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

(Na+, K+)- ATPase kinetics in Macrobrachium pantanalense: highlighting intra- and interspecific variation within the Macrobrachium amazonicum complex

The Macrobrachium amazonicum complex is composed of at least the Macrobrachium amazonicum and Macrobrachium pantanalense species, with the latter described from specimens originally identified as part of an endemic M. amazonicum population in the Brazilian Pantanal region. While there may be a reproductive barrier between these two Macrobrachium species, both are phylogenetically close, with small genetic distance. However, there is currently no available biochemical information of Macrobrachium pantanalense (Na+, K+)-ATPase. Here, we report the kinetic characteristics of the gill (Na+, K+)-ATPase in two populations of M. pantanalense from Baiazinha Lagoon (Miranda, MS, Brazil) and Araguari River (Uberlândia, MG, Brazil), and compare them with Macrobrachium amazonicum populations from the Paraná-Paraguay River Basin. (Na+, K+)-ATPase activities were 67.9 ± 3.4 and 93.3 ± 4.1 nmol Pi min-1 mg-1 protein for the Baiazinha Lagoon and Araguari River populations, respectively. Two ATP hydrolyzing sites were observed for the Araguari River population while a single ATP site was observed for the Baiazinha Lagoon shrimps. Compared to the Araguari River population, a 3-fold greater apparent affinity for Mg2+ and Na+ was estimated for the Baiazinha Lagoon population, but no difference in K+ affinity and ouabain inhibition was seen. The kinetic differences observed in the gill (Na+, K+)-ATPase between the two populations of M. pantanalense, compared with those of various M. amazonicum populations, highlight interspecific divergence within the Macrobrachium genus, now examined from a biochemical perspective.

Crystal structures of Na+ ,K+ -ATPase reveal the mechanism that converts the K+ -bound form to Na+ -bound form and opens and closes the cytoplasmic gate

Na+ ,K+ -ATPase (NKA) plays a pivotal role in establishing electrochemical gradients for Na+ and K+ across the cell membrane by alternating between the E1 (showing high affinity for Na+ and low affinity for K+ ) and E2 (low affinity to Na+ and high affinity to K+ ) forms. Presented here are two crystal structures of NKA in E1·Mg2+ and E1·3Na+ states at 2.9 and 2.8 Å resolution, respectively. These two E1 structures fill a gap in our description of the NKA reaction cycle based on the atomic structures. We describe how NKA converts the K+ -bound E2·2K+ form to an E1 (E1·Mg2+ ) form, which allows high-affinity Na+ binding, eventually closing the cytoplasmic gate (in E1 ~ P·ADP·3Na+ ) after binding three Na+ , while keeping the extracellular ion pathway sealed. We now understand previously unknown functional roles for several parts of NKA and that NKA uses even the lipid bilayer for gating the ion pathway.

A Kinetic Characterization of the Gill (Na+, K+)-ATPase from the Semi-terrestrial Mangrove Crab Cardisoma guanhumi Latreille, 1825 (Decapoda, Brachyura)

We provide a kinetic characterization of (Na⁺, K⁺)-ATPase activity in a posterior gill microsomal fraction from the semi-terrestrial mangrove crab Cardisoma guanhumi. Sucrose density gradient centrifugation reveals two distinct membrane fractions showing considerable (Na⁺, K⁺)-ATPase activity, but also containing other microsomal ATPases. The (Na⁺, K⁺)-ATPase, notably immuno-localized to the apical region of the epithelial pillar cells, and throughout the pillar cell bodies, has an M _r of around 110 kDa and hydrolyzes ATP with V _M = 146.8 ± 6.3 nmol Pi min^-1 mg protein^-1 and K _M = 0.05 ± 0.003 mmol L^-1 obeying Michaelis-Menten kinetics. While stimulation by Na⁺ (V _M = 139.4 ± 6.9 nmol Pi min^-1 mg protein^-1, K _M = 4.50 ± 0.22 mmol L^-1) also follows Michaelis-Menten kinetics, modulation of (Na⁺, K⁺)-ATPase activity by MgATP (V _M = 136.8 ± 6.5 nmol Pi min^-1 mg protein^-1, K _0.5 = 0.27 ± 0.04 mmol L^-1), K⁺ (V _M = 140.2 ± 7.0 nmol Pi min^-1 mg protein^-1, K _0.5 = 0.17 ± 0.008 mmol L^-1), and NH₄⁺ (V _M = 149.1 ± 7.4 nmol Pi min^-1 mg protein^-1, K _0.5 = 0.60 ± 0.03 mmol L^-1) shows cooperative kinetics. Ouabain (K _I = 52.0 ± 2.6 µmol L^-1) and orthovanadate (K _I = 1.0 ± 0.05 µmol L^-1) inhibit total ATPase activity by around 75%. At low Mg²⁺ concentrations, ATP is an allosteric modulator of the enzyme. This is the first study to provide a kinetic characterization of the gill (Na⁺, K⁺)-ATPase in C. guanhumi, and will be useful in better comprehending the biochemical underpinnings of osmoregulatory ability in a semi-terrestrial mangrove crab.

