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PubMed1991

Structural and kinetic studies of the Fab fragment of a monoclonal anti-spin label antibody by nuclear magnetic resonance.

Nuclear magnetic resonance has been used to study the structure of the anti-spin label antibody AN02 combining site and kinetic rates for the hapten-antibody reaction. The association reaction for the hapten dinitrophenyl-diglycine (DNP-diGly) is diffusion-limited. The activation enthalpy for association, 5.1 kcal/mol, is close to the activation enthalpy for diffusion in water. Several reliable resonance assignments have been made with the aid of recently reported crystal structure. Structural data deduced from the nuclear magnetic resonance (n.m.r.) spectra compare favorably with the crystal structure in terms of the combining site amino acid composition, distances of tyrosine residues from the unpaired electron of the hapten, and residues in direct contact with the hapten. Evidence is presented that a single binding site region tyrosine residue can assume two distinct conformations on binding of DNP-diGly. The AN02 antibody is an autoantibody. Dimerization of the Fab fragments is blocked by the hapten DNP-diGly. The n.m.r. spectra suggests that some of the amino acid residues involved in the binding of the DNP-hapten are also involved in the Fab dimerization.

Antibodies, Monoclonal↗

PubMed1983

The spin-label approach to labeling membrane protein sulfhydryl groups.

Labeling experiments with viable membrane preparations are optimally undertaken where conditions are extremely mild (minimal excess of modification reagent, very short reaction times) in order to reduce the possibilities of introducing artifacts (cell aging and death, denaturation, etc.). Furthermore, specificity of labeling is a critical requirement where the system is complex and contains many proteins. The spin label IV described above offers several advantages over previously employed reagents. By virtue of its reversibility and ease of quantitation, one may repetitively label and remove label from the same membrane preparation in order to check reproducibility while using the same sample as a control.

Chemical Phenomena↗

PubMed2006

The interaction of Fusarium solani pisi cutinase with long chain spin label esters.

We here present a study of the interaction between the Fusarium solani pisi cutinase mutant S120A and spin-labeled 4,4-dimethyloxazoline-N-oxyl-(DOXYL)-stearoyl-glycerol substrates in a micellar system. The interaction is detected by NMR measuring changes in chemical shift for 1H and 15N as well as relaxation parameters for backbone 1H (T1) and 15N (T1, T2) atoms as well as for side chain methyl groups 1H (T1). The detected interaction shows a weak binding of cutinase to the lipid micelles. Structural and mobility changes are located inside and around the active site, its flanking loops, and the oxyanion hole, respectively. Relaxation changes in the amino acid pairs Ser 92, Ala 93 and Thr 173, Gly 174 positioned at the edge of each of the active site flanking loops make these residues prime candidates for hinges, allowing for structural rearrangement during substrate binding. The cutinase mutant S120A used carries a 15 amino acid pro-peptide; the significance of this pro-peptide was so far undetermined. We show here that the pro-peptide is affected by the presence of the micellar substrate. Relaxation enhancements indicative of spatial proximity between the DOXYL group in the lipid chain and some hydrophobic residues surrounding the active site could be found.

Alanine↗

PubMed1988

Evidence for bidirectional transverse diffusion of spin-labeled phospholipids in the plasma membrane of guinea pig blood cells.

The distribution and transverse diffusion kinetics of four spin-labeled phospholipid analogues (two with choline heads: phosphatidylcholine (PC) and sphingomyelin (SM); two with amino heads: phosphatidylserine (PS) and phosphatidylethanolamine (PE) were studied in the plasma membrane of guinea pig blood cells: erythrocytes, reticulocytes, and leukemic lymphocytes. Nitroxide reduction by the internal content of the cells was used as an indicator to determine the phospholipids that penetrated the cells. The reduction rates were in the order, PS greater than PE greater than PC greater than SM in all cells. Reoxidation of phospholipids extracted by serum albumin revealed the distribution of the phospholipids at a given time. In all cells, the distribution equilibrium was reached in less than 2 h and the amounts left in the external leaflet were in the following proportional order: PS less than PE less than PC less than SM. In the erythrocytes and especially in the reticulocytes, the shape change induced by adding phospholipids relaxed partially or completely at a lower speed but kept the same proportional order as at equilibrium. All the results were analyzed quantitatively with a simple kinetic model including the rates of transverse diffusion (flip and flop), the exchange between plasma membrane and internal membranes, and the reduction rate of free radicals (determined in either the internal or external membrane leaflet). The calculated rate constants of transverse diffusion varied from 2 x 10(-3) to 1.2 x 10(-1) min-1 for the flip and from 4 x 10(-3) to 1.2 x 10(-1) for the flop, depending on the polar head and the cell type. Possible interpretations of the external phospholipid reduction mechanism and cell deformation are discussed.

