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PubMed1999

Role of glutamate-59 hydrogen bonded to N(3)H of the flavin mononucleotide cofactor in the modulation of the redox potentials of the Clostridium beijerinckii flavodoxin. Glutamate-59 is not responsible for the pH dependency but contributes to the stabilization of the flavin semiquinone.

The midpoint potentials for both redox couples of the noncovalently bound flavin mononucleotide (FMN) cofactor in the flavodoxin are known to be pH dependent. While the pH dependency for the oxidized-semiquinone (ox/sq) couple is consistent with the formation of the blue neutral form of the flavin semiquinone, that of the semiquinone-hydroquinone (sq/hq) couple is more enigmatic. The apparent pK(a) of 6.7 for this couple in the flavodoxin from Clostridium beijerinckii has been attributed to the ionization of the FMN(HQ); however, nuclear magnetic resonance data strongly suggest the FMN(HQ) remains anionic over the entire pH range testable. As an alternative explanation, a specific glutamate residue (Glu59 in this flavodoxin), which is hydrogen-bonded to N(3)H of the FMN, has been postulated to be the primary redox-linked proton acceptor responsible for the pH effect in some flavodoxins. This model was directly tested in this study by permanently neutralizing Glu59 by its replacement with glutamine. This conservative substitution resulted in an increase of 86 mV (at pH 7) in midpoint potential of the sq/hq couple; however, the pH dependency of this couple was not altered. Thus, the redox-linked protonation of Glu59 clearly cannot be responsible for this effect as proposed. The pH dependency of the ox/sq couple was also similar to wild type, but the midpoint potential has decreased by 65 mV (pH 7). The K(d) values for the oxidized, semiquinone, and hydroquinone complexes increased by 43-, 590-, and 20-fold, respectively, relative to the wild type. Thus, the Glu59 to glutamine substitution substantially effects the stability of the semiquinone but, on a relative basis, slightly favors the formation of the hydroquinone. On the basis of (1)H-(15)N HSQC nuclear magnetic resonance spectroscopic studies, the increased temperature coefficients for the protons on N(3) and N(5) of the reduced FMN in E59Q suggest that the hydrogen-bonding interactions at these positions are significantly weakened in this mutant. The increase for N(5)H correlates with the reduced stability of the FMN(SQ) and the more negative midpoint potential for the ox/sq couple. On the basis of the X-ray structure, an "anchoring" role is proposed for the side chain carboxylate of Glu59 that stabilizes the structure of the 50's loop in such a way so as to promote the crucial hydrogen-bonding interaction that stabilizes the flavin semiquinone, contributing to the low potential of this flavodoxin.

Circular Dichroism↗
PubMed1976

The effect of medium redox potential on the folate-limited growth of Lactobacillus casei var. rhamnosus.

The medium redox potential (Eh) influenced the folate-limited growth of Lactobacillus casei; the growth response was maximal at an Eh of +120 mV (pH 6-35). At raised Eh serum folate would support less growth than pteroylglutamic acid, and the response to N5-methyl tetrahydrofolic acid was intermediate between them. Pteroylglutamic acid was not destroyed during 24 h incubation at 37 degrees C in medium with Eh values between +40 and +440 mV. Destruction of N5-methyl tetrahydrofolic acid occurred within 24 h when the medium Eh was greater than +125 mV. Folate was taken up rapidly by L. casei with an Eh optimum at +270 mV.

Ascorbic Acid↗
PubMed1999

Glutathione redox potential in response to differentiation and enzyme inducers.

The reduced glutathione (GSH)/oxidized glutathione (GSSG) redox state is thought to function in signaling of detoxification gene expression, but also appears to be tightly regulated in cells under normal conditions. Thus it is not clear that the magnitude of change in response to physiologic stimuli is sufficient for a role in redox signaling under nontoxicologic conditions. The purpose of this study was to determine the change in 2GSH/GSSG redox during signaling of differentiation and increased detoxification enzyme activity in HT29 cells. We measured GSH, GSSG, cell volume, and cell pH, and we used the Nernst equation to determine the changes in redox potential Eh of the 2GSH/GSSG pool in response to the differentiating agent, sodium butyrate, and the detoxification enzyme inducer, benzyl isothiocyanate. Sodium butyrate caused a 60-mV oxidation (from -260 to -200 mV), an oxidation sufficient for a 100-fold change in protein dithiols:disulfide ratio. Benzyl isothiocyanate caused a 16-mV oxidation in control cells but a 40-mV oxidation (to -160 mV) in differentiated cells. Changes in GSH and mRNA for glutamate:cysteine ligase did not correlate with Eh; however, correlations were seen between Eh and glutathione S-transferase (GST) and nicotinamide adenine dinucleotide phosphate (NADPH):quinone reductase activities (N:QR). These results show that 2GSH/GSSG redox changes in response to physiologic stimuli such as differentiation and enzyme inducers are of a sufficient magnitude to control the activity of redox-sensitive proteins. This suggests that physiologic modulation of the 2GSH/GSSG redox poise could provide a fundamental parameter for the control of cell phenotype.

