Magnetic resonance and kinetic studies of the mechanism of membrane-bound sodium and potassium ion- activated adenosine triphosphatase

Structure and function of the P-type ATPases

The P-type ATPases are integral membrane proteins that generate essential transmembrane ion gradients in virtually all living cells. The structures of two of these have recently been elucidated at a resolution of 8 A. When considered together with the large body of biochemical information that has accrued for these transporters and for enzymes in general, this new structural information is providing tantalizing insights regarding the molecular mechanism of active ion transport catalyzed by these proteins.

A proposal for the Mg2+ binding site of P-type ion motive ATPases and the mechanism of phosphoryl group transfer

Mutations of D586 in the DPPR sequence of sodium pump decrease the enzyme's affinity for inorganic phosphate [Farley R. A., Heart, E., Kabalin, M., Putnam, D., Wang, K., Kasho, V. N., and Faller, L. D. (1997) Biochemistry 36, 941-951]. Therefore, it was proposed that D586 coordinates the Mg2+ required for catalytic activity. This hypothesis is tested (1) by determining the substrate for catalysis of 18O exchange between inorganic phosphate and water and (2) by comparing conserved amino acid sequences in P-type pumps with the primary structures of enzymes of known tertiary structure that catalyze phosphoryl group transfer. From the isotope exchange data, it is concluded that the Mg2+-dependent and Na+- and K+-stimulated ATPase binds Mg2+ before inorganic phosphate. Sequence homology is demonstrated between the conserved DPPR and MV(I,L)TGD sequences of P-type pumps and two conserved adenylate kinase sequences that coordinate Mg2+ and/or bind nucleotide in the crystal structure of the yeast enzyme. A model for the Mg2+ site of P-type pumps and the mechanism of phosphoryl group transfer is proposed and tested by demonstrating that the conserved sequences are also structurally homologous.

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.

Nuclear Overhauser effect studies of the conformation of Co(NH3)4ATP bound to kidney Na,K-ATPase

Transferred nuclear Overhauser effect measurements (in the two-dimensional mode) have been used to determine the three-dimensional conformation of an ATP analogue, Co(NH3)4ATP, at the active site of sheep kidney Na,K-ATPase. Previous studies have shown that Co(NH3)4ATP is a competitive inhibitor with respect to MnATP for the Na,K-ATPase [Klevickis, C., & Grisham, C.M. (1982) Biochemistry 21, 6979. Gantzer, M.L., et al. (1982) Biochemistry 21, 4083]. Nine unique proton-proton distances on ATPase-bound Co(NH3)4ATP were determined from the initial build-up rates of the cross-peaks of the 2D-TRNOE data sets. These distances, taken together with previous 31P and 1H relaxation measurements with paramagnetic probes, are consistent with a single nucleotide conformation at the active site. The bound Co(NH3)4ATP) adopts an anti conformation, with a glycosidic torsion angle of 35 degrees, and the conformation of the ribose ring is slightly N-type (C2'-exo, C3'-endo). The delta and gamma torsional angles in this conformation are 100 degrees and 178 degrees, respectively. The nucleotide adopts a bent configuration, in which the triphosphate chain lies nearly parallel to the adenine moiety. Mn2+ bound to a single, high-affinity site on the ATPase lies above and in the plane of the adenine ring. The distances from enzyme-bound Mn2+ to N6 and N7 are too large for first coordination sphere complexes, but are appropriate for second-sphere complexes involving, for example, intervening hydrogen-bonded water molecules. The NMR data also indicate that the structure of the bound ATP analogue is independent of the conformational state of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

1H nuclear magnetic resonance studies of the conformation of an ATP analogue at the active site of Na,K-ATPase from kidney medulla

1H nuclear magnetic relaxation measurements have been used to determine the three-dimensional conformation of an ATP analogue, Co(NH3)4ATP, at the active site of sheep kidney Na,K-ATPase. Previous studies have shown that Co(NH3)4ATP is a competitive inhibitor with respect to MnATP for the Na,K-ATPase [Klevickis, C., & Grisham, C. M. (1982) Biochemistry 21, 6979; Gantzer, M. L., Klevickis, C., & Grisham, C. M. (1982) Biochemistry 21, 4083] and that Mn2+ bound to a single, high-affinity site on the ATPase can be an effective paramagnetic probe for nuclear relaxation studies of the Na,K-ATPase [O'Connor, S. E., & Grisham, C. M. (1979) Biochemistry 18, 2315]. From the paramagnetic effect of Mn2+ bound to the ATPase on the longitudinal relaxation rates of the protons of Co(NH3)4ATP at the substrate site (at 300 and 361 MHz), Mn-H distances to seven protons on the bound nucleotide were determined. Taken together with previous 31P nuclear relaxation data, these measurements are consistent with a single nucleotide conformation at the active site. The nucleotide adopts a bent configuration, in which the triphosphate chain lies nearly parallel to the adenine moiety. The glycosidic torsion angle is 35 degrees, and the conformation of the ribose ring is slightly N-type (C2'-exo, C3'-endo). The delta and gamma torsional angles in this conformation are 100 degrees and 178 degrees, respectively. The bound Mn2+ lies above and in the plane of the adenine ring. The distances from Mn2+ to N6 and N7 are too large for first coordination sphere complexes but are appropriate for second-sphere complexes involving, for example, intervening hydrogen-bonded water molecules.(ABSTRACT TRUNCATED AT 250 WORDS)