Animals↗

PubMed1995

A method for distance determination in proteins using a designed metal ion binding site and site-directed spin labeling: evaluation with T4 lysozyme.

The use of molecular genetics to introduce both a metal ion binding site and a nitroxide spin label into the same protein opens the use of paramagnetic metalnitroxyl interactions to estimate intramolecular distances in a wide variety of proteins. In this report, a His-Xaa3-His metal ion binding motif was introduced at the N terminus of the long interdomain helix of T4 lysozyme (Lys-65 --> His/Gln-69 --> His) of three mutants, each containing a single nitroxide-labeled cysteine residue at position 71, 76, or 80. The results show that Cu(II)-induced relaxation effects on the nitroxide can be quantitatively analyzed in terms of interspin distance in the range of 10-25 A using Redfield theory, as first suggested by Leigh [Leigh, J.S. (1970) J. Chem. Phys. 52, 2608-2612]. Of particular interest is the observation that distances can be determined both under rigid lattice conditions in frozen solution and in the presence of motion of the spins at room temperature under physiological conditions. The method should be particularly attractive for investigating structure in membrane proteins that are difficult to crystallize. In the accompanying paper, the technique is applied to a polytopic membrane protein, lactose permease.

Bacteriophage T4↗

PubMed1980

Specific spin-labeling at trypsin active site. Application of 'inverse substrate' to the structural analysis of the active site.

Specific and reversible spin-labeling of trypsin (EC 3.4.21.4) active site was carried out by 'inverse substrate', 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl and 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolinyloxyl-p-amidinophenyl esters. The paramagnetic resonance spectra of the labeled trypsin in the form of acyl enzyme intermediate were investigated. The 2T value, separation of the outer hyperfine extrema, was observed to be sensitive to pH of the medium. These results are discussed in terms of pH-dependent conformational change at the vicinity of active site.

Acylation↗

PubMed1999

Structure of the KcsA potassium channel from Streptomyces lividans: a site-directed spin labeling study of the second transmembrane segment.

KcsA is a prokaryotic potassium channel. The present study employs cysteine scanning mutagenesis and site-directed spin labeling to investigate the structure of the second transmembrane segment (residues 82-120) in functional tetrameric channels reconstituted in lipid bilayers. Spin-spin interactions are observed between nitroxide side chains at symmetry-related sites close to the 4-fold axis of symmetry. To aid in quantitative analysis of these interactions, a new diamagnetic analogue of the nitroxide side chain is used to prepare magnetically dilute samples with constant structure. Using constraints imposed by the spin-spin interactions, a packing model for this segment is deduced that is in excellent agreement with the recently reported crystal structure [Doyle, D., et al. (1998) Science 280, 69-77]. The relatively immobilized state of the nitroxide side chains suggests that the channel is rigid on the electron paramagnetic resonance time scale. Moreover, the poor sulfhydryl reactivity of the cysteine at many locations indicates that the channel is not subject to the low-frequency fluctuations that permit reaction of buried cysteines. At sites expected to be located in the pore, the accessibility of the side chains to collision with O(2) or nickel(II) ethylenediaminediacetate is low. This inaccessibility, together with the generally low mobility of the side chains throughout the sequence, makes it difficult to detect the presence of the pore based on these measurements. However, the presence of a solvated pore can be directly demonstrated using a polarity parameter deduced from the EPR spectra recorded at low temperature. These measurements also reveal the presence of a polarity gradient in the phospholipid bilayer.

Amino Acid Sequence↗

PubMed1979

Molecular mechanism of general anesthesia: II. Spin label studies on synaptic membranes.

In this communication we report the effects of general anesthetics on the mobility and order of spin labeled stearic acid derivatives in synaptic membranes and in bilayers formed from the lipids extracted therefrom. The anesthetics studied abolish the immobilization induced by synaptic membrane proteins on the membrane lipids : this effect, observed particularly in the bilayer core, is interpreted as a labilization of lipid-protein interactions induced by anesthetics.

Anesthesia, General↗

PubMed1993

Preparation and characterization of a bifunctionally spin-labeled mutant of murine epidermal growth factor for saturation-transfer electron paramagnetic resonance studies of the growth factor/receptor complex.

