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

Solid-Supported Assembly Composed of n-Octyl-β-D-glucopyranoside and 1,2-Dioleoyl-sn-glycero-3-phosphocholine in Equilibrium with Its Ambient Aqueous Solution System Including Dispersed Assembly

In the last few decades, the preparation of solid-supported lipid bilayers by immersing a solid substrate in an aqueous solution where the lipid is dissolved with the aid of a surfactant, followed by dilution of the solution, has been reported. In this study, we attempted to interpret the evolution of supported surfactant/lipid assemblies towards the supported lipid bilayer in terms of a phase equilibrium between the supported assembly phase and its ambient solution system consisting of the dispersed surfactant/lipid assembly phase and the bulk solution phase comprising monomeric surfactant and lipid. We characterized the supported assembly formed on hydrophilized Ge or mica substrates in equilibrium with aqueous solutions containing various concentrations of the nonionic surfactant, n-octyl-β-D-glucopyranoside (OG) and the amphoteric phospholipid, 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), using interaction-force-profile measurements by atomic force microscopy (AFM), and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). We also investigated the ambient solution system using equilibrium dialysis to obtain the partition equilibrium profile of OG between the bulk solution and dispersed assembly phases in the micellar or vesicular states. These studies indicate that the properties of the supported assembly depend on the composition of the dispersed assembly and concentration of monomerically dissolved OG. Further, a type of micellar-bilayer state transition occurs in the supported assembly, roughly synchronized with that in the dispersed assembly.

Interfacial Enzymes: Membrane Binding, Orientation, Membrane Insertion, and Activity

Most interfacial enzymes undergo activation upon membrane binding. Interfacial activation is determined not only by the binding strength but also by the specific mode of protein-membrane interactions, including the angular orientation and membrane insertion of the enzymes. This chapter describes biophysical techniques to quantitatively evaluate membrane binding, orientation, membrane insertion, and activity of secreted phospholipase A2 (PLA2) and lipoxygenase (LO) enzymes. Procedures for recombinant production and purification of human pancreatic PLA2 and human 5-lipoxygenase (5-LO) are also presented. Several methods for measurements of membrane binding of peripheral proteins are described, i.e., fluorescence resonance energy transfer (FRET) from tryptophan or tyrosine residues of the protein to a fluorescent lipid in vesicles, changes in fluorescence of an environment-sensitive fluorescent lipid upon binding of proteins to membranes, and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. These methods produce the apparent binding constant, the protein-to-lipid binding stoichiometry, and the Hill cooperativity coefficient. Experimental procedures for segmental isotope labeling of proteins and determination of the orientation of membrane-bound proteins by polarized ATR-FTIR spectroscopy are described. Furthermore, evaluation of membrane insertion of peripheral proteins by a fluorescence quenching technique is outlined. Combination of the orientation and membrane insertion provides a unique configuration of the protein-membrane complex and hence elucidates certain details of the enzyme function, such as the modes of acquisition of a membrane-residing substrate and product release. Finally, assays for determination of the activities of secreted PLA2, soybean LO, and human 5-LO are described.

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.

Preparation and adsorption of refined polyelectrolyte complex nanoparticles

We report on bulk and surface properties of centrifuged nonstoichiometric polyelectrolyte complex (PEC) dispersions. PECs were prepared by mixing poly(diallyldimethylammonium chloride) (PDADMAC) and sodium poly(maleic acid-co-alpha-methylstyrene) (PMA-MS) at the monomolar mixing ratio of 0.6 and polymer concentration >/=1 mmol/l. Centrifugation of initial PEC dispersions revealed three phases: supernatant (SUP), coacervate (COAC), and an insoluble precipitate. Mass, turbidity, particle hydrodynamic radii (R(h)), and the titratable charge amount were determined for those phases. The turbid COAC phase consisted of 200-nm nanoparticles and carried 60% of the polymer mass and 20% of the titratable charge amount of the initial PEC dispersion. The SUP phase showed no turbidity and no such nanoparticles, but carried 80% of the initial titratable charge amount, presumably caused by excess polycations. Furthermore, linear dependences of turbidity and R(h) on COAC concentration was observed. COAC adsorption was studied at polyelectrolyte multilayer (PEM) modified silicon surfaces in dependence on both adsorption time and concentration using attenuated total-reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. The adsorption data were fitted by the simple Langmuir model. Comparison of COAC particles and polystyrene latices revealed similar adsorption features. SEM and AFM measurements resulted in hemispherically shaped adsorbed COAC particles with coverages >/=25%, whose calculated volumes correlated well with those in dispersion obtained by PCS.

