Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Redox potential”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 73 records · Page 4Linked to original sources
PubMed2000

Redox potential measurements of the Mycobacterium tuberculosis heme protein KatG and the isoniazid-resistant enzyme KatG(S315T): insights into isoniazid activation.

Mycobacterium tuberculosis KatG is a multifunctional heme enzyme responsible for activation of the antibiotic isoniazid. A KatG(S315T) point mutation is found in >50% of isoniazid-resistant clinical isolates. Since isoniazid activation is thought to involve an oxidation reaction, the redox potential of KatG was determined using cyclic voltammetry, square wave voltammetry, and spectroelectrochemical titrations. Isoniazid activation may proceed via a cytochrome P450-like mechanism. Therefore, the possibility that substrate binding by KatG leads to an increase in the heme redox potential and the possibility that KatG(S315T) confers isoniazid resistance by altering the redox potential were examined. Effects of the heme spin state on the reduction potentials of KatG and KatG(S315T) were also determined. Assessment of the Fe(3+)/Fe(2+) couple gave a midpoint potential of ca. -50 mV for both KatG and KatG(S315T). In contrast to cytochrome P450s, addition of substrate had no significant effect on either the KatG or KatG(S315T) redox potential. Conversion of the heme to a low-spin configuration resulted in a -150 to -200 mV shift of the KatG and KatG(S315T) redox potentials. These results suggest that isoniazid resistance conferred by KatG(S315T) is not mediated through changes in the heme redox potential. The redox potentials of isoniazid were also determined using cyclic and square wave voltammetry, and the results provide evidence that the ferric KatG and KatG(S315T) midpoint potentials are too low to promote isoniazid oxidation without formation of a high-valent enzyme intermediate such as compounds I and II or oxyferrous KatG.

Amino Acid Substitution↗
PubMed2001

Metabolic flux analysis of Clostridium thermosuccinogenes: effects of pH and culture redox potential.

Clostridium thermosuccinogenes are anaerobic thermophilic bacteria that ferment various carbohydrates to succinate and acetate as major products and formate, lactate, and ethanol as minor products. Metabolic carbon flux analysis was used to evaluate the effect of pH and redox potential on the batch fermentation of C. thermosuccinogenes. In a first study, the effects of four pH values (6.50, 6.75, 7.00, and 7.25) on intracellular carbon flux at a constant redox potential of -275 mV were compared. The flux of carbon toward succinate and formate increased whereas the flux to lactate decreased significantly with a pH increase from 6.50 to 7.25. Both specific growth rate and specific rate of glucose consumption were unaffected by changes in pH. The fraction of carbon flux at the phosphoenolpyruvate (PEP) node flowing to oxaloacetate increased with an increase in pH. At the pyruvate node, the fraction of flux to formate increased with increasing pH. At the acetyl CoA node, the fraction of flux to acetate increased significantly with an increase in pH. A second study elucidated the effect of four controlled culture redox potentials (-225, -250, -275, and -310 mV) on metabolic carbon flux at a constant pH of 7.25. Lower values of culture redox potential were correlated with increased succinate, acetate, and formate fluxes and decreased ethanol and hydrogen fluxes in C. thermosuccinogenes. Lactate formation was not significantly influenced by redox potential. At the PEP node, the fraction of carbon to oxaloacetate increased with a decrease in redox potential. At the pyruvate node, the fraction of carbon to formate increased, while at the acetyl CoA node, the fraction of carbon flux to acetate increased with reduced redox potential. The presence of hydrogen in the headspace or the addition of nicotinic acid to the growth media resulted in increased hydrogen and ethanol fluxes and decreased succinate, acetate, formate, and lactate fluxes.

Acetic Acid↗
PubMed1983

Nitrogenase reduction by electron carriers: influence of redox potential on activity and the ATP/2e- ratio.

