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Query: UNIPROT:O14944 (EPR)
 13,097 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The electronic term scheme of ferric iron in metmyoglobin, metmyoglobin fluoride, and methemoglobin is evaluated by a Hamiltonian which involves the Coulomb repulsion of the 3d electrons, their interaction with the C2v-coordinated ligands, and spin-orbit coupling. The adjustable parameters of the theory were determined by a least squares fit to experimental EPR, susceptibility, and far-infrared data reported in the literature. According to these results, the structural properties of the ferric ion and its neighboring ligands were discussed by means of group theoretical arguments: An increasing out of plane position of the ferric ion is found in the sequence metHb--metMb--MbF which corresponds to an increasing binding strength with the axial ligands.
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PMID:Electronic structure and spatial arrangement of C2v-coordinated ferric iron in metmyoglobin, metmyoglobin fluoride, and methemoglobin. 17 97

Human apohemoglobin (globin) was spin-labeled at the beta-93 sulfhydryl groups with 2,2,5,5-tetramethyl-3-aminopyrrolidine-I-oxyl. Spin-labeled globin exhibited an EPR spectra that is less immobilized than that of spin-labeled hemoglobin, indicating the conformational difference in the vicinity of the label between hemoglobin and globin. Spectrophotometric titration of spin-labeled globin with protohemin showed that 1 mol of globin (on the tetramer basis) combines with 4 mol of hemin, producing a holomethemoglobin spectrophotometrically indistinguishable from native methemoglobin. The EPR spectrum was also changed strikingly upon the addition of protohemin. This change, however, was not proportional to the amount of hemin added, but marked changes occurred after 3 to 4 mol of hemin were mixed with 1 mol of spin-labeled globin. The EPR spectrum of spin-labeled hemoglobin thus prepared was identical with that prepared by direct spin labeling to methemoglobin. These results suggest the preferential binding of hemin to alpha-globin chains in the course of heme binding by globin. This assumption was further confirmed by preparing spin-labeled semihemoglobin in which only one kind of chain contained hemin (alpha h betaO SL and alpha O beta h SL). The EPR spectrum of the alpha h beta O SL molecule showed a slightly immobilized EPR spectrum, similar to that of spin-labeled globin mixed with 50% of the stoichiometric amount of hemin. On the other hand, the alpha O beta h SL molecule showed a distinctly different EPR signal from that of globin half-saturated with hemin, and showed an intermediate spectrum between those of beta h SL and alpha h beta h SL. These results indicate that heme binding to globin chains brings about a major conformational change in the protein moiety and that chain-chain association plays a secondary role. We conclude that hemin binds preferentially to alpha-globin chains and that the conformation of globin changes rapidly to that of methemoglobin after all four hemes are attached to globin heme pockets.
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PMID:Spin label studies on conformational changes of aphohemoglobin due to heme binding. 18 21

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.
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PMID:Electron paramagnetic resonance study of carp methemoglobin. 19 Feb 29

The low temperature EPR spectrum of a quickly reacted mixture of oxyhemoglobin and phenylhydrazine was studied. With the use of a computer, the spectral contribution of methemoglobin in the region of g = 2 was subtracted. The remaining spectrum was that of an axial free radical (g perpendicular = 2.00, g parallel = 2.06) having the magnetic parameters of superoxide anion. In the presence of superoxide dismutase, this axial radical was not seen, confirming that superoxide anion is indeed generated by the reaction.
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PMID:The detection of superoxide anion from the reaction of oxyhemoglobin and phenylhydrazine using EPR spectroscopy. 22 81

The properties of human methemoglobin have been investigated under a wide variety of conditions to determine its conformation and to test for evidence of the T state conformation which has been proposed by Perutz to exist in the presence of high spin ligands and inositol hexaphosphate (IHP). Subunit dissociation was measured as a criterion for the T state since marked differences in the tetramer-dimer equilibrium exist for oxyhemoglobin (R state) and deoxyhemoglobin (T state). In the absence of IHP, complexes of methemoglobin with both high spin ligands (water, fluoride) or low spin ligands (azide, cyanide) show extensive dissociation in 2,2-bis(hydroxymethyl)-2,2',2"-nitriloethanol buffers, pH 6, 0.1 M NaCl, with values of the tetramer-dimer dissociation constant (K4,2) near 10-5 M. The addition of IHP lowers K4,2 to a value near 10-5 M for all forms of methemoglobin. Combination of IHP with methemoglobin promotes a conformational change, but the change is apparently independence of spin state. The conformation acquired in the presence of IHP is not identical with the T state (K4,2 similar to 10-12 M) and can also occur with hemoglobin in the ferrous form, as revealed by a substantial reduction in K4,2 for CO-hemoglobin upon addition of IHP. Subunit dissociation has also been measured using the haptoglobin reaction, since haptoglobin binds only to hemoglobin dimers. The haptoglobin experiments give results that are qualitatively in agreement with the conclusions reached by ultracentrifuge measurements. Similar results are also obtained by estimating the degree of dissociation on the basis of the material which aggregates following mixing with dithionite. The effect of IHP on azide-binding kinetics with methemoglobin has also been examined. Changes in reactivity is observed upon addition of IHP, but the principal effect is observed upon addition of IHP, but the principal effect is an enhancement of the rate of reaction of the beta chains. Changes in the reactivity of the beta93 sulfhydryl group of methemoglobin also accompany addition of IHP, but in a manner which is largely independent of the spin state of the iron. Similar changes are again found with CO-hemoglobin upon addition of IHP. The rate of binding of bromthymol blue also shows some changes upon addition of IHP, but the changes are more pronounced for deoxyhemoglobin than for methemoglobin. Since the results obtained did not appear to indicate a significant role for spin state in the changes observed, additional studies were undertaken using EPR spectroscopy.
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PMID:Conformation and spin state in methemoglobin. 23 44