Modulation of the Na,K-ATPase by Magnesium Ions

Since the beginning of investigations of the Na,K-ATPase, it has been well-known that Mg²⁺ is an essential cofactor for activation of enzymatic ATP hydrolysis without being transported through the cell membrane. Moreover, experimental evidence has been collected through the years that shows that Mg²⁺ ions have a regulatory effect on ion transport by interacting with the cytoplasmic side of the ion pump. Our experiments allowed us to reveal the underlying mechanism. Mg²⁺ is able to bind to a site outside the membrane domain of the protein's α subunit, close to the entrance of the access channel to the ion-binding sites, thus modifying the local concentration of the ions in the electrolyte, of which Na⁺, K⁺, and H⁺ are of physiological interest. The decrease in the concentration of these cations can be explained by electrostatic interaction and estimated by the Debye-Hückel theory. This effect provokes the observed apparent reduction of the binding affinity of the binding sites of the Na,K-ATPase in the presence of various Mg²⁺ concentrations. The presence of the bound Mg²⁺, however, does not affect the reaction kinetics of the transport function of the ion pump. Therefore, stopped-flow experiments could be performed to gain the first insight into the Na⁺ binding kinetics on the cytoplasmic side by Mg²⁺ concentration jump experiments.

Sulfate acts as phosphate analog on the monomeric catalytic fragment of the CPx-ATPase CopB from Sulfolobus solfataricus

The crystal structure of the catalytic fragment of a Sulfolobus solfataricus P-type ATPase, CopB-B, was determined with a 2.6 A resolution. CopB-B is the major soluble fragment of the archaeal CPx-ATPase CopB and is comprized of a nucleotide and a phosphorylation domain. In the crystalline state two molecules of CopB-B are in close contact to each other, although the presence of dimers in free solution could be ruled out by analytical ultracentrifugation. The overall architecture of CopB-B is similar to that of other P-type ATPases such as Ca-ATPase. Short peptide segments are linking the nucleotide binding to the phosphorylation domain. CopB-B exhibits 33% sequence identity (of 216 aligned residues) with the respective fragment of the Archaeoglobus fulgidus ATPase CopA. The CopB-B nucleotide-binding domain has the most primitive fold yet identified for this enzyme class. It is 24% identical to the nucleotide-binding domain of the disease-related Wilson ATPase ATP7B (80 structurally aligned residues). Structural superposition with Ca-ATPase suggests a putative nucleotide-binding site in CopB-B. The phosphorylation domain of CopB-B is structurally related to the corresponding part of Ca-ATPase in the anion-bound E2 state. In CopB-B crystals, a bound sulfate anion was identified at the phosphate-binding location. In solution state, the potential binding of CopB-B to phosphate was probed with (32)P(i). Bound phosphate could be readily displaced by orthovanadate at submillimolar concentration as well as by sulfate at millimolar concentration. It is possible therefore to assign the structure of the sulfate-bound phosphorylation domain of CopB-B to a state related to the E2.P(i) intermediate state of the catalytic cycle.

Evidence Calcium Pump Binds Magnesium before Inorganic Phosphate

Calcium pump-catalyzed (18)O exchange between inorganic phosphate and water was studied to test the hypothesis that all P-type pumps bind Mg(2+) before P(i) and validate utilization of the rate equation for ordered binding to interpret differences between site-directed mutants and wild-type enzyme. The results were remarkably similar to those obtained earlier with sodium pump (Kasho, V. N., Stengelin, M., Smirnova, I. N., and Faller, L. D. (1997) Biochemistry 36, 8045- 8052). The equation for ordered binding of Mg(2+) before P(i) fit the data best with only a slight chance (0.6%) of P(i) binding to apoenzyme. Therefore, P(i) is the substrate, and Mg(2+) is an obligatory cofactor. The intrinsic Mg(2+) dissociation constant from metalloenzyme (K(M) = 3.5 +/- 0.3 mm) was experimentally indistinguishable from the sodium pump value. However, the half-maximal concentration for P(i) binding to metalloenzyme ((K(p)(')=6.3+/-0.6 mM)) was significantly higher ( approximately 6-fold), and the probability of calcium pump forming phosphoenzyme from bound P(i) (P(c) = 0.04 +/- 0.03) was significantly lower ( approximately 6-fold) than for the sodium pump. From estimates of the rate constants for phosphorylation and dephosphorylation, the calcium pump appears to catalyze phosphoryl group transfer less efficiently than the sodium pump. Ordered binding of Mg(2+) before P(i) implies that both calcium pump and sodium pump form a ternary enzyme.metal.phosphate complex, consistent with molecular structures of other haloacid dehalogenase superfamily members that were crystallized with Mg(2+) and phosphate, or a phosphate analogue, bound.

