The role of tightly bound ADP on chloroplast ATPase

C-terminal regulatory domain of the ε subunit of Fo F1 ATP synthase enhances the ATP-dependent H+ pumping that is involved in the maintenance of cellular membrane potential in Bacillus subtilis

The ε subunit of F_oF₁‐ATPase/synthase (F_oF₁) plays a crucial role in regulating F_oF₁ activity. To understand the physiological significance of the ε subunit‐mediated regulation of F_oF₁ in Bacillus subtilis, we constructed and characterized a mutant harboring a deletion in the C‐terminal regulatory domain of the ε subunit (ε^∆C). Analyses using inverted membrane vesicles revealed that the ε^∆C mutation decreased ATPase activity and the ATP‐dependent H⁺‐pumping activity of F_oF₁. To enhance the effects of ε^∆C mutation, this mutation was introduced into a ∆rrn8 strain harboring only two of the 10 rrn (rRNA) operons (∆rrn8 ε^∆C mutant strain). Interestingly, growth of the ∆rrn8 ε^∆C mutant stalled at late‐exponential phase. During the stalled growth phase, the membrane potential of the ∆rrn8 ε^∆C mutant cells was significantly reduced, which led to a decrease in the cellular level of 70S ribosomes. The growth stalling was suppressed by adding glucose into the culture medium. Our findings suggest that the C‐terminal region of the ε subunit is important for alleviating the temporal reduction in the membrane potential, by enhancing the ATP‐dependent H⁺‐pumping activity of F_oF₁.

The effect of medium viscosity on kinetics of ATP hydrolysis by the chloroplast coupling factor CF1

The coupling factor CF1 is a catalytic part of chloroplast ATP synthase which is exposed to stroma whose viscosity is many-fold higher than that of reaction mixtures commonly used to measure kinetics of CF1-catalyzed ATP hydrolysis. This study is focused on the effect of medium viscosity modulated by sucrose or bovine serum albumin (BSA) on kinetics of Ca(2+)- and Mg(2+)-dependent ATP hydrolysis by CF1. These agents were shown to reduce the maximal rate of Ca(2+)-dependent ATPase without changing the apparent Michaelis constant (К m), thus supporting the hypothesis on viscosity dependence of CF1 activity. For the sulfite- and ethanol-stimulated Mg(2+)-dependent reaction, the presence of sucrose increased К m without changing the maximal rate that is many-fold as high as that of Ca(2+)-dependent hydrolysis. The hydrolysis reaction was shown to be stimulated by low concentrations of BSA and inhibited by its higher concentrations, with the increasing maximal reaction rate estimated by extrapolation. Sucrose- or BSA-induced inhibition of the Mg(2+)-dependent ATPase reaction is believed to result from diffusion-caused deceleration, while its BSA-induced stimulation is probably caused by optimization of the enzyme structure. Molecular mechanisms of the inhibitory effect of viscosity are discussed. Taking into account high protein concentrations in the chloroplast stroma, it was suggested that kinetic parameters of ATP hydrolysis, and probably those of ATP synthesis in vivo as well, must be quite different from measurements taken at a viscosity level close to that of water.

Kinetic and hysteretic behavior of ATP hydrolysis of the highly stable dimeric ATP synthase of Polytomella sp

The F1FO-ATP synthase of the colorless alga Polytomella sp. exhibits a robust peripheral arm constituted by nine atypical subunits only present in chlorophycean algae. The isolated dimeric enzyme exhibits a latent ATP hydrolytic activity which can be activated by some detergents. To date, the kinetic behavior of the algal ATPase has not been studied. Here we show that while the soluble F1 sector exhibits Michaelis-Menten kinetics, the dimer exhibits a more complex behavior. The kinetic parameters (Vmax and Km) were obtained for both the F1 sector and the dimeric enzyme as isolated or activated by detergent, and this activation was also seen on the enzyme reconstituted in liposomes. Unlike other ATP synthases, the algal dimer hydrolyzes ATP on a wide range of pH and temperature. The enzyme was inhibited by oligomycin, DCCD and Mg-ADP, although oligomycin induced a peculiar inhibition pattern that can be attributed to structural differences in the algal subunit-c. The hydrolytic activity was temperature-dependent and exhibited activation energy of 4 kcal/mol. The enzyme also exhibited a hysteretic behavior with a lag phase strongly dependent on temperature but not on pH, that may be related to a possible regulatory role in vivo.