The variation with redox potential of nitrogenase activity and the ratio of ATP hydrolyzed per two electrons transferred were measured using two systems: the dithionite/bisulfite couple at pH 7.4; and H2, hydrogenase, and ferredoxin at pH 8.5. In both cases, the variation in nitrogenase activity with redox potential followed a theoretical Nernst plot for a two-electron process with an apparent midpoint potential of about -470 mV. The ratio ATP/2e- was about 4 under highly reducing conditions. However, above the apparent midpoint potential, the ratio ATP/2e- increased drastically, reaching values as high as 20. These data imply that a low redox potential must be maintained for efficient nitrogen fixation in vitro and in vivo.

Adenosine Triphosphate↗
PubMed2000

Cytochrome b562 variants: a library for examining redox potential evolution.

A general understanding of how cytochromes evolve within a fixed structure to optimize redox potential for specific bioenergetic processes does not exist. Toward this end, a library approach is used to investigate the range and distribution of redox potential which occurs when all sequence space available through mutation at two positions is examined within a fixed structural motif. Random mutation of Phe61 and Phe65 of cytochrome b562 (E. coli), and subsequent examination of a statistically significant sampling of this library, demonstrates that the redox potential can vary over 100 mV (>25% of the known accessible potential in native proteins with axial His-Met ligation) through mutation at these two positions. The redox potential of the wild-type protein occurs at an extremum of the distribution observed, indicating that Phe61 and Phe65 were most likely naturally selected to differentially stabilize the reduced state of the protein. At the other extremum, a compositionally conservative set of mutations (F61I, F65Y) leads to a 100 mV shift in the redox equilibrium toward the oxidized state. NMR analyses indicate that a charge-dipole interaction which results from mutation of phenylalanine to tyrosine at position 65 may be responsible.

Amino Acid Substitution↗
PubMed2002

Pyridine nucleotide redox potential in erythrocytes of saudi subjects with sickle cell disease.

Key antioxidant defense reactions in RBCs are linked to nicotinamide adenine dinucleotide (NAD(+)) and nicotinamide adenine dinucleotide phosphate (NADP(+)). The ratio of reduced (NAD(P)H) to total pyridine nucleotide pool [(NAD(P)(T) i.e. NAD(P)H plus NAD(P)(+)], which is known as redox potential, reflects the redox status of erythrocytes. Through the utilization of a spectrophotometric cycling assay, the NAD(+) and NADP(+) redox potentials were determined in erythrocytes from normal Saudi subjects and those with sickle cell disease (SCD). The sickle erythrocytes NADH/NAD(T) ratio (0.386) was reduced compared to the normal ratio (0.464). This reduction appears to be due to a significant increase in oxidized NAD(+) in sickle RBCs. However, the value of the redox potential of erythrocytes in Saudi subjects with SCD is slightly higher than that reported for comparable African Americans. Although the high fetal hemoglobin is the main factor in the amelioration of the clinical course exhibited by Saudi sickle cell patients, it is assumed that the slight improvement in the redox potential may play a part in this process. This is supported by a relatively uninhibited glycolytic pathway in the erythrocytes of Saudi subjects with SCD with a higher level of NADH than their African American counterparts.

Black or African American↗
PubMed2002

Density functional calculation of p K(a) values and redox potentials in the bovine Rieske iron-sulfur protein.

The redox potential of the Rieske iron-sulfur protein depends on pH. It has been proposed that the histidines coordinating one of the irons are responsible for this pH dependence, but an experimental proof for this proposal is still lacking. In this work, we present a density functional/continuum electrostatics calculation of the p K(a) values of the histidines in the Rieske iron-sulfur center. The calculated apparent p K(a) values are 6.9 and 8.8 in the oxidized state, which are in good agreement with the corresponding experimental values of 7.5 and 9.2 and the measured pH dependence of the redox potential. Neither of these two p K(a) values can, however, be assigned to only one of the histidines. We find that both histidines titrate over a wide pH range in the oxidized state. Reduction of the iron-sulfur center shifts the p K(a) values to 11.3 and 12.8, thus above 10.0 as found experimentally. The results provide a complete picture of the coupling of proton and electron binding, showing strongly cooperative binding of protons at electrode potentials near the redox midpoint potential of the cluster. The potential biological function of the low p K(a) value of the histidines and the shift upon reduction are briefly discussed.

