Application of the theory of enzyme subunit interactions to ATP-hydrolyzing enzymes. The case of Na,K-ATPase

Mechanism of allosteric effects of ATP on the kinetics of P-type ATPases

The roles of allosteric effects of ATP and protein oligomerisation in the mechanisms of P-type ATPases belong to the most controversial and least well understood topics in the field. Recent crystal structural and kinetic data, however, now allow certain hypotheses to be definitely excluded and consistent hypotheses to be developed. The aim of this review is to critically discuss recent results and, in the light of them, to present a set of conclusions which could form the basis of future research. The major conclusions are: (1) at saturating ATP concentrations P-type ATPases function as monomeric enzymes, (2) the catalytic units of P-type ATPases only possess a single ATP binding site, (3) at non-saturating ATP concentrations P-type ATPases exist as diprotomeric (or higher oligomeric) complexes, (4) protein-protein interactions within a diprotomeric complex enhances the enzymes' ATP binding affinity, (5) ATP binding to both protomers within a diprotomeric complex causes it to dissociate into two separate monomers. The physiological role of protein-protein interactions within a diprotomer may be to enhance ATP binding affinity so as to scavenge ATP and maximize the ion pumping rate under hypoxic or anoxic conditions. For the first time a structural basis for the well-known ATP allosteric acceleration of the E2 --> E1 transition is presented. This is considered to be due to a minimization of steric hindrance between neighbouring protomers because of the ability of ATP to induce a compact conformation of the enzymes' cytoplasmic domains.

Affinity labelling with MgATP analogues reveals coexisting Na+ and K+ forms of the alpha-subunits of Na+/K+-ATPase

To test the hypothesis that Na+/K+-ATPase works as an (alpha beta)2-diprotomer with interacting catalytic alpha-subunits, tryptic digestion of pig kidney enzyme, that had been inactivated with substitution-inert MgATP complex analogues, was performed. This led to the demonstration of coexisting C-terminal Na+-like 80-kDa as well as K+-like 60-kDa peptides and N-terminal 40-kDa peptides of the alpha-subunit. To localize the ATP binding sites on tryptic peptides, studies with radioactive MgATP complex analogues were performed: Co(NH3)4-8-N3-ATP specifically modified the E2ATP (low affinity) binding site of Na+/K+-ATPase with an inactivation rate constant (k2) of 12 x 10-3.min-1 at 37 degrees C and a dissociation constant (Kd) of 207 +/- 28 microm. Tryptic digestion of the [gamma32P]Co(NH3)4-8-N3-ATP-inactivated and photolabelled alpha-subunit (Mr = 100 kDa) led, in the absence of univalent cations, to a K+-like C-terminal 60-kDa fragment which was labelled in addition to an unlabelled Na+-like C-terminal 80-kDa fragment. Tryptic digestion of [alpha32P]-or [gamma32P]Cr(H2O)4ATP - bound to the E1ATP (high affinity) site - led to the labelling of a Na+-like 80-kDa fragment besides the immediate formation of an unlabelled K+-like N-terminal 40-kDa fragment and a C-terminal 60-kDa fragment. Because a labelled Na+-like 80-kDa fragment cannot result from an unlabelled K+-like 60-kDa fragment, and because unlabelled alpha-subunits did not show any catalytic activity, the findings are consistent with a situation in which Na+- and K+-like conformations are stabilized by tight binding of substitution-inert MgATP complex analogues to the E1ATP and E2ATP sites. Hence, all data are consistent with the hypothesis that ATP binding induces coexisting Na+ and K+ conformations within an (alphabeta)2-diprotomeric Na+/K+-ATPase.

