Characterization of the catalytic and noncatalytic ADP binding sites of the F1-ATPase from the thermophilic bacterium, PS3

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.

Inherent conformational flexibility of F1-ATPase α-subunit

The core of F1-ATPase consists of three catalytic (β) and three noncatalytic (α) subunits, forming a hexameric ring in alternating positions. A wealth of experimental and theoretical data has provided a detailed picture of the complex role played by catalytic subunits. Although major conformational changes have only been seen in β-subunits, it is clear that α-subunits have to respond to these changes in order to be able to transmit information during the rotary mechanism. However, the conformational behavior of α-subunits has not been explored in detail. Here, we have combined unbiased molecular dynamics (MD) simulations and calorimetrically measured thermodynamic signatures to investigate the conformational flexibility of isolated α-subunits, as a step toward deepening our understanding of its function inside the α3β3 ring. The simulations indicate that the open-to-closed conformational transition of the α-subunit is essentially barrierless, which is ideal to accompany and transmit the movement of the catalytic subunits. Calorimetric measurements of the recombinant α-subunit from Geobacillus kaustophilus indicate that the isolated subunit undergoes no significant conformational changes upon nucleotide binding. Simulations confirm that the nucleotide-free and nucleotide-bound subunits show average conformations similar to that observed in the F1 crystal structure, but they reveal an increased conformational flexibility of the isolated α-subunit upon MgATP binding, which might explain the evolutionary conserved capacity of α-subunits to recognize nucleotides with considerable strength. Furthermore, we elucidate the different dependencies that α- and β-subunits show on Mg(II) for recognizing ATP.

Structure of a thermophilic F1-ATPase inhibited by an ε-subunit: deeper insight into the ε-inhibition mechanism

F1-ATPase (F1) is the catalytic sector in F(o)F1-ATP synthase that is responsible for ATP production in living cells. In catalysis, its three catalytic β-subunits undergo nucleotide occupancy-dependent and concerted open-close conformational changes that are accompanied by rotation of the γ-subunit. Bacterial and chloroplast F1 are inhibited by their own ε-subunit. In the ε-inhibited Escherichia coli F1 structure, the ε-subunit stabilizes the overall conformation (half-closed, closed, open) of the β-subunits by inserting its C-terminal helix into the α3β3 cavity. The structure of ε-inhibited thermophilic F1 is similar to that of E. coli F1, showing a similar conformation of the ε-subunit, but the thermophilic ε-subunit stabilizes another unique overall conformation (open, closed, open) of the β-subunits. The ε-C-terminal helix 2 and hook are conserved between the two structures in interactions with target residues and in their positions. Rest of the ε-C-terminal domains are in quite different conformations and positions, and have different modes of interaction with targets. This region is thought to serve ε-inhibition differently. For inhibition, the ε-subunit contacts the second catches of some of the β- and α-subunits, the N- and C-terminal helices, and some of the Rossmann fold segments. Those contacts, as a whole, lead to positioning of those β- and α- second catches in ε-inhibition-specific positions, and prevent rotation of the γ-subunit. Some of the structural features are observed even in IF1 inhibition in mitochondrial F1.

Characteristics of protection by MgADP and MgATP of α3β3γ subcomplex of thermophilic Bacillus PS3 βY341W-mutant F1-ATPase from inhibition by 7-chloro-4-nitrobenz-2-oxa-1,3-diazole support a bi-site mechanism of catalysis

MgADP and MgATP binding to catalytic sites of βY341W-α(3)β(3)γ subcomplex of F(1)-ATPase from thermophilic Bacillus PS3 has been assessed using their effect on the enzyme inhibition by 7-chloro-4-nitrobenz-2-oxa-1,3-diazole (NBD-Cl). It was assumed that NBD-Cl can inhibit only when catalytic sites are empty, and inhibition is prevented if a catalytic site is occupied with a nucleotide. In the absence of an activator, MgADP and MgATP protect βY341W-α(3)β(3)γ subcomplex from inhibition by NBD-Cl by binding to two catalytic sites with an affinity of 37 µM and 12 mM, and 46 µM and 15 mM, respectively. In the presence of an activator lauryldimethylamine-N-oxide (LDAO), MgADP protects βY341W-α(3)β(3)γ subcomplex from inhibition by NBD-Cl by binding to a catalytic site with a K(d) of 12 mM. Nucleotide binding to a catalytic site with affinity in the millimolar range has not been previously revealed in the fluorescence quenching experiments with βY341W-α(3)β(3)γ subcomplex. In the presence of activators LDAO or selenite, MgATP protects βY341W-α(3)β(3)γ subcomplex from inhibition by NBD-Cl only partially, and the enzyme remains sensitive to inhibition by NBD-Cl even at MgATP concentrations that are saturating for ATPase activity. The results support a bi-site mechanism of catalysis by F(1)-ATPases.

