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1.
A kinetic analysis of ATP binding to noncatalytic sites of chloroplast coupling factor CF1 was made. The ATP binding proved to be unaffected by reduction of the disulfide bridge of the CF1 -subunit. The first-order equation describing nucleotide binding to noncatalytic sites allowed for two vacant nucleotide binding sites different in their kinetics. As suggested by nucleotide concentration dependence of the rate of nucleotide binding, the tight binding was preceded by rapid reversible binding of nucleotides. Preincubation of CF1 with Mg2+ resulted in a decreased rate of ATP binding. ATP dissociation from noncatalytic sites was described by the first order equation for similar sites with a dissociation rate constant k d (ATP) 10–3 min–1. Noncatalytic sites of CF1 were shown to be not homogeneous. One of them retained the major part of endogenous ADP after precipitation of CF1 with ammonium sulfate. Its two other sites differed in kinetic parameters and affinity for ATP. Anions of phosphate, sulfite, and especially, pyrophosphate inhibited the interaction between ATP and the noncatalytic sites.  相似文献   

2.
Nucleotide binding properties of two vacant noncatalytic sites of thioredoxin-activated chloroplast coupling factor 1 (CF1) were studied. Kinetics of nucleotide binding to noncatalytic sites is described by the first-order equation that allows for two nucleotide binding sites that differ in kinetic features. Dependence of the nucleotide binding rate on nucleotide concentration suggests that tight nucleotide binding is preceded by rapid reversible binding of nucleotides. ADP binding is cooperative. The preincubation of CF1 with Mg2+ produces only slight effect on the rate of ADP binding and decreases the ATP binding rate. The ATP and ADP dissociation from noncatalytic sites is described by the first-order equation for similar sites with dissociation rate constants k−2(ADP)=1.5×10−1 min−1 and k−2(ATP)≅10−3 min−1, respectively. As follows from the study, the noncatalytic sites of CF1 are not homogeneous. One of them retains the major part of endogenous ADP after CF1 precipitation with ammonium sulfate. Its other two sites can bind both ADP and ATP but have different kinetic parameters and different affinity for nucleotides.  相似文献   

3.
SolubilizedRhodospirillum rubrum RrF1-ATPase, depleted of loosely bound nucleotides, retains 2.6 mol of tightly bound ATP and ADP/mol of enzyme. Incubation of the depleted RrF1 with Mg2+-ATP or Mg2+-AMP-PNP, followed by passage through two successive Sephadex centrifuge columns, results in retention of a maximal number of 4 mol of tightly bound nucleotides/mol of RrF1. They include 1.5 mol of nonexchangeable ATP, whereas all tightly bound ADP is fully exchangeable. A similar retention of only four out of the six nucleotide binding sites present on CF1 has been observed after its passage through one or two centrifuge columns. These results indicate that the photosynthetic, unlike the respiratory, F1-ATPases have fasterk off constants for two of the Mg-dependent nucleotide binding sites. This could be the reason for the tenfold lower Mg2+ than Ca2+-ATPase activity observed with native RrF1, as with -depleted, activated CF1. An almost complete conversion of both RrF1 and CF1 from Ca2+- to Mg2+-dependent ATPases is obtained upon addition of octylglucoside, at concentrations below its CMC, to the ATPase assay medium. Thus, octylglucoside seems to affect directly the RrF1 and CF1 divalent cation binding site(s), in addition to its proposed role in relieving their inhibition by free Mg2+ ions. The RrF1-ATPase activity is 30-fold more sensitive than CF1 to efrapeptin, and completely resistant to either inhibition or stimulation by the CF1 effector, tentoxin. Octylglucoside decreases the inhibition by efrapeptin and tentoxin, but exposes on CF1 a low-affinity, stimulatory site for tentoxin.Abbreviations: CF1, EcF1, MF1, and TF1, the soluble F1-ATPase from chloroplasts, PE. coli, mitochondria,R. rubrum, and the thermophilic bacterium PS3, respectively: AMP-PNP, adenylyl-, -imidodiphosphate; CMC, critical micellar concentration; DTT, dithiothreitol, LDAO, lauryl dimethylamine oxide.Dedicated to Professor Achim Trebst in honor of this 65th birthday.  相似文献   