Adenocarcinoma↗
PubMed1983

Redox potential dependence of photophosphorylation and electron transfer in continuous illumination of Rhodopseudomonas sphaeroides chromatophores.

The dependence on redox potential (Eh) of the steady-state photophosphorylation rate in chromatophores of Rhodopseudomonas sphaeroides Ga was measured using slowly equilibrating (and hence less interfering) redox mediators. The remaining interference of the mediators was taken into account by extrapolating to zero mediator concentration. The extents of cytochrome redox reactions (in the presence of antimycin) and of the carotenoid shift were similarly measured. The redox titration of cytochrome c oxidation is consistent with a requirement for prior cytochrome c2 reduction and prior Q1 (primary electron acceptor) oxidation, while the titration of cytochrome b reduction is consistent with a requirement for prior (BChl)2 reduction and prior cytochrome b560 oxidation. The steady-state carotenoid shift extent is a much more broadly peaked function of Eh than is the extent following a single-turnover flash, indicating that the transmembrane electrical potential difference can be built up to significant levels by minimal rates of electron flow. The photophosphorylation rate, in contrast, is much more strongly Eh dependent, supporting the concept of a threshold membrane energization below which phosphorylation cannot occur. Earlier work by others showing a requirement for equilibrium cytochrome c2 reduction and Q1 oxidation is clearly confirmed. A much greater enhancement in phosphorylation rate was found at low Eh than has heretofore been reported. This threefold enhancement is discussed in relation to the equilibrium redox states of the ubiquinone-10 complement of the chromatophore membrane.

Bacterial Chromatophores↗
PubMed1997

Protein contributions to redox potentials of homologous rubredoxins: an energy minimization study.

The energetic contributions of the protein to the redox potential in an iron-sulfur protein are studied via energy minimization, comparing homologous rubredoxins from Clostridium pasteurianum, Desulfovibrio gigas, Desulfovibrio vulgaris, and Pyrococcus furiosus. The reduction reaction was divided into 1) the change in the redox site charge without allowing the protein to respond and 2) the relaxation of the protein in response to the new charge state, focusing on the latter. The energy minimizations predict structural relaxation near the redox site that agrees well with that in crystal structures of oxidized and reduced P. furiosus rubredoxin, but underpredicts it far from the redox site. However, the relaxation energies from the energy-minimized structures agree well with those from the crystal structures, because the polar groups near the redox site are the main determinants and the charged groups are all located at the surface and thus are screened dielectrically. Relaxation energies are necessary for good agreement with experimentally observed differences in reduction energies between C. pasteurianum and the other three rubredoxins. Overall, the relaxation energy is large (over 500 mV) from both the energy-minimized and the crystal structures. In addition, the range in the relaxation energy for the different rubredoxins is large (300 mV), because even though the structural perturbations of the polar groups are small, they are very near the redox site. Thus the relaxation energy is an important factor to consider in reduction energetics.

Amino Acid Sequence↗
PubMed2000

Redox potentials and their pH dependence of D-amino-acid oxidase of Rhodotorula gracilis and Trigonopsis variabilis.

The redox potentials and pH characteristics of D-amino-acid oxidase (EC 1.4.3.3; DAAO) from the yeast Rhodotorula gracilis and Trigonopsis variabilis were measured in the pH range 6.5-8.5 at 15 degrees C. In the free enzyme form, the anionic red semiquinone is quantitatively formed in both DAAOs, indicating that a two single-electron transfer mechanism is active. The semiquinone species is also thermodynamically stable, as indicated by the large separation of the single-electron transfer potentials. The first electron potential is pH-independent, while the second electron transfer is pH-dependent exhibiting a approximately -60 mV/pH unit slope, consistent with a one-electron/one-proton transfer. In the presence of the substrate analogue benzoate, the two-electron transfer is the thermodynamically favoured process for both DAAOs, with only a quantitative difference in the stabilization of the anionic semiquinone. Clearly binding of the substrate (or substrate analogue) modulates the redox properties of the two enzymes. In both cases, in the presence and absence of benzoate, the slope of Em vs. pH (-30 mV/pH unit) corresponds to an overall two-electron/one-proton transfer in the reduction to yield the anionic reduced flavin. This behaviour is similar to that reported for DAAO from pig kidney. The differences in potentials and the stability of the semiquinone intermediate measured for the three DAAOs probably stem from different isoalloxazine environments. In the case of R. gracilis DAAO, the low stability of the semiquinone form in the DAAO-benzoate complex can be explained by the shift in position of the side chain of Arg285 following substrate analogue binding.