Regulation of the ATPase activity of ABCE1 from Pyrococcus abyssi by Fe-S cluster status and Mg²⁺: implication for ribosomal function

Ribosomal function is dependent on multiple proteins. The ABCE1 ATPase, a unique ABC superfamily member that bears two Fe₄S₄ clusters, is crucial for ribosomal biogenesis and recycling. Here, the ATPase activity of the Pyrococcus abyssi ABCE1 (PabABCE1) was studied using both apo- (without reconstituted Fe-S clusters) and holo- (with full complement of Fe-S clusters reconstituted post-purification) forms, and is shown to be jointly regulated by the status of Fe-S clusters and Mg²⁺. Typically ATPases require Mg²⁺, as is true for PabABCE1, but Mg²⁺ also acts as a negative allosteric effector that modulates ATP affinity of PabABCE1. Physiological [Mg²⁺] inhibits the PabABCE1 ATPase (K(i) of ∼1 μM) for both apo- and holo-PabABCE1. Comparative kinetic analysis of Mg²⁺ inhibition shows differences in degree of allosteric regulation between the apo- and holo-PabABCE1 where the apparent ATP K(m) of apo-PabABCE1 increases >30-fold from ∼30 μM to over 1 mM with M²⁺. This effect would significantly convert the ATPase activity of PabABCE1 from being independent of cellular energy charge (φ) to being dependent on φ with cellular [Mg²⁺]. These findings uncover intricate overlapping effects by both [Mg²⁺] and the status of Fe-S clusters that regulate ABCE1's ATPase activity with implications to ribosomal function.

Characterization of the relationship between ADP- and epsilon-induced inhibition in cyanobacterial F1-ATPase

The ATPase activity of chloroplast and bacterial F(1)-ATPase is strongly inhibited by both the endogenous inhibitor ε and tightly bound ADP. Although the physiological significance of these inhibitory mechanisms is not very well known for the membrane-bound F(0)F(1), these are very likely to be important in avoiding the futile ATP hydrolysis reaction and ensuring efficient ATP synthesis in vivo. In a previous study using the α(3)β(3)γ complex of F(1) obtained from the thermophilic cyanobacteria, Thermosynechococcus elongatus BP-1, we succeeded in determining the discrete stop position, ∼80° forward from the pause position for ATP binding, caused by ε-induced inhibition (ε-inhibition) during γ rotation (Konno, H., Murakami-Fuse, T., Fujii, F., Koyama, F., Ueoka-Nakanishi, H., Pack, C. G., Kinjo, M., and Hisabori, T. (2006) EMBO J. 25, 4596-4604). Because γ in ADP-inhibited F(1) also pauses at the same position, ADP-induced inhibition (ADP-inhibition) was assumed to be linked to ε-inhibition. However, ADP-inhibition and ε-inhibition should be independent phenomena from each other because the ATPase core complex, α(3)β(3)γ, also lapses into the ADP-inhibition state. By way of thorough biophysical and biochemical analyses, we determined that the ε subunit inhibition mechanism does not directly correlate with ADP-inhibition. We suggest here that the cyanobacterial ATP synthase ε subunit carries out an important regulatory role in acting as an independent "braking system" for the physiologically unfavorable ATP hydrolysis reaction.

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.

Influence of ATP and ADP on dissociation of the V-ATPase into its V(1) and V(O) complexes

Although the reversible dissociation of the V(1)V(O) holoenzyme into its V(1) and V(O) complexes is a general mechanism for the regulation of V-ATPases, important aspects are still not understood. By analyzing the endogenous nucleotide content of the V(1)V(O) holoenzyme and of the V(1) complex, both purified from Manduca sexta larval midgut, we found that the V(1) complex contained 1.7 molec. of ADP, whereas only 0.3 molec. of ADP were bound to the V(1)V(O) holoenzyme. By contrast, both proteins contained only negligible amounts of ATP. Incubation of the V(1)V(O) holoenzyme with various adenine nucleotides revealed that ATP hydrolysis, leading to a state containing tightly bound ADP is necessary for its dissociation.