In this report we describe the production of a [Lys3,Tyr22] murine epidermal growth factor (mEGF) mutant for spin-labeling with bis(sulfo-N-succinimidyl)-[15N,2H16]doxyl-2-spiro-4'-pimelate ([15N,2H16]BSSDP) in order to study the rotational dynamics of the EGF/EGF receptor complex by saturation-transfer electron paramagnetic resonance (ST-EPR). Previous results [Faulkner-O'Brien et al. (1991) Biochemistry 30,8976-8985] indicated that the reaction of [15N,2H16]BSSDP with wild-type mEGF did not yield a product useful for ST-EPR studies of the EGF/EGF receptor complex because the major product, in which [15N,2H16]BSSDP was attached only at the amino terminus of mEGF, lacked rigid motional coupling of the spin probe to the protein, and the more tightly coupled bidentate product was unstable. Using oligonucleotide-mediated site-directed mutagenesis of a synthetic gene for mEGF, we replaced Tyr3 with Lys and His22 with Tyr in wild-type mEGF to produce a mutant mEGF suitable for [15N,2H16]BSSDP labeling. The [Lys3,Tyr22] mEGF was expressed in Escherichia coli HB101 transformed with a pIN-III-ompA3-[Lys3,Tyr22] mEGF plasmid and was purified from the bacterial periplasm using a simple two step purification method. The [15N,2H16]BSSDP reacted with [Lys3,Tyr22]mEGF in high yield, and EPR analysis of the major product revealed tight motional coupling between the spin probe and the protein. Biological activity, as assessed by stimulation of EGF receptor autophosphorylation and dimerization, was not affected by either the mutations or the addition of the spin label. The [15N,2H16]BSSDP-modified [Lys3,Tyr22] mEGF was shown to be equipotent with mEGF in EGF receptor competition binding assays using A431 cells; in EPR studies, mEGF also was shown to specifically block [15N,2H16]BSSDP-modified [Lys3,Tyr22]mEGF binding to the EGF receptor in A431 membrane vesicles. Using the [15N,2H16]BSSDP-modified [Lys3,Tyr22]mEGF, we now report the first measurement of the rotational dynamics of the EGF/EGF receptor complex in A431 membrane vesicles by ST-EPR.

Amino Acid Sequence↗

PubMed1998

BASE imaging: a new spin labeling technique for measuring absolute perfusion changes.

A new technique for magnetic resonance imaging of absolute perfusion changes that uses magnetically labeled tissue water proton spins as a freely diffusible tracer is described. It consists of unprepared basis (BA) images that serve as a reference and selective (SE) inversion prepared images that are sensitive to perfusion changes. In the present study, the BASE technique was applied to functional neuroimaging. BA and SE images were alternatingly and repeatedly acquired during periods of visual stimulation and control. Visual stimulation was achieved with an alternating black/white checkerboard operating at a frequency of 8 Hz. Maps of the absolute cerebral blood flow changes (deltaCBF) were calculated from the image intensities of the corresponding BA and SE images. The individual mean values of deltaCBF measured in five healthy volunteers ranged from 69 +/- 18 to 99 +/- 26 ml/min/100 g. Since the BASE technique does not require nonselective spin inversion, it can be used with small transmit/receive head coils (e.g., surface coils). In addition, the BASE technique is robust against a mismatch of the inversion and detection slice profiles.

Brain↗

PubMed2006

Molecular motion of spin labeled side chains in the C-terminal domain of RGL2 protein: a SDSL-EPR and MD study.

Five singly spin labeled side chains at surface sites in the C-terminal domain of RGL2 protein have been analyzed to investigate the general relationship between nitroxide side chain mobility and protein structure. At these sites, the structural perturbation produced by replacement of a native residue with a nitroxide side chain appears to be very slight at the level of the backbone fold. The primary determinants of the nitroxide side chain mobility are backbone dynamics and tertiary interactions. On the exposed surfaces of alpha-helices, the side chain mobility is not restricted by tertiary interactions but appears to be determined by backbone dynamics, while in loop sites, the side chain mobility is even higher. For a better understanding of the changes in the EPR spectral line shape, molecular dynamics simulations were performed and found in agreement with EPR spectral data.

Electron Spin Resonance Spectroscopy↗

PubMed2001

Arterial spin labeling in combination with a look-locker sampling strategy: inflow turbo-sampling EPI-FAIR (ITS-FAIR).

