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 361 records · Page 20Linked to original sources
PubMed1976

The effect of diabetes, insulin, and the redox potential on leucine metabolism by isolated rat hemidiaphragm.

The oxidation of leucine by hemidiaphragms of control and diabetic rats was studied in vitro. Rats were rendered diabetic with streptozotocin. Hemidiaphragms of diabetic rats produced approximately 50% more 14CO2 during incubation with 0.1 mM [1-14C]leucine than did control muscles. This was observed during incubation with or without glucose and in the presence or absence of a full complement of plasma amino acids. The concentration of leucine in the tissue water of hemidiaphragms from diabetic rats was greater than that in the control muscles before incubation. The specific activity of leucine at the end of 60 min incubation was not significantly different in diabetic and control muscles, indicating that the increased 14CO2 production represented stimulation of leucine oxidation. Hemidiaphragms of diabetic rats released more leucine into the medium during incubation than did control muscles. The stimulating effect of diabetes on leucine oxidation in vitro was reversible by insulin therapy prior to sacrifice. The addition of 5 mM pyruvate to a medium containing glucose inhibited 14CO2 production from [14C]leucine in control muscles, but stimulated leucine oxidation by hemidiaphragms of diabetic rats. Leucine oxidation by hemidiaphragms of diabetic rats was markedly stimulated by the addition of an electron acceptor, 0.02 mM methylene blue, suggesting that the NADH/NAD ratio may be rate-limiting for branched chain amino acid oxidation in muscles of diabetic rats, but not in muscles of controls. We suggest that the accelerated oxidation of branched chain amino acids by muscles may play a role in the acceleration of the muscle protein catabolism and gluconeogenesis which develop during insulin deficiency. The restraining effect of the cellular redox potential on branched chain amino acid oxidation may play a role in the eventual deceleration of protein catabolism during a prolonged fast.

Amino Acids↗
PubMed2001

Protein control of the redox potential of the primary quinone acceptor in reactioncCenters from Rhodobacter sphaeroides.

The role of the protein environment in determining the redox midpoint potential (E(m)) of Q(A), the primary quinone of bacterial reaction centers, was investigated by mutation of isoleucine at position 265 of the M subunit in Rhodobacter sphaeroides. Isoleucine was changed to threonine, serine, and valine, yielding mutants M265IT, M265IS, and M265IV, respectively. All three mutants, with smaller residues replacing isoleucine, exhibited decreased binding affinities of the Q(A) site for various quinone analogues, consistent with an enlargement or loosening of the headgroup binding domain and a decrease in the van der Waals contact for small quinones. In all other respects, M265IV was like the wild type, but the polar mutants, M265IT and M265IS, had unexpectedly dramatic decreases in the redox midpoint potential of Q(A), resulting in faster rates of P(+)Q(A)(-) charge recombination. For both anthraquinone and native ubiquinone, the in situ E(m) of Q(A) was estimated to be approximately 100 and 85 mV lower in M265IT and M265IS, respectively. The effect on E(m)(Q(A)) indicates destabilization of the semiquinone or stabilization of the quinone. This is suggested to arise from either (i) electrostatic interaction between the partial charges or dipole of the residue hydroxyl group and the charge distribution of quinone and semiquinone states with particular influence near the C4 carbonyl group or (ii) from hydrogen bonding interactions between the hydroxyl oxygen and the N(delta)H of histidine M219, causing a weakening of the hydrogen bond to the C4 carbonyl. The rate of the first electron transfer (k(AB)(()(1)())) in the polar mutants was the same as in the wild type at low pH but decelerated at higher pH with altered pH dependence. The rate of the second electron transfer (k(AB)(()(2)())) was 3-4-fold greater than in the wild type over the whole pH range from 4 to 11, consistent with a larger driving force for electron transfer derived from the lower E(m) of Q(A).

Amino Acid Substitution↗
PubMed1991

Reductive alkylation of DNA by mitomycin A, a mitomycin with high redox potential.

The mitomycins are a group of antitumor antibiotics that covalently bind to DNA upon reductive activation. Mitomycin A (1b; MA) is more toxic than its clinically useful mitomycin C (1a; MC). The greater toxicity of mitomycin A has been previously attributed to its higher reduction potential. In this report, the DNA alkylation products of reductively activated MA were isolated and characterized by conversion to the known 7-amino mitosene-deoxyguanosine adducts. The three major adducts formed were identified as a monoadduct, N2-(2"beta-amino-7"-methoxymitosen-1"alpha-yl)- 2'-deoxyguanosine (5), a decarbamoyl monoadduct, N2-(2"beta-amino-10"-decarbamoyl-7"-methoxymitosen-1"alpha-y l)-2'- deoxyguanosine (6), and a bisadduct, N2-(2"beta-amino-10"-deoxyguanosin-N2-yl-7-methoxymitosen-1" alpha- yl)-2'-deoxyguanosine (7). Under all reductive activation conditions employed, MA selectively alkylated the 2-amino group of guanine in DNA, like MC. In addition, both MA and MC alkylated DNA and cross-linked oligonucleotides to a similar extent. However, variations in the reductive activation conditions (H2/PtO2, Na2S2O4, or enzymatic) affected the distribution of the three major MA adducts in a different manner than the distribution of MC adducts was affected. A mechanism is proposed wherein the 7-methoxy substituent of MA allows initial indiscriminate activation of either of the drugs' two electrophilic sites. While oxygen inhibited cross-linking by MC, similar aerobic conditions exhibited little influence on the cross-linking ability of MA. Hence, the greater toxicity of MA may be influenced by increased and nonselective activation and cross-link formation in both aerobic and anaerobic cells. This effect is a direct consequence of the higher redox potential of MA as compared to MC.

