The reaction sequence of the Na+/K(+)-ATPase: rapid kinetic measurements distinguish between alternative schemes

The Kinetic Reaction Mechanism of the Vibrio cholerae Sodium-dependent NADH Dehydrogenase

The sodium-dependent NADH dehydrogenase (Na(+)-NQR) is the main ion transporter in Vibrio cholerae. Its activity is linked to the operation of the respiratory chain and is essential for the development of the pathogenic phenotype. Previous studies have described different aspects of the enzyme, including the electron transfer pathways, sodium pumping structures, cofactor and subunit composition, among others. However, the mechanism of the enzyme remains to be completely elucidated. In this work, we have studied the kinetic mechanism of Na(+)-NQR with the use of steady state kinetics and stopped flow analysis. Na(+)-NQR follows a hexa-uni ping-pong mechanism, in which NADH acts as the first substrate, reacts with the enzyme, and the oxidized NAD leaves the catalytic site. In this conformation, the enzyme is able to capture two sodium ions and transport them to the external side of the membrane. In the last step, ubiquinone is bound and reduced, and ubiquinol is released. Our data also demonstrate that the catalytic cycle involves two redox states, the three- and five-electron reduced forms. A model that gathers all available information is proposed to explain the kinetic mechanism of Na(+)-NQR. This model provides a background to understand the current structural and functional information.

Effect of ADP on Na(+)-Na(+) exchange reaction kinetics of Na,K-ATPase

The whole-cell voltage-clamp technique was used in rat cardiac myocytes to investigate the kinetics of ADP binding to phosphorylated states of Na,K-ATPase and its effects on presteady-state Na(+)-dependent charge movements by this enzyme. Ouabain-sensitive transient currents generated by Na,K-ATPase functioning in electroneutral Na(+)-Na(+) exchange mode were measured at 23 degrees C with pipette ADP concentrations ([ADP]) of up to 4.3 mM and extracellular Na(+) concentrations ([Na](o)) between 36 and 145 mM at membrane potentials (V(M)) from -160 to +80 mV. Analysis of charge-V(M) curves showed that the midpoint potential of charge distribution was shifted toward more positive V(M) both by increasing [ADP] at constant Na(+)(o) and by increasing [Na](o) at constant ADP. The total quantity of mobile charge, on the other hand, was found to be independent of changes in [ADP] or [Na](o). The presence of ADP increased the apparent rate constant for current relaxation at hyperpolarizing V(M) but decreased it at depolarizing V(M) as compared to control (no added ADP), an indication that ADP binding facilitates backward reaction steps during Na(+)-Na(+) exchange while slowing forward reactions. Data analysis using a pseudo three-state model yielded an apparent K(d) of approximately 6 mM for ADP binding to and release from the Na,K-ATPase phosphoenzyme; a value of 130 s(-1) for k(2), a rate constant that groups Na(+) deocclusion/release and the enzyme conformational transition E(1) approximately P --> E(2)-P; a value of 162 s(-1)M(-1) for k(-2), a lumped second-order V(M)-independent rate constant describing the reverse reactions; and a Hill coefficient of approximately 1 for Na(+)(o) binding to E(2)-P. The results are consistent with electroneutral release of ADP before Na(+) is deoccluded and released through an ion well. The same approach can be used to study additional charge-moving reactions and associated electrically silent steps of the Na,K-pump and other transporters.

Charge translocation by the Na+/K+-ATPase investigated on solid supported membranes: rapid solution exchange with a new technique

Adsorption of Na+/K+-ATPase containing membrane fragments from pig kidney to lipid membranes allows the detection of electrogenic events during the Na+/K+-ATPase reaction cycle with high sensitivity and time resolution. High stability preparations can be obtained using solid supported membranes (SSM) as carrier electrodes for the membrane fragments. The SSMs are prepared using an alkanethiol monolayer covalently linked to a gold surface on a glass substrate. The hydrophobic surface is covered with a lipid monolayer (SAM, self-assembled monolayer) to obtain a double layer system having electrical properties similar to those of unsupported bilayer membranes (BLM). As we have previously shown (, Biophys. J. 64:384-391), the Na+/K+-ATPase on a SSM can be activated by photolytic release of ATP from caged ATP. In this publication we show the first results of a new technique which allows rapid solution exchange at the membrane surface making use of the high mechanical stability of SSM preparations. Especially for substrates, which are not available as a caged substance-such as Na+ and K+-this technique is shown to be capable of yielding new results. The Na+/K+-ATPase was activated by rapid concentration jumps of ATP and Na+ (in the presence of ATP). A time resolution of up to 10 ms was obtained in these experiments. The aim of this paper is to present the new technique together with the first results obtained from the investigation of the Na+/K+-ATPase. A comparison with data taken from the literature shows considerable agreement with our experiments.

