Pressure effects on sarcoplasmic reticulum: a Fourier transform infrared spectroscopic study

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.

Structure and dynamics of membrane proteins as studied by infrared spectroscopy

Infrared (IR) spectroscopy is a useful technique in the study of protein conformation and dynamics. The possibilities of the technique become apparent specially when applied to large proteins in turbid suspensions, as is often the case with membrane proteins. The present review describes the applications of IR spectroscopy to the study of membrane proteins, with an emphasis on recent work and on spectra recorded in the transmission mode, rather than using reflectance techniques. Data treatment procedures are discussed, including band analysis and difference spectroscopy methods. A technique for the analysis of protein secondary and tertiary structures that combines band analysis by curve-fitting of original spectra with protein thermal denaturation is described in detail. The assignment of IR protein bands in H2O and in D2O, one of the more difficult points in protein IR spectroscopy, is also reviewed, including some cases of unclear assignments such as loops, beta-hairpins, or 3(10)-helices. The review includes monographic studies of some membrane proteins whose structure and function have been analysed in detail by IR spectroscopy. Special emphasis has been made on the role of subunit III in cytochrome c oxidase structure, and the proton pathways across this molecule, on the topology and functional cycle of sarcoplasmic reticulum Ca(2+)-ATPase, and on the role of lipids in determining the structure of the nicotinic acetylcholine receptor. In addition, shorter descriptions of retinal proteins and references to other membrane proteins that have been studied less extensively are also included.

Inhibition of the cardiac sarcoplasmic reticulum Ca2+-ATPase by glucose 6-phosphate is Ca2+ dependent

Defects in the structure or function of the cardiac sarcoplasmic reticulum (CSR) Ca2+-ATPase presumably contribute to the Ca2+ imbalance in the diabetic myocardium. The susceptibility to nonenzymatic protein glycation by glucose metabolites is suggested due to the relatively high percent of target lysines and arginines (approaching 15 mol%) at the ATP binding and phosphorylation domains. Brief incubations (15 min) of CSR microsomes at 24 degrees C in the presence of 5.0 mM glucose 6-phosphate (Glc6P) inhibited Ca2+-dependent ATPase maximal activity relative to controls. Inhibition was only observed when incubations contained 0.1 mM CaCl2 (1.86 micromol ATP hydrolyzed x mg-1 x min-1, +Glc6P versus 2.78, control). Nonconvergent regression lines drawn from maximal velocities as a function of CSR microsome concentration indicate an irreversible mechanism of inhibition which is supported by an observed depletion in CSR amine content (2.98 micromol -NH2 groups/mg microsomal protein, +Glc6P versus 3.34, control). Glucose 6-phosphate (5.0 mM) in Ca2+-free incubations (plus 0.1 mM EGTA) had no affect on either enzyme activity or total amine content. These data suggest that the E1 but not the E2 conformation of the CSR Ca2+-ATPase is susceptible to Glc6P-mediated modification resulting in diminished maximal Ca2+-dependent ATPase activity.

Effect of high hydrostatic pressure on the BK channel in bovine chromaffin cells

The activity of the BK channel of bovine chromaffin cells was studied at high hydrostatic pressure, using inside-out patches in symmetrical KCl solution, Ca2+-free and at V(H) = -60 to -40 mV. Pressure increased the probability of channels being open (900 atm increasing the probability 30-fold), and it increased the minimum number of channels apparent in the patches. The pressure activation of the channel was reversed on decompression. Channel conductance was unaffected. It was shown that pressure did not act by raising the temperature, or by affecting [Ca] or pH, or the order of the membrane bilayer, and it was concluded that pressure most likely acted directly on the channel proteins and/or their modulating reactions.

Structure-function relationships in the Ca(2+)-ATPase of sarcoplasmic reticulum: facts, speculations and questions for the future

Structural data on the Ca(2+)-ATPase of sarcoplasmic reticulum are integrated with kinetic data on Ca2+ transport. The emphasis is upon ATPase-ATPase interactions, the requirement for phospholipids, and the mechanism of Ca2+ translocation. The possible role of cytoplasmic [Ca2+] in the regulation of the synthesis of Ca(2+)-ATPase is discussed.

