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Keiji Shikama - One of the best experts on this subject based on the ideXlab platform.
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the swinging movement of the distal Histidine Residue and the autoxidation reaction for midge larval hemoglobins
FEBS Journal, 2003Co-Authors: Satoshi Kamimura, Ariki Matsuoka, Kiyohiro Imai, Keiji ShikamaAbstract:Some insects have a globin exclusively in their fast-growing larval stage. This is the case in the 4th-instar larva of Tokunagayusurika akamusi, a common midge found in Japan. In the polymorphic hemoglobin comprised of 11 separable components, hemoglobin VII (Ta-VII Hb) was of particular interest. When its ferric met-form was exposed to pH 5.0 from 7.2, the distal Histidine was found to swing away from the E7 position. As a result, the iron(III) was converted from a hexacoordinate to a pentacoordinate form by a concomitant loss of the axial water ligand. The corresponding spectral changes in the Soret band were therefore followed by stopped-flow and rapid-scan techniques, and the observed first-order rate constants of k(out) = 25 s(-1) and kin = 128 s(-1) were obtained for the outward and inward movements, respectively, of the distal Histidine Residue in 0.1 m buffer at 25 degrees C. For O2 affinity, Ta-VII Hb showed a value of P50 = 1.7 Torr at pH 7.4, accompanied with a remarkable Bohr effect (deltaH+ = -0.58) almost equal to that of mammalian hemoglobins. We have also investigated the stability property of Ta-VII HbO2 in terms of the autoxidation rate over a wide range of pH from 4 to 11. The resulting pH-dependence curve was compared with those of another component Ta-V HbO2 and sperm whale MbO2, and described based on a nucleophilic displacement mechanism. In light of the O2 binding affinity, Bohr effect and considerable stability of the bound O2 against acidic autoxidation, we conclude that T. akamusi Hb VII can play an important role in O2 transport and storage as the major component in the larval hemolymph.
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dual nature of the distal Histidine Residue in the autoxidation reaction of myoglobin and hemoglobin comparison of the h64 mutants
FEBS Journal, 2000Co-Authors: Tatsuya Suzuki, Ariki Matsuoka, Yohei Watanabe, Masashi Nagasawa, Keiji ShikamaAbstract:The oxygenated form of myoglobin or hemoglobin is oxidized easily to the ferric met-form with generation of the superoxide anion. To make clear the possible role(s) of the distal Histidine (H64) Residue in the reaction, we have carried out detailed pH-dependence studies of the autoxidation rate, using some typical H64 mutants of sperm whale myoglobin, over the wide range of pH 5–12 in 0.1 m buffer at 25 °C. Each mutation caused a dramatic increase in the autoxidation rate with the trend H64V ≥ H64G ≥ H64L ≫ H64Q > H64 (wild-type) at pH 7.0, whereas each mutant protein showed a characteristic pH-profile which is essentially different from that of the wild-type or native sperm whale MbO2. In particular, all the mutants have lost the acid-catalyzed process that can play a dominant role in the autoxidation reaction of most mammalian myoglobins or hemoglobins. Kinetic analyses of various types of pH-profiles lead us to conclude that the distal Histidine Residue can play a dual role in the nucleophilic displacement of O2− from MbO2 or HbO2 in protic, aqueous solution. One is in a proton-relay mechanism via its imidazole ring, and the other is in the maximum protection of the FeO2 center against a water molecule or an hydroxyl ion that can enter the heme pocket from the surrounding solvent.
Karen A Maubach - One of the best experts on this subject based on the ideXlab platform.