4.
The regulatory effects of malate on chloroplast Mg2+-ATPase were investigated and the mechanism was discussed. Malate stimulated methanol-activated membrane-bound and isolated CF1 Mg2+-ATPase activity. The subunit of CF1 may be involved in malate regulation of the enzyme function. Modification of subunit at one site of the peptide by NEM may affect malate stimulation of ATPase while at another site may have no effect. The effect of malate on the Mg2+-ATPase was also controlled by the Mg2+/ATP ratio in the reaction medium. The enhancing effect of malate on Mg2+-ATPase activity depended on the presence of high concentrations of Mg2+ in the reaction mixture. Kinetic study showed that malate raised the Vmax of catalysis without affecting the Km for Mg2+ ATP. The experiments imply that the stimulation of Mg2+-ATPase by malate is probably correlated with the Pi binding site on the enzyme. The regulation of ATPase activity by malate in chloroplasts may be relevant to its function in vivo.Abbreviations CF1 chloroplast coupling factor 1 - CF1 (-) and CF1 (-) CF1 deficient in the and subunit - MF1 mitochondria coupling factor 1 - NEM N-ethylmaleimide - PMS phenazine methosulfate - OG n-octyl--d-glucopyranoside  相似文献   

5.
The ATPase activity of the chloroplast coupling factor 1 (CF1) isolated from the green alga Dunaliella is completely latent. A brief heat treatment irreversibly induces a Ca2+ -dependent activity. The Ca2+ dependent ATPase activity can be reversibly inhibited by ethanol, which changes the divalent cation dependency from Ca2+ to Mg2+. Both the Ca2+ -dependent and Mg2+ -dependent ATPase activities of heat-treated Dunaliella CF1 are inhibited by monospecific antisera directed against Chlamydomonas reinhardi CF1. However, when assayed under identical conditions, the Ca2+ -dependent ATPase activity is significantly more sensitive to inhibition by the antisera than is the Mg2+ -dependent activity. These data are interpreted as indicating that soluble Dunaliella CF1 can exist in a variety of conformations, at least one of which catalyzes a Ca2+ -dependent ATPase and two or more of which catalyze an Mg2+ -dependent ATPase.  相似文献   

6.
(1) Octylglucoside stimulates an Mg2+-specific ATPase activity with CF1 preparations from different higher plants and the alga Chlamydomonas reinhardii. (2) Tentoxin at high concentrations (10?4–10?3 M) in the presence of octylglucoside further stimulates the Mg2+-ATPase activity of CF1 from tentoxin-sensitive species and inhibits the activity of CF1 from tentoxin-resistant species. The extent of tentoxin stimulation and inhibition varies among species. A maximal stimulation of over 2-fold was obtained with spinach CF1 and a maximal inhibition of 50% was obtained with C. reinhardii CF1. In Nicotiana spp., tentoxin had only a marginal effect on the Mg2+-ATPase activity induced by octylglucoside.  相似文献   

7.
The chloroplast coupling factor (CF1) was dissociated into subunits by the freezing-thawing procedure in the presence of 0.5 M NaBr and the subunit was purified by ion-exchange chromatography on a DEAE-cellulose column. The subunit did not catalyze ATP hydrolysis either in the presence or in the absence of reagents known to activate Mg2+-dependent ATPase activity of CF1. However, it manifested appreciable adenylate kinase-like and ATP-ADP -phosphate exchange activities. The adenylate kinase-like activity only slightly depended on Mg2+ ions. Ethanol, and especially diadenosine pentaphosphate, inhibited the reaction effectively. In contrast, the ATP-ADP exchange activity was Mg2+-dependent. Ethanol and diadenosine pentaphosphate were poor inhibitors. Sulfite, the CF1-ATPase activator, and quercetin, its inhibitor, had a minor effect on catalytic activity of the subunit.Abbreviations CF chloroplast coupling factor 1 - RBP carboxylase-ribulose-1,5-bisphosphate carboxylase - TLC thin layer chromatography - MES morpholinoethane sulfonic acid - PMSF phenylmethylsulfonyl fluoride - AP5A diadenosine pentaphosphate, P1, P5-bis(5-adenosyl)pentaphosphate - DCCD N1N-dicyclohexylcarbodiimide - SDS sodium dodecylsulfate - PAAG polyacrylamide gel  相似文献   

