Modification of ligand binding to the Na+/K+-activated ATPase

The importance of polarity in the evolution of the K+ binding site of pyruvate kinase

In a previous phylogenetic study of the family of pyruvate kinase, we found one cluster with Glu117 and another with Lys117. Those sequences with Glu117 have Thr113 and are K+-dependent, whereas those with Lys117 have Leu113 and are K+-independent. The carbonyl oxygen of Thr113 is one of the residues that coordinate K+ in the active site. Even though the side chain of Thr113 does not participate in binding K+, the strict co-evolution between position 117 and 113 suggests that T113 may be the result of the evolutionary pressure to maintain the selectivity of pyruvate kinase activity for K+. Thus, we explored if the replacement of Thr113 by Leu alters the characteristics of the K+ binding site. We found that the polarity of the residue 113 is central in the partition of K+ into its site and that the substitution of Thr for Leu changes the ion selectivity for the monovalent cation with minor changes in the binding of the substrates. Therefore, Thr113 is instrumental in the selectivity of pyruvate kinase for K+.

Divalent cation interactions with Na,K-ATPase cytoplasmic cation sites: implications for the para-nitrophenyl phosphatase reaction mechanism

The interactions of divalent cations with the adenosine triphosphatase (ATPase) and para-nitrophenyl phosphatase (pNPPase) activity of the purified dog kidney Na pump and the fluorescence of fluorescein isothiocyanate (FITC)-labeled pump were determined. Sr(2+) and Ba(2+) did not compete with K(+) for ATPase (an extracellular K(+) effect). Sr(2+) and Ba(2+) did compete with Na(+) for ATPase (an intracellular Na(+) effect) and with K(+) for pNPPase (an intracellular K(+) effect). These results suggest that Ba(2+) or Sr(2+) can bind to the intracellular transport site, yet neither Ba(2+) nor Sr(2+) was able to activate pNPPase activity; we confirmed that Ca(2+) and Mn(2+) did activate. As another measure of cation binding, we observed that Ca(2+) and Mn(2+), but not Ba(2+), decreased the fluorescence of the FITC-labeled pump; we confirmed that K(+) substantially decreased the fluorescence. Interestingly, Ba(2+) did shift the K(+) dose-response curve. Ethane diamine inhibited Mn(2+) stimulation of pNPPase (as well as K(+) and Mg(2+) stimulation) but did not shift the 50% inhibitory concentration (IC(50)) for the Mn(2+)-induced fluorescence change of FITC, though it did shift the IC(50) for the K(+)-induced change. These results suggest that the Mn(2+)-induced fluorescence change is not due to Mn(2+) binding at the transport site. The drawbacks of models in which Mn(2+) stimulates pNPPase by binding solely to the catalytic site vs. those in which Mn(2+) stimulates by binding to both the catalytic and transport sites are presented. Our results provide new insights into the pNPPase kinetic mechanism as well as how divalent cations interact with the Na pump.

Extracellular terbium and divalent cation effects on the red blood cell Na pump and chrysoidine effects on the renal Na pump

We examined the effect of extracellular terbium (Tb(3+)) and divalent metal cations (Ca(2+), Sr(2+), and Ba(2+)) on (86)Rb(+) influx into rabbit and human red blood cells. We found that Tb(3+) at 15 and 25 microM was a non-competitive inhibitor of (86)Rb(+) influx suggesting that Tb(3+) is not binding to the transport site. This result reduces the usefulness of Tb(3+) as a potential probe for the E(out) conformation (the conformation with the transport site facing extracellularly). Ba(2+), Sr(2+) and Ca(2+), at concentrations >50 mM, had minimal effects on Rb(+) influx into red blood cells (1 mM Rb-out). This suggests that the outside transport site is very specific for monovalent cations over divalent cations, in contrast to the inside transport site. We also found that chrysoidine (4-phenylazo-m-phenylenediamine) competes with Na(+) for ATPase activity and K(+) for pNPPase activity suggesting it is binding to the E(in) conformation. Chrysoidine and similar compounds may be useful as optical probes of the E(in) conformation.

Pseudosubstrate hydrolysis by the erythrocyte plasma membrane Ca(2+)-ATPase: kinetic evidence for a modified E1 conformation in dimethylsulfoxide