Molecular distance measurements reveal an (alpha beta)2 dimeric structure of Na+/K+-ATPase. High affinity ATP binding site and K+-activated phosphatase reside on different alpha-subunits

ATP hydrolysis by Na+/K+-ATPase proceeds via the interaction of simultaneously existing and cooperating high (E1ATP) and low (E2ATP) substrate binding sites. It is unclear whether both ATP sites reside on the same or on different catalytic alpha-subunits. To answer this question, we looked for a fluorescent label for the E2ATP site that would be suitable for distance measurements by Förster energy transfer after affinity labeling of the E1ATP site by fluorescein 5'-isothiocyanate (FITC). Erythrosin 5'-isothiocyanate (ErITC) inactivated, in an E1ATP site-blocked enzyme (by FITC), the residual activity of the E2ATP site, namely K+-activated p-nitrophenylphosphatase in a concentration-dependent way that was ATP-protectable. The molar ratios of FITC/alpha-subunit of 0.6 and of ErITC/alpha-subunit of 0.48 indicate 2 ATP sites per (alpha beta)2 diprotomer. Measurements of Förster energy transfer between the FITC-labeled E1ATP and the ErITC-labeled or Co(NH3)4ATP-inactivated E2ATP sites gave a distance of 6.45 +/- 0.64 nm. This distance excludes 2 ATP sites per alpha-subunit since the diameter of alpha is 4-5 nm. Förster energy transfer between cardiac glycoside binding sites labeled with anthroylouabain and fluoresceinylethylenediamino ouabain gave a distance of 4.9 +/- 0.5 nm. Hence all data are consistent with the hypothesis that Na+/K+-ATPase in cellular membranes is an (alpha beta)2 diprotomer and works as a functional dimer (Thoenges, D., and Schoner, W. (1997) J. Biol. Chem. 272, 16315-16321).

2'-O-Dansyl analogs of ATP bind with high affinity to the low affinity ATP site of Na+/K+-ATPase and reveal the interaction of two ATP sites during catalysis

Na+/K+-transport through mammalian cell membranes by Na+/K+-ATPase (EC 3.6.1.37) needs the interaction of ATP sites with different binding affinities during catalysis: one with catalytic (high affinity site) and one with regulatory properties (low affinity site). To find affinity labels for the latter one, the effects of 2'-O-dansylated ATP analogs on Na+/K+-ATPase and its partial activities were analyzed. DANS-ATP (2'-O-(6-dimethylaminonaphthalenesulfonyl)adenosine 5'-triphosphate) inhibited noncompetitively at low ATP concentrations and competitively at high ATP concentrations the Na+/K+-activated hydrolysis of ATP under turnover conditions. It interacted preferentially with the low affinity ATP site as shown by its protective effect against the inactivation of Na+/K+-ATPase by Co(NH3)4ATP and Cr(H2O)4ATP. DANS-N3-ATP, however, inactivated Na+/K+-ATPase. The initial velocity of inactivation shows a sigmoid concentration dependence that was converted to a hyperbola in the presence of ATP. DANS-N3-ATP inhibited competitively the K+-activated hydrolysis of p-nitrophenyl phosphate in a fluorescein isothiocyanate-blocked enzyme but did not effect Na+-dependent phosphoenzyme formation from [gamma-32P]ATP in a Co(NH3)4PO4-blocked enzyme. These effects could be described by a Koshland-Némethy-Filmer model assuming two nucleotide binding sites in strong cooperation. Fitting all data to this model revealed that ATP was bound in a negative cooperative way with a Kd = 0.3-1 microM to the first site and a Kd = 100-120 microM to the second site of the enzyme containing already one ATP bound. The hydrolysis of ATP through a pathway with two ATP bound was 30 times faster than hydrolysis with one ATP bound. DANS-N3-ATP bound in a positive cooperative way with a Kd = 500 +/- 100 microM to the first site and a Kd = 2.5 +/- 0.5 microM to the second site containing already one DANS-N3-ATP bound. Therefore, DANS-N3-ATP may be an useful affinity marker of the low affinity, regulatory ATP site.