Animals↗
PubMed1992

Protein control of iron-sulfur cluster redox potentials.

The relationship between the three-dimensional structures of iron-sulfur proteins and the redox potentials of their iron-sulfur clusters is of fundamental importance. We report calculations of the redox potentials of the [Fe4S4(S-cys)4]-2/-3 couple in four crystallographically characterized proteins: Azotobacter vinelandii ferredoxin I, Peptococcus aerogenes ferredoxin, Bacillus thermoproteolyticus ferredoxin, and Chromatium vinosum high potential iron protein (HiPIP). Our calculations use the "protein dipoles Langevin dipoles" microscopic electrostatic model, which includes both protein and solvent water. The variations in calculated redox potentials are in excellent agreement with experimental data. In particular, our results confirm the important role of amide groups close to the cluster in separating the potential of C. vinosum HiPIP from those of the other three proteins. However, the potentials of these latter exhibit a substantial range despite extremely similar amide group environments of their clusters. Our results show that the potentials in these proteins are tuned in part by varying the access of solvent water to the neighborhood of the cluster. Our calculations provide the first successful quantitative modeling of the protein control of iron-sulfur cluster redox potentials.

Azotobacter vinelandii↗
PubMed1986

Relationship between inhibition of mitochondrial respiration by naphthoquinones, their antitumor activity, and their redox potential.

The physicochemical properties of a series of 1,4-naphthoquinones were correlated with their activities against Sarcoma-180 by Hodnett et al. [J. med. Chem. 26, 570 (1983)]. Redox potential was the most important molecular parameter determining antitumor activity in this series of compounds, suggesting that interference with electron transport contributes to their cytotoxicity. We evaluated this same series of quinones for their abilities to inhibit the beef heart mitochondrial succinoxidase and NADH-oxidase enzyme systems. They exhibited a broad range of inhibitory potencies. There was a strong relationship between succinoxidase inhibition, antitumor activity (T/C ratio), and redox potential. The redox potentials of the quinones which inhibited succinoxidase lay within the narrow range of endogenous components of the respiratory chain. In contrast, inhibition of NADH-oxidase was related to redox potential but did not significantly predict antitumor activity. These results suggest that inhibition of mitochondrial succinoxidase may be a useful preliminary screen for antitumor activity.

Animals↗
PubMed2003

Redox potential of quinones in photosynthetic reaction centers from Rhodobacter sphaeroides: dependence on protonation of Glu-L212 and Asp-L213.

The absolute values of the one-electron redox potentials of the two quinones (Q(A) and Q(B)) in bacterial photosynthetic reaction centers from Rhodobacter sphaeroides were calculated by evaluating the electrostatic energies from the solution of the linearized Poisson-Boltzmann equation at pH 7.0. The redox potential for Q(A) was calculated to be between -173 and -160 mV, which is close to the lowest measured values that are assumed to refer to nonequilibrated protonation patterns in the redox state Q(A)(-). The redox potential of quinone Q(B) is found to be about 160-220 mV larger for the light-exposed than for the dark-adapted structure. These values of the redox potentials are obtained if Asp-L213 is nearly protonated (probability 0.75-1.0) before and after electron transfer from Q(A) to Q(B), while Glu-L212 is partially protonated (probability 0.6) in the initial state Q(A)(-)Q(B)(0) and fully protonated in the final state Q(A)(0)Q(B)(-). Conversely, if the charge state of the quinones is varied from Q(A)(-)Q(B)(0) to Q(A)(0)Q(B)(-) corresponding to the electron transfer from Q(A) to Q(B), Asp-L213 remains protonated, while Glu-L212 changes its protonation state from 0.15 H(+) to fully protonated. In agreement with results from FTIR spectra, there is proton uptake at Glu-L212 going along with the electron transfer, whereas Asp-L213 does not change its protonation state. However, in our simulations Asp-L213 is considered to be protonated rather than ionized as deduced from FTIR spectra. The calculated redox potential of Q(A) shows little dependence on the charge state of Asp-L213, which is due to a strong coupling with the protonation state of Asp-M17 but increases by 50 mV if Glu-L212 changes from the ionized to the protonated charge state. Both are in agreement with fluorescence measurements observing the decay of SP(+)Q(A)(-) in a wide pH regime. The computed difference in redox potential of Q(B) in the light-exposed and dark-adapted structure was traced back to the hydrogen bond of Q(B) with His-L190 that is lost in the dark-adapted structure and the charge of the non-heme iron atom, which is closer to Q(B) in the light-exposed than in the dark-adapted structure.