Implications of threonine hydrogen bonding in the glycophorin A transmembrane helix dimer

The transmembrane helix of glycophorin A contains a seven-residue motif, LIxxGVxxGVxxT, that mediates protein dimerization. Threonine is the only polar amino acid in this motif with the potential to stabilize the dimer through hydrogen-bonding interactions. Polarized Fourier transform infrared spectroscopy is used to establish a robust protocol for incorporating glycophorin A transmembrane peptides into membrane bilayers. Analysis of the dichroic ratio of the 1655-cm(-1) amide I vibration indicates that peptides reconstituted by detergent dialysis have a transmembrane orientation with a helix crossing angle of <35 degrees. Solid-state nuclear magnetic resonance spectroscopy is used to establish high resolution structural restraints on the conformation and packing of Thr-87 in the dimer interface. Rotational resonance measurement of a 2.9-A distance between the gamma-methyl and backbone carbonyl carbons of Thr-87 is consistent with a gauche- conformation for the chi1 torsion angle. Rotational-echo double-resonance measurements demonstrate close packing (4.0 +/- 0.2 A) of the Thr-87 gamma-methyl group with the backbone nitrogen of Ile-88 across the dimer interface. The short interhelical distance places the beta-hydroxyl of Thr-87 within hydrogen-bonding range of the backbone carbonyl of Val-84 on the opposing helix. These results refine the structure of the glycophorin A dimer in membrane bilayers and highlight the complementary role of small and polar residues in the tight association of transmembrane helices in membrane proteins.

Toward understanding interfacial activation of secretory phospholipase A2 (PLA2): membrane surface properties and membrane-induced structural changes in the enzyme contribute synergistically to PLA2 activation

Phospholipase A2 (PLA2) hydrolyzes phospholipids to free fatty acids and lysolipids and thus initiates the biosynthesis of eicosanoids and platelet-activating factor, potent mediators of inflammation, allergy, apoptosis, and tumorigenesis. The relative contributions of the physical properties of membranes and the structural changes in PLA2 to the interfacial activation of PLA2, that is, a strong increase in the lipolytic activity upon binding to the surface of phospholipid membranes or micelles, are not well understood. The present results demonstrate that both binding of PLA2 to phospholipid bilayers and its activity are facilitated by membrane surface electrostatics. Higher PLA2 activity toward negatively charged membranes is shown to result from stronger membrane-enzyme electrostatic interactions rather than selective hydrolysis of the acidic lipid. Phospholipid hydrolysis by PLA2 is followed by preferential removal of the liberated lysolipid and accumulation of the fatty acid in the membrane that may predominantly modulate PLA2 activity by affecting membrane electrostatics and/or morphology. The previously described induction of a flexible helical structure in PLA2 during interfacial activation was more pronounced at higher negative charge densities of membranes. These findings identify a reciprocal relationship between the membrane surface properties, strength of membrane binding of PLA2, membrane-induced structural changes in PLA2, and the enzyme activation.

Infrared dichroism from the X-ray structure of bacteriorhodopsin

A detailed comparison with the three-dimensional protein structure provides a stringent test of the models and parameters commonly used in determining the orientation of the alpha-helices from the linear dichroism of the infrared amide bands, particularly in membranes. The order parameters of the amide vibrational transition moments are calculated for the transmembrane alpha-helices of bacteriorhodopsin by using the crystal structure determined at a resolution of 1.55 A (PDB accession number 1C3W). The dependence on the angle delta(M) that the transition moment makes with the peptide carbonyl bond is fit by the expression ((3)/(2)S(alpha) cos(2) alpha)cos(2)(delta(M) + beta) - 1/2S(alpha), where S(alpha) (0.91) is the order parameter of the alpha-helices, alpha (13 degrees ) is the angle that the peptide plane makes with the helix axis, and beta (11 degrees ) is the angle that the peptide carbonyl bond makes with the projection of the helix axis on the peptide plane. This result is fully consistent with the model of nested axial distributions commonly used in interpreting infrared linear dichroism of proteins. Comparison with experimental infrared dichroic ratios for bacteriorhodopsin yields values of Theta(A) = 33 +/- 1 degree, Theta(I) = 39.5 +/- 1 degree, and Theta(II) = 70 +/- 2 degrees for the orientation of the transition moments of the amide A, amide I, and amide II bands, respectively, relative to the helix axis. These estimates are close to those found for model alpha-helical polypeptides, indicating that side-chain heterogeneity and slight helix imperfections are unlikely to affect the reliability of infrared measurements of helix orientations.