Ascomycota↗
PubMed1984

Reactivation of the hydrogenase from Desulfovibrio gigas by hydrogen. Influence of redox potential.

The specific activity of the periplasmic hydrogenase from Desulfovibrio gigas is increased approximately 10-fold in the H2 utilization assay with benzyl viologen by several hours of incubation under an atmosphere of H2. After a variable lag phase during which residual traces of O2 are removed, the reversible activation is exponential. The extent of activation is dependent on pH and the redox potential of the incubation medium. A tentative model based on the existence of a monoelectronic redox center is proposed as shown in the following equation: (formula; see text) The potential of this redox couple was determined to be -310 mV (pH = 7; T = 298 K) versus the normal hydrogen electrode.

Desulfovibrio↗
PubMed1993

Redox potential and electrostatic effects in competitive inhibition of dual-path electron transfer reactions of spinach plastocyanin.

Redox inactive ions with high positive charges lower the rate constant for oxidation of several plant plastocyanins (PC) by small positively charged inorganic reaction partners. The rate constant decrease is commonly attributed to competitive inhibition where the redox inactive ions are bound to the negatively charged remote electron transfer (ET) site of PC and block this site sterically. We have investigated the effects of the inhibitor [NH3)5Co(NH2)Co(NH3)5]5+ on the ET reactions of spinach PC with [Co(phen)3]3+ (phen = 1,10-phenanthroline) and the electrically neutral analogue [Co(phen-SO3)3] (phen-SO3 = 5-sulfonato-1,10-phenanthroline) at the ionic strengths mu = 0.1 M and 0.03 M. Inhibition of the [Co(phen)3]3+ reactions is notably smaller for PC(II) reduction than for PC(I) oxidation. This is indicative of a redox potential increase of PC(II)/PC(I) on inhibitor attachment. The effect amounts to 16 mV at mu = 0.1 M and 31 mV at mu = 0.03 M. These data, and analysis in terms of ET theory show that inhibition cannot be caused solely by steric blocking. Driving force and interreactant electrostatic work terms are equally important. The PC(I)/[Co(phen-SO3)3] reaction exhibits a more entangled pattern. The rate constant first increases slightly with increasing inhibitor concentration, then drops, and approaches a constant value not far from the original value. This pattern is in line with association between the negatively charged -SO3- groups of the Co(III) complex and the inhibitor, and ET of the associate at both ET sites of PC.

Binding, Competitive↗
PubMed2005

Redox potential characterization and soil greenhouse gas concentration across a hydrological gradient in a Gulf coast forest.

Soil redox potential (Eh), concentrations of oxygen (O2) and three greenhouse gases (CO2, CH4, and N2O) were measured in the soil profile of a coastal forest at ridge, transition, and swamp across a hydrological gradient. The results delineated a distinct boundary in soil Eh and O2 concentration between the ridge and swamp with essentially no overlap between the two locations. Critical soil Eh to initiate significant CH4 production under this field conditions was about +300 mV, much higher than in the homogenous soils (about -150 mV). The strength of CH4 source to the atmosphere was strong for the swamp, minor for the transition, and negligible or even negative (consumption) for the ridge. Maximum N2O concentration in the soils was found at about Eh +250 mV, and the soil N2O emission was estimated to account for less than 4% for the ridge and transition, and almost negligible for the swamp in the cumulative global warming potential (GWP) of these three gases. The dynamic nature of this study site in response to water table fluctuations across a hydrological gradient makes it an ideal model of impact of future sea level rise to coastal ecosystems. Soil carbon (C) sequestration potential due to increasing soil water content upon sea level rise and subsidence in this coastal forest was likely limited and temporal, and at the expense of increasing soil CH4 production and emission.

Carbon Dioxide↗
PubMed1995

Control of the redox potential in c-type cytochromes: importance of the entropic contribution.