Kinetics of Na(+)-dependent conformational changes of rabbit kidney Na+,K(+)-ATPase

The kinetics of Na(+)-dependent partial reactions of the Na+,K(+)-ATPase from rabbit kidney were investigated via the stopped-flow technique, using the fluorescent labels N-(4-sulfobutyl)-4-(4-(p-(dipentylamino)phenyl)butadienyl)py ridinium inner salt (RH421) and 5-iodoacetamidofluorescein (5-IAF). When covalently labeled 5-IAF enzyme is mixed with ATP, the two labels give almost identical kinetic responses. Under the chosen experimental conditions two exponential time functions are necessary to fit the data. The dominant fast phase, 1/tau 1 approximately 155 s-1 for 5-IAF-labeled enzyme and 1/tau 1 approximately 200 s-1 for native enzyme (saturating [ATP] and [Na+], pH 7.4 and 24 degrees C), is attributed to phosphorylation of the enzyme and a subsequent conformational change (E1ATP(Na+)3-->E2P(Na+)3 + ADP). The smaller amplitude slow phase, 1/tau 2 = 30-45 s-1, is attributed to the relaxation of the dephosphorylation/rephosphorylation equilibrium in the absence of K+ ions (E2P<==>E2). The Na+ concentration dependence of 1/tau 1 showed half-saturation at a Na+ concentration of 6-8 mM, with positive cooperatively involved in the occupation of the Na+ binding sites. The apparent dissociation constant of the high-affinity ATP-binding site determined from the ATP concentration dependence of 1/tau 1 was 8.0 (+/- 0.7) microM. It was found that P3-1-(2-nitrophenyl)ethyl ATP, tripropylammonium salt (NPE-caged ATP), at concentrations in the hundreds of micromolar range, significantly decreases the value of 1/tau 1, observed. This, as well as the biexponential nature of the kinetic traces, can account for previously reported discrepancies in the rates of the reactions investigated.

Transient kinetics of substrate binding to Na+/K(+)-ATPase measured by fluorescence quenching

This paper examines the transient kinetics of substrate binding to the Na+/K(+)-ATPase labelled with iodoacetamidofluorescein (IAF) using fluorescence quenching by trinitrophenyl-ATP (TNP-ATP). Earlier work (E.H. Hellen, P.R. Pratap, 1996, Fluorescence quenching of IAF-Na+/K(+)-ATPase via energy transfer to TNP-labelled nucleotide, Proceedings of the VIIIth International Conference on the Na+/K(+)-ATPase, in press) has shown that TNP-nucleotide binds to specific sites (from which unlabelled nucleotide can displace it) and nonspecific sites (from which unlabelled nucleotide cannot displace it). Under stopped-flow conditions, quenching of IAF-enzyme fluorescence was well described by a stretched exponential (F(t) = F infinity + delta F exp[-Bt alpha]). Physically, this function may be interpreted in terms of its inverse Laplace transform phi (k), which describes a distribution of rate-constants; alpha reflects the width of this distribution. As TNP-ATP concentration increased, alpha decreased, reflecting TNP-ATP binding to sites with higher energy barriers. alpha decreased by about the same amount with increasing [TNP-ATP] in the presence of saturating ATP, indicating that the distribution of rate-constants is largely associated with the nonspecific binding sites. However, alpha was significantly less than 1 for ATP-induced fluorescence recovery in the presence of TNP-ATP, indicating that rate-constants associated with specific binding site are also distributed. The distribution of rate-constants for binding to the specific site indicates a distribution in the energy of the transition state for substrate binding. These results suggest that the specific binding site (in either the empty or the full state) may exist in a series of conformations separated by small energy barriers. However, the energy barriers for binding associated with these conformations are significantly distributed.

Kinetics of conformational changes associated with potassium binding to and release from Na+/K(+)-ATPase

The Na+/K(+)-ATPase functions in cells to couple energy from the hydrolysis of ATP to the transport Na+ out and K+ in. The fluorescent probe IAF (iodoacetamidofluorescein) covalently binds to this enzyme, reporting conformational changes without inhibiting enzyme activity. This paper describes experiments using dog kidney enzyme labeled with IAF to examine kinetics of conformational changes resulting from added Na+ and K+, measured in terms of steady-state and stopped-flow fluorescence changes. Kinetics of these fluorescence changes were examined as a function of temperature from two initial conditions: (a) enzyme in the high-fluorescence form (E(high)) was rapidly mixed with varying [K+]; and (b) enzyme in the low-fluorescence form (E(low)) was rapidly mixed with varying [ATP]. These experiments showed: (1) The rate constant for the fluorescence change from E(high) to E(low) was much larger than that for the opposite transition, E(low) to E(high); (2) the apparent free energy of activation (Ea(app)) for the two transitions were different (as estimated from Arrhenius plots); (3) under steady-state conditions, IAF fluorescence did not change when ATP was added to E(low)(K+) in the absence of Na+; (4) the apparent free energy of activation was independent of [K+] for the E(high) to E(low) transition (at 16.4 kcal/mol) but increased with [ATP] for the E(low) to E(high) transition; (5) Ea(app) for the E(low) to E(high) transition with 1 mM ATP was approximately the same as that in the absence of ATP (34 kcal/mol). These results can be interpreted as: (i) in the transition from E(low) to E(high), IAF reported a conformational change that occurred after K+ release to the intracellular side and which is involved in Na+ binding; (ii) Ea(app) increased with [ATP], while increasing the entropy of the transition state. Thus, ATP appeared to destabilize the enzyme during the transition from E(low) to E(high).

