2,4-Dinitrophenol causes a marked increase in the apparent Km of Pi and of ADP for oxidative phosphorylation

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.

Modulation of the mitochondrial permeability transition by cyclophilin D: moving closer to F(0)-F(1) ATP synthase?

Cyclophilin D was recently shown to mask an inhibitory site of the mitochondrial permeability transition pore (PTP) for phosphate, and to constitutively bind F(0)-F(1) ATP synthase resulting in the slowing of ATP synthesis and hydrolysis rates, thus regulating matrix adenine nucleotide levels. Here we review the striking similarities of the factors affecting the threshold for PTP induction, to those affecting binding of phosphate to formerly proposed sides on F(1)-ATPase affecting ATP hydrolytic activity, including critical arginine residues, matrix pH, [Mg(2+)], adenine nucleotides and proton motive force. Based on these similarities, we scrutinize the hypothesis that in depolarized mitochondria exhibiting reversal of F(0)-F(1) ATP synthase operation, the genetic ablation of cyclophilin D or its inhibition by cyclosporin A results in accelerated proton pumping by ATP hydrolysis, opposing a further decrease in membrane potential and promoting high matrix phosphate levels, both negatively affecting the probability of PTP opening.

Paracoccus denitrificans proton-translocating ATPase: kinetics of oxidative phosphorylation

The initial rates of ATP synthesis catalyzed by tightly coupled Paracoccus denitrificans plasma membrane were measured. The reaction rate was hyperbolically dependent on the substrates, ADP and inorganic phosphate (P(i)). Apparent K(m) values for ADP and P(i) were 7-11 and 60-120 µM, respectively, at saturating concentration of the second substrate (pH 8.0, saturating Mg²(+)). These values were dependent on coupling efficiency. The substrate binding in the ATP synthesis reaction proceeds randomly: K(m) value for a given substrate was independent of the concentration of the other one. A decrease of electrochemical proton gradient by the addition of malonate (when succinate served as the respiratory substrate) or by a decrease of steady-state level of NADH (when NADH served as the respiratory substrate) resulted in a proportional decrease of the maximal rates and apparent K(m) values for ADP and P(i) (double substitution, ping-pong mechanism). The kinetic scheme for ATP synthesis was compared with that described previously for the proton-translocating ATP hydrolysis catalyzed by the same enzyme preparation (T. V. Zharova and A. D. Vinogradov (2006) Biochemistry, 45, 14552-14558).

Proton transport coupled ATP synthesis by the purified yeast H+ -ATP synthase in proteoliposomes

The H(+)/ATP synthase from yeast mitochondria, MF₀F₁, was purified and reconstituted into liposomes prepared from phosphatidylcholine and phosphatidic acid. Analysis by mass spectrometry revealed the presence of all subunits of the yeast enzyme with the exception of the K-subunit. The MF₀F₁ liposomes were energized by acid-base transitions (DeltapH) and a K(+)/valinomycin diffusion potential (Deltaphi). ATP synthesis was completely abolished by the addition of uncouplers as well as by the inhibitor oligomycin. The rate of ATP synthesis was optimized as a function of various parameters and reached a maximum value (turnover number) of 120s⁻¹ at a transmembrane pH difference of 3.2 units (at pH(in)=4.8 and pH(out)=8.0) and a Deltaphi of 133mV (Nernst potential). Functional studies showed that the monomeric MF₀F₁, was fully active in ATP synthesis. The turnover increased in a sigmoidal way with increasing internal and decreasing external proton concentration. The dependence of the turnover on the phosphate concentration and the dependence of K(M) on pH(out) indicated that the substrate for ATP synthesis is the monoanionic phosphate species H₂PO⁻₄.

Inhibitory Mg-ADP-fluoroaluminate complexes bound to catalytic sites of F(1)-ATPases: are they ground-state or transition-state analogs?

