Lack of gross protein structure changes in the working cycle of (Na+, K+)-dependent adenosinetriphosphatase. Evidence from infrared and intrinsic fluorescence spectroscopy data

Alexander Spirin on Molecular Machines and Origin of Life

Once it was believed that ribosomal RNA encodes proteins, and GTP hydrolysis supplies the energy for protein synthesis. Everything has changed, when Alexander Spirin joined the science. It turned out that proteins are encoded by a completely different RNA, and GTP hydrolysis only accelerates the process already provided with energy. It was Spirin who first put forward the idea of a Brownian ratchet and explained how and why molecular machines could arise in the RNA world.

Structural changes in the catalytic cycle of the Na+,K+-ATPase studied by infrared spectroscopy

Pig kidney Na(+),K(+)-ATPase was studied by means of reaction-induced infrared difference spectroscopy. The reaction from E1Na(3)(+) to an E2P state was initiated by photolysis of P(3)-1-(2-nitrophenyl)ethyl ATP (NPE caged ATP) in samples that contained 3 mM free Mg(2+) and 130 mM NaCl at pH 7.5. Release of ATP from caged ATP produced highly detailed infrared difference spectra indicating structural changes of the Na(+),K(+)-ATPase. The observed transient state of the enzyme accumulated within seconds after ATP release and decayed on a timescale of minutes at 15 degrees C. Several controls ensured that the observed difference signals were due to structural changes of the Na(+),K(+)-ATPase. Samples that additionally contained 20 mM KCl showed similar spectra but less intense difference bands. The absorbance changes observed in the amide I region, reflecting conformational changes of the protein backbone, corresponded to only 0.3% of the maximum absorbance. Thus the net change of secondary structure was concluded to be very small, which is in line with movement of rigid protein segments during the catalytic cycle. Despite their small amplitude, the amide I signals unambiguously reveal the involvement of several secondary structure elements in the conformational change. Similarities and dissimilarities to corresponding spectra of the Ca(2+)-ATPase and H(+),K(+)-ATPase are discussed, and suggest characteristic bands for the E1 and E2 conformations at 1641 and 1661 cm(-1), respectively, for alphabeta heterodimeric ATPases. The spectra further indicate the participation of protonated carboxyl groups or lipid carbonyl groups in the reaction from E1Na(3)(+) to an E2P state. A negative band at 1730 cm(-1) is in line with the presence of a protonated Asp or Glu residue that coordinates Na(+) in E1Na(3)(+). Infrared signals were also detected in the absorption regions of ionized carboxyl groups.

FTIR study of ATP-induced changes in Na+/K+-ATPase from duck supraorbital glands

The Na+/K+-ATPase uses energy from the hydrolysis of ATP to pump Na+ ions out of and K+ ions into the cell. ATP-induced conformational changes in the protein have been examined in the Na+/K+-ATPase isolated from duck supraorbital salt glands using Fourier transform infrared spectroscopy. Both standard transmission and attenuated total internal reflection sample geometries have been employed. Under transmission conditions, enzyme at 75 mg/ml was incubated with dimethoxybenzoin-caged ATP. ATP was released by flashing with a UV laser pulse at 355 nm, which resulted in a large change in the amide I band. The absorbance at 1659 cm(-1) decreased with a concomitant increase in the absorbance at 1620 cm(-1). These changes are consistent with a partial conversion of protein secondary structure from alpha-helix to beta-sheet. The changes were approximately 8% of the total absorbance, much larger than those seen with other P-type ATPases. Using attenuated total internal reflection Fourier transform infrared spectroscopy, the decrease in absorbance at approximately 1650 cm(-1) was titrated with ATP, and the titration midpoint K0.5 was determined under different ionic conditions. In the presence of metal ions (Na+, Na+ and K+, or Mg2+), K0.5 was on the order of a few microM. In the absence of these ions, K0.5 was an order of magnitude lower (0.1 microM), indicating a higher apparent affinity. This effect suggests that the equilibrium for the ATP-induced conformational changes is dependent on the presence of metal ions.

Secondary structural analysis of the Na(+),K(+)-ATPase and its Na(+) (E(1)) and K(+) (E(2)) complexes by FTIR spectroscopy