Investigation of ion binding to the cytoplasmic binding sites of the Na,K-pump

A dual-wavelength fluorimeter was constructed, which used two light emitting diodes (LEDs) to excite the fluorescence dye RH 421 alternately with two different wavelengths. The ratio of the emissions at the two excitation wavelengths provided a drift-insensitive signal, which allowed detection of very small changes of the fluorescence intensity. Those small changes were induced by ion binding and release in conformation E1 of the Na,K-ATPase. Titration experiments were performed to determine equilibrium dissociation constants (+/- standard deviation) for each step in the complete binding and release sequence: 0.12 +/- 0.01 mM (E2(K2)<==>KE1), 0.08 +/- 0.01 mM (KE1<==>E1A), 3.0 +/- 0.2 mM (NaE1<==>E1), 5.2 +/- 0.4 mM (Na2E1<==>NaE1) and 6.5 +/- 0.4 mM (Na3E1<==>Na2E1) at pH 7.2 and T = 16 degrees C. These numbers show that the affinities of the binding sites exposed to the cytoplasm, are higher for K+ than for Na+ ions, similar to what was found on the extracellular side. The physiological requirement for extrusion of Na+ from the cytoplasm, and for import of K+ from the extracellular medium seems to be facilitated not by favorable binding affinities in state E1 but by the two ATP-driven reaction steps of the cycle, E2(K2) + ATP-->K2E1.ATP and Na3E1.ATP<==>(Na3) E1-P, which border the ion exchange reactions at the binding sites in conformation E1.

Rapid kinetic analyses of the Na+/K(+)-ATPase distinguish among different criteria for conformational change

The Na+/K(+)-ATPase couples the hydrolysis of ATP to the transport of Na+ and K+ via a phosphorylated intermediate and conformational changes. In order to identify these conformational changes, we have probed the sequence of steps from EP(3Na+ in) to EP + 3Na+ out with three fluorescent probes (IAF: 5-iodoacetamidofluorescein; BIPM: N-[p-(2-benzimidazolyl)phenyl]maleimide; and RH421) and the sensitivity of their fluorescence change to oligomycin and divalent cations (Ca2+ and Mn2+). The magnitude (% delta F) and rate constant (k(obs)) of ATP-induced fluorescence changes were measured on a fluorescence stopped-flow apparatus, and yielded the following results. (a) With RH421, k(obs) and % delta F varied with [Na+] (maximal k(obs) = 100 s-1, K1/2 = 6 mM; % delta Fmax = 6%, K1/2 = 1 mM); these values are comparable to those previously reported using IAF-labeled enzyme (Pratap, P.R., Robinson, J.D. and Steinberg, M.I. (1991) Biochim. Biophys. Acta 1069, 288-298). (b) With BIPM-labeled enzyme k(obs) did not vary with [Na+] over the range tested, and was twice as high as the maximum k(obs) for RH421. (c) Treatment with oligomycin reduced k(obs) for all three probes to about the same level (approximately 1-2 s-1) while % delta Fmax was largely unaffected. (d) Replacing Mg2+ with Ca2+ had similar effects to treatment with oligomycin. (e) RH421 fluorescence change was completely abolished in the presence of oligomycin and Ca2+. (f) Replacing Mg2+ with Mn2+ decreased IAF fluorescence, i.e., put the enzyme in an E2-like form, reduced k(obs), and rendered oligomycin less effective in reducing k(obs). From these results, we conclude: (a) the release of the second/third Na+ is the rate-limiting step for the conformational change measured by IAF and charge transfer measured with RH421; (b) BIPM indicates an earlier step, either the deocclusion of Na+ and/or the release of the first Na+; (c) oligomycin blocks Na+ deocclusion, and this step is sensitive to the divalent cation used to activate enzyme phosphorylation; and (d) Ca2+ slows the step reported by IAF as well. These experiments indicate that a simple model with two conformations (E1 and E2) is insufficient to explain transient kinetic data.