Met23Lys mutation in subunit gamma of F(O)F(1)-ATP synthase from Rhodobacter capsulatus impairs the activation of ATP hydrolysis by protonmotive force

H(+)-F(O)F(1)-ATP synthase couples proton flow through its membrane portion, F(O), to the synthesis of ATP in its headpiece, F(1). Upon reversal of the reaction the enzyme functions as a proton pumping ATPase. Even in the simplest bacterial enzyme the ATPase activity is regulated by several mechanisms, involving inhibition by MgADP, conformational transitions of the epsilon subunit, and activation by protonmotive force. Here we report that the Met23Lys mutation in the gamma subunit of the Rhodobacter capsulatus ATP synthase significantly impaired the activation of ATP hydrolysis by protonmotive force. The impairment in the mutant was due to faster enzyme deactivation that was particularly evident at low ATP/ADP ratio. We suggest that the electrostatic interaction of the introduced gammaLys23 with the DELSEED region of subunit beta stabilized the ADP-inhibited state of the enzyme by hindering the rotation of subunit gamma rotation which is necessary for the activation.

Regulatory interplay between proton motive force, ADP, phosphate, and subunit epsilon in bacterial ATP synthase

ATP synthase couples transmembrane proton transport, driven by the proton motive force (pmf), to the synthesis of ATP from ADP and inorganic phosphate (P(i)). In certain bacteria, the reaction is reversed and the enzyme generates pmf, working as a proton-pumping ATPase. The ATPase activity of bacterial enzymes is prone to inhibition by both ADP and the C-terminal domain of subunit epsilon. We studied the effects of ADP, P(i), pmf, and the C-terminal domain of subunit epsilon on the ATPase activity of thermophilic Bacillus PS3 and Escherichia coli ATP synthases. We found that pmf relieved ADP inhibition during steady-state ATP hydrolysis, but only in the presence of P(i). The C-terminal domain of subunit epsilon in the Bacillus PS3 enzyme enhanced ADP inhibition by counteracting the effects of pmf. It appears that these features allow the enzyme to promptly respond to changes in the ATP:ADP ratio and in pmf levels in order to avoid potentially wasteful ATP hydrolysis in vivo.

The regulator of the F1 motor: inhibition of rotation of cyanobacterial F1-ATPase by the epsilon subunit

The chloroplast-type F(1) ATPase is the key enzyme of energy conversion in chloroplasts, and is regulated by the endogenous inhibitor epsilon, tightly bound ADP, the membrane potential and the redox state of the gamma subunit. In order to understand the molecular mechanism of epsilon inhibition, we constructed an expression system for the alpha(3)beta(3)gamma subcomplex in thermophilic cyanobacteria allowing thorough investigation of epsilon inhibition. epsilon Inhibition was found to be ATP-independent, and different to that observed for bacterial F(1)-ATPase. The role of the additional region on the gamma subunit of chloroplast-type F(1)-ATPase in epsilon inhibition was also determined. By single molecule rotation analysis, we succeeded in assigning the pausing angular position of gamma in epsilon inhibition, which was found to be identical to that observed for ATP hydrolysis, product release and ADP inhibition, but distinctly different from the waiting position for ATP binding. These results suggest that the epsilon subunit of chloroplast-type ATP synthase plays an important regulator for the rotary motor enzyme, thus preventing wasteful ATP hydrolysis.

The role of subunit epsilon in the catalysis and regulation of FOF1-ATP synthase

The regulation of ATP synthase activity is complex and involves several distinct mechanisms. In bacteria and chloroplasts, subunit epsilon plays an important role in this regulation, (i) affecting the efficiency of coupling, (ii) influencing the catalytic pathway, and (iii) selectively inhibiting ATP hydrolysis activity. Several experimental studies indicate that the regulation is achieved through large conformational transitions of the alpha-helical C-terminal domain of subunit epsilon that occur in response to membrane energization, change in ATP/ADP ratio or addition of inhibitors. This review summarizes the experimental data obtained on different organisms that clarify some basic features as well as some molecular details of this regulatory mechanism. Multiple functions of subunit epsilon, its role in the difference between the catalytic pathways of ATP synthesis and hydrolysis and its influence on the inhibition of ATP hydrolysis by ADP are also discussed.

