Electron paramagnetic resonance measurements of the hydration of Mn(II) in ternary complexes with GDP and ras p21 proteins

Electron paramagnetic resonance (EPR) spectroscopy has been used to determine the hydration numbers of Mn(II) in complexes with GDP and three forms of ras p21. EPR signals of Mn(II) in the GDP complex with viral-Harvey p21pRAS1 (Arg 12, Thr 59), p21EC (Gly 12, Thr 59), and p21EJ (Val 12, Thr 59) have narrow line-widths that permit ready observation of inhomogeneous broadening from unresolved superhyperfine coupling with the nuclear spin of 17O of directly coordinated oxygen ligands. Quantitative analysis of the lineshapes for the samples in H2 17O-enriched water indicates that four water ligands coordinate to the metal ion in the GDP complexes with all three proteins. The four solvent ligands, together with an oxygen from the beta-phosphate group of GDP, leave space for only one ligand from the protein. An X-ray diffraction-derived model for the MgII beta-gamma-imidoguanosine-5'-triphosphate complex with p21 shows coordination of Mg(II) to the beta- and gamma-phosphate groups of the nucleotide as well as to the hydroxyl groups of Thr 35 and Ser 17 (Pai, E.F., Kabusch, W., Krengel, U., Holmes, K. H., John, J., and Wittinghofer, A., 1989, Nature (London) 341, 209-214). Thus, upon conversion of the nucleotide from a triphosphate to a diphosphate, solvent replaces both the gamma-phosphate of the nucleotide and one of the protein ligands. The EPR results are consistent with a recent X-ray crystallographic model for the p21-MgIIGDP complex (Milburn, M. V., Tong, L., DeVos, A. M., Brunger, A., Yamaizumi, Z., Nishimura, S., and Kim, S.-H., 1990, Science 247, 939-945). EPR spectra of complexes with the three forms of ras p21 differ with respect to the intrinsic linewidths of the EPR signals. These subtle differences in linewidth appear to originate from slight differences in local disorder near the metal-nucleotide binding site.     

Electron paramagnetic resonance studies of spectrin-phospholipid associations

The interaction of spectrin, a peripheral cytoplasmic protein of the erythrocyte membrane, with synthetic phospholipids was characterized by density gradient centrifugation, electron microscopy, and the paramagnetic resonance of nitroxide spin labels. The organic solvent 2-chloroethanol, which favors the stability of hydrophobic surfaces on proteins, was utilized in the formation of the protein-lipid systems. Spectrin, upon dialysis to remove 2-chloroethanol, was found to associate into extensive network-like aggregates and in the presence of dipalmitoylphosphatidylcholine, the spectrin aggregates were found to associate with liposomes formed during dialysis. This interaction, which was significantly enhanced by the presence of dipalmitoylphosphatidylethanolamine, was found to reduce the mobility of fatty acid spin labels incorporated into the lipid regions of the lipid-protein associations. Evidence was found which suggests that spectrin tends to stabilize the phospholipid vesicles against fusion and decrease lipid mobility, particularly near the polar bilayer surfaces.     

Electron paramagnetic resonance studies of the iron-sulfur centers from complex I of Rhodothermus marinus

Rhodothermus marinus, a thermohalophilic gram negative bacterium, contains a type I NADH/quinone oxidoreductase (complex I). Its purification was optimized, yielding large amounts of pure and active protein. Furthermore, the stoichiometry of NADH oxidation and quinone reduction was shown to be 1:1. The large amounts of protein enabled a thorough characterization by electron paramagnetic resonance (EPR) spectroscopy at different temperatures and microwave powers, using NADH, NADPH, and dithionite as reducing agents. A minimum of two [2Fe-2S](2+/1+) and four [4Fe-4S](2+/1+) centers were observed in the purified complex. Redox titrations monitored by EPR spectroscopy made possible the determination of the reduction potentials of the iron-sulfur centers; with the exception of one of the [4Fe-4S](2+/1+) centers, which has a lower reduction potential, all the other centers have reduction potentials of -240 +/- 20 mV, pH 7.5.     

