role as energy supply for ATP synthesis. In the present study, we focused on the relationship between the proton translocation via the membrane sector of ATP synthase, F(o), and the conformational change of the central stalk subunit gamma. The conformational change of CF(1) mainly at the gamma subunit was induced by the proton flow via F(o) in the absence of substrates. The effects of inhibitors on CF(o) or CF(1) for this conformational change were also examined. The observed conformational change was partially suppressed by ADP binding. From these results, we propose the Delta mu H(+)-dependent conformational change of CF(1) on the enzyme activation process, which is affected by both ADP binding to the catalytic sites and proton flow via F(o) portion.

Reversible ischemic inhibition of F(1)F(0)-ATPase in rat and human myocardium

The physiological role of F(1)F(0)-ATPase inhibition in ischemia may be to retard ATP depletion although views of the significance of IF(1) are at variance. We corroborate here a method for measuring the ex vivo activity of F(1)F(0)-ATPase in perfused rat heart and show that observation of ischemic F(1)F(0)-ATPase inhibition in rat heart is critically dependent on the sample preparation and assay conditions, and that the methods can be applied to assay the ischemic and reperfused human heart during coronary by-pass surgery. A 5-min period of ischemia inhibited F(1)F(0)-ATPase by 20% in both rat and human myocardium. After a 15-min reperfusion a subsequent 5-min period of ischemia doubled the inhibition in the rat heart but this potentiation was lost after 120 min of reperfusion. Experiments with isolated rat heart mitochondria showed that ATP hydrolysis is required for effective inhibition by uncoupling. The concentration of oligomycin for 50% inhibition (I(50)) for oxygen consumption was five times higher than its I(50) for F(1)F(0)-ATPase. Because of the different control strengths of F(1)F(0)-ATPase in oxidative phosphorylation and ATP hydrolysis an inhibition of the F(1)F(0)-ATPase activity in ischemia with the resultant ATP-sparing has an advantage even in an ischemia/reperfusion situation.

Triphenyltin salicylate-antimicrobial effect and resistance--the pyrophosphatase connection

The effect of Triphenyltin salicylate (TPS) was tested against six bacteria, Escherichia coli, Staphylococcus aureus, Shigella flexneri, Pseudomonas aeruginosa, Klebsiella pneumoniae and Salmonella typhi and five fungi, Aspergillus flavus, Aspergillus fumigatus, Aspergillus niger, Rhodotorula spp. and Saccharomyces spp. Sensitivity tests were determined with 5-500 microg/ml of TPS. All organisms were sensitive to the compound except Klebsiella pneumoniae, Pseudomonas aeruginosa, Rhodotorula spp. and Saccharomyces spp. The minimum dose of TPS that can kill 50% of the susceptible microorganisms is in the range 5-50 microg/ml. Membrane bound pyrophosphatase(s) from the organisms was non-competitively inhibited by 5 microM TPS with Ki values of 7.6, 18, 8.8 and 6.9 microM for Escherichia coli, Shigella flexneri, Aspergillus niger, and Aspergillus fumigatus, respectively. The physiological index of efficiency of the enzyme (Vmax/KM) for TPS susceptible organisms was reduced by 17-68% in the presence of 5-10 microM of the compound. In contrast the index for the non-susceptible organisms was unaffected. The mode of action of TPS is discussed.

Isolation and characterization of mitochondrial F(1)-ATPase from crayfish (Orconectes virilis) gills