The purified Ca(2+)-ATPase of pig red cells displays a phosphatase activity towards p-nitrophenylphosphate which is inhibited by Ca2+ in the absence of solvents, and activated by calmodulin. This activity has been attributed to the E2 conformation of the enzyme. Here we show that the pNPPase activity in the absence of Ca2+ is stimulated 10-25-fold by the presence of the organic solvent dimethylsulfoxide (Me2SO). This is an activation that surpasses by severalfold that induced by calmodulin in the absence of the solvent. At 30% Me2SO, activation by calmodulin disappears. In the absence of calmodulin and at pH 7.2, the Ca2+ concentration needed for half-maximal inhibition of the pNPPase activity (K1) increases from 130 microM in the absence of Me2SO to 860 microM at 30% Me2SO. This effect of Me2SO is enhanced at pH 8.0: the K for Ca2+ increases from 2.7 microM in the absence of the solvent to 2.0 mM in its presence. However, the K0.5 for Ca2+ activation of the ATPase activity decreases from 8.3 to 2.6 microM following addition of the same Me2SO concentration. This indicates that, even in the presence of Me2SO, microM Ca2+ concentrations shift the equilibrium towards E1 but the decrease in activity that would be expected if pNPP hydrolysis were catalysed exclusively by the E2 conformation is not observed. The affinity for pNPP as a substrate increases from 2.6 mM in the absence of Me2SO to 1.6 mM in the presence of 20% Me2SO. These results suggest that Me2SO induces multiple effects in the Ca(2+)-ATPase that (i) increase the reactivity of E2 towards substrate: (ii) surpass the activation by calmodulin and, (iii) allow the enzyme to hydrolyze pNPP even when Ca2+ is bound to the high-affinity sites of the enzyme. The change in reactivity is attributed to an increase on substrate catalysis rather than on pNPP binding.

The effect of dimethylsulfoxide on the substrate site of Na+/K(+)-ATPase studied through phosphorylation by inorganic phosphate and ouabain binding

To obtain further information on the role of H2O at the substrate site of Na+/K(+)-ATPase, we have studied the enzymes reaction with P(i) and ouabain in 40% (v/v) Me2SO (dimethylsulfoxide). When the enzyme (E) was incubated with ouabain (O) for 5 min in a 40% (v/v) Me2SO-medium with 5 mM MgCl2 and 0.5 mM KCl (but no phosphate), ouabain was bound (as EO). Subsequent incubation with P(i) showed that E, but not EO, was rapidly phosphorylated (to EP). Long-time phosphorylation revealed that EO is also phosphorylated by P(i) albeit very slowly (t1/2 about 60 min) and that binding of ouabain to EP also is very slow. The EOP complex is stable, i.e., the t1/2 for the loss of P(i) is >> 60 min in contrast to about 1 min in water. These results in 40% Me2SO are distinctly different from what would be obtained in a watery milieu: ouabain would bind slowly and inefficiently in the absence of P(i), and ouabain would catalyse phosphorylation from P(i) rather than retard it. Equilibrium binding of [3H]ouabain to E and EP in water or 40% Me2SO confirmed these observations: Kdiss in water is 11 microM and 12 nM for EO and EOP, respectively, whereas in Me2SO they are 112 nM and 48 nM. It is suggested that the primary effect of the lowered water activity in 40% Me2SO is a rearrangement of the substrate site so that it also in the absence of P(i) attains a transition state configuration corresponding to the phosphorylated conformation. This would be sensed by the ouabain binding site and lead to high affinity ouabain binding in the absence of P(i).

Role of Mg2+ ions in the conformational change reported by fluorescein 5'-isothiocyanate modification of Na+,K(+)-ATPase

The role of Mg2+ in the conformational change reported by fluorescein 5'-isothiocyanate modification of Na,K-ATPase has been studied by stopped-flow fluorometry. K+ causes a fluorescence quench that is reversed by Na+. The principal experimental observations are as follows: (1) Mg2+ decreases the apparent affinity of the enzyme for K+ but does not affect the maximum rate of the K+ quench. (2) The amplitude of the K+ quench depends hyperbolically on the K+ concentration, and the maximum amplitude is unaffected by the Mg2+ concentration. (3) The rate at which Na+ reverses the K+ quench depends inversely on the Mg2+ concentration. (4) The amplitude of the Na+ reversal also decreases with increasing Mg2+ concentration. The data are quantitatively explained by a model that assumes only two enzyme conformations, detectable by their fluorescence emission. Mg2+ increases Kd for K+ from 14 to 223 mM. At 22 degrees C, Kd = 0.16 mM for Mg2+ dissociation from E1, and the heat of Mg2+ binding, delta H degrees, is 11.4 kcal mol-1. Kd is more than an order of magnitude larger for Mg2+ dissociation from E2K. Mg2+ binding does not affect the forward (E1K-->E2K) rate constant (kf), but decreases the reverse rate constant (kr) thus increasing the equilibrium constant for the reaction (Kc = kf/kr) 6-fold. Therefore, Mg2+ is not directly involved in the conformational transition, but the study supports proposals that Mg2+ binding and release may help to regulate the transport cycle by shifting the distribution of enzyme between E1 and E2 conformers.