The Occlusion of Rb(+) in the Na(+)/K(+)-ATPase. II. The effects of Rb(+), Na(+), Mg2(+), or ATP on the equilibrium between free and occluded Rb(+)

We used the direct route of occlusion to study the equilibrium between free and occluded Rb(+) in the Na(+)/K(+)-ATPase, in media with different concentrations of ATP, Mg(2+), or Na(+). An empirical equation, with the restrictions imposed by the stoichiometry of ligand binding was fitted to the data. This allowed us to identify which states of the enzyme were present in each condition and to work out the schemes and equations that describe the equilibria between the ATPase, Rb(+), and ATP, Mg(2+), or Na(+). These equations were fitted to the corresponding experimental data to find out the values of the equilibrium constants of the reactions connecting the different enzyme states. The three ligands decreased the apparent affinity for Rb(+) occlusion without affecting the occlusion capacity. With [ATP] tending to infinity, enzyme species with one or two occluded Rb(+) seem to be present and full occlusion seems to occur in enzymes saturated with the nucleotide. In contrast, when either [Mg(2+)] or [Na(+)] tended to infinity no occlusion was detectable. Both Mg(2+) and Na(+) are displaced by Rb(+) through a process that seems to need the binding and occlusion of two Rb(+), which suggests that in these conditions Rb(+) occlusion regains the stoichiometry of the physiological operation of the Na(+) pump.

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.

Preparation of Na,K-ATPase specifically modified on the anti-fluorescein antibody-inaccessible site by fluorescein 5'-isothiocyanate

Specific labeling is required for energy transfer measurements and to avoid artifacts in the use of fluorophores as reporter groups. Therefore, a method for specific modification by one of the most popular reagents for P-type ATPases (fluorescein 5'-isothiocyanate) has been developed. Sulfhydryl reagents protected against modification of cysteine residues, and treatment with dithiothreitol eliminated a slow doubling of the fluorescence of conventionally modified Na,K-ATPase upon dilution that is attributed to disappearance of self-energy transfer. Removal of nonspecifically bound fluorescein was also confirmed by titration of the modified Na, K-ATPase with anti-fluorescein antibody and by time resolution of the fluorescence change when the modified enzyme was mixed with Na(+) in a stopped-flow instrument. The only fluorescence change when specifically modified Na,K-ATPase was mixed with Na(+) was the signal from fluorescein at the antibody-inaccessible, substrate-protectable site that reports the conformational change in unphosphorylated enzyme. The magnitude of the fluorescence change reporting the conformational change increased from between 8 and 12% to between 25 and 30% without affecting the kinetic constants estimated from titrations with Na(+) and K(+). The method should be generally applicable to the preparation of specifically labeled P-type pumps for use in kinetic and equilibrium titrations or energy transfer measurements.

Extracellular Mg2+ regulates the intracellular Na+ concentration in rat sublingual acini

The intracellular free Na+ concentration ([Na+]i) increases during muscarinic stimulation in salivary acinar cells. The present study examined in rat sublingual acini the role of extracellular Mg2+ in the regulation of the stimulated [Na+]i increase using the fluorescent sodium indicator benzofuran isophthalate (SBFI). The muscarinic induced rise in [Na+]i was approximately 4-fold greater in the absence of extracellular Mg2+. When Na+ efflux was blocked by the Na+,K+-ATPase inhibitor ouabain, the stimulated [Na+]i increase was comparable to that seen in an Mg2+-free medium. Moreover, ouabain did not add further to the stimulated [Na+]i increase in an Mg2+-free medium suggesting that removal of extracellular Mg2+ may inhibit the Na+ pump. In agreement with this assumption, ouabain-sensitive Na+ efflux and rubidium uptake were reduced by extracellular Mg2+ depletion. Our results suggest that extracellular Mg2+ may regulate [Na+]i in sublingual salivary acinar cells by modulating Na+ pump activity.

Time resolution of fluorescence changes observed in titrations of fluorescein 5'-isothiocyanate-modified Na,K-ATPase with monovalent cations

Equilibrium fluorometric titrations of fluorescein 5'-isothiocyanate-modified Na,K-ATPase with cations have usually been interpreted by assuming that an enhancement reports the conformational change from E2 to E1. We report time resolution of the fluorescence change into three phases when fluorophore-modified enzyme is mixed with the chloride salt of either sodium or choline in a stopped-flow instrument. The first phase is an increase in fluorescence within the dead time of the instrument that is also observed when fluorescein 5'-isothiocyanate (FITC) reacted with lysine is substituted for fluorescein-labeled enzyme. The other two phases occur on millisecond and second time scales. Three phases are also observed when fluorophore-modified enzyme preincubated in KCl is mixed with NaCl, but in this case the slowest phase is absent when choline chloride replaces NaCl. The two faster effects in phases one and two can be eliminated either by controlling the ionic strength or by anti-fluorescein antibody. Labeling the enzyme with fluorescein 5'-isothiocyanate in the presence of its substrate, adenosine 5'-triphosphate, practically eliminates the slowest effect. These results demonstrate that fluorescein reports three events that occur on three different time scales. The fastest phase reports the ionic strength jump of unbound fluorophore. The intermediate phase reports the ionic strength jump of fluorescein at "antibody-accessible" sites [Abbott, A. J., Amler, E., & Ball, W. J., Jr. (1991) Biochemistry 30, 1692-1701]. Only the slowest phase reports the enzyme conformational change implicated in transport.(ABSTRACT TRUNCATED AT 250 WORDS)