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role of the Histidine Residue at position 105 in the human α5 containing gabaa receptor on the affinity and efficacy of benzodiazepine site ligands
British Journal of Pharmacology, 2002Co-Authors: M D Kelly, Alison J Smith, G Banks, Peter B Wingrove, P W Whiting, John R Atack, Guy R Seabrook, Karen A MaubachAbstract:1. A Histidine Residue in the N-terminal extracellular region of alpha 1,2,3,5 subunits of the human GABA(A) receptor, which is replaced by an arginine in alpha 4 and alpha 6 subunits, is a major determinant for high affinity binding of classical benzodiazepine (BZ)-site ligands. The effect of mutating this Histidine at position 105 in the alpha 5 subunit to an arginine (alpha 5H105R) on BZ-site pharmacology has been investigated using radioligand binding on HEK293 and L(tk-) cells and two electrode voltage clamp recording on Xenopus oocytes in which GABA(A) receptors of subtypes alpha 5, alpha 5H105R, alpha 4 and alpha 6 were co-expressed with beta 3 gamma 2s. 2. The classical BZs, diazepam and flunitrazepam (full agonists on the alpha 5 receptor) showed negligible affinity and therefore negligible efficacy on alpha 5H105R receptors. The beta-carbolines DMCM and beta CCE (inverse agonists on the alpha 5 receptor) retained some affinity but did not exhibit inverse agonist efficacy at alpha 5H105R receptors. Therefore, the alpha 5H105R mutation confers an alpha 4/alpha 6-like pharmacology to the classical BZs and beta-carbolines. 3. Ro15-4513, flumazenil, bretazenil and FG8094, which share a common imidazobenzodiazepine core structure, retained high affinity and were higher efficacy agonists on alpha 5H105R receptors than would be predicted from an alpha 4/alpha 6 pharmacological profile. This effect was antagonized by DMCM, which competes for the BZ-site and therefore is likely to be mediated via the BZ-site. 4. These data indicate that the conserved Histidine Residue in the alpha subunit is not only a key determinant in the affinity of BZ-site ligands on alpha 5 containing GABA(A) receptors, but also influences ligand efficacy.
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Role of the Histidine Residue at position 105 in the human alpha 5 containing GABA(A) receptor on the affinity and efficacy of benzodiazepine site ligands.
2002Co-Authors: Smith A, Banks G, Wingrove P, Atack J, P W Whiting, Gr Seabrook, Karen A MaubachAbstract:1. A Histidine Residue in the N-terminal extracellular region of alpha 1,2,3,5 subunits of the human GABA(A) receptor, which is replaced by an arginine in alpha 4 and alpha 6 subunits, is a major determinant for high affinity binding of classical benzodiazepine (BZ)-site ligands. The effect of mutating this Histidine at position 105 in the alpha 5 subunit to an arginine (alpha 5H105R) on BZ-site pharmacology has been investigated using radioligand binding on HEK293 and L(tk-) cells and two electrode voltage clamp recording on Xenopus oocytes in which GABA(A) receptors of subtypes alpha 5, alpha 5H105R, alpha 4 and alpha 6 were co-expressed with beta 3 gamma 2s. 2. The classical BZs, diazepam and flunitrazepam (full agonists on the alpha 5 receptor) showed negligible affinity and therefore negligible efficacy on alpha 5H105R receptors. The beta-carbolines DMCM and beta CCE (inverse agonists on the alpha 5 receptor) retained some affinity but did not exhibit inverse agonist efficacy at alpha 5H105R receptors. Therefore, the alpha 5H105R mutation confers an alpha 4/alpha 6-like pharmacology to the classical BZs and beta-carbolines. 3. Ro15-4513, flumazenil, bretazenil and FG8094, which share a common imidazobenzodiazepine core structure, retained high affinity and were higher efficacy agonists on alpha 5H105R receptors than would be predicted from an alpha 4/alpha 6 pharmacological profile. This effect was antagonized by DMCM, which competes for the BZ-site and therefore is likely to be mediated via the BZ-site. 4. These data indicate that the conserved Histidine Residue in the alpha subunit is not only a key determinant in the affinity of BZ-site ligands on alpha 5 containing GABA(A) receptors, but also influences ligand efficacy
Fraser A Armstrong - One of the best experts on this subject based on the ideXlab platform.
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control of myoglobin electron transfer rates by the distal nonbound Histidine Residue
Journal of the American Chemical Society, 1996Co-Authors: Bruce R Van Dyke, And Paul Saltman, Fraser A ArmstrongAbstract:Changing the distal Histidine (H64) of sperm-whale myoglobin into any of the following Residuesvaline, leucine, methionine, glycine, or phenylalaninecauses a dramatic improvement in the reversibility (electron-transfer kinetics) and reproducibility of the direct electrochemistry. Cyclic voltammograms of native or wild-type recombinant myoglobin are irreversible (in the electrochemical sense) and critically dependent on the condition of the sample and electrode. By contrast, the H64 mutants display quasi-reversible electrochemistry much more typical of results obtained with true electron-transfer proteins. The difference in activity correlates sharply with alterations to the distal-pocket H-bonding network, which in the native protein comprises the H2O that is coordinated to Fe(III), the Ne of H64, and the “lattice” extending from arginine-45 to the heme periphery. It is proposed that this H-bond network increases the electron-transfer activation energy by coupling the displacement of Fe(III)-coordinated H...