8.
A modified ‘cold chase’ technique was used to study tight [14C]ADP and [14C]ATP binding to noncatalytic sites of chloroplast ATP synthase (CF0F1). The binding was very low in the dark and sharply increased with light intensity. Dissociation of labeled nucleotides incorporated into noncatalytic sites of CF0F1 or CF1 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 μM nucleotides leads to filling of up to two noncatalytic sites of CF0F1. 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 CF1 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 α subunits induced by its interaction with the δ subunit and/or subunit I–II when CF1 becomes bound to a thylakoid membrane.  相似文献   

9.
Interaction between F(1)-ATPase activity stimulating oxyanions and noncatalytic sites of coupling factor CF(1) was studied. Carbonate, borate and sulfite anions were shown to inhibit tight binding of [14C]ATP and [14C]ADP to CF(1) noncatalytic sites. The demonstrated change of their inhibitory efficiency in carbonate-borate-sulfite order coincides with the previously found change in efficiency of these anions as stimulators of CF(1)-ATPase activity [Biochemistry (Mosc.) 43 (1978) 1206-1211]. Inhibition of tight nucleotide binding to noncatalytic sites was accompanied by stimulation of nucleotide binding to catalytic sites. This suggests that stimulation of CF(1)-ATPase activity is caused by interaction between oxyanions and noncatalytic sites. A most efficient stimulator of CF(1)-ATPase activity, sulfite oxyanion, appeared to be a competitive inhibitor with respect to ATP and a partial noncompetitive inhibitor with respect to ADP. The inhibition weakened with increasing time of CF(1) incubation with sulfite and nucleotides. Sulfite is believed to inhibit fast reversible interaction between nucleotides and noncatalytic sites and to produce no effect on subsequent tight binding of nucleotides. A possible mechanism of the oxyanion-stimulating effect is discussed.  相似文献   

10.
The Mg2+ dependent asymmetry of the F1-ATPase catalytic sites leads to the differences in affinity for nucleotides and is an essential component of the binding-change mechanism. Changes in metal ligands during the catalytic cycle responsible for this asymmetry were characterized by vanadyl (V IV + O)2+, a functional surrogate for Mg2+. The 51V-hyperfine parameters derived from EPR spectra of VO2+ bound to specific sites on F1 provide a direct probe of the metal ligands. Site-directed mutations of metal ligand residues cause measurable changes in the 51V-hyperfine parameters of the bound VO2+, thereby providing a means to identification. Initial binding of the metal–nucleotide to the low-affinity catalytic site conformation results in metal coordination by hydroxyl groups from the P-loop threonine and catch-loop threonine. Upon conversion to the high-affinity conformation, carboxyl groups from the Walker homology B aspartate and MF1E197 become ligands in lieu of the hydroxyl groups.  相似文献   

11.
The isolation of the chloroplast ATP synthase complex (CF0-CF1) and of CF1 from Dunaliella bardawil is described. The subunit structure of the D. bardawil ATPase differs from that of the spinach in that the D. bardawil α subunit migrates ahead of the β subunit and ε-migrates ahead of subunit II of CF0 when separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The CF1 isolated from D. bardawil resembles the CF1 isolated from Chladmydomonas reinhardi in that a reversible, Mg2+-dependent ATPase is induced by selected organic solvents. Glycerol stimulates cyclic photophosphorylation catalyzed by D. bardawil thylakoid membranes but inhibits photophosphorylation catalyzed by spinach thylakoid membranes. Glycerol (20%) also stimulates the rate of ATP-Pi exchange catalyzed by D. bardawil CF0-CF1 proteoliposomes but inhibits the activity with the spinach enzyme. The ethanol-activated, Mg2+-ATPase of the D. bardawil CF1 is more resistant to glycerol inhibition than the octylglucoside-activated, Mg2+-ATPase of spinach CF1 or the ethanol-activated, Mg2+-dependent ATPase of the C. reinhardi CF1. Both cyclic photophosphorylation and ATP-Pi exchange catalyzed by D. bardawil CF0-CF1 are more sensitive to high concentrations of NaCl than is the spinach complex.  相似文献   

