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

Disease mutations of human α3 Na+/K+-ATPase define extracellular Na+ binding/occlusion kinetics at ion binding site III

Na⁺/K⁺-ATPase, which creates transmembrane electrochemical gradients by exchanging 3 Na⁺ for 2 K⁺, is central to the pathogenesis of neurological diseases such as alternating hemiplegia of childhood. Although Na⁺/K⁺-ATPase has 3 distinct ion binding sites I-III, the difficulty of distinguishing ion binding events at each site from the others hinders kinetic study of these transitions. Here, we show that binding of Na⁺ at each site in the human α3 Na⁺/K⁺-ATPase can be resolved using extracellular Na⁺-mediated transient currents. When Na⁺/K⁺-ATPase is constrained to bind and release only Na⁺, three kinetic components: fast, medium, and slow, can be isolated, presumably corresponding to the protein dynamics associated with the binding (or release depending on the voltage step direction) and the occlusion (or deocclusion) of each of the 3 Na⁺. Patient-derived mutations of residues which coordinate Na⁺ at site III exclusively impact the slow component, demonstrating that site III is crucial for deocclusion and release of the first Na⁺ into the extracellular milieu. These results advance understanding of Na⁺/K⁺-ATPase mutation pathogenesis and provide a foundation for study of individual ions' binding kinetics.

Kinetic comparisons of heart and kidney Na+,K(+)-ATPases

Most kinetic measurements of the partial reactions of Na(+),K(+)-ATPase have been conducted on enzyme from mammalian kidney. Here we present a kinetic model that is based on the available equilibrium and kinetic parameters of purified kidney enzyme, and allows predictions of its steady-state turnover and pump current in intact cells as a function of ion and ATP concentrations and the membrane voltage. Using this model, we calculated the expected dependence of the pump current on voltage and extracellular Na(+) concentration. The simulations indicate a lower voltage dependence at negative potentials of the kidney enzyme in comparison with heart muscle Na(+),K(+)-ATPase, in agreement with experimental results. The voltage dependence is enhanced at high extracellular Na(+) concentrations. This effect can be explained by a voltage-dependent depopulation of extracellular K(+) ion binding sites on the E2P state and an increase in the proportion of enzyme in the E1P(Na(+))(3) state in the steady state. This causes a decrease in the effective rate constant for occlusion of K(+) by the E2P state and hence a drop in turnover. Around a membrane potential of zero, negligible voltage dependence is observed because the voltage-independent E2(K(+))(2) → E1 + 2K(+) transition is the major rate-determining step.

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.

Identification of potential regulatory sites of the Na+,K+-ATPase by kinetic analysis

Kinetic models are presented that allow the Na(+),K(+)-ATPase steady-state turnover number to be estimated at given intra- and extracellular concentrations of Na(+), K(+), and ATP. Based on experimental transient kinetic data, the models utilize either three or four steps of the Albers-Post scheme, that is, E(2) --> E(1), E(1) --> E(2)P (or E(1) --> E(1)P and E(1)P --> E(2)P), and E(2)P --> E(2), which are the major rate-determining steps of the enzyme cycle. On the time scale of these reactions, the faster binding steps of Na(+), K(+), and ATP to the enzyme are considered to be in equilibrium. Each model was tested by comparing calculations of the steady-state turnover from rate constants and equilibrium constants for the individual partial reactions with published experimental data of the steady-state activity at varying Na(+) and K(+) concentrations. To provide reasonable agreement between the calculations and the experimental data, it was found that Na(+)/K(+) competition for cytoplasmic binding sites was an essential feature required in the model. The activity was also very dependent on the degree of K(+)-induced stimulation of the reverse reaction E(1) --> E(2). Taking into account the physiological substrate concentrations, the models allow the most likely potential sites of short-term Na(+),K(+)-ATPase regulation to be identified. These were found to be (a) the cytoplasmic Na(+) and K(+) binding sites, via changes in Na(+) or K(+) concentration or their dissociation constants, (b) ATP phosphorylation (as a substrate), via a change in its rate constant, and (c) the position of the E(2)<==>E(1) equilibrium.

Mechanism of the rate-determining step of the Na(+),K(+)-ATPase pump cycle

The kinetics of the E(2) --> E(1) conformational change of unphosphorylated Na(+),K(+)-ATPase from rabbit kidney and shark rectal gland were investigated via the stopped-flow technique using the fluorescent label RH421 (pH 7.4, 24 degrees C). The enzyme was pre-equilibrated in a solution containing 25 mM histidine and 0.1 mM EDTA to stabilize initially the E(2) conformation. When rabbit kidney enzyme was mixed with NaCl alone, tris ATP alone or NaCl, and tris ATP simultaneously, a fluorescence decrease was observed. The reciprocal relaxation time, 1/tau, of the fluorescent transient was found to increase with increasing NaCl concentration and reached a saturating value in the presence of 1 mM tris ATP of 54 +/- 3 s(-1) in the case of rabbit kidney enzyme. The experimental behavior could be described by a binding of Na(+) to the enzyme in the E(2) state with a dissociation constant of 31 +/- 7 mM, which induces a subsequent rate-limiting conformational change to the E(1) state. Similar behavior, but with a decreased saturating value of 1/tau, was found when NaCl was replaced by choline chloride. Analogous experiments performed with enzyme from shark rectal gland showed similar effects, but with a significantly lower amplitude of the fluorescence change and a higher saturating value of 1/tau for both the NaCl and choline chloride titrations. The results suggest that Na(+) ions or salt in general play a regulatory role, similar to that of ATP, in enhancing the rate of the rate-limiting E(2) --> E(1) conformational transition by interaction with the E(2) state.

Rate limitation of the Na(+),K(+)-ATPase pump cycle

The kinetics of Na(+)-dependent phosphorylation of the Na(+),K(+)-ATPase by ATP were investigated via the stopped-flow technique using the fluorescent label RH421 (saturating [ATP], [Na(+)], and [Mg(2+)], pH 7.4, and 24 degrees C). The well-established effect of buffer composition on the E(2)-E(1) equilibrium was used as a tool to investigate the effect of the initial enzyme conformation on the rate of phosphorylation of the enzyme. Preincubation of pig kidney enzyme in 25 mM histidine and 0.1 mM EDTA solution (conditions favoring E(2)) yielded a 1/tau value of 59 s(-1). Addition of MgCl(2) (5 mM), NaCl (2 mM), or ATP (2 mM) to the preincubation solution resulted in increases in 1/tau to values of 129, 167, and 143 s(-1), respectively. The increases can be attributed to a shift in the enzyme conformational equilibrium before phosphorylation from the E(2) state to an E(1) or E(1)-like state. The results thus demonstrate conclusively that the E(2) --> E(1) transition does in fact limit the rate of subsequent reactions of the pump cycle. Based on the experimental results, the rate constant of the E(2) --> E(1) transition under physiological conditions could be estimated to be approximately 65 s(-1) for pig kidney enzyme and 90 s(-1) for enzyme from rabbit kidney. Taking into account the rates of other partial reactions, computer simulations show these values to be consistent with the turnover number of the enzyme cycle (approximately 48 s(-1) and approximately 43 s(-1) for pig and rabbit, respectively) calculated from steady-state measurements. For enzyme of the alpha(1) isoform the E(2) --> E(1) conformational change is thus shown to be the major rate-determining step of the entire enzyme cycle.

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.