Schemes are proposed for coupling sequential opening and closing the three catalytic sites of F(1) to rotation of the gamma subunit during ATP synthesis and hydrolysis catalyzed by the F(o)F(1)-ATP synthase. A prominent feature of the proposed mechanisms is that the transition state during ATP synthesis is formed when a catalytic site is in the process of closing and that the transition state during ATP hydrolysis is formed when a catalytic site is in the process of opening. The unusual kinetics of formation of Mg-ADP-fluoroaluminate complexes in one or two catalytic sites of nucleotide-depleted MF(1) and wild-type and mutant alpha(3)beta(3)gamma subcomplexes of TF(1) are also reviewed. From these considerations, it is concluded that Mg-ADP-fluoroaluminate complexes formed at catalytic sites of isolated F(1)-ATPases or F(1) in membrane-bound F(o)F(1) are ground-state analogs.

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.

Catalytic site forms and controls in ATP synthase catalysis

A suggested minimal scheme for substrate binding by and interconversion of three forms of the catalytic sites of the ATP synthase is presented. Each binding change, that drives simultaneous interchange of the three catalytic site forms, requires a 120 degrees rotation of the gamma with respect to the beta subunits. The binding of substrate(s) at two catalytic sites is regarded as sufficing for near maximal catalytic rates to be attained. Although three sites do not need to be filled for rapid catalysis, during rapid bisite catalysis some enzyme may be transiently present with three sites filled. Forms with preferential binding for ADP and P(i) or for ATP are considered to arise from the transition state and participate in other steps of the catalysis. Intermediate forms and steps that may be involved are evaluated. Experimental evidence for energy-dependent steps and for control of coupling to proton translocation and transition state forms are reviewed. Impact of relevant past data on present understanding of catalytic events is considered. In synthesis a key step is suggested in which proton translocation begins to deform an open site so as to increase the affinity for ADP and P(i), that then bind and pass through the transition state, and yield tightly bound ATP in one binding change. ADP binding appears to be a key parameter controlling rotation during synthesis. In hydrolysis ATP binding to a loose site likely precedes any proton translocation, with proton movement occurring as the tight site form develops. Aspects needing further study are noted. Characteristics of the related MgADP inhibition of the F(1) ATPases that have undermined many observations are summarized, and relations of three-site filling to catalysis are assessed.

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.

Physical mechanism of ATP formation in biomembranes

Membrane-bound and isolated H+ ATPases of various origin are able to synthesize ATP from ADP and Pi after a jump-like pH increase. In the course of this increase the pH of solution (or suspension) must cross a value corresponding to pK of certain acid groups in the catalytic component of ATPase. In the case of isolated soluble enzymes it is possible to obtain up to 10 ATP molecules per one pH jump per one enzyme molecule. A physical mechanism of this phenomenon as well as of oxidative and photosynthetic phosphorylation is suggested.

Activity of ATP synthetase complex after low temperature treatment or freeze-drying of mitochondria isolated from skeletal muscles

The influence of freezing, thawing, or freeze-drying on ATP synthetase complex of isolated skeletal muscle mitochondria was studied. Cooling to -60 or to -196 degrees C and rapid thawing did not change activity significantly. Slow warming stimulated the release of latent ATP-ase activity and decreased ATP synthesis. These changes were more pronounced after freeze-drying.

Simultaneous synthesis and hydrolysis of ATP regulated by the inhibitor protein in submitochondrial particles

Coupled submitochondrial particles from bovine heart with ATP synthases devoid of control by the inhibitor protein of Pullman and Monroy [J. Biol. Chem. 238, 3762-3769 (1963)] can be prepared by incubation of Mg-ATP particles in 50 mM phosphate, 250 mM sucrose, and greater than 95% D2O (pD 7.8) at 38 degrees C. As monitored with oxonol, the respiring particles build up and maintain a delta psi about 5-10% lower than that of the starting preparation. With oligomycin delta psi of the two preparations is the same. In the presence of an ATP trap (hexokinase and glucose), the two types of particles carry out oxidative phosphorylation at comparable rates. Low concentrations of oligomycin induce a small enhancement of the rate of ATP synthesis in non-controlled particles. In the absence of an ATP trap, net accumulation of ATP, as driven by electron transport in particles without control by the inhibitor protein, is low. Apparently this is due to lack of control by the inhibitor protein of ATP hydrolysis that occurs during oxidative phosphorylation.