Na+/K(+)-ATPase with a blocked E1ATP site still allows backdoor phosphorylation of the E2ATP site

The role of simultaneously existing ATP-binding sites in the catalytic process of Na+/K(+)-ATPase is unclear. In order to learn whether blocking the E1ATP site affects the properties of the E2ATP site, the E1ATP site was inactivated by either fluorescein 5'-isothiocyanate, the non-phosphorylating Cr(H2O)4AdoPP[CH2]P or the phosphorylating Cr(H2O)4ATP. The properties of the remaining E2ATP site were studied by measuring 'backdoor phosphorylation' in the presence of ouabain, or K(+)-activated hydrolysis of p-nitrophenyl phosphate. The involvement of the E2ATP site was further tested by the effects of Co(NH3)4ATP, a specific inactivator of this site. When the E1ATP site was inactivated by fluorescein 5'-isothiocyanate or the non-phosphorylating Cr(H2O)4AdoPP[CH2]P, backdoor phosphorylation and the activity of K(+)-activated p-nitrophenylphosphatase remained unchanged. Both processes were lost, however, when the E2ATP site was additionally inactivated by Co(NH3)4ATP. Inactivation of the E1ATP site by fluorescein 5'-isothiocyanate or Cr(H2O)4AdoPP[CH2]P decreased the affinity of the p-nitrophenylphosphatase activity of the E2ATP site for the substrate p-nitrophenyl phosphate by four times. This is consistent with a former report showing that dephosphorylation in a fluorescein 5'-isothiocyanate-inactivated Na+/K(+)-ATPase has a lowered sensitivity for ATP [Scheiner-Bobis, G., Antonipillai, J. & Farley, R. A. (1993) Biochemistry 32, 9592-9599]. Inactivation of the E1ATP site by the phosphorylating Cr(H2O)4ATP, however, led to a loss of the property of the E2ATP site to hydrolyse K(+)-dependent p-nitrophenyl phosphate and to achieve backdoor phosphorylation. Evidently, ATP sites coexist in Na+/K(+)-ATPase, and binding of ATP to one site affects the property of the other site [Scheiner-Bobis, G., Esmann, M. & Schoner, W. (1989) Eur. J. Biochem. 183, 173-178]. Although the enzyme can be phosphorylated from both ATP sites, phosphorylation of the E1ATP site excludes the phosphorylation of the E2ATP site.

Functional coupling of phosphorylation and nucleotide binding sites in the proteolytic fragments of Na+/K(+)-ATPase

Cleavage of the alpha-subunit of Na+/K(+)-ATPase by trypsin at Arg438-Ala439 causes enzyme inhibition which has been suggested to be due to altered alignment of phosphorylation site on the 48-kDa N-terminal fragment with nucleotide binding site on the 64-kDa C-terminal fragment. Our aims were to test this hypothesis and to assess the effect of the cleavage on the enzyme's two ATP sites. Na(+)-dependent phosphorylation of the partially cleaved enzyme by ATP showed that K0.5 values of ATP for phosphorylations of intact alpha and 48-kDa peptide were the same (0.4 microM). Unchanged interactions among the residues across the cleavage site were also indicated by data showing that reaction of fluorescein isothiocyanate with the 64-kDa peptide blocked phosphorylation of the 48-kDa peptide by ATP. ATP is known to block the reaction of fluorescein isothiocyanate with the enzyme. Experiments on the partially cleaved enzyme showed that K0.5 of ATP for protection of alpha was 30-60 microM, and the value for the protection of interacting 48-kDa and 64-kDa peptides was 1-3 mM. Evidently, while the cleavage does not affect the high affinity catalytic site, it disrupts the allosteric low affinity ATP site. Experiments on reconstituted preparations showed that the cleavage abolished ATP-dependent Na+/K+ exchange, Pi+ATP-dependent Rb+/Rb+ exchange, ATP-dependent Na+/Na+ exchange, and ADP+ATP-dependent Na+/Na+ exchange activities. Selective disruption of the low affinity ATP site accounts for the inhibitions of all functions involving K+(Rb+), based on the established role of this site in the control of K+ access channels. Cleavage-induced inhibitions of other activities, however, suggest additional roles of the low affinity ATP site in the reaction cycle.