Model of anion and monovalent cation transport as neutral ion pairs through lipophilic water channels of the Na,K ATPase complex

A model of anion and monovalent cation transport through a lipophilic water channel of the Na,K ATPase complex is presented. Literature data for the Na,K ATPase cation binding sites are combined with data for the anion binding sites of Band 3 to obtain adjacent cation and anion combining sites at the inner and outer channel mouths. Cations and anions form neutral ion pairs or undissociated acids at these sites and then partition much more favorably into lipophilic channel water, passing through the channel in diffusive fashion. Cation movements in an "uphill" direction occur without an enzyme translocating moiety and its specific energetic requirement. The pertinent factors are the exclusion of unpaired cations by the tight channel and the site selectivity or pickup ratios for Na/K at each side which dominate over bulk and transmembrane concentration ratios. ATP hydrolysis provides phosphate for ion pairing.

A simple method for the purification of rat brain Na+,K+-adenosine triphosphatase (ATPase)

Several methods of purification of Na+,K+-adenosine triphosphatase (ATPase) have been previously described for a wide variety of tissues. In general, highest activity preparations have necessitated large amounts of tissue and many purification steps. This article describes a technique that allows partial purification of Na+,K+-ATPase from as few as 15 rat brains and should be of interest to investigators of the pharmacology of this particular enzyme system. In this modified version of the Jorgensen procedure (Biochim Biophys Acta 356:36--52, 1974) we purified the Na+,K+-ATPase from 15--90 rat brains, and obtained enzyme preparations with a mean specific activity of 552 +/- 37.6 mumol Pi/mg of protein/hr (95.5% ouabain sensitive). This "purified" enzyme had an activity ratio (Mg2+ + Na+ + K+)/(Mg2+ + Na+) of 47.4 +/- 12.3 SEM, compared to 3.29 +/- 0.17 SEM for the untreated microsomes. Ouabain inhibited the "purified" enzyme with an I50 of 6 X 10(-9) M. Ouabain binding (644 pmol/mg of protein) yielded a turnover number of 13,700 min-1. Sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis of the enzyme revealed predominantly the alpha and beta subunits with some minor contaminant bands. Previous methods of purification of rat brain Na+,K+-ATPase have employed sodium deoxycholate and high concentrations of NaI; the reported specific activity obtained was generally 150--350 mumol Pi/mg of protein/hr. We have employed higher SDS concentrations than in Jorgensen's technique for rabbit kidney but the procedure is simpler because sucrose gradients are not used. Final wash steps also include 10--20% glycerol in the media. These modifications have yielded Na+,K+-ATPase of significantly higher specific activity than previously reported for rat brain.

Characterization of ATP binding sites of sheep kidney medulla (Na+ + K+)--ATPase using CrATP

The nucleotide substrate sites of sheep kidney medulla (NA+ + K+)-ATPase are characterized using CrATP, a paramagnetic, substitution-inert substrate analogue probe. The paramagnetic effect of CrATP on 1/T1 of water protons of water protons is enhanced upon complexation with the enzyme. Titrations of the enzyme with CrATP in the presence of Na+ and K+ yielded characteristic enhancements for the binary enzyme-CrATP and ternary enzyme-Mg2+-CrATP complexes of 3.3 and 3.6 and dissociation constants for CrATP of 5 and 12 microM, respectively. Substitution of Li+ for K+ in these titrations did not substantially alter the titration behavior. From the frequency dependence of 1/T1, the correlation time, tau c, for the dipolar water proton-CrATP interaction is 2.7 x 10(-10) sec, indicating that tau c is dominated by tau s, the electron spin relaxation time of Cr3+. The paramagnetic effect of enzyme-bound Mn2+ on 1/T1 of water protons decreases upon the addition of CrATP. Titration of the binary enzyme-Mn2+ complex with CrATP decreases the characteristic enhancement due to Mn2+ from 6.6-8.0 to 1.5. The failure to observe free Mn2+ epr signals in solutions of the ATPase, Mn2+, and CrATP demonstrate that this decrease in epsilon Mn is due to cross-relaxation between Mn2+ and Cr3+ bound simultaneously to the enzyme, and not to displacement of Mn2+ from the enzyme by CrATP. The relaxation rate, 1/T1, of 7Li+ is increased upon addition of CrATP to solutions of the ATPase, indicating that the sites for Li+ and CrATP are close on the enzyme. A Cr3+-Li+ distance of 4.8 +/- 0.5 angstrom is calculated from that data.

Paramagnetic probes in NMR and EPR studies of membrane enzymes

The applications of paramagnetic probes to problems of structure and mechanism are discussed from the point of view of the membrane enzymologist. Problems unique to membrane systems are discussed, and a variety of nuclear and paramagnetic probes are evaluated. Three membrane ATPase (kidney (Na+ + K+)-ATPase, Ca2+-ATPase from sarcoplasmic reticulum and Mg2+-ATPase from kidney) are used to describe the types of experiments which can be done, the information which can be obtained and the limitations involved. Nuclear relaxation studies employing 1H, 7Li+, 31P and 205Tl+ nuclei are described. The advantages and disadvantages of Mn2+, Gd3+ and Cr3+ as paramagnetic probes are discussed in terms of the three ATPases. The theory and interpretation of Mn2+ and Gd3+ EPR spectra are evaluated in studies with the (Na+ + K+)-ATPase and Ca2+-ATPase, respectively.