A soluble F(1)-ATPase was isolated from the mitochondria of crayfish (Orconectes virilis) gill tissue. The maximal mitochondrial disruption rate (95%) was obtained by sonicating for 4 min at pH 8.6. A 15-fold purification was estimated. The properties for both soluble and membrane-bound enzyme were studied. Both enzyme forms were stable at 4 to -70 degrees C when kept in 20% glycerol. Soluble F(1)-ATPase was more stable at room temperature than membrane-bound enzyme. It displayed a narrower pH profile (pK(1) =6.58, pK(2)=7.68) and more acid pH optimum (7.13) than membrane-bound enzyme (pK(1)=6.42, pK(2)=8.55, optimum pH 7.49). The anion-stimulated activities were in the order HCO(3)(-)>SO(4)(2-)>Cl(-). The apparent K(a) values for soluble enzyme were 11.4, 11.2, and 10.9 mM, respectively, but the K(a) of HCO(3)(-) for membrane-bound enzyme (14.9 mM) was higher than for soluble enzyme. Oligomycin and DCCD inhibited membrane-bound F(1)-ATPase with I(50) of 18.6 ng/ml and 2.2 microM, respectively, but were ineffective in inhibiting soluble enzyme. Both enzyme forms shared identical sensitivity to DIDS (I(50)=12.5 microM) and vanadate (I(50)=9.0 mM). Soluble ATPase was significantly more sensitive to pCMB (I(50)=0.15 microM) and NO(3)(-) (I(50)=28.6 mM) than membrane-bound enzyme (I(50)=1.04 microM pCMB and 81.5 mM NO(3)(-)). In addition, soluble F(1)-ATPase was slightly more sensitive to azide (I(50)=91.8 microM) and NBD-Cl (I(50)=9.18 microM) than membrane-bound enzyme (I(50)=111.6 microM azide and 12.88 microM NBD-Cl). These data suggest a conformational change transmission between F(0) and F(1) sectors and slight conformational differences between soluble F(1) and membrane-bound F(1). In addition, an unmodified F(0) stabilizes F(1) and decreases F(1) sensitivities to inhibitors and modulators.

Rotational coupling in the F0F1 ATP synthase

The F0F1 ATP synthase is a large multisubunit complex that couples translocation of protons down an electrochemical gradient to the synthesis of ATP. Recent advances in structural analyses have led to the demonstration that the enzyme utilizes a rotational catalytic mechanism. Kinetic and biochemical evidence is consistent with the expected equal participation of the three catalytic sites in the alpha 3 beta 3 hexamer, which operate in sequential, cooperative reaction pathways. The rotation of the core gamma subunit plays critical roles in establishing the conformation of the sites and the cooperative interactions. Mutational analyses have shown that the rotor subunits are responsible for coupling and in doing so transmit specific conformational information between transport and catalysis.

One of the non-exchangeable nucleotides of the mitochondrial F1-ATPase is bound at a beta-subunit: evidence for a non-rotatory two-site catalytic mechanism

In active MF1, one of the two non-exchangeable tightly bound adenine nucleotides is an ATP, while the other is an ADP. The respective sites are called the T-site and the D-site. The activity of the enzyme correlates linearly with the amount of bound ATP, ADP at the T-site being inhibitory. When MF1 is stored at room temperature in 50% glycerol and 100 mM Tris-HCl (pH 7.3) after slow passage through a Sephadex column, the tightly bound ATP is slowly dephosphorylated to ADP which is subsequently released, without effect on activity. When enzyme with about one residual ADP left (at the D-site) was incubated at pH 7.3, after dilution of the glycerol, with 400 &mgr;M [14C]ATP under varying conditions, the amount of tightly bound nucleotide triphosphate again correlated well with activity, the residual ADP being bound at the D-site. Optimal results were obtained when the incubation was performed in the presence of a regenerating system. Binding of 2-azido-ATP instead of ATP to the T-site as a triphosphate, as indicated by the specific activity of the enzyme, appeared to be optimal when the binding was performed at pH 6.4 in the absence of Mg2+ and with high concentrations of the nucleotide. Under such conditions, 3 mol 2-azido-AXP per mol F1 remained tightly bound after ammonium sulfate precipitation and column centrifugation, in addition to about one residual ADP at the D-site. After a 2-min period of turnover with ATP/Mg2+ as substrate two mol 2-azido-AXP were left on the enzyme, of which one was bound at a beta-site. These results show that one of the non-catalytic nucleotide binding sites that contain tightly bound nucleotides, is a beta-site, in conflict with the requirements for a rotatory tri-site mechanism for ATP hydrolysis. This beta-site can further be identified with the T-site. The validity of these conclusions for F1 from other sources and for catalysis by membrane-bound enzyme is discussed.