12.
Hassan M. Younis  John S. Boyer 《BBA》1979,548(2):328-340
(1) Photophosphorylation, fCa2+-ATPase and Mg2+-ATPase activities of isolated chloroplasts were inhibited 55–65% when the chemical potential of water was decreased by dehydrating leaves to water potentials (ψw) of ?25 bars before isolation of the plastids. The inhibition could be reversed in vivo by rehydrating the leaves.(2) These losses in activity were reflected in coupling factor (CF1) isolated from the leaves, since CF1 from leaves with low ?w had less Ca2+-ATPase activity than control CF1 and did not recouple phosphorylation in CF1-deficient chloroplasts. In contrast, CF1 from leaves having high ?w only partially recoupled phosphorylation by CF1-deficient chloroplasts from leaves having low ?w. This indicated that low ?w affected chloroplast membranes as well as CF1 itself.(3) Coupling factor from leaves having low ψw had the same number of subunits, and the same electrophoretic mobility, and could be obtained with the same yields as CF1 from control leaves. However, direct measurements of fluorescence polarization, ultraviolet absorption, and circular dichroism showed that CF1 from leaves having low ?w differed from control CF1. The CF1 from leaves having low ?w also had decreased ability to bind fluorescent nucleotides (?-ATP and ?-ADP).(4) Exposure of isolated CF1 to low ?w in vitro by preincubation in sucrose-containing media inhibited the Ca2+-ATPase activity of the protein in subsequent assays without sucrose. Inclusion of 5 or 10 mM Mg2+ in the preincubation medium markedly inhibited Ca2+-ATPase activity.(5) These results show that CF1 undergoes changes in cells which alter its phosphorylating ability. Since low cell ?w changed the spectroscopic properties but not other protein properties of CF1, the changes were most likely caused by altered conformation of the protein. This decreased the binding of nucleotides and, in turn, photophosphorylation. The inhibition of ATPase activity in CF1 in vitro at low ?w and high ion concentration mimicked the change in activity seen in vivo.  相似文献   

13.
This study investigated the effects of tetracycline on photophosphorylation, electron transport and P/O ratio of spinach chloroplasts. When chloroplast preparations were treated with low concentrations of tetracycline, non-cyclic and cyclic photophosphorylation activities increased, electron transport rates and P/O ratios improved, chloroplast ms-DLE also improved, and the Mg2+-ATPase activity of CF1 increased in comparison to the control. These results indicate that spinach chloroplasts are sensitive to tetracycline. Next, we used the fluorescence emission spectra of CF1 to examine the possible binding sites for tetracycline. The fluorescence emission spectra of CF1 treated with glutaraldehyde, NEM and TNBS, which interact with CF1 across its whole structure, at the γ subunit and at the β subunit, respectively, were compared with that of control CF1. The peak sites of the various fluorescence emission spectra were the same, but the peak values for CF1 treated with glutaraldehyde, NEM and TNBS were lower than that of control CF1. The peak value of CF1 treated with 50 μM tetracycline was very similar to that of CF1 treated with NEM. The above results indicate that the acting site of tetracycline may be at or near the γ subunit of CF1, and allows the creation of a model in which tetracycline binding strengthens the subunit interactions of ATP synthase, enlarges the proton motive force across the thylakoid membrane, and allows the excess proton motive force to increase ATP formation and improve the P/O ratio. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   

14.
Effect of nano-TiO2 on photochemical reaction of chloroplasts of spinach   总被引:1,自引:0,他引:1  
The effects of nano-TiO2 (rutile) on the photochemical reaction of chloroplasts of spinach were studied. The results showed that when spinach was treated with 0.25% nano-TiO2, the Hill reaction, such as the reduction rate of FeCy, and the rate of evolution oxygen of chloroplasts was accelerated and noncyclic photophosphorylation (nc-PSP) activity of chloroplasts was higher than cyclic photophosphorylation (c-PSP) activity, the chloroplast coupling was improved and activities of Mg2+-ATPase and chloroplast coupling factor I (CF1)-ATPase on the thylakoid membranes were obviously activated. It suggested that photosynthesis promoted by nano-TiO2 might be related to activation of photochemical reaction of chloroplasts of spinach.  相似文献   