A bi-site mechanism for Escherichia coli F1-ATPase accounts for the observed positive catalytic cooperativity

Nucleotide binding to nucleotide-depleted F(1)-ATPase from Escherichia coli (EcF(1)) during MgATP hydrolysis in the presence of excess epsilon subunit has been studied using a combination of centrifugal filtration and column-centrifugation methods. The results show that nucleotide-binding properties of catalytic sites on EcF(1) are affected by the state of occupancy of noncatalytic sites. The ATP-concentration dependence of catalytic-site occupancy during MgATP hydrolysis demonstrates that a bi-site mechanism is responsible for the positive catalytic cooperativity observed during multi-site catalysis by EcF(1). The results suggest that a bi-site mechanism is a general feature of F(1) catalysis.

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.

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.

Redox properties of iron in the binding site(s) of F1ATPase from mammalian mitochondria and thermophilic bacterium PS3: a comparative study

Iron ions in the two iron centers of beef heart mitochondrial F1ATPase, which we have been recently characterized (FEBS Letters 1996, 379, 231-235), exhibit different redox properties. In fact, the ATP-dependent site is able to maintain iron in the redox state of Fe(II) even in the absence of reducing agents, whereas in the nucleotide-independent site iron is oxidized to Fe(III) upon removal of the reductant. Fe(III) ions in the two sites display different reactivity towards H2O2, because only Fe(III) bound in the nucleotide-independent site rapidly reacts with H2O2 thus mediating a 30% enzyme inactivation. Thermophilic bacterium PS3 bears one Fe(III) binding site, which takes up Fe(III) either in the absence or presence of nucleotides and is unable to maintain iron in the redox state of Fe(II) in the absence of ascorbate. Fe(III) bound in thermophilic F1ATPase in a molar ratio 1:1 rapidly reacts with H2O2 mediating a 30% enzyme inactivation. These results support the presence in mitochondrial and thermophilic F1ATPase of a conserved site involved in iron binding and in oxidative inactivation, in which iron exhibits similar redox properties. On the other hand, at variance with thermophilic F1ATPase, the mitochondrial enzyme has the possibility of maintaining one equivalent of Fe(II) in its peculiar ATP-dependent site, besides one equivalent of Fe(III) in the conserved nucleotide-independent site. In this case mitochondrial F1ATPase undergoes a higher inactivation (75%) upon exposure to H2O2. Under all conditions the inactivation is significantly prevented by PBN and DMSO but not by Cu, Zn superoxide dismutase, thus suggesting the formation of OH radicals as mediators of the oxidative damage. No dityrosines, carbonyls or oxidized thiols are formed. In addition, in any cases no protein fragmentation or aggregation is observed upon the treatment with H2O2.

Mutations in the nucleotide binding domain of the alpha subunits of the F1-ATPase from thermophilic Bacillus PS3 that affect cross-talk between nucleotide binding sites