ADP and ATP binding to noncatalytic sites of thiol-modulated chloroplast ATP synthase

A modified 'cold chase' technique was used to study tight [(14)C]ADP and [(14)C]ATP binding to noncatalytic sites of chloroplast ATP synthase (CF(0)F(1)). The binding was very low in the dark and sharply increased with light intensity. Dissociation of labeled nucleotides incorporated into noncatalytic sites of CF(0)F(1 )or CF(1) reconstituted with EDTA-treated thylakoid membranes was also found to be light-dependent. Time dependence of nucleotide dissociation is described by the first order equation with a k (d) of about 5 min(-1). The exposure of thylakoid membranes to 0.7-24.8 muM nucleotides leads to filling of up to two noncatalytic sites of CF(0)F(1). The sites differ in their specificity: one preferentially binds ADP, whereas the other - ATP. A much higher ATP/ADP ratio of nucleotides bound at noncatalytic sites of isolated CF(1) dramatically decreases upon its reconstitution with EDTA-treated thylakoid membranes. It is suggested that the decrease is caused by conformational changes in one of the alpha subunits induced by its interaction with the delta subunit and/or subunit I-II when CF(1) becomes bound to a thylakoid membrane.

Regulation of the F0F1-ATP synthase: the conformation of subunit epsilon might be determined by directionality of subunit gamma rotation

F(0)F(1)-ATP synthase couples ATP synthesis/hydrolysis with transmembrane proton transport. The catalytic mechanism involves rotation of the gamma epsilon c(approximately 10)-subunits complex relative to the rest of the enzyme. In the absence of protonmotive force the enzyme is inactivated by the tight binding of MgADP. Subunit epsilon also modulates the activity: its conformation can change from a contracted to extended form with C-terminus stretched towards F(1). The latter form inhibits ATP hydrolysis (but not synthesis). We propose that the directionality of the coiled-coil subunit gamma rotation determines whether subunit epsilon is in contracted or extended form. Block of rotation by MgADP presumably induces the extended conformation of subunit epsilon. This conformation might serve as a safety lock, stabilizing the ADP-inhibited state upon de-energization and preventing spontaneous re-activation and wasteful ATP hydrolysis. The hypothesis merges the known regulatory effects of ADP, protonmotive force and conformational changes of subunit epsilon into a consistent picture.

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

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

The participation of metals in the mechanism of the F(1)-ATPase

The Mg(2+) cofactor of the F(1)F(0) ATP synthase is required for the asymmetry of the catalytic sites that leads to the differences in affinity for nucleotides. Vanadyl (V(IV)=O)(2+) is a functional surrogate for Mg(2+) in the F(1)-ATPase. The (51)V-hyperfine parameters derived from EPR spectra of VO(2+) bound to specific sites on the enzyme provide a direct probe of the metal ligands at each site. Site-directed mutations of residues that serve as metal ligands were found to cause measurable changes in the (51)V-hyperfine parameters of the bound VO(2+), thereby providing a means by which metal ligands were identified in the functional enzyme in several conformations. At the low-affinity catalytic site comparable to beta(E) in mitochondrial F(1), activation of the chloroplast F(1)-ATPase activity induces a conformational change that inserts the P-loop threonine and catch-loop tyrosine hydroxyl groups into the metal coordination sphere thereby displacing an amino group and the Walker homology B aspartate. Kinetic evidence suggests that coordination of this tyrosine by the metal when the empty site binds substrate may provide an escapement mechanism that allows the gamma subunit to rotate and the conformation of the catalytic sites to change, thereby allowing rotation only when the catalytic sites are filled. In the high-affinity conformation analogous to the beta(DP) site of mitochondrial F(1), the catch-loop tyrosine has been displaced by carboxyl groups from the Walker homology B aspartate and from betaE197 in Chlamydomonas CF(1). Coordination of the metal by these carboxyl groups contributes significantly to the ability of the enzyme to bind the nucleotide with high affinity.