Electron paramagnetic resonance studies of Pseudomonas cytochrome c peroxidase

The EPR spectrum at 15 K of Pseudomonas cytochrome c peroxidase, which contains two hemes per molecule, is in the totally ferric form characteristic of low-spin heme giving two sets of g-values with gz 3.26 and 2.94. These values indicate an imidazole-nitrogen : heme-iron : methionine-sulfur and an imidazole-nitrogen : heme-iron : imidazole-nitrogen hemochrome structure, respectively. The spectrum is essentially identical at pH 6.0 and 4.6 and shows only a very small amount of high-spin heme iron (g 5--6) also at 77 K. Interaction between the two hemes is shown to exist by experiments in which one heme is reduced. This induces a change of the EPR signal of the other (to gz 2.83, gy 2.35 and gx 1.54), indicative of the removal of a histidine proton from that heme, which is axially coordinated to two histidine residues. If hydrogen peroxide is added to the partially reduced protein, its EPR signal is replaced by still other signals (gz 3.5 and 3.15). Only a very small free radical peak could be observed consistent with earlier mechanistic proposals. Contrary to the EPR spectra recorded at low temperature, the optical absorption spectra of both totally oxidized and partially reduced enzyme reveal the presence of high-spin heme at room temperature. It seems that a transition of one of the heme c moieties from an essentially high-spin to a low-spin form takes place on cooling the enzyme from 298 to 15 K.     

Electron paramagnetic resonance studies of membrane proteins in hepatic microsomes

Hepatic microsomal membranes, prepared under various conditions that yield either ‘intact’ or ‘disrupted’ microsomal vesicles, have been labeled via the sulfhydryl groups of intrinsic membrane proteins using nitroxide analogs of $\text{N-}\text{ethylmaleimide}$. Electron paramagnetic resonance spectra revealed the presence of two dominant classes of bound label corresponding to differing degrees of immobilization, the ratio of which were quantitated using a parameter designated the ‘$\text{W/S}$’ ratio. For latent microsomes, the value of this parameter was determined to be $\text{0.65 ± 0.02}$ and was influenced by factors such as label/protein ratio, incubation period, nitroxide structure, temperature and pH. The $\text{W/S}$ ratio was also sensitive to the degree of membrane integrity as revealed by the latency of mannose 6-phosphate activity of glucose-6-phosphohydrolase. In addition, membrane disruption resulted in a corresponding decrease in the order parameter for nitroxide-labeled fatty acids intercalated within the lipid bilayer. The $\text{W/S}$ ratio was observed to be dependent upon the method of microsome preparation yielding values of $\text{1.02 ± 0.02}$ for ‘hypertonically disrupted’ vesicles and $\text{1.28 ± 0.02}$ for ‘mechanically disrupted’ vesicles. Microsomal marker enzymes such as cytochrome $\text{P-450}$ and FAD-containing monooxygenase retained significant levels of functionally following nitroxide incorporation.     

Electron paramagnetic resonance studies of zinc-substituted reaction centers from Rhodopseudomonas viridis.