15.
M B Murataliev 《Biochemistry》1992,31(51):12885-12892
The evidence is presented that the ADP- and Mg(2+)-dependent inactivation of MF1-ATPase during MgATP hydrolysis requires binding of ATP at two binding sites: one is catalytic and the second is noncatalytic. Binding of the noncatalytic ATP increases the rate of the inactive complex formation in the course of ATP hydrolysis. The rate of the enzyme inactivation during ATP hydrolysis depends on the medium Mg2+ concentration. High Mg2+ inhibits the steady-state activity of MF1-ATPase by increasing the rate of formation of inactive enzyme-ADP-Mg2+ complex, thereby shifting the equilibrium between active and inactive enzyme forms. The Mg2+ needed for MF1-ATPase inactivation binds from the medium independent from the MgATP binding at either catalytic or noncatalytic sites. The inhibitory ADP molecule arises at the MF1-ATPase catalytic site as a result of MgATP hydrolysis. Exposure of the native MF1-ATPase with bound ADP at a catalytic site to 1 mM Mg2+ prior to assay inactivates the enzymes with kinact 24 min-1. The maximal inactivation rate during ATP hydrolysis at saturating MgATP and Mg2+ does not exceed 10 min-1. The results show that the rate-limiting step of the MF1-ATPase inactivation during ATP hydrolysis with excess Mg2+ precedes binding of Mg2+ and likely is the rate of formation of enzyme with ADP bound at the catalytic site without bound P(i). This complex binds Mg2+ resulting in inactive MF1-ATPase.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   

16.
In order to get insight into the origin of apparent negative cooperativity observed for F1-ATPase, we compared ATPase activity and ATPMg binding of mutant subcomplexes of thermophilic F1-ATPase, α(W463F)3β(Y341W)3γ and α(K175A/T176A/W463F)3β(Y341W)3γ. For α(W463F)3β(Y341W)3γ, apparent Km's of ATPase kinetics (4.0 and 233 μM) did not agree with apparent Km's deduced from fluorescence quenching of the introduced tryptophan residue (on the order of nM, 0.016 and 13 μM). On the other hand, in case of α(K175A/T176A/W463F)3β(Y341W)3γ, which lacks noncatalytic nucleotide binding sites, the apparent Km of ATPase activity (10 μM) roughly agreed with the highest Km of fluorescence measurements (27 μM). The results indicate that in case of α(W463F)3β(Y341W)3γ, the activating effect of ATP binding to noncatalytic sites dominates overall ATPase kinetics and the highest apparent Km of ATPase activity does not represent the ATP binding to a catalytic site. In case of α(K175A/T176A/W463F)3β(Y341W)3γ, the Km of ATPase activity reflects the ATP binding to a catalytic site due to the lack of noncatalytic sites. The Eadie-Hofstee plot of ATPase reaction by α(K175A/T176A/W463F)3β(Y341W)3γ was rather linear compared with that of α(W463F)3β(Y341W)3γ, if not perfectly straight, indicating that the apparent negative cooperativity observed for wild-type F1-ATPase is due to the ATP binding to catalytic sites and noncatalytic sites. Thus, the frequently observed Km's of 100-300 μM and 1-30 μM range for wild-type F1-ATPase correspond to ATP binding to a noncatalytic site and catalytic site, respectively.  相似文献   

17.
Calcium is actively transported into intracellular organelles and out of the cytoplasm by Ca2+/Mg2+-ATPases located in the endoplasmic reticulum and plasma membranes. We studied the effects of aluminum on calcium transport in the adult rat brain. We examined 45Ca-uptake in microsomes and Ca2+-ATPase activity in microsomes and synaptosomes isolated from the frontal cortex and cerebellum of adult male Long-Evans rats. ATP-dependent45Ca-uptake was similar in microsomes from both brain regions. The addition of 50-800 μM AICI3 resulted in a concentration-dependent inhibition of 45Ca-uptake. Mg2+-dependent Ca2+-ATPase activity was significantly lower in synaptosomes compared to microsomes in both frontal cortex and cerebellum. In contrast to the uptake studies, AICI3 stimulated Mg2+-dependent Ca2+-ATPase activity in both microsomes and synaptosomes from both brain regions. To determine the relationship between aluminum and Mg2+, we measured ATPase activity in the presence of increasing concentrations of Mg2+ or AICI3. Maximal ATPase activity was obtained between 3 and 6 mM Mg2+. When we substituted AICI3 for Mg2+, ATPase activity was also stimulated in a concentration-dependent manner, but to a greater extent than with Mg2+. One interpretation of these data is that aluminum acts at multiple sites to displace both Mg2+ and Ca2+, increasing the activity of the Ca2+-ATPase, but disrupting transport of calcium.  相似文献   

