Eosin as a probe for conformational transitions and nucleotide binding in Na,K-ATPase

Electrostatic Stabilization Plays a Central Role in Autoinhibitory Regulation of the Na+,K+-ATPase

The Na⁺,K⁺-ATPase is present in the plasma membrane of all animal cells. It plays a crucial role in maintaining the Na⁺ and K⁺ electrochemical potential gradients across the membrane, which are essential in numerous physiological processes, e.g., nerve, muscle, and kidney function. Its cellular activity must, therefore, be under tight metabolic control. Consideration of eosin fluorescence and stopped-flow kinetic data indicates that the enzyme's E2 conformation is stabilized by electrostatic interactions, most likely between the N-terminus of the protein's catalytic α-subunit and the adjacent membrane. The electrostatic interactions can be screened by increasing ionic strength, leading to a more evenly balanced equilibrium between the E1 and E2 conformations. This represents an ideal situation for effective regulation of the Na⁺,K⁺-ATPase's enzymatic activity, because protein modifications, which perturb this equilibrium in either direction, can then easily lead to activation or inhibition. The effect of ionic strength on the E1:E2 distribution and the enzyme's kinetics can be mathematically described by the Gouy-Chapman theory of the electrical double layer. Weakening of the electrostatic interactions and a shift toward E1 causes a significant increase in the rate of phosphorylation of the enzyme by ATP. Electrostatic stabilization of the Na⁺,K⁺-ATPase's E2 conformation, thus, could play an important role in regulating the enzyme's physiological catalytic turnover.

Long-Range Effects of Na(+) Binding in Na,K-ATPase Reported by ATP

This paper addresses the question of long-range interactions between the intramembranous cation binding sites and the cytoplasmic nucleotide binding site of the ubiquitous ion-transporting Na,K-ATPase using (13)C cross-polarization magic-angle spinning (CP-MAS) solid-state nuclear magnetic resonance. High-affinity ATP binding is induced by the presence of Na(+) as well as of Na-like substances such as Tris(+), and these ions are equally efficient promoters of nucleotide binding. CP-MAS analysis of bound ATP with Na,K-ATPase purified from pig kidney membranes reveals subtle differences in the nucleotide interactions within the nucleotide site depending on whether Na(+) or Tris(+) is used to induce binding. Differences in chemical shifts for ATP atoms C1' and C5' observed in the presence of Na(+) or Tris(+) suggest alterations in the residues surrounding the bound nucleotide, hydrogen bonding, and/or conformation of the ribose ring. This is taken as evidence of a long-distance communication between the Na(+)-filled ion sites in the membrane interior and the nucleotide binding site in the cytoplasmic domain and reflects the first conformational change ultimately leading to phosphorylation of the enzyme. Stopped-flow fluorescence measurements with the nucleotide analogue eosin show that the dissociation rate constant for eosin is larger in Tris(+) than in Na(+), giving kinetic evidence of the difference in structural effects of Na(+) and Tris(+). According to the recent crystal structure of the E1·AlF4(-)·ADP·3Na(+) form, the coupling between the ion binding sites and the nucleotide side is mediated by, among others, the M5 helix.

Na,K-ATPase structure/function relationships probed by the denaturant urea

Urea interacts with the Na,K-ATPase, leading to reversible as well as irreversible inhibition of the hydrolytic activity. The enzyme purified from shark rectal glands is more sensitive to urea than Na,K-ATPase purified from pig kidney. An immediate and reversible inhibition under steady-state conditions of hydrolytic activity at 37°C is demonstrated for the three reactions studied: the overall Na,K-ATPase activity, the Na-ATPase activity observed in the absence of K+ as well as the K+-dependent phosphatase reaction (K-pNPPase) seen in the absence of Na+. Half-maximal inhibition is seen with about 1M urea for shark enzyme and about 2M urea for pig enzyme. In the presence of substrates there is also an irreversible inhibition in addition to the reversible process, and we show that ATP protects against the irreversible inhibition for both the Na,K-ATPase and Na-ATPase reaction, whereas the substrate paranitrophenylphosphate leads to a slight increase in the rate of irreversible inhibition of the K-pNPPase. The rate of the irreversible inactivation in the absence of substrates is much more rapid for shark enzyme than for pig enzyme. The larger number of potentially urea-sensitive hydrogen bonds in shark enzyme compared to pig enzyme suggests that interference with the extensive hydrogen bonding network might account for the higher urea sensitivity of shark enzyme. The reversible inactivation is interpreted in terms of domain interactions and domain accessibilities using as templates the available crystal structures of Na,K-ATPase. It is suggested that a few interdomain hydrogen bonds are those mainly affected by urea during reversible inactivation.

Anion interactions with Na,K-ATPase: simultaneous binding of nitrate and eosin

Nucleotide binding affinity to Na,K-ATPase is reduced by a number of anions such as nitrate and perchlorate in comparison with affinity in the presence of chloride (all with sodium as the cation). The reduction correlates with the position of these anions in the Hofmeister series. Transient kinetic experiments using the fluorescent dye eosin-which binds to the nucleotide site of the Na,K-ATPase-show that simultaneous anion binding, exemplified with nitrate, and eosin binding is possible. The effect of nitrate on eosin binding is reflected in a decreased binding-rate constant and an increased dissociation rate constant, leading to a decreased equilibrium binding constant for eosin. Since eosin binding is analogous with nucleotide binding to Na,K-ATPase, the results suggest the simultaneous presence of nucleotide and anion binding sites.