Structural difference in the H+,K+-ATPase between the E1 and E2 conformations. An attenuated total reflection infrared spectroscopy, UV circular dichroism and raman spectroscopy study

Conformational changes taking place in the gastric H+,K+-ATPase when shifting from the K+-induced E2 form to the E1 form upon replacing K+ ions by Na+ were investigated by different spectroscopic approaches. No significant secondary-structure change or secondary-structure reorientation with respect to the membrane plane could be measured by attenuated total reflection Fourier transform infrared spectroscopy of oriented films. Circular dichroism and Raman spectra obtained on tubulovesicle suspensions indicated no significant secondary structure or tyrosine and tryptophan side-chain environment changes in tubulovesicle suspensions. The smallest observable structural changes are discussed in term of the number of amino-acid residues involved for each technique.

Quantitative orientation measurements in thin lipid films by attenuated total reflection infrared spectroscopy

Quantitative orientation measurements by attenuated total reflectance (ATR) infrared spectroscopy require the accurate knowledge of the dichroic ratio and of the mean-square electric fields along the three axes of the ATR crystal. In this paper, polarized ATR spectra of single supported bilayers of the phospholipid dimyristoylphosphatidic acid covered by either air or water have been recorded and the dichroic ratio of the bands due to the methylene stretching vibrations has been calculated. The mean-square electric field amplitudes were calculated using three formalisms, namely the Harrick thin film approximation, the two-phase approximation, and the thickness- and absorption-dependent one. The results show that for dry bilayers, the acyl chain tilt angle varies with the formalism used, while no significant variations are observed for the hydrated bilayers. To test the validity of the different formalisms, s- and p-polarized ATR spectra of a 40-A lipid layer were simulated for different acyl chain tilt angles. The results show that the thickness- and absorption-dependent formalism using the mean values of the electric fields over the film thickness gives the most accurate values of acyl chain tilt angle in dry lipid films. However, for lipid monolayers or bilayers, the tilt angle can be determined with an acceptable accuracy using the Harrick thin film approximation. Finally, this study shows clearly that the uncertainty on the determination of the tilt angle comes mostly from the experimental error on the dichroic ratio and from the knowledge of the refractive index.

Planar lipid bilayers on solid supports from liposomes--factors of importance for kinetics and stability

One method to create planar lipid bilayers on solid substrates involves the transfer of lipids from liposomes to the support. We have varied the composition of liposomes systematically using factorial experimental designs and analyzed the adsorption behaviour of lipids from these liposomes onto solid supports. The hydrophilic supports were either used plain or modified with a monolayer of a lipid mixture, exposing hydrophobic groups. The monolayer-covered supports were used to identify factors important for adhesion and stability. Lipid adsorption kinetics was primarily studied on plain silicon supports in an ellipsometric cell or on a silicon nitride surface in a resonant mirror system (IAsys), using the systematic approach. Saturated phospholipids were essential for the required stability. Mixtures of dipalmitoylphosphatidylcholine, dipalmitoylphosphatidylglycerol, dipalmitoylphosphatidylethanolamine and cholesterol in combination with proteins were investigated in further detail as regards kinetics. The propensity to form a supported planar bilayer could be manipulated by the presence of calcium ions.