Inactivation of MF1 (bovine mitochondrial F1-ATPase) with 5'-p-fluorosulfonylbenzoylethenoadenosine is caused by labeling alpha Y244 [Verburg, J. G., and Allison, W. S. (1990) J. Biol. Chem. 265, 8065-8074]. In the crystal structure [Abrahams, J.P., Leslie, A. G. W., Lutter, R., and Walker, J. E. (1994) Nature 370, 621-628], alpha Y244 is hydrogen bonded to alpha R304 which is also hydrogen bonded to alpha Y300. The catalytic properties of mutant alpha 3 beta 3 gamma subcomplexes of the TF1-ATPase from the thermophilic Bacillus PS3 containing the alpha F244C, alpha R304C, and alpha Y300C substitutions have been examined. Each has unique features for hydrolyzing ATP and forming inhibitory ADP-fluoroaluminate complexes in catalytic sites. Unlike wild-type, the (alpha R304C)3 beta 3 gamma and (alpha Y300C)3 beta 3 gamma subcomplexes entrap inhibitory MgADP in a catalytic site during turnover which fails to dissociate when ATP binds to noncatalytic sites. Although the hydrolytic properties of the (alpha F244C)3 beta 3 gamma subcomplex and wild-type are similar, the mutant forms ADP-fluoroaluminate complexes 7 times faster than wild-type when Al3+ and F- are added to it in the presence of excess ADP and Mg2+. It also resists inhibition by high Mg2+ concentrations in the assay medium. At least one noncatalytic site of the (alpha F244C)3 beta 3 gamma subcomplex has increased affinity for ADP, indicating that the enhanced rate of formation of the ADP-fluoroaluminate complex reflects augmented cooperativity between noncatalytic and catalytic sites. The rate of formation of the ADP-fluoroaluminate complex in (alpha Y300C)3 beta 3 gamma increases only 40% when MgADP in bound to two catalytic sites rather than one, compared to a 9-fold increase exhibited by wild type. When Al3+ and F- are added to the (alpha Y300C)3 beta 3 gamma subcomplex after incubation with excess ADP and Mg2+, ADP-fluoroaluminate complexes are formed in three catalytic sites rather than two observed with the other subcomplexes. Reconciliation of the catalytic properties of the mutant subcomplexes in terms of the crystal structure suggests that alpha F244, alpha R304, and alpha Y300 of TF1 are part of a pathway that propagates conformational signals from one catalytic site to another.

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.

ADP-fluoroaluminate complexes are formed cooperatively at two catalytic sites of wild-type and mutant alpha3beta3gamma subcomplexes of the F1-ATPase from the thermophilic Bacillus PS3

Addition of Al3+ and F- to the alpha3beta3gamma subcomplex of the TF1-ATPase containing MgADP in one catalytic site causes slow, complete inactivation as the ADP-fluoroaluminate complex is formed. This conflicts with the "bisite" stochastic model suggested earlier (Issartel, J. P., Dupuis, A., Lunardi, J. & Vignais, P. V. (1991) Biochemistry 30, 4726-4733] on the finding that complete inactivation of the bovine mitochondrial F1-ATPase by Al3+, F-, Mg2+, and excess ADP occurs as ADP-fluoroaluminate complexes form in two catalytic sites. When Al3+ and F- were added to alpha3beta3gamma containing MgADP in two catalytic sites, inactivation accelerated 8-fold, indicating catalytic to catalytic site cooperativity. When added to alpha3beta3gamma containing MgADP bound to one or two catalytic sites prior to addition of Al3+ and F-, phosphate inhibits formation of the ADP-fluoroaluminate complex. When introduced after adding 200 microM ADP plus Mg2+ to alpha3beta3gamma, but before adding Al3+ and F-, phosphate accelerated formation of the ADP-fluoroaluminate complex 3-fold. Sulfite accelerated formation of the ADP-fluoroaluminate complex 9-fold when 200 microM ADP plus Mg2+ was added to alpha3beta3gamma before adding Al3+ and F-. The accelerations induced by phosphate or sulfite in the presence of excess ADP and Mg2+ suggest noncatalytic to catalytic site cooperativity. When Al3+ and F- were added to the (alphaD261N)3beta3gamma subcomplex containing MgADP in a single catalytic site, the ADP-fluoroaluminate complex formed at least 10-fold more slowly than observed with wild-type under the same conditions. Therefore, the catalytic site containing MgADP recognizes the alphaD261N substitution when noncatalytic sites are empty. Cross-linking alpha to gamma or beta to gamma by oxidizing the (alphaA396C)3beta3(gammaA22C) and alpha3(betaD390C)3(gammaS90C) subcomplexes, respectively, abolishes cooperative formation of ADP-fluoroaluminate complexes in two catalytic sites. ADP-fluoroaluminate complex formation is restricted to a single catalytic site in the oxidized double mutants. The alpha3beta3delta subcomplex does not form an inhibitory ADP-fluoroaluminate complex under any of the conditions examined for the alpha3beta3gamma subcomplexes.