The structure and interactions of Ca(2+)-ATPase

Electron crystallographic studies on membrane crystals of Ca(2+)-ATPase reveal different patterns of ATPase-ATPase interactions depending on enzyme conformation. Physiologically relevant changes in Ca2+ concentration and membrane potential affect these interactions. Ca2+ induced difference FTIR spectra of Ca(2+)-ATPase triggered by photolysis of caged Ca2+ are consistent with changes in secondary structure and carboxylate groups upon Ca2+ binding; the changes are reversed during ATP hydrolysis suggesting that a phosphorylated enzyme form of low Ca2+ affinity is the dominant intermediate during Ca2+ transport. A two-channel model of Ca2+ translocation is proposed involving the membrane-spanning helices M2-M5 and M4, M5, M6 and M8 respectively, with separate but interacting Ca2+ binding sites.

Structural changes of sarcoplasmic reticulum Ca(2+)-ATPase upon Ca2+ binding studied by simultaneous measurement of infrared absorbance changes and changes of intrinsic protein fluorescence

Ca2+ binding to sarcoplasmic reticulum Ca(2+)-ATPase was investigated by Fourier transform infrared (FTIR) spectroscopy using the photolytic release of Ca2+ from the photolabile Ca2+ chelator 1-(2-nitro-4,5-dimethoxy)-N,N,N',N',- tetrakis[(oxycarbonyl)]methyl-1,2-ethandiamine (DM-nitrophen). IR absorbance changes in 1H2O and 2H2O were detected in the spectral region from 1800 cm-1 to 1200 cm-1, reflecting photolysis of DM-nitrophen and Ca2+ binding to the Ca(2+)-ATPase. As an independent probe for protein conformational changes, intrinsic fluorescence changes upon Ca2+ release were monitored simultaneously to the FTIR measurements. Both the IR absorbance changes and the fluorescence intensity changes correlated well with the Ca2+ binding activity of the ATPase in this specific step. Ca2+ binding caused IR difference bands mainly in the region of amide I absorption of the polypeptide backbone, reflecting conformational changes of the protein. The small amplitude of the signals indicates that only a few residues perform local structural changes such as changes of bond angles or hydrogen bonding. Other absorbance changes appearing above 1700 cm-1 can be assigned to Ca2+ binding to Glu or Asp side chain carboxyl groups and concomitant deprotonation of these residues. This assignment is strengthened by downshifts of these bands by 4 cm-1 to 6 cm-1 upon 1H2O/2H2O exchange. This is in line with results of mutagenesis studies where such residues containing carboxyl groups were associated with the high affinity Ca2+ binding site (Clarke, D.M., Loo, T.W. and MacLennan, D.H. (1990) J. Biol. Chem. 265, 6262-6267).

Changes of protein structure, nucleotide microenvironment, and Ca(2+)-binding states in the catalytic cycle of sarcoplasmic reticulum Ca(2+)-ATPase: investigation of nucleotide binding, phosphorylation and phosphoenzyme conversion by FTIR difference spectroscopy

Changes of infrared absorbance of sarcoplasmic reticulum Ca(2+)-ATPase (EC 3.6.1.38) associated with partial reactions of its catalytic cycle were investigated in the region from 1800 to 950 cm-1 in H2O and 2H2O. Starting from Ca2E1, 3 reaction steps were induced in the infrared cuvette via photolytic release of ATP and ADP: (a) nucleotide binding, (b) formation of the ADP-sensitive phosphoenzyme (Ca2E1P) and (c) formation of the ADP-insensitive phosphoenzyme (E2P). All reaction steps caused distinct changes of the infrared spectrum which were characteristic for each reaction step but comparable for all steps in the number and magnitude of the changes. Most pronounced were absorbance changes in the amide I spectral region sensitive to protein secondary structure. However, they were small--less than 1% of the total protein absorbance--indicating that the reaction steps are associated with small and local conformational changes of the polypeptide backbone instead of a large conformational rearrangement. Especially, there is no outstanding conformational change associated with the phosphoenzyme conversion Ca2E1P-->E2P. ADP-binding induces conformational changes in the ATPase polypeptide backbone with alpha-helical structures and presumably beta-sheet or beta-turn structures involved. Phosphorylation is accompanied by the appearance of a keto group vibration that can tentatively be assigned to the phosphorylated residue Asp351. Phosphoenzyme conversion and Ca(2+)-release produce difference signals which can be explained by the release of Ca2+ from carboxylate groups and a change of hydrogen bonding or protonation state of carboxyl groups.