The Na(+),K(+)-ATPase is an integral membrane protein which transports sodium and potassium cations against an electrochemical gradient. The transport of Na(+) and K(+) ions is presumably connected to an oscillation of the enzyme between the two conformational states, the E(1) (Na(+)) and the E(2) (K(+)) conformations. The E(1) and E(2) states have different affinities for ligand interaction. However, the determination of the secondary structure of this enzyme in its sodium and potassium forms has been the subject of much controversy. This study was designed to provide a quantitative analysis of the secondary structure of the Na(+),K(+)-ATPase in its sodium (E(1)) and potassium (E(2)) states in both H(2)O and D(2)O solutions at physiological pH, using Fourier transform infrared (FTIR) with its self-deconvolution and second derivative resolution enhancement methods, as well as curve-fitting procedures. Spectroscopic analysis showed that the secondary structure of the sodium salt of the Na(+),K(+)-ATPase in H(2)O solution contains alpha-helix 19.8+/-1%, beta-sheet 25.6+/-1%, turn 9.1+/-1%, and beta-anti 7.5+/-1%, whereas in D(2)O solution, the enzyme shows alpha-helix 16.8+/-1%, beta-sheet 24.5+/-1.5%, turn 10.9+/-1%, beta-anti 9.8+/-1%, and random coil 38.0+/-2%. Similarly, the potassium salt in H(2)O solution contains alpha-helix 16.6+/-1%, beta-sheet 26.4+/-1.5%, turn 8.9+/-1%, and beta-anti 8.1+/-1%, while in D(2)O solution it shows alpha-helix 16.2+/-1%, beta-sheet 24.5+/-1.5%, turn 10.3+/-1%, beta-anti 9.0+/-1%, and random coil 40+/-2%. Thus the main differences for the sodium and potassium forms of the Na(+),K(+)-ATPase are alpha-helix 3.2% in H(2)O and 0.6% in D(2)O, beta-sheet (pleated and anti) 1.5% in H(2)O and random structure 2% (D(2)O), while for other minor components (turn structure), the differences are less than 1%.

The effects of anions on the solution structure of Na,K-ATPase

Anions interact with protein to induce structural changes at ligand binding sites. The effects of anion complexation include structural stabilization and promote cation-protein interaction. This study was designed to examine the interaction of aspirin and ascorbate anions with the Na+, K+-dependent adenosine triphosphatase (Na,K-ATPase) in H2O and D2O solutions at physiological pH, using anion concentrations of 0.1 microM to 1 mM with final protein concentration of 0.5 to 1 mg/ml. Absorption spectra and Fourier transform infrared (FTIR) difference spectroscopy with its self-deconvolution, second derivative resolution enhancement and curve-fitting procedures were applied to characterize the anion binding mode, binding constant, and the protein secondary structure in the anion-ATPase complexes. Spectroscopic evidence showed that the anion interaction is mainly through the polypeptide C=O and C-N groups with minor perturbation of the lipid moiety. Evidence for this came from major spectral changes (intensity variations) of the protein amide I and amide II vibrations at 1651 and 1550 cm(-1). respectively. The anion-ATPase binding constants were K=6.45 x 10(3) M(-1) for aspirin and K=1.04 x 10(4) M(-1) for ascorbate complexes. The anion interaction resulted in major protein secondary structural changes from that of the alpha-helix 19.8%; beta-pleated sheet 25.6%; turn 9.1%; beta-antiparallel 7.5% and random 38% in the free Na,K-ATPase to that of the alpha-helix 24-26%; beta-pleated 17-18%; turn 8%; beta-antiparallel 5-3% and random 45.0% in the anion-ATPase complexes.

Polarized infrared absorption of Na+/K+-ATPase studied by attenuated total reflection spectroscopy

Na+/K+-ATPase can be isolated from the outer medulla of mammalian kidney in the form of flat membrane fragments containing the enzyme in a density of 10(3)-10(4) protein molecules per microm2 (Deguchi et al. (1977) J. Cell. Biol. 75, 619-634). In this paper we show that these membrane fragments can be bound to a germanium plate coated with a phospholipid bilayer. With this system infrared spectroscopic studies of the enzyme have been carried out using the technique of attenuated total reflection (ATR). At a coverage of the lipid surface corresponding to 30-40% of a monolayer of membrane fragments, characteristic infrared bands of the protein such as the amide I and II bands can be resolved. About 24% of the NH-groups of the peptide backbone are found to be resistant to proton/deuterium exchange within a time period of several days. Evidence for orientation of the protein with respect to the supporting lipid layer is obtained from experiments with polarized light, the largest polarization effects being associated with the -COO- band at 1400 cm-1. Experiments with aqueous media of different ionic composition indicate that the average orientation of transition moments changes when K+ in the medium is replaced by Tris+ or Na+.

Fourier transform infrared spectroscopic studies on gastric H+/K+-ATPase

Suspensions of membrane-bound H+/K+-ATPase in both H2O and 2H2O were investigated using Fourier transform infrared (FT-IR) spectroscopy. Second-derivative techniques were used to reveal the overlapping bands in the 1800-1500 cm-1 region. Analysis of the amide I band shows that the protein component contains substantial amounts of both alpha-helical and beta-sheet structures. Addition of 10 mM KCl to a suspension in 2H2O does not significantly affect the amide I band, indicating that the E1-E2 conformational transition of the enzyme, induced by K+, does not involve a gross change in protein secondary structure. Analysis of the amide II band in the spectra of suspensions in 2H2O shows that inhibition of the enzyme with omeprazole increases the rate of 1H-2H exchange, indicating an increase in conformational flexibility. Furthermore, an additional feature at 1628 cm-1 in the spectra of the inhibited samples in 2H2O could either support a conformational change or arise from a vibrational mode of omeprazole in its enzyme-bound form. The frequency of the band due to the symmetric stretching vibrations of the methylene groups of the lipid acyl chains increases steadily with increasing temperature indicating that there is no co-operative melting process in the lipid component of the membrane over the temperature range 9-50 degrees C. For comparison, FT-IR studies on aqueous suspensions of Na+/K+-ATPase were also carried out. These show that the protein components in the Na+/K+- and H+/K+-ATPases have similar secondary structures.