Effect of nucleotides on the thermal stability and on the deuteration kinetics of the thermophilic F0F1 ATP synthase

Differential scanning calorimetry has been used to characterize the influence of specific nucleotide binding on the thermal unfolding of the F0F1-type ATP synthase from the thermophilic Bacillus PS3 (TF0F1). The calorimetric trace shows an irreversible and kinetically controlled endothermic transition for TF0F1 in the absence of nucleotides. The thermal denaturation occurs at a transition temperature (t(m)) of 81.7 degrees C. The remarkable thermostability of this enzyme was decreased upon tight binding of Mg2+ x ATP to noncatalytic sites, whereas binding of Mg2+ x ADP increased the temperature at which thermal denaturation occurred. At high temperatures, an exothermic transition due to aggregation processes was also affected by nucleotide binding. With the aim to correlate these thermal effects with possible structural differences among the various forms of TF0F1, Fourier transform infrared spectroscopy was carried out. Hydrogen/deuterium exchange was clearly affected by specific nucleotide occupancy. As illustrated by the total extent of protons exchanged, our results demonstrate that more peptide groups are exposed to the medium in the presence of Mg2+ x ATP than in the presence of Mg2+ x ADP. Therefore, consistent with microcalorimetric data, binding of Mg2+ x ADP induces conformational changes which shield amide protons to more buried hydrogen-bonded structures, whereas binding of Mg2+ x ATP results in a more open or flexible structure.

The alpha3beta3gamma subcomplex of the F1-ATPase from the thermophilic bacillus PS3 with the betaT165S substitution does not entrap inhibitory MgADP in a catalytic site during turnover

The hydrolytic properties of the mutant alpha3(betaT165S)3gamma and wild-type alpha3beta3gamma subcomplexes of TF1 have been compared. Whereas the wild-type complex hydrolyzes 50 microM ATP in three kinetic phases, the mutant complex hydrolyzes 50 microM ATP with a linear rate. After incubation with a slight excess of ADP in the presence of Mg2+, the wild-type complex hydrolyzes 2 mM ATP with a long lag. In contrast, prior incubation of the mutant complex under these conditions does not affect the kinetics of ATP hydrolysis. The ATPase activity of the wild-type complex is stimulated 4-fold by 0. 1% lauryl dimethylamine oxide, whereas this concentration of lauryl dimethylamine oxide inhibits the mutant complex by 25%. Compared with the wild-type complex, the activity of the mutant complex is much less sensitive to turnover-dependent inhibition by azide. This comparison suggests that the mutant complex does not entrap substantial inhibitory MgADP in a catalytic site during turnover, which is supported by the following observations. ATP hydrolysis catalyzed by the wild-type complex is progressively inhibited by increasing concentrations of Mg2+ in the assay medium, whereas the mutant complex is insensitive to increasing concentrations of Mg2+. A Lineweaver-Burk plot constructed from rates of hydrolysis of 20-2000 microM ATP by the wild-type complex is biphasic, exhibiting apparent Km values of 30 microM and 470 microM with corresponding kcat values of 26 and 77 s-1. In contrast, a Lineweaver-Burk plot for the mutant complex is linear in this range of ATP concentration, displaying a Km of 133 microM and a kcat of 360 s-1.

Catalytic and EPR studies of the beta E204Q mutant of the chloroplast F1-ATPase from Chlamydomonas reinhardtii