Metal ligation by Walker homology B aspartate betaD262 at site 3 of the latent but not activated form of the chloroplast F(1)-ATPase from Chlamydomonas reinhardtii

Site-directed mutations D262C, D262H, D262N, and D262T were made to the beta subunit Walker Homology B aspartate of chloroplast F(1)-ATPase in Chlamydomonas. Photoautotrophic growth and photophosphorylation rates were 3-14% of wild type as were ATPase activities of purified chloroplast F(1) indicating that betaD262 is an essential residue for catalysis. The EPR spectrum of vanadyl bound to Site 3 of chloroplast F(1) as VO(2+)-ATP gave rise to two EPR species designated B and C in wild type and mutants. (51)V-hyperfine parameters of species C, present exclusively in the activated enzyme state, did not change significantly by the mutations examined indicating that it is not an equatorial ligand to VO(2+), nor is it hydrogen-bonded to a coordinated water at an equatorial position. Every mutation changed the ratio of EPR species C/B and/or the (51)V-hyperfine parameters of species B, the predominant conformation of VO(2+)-nucleotide bound to Site 3 in the latent (down-regulated) state. The results indicate that the Walker Homology B aspartate coordinates the metal of the predominant metal-nucleotide conformation at Site 3 in the latent state but not in the conformation present exclusively upon activation and elucidates one of the specific changes in metal ligation involved with activation.

Interaction of the clathrin-coated vesicle V-ATPase with ADP and sodium azide

The kinetics of adenosine triphosphate (ATP)-dependent proton transport into clathrin-coated vesicles from bovine brain have been studied. We observe that the vacuolar proton-translocating ATPase (V-ATPase) from clathrin-coated vesicles is subject to two different types of inhibition by ADP. The first is competitive inhibition with respect to ATP, with a Ki for ADP of 11 microM. The second type of inhibition occurs after preincubation of the V-ATPase in the presence of ADP and Mg2+, which results in inhibition of the initial rate of proton transport followed by reactivation over the course of several minutes. The second effect is observed at ADP concentrations as low as 0.1-0.2 microM, indicating that a high affinity inhibitory complex is formed between ADP and the V-ATPase and is only slowly dissociated after the addition of ATP. We have further investigated the effect of sodium azide, an inhibitor of the F-ATPases that has been shown to stabilize an inactive complex between ADP and the F1-F0-ATP synthase (F-ATPase). We observed that azide inhibited ATP-dependent proton transport by the purified, reconstituted V-ATPase with a K0.5 of 0.2-0.4 mM but had no effect on ATP hydrolysis. Azide was shown not to increase the passive proton permeability of reconstituted vesicles and did not stimulate ATP hydrolysis by the reconstituted enzyme, in contrast with CCCP, which both abolished the proton gradient and stimulated hydrolysis. Thus, azide does not appear to act as a simple uncoupler of proton transport and ATP hydrolysis. Rather, azide may have some more direct effect on V-ATPase activity. Possible mechanisms by which azide could exert this effect on the V-ATPase and the contrasting effects of azide on the F- and V-ATPases are discussed.

Mutagenesis of beta-Glu-195 of the Rhodospirillum rubrum F1-ATPase and its role in divalent cation-dependent catalysis

We introduced mutations at the fully conserved residue Glu-195 in subunit beta of Rhodospirillum rubrum F1-ATPase. The activities of the expressed wild type (WT) and mutant beta subunits were assayed by following their capacity to assemble into the earlier prepared beta-depleted, membrane-bound R. rubrum enzyme (Philosoph, S., Binder, A., and Gromet-Elhanan, Z. (1977) J. Biol. Chem. 252, 8742-8747) and to restore ATP synthesis and/or hydrolysis activity. All three mutations, beta-E195K, beta-E195Q, and beta-E195G, were found to bind as the WTbeta into the beta-depleted enzyme. They restored between 30 and 60% of the WT restored photophosphorylation activity and 16, 45, and 105%, respectively of the CaATPase activity. The mutants required, however, much higher concentrations of divalent cations and could not restore any significant MgATPase or MnATPase activities. Only beta-E195G could restore some of these activities when assayed in the presence of 100 mM sulfite and high MgCl2 or MnCl2 concentrations. These results suggest that the observed difference in restoration of ATP synthesis and CaATPase, as compared with MgATPase and MnATPase, can be due to the tight regulation of the last two activities, resulting in their inhibition at cation/ATP ratios above 0.5. The R. rubrum F1beta-E195 is equivalent to the mitochondrial F1beta-E199, which points into the tunnel leading to the F1 catalytic nucleotide binding sites (Abrahams, J. P., Leslie, A. G. W., Lutter, R., and Walker, J. E. (1994) Nature 370, 621-628). Our findings indicate that this residue, although not an integral part of the F1 catalytic sites, affects divalent cation binding and release of inhibitory MgADP, suggesting its participation in the interconversion of the F1 catalytic sites between different conformational states.