Fourier transform infrared spectroscopy and site-directed isotope labeling as a probe of local secondary structure in the transmembrane domain of phospholamban

Phospholamban is a 52-amino acid residue membrane protein that regulates Ca(2+)-ATPase activity in the sarcoplasmic reticulum of cardiac muscle cells. The hydrophobic C-terminal 28 amino acid fragment of phospholamban (hPLB) anchors the protein in the membrane and may form part of a Ca(2+)-selective ion channel. We have used polarized attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy along with site-directed isotope labeling to probe the local structure of hPLB. The frequency and dichroism of the amide I and II bands appearing at 1658 cm-1 and 1544 cm-1, respectively, show that dehydrated and hydrated hPLB reconstituted into dimyristoylphosphatidycholine bilayer membranes is predominantly alpha-helical and has a net transmembrane orientation. Specific local secondary structure of hPLB was probed by incorporating 13C at two positions in the protein backbone. A small band seen near 1614 cm-1 is assigned to the amide I mode of the 13C-labeled amide carbonyl group(s). The frequency and dichroism of this band indicate that residues 39 and 46 are alpha-helical, with an axial orientation that is approximately 30 degrees relative to the membrane normal. Upon exposure to 2H2O (D2O), 30% of the peptide amide groups in hPLB undergo a slow deuterium/hydrogen exchange. The remainder of the protein, including the peptide groups of Leu-39 and Leu-42, appear inaccessible to exchange, indicating that most of the hPLB fragment is embedded in the lipid bilayer. By extending spectroscopic characterization of PLB to include hydrated, deuterated as well as site-directed isotope-labeled hPLB films, our results strongly support models of PLB that predict the existence of an alpha-helical hydrophobic region spanning the membrane domain.

Strong hydrogen bonding interactions involving a buried glutamic acid in the transmembrane sequence of the neu/erbB-2 receptor

The receptor tyrosine kinase encoded by the neu/erbB-2 proto-oncogene is constitutively activated by a single valine to glutamic acid substitution at position 664 in the predicted membrane-spanning sequence of the receptor. We have explored the structural changes involved in receptor activation with polarized FTIR and magic angle spinning NMR spectroscopy. The hydrophobic transmembrane sequence folds into a well-defined alpha-helical structure spanning the membrane bilayer. Measurements of the pKa and 13C chemical shift anisotropy of Glu 664 reveal that the side chain carboxyl group is protonated and strongly hydrogen bonded. These studies provide direct evidence for glutamate hydrogen-bonding interactions in the mechanism of receptor dimerization and activation.

Orientational order determination by internal reflection infrared spectroscopy

PATIR-FTIR spectroscopy is a powerful technique for the determination of molecular order in thin films such as supported lipid membranes, but it relies on electromagnetic theory which is incomplete and potentially misleading. A complete derivation of the current theory for two, three and four phase system has been reported. The two phase and thin film approximations most commonly used in practice have been shown to represent the thickness-dependent expressions from which they are derived with a high degree of accuracy. However, these expressions are based on the macroscopic behavior of dielectric materials, and may not be accurate when applied to microscopic circumstances. The potential error introduced is qualitatively and quantitatively significant. Further experimental and theoretical work is needed to verify the accuracy of this theory, or to refute and refine it. This effort to do this is warranted by the power and increasing popularity of the technique.

Orientation of fusion-active synthetic peptides in phospholipid bilayers: determination by Fourier transform infrared spectroscopy

A group of synthetic peptides having an amino acid sequence related to the N-terminal region of the influenza virus hemagglutinin HA-2 chain can induce phospholipid membrane fusion in a pH-dependent manner. These peptides bind to membranes to form alpha-helices even at pH's where no fusion activity is seen. We determined the orientation of these alpha-helical peptides in lipid multibilayers using attenuated total reflection infrared spectroscopy and found that the peptide alpha-helices took a preferential orientation, the helix axis being about 70 degrees from the normal of the membrane plane, or in other words rather parallel to the membrane plane. The orientation was almost independent of pH and a modification of the N-terminal amino group which reduced the fusion activity of the peptides. The determination was carried out for peptides in lipid multibilayers in dry or hydrated (membranes equilibrated with D2O vapor) conditions. Although a slight decrease in the helix orientation angle from the membrane normal was noticed for a hydrated system, the difference between the results for dry and hydrated conditions was small.