The mutation E204Q in the beta subunit of the chloroplast F1-ATPase was made by biolistic transformation of Chlamydomonas reinhardtii. The yield of chloroplast F1-ATPase (CF1) purified from thylakoids was unaltered, suggesting that the mutation did not affect protein assembly. However, photoautotrophic growth of Chlamydomonas strains containing beta E204Q was virtually abolished, and the effect of the mutation on the light-driven ATPsynthase activity catalyzed by purified thylakoids was comparable to the change in the photoautotrophic growth rate. The loss of ATPsynthase activity in the mutant was not the result of uncoupling. Addition of wild-type CF1 to mutant thylakoids depleted of CF1 reconstituted ATPsynthase activity indicating that the mutation did not affect assembly of F0. Furthermore, the mutant CF1F0 was capable of catalyzing ATPase-dependent proton pumping as measured by fluorescence quenching of 9-amino acridine. Although the mutation significantly affected the apparent kcat/K(m) of the Mg(2+)-ATPase activity of the purified CF1-ATPase, no significant effect on the apparent kcat was observed with the mutant compared to wild-type. No significant changes in the ability of Mg2+ or Mn2+ to serve either as a cofactor or as an inhibitor of ATPase activity were observed in the mutants relative to the wild-type CF1-ATPase. EPR spectra were also taken of VO2+ bound at catalytic site 3 in its latent form. In a large fraction of the latent enzyme, a carboxyl group has displaced the nucleotide-phosphate coordination to the metal which results in the free-metal inhibited form (M3). No significant effects on the gII and AII 51V hyperfine parameters were observed between wild-type and mutant. However, the mutation increased the abundance of the M3 form relative to the M3-N3 form (metal-nucleotide-coordinated form). On the basis of these results, beta E204 is not the carboxyl group that displaces the nucleotide phosphate as a ligand to form the free-metal inhibited enzyme form which predominates in site 3 in the latent state. Instead, the data are consistent with a role in which beta E204 is essential to protonate an inorganic phosphate-oxygen to make that oxygen a good leaving group to facilitate ATP synthesis and, via this role in H-bonding, increases the abundance of the functional metal-nucleotide complex bound to the catalytic site.

Blocking one non-catalytic ADP binding site results in complete inhibition of the F-type ATPase from the thermophilic Bacillus PS3

The F-type ATPase, TF0F1, from the thermophilic Bacillus PS3, which is free of nucleotides after isolation, was specifically loaded with one 2-azido ADP on a non-catalytic site. The enzyme was covalently modified to various extents and the rate of ATP synthesis and ATP hydrolysis was measured. Both ATP synthesis and ATP hydrolysis extrapolated to zero for covalently binding one nucleotide per enzyme. This was interpreted such that the non-catalytic sites are involved in the coupled catalytic process.

ATP synthesis by the F0F1-ATPase from the thermophilic Bacillus PS3 co-reconstituted with bacteriorhodopsin into liposomes. Evidence for stimulation of ATP synthesis by ATP bound to a noncatalytic binding site

F-type ATPase from the thermophilic Bacillus PS3, TF0F1, which was essentially free of bound nucleotides after isolation and purification, was co-reconstituted into liposomes with the light-driven proton pump bacteriorhodopsin. The time course of the light-induced ATP synthesis was biphasic; an initial slow phase accelerated to a final steady-state rate two to three times faster. Adding ATP before initiating the reaction suppressed the slow phase, suggesting that the state of occupancy of specific sites by ATP regulated the synthetic activity of TF0F1. Incubating the purified TF0F1 with ADP and ATP revealed one ADP and two ATP binding sites that were stable to gel filtration. We analyzed the time courses of light-induced ATP synthesis for the enzyme with different nucleotide content, after co-reconstitution into liposomes with bacteriorhodopsin. The two ATP sites were identified to have regulatory function. A complex containing TF0F1.ADP, 1:1, was co-reconstituted with various quantities of ATP to obtain a range of molar ratios of TF0F1.ADP:ATP of between 1:0 and 1:1.7. It was found that the initial rate of ATP synthesis increased with the level of ATP bound to the enzyme. After binding one ATP, a stimulation of ATP synthesis by a factor of 2 was observed. The second ATP site also exhibited regulatory properties. It stimulated ATP synthesis but to a much smaller extent; the stimulation did not exceed 20%. Binding of the photoreactive analogues 2-azido-[alpha-32P]ADP and 2-azido-[alpha-32P]ATP to the TF0F1 and their effects on the rate of ATP synthesis are described further.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of the wild-type and the Cys-/Trp-less alpha 3 beta 3 gamma complex of thermophilic F1-ATPase in Escherichia coli

The alpha, beta and gamma subunits of F1-ATPase from thermophilic Bacillus PS3 were expressed in Escherichia coli cells simultaneously in large amounts. Most of the expressed subunits assembled into a form of alpha 3 beta 3 gamma complex in E. coli cells and this complex was easily purified to homogeneity. The recombinant alpha 3 beta 3 gamma complex thus obtained showed similar enzymatic properties to the alpha 3 beta 3 gamma complex obtained by in vitro reconstitution from individual subunits (Yokoyama, K. et al. (1989) J. Biol. Chem. 264, 21837-21841) except that the former had several-fold higher ATPase activity than the latter. Using this expression system, a mutant alpha 3 beta 3 gamma complex with no Trp and Cys was generated by replacing alpha Cys193 and alpha Trp463 with Ser and Phe, respectively. This mutant complex was functionally intact, indicating both residues are not essential for catalysis. The Cys-/Trp-less complex is a convenient 'second wild type' enzyme from which one can generate mutants with Trp (as a fluorescent probe) or Cys (as an acceptor of a variety of probes) at desired positions without concern for 'background' Trp and Cys residues.