The effect of high pressure on the conformation, interactions and activity of the Ca(2+)-ATPase of sarcoplasmic reticulum

High pressure (100-150 MPa) increases the intensity and polarization of fluorescence of FITC-labeled Ca(2+)-ATPase in a medium containing 0.1 mM Ca2+, suggesting a reversible pressure-induced transition from the E1 into an E2-like state with dissociation of ATPase oligomers. Under similar conditions but using unlabeled sarcoplasmic reticulum vesicles, high pressure caused the reversible release of Ca2+ from the high-affinity Ca2+ sites of Ca(2+)-ATPase, as indicated by changes in the fluorescence of the Ca2+ indicator, Fluo-3; this was accompanied by reversible inhibition of the Ca(2+)-stimulated ATPase activity measured in a coupled enzyme system of pyruvate kinase and lactate dehydrogenase, and by redistribution of Prodan in the lipid phase of the membrane, as shown by marked changes in its fluorescence emission characteristics. In a Ca(2+)-free medium where the equilibrium favors the E2 conformation of Ca(2+)-ATPase the fluorescence intensity of FITC-ATPase was not affected or only slightly reduced by high pressure. The enhancement of TNP-AMP fluorescence by 100 mM inorganic phosphate in the presence of EGTA and 20% dimethylsulfoxide was essentially unaffected by 150 MPa pressure at pH 6.0 and was only slightly reduced at pH 8.0. As the enhancement of TNP-AMP fluorescence by Pi is associated with the Mg(2+)-dependent phosphorylation of the enzyme and the formation of Mg.E2-P intermediate, it appears that the reactions of Ca(2+)-ATPase associated with the E2 state are relatively insensitive to high pressure. These observations suggest that high pressure stabilizes the enzyme in an E2-like state characterized by low reactivity with ATP and Ca2+ and high reactivity with Pi. The transition from the E1 to the E2-like state involves a decrease in the effective volume of Ca(2+)-ATPase.

Ca2+ release from caged-Ca2+ alters the FTIR spectrum of sarcoplasmic reticulum

Light-induced Ca2+ release from the Ca2+ complex of Nitr-5 altered the FTIR spectra of sarcoplasmic reticulum vesicles and purified Ca(2+)-ATPase preparations. The principal changes seen in difference spectra obtained after and before illumination in the presence of Nitr-5.Ca2+ consisted of an increase in absorbance at 1663 and 1676 cm-1 and a decrease in absorbance at 1653 cm-1. The light-induced changes in FTIR spectra were prevented by vanadate or EGTA, indicating that they were associated with the formation of Ca2E1 enzyme intermediate. Other light-induced changes in the FTIR spectra at 1600-1250 cm-1 were not clearly related to the sarcoplasmic reticulum, and were attributed to photolysis of Nitr-5. The difference absorbance bands are narrow, suggesting that they originate from changes in side chain vibrations, although some changes in secondary structures may also contribute.

Infrared spectroscopic signals arising from ligand binding and conformational changes in the catalytic cycle of sarcoplasmic reticulum calcium ATPase

Fourier transform infrared spectroscopy was used to investigate ligand binding and conformational changes in the Ca2(+)-ATPase of sarcoplasmic reticulum during the catalytic cycle. The ATPase reaction was started in the infrared sample by release of ATP from the inactive, photolabile ATP derivative P3-1-(2-nitro)phenylethyladenosine 5'-triphosphate (caged ATP). Absorption spectroscopy in the visible spectral region using the Ca2(+)-sensitive dye Antipyrylazo III ensured that the infrared samples were able to transport Ca2+ in spite of their low water content, which is required for mid-infrared measurements (1800-950 cm-1). Small, but characteristic and highly reproducible infrared absorbance changes were observed upon ATP release. These infrared absorbance changes exhibit different kinetic properties. Comparison with model compound infrared spectra indicates that they are related to photolysis of caged ATP, hydrolysis of ATP in consequence of ATPase activity and to molecular changes in the active ATPase. The absorbance changes due to alterations in the ATPase were observed mainly in the region of Amide I and Amide II protein absorbance and presumably reflect the molecular processes upon phosphoenzyme formation. Since the absorbance changes were small compared to the overall ATPase absorbance, no major rearrangement of ATPase conformation as the result of catalysis could be detected.