Amino Acid Substitution↗
PubMed1990

Electrostatic and redox potential effects on the rat of electron-transfer reaction of nicotinamide adenine dinucleotides with 1-substituted 5-ethylphenazines.

The effects of redox potential and electric charge on the rate of electron-transfer reaction by a two-electron process were investigated. For electron donors, beta-NADH, beta-NADPH and alpha-NADH were used; they have similar structures but different charges and different redox potentials. For electron acceptors, the following 5-ethylphenazine derivatives were used: 1-(3-carboxypropyloxy)-5-ethylphenazine, 1-(3-ethoxycarbonylpropyloxy)-5-ethylphenazine, and 1-[N-(2-aminoethyl)carbamoylpropyloxy]-5-ethylphenazine. They have similar structures and different charges. Using these donors and acceptors, the potential and the charge effects were estimated separately. In the potential effect, a linear free energy relationship was observed for the change in the redox potential of the donor with a Brønsted slope of about unity. On the other hand, the slope for the change in the potential of the acceptor was about 0.5. These results show that the potential effect due to electron donors is different from that due to electron acceptors. A linear relationship was also observed between activation free energy and electrostatic force (or potential). The redox potential effect and the electrostatic effect are independent and additive. New theory for the mechanism of electron-transfer reactions is needed to explain these results.

Electricity↗
PubMed1990

Estimation of microscopic redox potentials of a tetraheme protein, cytochrome c3 of Desulfovibrio vulgaris, Miyazaki F, and partial assignments of heme groups.

The microscopic formal redox potentials of a tetraheme protein, cytochrome c3 from Desulfovibrio vulgaris, Miyazaki F, were estimated from the chemical shifts of the heme methyl signals in its 1H NMR spectrum. All chemical shifts in the five macroscopic oxidation states were determined for eight of the heme methyl protons by the saturation-transfer method. The electron-distribution probability at each heme in each oxidation state was estimated directly from the chemical shifts. To minimize errors due to interheme pseudocontact contributions, the average electron-distribution probability was used for calculation of the microscopic formal redox potentials. By introducing interacting potentials, 32 parameters were reduced to 10. The 10 parameters were determined analytically from the 9 independent electron-distribution probabilities and 2 macroscopic formal redox potentials. The results showed the presence of a strong positive interaction between a pair of particular hemes. The microscopic formal redox potential changes dramatically with the extent of reduction because of the intramolecular interheme interactions. NMR signals of two hemes were assigned to particular hemes in the crystal structures by nuclear Overhauser effect experiments. The results showed that the hemes with the highest and lowest redox potentials in the one-electron reduction process correspond to hemes I and IV in the crystal structure.

Cytochrome c Group↗
PubMed2006

Redox potentials of the blue copper sites of bilirubin oxidases.