Covalent modification of catalytic sites on membrane-bound beef heart mitochondrial ATPase by 2-azido-adenine nucleotides

Incubation in the dark of 32P-labeled 2-azido-adenine nucleotides with submitochondrial particles from beef heart led to tight binding of the label by membrane-bound F1. That is, the label remained with the particles following two passages through centrifuge columns. After removal of free nucleotides and ultraviolet irradiation, the radioactive label was covalently bound exclusively to the beta subunit of the ATPase. Extraction of the modified enzyme from the membrane with chloroform followed by tryptic digestion and separation of peptides by reverse-phase high-pressure liquid chromatography indicated that the radioactive label had been inserted into a peptide fragment that included part of the catalytic site. Covalent modification of catalytic sites by 2-azido-ADP was accompanied by parallel inhibition of both ATP synthesis and ATP hydrolysis by submitochondrial particles. Estimation of the likely amount of F1 participating in the reaction and extrapolation to complete inhibition suggested that modification of no more than a single site was sufficient to block both reactions. The results support suggestions of cooperative interactions between catalytic sites as well as a single catalytic pathway for both enzymic reactions.

Structural model of the ATP-binding domain of the F1-beta subunit based on analogy to the RecA protein

In contrast to the previous topological model of the ATP binding domain of the F1-ATPase beta subunit based on analogies to those of ras p21 and adenylate kinase, a more consistent model can be constructed with the known structure of the recA protein as a reference. The secondary structure of the F1-ATPase beta subunit predicted from the primary structure agrees well with that of the recA protein. The topology includes a repetitive beta alpha c beta alpha beta alpha beta alpha beta structure where all beta strands are parallel and surround the central alpha c helix above which bound ATP is located. Several residues thought to be located at catalytic site as reported in genetic and chemical labeling work can be consistently positioned in this model.

In vivo affinity label of a protein expressed in Escherichia coli. Coenzyme A occupied the AT(D)P binding site of the mutant F1-ATPase beta subunit (Y307C) through a disulfide bond

When Tyr-307 of the beta subunit of F1-ATPase from a thermophilic Bacillus strain PS3 is replaced by cysteine and expressed in Escherichia coli cells, about a half population of the mutant beta subunit are labeled by Coenzyme A at Cys-307 through a disulfide bond which is cleavable by reducing treatment. The mutant beta subunit can be reconstituted into the alpha 3 beta 3 complex of which ATPase activity is stimulated two-fold by reducing treatment either prior or after reconstitution. Since Tyr-307 has been supposed to be located at one of subdomains which form the ATP binding site of the beta subunit, Coenzyme A binds to the mutant beta subunit as an AT(D)P analogue in E. coli cells and then covalently attaches to Cys-307.

Functional sites in F1-ATPases: location and interactions

This review focuses on the location and interaction of three functional sites in F1-ATPases. These are catalytic sites which are located in beta subunits, noncatalytic nucleotide-binding sites which are located at interfaces of alpha and beta subunits and modulate the hydrolytic activity of the enzyme, and a site that binds inhibitory amphipathic cations which is at an interface of alpha and beta subunits. The latter site may participate in transmission of conformational signals between catalytic sites in F1 and the proton-conducting apparatus of F0 in the intact ATP synthases.

The ATP synthase (F0-F1) complex in oxidative phosphorylation

The transmembrane electrochemical proton gradient generated by the redox systems of the respiratory chain in mitochondria and aerobic bacteria is utilized by proton translocating ATP synthases to catalyze the synthesis of ATP from ADP and P(i). The bacterial and mitochondrial H(+)-ATP synthases both consist of a membranous sector, F0, which forms a H(+)-channel, and an extramembranous sector, F1, which is responsible for catalysis. When detached from the membrane, the purified F1 sector functions mainly as an ATPase. In chloroplasts, the synthesis of ATP is also driven by a proton motive force, and the enzyme complex responsible for this synthesis is similar to the mitochondrial and bacterial ATP synthases. The synthesis of ATP by H(+)-ATP synthases proceeds without the formation of a phosphorylated enzyme intermediate, and involves co-operative interactions between the catalytic subunits.