Tryptophan fluorescence of mitochondrial complex III reconstituted in phosphatidylcholine bilayers

The tryptophan intrinsic fluorescence of mitochondrial complex III reconstituted in phosphatidylcholine bilayers was examined at different temperatures. Absorption and emission maxima occur at 277 and 332 nm, irrespective of temperature or lipid:protein ratio even if there are indications (from fluorescence quenching) of protein conformational changes as a function of lipid:protein ratio. Low values of Trp fluorescence quantum yield in complex III (0.008-0.010) are probably due to the neighborhood of the heme groups. The temperature-dependent decrease of fluorescence intensity is nonlinear; the corresponding Arrhenius plots show "breaks" or discontinuities that could be interpreted as thermally dependent changes in protein conformation. However, no temperature-dependent changes in fluorescence quenching have been observed that may be related to protein conformational changes. In addition, Arrhenius plots of the fluorescence intensity of simple molecules, such as Trp or 1-anilino-8-naphthalene sulfonate in the presence of aqueous phospholipid dispersions, also show breaks in the same temperature range. Stern-Volmer plots of acrylamide and iodide quenching were also nonlinear, indicating large differences in quenching constants for the various tryptophanyl residues. The quenching results also suggest that, at high lipid:protein ratios, the microviscosity of the protein matrix is higher than that in lipid-poor systems. Comparison of quenching efficiencies of iodide and acrylamide suggest that no significant fraction of the fluorophores occurs in the neighborhood of charged residues.

Circular dichroism of the two major conformational states of mammalian (Na+ + K+)-ATPase

No alteration in the circular dichroic spectrum of fully active, membrane-bound (Na+ + K+)-ATPase is observed when the protein is cycled between the two major conformational states, E1 and E2. This finding is in agreement with the infrared study by Chetverin and Brazhnikov (J. Biol. Chem. 260 (1985) 7817) and demonstrates that any difference in secondary structure between the two conformers must be less than 2%.

Fluorescence study of brevin, the Mr 92 000 actin-capping and -fragmenting protein isolated from serum. Effect of Ca2+ on protein conformation

The fluorescence characteristics of brevin and the effects of Ca2+ on the protein conformation were fully investigated. Brevin contains 18 tryptophans and 27 tyrosines. Analysis of the fluorescence spectra and the accessibility to quenching molecules indicate that the emitting tryptophans are located in a hydrophobic environment (lambda max = 324 nm) close to the protein surface. In native brevin, tyrosyl residues do not contribute to the fluorescence emission. Partial quenching of these chromophores has to be attributed to tyrosine----tryptophan resonance energy transfer which is highly efficient. The effect of brevin on actin polymerization has been shown to be Ca2+ sensitive [Harris, D. A., & Schwartz, J. H. (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 6798-6802; Thorstensson, R., Utter, G., & Norberg, R. (1982) Eur. J. Biochem. 126, 11-16; Wilkins, J. A., Schwartz, J. H. & Harris, D. A. (1983) Cell Biol. Int. Rep. 7, 1097-1104; Harris, H. E., & Weeds, A. G. (1983) Biochemistry 22, 2728-2741] and brevin binding to hydrophobic matrices to be Ca2+ dependent (Z. Soua, personal communication). Ca2+ binding to brevin decreases the tryptophan fluorescence polarization degree (without affecting the excited-state lifetime), which suggests a higher chromophore mobility. This effect may be partly related to the slight unshielding of the tryptophan residues observed in fluorescence quenching experiments. Moreover, the reactivity of brevin sulfhydryl groups toward 5,5'-dithiobis(2-nitrobenzoic acid) increases in the presence of Ca2+. On the other hand, fluorescence spectra, quantum yields, excited-state lifetimes, and thermostability remain unchanged.(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.

Small differences in tryptophan fluorescence spectra of 'sodium' and 'potassium' forms of (Na+, K+)-dependent adenosinetriphosphatase

A detailed comparative analysis of tryptophan fluorescence spectra of 'sodium' and 'potassium' forms of (Na+, K+)-activated ATPase was carried out. The 'potassium' form spectrum is shifted relative to that of the 'sodium' form by approximately 0.5-1 nm towards shorter wavelengths. The maximal amplitude of the difference spectrum for these forms makes up about 2% of maximal fluorescence intensity of any of the forms. The shape of the difference spectrum does not depend on the solution temperature or ionic strength. The spectral differences between the forms are reversible upon addition of a functionally opposite cation (K+ for 'sodium' form and vice versa) into the medium. The results suggest that if the differences in fluorescence spectra of the 'sodium' and 'potassium' forms of (Na+, K+)-ATPase resulted from the differences in the protein structure, they may be caused by an alteration in local environment of no more than one or two tryptophan residues.