The role of Mg2+ and K+ in the phosphorylation of Na+,K(+)-ATPase by ATP in the presence of dimethylsulfoxide but in the absence of Na+

We have previously demonstrated that Na+,K(+)-ATPase can be phosphorylated by 100 microM ATP and 5 mM Mg2+ and in the absence of Na+, provided that 40% dimethylsulfoxide (Me2SO) is present. Phosphorylation was stimulated by K+ up to a steady-state level of about 50% of Etot (Barrabin et al. (1990) Biochim. Biophys. Acta 1023, 266-273). Here we describe the time-course of phosphointermediate (EP) formation and of dephosphorylation of EP at concentrations of Mg2+ from 0.1 to 5000 microM and of K+ from 0.01 to 100 mM. The results were simulated by a simplified version of the commonly accepted Albers-Post model, i.e. a 3-step reaction scheme with a phosphorylation, a dephosphorylation and an isomerization/deocclusion step. Furthermore it was necessary to include an a priori, Mg(2+)- and K(+)-independent, equilibration between two enzyme forms, only one of which (constituting 14% of Etot) reacted directly with ATP. The role of Mg(2+) was two-fold: At low Mg2+, phosphorylation was stimulated by Mg2+ due to formation of the substrate MgATP, whereas at higher concentrations it acted as an inhibitor at all three steps. The affinity for the inhibitory Mg(2+)-binding was increased several-fold, relative to that in aqueous media, by dimethylsulfoxide. K+ stimulated dephosphorylation at all Mg(2+)-concentrations, but at high, inhibitory [Mg2+], K+ also stimulated the phosphorylation reaction, increasing both the rate coefficient and the steady-state level of EP. Generally, the presence of Me2SO seems to inhibit the dephosphorylation step, the isomerization/deocclusion step, and to a lesser extent (if at all) the phosphorylation reaction, and we discuss whether this reflects that Me2SO stabilizes occluded conformations of the enzyme even in the absence of monovalent cations. The results confirm and elucidate the stimulating effect of K+ on EP formation from ATP in the absence of Na+, but they leave open the question of the molecular mechanism by which Me2SO, inhibitory Mg2+ and stimulating K+ interact with the Na+,K(+)-ATPase.

Effects of choline on Na(+)- and K(+)-interactions with the Na+/K(+)-ATPase

Choline chloride, 100 mM, stimulates Na+/K(+)-ATPase activity of a purified dog kidney enzyme preparation when Na+ is suboptimal (9 mM Na+ and 10 mM K+) and inhibits when K+ is suboptimal (90 mM Na+ and 1 mM K+), but has a negligible effect at optimal concentrations of both (90 mM Na+ and 10 mM K+). Stimulation occurs at low Na+ to K+ ratios, but not at those same ratios when the actual Na+ concentration is high (90 mM). Stimulation decreases or disappears when incubation pH or temperature is increased or when Li+ is substituted for K+ or Rb+. Choline+ also reduces the Km for MgATP at the low ratio of Na+ to K+ but not at the optimal ratio. In the absence of K+, however, choline+ does not stimulate at low Na+ concentrations: either in the Na(+)-ATPase reaction or in the E1 to E2P conformational transition. Together, these observations indicate that choline+ accelerates the rate-limiting step in the Na+/K(+)-ATPase reaction cycle, K(+)-deocclusion; consequently, optimal Na+ concentrations reflect Na+ accelerating that step also. Thus, the observed K0.5 for Na+ includes high-affinity activation of enzyme phosphorylation and low-affinity acceleration of K(+)-deocclusion. Inhibition of Na+/K(+)-ATPase and K(+)-nitrophenylphosphatase reactions by choline+ increases as the K(+)-concentration is decreased; the competition between choline+ and K+ may represent a similar antagonism between conformations selected by choline+ and by K+.

Na+/K(+)-ATPase: modes of inhibition by Mg2+

Adding 15 mM free Mg2+ decreased Vmax of the Na+/K(+)-ATPase reaction. Mg2+ also decreased the K0.5 for K+ activation, as a mixed inhibitor, but the increased inhibition at higher K+ concentrations diminished as the Na+ concentration was raised. Inhibition was greater with Rb+ but less with Li+ when these cations substituted for K+ at pH 7.5, while at pH 8.5 inhibition was generally less and essentially the same with all three cations: implying an association between inhibition and ion occlusion. On the other hand, Mg2+ increased the K0.5 for Na(+)-activation of the Na+/K(+)-ATPase and Na(+)-ATPase reactions, as a mixed inhibitor. Changing incubation pH or temperature, or adding dimethylsulfoxide affected inhibition by Mg2+ and K0.5 for Na+ diversely. Presteady-state kinetic studies on enzyme phosphorylation, however, showed competition between Mg2+ and Na+. In the K(+)-phosphatase reaction catalyzed by this enzyme Mg2+ was a (near) competitor toward K+. Adding Na+ with K+ inhibited phosphatase activity, but under these conditions 15 mM Mg2+ stimulated rather than inhibited; still higher Mg2+ concentrations then inhibited with K+ plus Na+. Similar stimulation and inhibition occurred when Mn2+ was substituted for Mg2+, although the concentrations required were an order of magnitude less. In all these experiments no ionic substitutions were made to maintain ionic strength, since alternative cations, such as choline, produced various specific effects themselves. Kinetic analyses, in terms of product inhibition by Mg2+, require Mg2+ release at multiple steps. The data are accommodated by a scheme for the Na+/K(+)-ATPase with three alternative points for release: before MgATP binding, before K+ release and before Na+ binding. The latter alternatives necessitate two Mg2+ ions bound simultaneously to the enzyme, presumably to divalent cation-sites associated with the phosphate and the nucleotide domains of the active site.