Human α2-HS-glycoprotein/bovine fetuin homologue in mice: identification and developmental regulation of the gene

Human alpha 2-HS-glycoprotein (AHSG) is a plasma protein synthesized in liver and selectively concentrated in bone matrix. It has been reported to be involved in bone formation and resorption as well as immune responses. Recently, AHSG was found to be the species equivalent protein of fetuin, the major fetal serum protein in cattle and sheep. The function and regulation of AHSG/fetuin in different species are not understood. We have isolated a liver cDNA clone that encodes the human AHSG/bovine fetuin homologue in the mouse. The AHSG/fetuin gene may have a role in differentiation since it is expressed in mouse limb buds and brain only at certain stages during development. Mouse liver AHSG/fetuin mRNA was present at low level at 12 days gestation but its level increased during the late part of gestation and peaked between 1 to 3 months after birth. The regulation of mouse AHSG/fetuin synthesis during development was found to be significantly different from that of sheep and bovine fetuin. Compared to fetuin, which is reduced in adult to 1 to 2% of the fetal level, mouse AHSG synthesis subsides only 50% 4 months after birth.

Changes in the adenine nucleotide content of beef-heart mitochondrial F1 ATPase during ATP synthesis in dimethyl sulfoxide

Beef-heart mitochondrial F1 ATPase can be induced to synthesize ATP from ADP and inorganic phosphate in 30% Me2SO. We have analyzed the adenine nucleotide content of the F1 ATPase during the time-course of ATP synthesis, in the absence of added medium nucleotide, and in the absence and presence of 10 mM inorganic phosphate. The enzyme used in these investigations was either pretreated or not pretreated with ATP to produce F1 with a defined nucleotide content and catalytic or noncatalytic nucleotide-binding site occupancy. We show that the mechanism of ATP synthesis in Me2SO involves (i) an initial rapid loss of bound nucleotide(s), this process being strongly influenced by inorganic phosphate; (ii) a rebinding of lost nucleotide; and (iii) synthesis of ATP from bound ADP and inorganic phosphate.

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.

Uni-site ATP synthesis in thylakoids

Uni-site ATP synthesis was measured with thylakoids. The membrane-bound ATP-synthase, CF0F1, was brought into the active, reduced state by illumination in the presence of thioredoxin, dithiothreitol and phosphate. This enzyme contains two tightly bound ATP per CF0F1. ATP was released from the enzyme when ADP was added in substoichiometric amounts during illumination. Experiments with [14C]ADP indicated that after binding the same nucleotide was phosphorylated and released as [14C]ATP, i.e. only one site is involved in ATP-synthesis ('uni-site ATP-synthesis'). The two tightly bound ATP are not involved in the catalytic turnover. The rate constant for ADP binding was (4 +/- 2) x 10(6) M-1s-1. Compared to deenergized conditions the rate constant for ADP binding and that for ATP-release were drastically increased, i.e. membrane energization increased the rate constants for the ATP-synthesis direction.

Aromatic rings of tyrosine residues at adenine nucleotide binding sites of the beta subunits of F1-ATPase are not necessary for ATPase activity

Using site-directed mutagenesis, Tyr-307, Tyr-341, or Tyr-364, supposedly located at the adenine nucleotide binding site(s) of the beta subunits of F1-ATPase from the thermophilic bacterium PS3, was replaced with Phe or Cys. The alpha 3 beta 3 complexes reconstituted from the alpha subunits and individual mutant beta subunits hydrolyzed ATP. Thus, neither the hydroxyl groups nor the aromatic rings in these positions are required for ATPase activity of F1-ATPase.

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.