Nonenzymatic reduction of methemoglobin (met-Hb) is effectively catalyzed by NADH and riboflavin (RF). At low concentrations of met-Hb the EPR spectra of the ternary complex are characterized by an increased, whereas those at high met-Hb concentrations--by a decreased intensity of the RF semiquinone signal, which suggests that the met Hb reduction by a Leu-form of RF proceeds at a higher rate than that by the RF semiquinone radical. Riboflavin also accelerates the met-Hb reduction by a NADH-Fe2+ or NADH-Fe(2+)-EDTA mixture. It was found that the rate of met-Hb reduction by the organic radical increases in the following order: NAD. < RF H. < RF H2.
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PMID:[Nonenzymatic reduction of methemoglobin by free radical forms of NADH and riboflavin]. 134 94

Temperature-dependent EPR and temperature-jump measurements have been carried out, in order to examine the high-spin to low-spin transition of aquomethemogobin (pH 6.0). Relaxation rates and equilibrium constants could be determined as a function of temperature. As a reaction mechanism for the high-spin to low-spin transition, the binding of N epsilon of His E7 to the heme iron had been proposed; the same mechanism had been suggested for the ms-effect, found in temperature-jump experiments on aquomethemoglobin. A comparison of the thermodynamic quantities, deduced form the measurements in this paper, gives evidence that indeed the same reaction is investigated in both cases. Our results and most of the findings of earlier studies on the spin-state transitions of aquomethemoglobin, using susceptibility, optical, or EPR measurements, can be explained by the transition of methemoglobin with H2O as ligand (with high-spin state at all temperatures) and methemoglobin with ligand N epsilon of His E7 (with a low-spin ground state). Thermal fluctuations of large amplitude have to be postulated for the reaction to take place, so this reaction may be understood as a probe for the study of protein dynamics.
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PMID:Conformational transition of aquomethemoglobin: intramolecular histidine E7 binding reaction to the heme iron in the temperature range between 220 K and 295 K as seen by EPR and temperature-jump measurements. 254 30

The residual motion of spin labels bound to cysteine beta 93 of methemoglobin and oxyhemoglobin has been analyzed as a function of temperature and hydration. The rotational diffusion of the whole protein molecule has been prevented or restricted by crystallization, lyophilization or by high viscosity of the solution. The residual motion of the labels is characterized by an angle of the limited motion cone and their rotational correlation time using computer simulations of the EPR spectra. Two types of motion can be separated due to different correlation times and different dependences on temperature and hydration. One of these motional mechanisms can be shown to be determined by protein fluctuations. Correlation times of these fluctuations decrease from 2 X 10(-8) s at T = 220 K to 10(-9) s at T = 300 K in the samples of high water concentration. Strong correlation between the properties of the hydration shell and these fluctuations are observed.
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PMID:Residual motion of hemoglobin-bound spin labels as a probe for protein dynamics. 254 17

In an effort to elucidate the mechanism of the initiation reaction of the denaturation of oxyhemoglobin, I, to methemoglobin, II, by hydrazines, we have investigated by electron paramagnetic resonance, EPR, and visible spectrophotometry at 22 degrees C, pH 7.4, the reaction of I with 1,1-diphenyl-2-picrylhydrazine, III, and 2,2-diphenyl-1-picrylhydrazyl, IV, in dimethylsulphoxide/buffer and methanol/buffer mixtures, these organic solvents included at a concentration of 10 v/v% to render III and IV soluble while not causing appreciable denaturation of I. In both buffer mixtures, the results obtained were the same. For the I/III reaction mixtures, although the spectrophotometric data obtained showed denaturation to occur, there was no EPR evidence for formation of IV, contrary to expectation based on the chemical structure of III. The EPR observations on each I/IV reaction mixture showed a rapid decrease in IV signal intensity to a value that, depending on the initial reactant concentrations, was either below the detection limit or, when measurable, constant with time. The results of similar EPR measurements on analogous II/IV reaction mixtures were the same. These EPR results are compatible with the idea that IV forms a complex with the protein moiety of I and II, and show that the I/III reaction could yield IV and thus involve a one-electron transfer process.
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PMID:The reaction of oxyhemoglobin with 1,1-diphenyl-2-picrylhydrazine and 2,2-diphenyl-1-picrylhydrazyl. 283 Mar 66

The oxidation reaction with nitrite of the dimeric and tetrameric hemoglobins from the mollusc Scapharca inaequivalvis has been studied kinetically and at equilibrium. In line with previous findings obtained with ferricyanide as oxidant, in both proteins the stable oxidation product is a hemichrome, although the nitrite-methemoglobin complex is formed in significant amount when excess nitrite is employed. The reaction kinetics are characterized by a lag period followed by an autocatalytic phase, as in the case of human hemoglobin. However, with respect to human hemoglobin, in the two molluscan proteins the lag phase is prolonged significantly due to the instability of their met-form, an obligatory intermediate for the onset of autocatalysis. All the data obtained in spectrophotometric, EPR and sedimentation velocity experiments under a variety of experimental conditions conform to the reaction mechanism proposed for human hemoglobin (Spagnuolo et al., Biochim. Biophys. Acta 911 (1987) 59-63) provided hemichrome formation and nitrite binding are taken into account.
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PMID:Oxidation reaction of Scapharca inaequivalvis hemoglobins with nitrite. 284 66


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