Arterial spin labeling (ASL) permits quantification of tissue perfusion without the use of MR contrast agents. With standard ASL techniques such as flow-sensitive alternating inversion recovery (FAIR) the signal from arterial blood is measured at a fixed inversion delay after magnetic labeling. As no image information is sampled during this delay, FAIR measurements are inefficient and time-consuming. In this work the FAIR preparation was combined with a Look-Locker acquisition to sample not one but a series of images after each labeling pulse. This new method allows monitoring of the temporal dynamics of blood inflow. To quantify perfusion, a theoretical model for the signal dynamics during the Look-Locker readout was developed and applied. Also, the imaging parameters of the new ITS-FAIR technique were optimized using an expression for the variance of the calculated perfusion. For the given scanner hardware the parameters were: temporal resolution 100 ms, 23 images, flip-angle 25.4 degrees. In a normal volunteer experiment with these parameters an average perfusion value of 48.2 +/- 12.1 ml/100 g/min was measured in the brain. With the ability to obtain ITS-FAIR time series with high temporal resolution arterial transit times in the range of -138 - 1054 ms were measured, where nonphysical negative values were found in voxels containing large vessels.

Adult↗

PubMed1975

Investigations of thermotropic phase changes in peripheral nerve of frog and rat. A spin label study.

The temperature-dependent fluidity of myelin of frog and rat peripheral nerve (Nervus ischiadicus) was studied using the spin label technique. In frog nerve a phase change was detected at 38 degree C. In rat nerve no sharp phase change could be established, and the lipid-depleted frog and rat nerve also showed no transition. From the spectral data, it was concluded that in frog and rat nerve the lipid-protein interactions are different, i.e. species dependent. Ca-2+-depletion of frog nerve caused a loss of transition, while rat nerve remained unaffected. Thus it was indicated that, in frog nerve, Ca-2+ is involved in the phase change. In the total lipid extract of frog nerve a phase change centered at 32 degree C occurred, while the total lipid extract of rat nerve again showed no transition. It is suggested that a connexion exists between our results and investigations on the temperature dependence of an axonal conduction block of nerve.

Animals↗

PubMed1985

Spin label study of the effect of ticlopidine on platelets.

The effect of ticlopidine on platelet membrane fluidity was investigated using a spin label technique. Ticlopidine, when orally administered to rats, increased both the order parameter and the motion parameter, indicating a decrease in the membrane fluidity of platelets. On the other hand, the order parameter and the motion parameter decreased markedly when the platelets were aggregated by thrombin. Ticlopidine inhibited the thrombin-induced aggregation of platelets and caused a slight increase in order parameter and motion parameter in thrombin-aggregated platelets. Judging from sodium dodecyl sulfate polyacrylamide gel electrophoresis, ticlopidine did not modify the electrophoretic pattern of platelet proteins appreciably. Ticlopidine decreased cholesterol/phospholipids molar ratio and increased slightly total amounts of proteins of the platelets. These results indicate that the inhibitory action by ticlopidine was accompanied by changes in membrane fluidity, and these changes were due to a perturbation of the membrane phospholipid core of the platelets by ticlopidine and/or its metabolites.

Animals↗

PubMed1986

[Thermitase--a thermostable serine protease. VIII. Kinetic and ESR investigations on the interactions of enzymes with spin labeled peptide methyl ketones].

An attempt was made to study the structure of the active center of thermitase by means of spin labeled peptide analogues. For this purpose, peptide methyl ketones SL-Alan-PheCH3 of different chain length (n = 0, 1, 2, 3; SL = 2,2,5,5-Tetramethyl-pyrrolin-1-oxyl-3-carbonyl) were synthesized. Synthesis and physico-chemical properties of these compounds are described and inhibition constants Ki for the interaction of these compounds with thermitase were measured. SL-Ala2-PheCH3 and SL-Ala3-PheCH3 are strong reversible inhibitors of thermitase with Ki values of 8.9 X 10(-6) M and 4.8 X 10(-7) M, respectively, whereas the analogous compounds with n = 0 and n = 1 represent only reduced affinity for this enzyme. ESR spectra of SL-Ala2-PheCH3 and SL-Ala3-PheCH3 in the presence of thermitase reveal that a great part of the SL residues of enzyme bound inhibitor molecules is not measurably restricted in their mobility whereas another part is hindered by unspecific interaction with the protein. The kinetic and ESR data are discussed with regard to the substrate binding region of the enzyme.

Binding Sites↗

PubMed1984

Self-adaptive modification of red-cell membrane lipids in lecithin: cholesterol acyltransferase deficiency. Lipid analysis and spin labeling.