Martin Karplus - One of the best experts on this subject based on the ideXlab platform.
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hemoglobin bohr effects atomic origin of the Histidine Residue contributions
Biochemistry, 2013Co-Authors: Guishan Zheng, Michael Schaefer, Martin KarplusAbstract:The Bohr effect in hemoglobin, which refers to the dependence of the oxygen affinity on the pH, plays an important role in its cooperativity and physiological function. The dominant contribution to the Bohr effect arises from the difference in the pKa values of His Residues of the unliganded (deoxy) and liganded (carbonmonoxy) structures. Using recent high resolution structures, the Residue pKa values corresponding to the two structures are calculated. The method is based on determining the electrostatic interactions between Residues in the protein, relative to those of the Residue in solution, by use of the linearized finite difference Poisson–Boltzmann equation and Monte Carlo sampling of protonation states. Given that good agreement is obtained with the available experimental values for the contribution of His Residues in HbA to the Bohr effect, the calculated results are used to determine the atomic origin of the pKa shift between deoxy and carbonmonoxy HbA. The contributions to the pKa shift calculat...
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hemoglobin bohr effects analysis of the Histidine Residue contributions
2013Co-Authors: Guishan Zheng, Michael Schaefer, Martin KarplusAbstract:The Bohr effect in hemoglobin, which refers to the dependence of the oxygen affinity on the pH, plays an important role in its cooperativity and physiological function. The dominant contribution to the Bohr effect arises from the difference in the pKa values of this Residue of the unliganded (deoxy) and liganded (carbonmonoxy) structures. Using recent high resolution structures for the two states, the Residue pKa values corresponding to the two structures are calculated with a method based on determining the electrostatic interaction with the linearized finite difference Poisson-Boltzmann equation and Monte Carlo sampling of protonation states. Good agreement with the available experimental values is observed, so that a meaningful analysis of the origin of the pKa shift between the two structures can be made. By decomposing the contributions to the pKa shift, it is shown that the salt bridge involving His146 plays an important role in the alkaline Bohr effect, as suggested by Perutz, but many other interactions are significant as well. The method used is summarized and the implemented program is described in the Appendix.
Jacques Magdalou - One of the best experts on this subject based on the ideXlab platform.
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chemical modification of human udp glucuronosyltransferase ugt1 6 by diethyl pyrocarbonate possible involvement of a Histidine Residue in the catalytic process
Archives of Biochemistry and Biophysics, 1994Co-Authors: Eric Battaglia, M Pritchard, Mohamed Ouzzine, Sylvie Fournelgigleux, A Radominska, Gerard Siest, Jacques MagdalouAbstract:Chemical modification with diethyl pyrocarbonate (DEPC) of the recombinant human liver UDP-glucuronosyltransferase UGT1*6 in enriched membrane fractions from a V79 cell line resulted in a rapid inactivation of the glucuronidation reaction, measured with 4-methyl-umbelliferone as aglycone substrate, with a second-order rate constant of 3110 M-1.min-1 at pH 6.0 and 25 degrees C. The enzymatic activity was restored by hydroxylamine. Chemical modification with 0.2 mM DEPC for 60 s decreased the apparent Vmax 2.4-fold without significantly affecting the apparent Km toward 4-methylumbelliferone and UDP-glucuronic acid. Similarly, the binding of the photoactivatable cosubstrate analog [beta-32P]5-azido-UDP-glucuronic acid to the active site was not affected by the chemical modification. The enzyme was protected against this inactivation by 4-methylumbelliferone, suggesting that the modified Residue was located in or near the aglycone binding site. In contrast, the cosubstrate UDP-glucuronic acid potentiated the irreversible inhibition, indicating a conformational change in the protein upon binding. The pH-dependence of the inactivation was in agreement with the modification of an amino acid Residue with a pKa of 6.1. On the other hand, analysis of the variation of Vmax and Vmax/Km values of the glucuronidation reaction as a function of the pH revealed the presence of two essential Residues with a pKa within the range 5.7-6.0. The data of the chemical modification of the recombinant enzyme together with that of the pH dependence of the activity strongly suggest the involvement of a Histidine Residue, highly reactive toward DEPC, which could be the base catalyst of the glucuronidation reaction supported by human UGT1*6.