Rat testis P-450(17)alpha cDNA: the deduced amino acid sequence, expression and secondary structural configuration

A complete amino acid sequence for rat testis P-450(17)alpha was deduced from nucleotide analysis of a cDNA clone isolated from a rat Leydig cell cDNA library. This DNA clone, containing initiation and termination codons and a polyA tail, translated a polypeptide in COS-1 cells that expressed both 17 alpha-hydroxylase and 17,20 lyase activities. It exhibited significant similarity to the nucleotide and deduced amino acid sequences of the bovine and human cytochrome P-450(17)alpha, particularly with respect to the highly conserved regions and secondary structure. The P-450(17)alpha appears to be anchored to the membrane of the endoplasmic reticulum through two transmembrane regions, specifically the N terminal insertion peptide and the stop-transfer sequence. Hydropathic analysis indicates that the remainder of the C terminus is associated with the membrane through four hydrophobic clefts, including the putative steroid binding site.

Inactivation of the F1-ATPase from the thermophilic bacterium PS3 by 5'-p-fluorosulfonylbenzoylinosine at 65 degrees C is accompanied by modification of beta-tyrosine-364

A major radioactive peptide, T1, was resolved by high-performance liquid chromatography from a tryptic digest prepared from the F1-ATPase from the thermophilic bacterium PS3 which had been inactivated with p-fluorosulfonylbenzoyl[3H]inosine. Two radioactive peptides, T1P1 and T1P2, were isolated from a peptic digest of T1 by high-performance liquid chromatography. The sequences of T1P1 and T1P2 were shown to be E-E-H-X-Q-V-A-R and E-E-H-X-Q, respectively, where X corresponds to derivatized Tyr-364 of the beta subunit.

E. coli F1-ATPase: site-directed mutagenesis of the beta-subunit

Residues beta Glu-181 and beta Glu-192 of E. coli F1-ATPase (the DCCD-reactive residues) were mutated to Gln. Purified beta Gln-181 F1 showed 7-fold impairment of 'unisite' Pi formation from ATP and a large decrease in affinity for ATP. Thus the beta-181 carboxyl group in normal F1 significantly contributes to catalytic site properties. Also, positive catalytic site cooperativity was attenuated from 5 X 10(4)- to 548-fold in beta Gln-181 F1. In contrast, purified beta Gln-192 F1 showed only 6-fold reduction in 'multisite' ATPase activity. Residues beta Gly-149 and beta Gly-154 were mutated to Ile singly and in combination. These mutations, affecting residues which are strongly conserved in nucleotide-binding proteins, were chosen to hinder conformational motion in a putative 'flexible loop' in beta-subunit. Impairment of purified F1-ATPase ranged from 5 to 61%, with the double mutant F1 less impaired than either single mutant. F1 preparations containing beta Ile-154 showed 2-fold activation after release from membranes, suggesting association with F0 restrained turnover on F1 in these mutants.

Directed mutagenesis of the dicyclohexylcarbodiimide-reactive carboxyl residues in beta-subunit of F1-ATPase of Escherichia coli

Previous studies in which dicyclohexylcarbodiimide (DCCD) was used to inactivate F1-ATPase enzymes have suggested that two glutamate residues in the beta-subunit are essential for catalysis. In the Escherichia coli F1-ATPase, these are residues beta-Glu-181 and beta-Glu-192. Oligonucleotide-directed mutagenesis was used to change these residues to beta-Gln-181 and beta-Gln-192. The beta-Gln-181 mutation produced strong impairment of oxidative phosphorylation in vivo and also of ATPase and ATP-driven proton-pumping activities in membranes assayed in vitro. A low level of each activity was detected and an F1-ATPase appeared to be assembled normally on the membranes. Therefore, it is suggested that the carboxyl side chain at residue beta-181 is important, although not absolutely required, for catalysis in both directions on E. coli F1-ATPase. The beta-Gln-192 mutation produced partial inhibition of oxidative phosphorylation in vivo and membrane ATPase activity was reduced by 78%. These results contrast with the complete or near-complete inactivation seen when E. coli F1-ATPase is reacted with DCCD and imply that DCCD-inactivation is attributable more to the attachment of the bulky DCCD molecule than to the derivatization of the carboxyl side chain of residue beta-Glu-192. M. Ohtsubo and colleagues (Biochem. Biophys. Res. Commun. (1987) 146, 705-710) described mutagenesis of the F1-beta-subunit of thermophilic bacterium PS3. Mutations (Glu----Gln) of the residues homologous to Glu-181 and Glu-192 of E. coli F1-beta-subunit both caused total inhibition of ATPase activity. Therefore, there was a marked difference in results obtained when the same residues were modified in the PS3 and E. coli F1-beta-subunits.