Studies on the mechanism of oxidative phosphorylation: effects of specific F0 modifiers on ligand-induced conformation changes of F1

Aurovertin is a fluorescent antibiotic that binds to the catalytic beta subunits of the mitochondrial F1-ATPase and inhibits ATP synthesis and hydrolysis. ATP, ADP, and membrane energization in submitochondrial particles (SMP) alter the fluorescence of F1-bound aurovertin. These fluorescence changes are considered to be in response to the conformation changes of F1-ATPase. This paper shows that the ATP-induced fluorescence change of aurovertin bound to SMP or complex V (purified ATP synthase complex F0-F1) is inhibited when these preparations are pretreated with oligomycin or N,N'-dicyclohexylcarbodiimide (DCCD). This inhibition is not seen with isolated F1-ATPase. These and other results have suggested that modifications of the DCCD-binding protein in the membrane sector (F0) of the ATP synthase complex are communicated to F1, thereby altering the binding characteristics of ATP to the beta subunits. By analogy, it is proposed that modifications (e.g., protonation/deprotonation) of the DCCD-binding protein effected by protonic energy alter the conformation of F1 and bring about the substrate/product binding changes that appear to be essential features of the mechanism and regulation of oxidative phosphorylation.

Functional changes in mitochondrial properties as a result of their membrane cryodestruction. II. Influence of freezing and thawing on ATP complex activity of intact liver mitochondria

The influence of the freeze-thawing rates on ATP synthetase (ATPase) complex of intact liver mitochondria was investigated. It was shown that the increase in latent ATPase activity and decrease in ATP synthetase activity resulted from an influence on the inner mitochondrial membrane. An increase in freeze-thawing rates led to the preservation of ATP synthetase activity and ATP hydrolysis reduction. Kinetic parameter changes of the ATP synthetase reaction resulted from an insignificant nonspecific increase in the inner mitochondrial membrane permeability and changes in its electrochemical potential level.

Steady-state kinetics of the overall oxidative phosphorylation reaction in heart mitochondria. Determination of the coupling relationships between the respiratory reactions and miscellaneous observations concerning rate-limiting steps

The linear sequence of steps involved in the oxidation of extramitochondrial succinate by O2 in bovine heart mitochondria was examined by a steady-state kinetic method to determine whether or not freely diffusible intermediates occur between the various inhibitor-sensitive steps. The kinetic method is based on the facts (1) that if two inhibitor-sensitive steps within a sequence are linked by a freely diffusible intermediate, inhibition of one will make the other less rate limiting in the overall reaction and thus will increase the amount of inhibitor of the other step required for half-maximal inhibition of the overall reaction, and (2) that if the two steps are not linked in this manner, inhibition of one will make the other more rate limiting and thus will decrease the amount of inhibitor of the other required for half-maximal inhibition. These two types of "coupling relationships" between steps were designated as "sequential" and "fixed," respectively. The results indicate the existence of freely diffusible intermediates (sequential coupling relationships) between the succinate transport and succinate dehydrogenase reactions, between the succinate dehydrogenase and cytochrome bc1 reactions, and between the cytochromes bc1 and aa3 reactions. Uncoupling respiration from phosphorylation results in the coupling relationship between the bc1 and aa3 reactions becoming partially fixed. This change is accompanied by marked decreases in the degrees to which the bc1 and aa3 reactions limit the overall reaction and appears to account for the large uncoupler-induced releases of inhibition at the levels of the bc1 and aa3 reactions observed previously by others. It is suggested that cytochrome c is the freely diffusible intermediate between the bc1 and aa3 reactions and that the uncoupler-induced changes occur as a result of formation of functional and highly efficient supercomplexes between cytochrome c and the cytochromes bc1 and aa3 complexes.