Simultaneous binding of phosphate and TNP-ADP to FITC-modified NA+,K(+)-ATPase

Double-reciprocal plots of the rate of ATP hydrolysis by Na+,K(+)-ATPase versus ATP concentration are not linear, and may reflect either two distinct binding sites for ATP or a single ATP binding site whose affinity for the nucleotide alternates between high-affinity and low-affinity states. In order to determine whether multiple nucleotides or nucleotide analogs can bind simultaneously to Na,+,K(+)-ATPase, the effects of nucleotides on the hydrolysis of p-nitrophenyl phosphate and on the dephosphorylation rate of Na+,K(+)-ATPase modified by fluorescein 5'-isothiocyanate (FITC) were measured. FITC blocks the high-affinity binding site for ATP on the Na+K(+)-ATPase and inhibits ATP hydrolysis at ATP concentrations as high as 8.3 mM. The hydrolysis of p-nitrophenyl phosphate and phosphoenzyme formation from inorganic phosphate and Mg2+ were not affected by FITC modification. The p-nitrophenylphosphatase activity of unmodified Na+,K(+)-ATPase was stimulated by low concentrations of ATP (10-100 microM) and other nucleotides, and was inhibited at higher nucleotide concentrations. In contrast, there was no effect on p-nitrophenyl phosphate hydrolysis by FITC-modified Na,K(+)-ATPase at ATP concentrations less than 100 microM. The hydrolysis of p-nitrophenyl phosphate by FITC-modified Na+,K(+)-ATPase was inhibited at ATP concentrations greater than 100 microM. These observations demonstrate that the effects of ATP acting at high-affinity sites are absent in FITC-modified Na+,K(+)-ATPase but the effects of ATP acting at low-affinity sites are still observed. In unmodified Na+,K(+)-ATPase, the rate of dephosphorylation of the phosphoenzyme formed from inorganic phosphate and Mg2+ was inhibited by ATP.(ABSTRACT TRUNCATED AT 250 WORDS)

Modification of the E1ATP binding site of Na+/K(+)-ATPase by the chromium complex of adenosine 5'-[beta,gamma-methylene]triphosphate blocks the overall reaction but not the partial activities of the E2 conformation

The chromium complex of adenosine 5'-[beta,gamma-methylene]triphosphate, Cr(H2O)4AdoPP[CH2]P, inactivates Na+/K(+)-ATPase from pig kidney at 37 degrees C with an inactivation velocity constant of 7.1 x 10(-3) min-1 by binding to the high-affinity ATP site (E1ATP site). The dissociation constant (Kd) of the analogue at this site is 26 microM, and of ATP 0.8 microM. Inactivation of the overall reaction of Na+/K(+)-ATPase by Cr(H2O)4AdoPP[CH2]P did not alter the activities of the E2 conformational state such as K(+)-activated p-nitrophenylphosphatase, 86Rb+ occlusion and [3H]ouabain binding by the 'backdoor' phosphorylation. However, [3H]ouabain binding via the forwards reaction from E1ATP in the presence of Na+ + Mg2+ is inhibited. K(+)-activated p-nitrophenylphosphatase activity of the Cr(H2O)4AdoPP[CH2]P-inactivated enzyme decreases when an MgATP analogue, the tetraammine cobalt complex of ATP, Co(NH3)4ATP, is used additionally to inactivate the E2ATP site. The enzyme activity of K(+)-activated phosphatase is also lost if the beta,gamma-bidentate chromium(III) complex of ATP, Cr(H2O)4ATP, which may form a stable E1-chromo-phosphointermediate, is used for the inactivation of Na+/K(+)-ATPase. We conclude that the phenomenon of a blockade of the overall reaction of Na+/K(+)-ATPase by the formation of a stable E1.CrAdoPP[CH2]P complex, leading thereby to a loss of the partial activities of the E1 conformation, but not of the E2 conformation, is consistent with the postulate of an (alpha beta)2 diprotomeric nature of the sodium pump. The observation, moreover, that treatment of the sodium pump with Cr(H2O)4ATP but not with Cr(H2O)4AdoPP[CH2]P leads to an inactivation of K(+)-activated phosphatase seems to indicate that the formation of a E1-phosphointermediate affects the E2ATP site.