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.

Protonation-dependent inactivation of Na,K-ATPase by hydrostatic pressure developed at high-speed centrifugation

Irreversible inactivation of membranous Na,K-ATPase by high-speed centrifugation in dilute aqueous solutions depends markedly on the protonation state of the protein. Pig kidney Na,K-ATPase is irreversibly inactivated at pH 5 but is fully protected at pH 7 and above. Shark rectal gland Na,K-ATPase is irreversibly inactivated at neutral or acidic pH and partially protected at an alkaline pH. The overall Na,K-ATPase activity and the K-dependent pNPPase activity were denatured in parallel. Cryoprotectants such as glycerol or sucrose at concentrations of 25-30% fully protect both enzymes against inactivation. The specific ligands NaCl and KCl protect the Na,K-ATPase activity partially and the pNPPase activity fully at concentrations of 0.2-0.3 M. Electron microscope analysis of the centrifuged Na,K-ATPase membranes revealed that the ultrastructure of the native membranes is preserved upon inactivation. It was also observed that the sarcoplasmic reticulum Ca-ATPase and hog gastric H, K-ATPase are susceptible to inactivation by high-speed centrifugation in a pH-dependent fashion. H,K-ATPase is protected at alkaline pH, whereas Ca-ATPase is protected only in the neutral pH range.

Conformational changes of transcobalamin induced by aquocobalamin binding. Mechanism of substitution of the cobalt-coordinated group in the bound ligand

Binding of aquo-, cyano-, or azidocobalamin (Cbl.OH(2), Cbl.CN, and Cbl.N(3), respectively) to the recombinant human transcobalamin (TC) and haptocorrin from human plasma was investigated via stopped-flow spectroscopy. Association of cobalamins with haptocorrin always proceeded in one step. TC, however, displayed a certain selectivity for the ligands: Cbl.CN or Cbl.N(3) bound in one step with k(+1) = 1 x 10(8) M(-1) s(-1) (20 degrees C), whereas binding of Cbl.OH(2) under the same conditions occurred in two steps with k(+1) = 3 x 10( 7) M(-1) s(-1) (E(a) = 30 kJ/mol) and k(+2) = 0.02 s(-1) (E(a) = 120 kJ/mol). The second step of Cbl.OH(2) binding was interpreted as a transformation of the initial "open" intermediate TC.Cbl.OH(2) to the "closed" conformation TC(Cbl) with displaced water. The backward transition from the closed to the open conformation was the reason for the identical rate-limiting steps during substitution of H(2)O in TC.Cbl.OH(2) for cyanide or azide according to the reaction TC(Cbl) --> TC.Cbl.OH(2) + CN(-)/N(3)(-). The cyano and azido forms of holo-TC which were produced behaved as the open proteins. Different conformations of holo-TC, determined by the nature of the active group in the bound Cbl, may direct transportation of cobalamins in the organism.

The effect of ionic strength and specific anions on substrate binding and hydrolytic activities of Na,K-ATPase

The physiological ligands for Na,K-ATPase (the Na,K-pump) are ions, and electrostatic forces, that could be revealed by their ionic strength dependence, are therefore expected to be important for their reaction with the enzyme. We found that the affinities for ADP3-, eosine2-, p-nitrophenylphosphate, and V(max) for Na,K-ATPase and K+-activated p-nitrophenylphosphatase activity, were all decreased by increasing salt concentration and by specific anions. Equilibrium binding of ADP was measured at 0-0.5 M of NaCl, Na2SO4, and NaNO3 and in 0.1 M Na-acetate, NaSCN, and NaClO4. The apparent affinity for ADP decreased up to 30 times. At equal ionic strength, I, the ranking of the salt effect was NaCl approximately Na2SO4 approximately Na-acetate < NaNO3 < NaSCN < NaCl04. We treated the influence of NaCl and Na2SO4 on K(diss) for E x ADP as a "pure" ionic strength effect. It is quantitatively simulated by a model where the binding site and ADP are point charges, and where their activity coefficients are related to I by the limiting law of Debye and Hückel. The estimated net charge at the binding site of the enzyme was about +1. Eosin binding followed the same model. The NO3- effect was compatible with competitive binding of NO3- and ADP in addition to the general I-effect. K(diss) for E x NO3 was approximately 32 mM. Analysis of V(max)/K(m) for Na,K-ATPase and K+-p-nitrophenylphosphatase activity shows that electrostatic forces are important for the binding of p-nitrophenylphosphate but not for the catalytic effect of ATP on the low affinity site. The net charge at the p-nitrophenylphosphate-binding site was also about +1. The results reported here indicate that the reversible interactions between ions and Na,K-ATPase can be grouped according to either simple Debye-Hückel behavior or to specific anion or cation interactions with the enzyme.