18.
We have investigated the localization of a set of intrinsic ATPase activities associated with purified synaptic plasma membranes and consisting of (a) a Mg2+-ATPase; (b) an ATPase active at high concentrations of Ca2+ in the absence of Mg2+ (CaH-ATPase); (c) a Ca2+ requiring Mg2+-dependent ATPase (Ca + Mg)-ATPase, stimulated by calmodulin (Ca-CaM-ATPase); (d) a Ca2+-dependent ATPase stimulated by dopamine (DA-ATPase); and (e) the ouabain-sensitive (Na + K)-ATPase. The following results were obtained: (1) All ATPases are largely confined to the presynaptic membrane; (2) the DA-, (Ca + Mg)-, (Ca-CaM)-, and (Na + K)-ATPases are oriented with their ATP hydrolysis sites facing the synaptoplasm; (3) the Mg- and CaH-ATPases are oriented with their ATP hydrolysis sites on the junctional side of the presynaptic membrane and are therefore classified as ecto-ATPases of as yet unknown function.  相似文献   

19.
Catalytic and noncatalytic sites of the chloroplast coupling factor (CF1) were selectively modified by incubation with the dialdehyde derivative of fluorescent adenosine diphosphate analog 1,N6-ethenoadenosine diphosphate. The modified CF1 was reconstituted with EDTA-treated thylakoid membranes of chloroplasts. The effects of light-induced transmembrane proton gradient and phosphate ions on the fluorescence of 1,N6-ethenoadenosine diphosphate, covalently bound to the catalytic sites of ATP synthase, were studied. Quenching of fluorescence of covalently bound 1,N6-ethenoadenosine diphosphate was observed under illumination of thylakoid membranes with saturating white light. Addition of inorganic phosphate to the reaction mixture in the dark increased the fluorescence of the label. Quenching reappeared under repeated illumination; however, addition of phosphate ions had no effect on the fluorescence yield in this case. When 1,N6-ethenoadenosine diphosphate was covalently bound to noncatalytic sites of ATP synthase, no similar fluorescence changes were observed. The relation between the observed changes of 1,N6-ethenoadenosine diphosphate fluorescence and the mechanism of energy-dependent structural changes in the catalytic site of ATP synthase is discussed.  相似文献   

20.
Action of Cl? + HCO3 ?1 ions on Mg2+-ATPase from brain plasma membranes of fish and rats has been studied. Maximal effect of the anions on the “basal” Mg2+-ATPase activity is revealed in the presence of 10 mM Cl? and 3 mM HCO3 ?1 at physiological values of pH of incubation medium. The studied Cl?, HCO3 ?-activated Mg2+-ATPases of both animal species, by their sensitivity to SH-reagents (5,5-dithio-bis-nitrobenzoic acid, N-ethylmaleimide), oligomycin, and orthovanadate, are similar to transport ATPase of the P-type, but differ from them by molecular properties and by sensitivity to ligands of GABAA-receptors. It has been established that the sensitive to GABAA-ergic ligands, Cl?, HCO3 ?-activated Mg2+-ATPase from brain of the both animal species is protein of molecular mass around 300 kDa and of Stock’s radius 5.4 nm. In fish the enzyme is composed of one major unit of molecular mass approximately 56 kDa, while in rats-of three subunits of molecular masses about 57, 53, and 45 kDa. A functional and structural coupling of the ATP-hydrolyzing areas of the studied enzyme to sites of binding of GABAA-receptor ligands is suggested.  相似文献   

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