Annual review prize lecture. 'All hands to the sodium pump'

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Phosphate binding and ATP-binding sites coexist in Na+/K(+)-transporting ATPase, as demonstrated by the inactivating MgPO4 complex analogue Co(NH3)4PO4

Tetrammine cobalt(III) phosphate [Co(NH3)4PO4] inactivates Na+/K(+)-ATPase in the E2 conformational state, dependent on time and concentration, according to Eqn (1): Co(NH3)4PO4 + E2 Kd in equilibrium E2.Co(NH3)4PO4k2----E'2.Co(NH3)4PO4. The inactivation rate constant k2 for the formation of a stable E'2.Co(NH3)4PO4 at 37 degrees C was 0.057 min-1; the dissociation constant, Kd = 300 microM. The activation energy for the inactivation process was 149 kJ/mol. ATP and the uncleavable adenosine 5'-[beta, gamma-methylene]triphosphate competed with Co(NH3)4PO4 for its binding site with Ks = 0.41 mM and 5 mM, respectively. MgPO4 competed with Co(NH3)4PO4 linearly, with Ks = 50 microM, as did phosphate (Ks = 16 mM) and Mg2+ (Ks = 160 microM). It is concluded that the MgPO4 analogue binds to the MgPO4-binding subsite of the low-affinity ATP-binding site (of the E2 conformation). Also, Na+ (Ks = 860 microM) protected the enzyme against inactivation in a competitive manner. From the intersecting (slope and intercept linear) noncompetitive effect of Na+ against the inactivation by Co(NH3)4PO4, apparent affinities of K+ for the free enzyme of 41 microM, and for the E.Co(NH3)4PO4 complex of 720 microM, were calculated. Binding of Co(NH3)4PO4 to the enzyme inactivated Na+/K(+)-ATPase and K(+)-activated phosphatase, and, moreover, prevented the occlusion of 86Rb+; however, the activity of the Na(+)-ATPase, the phosphorylation capacity of the high-affinity ATP-binding site and the ATP/ADP-exchange reaction remained unchanged. With Co(NH3)432PO4 a binding capacity of 135 pmol unit enzyme was found. Phosphorylation and complete inactivation of the enzyme with Co(NH3)432PO4 or the 32P-labelled tetramminecobalt ATP ([gamma-32P]Co(NH3)4ATP) at the low-affinity ATP-binding site, allowed (independent of the purity of the Na+/K(+)-ATPase preparation) a further incorporation of radioactivity from 32P-labelled tetraaquachromium(III) ATP ([gamma-32P]CrATP) to the high-affinity ATP-binding site with unchanged phosphorylation capacity. However, inactivation and phosphorylation of Na+/K(+)-ATPase by [gamma-32P]CrATP prevented the binding of Co(NH3)4 32PO4 or [gamma-32P]Co(NH3)4ATP to the enzyme. [gamma-32P]CO(NH3)4ATP and Co(NH3)432PO4 are mutually exclusive. The data are consistent with the assumption of a cooperation of catalytic subunits within an (alpha,beta)2-diprotomer, which change their interactions during the Na+/K(+)-pumping process. Our findings seem not to support a symmetrical Repke and Stein model of enzyme action.

Blocking of Na+/K+ transport by the MgPO4 complex analogue Co(NH3)4PO4 leaves the Na+/Na(+)-exchange reaction of the sodium pump unaltered and shifts its high-affinity ATP-binding site to a Na(+)-like form