The enthalpic and entropic components of the redox free energy variation of cytochrome c553 from Desulfovibrio vulgaris Hildenborough and its mutant Y64V, flavocytochrome b2 from Saccharomyces cerevisiae, and the different hemes of cytochromes c3 from Desulfovibrio vulgaris Miyazaki and Desulfovibrio desulfuricans Norway have been determined in 0.1 M Tris-HCl pH 7.0 (7.6 for cytochromes c3) at 25 degrees C by using nonisothermal potentiometric titrations. The set of available experimental data demonstrates that the entropic component plays an important role in the control of the redox potential in c-type and b-type cytochromes. The variation of the entropic component within the class of cytochromes characterized by a positive value of E degrees ' is proposed to be mainly determined by the variation of the exposure of the heme propionates to the solvent. In the case of tetraheme cytochromes c3, the thermodynamic characteristics vary largely among the hemes belonging to the same molecule, which reflects the environmental peculiarities of each heme and also the heme-heme redox interactions. This study substantiates the existence of compensatory effects between large and opposite contributions to E degree ' predicted by all the current theoretical models which are based on electrostatic free energy calculations.

Animals↗
PubMed1989

Effect of redox potential on fixation of 137Cs in lake sediment.

Fixation of 137Cs was determined in lake sediment suspensions under controlled redox potentials in the laboratory. The activity of previously added 137Cs on various clay sites was determined by time-series selective extractions. Monovalent cations, particularly NH+4, were much more effective at displacing 137Cs than divalent cations or Na+. Ammonium ion (NH+4) and Na+ were used to extract 137Cs from selective and non-selective 137Cs-binding sites, respectively. The activity of water-soluble 137Cs and Na-extractable 137Cs was significantly higher under anaerobic redox conditions (-200 mV), when soluble NH+4 concentrations in the anaerobic suspensions were 1000 microM or higher. Activities of 137Cs were highest (initially 40-50% of the 137Cs added) on the NH+4-extractable site. Over the long term, activities of NH+4-extractable 137Cs decreased linearly to below 10% as 137Cs was fixed on inter-lattice sites. Water-soluble 137Cs was significantly correlated with Na-extractable 137Cs in short-term experiments, suggesting the existence of an equilibrium between the different clay sites. High concentrations of NH+4 under anaerobic redox conditions could shift this equilibrium, resulting in increases in water-soluble 137Cs and increases in the activities of 137Cs bound on non-selective clay sites. Additional studies are necessary to evaluate the interaction of 137Cs with sulfides, iron oxides, and other reactive chemical species which may attenuate 137Cs in sediment.

Cesium Radioisotopes↗
PubMed1996

Behavioral responses of Escherichia coli to changes in redox potential.

Escherichia coli bacteria sensed the redox state in their surroundings and they swam to a niche that had a preferred reduction potential. In a spatial redox gradient of benzoquinone/benzoquinol, E. coli cells migrated to form a sharply defined band. Bacteria swimming out of either face of the band tumbled and returned to the preferred conditions at the site of the band. This behavioral response was named redox taxis. Redox molecules, such as substituted quinones, that elicited redox taxis, interact with the bacterial electron transport system, thereby altering electron transport and the proton motive force. The magnitude of the behavioral response was dependent on the reduction potential of the chemoeffector. The Tsr, Tar, Trg, Tap, and CheR proteins, which have a role in chemotaxis, were not essential for redox taxis. A cheB mutant had inverted responses in redox taxis, as previously demonstrated in aerotaxis. A model is proposed in which a redox effector molecule perturbs the electron transport system, and an unknown sensor in the membrane detects changes in the proton motive force or the redox status of the electron transport system, and transduces this information into a signal that regulates phosphorylation of the CheA protein. A similar mechanism has been proposed for aerotaxis. Redox taxis may play an important role in the distribution of bacterial species in natural environments.

Bacterial Proteins↗
PubMed1985

A modified flavodoxin with altered redox potentials is less efficient in electron transfer to nitrogenase.

The flavodoxins of the Azotobacter vinelandii wild-type and a mutant strain TZN 200 have been studied. Although the primary structure of the two proteins is the same, the ability of the mutant flavodoxin to donate electrons to nitrogenase is reduced by 75%. One reason may be the raised mid-point potential of -435 mV for the semiquinone/hydroquinone couple in the mutant flavodoxin. The respective redox potential for the wild-type flavodoxin was found to be -480 mV. As shown by paper chromatography and light absorption spectroscopy, the structure of FMN is modified in the TZN 200 flavodoxin.

Azotobacter↗
PubMed2004

Redox potential regulates binding of universal minicircle sequence binding protein at the kinetoplast DNA replication origin.