Kinetic effects of chemical and physical uncoupling on the energy-transducing ATPase from spinach chloroplasts

Ammonium chloride, an uncoupler of photophosphorylation which stimulates the membrane-bound chloroplast coupling factor ATPase when added after light/dithiothreitol activation, causes a decrease in the number of extra water oxygens incorporated into the phosphate formed during ATP hydrolysis. This observation is in contrast to the long-reported insensitivity of intermediate Pi:H2O oxygen exchange to uncoupler dinitrophenol in the mitochondrial F1 ATPase system. The effect of ammonium chloride on the CF1-catalyzed oxygen exchange reaction is consistent with ATPase activity stimulation caused by increased partitioning forward of the enzyme . products complex. In line with the oxygen exchange data, ammonium chloride causes an increase in the apparent Km of the enzyme for substrate ATP. The effect of ammonium chloride on the pattern of the intermediate Pi:H2O oxygen exchange is not a threshold phenomenon; the extent of exchange decreases in a continuous fashion, paralleling the stimulation of ATPase activity. The uncoupler CF3OPhzC(CN)2 also decreases the extent of oxygen exchange upon stimulating the membrane-bound ATPase, while phlorizin, an energy-transfer inhibitor, has essentially no effect on exchange although it inhibits the ATPase reaction. Similar to the effect of chemical uncoupling on the membrane-bound enzyme, physical removal of the coupling factor ATPase from the thylakoid membrane also results in an increase in forward partitioning of the enzyme . ADP . Pi complex. The modulation of oxygen exchange observed by altering the degree of coupling is similar to that which accompanies changing ATP concentration in the mitochondrial ATPase system [Russo, J. A., Lamos, C. M. and Mitchell, R. A. (1978) Biochemistry 17,473-480 and Choate, G. L., Hutton, R. L. and Boyer, P. D. (1979) J. Biol. Chem. 254, 286-290]. However, the uncoupler modulation is not readily correlated with the degree to which multiple catalytic sites are occupied by substrate.

The effects of partial uncoupling upon the kinetics of ATP synthesis by vesicles from Paracoccus denitrificans and by bovine heart submitochondrial particles. Implications for the mechanism of the proton-translocating ATP synthase

1. Reduction in the magnitude of the respiration-dependent protonmotive force (proton electrochemical gradient in mV) of vesicles from Paracoccus denitrificans, and of submitochondrial particles, has been found to be paralleled small increases in S50% values for both ADP and Pi. For example, reduction of the protonmotive force of P. denitrificans vesicles from 145 mV to 110 mV was accompanied by an increase of S50% (ADP) from 8 microM to 18 microM, and an increase of S50% (Pi) from 0.33 mM to 1.4 mM. This result was obtained with partial uncoupling quantities of both carbonyl-cyanide p-trifluoromethoxyphenylhydrazone and of the synergistic combination of nigericin plus valinomycin in the presence of K+. In view of the similar effects of these two different methods of uncoupling it is concluded that the changes in S50% were a consequence of the diminished protonmotive force acting on the ATP synthase rather than of a secondary, direct interaction of the uncouplers with the enzyme. Changes in S50% rather than Km are described because under several sets of conditions double-reciprocal plots were nonlinear. 2. For equivalent attenuations in the rate of ATP synthesis by submitochondrial particles, 2,4-dinitrophenol caused much larger increases in S50% (ATP) than did carbonylcyanide p-trifluoromethoxyphenylhydrazone. Therefore it is concluded that the effect of 2,4-dinitrophenol was primarily a consequence of its previously recognized direct interaction with the F1 segment of the mitochondrial ATPase. The concentration range of 2,4-dinitrophenol that raised S50% (ADP) is similar to that which weakens the binding of ADP to a particular type of site on the purified F1 sector of ATP synthase. This correlation is consistent with such a site having a catalytic role during ATP synthesis. 3. A titration of the rate of ATP synthesis by vesicles of P. denitrificans with increasing quantities of carbonylcyanide p-trifluoromethoxyphenylhydrazone showed that the initial titres of the uncoupler caused large decreases in the rate of ATP synthesis for relatively small attenuations in the protonmotive force. Thus the initial 20 mV drop in the protonmotive force was accompanied by a reduction of more than 65% in the rate of ATP synthesis. Over the lowest range of values of protonmotive force that drove detectable rates of ATP synthesis however, the dependence of the rate was a less steep function of the protonmotive force. A plot of the logarithm of the rate of ATP synthesis against protonmotive force reveals a biphasic relationship. There does not appear to be a 'threshold' value of the protonmotive force below which ATP synthesis is blocked by kinetic factors. 4. The relationships of the protonmotive force with S50% values and with the rate of ATP synthesis (at near saturating concentrations of ADP and Pi) are discussed in relation to possible mechanisms for the coupling of proton translocation to ATP synthesis.