Inactivation of Na+/K(+)-ATPase activity by the MgPO4 complex analogue Co(NH3)4PO4 leads, in everted red blood cell vesicles, to the parallel inactivation of 22Na+/K+ flux and 86Rb/Rb+ exchange, but leaves the 22Na+/Na(+)-exchange activity and the uncoupled ATP-supported 22Na+ transport unaffected. Furthermore, inactivation of purified Na+/K(+)-ATPase by Co(NH3)4PO4 leads to a parallel decrease of the capacity of the [3H]ouabain receptor site, when binding was studied by the Mg2+/Pi-supported pathway (ouabain-enzyme complex II) but the capacity of the ouabain receptor site was unaltered, when the Na+/Mg2+/ATP-supported pathway (ouabain-enzyme complex I) was used. No change in the dissociation constants of either ouabain receptor complex was observed following inactivation of Na+/K(+)-ATPase. When eosin was used as a marker for the high-affinity ATP-binding site of the E1 conformation, formation of stable E'2.Co(NH3)4PO4 complex led to a shift in the high-affinity ATP-binding site towards the sodium form. This led to an increase in the dissociation constant of the enzyme complex with K+, from 1.4 mM with the unmodified enzyme to 280 mM with the Co(NH3)4PO4-inactivated enzyme. It was concluded, that the effects of Co(NH3)4PO4 on the partial activities of the sodium pump are difficult to reconcile with an alpha, beta-protomeric enzyme working according the Albers-Post scheme. The data are consistent with an alpha 2, beta 2 diprotomeric enzyme of interacting catalytic subunits working with a modified version of the Albers-Post model.

[32P]ATP synthesis in steady state from [32P]Pi and ADP by Na+/K(+)-ATPase from ox brain and pig kidney. Activation by K+

The ouabain-sensitive synthesis of [32P]ATP from [32P]Pi and ADP (vsyn) was measured in parallel with the ouabain-sensitive hydrolysis of [32P]ATP (vhy) at steady state, at varying concentrations of sodium, potassium, magnesium, inorganic phosphate, ADP, ATP and oligomycin, and at varying pH. Na+ was necessary for ATP synthesis, but vsyn was decreased by high sodium concentrations. Oligomycin, depending on the Na+ concentration, either decreased or did not affect vsyn. Potassium, at low concentrations (1-5 mM) increased vsyn at all magnesium and sodium concentrations tested, lower potassium concentrations being needed to activate vsyn at lower sodium concentrations. vsyn was optimal below pH 6.7, decreasing abruptly at higher values of pH. At pH 6.7, vsyn was a hyperbolic function of the concentration of inorganic phosphate. In the presence of potassium, half-maximal rate was obtained at [Pi] congruent to 40 mM, whereas a higher concentration was needed to obtain half-maximal rate in the absence of K+. In contrast, increasing the concentration of ADP caused a nonhyperbolic activation of vsyn, the pattern obtained in the presence of potassium being different from that obtained in its absence. Increasing the ATP concentration above 0.5 mM decreased vsyn. The data are used to elucidate (1) which reaction steps are involved in the ATP-synthesis catalysed by the Na+/K(+)-ATPase at steady state in the absence of ionic gradients and (2) the mechanism by which K+ ions stimulate the reaction.

Shift to the Na+ form of Na+/K+-transporting ATPase due to modification of the low-affinity ATP-binding site by Co(NH3)4ATP

1. Inactivation of purified Na+/K+-transporting ATPase by the MgATP complex analogue Co(NH3)4ATP, which binds to the low-affinity ATP-binding site, results in the concomitant inhibition of the K+-activated p-nitrophenylphosphatase, which is considered to be a partial reaction catalyzed by the enzyme in the E2 conformational state. 2. Complete inactivation of Na+/K+-transporting ATPase by Co(NH3)4ATP does not alter the ADP/ATP exchange reaction which is considered to be part of the catalytic activity in the E1 conformation. 3. The enzyme binds eosin at the high-affinity ATP-binding site as measured by the change in eosin fluorescence. Eosin binding to the Co(NH3)4ATP-inactivated enzyme is, in contrast to the untreated enzyme, not stimulated by Na1. Inactivation by Co(NH3)4ATP increased the half-maximal opposing effect of K+ on eosin binding from 1.1 mM in the control to 43.2 mM in the almost completely inactive enzyme. No eosin fluorescence changes were observed when the Co(NH3)4ATP-inactivated enzyme was treated subsequently with CrATP. This MgATP complex analogue forms a stable complex at the high-affinity ATP-binding site. CrATP thus abolishes eosin binding. 4. It is concluded, that Co(NH3)4ATP interacts with Na+/K+-transporting ATPase in the E2 conformation and arrests it there. This affects eosin binding to the high-affinity ATP-binding site, since the K+ sensitivity is lost. A possible interpretation of these differing effects of Co(NH3)4ATP on partial reactions of Na+/K+-transporting ATPase is that the sodium pump works as an (alpha,beta)2 diprotomer. It is likely that the arrest of one alpha,beta promoter in the E2 conformational state by occupancy of the low-affinity ATP-binding site with Co(NH3)4ATP induces the Na+ form (E1 form) in the corresponding alpha,beta promoter, as is indicated by the unaffected ADP/ATP exchange and the response of the eosin fluorescence on Na+ and K+.