The redox potentials of the multicopper redox enzyme bilirubin oxidase (BOD) from two organisms were determined by mediated and direct spectroelectrochemistry. The potential of the T1 site of BOD from the fungus Myrothecium verrucaria was close to 670 mV, whereas that from Trachyderma tsunodae was >650 mV vs. NHE. For the first time, direct electron transfer was observed between gold electrodes and BODs. The redox potentials of the T2 sites of both BODs were near 390 mV vs. NHE, consistent with previous finding for laccase and suggesting that the redox potentials of the T2 copper sites of most blue multicopper oxidases are similar, about 400 mV.

Amino Acid Sequence↗
PubMed2005

Post-translational regulation of mercaptopyruvate sulfurtransferase via a low redox potential cysteine-sulfenate in the maintenance of redox homeostasis.

3-Mercaptopyruvate sulfurtransferase (MST) (EC 2.8.1.2), a multifunctional enzyme, catalyzes a transsulfuration from mercaptopyruvate to pyruvate in the degradation process of cysteine. A stoichiometric concentration of hydrogen peroxide and of tetrathionate (S(4)O(6)(2-)) inhibited rat MST (k(i) = 3.3 min(-1), K(i) = 120.5 microM and k(i) = 2.5 min(-1), K(i) = 178.6 microM, respectively). The activity was completely restored by dithiothreitol or thioredoxin with a reducing system containing thioredoxin reductase and NADPH, but glutathione did not restore the activity. On the other hand, an excess molar ratio dose of hydrogen peroxide inactivated MST. Oxidation with a stoichiometric concentration of hydrogen peroxide protected the enzyme against reaction by iodoacetate, which modifies a catalytic Cys(247), suggesting that Cys(247) is a target of the oxidants. A matrix-assisted laser desorption/ionization-time-of-flight mass spectrometric analysis revealed that hydrogen peroxide- and tetrathionate-inhibited MSTs were increased in molecular mass consistent with the addition of atomic oxygen and with a thiosulfate (S(2)O(3)(-)), respectively. Treatment with dithiothreitol restored modified MST to the original mass. These findings suggested that there was no nearby cysteine with which to form a disulfide, and mild oxidation of MST resulted in formation of a sulfenate (SO(-)) at Cys(247), which exhibited exceptional stability and a lower redox potential than that of glutathione. Oxidative stress decreases MST activity so as to increase the amount of cysteine, a precursor of thioredoxin or glutathione, and furthermore, these cellular reductants restore the activity. Thus the redox state regulates MST activity at the enzymatic level, and on the other hand, MST controls redox to maintain cellular redox homeostasis.

Animals↗
PubMed1978

Redox potentials of the photosynthetic bacterial cytochromes c2 and the structural bases for variability.

The cytochromes c2 of the Rhodospirillaceae show a much greater variation in redox potential and its pH dependence than the mitochondrial cytochromes c that have been studied. It is proposed that the range of redox potential for cytochromes c2 functioning as the immediate electron donor to photo-oxidised bacteriochlorophyll may be 345-395 mV at pH 5. Closely related cytochromes c2 with different redox potentials show patterns of amino acid substitution which are consistent with changes in hydrophobicity near the haem being at least a partial determinant of redox potential. More distantly related cytochromes are difficult to compare because of the large number of amino acid substitutions and the probability that there are subtle changes in overall peptide chain folding. The redox potential versus pH curves can be analysed in terms of either one ionisation in the oxidised form or two in the oxidised form and one in the reduced. The pK in the oxidised form at higher pH values can be correlated with the pK for the disappearance or shift of the near infrared absorption band located near 695 nm. The structural bases of these ionisations are not known but the possible involvement of the haem propionate residues is discussed.

Amino Acids↗
PubMed2002

Effect of redox potential on activity of hydrogenase 1 and hydrogenase 2 in Escherichia coli.