Interaction of high-affinity nucleotide binding sites in mitochondrial ATP synthesis and hydrolysis

The present study contributes to the problem of the dynamic structure of mitochondrial F1-ATPase and the functional interrelation of so-called tight nucleotide binding sites. Nucleotide analogs are used as a tool to differentiate two distinct functional states of the membrane-bound enzyme, proposed to reflect corresponding conformational states; they reveal F1-ATPase as a "dual-state" enzyme: ATP-synthetase, and ATP-hydrolase. The analogs used are 3'-naphthoyl esters of AD(T)P, and 2'(3')-O-trinitrophenyl ethers of AD(T)P. Both types of analogs act inversely to each other with respect to their relative effects on oxidative phosphorylation and on ATPase in submitochondrial vesicles. The respective ratios of Ki versus both processes are 250/1 compared to 1/170. It is also shown that in the presence of the inhibitory 3'-esters oxidative phosphorylation deviates from linear kinetics and that these inhibitors induce a lag time of oxidative phosphorylation depending on the initial pattern of nucleotides available to energized submitochondrial vesicles. The duration of the lag time coincides with the time course of displacement of the analog from a tight binding site. The conclusions of the study are: (a) the catalytic sites of F1-ATP-synthetase are not operating independently from each other; they rather interact in a cooperative manner; (b) F1-ATPase as a "dual-state" enzyme exhibits highly selective responses to tight binding of nucleotides or analogs in its "energized" (membrane-bound) state versus its "nonenergized" state, respectively.

A hypothesis for the role of dithiol-disulfide interchange in solute transport and energy-transducing processes

We have recently shown that the physical mechanism for delta approximately mu H+-driven changes in the Km for three different transport systems is an oxidation-reduction reaction involving a dithiol-disulfide interconversion [Robillard, G.T. and Konings, W.N. (1981) Biochemistry, 20, 5025-5032; Konings, W.N. and Robillard, G.T. (1982) Proc. Natl Acad. Sci. USA, in the press]. Based on the similarities between the data from these three systems and published data from other systems, we now propose that dithiol-disulfide interchange may play a general role in membrane-related processes such as transport, energy transduction and hormone-receptor interactions. We propose that the affinities of the substrate-binding sites are regulated by a dithiol and a disulfide situated at different depths in the membrane. In addition we propose that the oxidation states of these two redox centers are coupled by dithiol-disulfide interchange such that, when one is oxidized, the other is reduced. Since a transmembrane electrical potential, delta psi, or a pH gradient, delta pH, can alter the redox state, it can change the affinity of the substrate-binding sites. The delta approximately mu H+-induced changes in affinity are sufficient to drive active transport (symport or antiport) and energy-transducing processes. A similar mechanism can be applied to transport systems driven by phosphorylated enzyme intermediates instead of delta approximately mu H+. Changes of the redox potential in a given compartment during metabolism could also control the affinity of ligand binding even in the absence of a delta approximately mu H+. The ligand-binding affinities of facilitated diffusion transport systems and receptor proteins may be regulated in this manner.

Substrate binding affinity changes in mitochondrial energy-linked reactions

The effects of uncouplers and valinomycin plus nigericin (in the presence of K+) were studied on the apparent Km for substrates and apparent Vmax of the following energy-linked reactions catalyzed by submitochondrial particles: oxidative phosphorylation, NTP-33Pi exchange, ATP-driven electron transfer from succinate to NAD, and respiration-driven transhydrogenation from NADH to 3-acetylpyridine adenine dinucleotide phosphate. In all cases, partially uncoupling (up to 90%) concentrations of uncouplers of valinomycin plus nigericin were found to decrease apparent Vmax and to increase apparent Km. Results plotted as ln (Vmax/Km) versus the concentration of uncouplers or ionophores showed a linear decrease of the former as a function of increasing perturbant concentration (i.e., decreasing free energy). Because Vmax/Km may be considered as a measure of the apparent first-order rate constant for enzyme-substrate interaction and reflects the affinity between enzyme and substrate to form a complex, the results are consistent with the interpretation that membrane energization leads to a change in enzyme conformation with the resultant increase in enzyme-substrate affinity and facilitation of the reaction rate under consideration. The significance of these findings with respect to the mechanism of action of the energy-transducing systems studied is discussed.