Catalytic activity of the alpha3beta3gamma complex of F1-ATPase without noncatalytic nucleotide binding site

A mutant alpha3beta3gamma complex of F1-ATPase from thermophilic Bacillus PS3 was generated in which noncatalytic nucleotide binding sites lost their ability to bind nucleotides. It hydrolyzed ATP at an initial rate with cooperative kinetics (Km(1), 4 microM; Km(2), 135 microM) similar to the wild-type complex. However, the initial rate decayed rapidly to an inactivated form. Since the inactivated mutant complex contained 1.5 mol of ADP/mol of complex, this inactivation seemed to be caused by entrapping inhibitory MgADP in a catalytic site. Indeed, the mutant complex was nearly completely inactivated by a 10 min prior incubation with equimolar MgADP. Analysis of the progress of inactivation after initiation of ATP hydrolysis as a function of ATP concentration indicated that the inactivation was optimal at ATP concentrations in the range of Km(1). In the presence of ATP, the wild-type complex dissociated the inhibitory [3H]ADP preloaded onto a catalytic site whereas the mutant complex did not. Lauryl dimethylamineoxide promoted release of preloaded inhibitory [3H]ADP in an ATP-dependent manner and partly restored the activity of the inactivated mutant complex. Addition of ATP promoted single-site hydrolysis of 2',3'-O-(2,4,6-trinitrophenyl)-ATP preloaded at a single catalytic site of the mutant complex. These results indicate that intact noncatalytic sites are essential for continuous catalytic turnover of the F1-ATPase but are not essential for catalytic cooperativity of F1-ATPase observed at ATP concentrations below approximately 300 microM.

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 chloroplast ATP synthase: structural changes during catalysis

This article summarizes some of the evidence for the existence of light-driven structural changes in the epsilon and gamma subunits of the chloroplast ATP synthase. Formation of a transmembrane proton gradient results in: (1) a changed in the position of the epsilon subunit such that it becomes exposed to polyclonal antibodies and to reagents which selectively modify epsilon Lys109; (2) enhanced solvent accessibility of several sulfhydryl residues on the gamma subunit; and (3) release/exchange of tightly bound ADP from the enzyme. Theses and related experimental observations can, at least partially, be explained in terms of two different bound conformational states of the epsilon subunit. Evidence for structural changes in the enzyme which are driven by light or nucleotide binding is discussed with special reference to the popular rotational model for catalysis.

Differences between two tight ADP binding sites of the chloroplast coupling factor 1 and their effects on ATPase activity

Purified chloroplast ATP synthase (CF1) contains 1.2-2 mol of tightly bound ADP/mol of enzyme that resists removal by gel filtration or dialysis. CF1 was depleted of its endogenous nucleotide by treatment with alkaline phosphatase. Tightly bound nucleotide was demonstrated not to have an essential structural role. CF1 depleted of endogenous nucleotide retains its ability to catalyze Ca2+- and Mg2+-dependent ATPase activity and is not more sensitive to cold inactivation than untreated CF1. 2'(3')-O-Trinitrophenyladenosine 5'-diphosphate (TNP-ADP) binds tightly to two sites on nucleotide-depleted CF1, binding to either site at a faster rate than that of exchange of bound nucleotide for medium nucleotide. The nucleotide-depleted enzyme binds about one additional mol of TNP-ADP/mol of CF1, indicating that there is a tight TNP-ADP binding site that does not exchange readily with medium nucleotide. It is MgADP in this nonexchanging site, not the easily exchanging ADP binding site, that is responsible for the MgADP-induced inhibition of the ATPase activity. The rate of exchange of tightly bound ADP from CF1 matches the rate at which the Mg2+ATPase activity of CF1 is activated but is not itself responsible for the activation.