This report elucidates the distinctions of redox properties between two uptake hydrogenases in Escherichia coli. Hydrogen uptake in the presence of mediators with different redox potential was studied in cell-free extracts of E. coli mutants HDK103 and HDK203 synthesizing hydrogenase 2 or hydrogenase 1, respectively. Both hydrogenases mediated H(2) uptake in the presence of high-potential acceptors (ferricyanide and phenazine methosulfate). H(2) uptake in the presence of low-potential acceptors (methyl and benzyl viologen) was mediated mainly by hydrogenase 2. To explore the dependence of hydrogen consumption on redox potential of media in cell-free extracts, a chamber with hydrogen and redox ( E(h)) electrodes was used. The mutants HDK103 and HDK203 exhibited significant distinctions in their redox behavior. During the redox titration, maximal hydrogenase 2 activity was observed at the E(h) below -80 mV. Hydrogenase 1 had maximum activity in the E(h) range from +30 mV to +110 mV. Unlike hydrogenase 2, the activated hydrogenase 1 retained activity after a fast shift of redox potential up to +500 mV by ferricyanide titration and was more tolerant to O(2). Thus, two hydrogenases in E. coli are complementary in their redox properties, hydrogenase 1 functioning at higher redox potentials and/or at higher O(2) concentrations than hydrogenase 2.

Electron Transport↗
PubMed2001

Artificial cytochrome b: computer modeling and evaluation of redox potentials.

We generated atomic coordinates of an artificial protein that was recently synthesized to model the central part of the native cytochrome b (Cb) subunit consisting of a four-helix bundle with two hemes. Since no X-ray structure is available, the structural elements of the artificial Cb were assembled from scratch using all known chemical and structural information available and avoiding strain as much as possible. Molecular dynamics (MD) simulations applied to this model protein exhibited root-mean-square deviations as small as those obtained from MD simulations starting with the crystal structure of the native Cb subunit. This demonstrates that the modeled structure of the artificial Cb is relatively rigid and strain-free. The model structure of the artificial Cb was used to determine the redox potentials of the two hemes by calculating the electrostatic energies from the solution of the linearized Poisson-Boltzmann equation (LPBE). The calculated redox potentials agree within 20 meV with the experimentally measured values. The dependence of the redox potentials of the hemes on the protein environment was analyzed. Accordingly, the total shift in the redox potentials is mainly due to the low dielectric medium of the protein, the protein backbone charges, and the salt bridges formed between the arginines and the propionic acid groups of the hemes. The difference in the shift of the redox potentials is due to the interactions with the hydrophilic side chains and the salt bridges formed with the propionic acids of the hemes. For comparison and to test the computational procedure, the redox potentials of the two hemes in the native Cb from the cytochrome bc(1) (Cbc(1)) complex were also calculated. Also in this case the computed redox potentials agree well with experiments.

Amino Acid Sequence↗
PubMed2005

Absolute standard redox potential of monolayer-protected gold nanoclusters.

The redox properties of monolayer-protected gold nanoclusters (MPCs) are considered from both the theoretical and experimental viewpoints. The "absolute standard redox potential" ([E0((z/z)-1))]abs) of MPCs is first derived from electrostatic considerations. A linear dependence of the absolute standard redox potential on the valence state of MPCs is theoretically predicted and verified experimentally. By employing ferricinium/ferrocene (Fc+/Fc) as a reference redox couple, the average valence state of MPCs at a given potential can be estimated.

Letter↗
PubMed1999

Role of redox potential and reactive oxygen species in stress signaling.

Stress-activated signaling cascades are affected by altered redox potential. Key contributors to altered redox potential are reactive oxygen species (ROS) which are formed, in most cases, by exogenous genotoxic agents including irradiation, inflammatory cytokines and chemical carcinogens. ROS and altered redox potential can be considered as the primary intracellular changes which regulate protein kinases, thereby serving as an important cellular component linking external stimuli with signal transduction in stress response. The mechanisms, which underlie the ROS-mediated response, involve direct alteration of kinases and transcription factors, and indirect modulation of cysteine-rich redox-sensitive proteins exemplified by thioredoxin and glutathione S-transferase. This review summarizes the current understanding of the mechanisms contributing to ROS-related changes in key stress activated signaling cascades.

Animals↗