The primary quinone acceptor radical anion Q(A)(-)(*) (a menaquinone-9) is studied in reaction centers (RCs) of Rhodopseudomonas viridis in which the high-spin non-heme Fe(2+) is replaced by diamagnetic Zn(2+). The procedure for the iron substitution, which follows the work of Debus et al. [Debus, R. J., Feher, G., and Okamura, M. Y. (1986) Biochemistry 25, 2276-2287], is described. In Rps. viridisan exchange rate of the iron of approximately 50% +/- 10% is achieved. Time-resolved optical spectroscopy shows that the ZnRCs are fully competent in charge separation and that the charge recombination times are similar to those of native RCs. The g tensor of Q(A)(-)(*) in the ZnRCs is determined by a simulation of the EPR at 34 GHz yielding g(x) = 2.00597 (5), g(y) = 2.00492 (5), and g(z) = 2.00216 (5). Comparison with a menaquinone anion radical (MQ(4)(-)(*)) dissolved in 2-propanol identifies Q(A)(-)(*) as a naphthoquinone and shows that only one tensor component (g(x)) is predominantly changed in the RC. This is attributed to interaction with the protein environment. Electron-nuclear double resonance (ENDOR) experiments at 9 GHz reveal a shift of the spin density distribution of Q(A)(-)(*) in the RC as compared with MQ(4)(-)(*) in alcoholic solution. This is ascribed to an asymmetry of the Q(A) binding site. Furthermore, a hyperfine coupling constant from an exchangeable proton is deduced and assigned to a proton in a hydrogen bond between the quinone oxygen and surrounding amino acid residues. By electron spin-echo envelope modulation (ESEEM) techniques performed on Q(A)(-)(*) in the ZnRCs, two (14)N nuclear quadrupole tensors are determined that arise from the surrounding amino acids. One nitrogen coupling is assigned to a N(delta)((1))-H of a histidine and the other to a polypeptide backbone N-H by comparison with the nuclear quadrupole couplings of respective model systems. Inspection of the X-ray structure of Rps. viridis RCs shows that His(M217) and Ala(M258) are likely candidates for the respective amino acids. The quinone should therefore be bound by two H bonds to the protein that could, however, be of different strength. An asymmetric H-bond situation has also been found for Q(A)(-)(*) in the RC of Rhodobacter sphaeroides. Time-resolved electron paramagnetic resonance (EPR) experiments are performed on the radical pair state P(960)(+) (*)Q(A)(-)(*) in ZnRCs of Rps. viridis that were treated with o-phenanthroline to block electron transfer to Q(B). The orientations of the two radicals in the radical pair obtained from transient EPR and their distance deduced from pulsed EPR (out-of-phase ESEEM) are very similar to the geometry observed for the ground state P(960)Q(A) in the X-ray structure [Lancaster, R., Michel, H. (1997) Structure 5, 1339].     

Electron paramagnetic resonance backbone dynamics studies on spin-labelled neuropeptide Y analogues.

Neuropeptide Y (NPY) is one of the most abundant peptides in the central nervous system of mammalians. NPY acts by binding to at least five G-protein coupled receptors (GPCRs) which have been named Y1, Y2, Y4, Y5 and Y6. Three spin-labelled NPY analogues containing the nitroxide group of the amino acid TOAC (2.2.6.6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid) as a paramagnetic probe were synthesized by solid-phase peptide synthesis. Synthetic problems owing to the sensitivity of nitroxide towards acidic and reducing conditions have been overcome by using a cleavage cocktail that contains anisole and cresol scavengers. Concerning the receptor binding preferences, the analogues [TOAC34]-pNPY and [Ala31, TOAC32]-pNPY showed a marked selectivity for the Y5 receptor, while [TOAC2]-pNPY maintained a significant binding also to the Y2 receptor subtype. The modifications of the native peptide structure caused by the introduction of TOAC were examined by circular dichroism. In order to determine the rotational correlation time of the spin probes, electron paramagnetic resonance measurements were performed in solution and in the presence of liposomes. This allowed us to evaluate the backbone dynamics of the different parts of the NPY molecule in the free and membrane bound states. The results of these studies showed that NPY Interacts with liposomes by using the C-terminal alpha-helix while the N-terminal tail retains a flexibility that is comparable to that of the peptide in solution as already shown by NMR studies on DPC micelles. Furthermore, we demonstrated that TOAC-labelllng is a valuable tool to investigate changes in the backbone conformation and dynamics. This may be of major importance for peptides and small proteins when they bind to cell membranes.     

Regulation of p21ras activity.

The ras genes encode GTP/GDP-binding proteins that participate in mediating mitogenic signals from membrane tyrosine kinases to downstream targets. The activity of p21ras is determined by the concentration of GTP-p21ras, which is tightly regulated by a complex array of positive and negative control mechanisms. GAP and NF1 can negatively regulate p21ras activity by stimulating hydrolysis of GTP bound to p21ras. Other cellular factors can positively regulate p21ras by stimulating GDP/GTP exchange.     