Kinetic mechanism of mitochondrial adenosine triphosphatase. ADP-specific inhibition as revealed by the steady-state kinetics

1. A substantial increase of the initial rate of ATP hydrolysis was observed after preincubation of bovine heart submitochondrial particles with phosphoenolpyruvate and pyruvate kinase. 2. The activation was accompanied by an increase of Vmax, without change of Km for ATP. 3. The activated particles catalysed the biphasic hydrolysis of ATP in the presence of an ATP-regenerating system; the initial rapid phase was followed by a second, slower, phase in a time-dependent fashion. 4. The higher the ATP concentration used as a substrate, the higher is the rate of transition between these two phases. 5. The particles catalysed the hydrolysis of ITP with a lag phase; after preincubation with phosphoenolpyruvate and pyruvate kinase, ITP was hydrolysed at a constant rate. 6. Qualitatively the same phenomena were observed when soluble mitochondrial ATPase (F1-ATPase) prepared by the conventional method in the presence of ATP was used as nucleotide triphosphatase. 7. A kinetic scheme is proposed, in which the intermediate active enzyme-product complex (E.ADP) formed during ATP hydrolysis is in slow equilibrium with the inactive E*.ADP complex forming as a result of dislocation of ADP from the active site of ATPase to the other site, which is not in rapid equilibrium with the surrounding medium.

Some factors affecting phosphate transport in a perfused rat heart preparation

Pi uptake by a perfused rat heart preparation did not require the presence of any other permeant anion, but was markedly dependent on the extracellular Na+ concentration and accelerated when tissue oxygenation was inadequate. Pi efflux was also independent of other permeant anions, but apparently varied with the intracellular Na+ concentration. Cardiac Pi efflux was not sensitive to a number of inhibitors that clock Cl- movement in heart and other tissues. Both uptake and efflux apparently proceed via a reversible electroneutral co-transport system linked to the transmembrane Na+ gradient. Pi uptake was independent of cardiac work load, but the efflux rate was sharply accelerated after an increase in aortic pressure development, with a slow return towards basal values during sustained periods of high work output. An inverted biphasic effect on the efflux rate was observed after a reduction in cardiac work load. Mild hypoxia and respiratory and metabolic acidosis each resulted in a transient acceleration of Pi efflux followed by a return towards basal values during prolonged exposure to the stimulus, whereas respiratory and metabolic alkalosis produced a similar but inverted response. The origin of these phasic effects on Pi efflux remains to be identified at present.

Correlation between ATP synthesis and the decay of the arotenoid band shift after single flash activation of chromatophores from Rhodopseudomonas capsulata

ATP synthesis and the acceleration of the decay of the carotenoid absorption band shift after single flash excitation of Rhodopseudomonas capsulata chromatophores were compared. The two processes behave similarly with respect to: (1) ADP and Pi concentration; (2) inhibition by efrapeptin and venturicidin, and (3) inhibition by valinomycin/K+ and by ionophores. Taken together with earlier evidence for the electrochromic nature of the carotenoid band shift the data support the contention that positive charge moves outwards across the chromatophore membrane during ATP synthesis and justify the method for determination of the H+/ATP ratio (Petty, K.M. and Jackson, J.B. (1979) FEBS Lett. 97, 367-372). The ability of nucleotide diphosphates in the presence of Pi and Mg2+ to give rise to the acceleration of the carotenoid shift decay closely correlates with the rate of phosphorylation of the nucleotides in steady-state light. Nucleotide triphosphates enhance the decay in parallel with their rate of hydrolysis. Adenylyl imidodiphosphate is itself without effect on the decay of the carotenoid shift and it does not prevent the ADP-induced acceleration. The analogue does prevent the ATP effect but only after repeated flashes.