Distinction between the intermediates in Na+-ATPase and Na+,K+-ATPase reactions. II. Exchange and hydrolysis kinetics at micromolar nucleotide concentrations

The ATP hydrolysis rate and the ADP-ATP exchange rate of (Na+ + K+)-ATPase from ox brain were measured at 10 microM Mg2+free and at micromolar concentrations of free ATP and ADP. (1) In the absence of K+, substrate inhibition of the hydrolysis rate was observed. It disappeared at low Na+ and diminished at increasing concentrations of ADP. This was interpreted in terms of free ATP binding to E1P. In support of this interpretation, free ATP was found to competitively inhibit ADP-ATP exchange. (2) In the presence of K+, substrate activation of the hydrolysis rate was observed. Increasing (microM) concentrations of ADP did not give rise to competitive inhibition in contrast to the situation in the absence of K+ (cf. 1, above). This was interpreted to show that at micromolar substrate, some low-affinity, high-turnover Na+ + K+ activity is possible, provided the Mg2+ concentration is low. (3) While small concentrations of K+ increased the hydrolysis rate (cf. 2) they decreased the rate of ADP-ATP exchange. To elucidate this phenomenon, parallel measurements of exchange and hydrolysis rates were performed over a wide range of ATP and ADP concentrations, with and without K+. If, in the presence and absence of K+, ADP (and ATP competing) are binding to the same phosphorylated intermediate for the backward reaction, it places quantitative restrictions on the ratio of rate constants with and without K+. The results did not conform to these restrictions, and the discrepancy is taken as evidence for the necessity for a bicyclic scheme for the action of the (Na+ + K+)-ATPase. (4) An earlier statement concerning the nature of the phosphoenzyme obtained in the presence of Na+ and K+ is amended.

Oligomycin inhibition of Na,K,ATPase. Analysis of half-of-sites moderator interaction with a dimeric enzyme

It is shown that the incomplete, uncompetitive inhibition pattern exhibited by oligomycin toward Na,K,ATPase cannot be explained by a single-cycle enzyme model. In contrast, the experimental data are easily explained in terms of a dimeric enzyme, only one subunit of which can bind oligomycin at a time, and that subunit is then rendered inactive. In a brief analysis of the model thus obtained by way of numerical examples it is shown that it may show activation at small concentrations of moderator, which disappears at higher concentrations, a property observed for the hydrolysis of p-nitro-phenylphosphate, which is also catalyzed by Na,K,ATPase.

(Na+ + K+)-ATPase: on the number of the ATP sites of the functional unit

Questions concerning the number of the ATP sites of the functional unit of (Na+ + K+)-ATPase (i.e., the sodium pump) have been at the center of the controversies on the mechanisms of the catalytic and transport functions of the enzyme. When the available data pertaining to the number of these sites are examined without any assumptions regarding the reaction mechanism, it is evident that although some relevant observations may be explained either by a single site or by multiple ATP sites, the remaining data dictate the existence of multiple sites on the functional unit. Also, while from much of the data it is clear that the multiple sites of the unit enzyme represent the interacting catalytic sites of an oligomer, it is not possible to rule out the existence of a distinct regulatory site for ATP in addition to the interacting catalytic sites. Regardless of the ultimate fate of the regulatory site, any realistic approach to the resolution of the kinetic mechanism of the sodium pump should include the consideration of the established site-site interactions of the oligomer.