Triphenyltin as an inhibitor of membrane-bound pyrophosphatase of Rhodospirillum rubrum

The effect of triphenyltin on the activity of membrane-bound pyrophosphatase of Rhodospirillum rubrum was investigated. Triphenyltin inhibits the hydrolysis of chromatophore membrane-bound pyrophosphatase in a pH-dependent pattern, being maximal at pH 9-10. At basic pH values, the inhibition produced by this organotin on membrane-bound pyrophosphatase is very similar to that produced on the chromatophore H+ATPase (I50 = 14.4 and 10 microM, respectively). Detergent-solubilized membrane-bound pyrophosphatase is also inhibited by triphenyltin, but the cytoplasmic enzyme of R. rubrum is inhibited only slightly. The inhibitory effect of triphenyltin on membrane-bound pyrophosphatase is the same with Mg-PPi or Zn-PPi, and is dependent on the chromatophore membrane concentration. Triphenyltin modified mainly the Vmax of the enzyme, and only slightly its Km. Free Mg2+ does not reverse the inhibition. Reducing agents prevent triphenyltin inhibition of the membrane-bound pyrophosphatase, but their effect is due to an alteration of the inhibitor, and not to a modification of thiol groups of the enzyme. The most likely site for triphenyltin inhibition in chromatophore membrane-bound pyrophosphatase is a component either within or closely associated with the membrane.

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.

The bound adenine nucleotides of purified bovine mitochondrial ATP synthase

The experiments in this study were directed towards defining the nucleotide content of purified beef-heart mitochondrial F1F0 ATP synthase during binding and hydrolysis of ATP. The purified, soluble synthase as prepared contained 2 mol ATP and 2 mol ADP/mol enzyme. Three of these four nucleotides were exchangeable on incubation with radiolabelled MgATP. Passage of the ATP synthase through a column of Sephadex G-50 readily removed 1 mol ADP/mol. The remaining bound nucleotides were not displaced by incubation with 1 mM GTP or 5 mM sodium sulfite, the latter an activator of the ATPase activity of the synthase. Incubation of the synthase with 250 microM MgATP in the presence of 3 mM sodium azide, an inhibitor of the ATPase, resulted in the transitory formation of a form of the enzyme in which 5-6 nucleotide-binding sites were loaded with ATP and/or ADP, thus showing that the ATP synthase, like the soluble F1 ATPase, contained a minimum of six nucleotide-binding sites. The presence of an ATP-regenerating system during incubation with MgATP resulted in the loading of 5-6 sites to yield a form of the enzyme containing 3-4 mol ATP and 2 mol ADP/mol synthase even after passage through a centrifuged column. Following hydrolysis of the medium MgATP, the enzyme reached a stable form containing 2 mol ATP and 2 mol ADP/mol synthase. Like the form of the enzyme originally prepared, 1 mol ADP/mol synthase was readily released. However, this ADP remained bound to the synthase in the presence of GTP if azide was present. These results are discussed in the context of current ideas about nucleotide-binding sites on the F1 ATPase portion of the F1F0 ATP synthase. It is concluded that the properties of the sites on the F1F0 synthase show some differences from those on the F1 ATPase.

Energetics of ATP dissociation from the mitochondrial ATPase during oxidative phosphorylation

The dissociation constant (KdATP) for ATP bound in the high affinity catalytic site of membrane-bound beef heart mitochondrial ATPase (F1) was calculated from the ratio of the rate constants for the reverse dissociation step (k-1) and the forward binding step (k+1). k-1 for ATP bound to submitochondrial particles or to submitochondrial particles washed with KCl so as to activate ATPase activity was accelerated by about five orders of magnitude during respiratory chain-linked oxidations of NADH. In the presence of NADH and 0.1 mM ADP, k-1 increased more than six orders of magnitude. These energy-dependent dissociations of ATP were sensitive to the uncoupler carbonyl cyanide p-trifluoromethyloxyphenylhydrazone. Only small changes in k+1 were observed in the presence of NADH or NADH and ADP. KdATP at 23 degrees C in the absence of NADH and ADP was 10(-12) M, in the presence of NADH, 3 microM, and in the presence of NADH and 0.1 mM ADP, 60 microM. Thus, the dissociation of ATP during the transition from non-energized to energized states was, under these conditions, accompanied by observed free energy changes of 8 and 9.7 kcal/mol, respectively.