3' Esters of ADP as energy-transfer inhibitors and probes of the catalytic site of oxidative phosphorylation

1. A large series of 3' esters of ADP has been synthesized. Several of these can serve as photoaffinity labels; others exhibit fluorescent properties. The corresponding AMP and ATP derivatives have also been synthesized in some cases. 2. The influence of the 3'-O-acyl nucleotides on energy-linked functions of beef-heart submitochondrial particles has been investigated. The following results were obtained. a) 3'Esters of ADP are powerful and highly specific inhibitors of oxidative phosphorylation. The inhibition is competitive to ADP and Ki values as low as 0.05 microM, for the 3'-O-(1)naphthoyl ester of ADP, could be observed. b) The inhibition of oxidative phosphorylation by 3' esters of ADP appears to be non-competitive versus inorganic phosphate. c) The nucleotide analogs are not phosphorylated themselves. The corresponding ATP analogs can not drive energy-linked process. d) The 3' esters of AMP are ineffective as inhibitors, whereas the ATP derivatives are only comparatively weak inhibitors. e) Uncoupled or solubilized ATPase is almost two orders of magnitude less sensitive against inhibition by 3' esters than coupled systems. The analogs exert maximal inhibition specifically in systems involving an 'energized' state of the coupling device. f) Azido-group-bearing analogs can be used for irreversible photoinactivation of the coupling ATPase. Photoinactivation also is most efficient when carried out with 'energized' particles. g) The inhibitory properties are similar also in ATP-driven NAD+ reduction by succinate, and in the uncoupler-sensitive ATP in equilibrium with Pi exchange. The required concentrations for half-maximal inhibition are somewhat higher than in oxidative phosphorylation, but lower than with uncoupled ATPase. 3. From molecular models, from substituent properties, and from the conditions required for inhibition it is concluded that these highly effective analogs of ADP may act as conformation-specific probes at the catalytic site of oxidative phosphorylation. The results are interpreted in terms of a model suggesting that, in the process of ATP synthesis, a hydrophobic cavity on the enzyme is exposed only in the energized state, accepting the large 3' substituent. The substituent is assumed to inhibit phosphoryl transfer and/or conformational transitions inherent in the process of ADP phosphorylation by steric hinderance.

Current-voltage relationships for the plasma membrane and its principal electrogenic pump in Neurospora crassa: I. Steady-state conditions

The nonlinear membrane current-voltage relationship (I-V curve) for intact hyphae of Neurospora crassa has been determined by means of a 3-electrode voltage-clamp technique, plus "quasi-linear" cable theory. Under normal conditions of growth and respiration, the membrane I-V curve is best described as a parabolic segment convex in the direction of depolarizing current. At the average resting potential of - 174 mV, the membrane conductance is approximately 190 micronhos/cm2; conductance increase to approximately 240 micronhos/cm2 at -300 mV, and decreases to approximately 130 micronhos/cm2 at 0 mV. Irreversible membrane breakdown occurs at potentials beyond this range. Inhibition of the primary electrogenic pump in Neurospora by ATP withdrawal (with 1 mM KCN) depolarizes the membrane to the range of -40 to -70 mV and reduces the slope of the I-V curve by a fixed scaling factor of approximately 0.8. For wild-type Neurospora, compared under control conditions and during steady-state inhibition by cyanide, the I-V difference curve--presumed to define the current-voltage curve for the electrogenic pump--is a saturation function with maximal current of approximately 20 muA/cm2, a half saturation potential near -300 mV, and a projected reversal potential of ca. -400 mV. This value is close to the maximal free energy available to the pump from ATP hydrolysis, so that pump stoichiometry must be close to 1 H+ extruded:1 ATP split. The time-courses of change in membrane potential and resistance with cyanide are compatible with the steady-state I-V curves, under the assumption the cyanide has no major effects other than ATP withdrawal. Other inhibitors, uncouplers, and lowered temperature all have more complicated effects. The detailed temporal analysis of voltage-clamp data showed three time-constants in the clamping currents: one of 10 msec, for charging the membrane capacitance (0.9 muF/cm/2); a second of 50-75 msec; and a third of 20-30 sec, perhaps representing changes of intracellular composition.