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.

The 'non-exchangeable' nucleotides of F1-F0ATP synthase. Cofactors in hydrolysis?

The F1-F0 ATP synthase bears 6 nucleotide binding sites, only 3 of which turn over during catalysis. The remaining 3 are occupied by slowly exchanging ATP in vivo, although at least 1 molecule is generally lost on isolation of the enzyme in the absence of nucleotide. It is proposed that the function of the slowly exchanging (NC) nucleotides is to participate in catalysis, the terminal phosphate of the bound ATP acting as an acid catalyst in the cleavage/synthesis of the phosphate anhydride bond in the catalytic sites. Such a role has been demonstrated for the bound pyridoxal phosphate moiety in glycogen phosphorylase. Evidence is presented that (i) the NC nucleotide spans the interface between an alpha subunit and its partner beta, interacting near the catalytic binding site on beta; (ii) the phosphate moieties of the catalyzed and NC nucleotide are close in space; and (iii) occupation of the NC nucleotide sites promotes ATP hydrolysis by F1 or its subfragments. All of these findings are required by the proposed mechanism. Relationships between phosphorylase and F1 structures are discussed.

The catalytic site is located on subunit I of the ATPase from Halobacterium saccharovorum. A direct photoaffinity labeling study

Nucleotide-binding sites of the ATPase from the halophilic archaebacterium Halobacterium saccharovorum were labeled by ultraviolet irradiation in the presence of [alpha-32P]ATP. A high-affinity site, located on subunit I (98 kDa), was identified as catalytic by the following criteria: ATP bound to subunit I was hydrolyzed and the cross-linked nucleotide was ADP; the specificity for ATP or ADP compared to that of other nucleotides was high; the tightly bound radionucleotide was exchangeable in the presence of excess unlabeled ATP and Mg2+; photolabeling of this site and enzyme inhibition due to tightly bound ADP were both dependent on the presence of Mg2+ and showed identical Kd values; treatment that restored the activity of the ADP-inhibited enzyme also led to the release of the tightly bound nucleotide from subunit I. In addition, a non-catalytic nucleotide-binding site was found, located on subunit II (71 kDa). This site did not hydrolyze ATP, its occupation was Mg2+ independent and the affinity for ATP and the nucleotide specificity were much lower than that of subunit I. We suspect that this site is nonspecific. These results indicate that H. saccharovorum ATPase is different from F1-ATPases which contain the catalytic site on the second largest subunit, but may be similar to other archaebacterial and vacuolar ATPases.

Adenine nucleotide-binding sites on mitochondrial F1-ATPase: studies of the inactive complex formed upon binding ADP at a catalytic site

ADP-induced inhibition of mitochondrial F1-ATPase has been studied. It is shown that in the presence of magnesium and the absence of light, the photoaffinity ADP analog, 2-azido-ADP, induces a reversible inhibition of native F1 that is indistinguishable from that obtained with ADP. Photolysis of the inactive complex results in the predominant labeling of a catalytic-site peptide identified previously (Cross et al., 1987, Proc. Natl. Acad. Sci. USA 84, 5715-5719). Dissociation of the inactive complex formed between F1 and ADP is biphasic with a rapid azide-insensitive phase followed by a slow azide-sensitive phase (k approximately 3 x 10(-3) s-1). It is also shown that incubation of the ADP-inhibited enzyme with EDTA or phosphate does not result in release or migration of ADP from the catalytic site. However, it does convert the complex to a form that reactivates in the presence of 100 microM ATP at a rate too rapid to observe using manual mixing.