Electron paramagnetic resonance studies and effects of vanadium in Saccharomyces cerevisiae

Vanadium uptake by whole cells and isolated cell walls of the yeast Saccharomyces cerevisiae was studied. When orthovanadate was added to wild-type S. cerevisiae cells growing in rich medium, growth was inhibited as a function of the VO₄ ^3- concentration and the growth was completely arrested at a concentration of 20 mM of VO₄ ^3- in YEPD. Electron paramagnetic resonance (EPR) spectroscopy was used to obtain structural and dynamic information about the cell-associated paramagnetic vanadyl ion. The presence of EPR signals indicated that vanadate was reduced by whole cells to the vanadyl ion. On the contrary, no EPR signals were detected after interaction of vanadate with isolated cell walls. A mobile and an immobile species associated in cells with small chelates and with macromolecular sites, respectively, were identified. The value of rotational correlation time _r indicated the relative motional freedom at the macromolecular site. A strongly immobilized vanadyl species bound to polar sites mainly through coulombic attractions was detected after interaction of VO²⁺ ions with isolated cell walls.     

Electron paramagnetic resonance study of carp methemoglobin.

The g anisotropy of the EPR spectra of carp azidomethemoglobin is found to be pH-dependent, whereas, the spectra of human azidomethemoglobin are not. The two hemoglobins have the same g values at alkaline pH values. Crystal field analysis yielded values of 2.25 and 3.31, respectively, for the rhombic distortion, V/lambda, and the tetragonal distortion, delta/lambda. The spin orbit coupling constant is lambda. At pH 4.0 the values of V/lambda and delta/lambda for carp azidomethemoglobin became 1.95 and 4.76, respectively, whereas those for the human hemoglobin are virtually unchanged. The results are interpreted to mean an increase of out-ofplane displacement of the iron atom and stabilization of the T form of carp azidomethemoglobin by high proton concentration. At pH 6.0 and lower, the EPR spectra of carp azidomethemoglobin showed the presence of about 1.5% of high spin species, the amount is not affected by excess of either inositol hexaphosphate or sodium azide. The EPR spectra of aquo- and fluoroderivatives of carp methemoglobin were not affected by pH changes.     

Electron paramagnetic resonance studies on the reduction of the components of complex I and transhydrogenase-inhibited complex I by NADH and NADPH.

NADH treatment of complex I at pH 7–8 results in the appearance of electron paramagnetic resonance (epr) signals at x band due to reduced ironsulfur centers 1, 2, 3 and 4, while NADPH treatment gives rise to the appearance of signals due to centers 2 and 3. Similar results are obtained with complex I preparations in which transhydrogenase activity from NADPH to NAD has been >95% inhibited by treatment of the complex with trypsin. At pH 6.5 and in the presence of rotenone, addition of NADPH to complex I or transhydrogenase-inhibited complex I results in partial reduction of iron-sulfur center 1 as well. These and other experiments with reduced 3-acetylpyridine adenine dinucleotide and NADPH + NAD as substrates have suggested that the differences in the reduction of complex I iron-sulfur centers by the above nucleotides are essentially quantitative and related to (a) the dehydrogenation rate of the nucleotides, and (b) autoxidation of complex I components under the epr experimental conditions.     

Electron paramagnetic resonance studies of membrane fluidity in ozone-treated erythrocytes and liposomes

Doxyl stearate spin probes which differed in the attachment of the nitroxide free radical to the fatty acid have been used to study membrane fluidity in ozone-treated bovine erythrocytes and liposomes. Analysis of EPR spectra of spin labels incorporated into lipid bilayer of the erythrocyte membranes indicates an increase in the mobility and decrease in the order of membrane lipids. In isolated erythrocyte membranes (ghosts) the most significant changes were observed for 16-doxylstearic acid. In intact erythrocytes statistically significant were differences for 5-doxylstearic acid. The effect of ozone on liposomes prepared from a lipid extract of erythrocyte lipids was marked in the membrane microenvironment sampled by all spin probes. Ozone apparently leads to alterations of membrane dynamics and structure but does not cause increased rigidity of the membrane.