Molecular changes in the sarcoplasmic reticulum calcium ATPase during catalytic activity. A Fourier transform infrared (FTIR) study using photolysis of caged ATP to trigger the reaction cycle

Fourier transform infrared spectroscopy was used to study ligand binding and conformational changes in the Ca2(+)-ATPase of sarcoplasmic reticulum. Novel in infrared difference spectroscopy, the catalytic cycle in the IR sample was started by photolytic release of ATP from an inactive, photolabile ATP-derivative (caged ATP). Small, but characteristic infrared absorbance changes were observed upon ATP release. On the basis of model spectra, the absorbance changes corresponding to the trigger and substrate reactions, i.e. to photolysis of caged ATP and hydrolysis of ATP, were separated from the absorbance changes due to the active ATPase reflecting formation of the phosphorylated Ca2E1P enzyme form. A major rearrangement of ATPase conformation as the result of catalysis can be excluded.

Structural dynamics of the Ca2(+)-ATPase of sarcoplasmic reticulum. Temperature profiles of fluorescence polarization and intramolecular energy transfer

The temperature dependence of fluorescence polarization and Förster-type resonance energy transfer (FRET) was analyzed in the Ca2(+)-ATPase of sarcoplasmic reticulum using protein tryptophan and site-specific fluorescence indicators such as 5-[2-[iodoacetyl)amino)ethyl]aminonaphthalene-1-sulfonic acid (IAEDANS), fluorescein 5'-isothiocyanate (FITC), 2',3'-O-(2,4,3-trinitrophenyl)adenosine monophosphate (TNP-AMP) or lanthanides (Pr3+, Nd3+) as probes. The normalized energy transfer efficiency between AEDANS bound at cysteine-670 and -674 and FITC bound at lysine-515 increases with increasing temperature in the range of 10-37 degrees C, indicating the existence of a relatively flexible structure in the region of the ATPase molecule that links the AEDANS to the FITC site. These observations are consistent with the theory of Somogyi, Matko, Papp, Hevessy, Welch and Damjanovich (Biochemistry 23 (1984) 3403-3411) that thermally induced structural fluctuations increase the energy transfer. Structural fluctuations were also evident in the energy transfer between FITC linked to the nucleotide-binding domain and Nd3+ bound at the putative Ca2+ sites. By contrast the normalized energy transfer efficiency between AEDANS and Pr3+ was relatively insensitive to temperature, suggesting that the region between cysteine-670 and the putative Ca2+ site monitored by the AEDANS-Pr3+ pair is relatively rigid. A combination of the energy transfer data with the structural information derived from analysis of Ca2(+)-ATPase crystals yields a structural model, in which the location of the AEDANS-, FITC- and Ca2+ sites are tentatively identified.

Emerging views on the structure and dynamics of the Ca2(+)-ATPase in sarcoplasmic reticulum

The ATP-dependent Ca2+ transport in sarcoplasmic reticulum involves transitions between several structural states of the Ca2(+)-ATPase, that occur without major changes in the secondary structure. The rates of these transitions are modulated by the lipid environment and by interactions between ATPase molecules. Although the Ca2(+)-ATPase restricts the rotational mobility of a population of lipids, there is no evidence for specific interaction of the Ca2(+)-ATPase with phospholipids. Fluorescence polarization and energy transfer (FET) studies, using site specific fluorescent indicators, combined with crystallographic, immunological and chemical modification data, yielded a structural model of Ca2(+)-ATPase in which the binding sites of Ca2+ and ATP are tentatively identified. The temperature dependence of FET between fluorophores attached to different regions of the ATPase indicates the existence of 'rigid' and 'flexible' regions within the molecule characterized, by different degrees of thermally induced structural fluctuations.