The ADP that binds tightly to nucleotide-depleted mitochondrial F1-ATPase and inhibits catalysis is bound at a catalytic site

Previous studies have shown that the initial complex formed when ADP binds to nucleotide-depleted F1-ATPase is transformed with a half time of 2 to 3 min to form with a much lower rate of ADP release. The ADP binding results in a strong inhibition of ATPase activity. The present paper reports appraisal of where the inhibitory ADP binds by use of the photoreactive ADP analog, 2-N3-ADP. In presence of Mg2+ the 2-N3-ADP like ADP induces reversible inhibition of nucleotide-depleted F1 (ndF1) with a Kd of about 10 nM. Photoirradiation of the inactive 2-N3-[beta-32P]ADP-ndF1 complex results in labeling of only the beta-subunit. The major labeled peptide isolated from a trypic digest consists of residues from Ala-338 to Arg-356, with Tyr-345 as the site of labeling. This identifies the site of the inhibitory ADP binding as one of the catalytic sites of the enzyme.

F1 ATPase from the thermophilic bacterium PS3 (TF1) shows ATP modulation of oxygen exchange

The ATPase from the ATP synthase of the thermophilic bacterium PS3 (TF1), unlike F1 ATPase from other sources, does not retain bound ATP, ADP, and Pi at a catalytic site under conditions for single-site catalysis [Yohda, M., & Yoshida, M. (1987) J. Biochem. 102, 875-883]. This raised a question as to whether catalysis by TF1 involved alternating participation of catalytic sites. The possibility remained, however, that there might be transient but catalytically significant retention of bound reactants at catalytic sites when the medium ATP concentration was relatively low. To test for this, the extent of water oxygen incorporation into Pi formed by ATP hydrolysis was measured at various ATP concentrations. During ATP hydrolysis at both 45 and 60 degrees C, the extent of water oxygen incorporation into the Pi formed increased markedly as the ATP concentration was lowered to the micromolar range, with greater modulation observed at 60 degrees C. Most of the product Pi formed arose by a single catalytic pathway, but measurable amounts of Pi were formed by a pathway with high oxygen exchange. This may result from the presence of some poorly active enzyme. The results are consistent with sequential participation of three catalytic sites on the TF1 as predicted by the binding change mechanism.

Selectivity of modification when latent and activated forms of the chloroplast F1-ATPase are inactivated by 7-chloro-4-nitrobenzofurazan

The characteristics and specificity of inactivation of the chloroplast F1-ATPase (CF1) with 7-chloro-4-nitrobenzofurazan (Nbf-Cl) have been investigated. Inactivation of the octylglucoside-dependent Mg2+-ATPase activity of latent CF1 by Nbf-Cl can be correlated with the formation of about 1.2 mol of Nbf-O-Tyr per mole of enzyme. Following inactivation of CF1 with [14C]Nbf-Cl, polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate revealed that the majority of the radioactive reagent incorporated is present in the beta subunit. Treatment of the enzyme with [14C]Nbf-Cl following dithiothreitol heat activation, led to similar labeling of the beta subunit and substantial incorporation of 14C into the gamma subunit. On complete inactivation, about 4 mol of Nbf-S-Cys is formed per mole of dithiothreitol-heat-activated CF1. Incorporation of 14C into the gamma subunit is prevented by prior treatment of the latent CF1 or of the dithiothreitol-heat-activated CF1 with iodoacetamide. Following incubation of the dithiothreitol-heat-activated CF1 with iodoacetamide, complete inactivation of the octylglucoside-dependent Mg2+-ATPase activity by Nbf-Cl can be correlated with the formation of about 1.2 mol of Nbf-O-Tyr per mole of enzyme. After stabilization of the [14C]Nbf-O-Tyr derivative by treatment with sodium dithionite, a labeled peptide was purified. Automatic Edman degradation of this peptide revealed the sequence V-X-V-P-A-D-(D). The majority of the radioactivity was cleaved in the second cycle, the position occupied in CF1 by Tyr-beta-328, which is homologous to Tyr-beta-311, the residue reactive with Nbf-Cl in the beef heart mitochondrial F1-ATPase. When CF1, modified at Tyr-beta-328 with Nbf-Cl, is incubated at pH 9.0, the Nbf-O-Tyr adduct is hydrolyzed, leading to concomitant recovery of the ATPase activity. In double labeling experiments, two-dimensional isoelectric focusing in the presence of urea followed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate indicates that 2-azido-ADP, covalently bound at the tight ADP binding site, and the tyrosine modified by [14C]Nbf-Cl are located in different beta subunits.

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.