In a patient with lecithin: cholesterol acyltransferase deficiency, free cholesterol was markedly increased, and esterified cholesterol was diminished. In the patient's plasma, an increase in phosphatidylcholine (PC) and a decrease in sphingomyelin were observed. Concomitantly, an increase in a shorter acyl chain 16:0 was noted in PC, sphingomyelin and phosphatidylethanolamine (PE). In contrast to these results, longer chains such as 22:0 and 24:0 were decreased, especially in sphingomyelin. Unsaturated double bonds such as 18:1 was also increased in PC and PE. In the red-cell membrane lipids, the increase in free cholesterol was counteracted by an increase in PC and by a decrease in sphingomyelin and PE, reflecting changes in the patient's plasma lipids. Increased 16:0 (in PC) and decreased 18:0 and 24:0 were observed. The increased plasma free cholesterol due to metabolic defect (lecithin: cholesterol acyltransferase deficiency) led to decreased red-cell membrane fluidity. This effect appeared to be counteracted by changing phospholipid composition (increased PC and decreased sphingomyelin and PE), by increasing shorter chains (16:0), by decreasing longer chains (18:0 and 24:0) and by increasing unsaturated double bonds (18:2). These results can be interpreted as a self-adaptive modification of lecithin: cholesterol acyltransferase deficiency-induced red-cell membrane abnormalities, to maintain normal membrane fluidity. This speculation was supported by the ESR spin-label studies on the patient's membrane lipids. The normal order parameters in intact red cells and in total lipid liposomes were decreased if cholesterol-depleted membrane liposomes were prepared. Thus, the hardening effect of cholesterol appeared to be counteracted by the softening effects described above. Overall membrane fluidity in intact red cells of the lecithin: cholesterol acyltransferase-deficient patient was maintained normally, judged by order parameters in ESR spin-label studies.

Cholesterol↗

PubMed2001

Molecular motion of spin labeled side chains in alpha-helices: analysis by variation of side chain structure.

Two single cysteine substitution mutants at helix surface sites in T4 lysozyme (D72C and V131C) have been modified with a series of nitroxide methanethiosulfonate reagents to investigate the structural and dynamical origins of their electron paramagnetic resonance spectra. The novel reagents include 4-substituted derivatives of either the pyrroline or pyrrolidine series of nitroxides. The spectral line shapes were analyzed as a function of side chain structure and temperature using a simulation method with a single order parameter and diffusion rates about three orthogonal axes as parameters. Taken together, the results provide strong support for an anisotropic motional model of the side chain, which was previously proposed from qualitative features of the spectra and crystal structures of spin labeled T4 lysozyme. Site-specific differences in apparent order parameter are interpreted in terms of backbone dynamics modes with characteristic correlation times in the nanosecond or faster time scale. The saturated 4-substituted pyrrolidine nitroxides are shown to be a suitable template for novel "functionalized" side chains designed to mimic salient features of the native side chains they replace.

Amino Acid Substitution↗

PubMed1990

Characterization of spin-labelled fatty acids and hematoporphyrin binding sites interactions in serum albumin.

Electron paramagnetic resonance (EPR) was used to investigate the spin-labelled fatty acid (SLFA) binding equilibrium to human (HSA) and bovine (BSA) serum albumin. The number of 5-doxyl stearate (5-DS) and 16-doxyl stearate (16-DS) binding sites on HSA and BSA were found to be equal, while the association constants, KA values (especially those of the primary binding site) were different. The applied EPR spectra analysis permitting a quantitative distinguishing between slow macromolecular rotation (pi c) and fast anisotropic motion (steric restriction, S) of bound SLFA, allowed SLFA oxazolidinyl ring mobility to be estimated. The 5-DS nitroxide radical is completely immobilized within the HSA protein matrix (S approximately 1.0, pi c approximately 56 +/- 1 ns). The 5-DS when bound to BSA exhibited the presence of more extensive fluctuations (lower S and pi c values) and its immersion depth with respect to BSA surface was calculated to be 4 +/- 2 A. The 16-DS oxazolidinyl radical bound to HSA was found to undergo moderated fluctuations (both S and pi c are smaller with respect to 5-DS) and it is buried deeper within the protein core (rimm = 10 +/- 2 A with respect to BSA surface). The tetrapyrrole ligands hematoporphyrin (Hp) and hematoporphyrin derivative (HpD) were found to induce well detectable changes in the SLFA binding patterns to serum albumin. The action mode was determined to be different for 16-DS (primary) and 5-DS (secondary) serum albumin binding sites: (i) 5-DS is extruded from several binding sites accompanied by an increase in KA in the remaining ones; (ii) simultaneous binding of 16-DS and Hp consists of cooperative and non-cooperative phases (both the number of the independent sites and the parameter of cooperativity, alpha, being dependent on Hp/HSA ratio); (iii) in principal the mobilities of 5-DS and 16-DS bound to HSA are changed, depending on the porphyrin/HSA ratio; and (iv) the effective immersion depth of the paramagnetic centres with respect to the protein surface is increased when Hp is present as a second ligand (rimm = 7 +/- 2 and 16 +/- 2 A for 5-DS and 16-DS, respectively).

Binding Sites↗