Kinetoplast DNA, the mitochondrial DNA of the trypanosomatid Crithidia fasciculata, is a remarkable structure containing 5,000 topologically linked DNA minicircles. Their replication is initiated at two conserved sequences, a dodecamer, known as the universal minicircle sequence (UMS), and a hexamer, which are located at the replication origins of the minicircle L- and H-strands, respectively. A UMS-binding protein (UMSBP), binds specifically the conserved origin sequences in their single stranded conformation. The five CCHC-type zinc knuckle motifs, predicted in UMSBP, fold into zinc-dependent structures capable of binding a single-stranded nucleic acid ligand. Zinc knuckles that are involved in the binding of DNA differ from those mediating protein-protein interactions that lead to the dimerization of UMSBP. Both UMSBP DNA binding and its dimerization are sensitive to redox potential. Oxidation of UMSBP results in the protein dimerization, mediated through its N-terminal domain, with a concomitant inhibition of its DNA-binding activity. UMSBP reduction yields monomers that are active in the binding of DNA through the protein C-terminal region. C. fasciculata trypanothione-dependent tryparedoxin activates the binding of UMSBP to UMS DNA in vitro. The possibility that UMSBP binding at the minicircle replication origin is regulated in vivo by a redox potential-based mechanism is discussed.

Animals↗
PubMed2005

Mediated spectroelectrochemical titration of proteins for redox potential measurements by a separator-less one-compartment bulk electrolysis method.

One-compartment bulk electrolysis and simultaneous spectroscopic measurements are realized in a conventional spectroscopic cuvette without separator by using a mesh-type working electrode with extremely large surface area and a wire-type counter electrode with very small surface area. Spectrophotometric monitoring revealed complete electrolysis in a first-order kinetics. This technique was applied to mediated titration of cytochrome c and bilirubin oxidase for determining their redox potentials. Kinetics for the solution redox reaction between protein and mediator is described. The subtraction of spectral background due to mediator adsorption is very easy because of high reproducibility. The experiments can be done under completely anaerobic conditions. Low-absorbance protein samples (of low concentrations or small absorption coefficients) and hydrophobic proteins (such as membrane-bound proteins) are acceptable for measurements.

Cytochromes c↗
PubMed2005

A crystallographic study of Cys69Ala flavodoxin II from Azotobacter vinelandii: structural determinants of redox potential.

Flavodoxin II from Azotobacter vinelandii is a "long-chain" flavodoxin and has one of the lowest E1 midpoint potentials found within the flavodoxin family. To better understand the relationship between structural features and redox potentials, the oxidized form of the C69A mutant of this flavodoxin was crystallized and its three-dimensional structure determined to a resolution of 2.25 A by molecular replacement. Its overall fold is similar to that of other flavodoxins, with a central five-stranded parallel beta-sheet flanked on either side by alpha-helices. An eight-residue insertion, compared with other long-chain flavodoxins, forms a short 3(10) helix preceding the start of the alpha3 helix. The flavin mononucleotide (FMN) cofactor is flanked by a leucine on its re face instead of the more conserved tryptophan, resulting in a more solvent-accessible FMN binding site and stabilization of the hydroquinone (hq) state. In particular the absence of a hydrogen bond to the N5 atom of the oxidized FMN was identified, which destabilizes the ox form, as well as an exceptionally large patch of acidic residues in the vicinity of the FMN N1 atom, which destabilizes the hq form. It is also argued that the presence of a Gly at position 58 in the sequence stabilizes the semiquinone (sq) form, as a result, raising the E2 value in particular.

Alanine↗
PubMedDate not supplied

'Wiring' of lactate oxidase within a low-redox potential electron-conducting hydrogel.

Lactate electrodes based on electron-conducting hydrogels made by cross-linking lactate oxidase and the redox polymer formed upon complexing polyvinyl imidazole with [Os(dmo-bpy)2Cl]+/2+ (dmo-bpy = 4,4'-dimethoxy-2,2'-bipyridine) on vitreous carbon electrode surfaces were investigated. The redox potential of the hydrogels was -69 mV, versus the standard calomel electrode (SCE), and their lactate electro-oxidation current reached a plateau at +50 mV (SCE). Urate and acetaminophen were not electro-oxidized at this potential at rates that would interfere with the lactate assays, but ascorbate was catalytically oxidized by the gel. At 6 mM lactate concentration, switching of the atmosphere from argon to O2, reduced the current by 40 percent, showing that the rate of electron transfer from the reduced enzyme to the gel was slow.

Cross-Linking Reagents↗