Orientation of functional and nonfunctional PTS permease signal sequences in lipid bilayers. A polarized attenuated total reflection infrared study

Synthetic peptides corresponding to the N-terminal 23 and 22 residues, respectively, of two integral plasma membrane proteins of Escherichia coli, namely the mannitol- and glucitol-specific permeases of the bacterial sugar phosphotransferase system, were incorporated into single planar phospholipid bilayers supported on germanium plates. Polarized attenuated total reflection infrared spectra were recorded, and order parameters were derived from the measured dichroic ratios. The order parameters of the two wild-type peptides which form amphiphilic alpha-helices in membranes were -0.4 to -0.5, indicating a preferential alignment of the alpha-helix long axis parallel to the membrane surface. Nonfunctional mutant peptides of the mannitol permease sequence in which serine-3 or aspartate-4 were substituted with prolines (S3P and D4P) or lysine (D4K), but which were still largely alpha-helical, exhibited peptide order parameters close to zero, indicating a high degree of disorder of these peptides in the lipid bilayers. The lipid was well ordered at low concentrations of peptides in the membranes but became disordered at high peptide concentrations. This effect of lipid disordering was more pronounced for the D4K mutant than for the wild-type mannitol peptide.

Orientation of melittin in phospholipid bilayers. A polarized attenuated total reflection infrared study

The helical order parameter of the 26-residue amphiphilic bee venom peptide melittin was measured by polarized attenuated total reflection infrared spectroscopy (ATR-IR) in dry phospholipid multibilayers (MBLs) and when bound to single supported planar bilayers (SPBs) under D2O. Melittin adopted an alpha-helical conformation in MBLs of dipalmitoyl-phosphatidylcholine (DPPC), 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC), a 4:1 mixture of POPC and 1-palmitoyl-2-oleoyl-phosphatidylglycerol (POPG), and when bound to SPBs of POPC:POPG (4:1). The order parameter of the alpha-helix in the bilayers depended mainly on the type of membrane preparation, and only little on the phospholipid composition of the bilayers. On hydrated SPBs, the helical order parameter was negative, indicating that the alpha-helix long axis of melittin was preferentially oriented parallel to the plane of the supported membrane. However, in dry MBLs, the helical order parameter was positive, indicating that the alpha-helix of melittin was preferentially oriented parallel to the phospholipid fatty acyl chains. It is concluded that the orientation of melittin in membranes depends on the degree of hydration of the model membranes rather than on the technique which is used for its determination. ATR-IR spectroscopy of polypeptides in or associated with supported planar membranes in D2O may become a useful tool for the determination of their orientation in and on membranes.

Polarized infrared attenuated total reflectance spectroscopy of the Ca(2+)-ATPase of sarcoplasmic reticulum

The mean orientations of the transition dipole moments associated with vibrational modes of the proteins and phospholipids of sarcoplasmic reticulum were determined on dry and hydrated membrane multilayers deposited on germanium or zinc selenide crystals, using polarized infrared attenuated total reflectance spectroscopy (P-IR-ATR). For preservation of the enzymatic activity of the Ca(2+)-ATPase the films were prepared from solutions containing 0.05 M KCl, 5 mM imidazole (pH 7.4), 0.5 mM MgCl2, 1-10 mM trehalose and dithiothreitol. The anisotropy was highest in dry films containing congruent to 7.5 micrograms protein/cm2, and decreased with increasing membrane thickness or hydration. The dichroic ratio of the CH2 vibrations (2923 cm-1) of extracted sarcoplasmic reticulum phospholipids on Ge plate was 1.56, compared with a dichroic ratio of 1.68 obtained on dry films of whole sarcoplasmic reticulum. The dichroic ratios of the amide I band (1650 cm-1) of the Ca(2+)-ATPase in the Ca2-E1 state and in the EGTA and vanadate stabilized E2-V state were nearly identical (1.60 vs. 1.62). The dichroism of the amide I, amide II and lipid CH2 vibrations was not affected by changes in the concentration of KCl (25-100 mM) or Ca2+ (approximately equal to 10(-8)-10(-4) M) and by the addition of vanadate (1 mM) or Pi (5 mM) in a calcium-free medium containing 0.5 mM EGTA. The dichroic ratio of the C-C (1033 cm-1) or CO stretching band (1046 cm-1) of trehalose incorporated into SR films was 1.2 on Ge plate; this corresponds to a mean angle of approximately 70 degrees between the plane of the trehalose ring and the normal of the film plane, suggesting that the trehalose molecules are surprisingly well oriented in the polar headgroup region of the phospholipids. The orientation of the trehalose was not affected by the presence of Ca(2+)-ATPase.