Alkylation↗
PubMed1989

Effect of redox potential on rat liver F1-ATPase.

Redox titration of F1-ATPase from rat liver mitochondria referred to the modification of the hydrolytic activity on Mg-ATP has resulted in a three-step pattern, with three distinct jumps of activity separated by clear plateaus. The measured potentials ranged from -400 mV to +400 mV and were obtained by the addition of dithionite and ferricyanide. Electron exchange was facilitated with a mixture of different redox mediators. At pH 7.4 the midpoint potentials were +210 mV, +40 mV and -230 mV. These three midpoint potentials were displaced towards more negative values by 2,4-dinitrophenol or by an increase of the pH of the medium. The titration curves were described by n = 2 Nernst equations.

Animals↗
PubMed1989

Pulse radiolytic measurement of redox potentials: the tyrosine and tryptophan radicals.

With the technique of pulse radiolysis we have measured the redox midpoint potentials of the tryptophan side chain neutral indolyl radical (1.05 +/- 0.01 V vs NHE, pH 7.0 and 25 degrees C) and the tyrosine side chain neutral phenoxy radical (0.94 +/- 0.01 vs NHE, pH 7.0 and 25 degrees C). These potentials were obtained by using a variety of inorganic reference compounds in both kinetic and equilibrium protocols. We compare these results with others already in the literature, and we also present data useful in establishing a pulse radiolysis redox reference scale over the range 0.42-1.28 V.

Free Radicals↗
PubMed1978

The redox potential of dithionite and SO-2 from equilibrium reactions with flavodoxins, methyl viologen and hydrogen plus hydrogenase.

1. It has been shown that redox equilibria can be formed between dithionite ion (plus SO-2) and (bi)sulphite, and the low-potential electron carriers flavodoxin and methyl viologen. The equilibria were established either by treating the oxidized electron carriers with dithionite, or by treating flavodoxin hydroquinone or methyl viologen semiquinone with (bi)sulphite. Similar redox equilibria were established between dithionite/(bi)sulphite and hydrogen using catalytic amounts of hydrogenase in the presence of a low-potential electron carrier. The effects of pH and temperature on the equilibria were determined. 2. The equilibria were analyzed to determine the redox potential of the dithionite/(bi)sulphite system. In accordance with the results of earlier kinetic studies, it was assumed that the reductant in dithionite solutions is the dissociation product SO-2. The calculated midpoint redox potential E' for the couple SO-2/HSO-3 at pH 7 and 25 degrees C was -0.66 V. The reductant is present largely as the dimer at concentrations of dithionite above about 10nM. Consequently, the midpoint potential, Em, of dithionite solutions becomes less negative as the concentration of dithionite is increased (deltaEm/deltalog S2O2-4 = 29 mV). The theoretical potential of a solution of 1 M S2O2-4 and 2 M (bi)sulphite at pH 7 was calculated to be -0.386V. This value is 59 mV more negative than that determined in 1911 by potentiometry, but considerably more positive than other values in the literature. The effects of pH on the equilibria showed that E' is controlled by the pK of (bi)sulphite at 6.9; the slope deltaE'/deltapH was -59 mV below the pK and -118 mV above the pK. The effects of temperature on the equilibria suggested that Em for dithionite changed by -1.6 mV/degrees C for a rise in temperature between 2 degrees C and 40 degrees C. If sodium dithionite is contaminated with small amounts of (bi)sulphite, its addition in large excess to a low potential electron carrier can cause oxidation of the carrier.

Anaerobiosis↗
PubMed1997

Influence of Asn/His L166 on the hydrogen-bonding pattern and redox potential of the primary donor of purple bacterial reaction centers.

The primary electron donor (P) of the photosynthetic reaction center (RC) from the purple bacterium Rhodobacter (Rb.) sphaeroides is constituted of two bacteriochlorophyll molecules in excitonic interaction. The C2 acetyl carbonyl group of one of the two bacteriochlorophyll molecules (PL), the one more closely associated with the L polypeptide subunit, is engaged in a hydrogen bond with histidine L168, while the other pi-conjugated carbonyl groups of P are free from such hydrogen-bonding interactions. The three-dimensional X-ray crystal structures of the RC from several strains of Rb. sphaeroides reveal that asparagine L166 probably interacts indirectly with P through His L168. Such an interaction is expected to modulate the hydrogen bond between P and His L168, a residue which is highly conserved in purple bacteria. RC mutants of Rb. sphaeroides where asparagine L166 was genetically replaced by leucine [NL(L166)], histidine [NH(L166)], and aspartate [ND(L166)] were studied using Fourier transform (FT) Raman spectroscopy. All of these mutations resulted in an increase in the strength of the hydrogen bond between His L168 and the acetyl carbonyl group of P(L), as observed in the FT Raman spectrum, by the 2-4 cm(-1) decrease in vibrational frequency of the 1620 cm(-1) band which has been assigned to this specific acetyl carbonyl group [Mattioli, T. A., Lin, X., Allen, J. P., & Williams, J. C. (1995) Biochemistry 34, 6142-6152]. At pH 8, the NH(L166) mutation showed the greatest change in the P0/P.+ redox midpoint potential (515 mV), increasing it by ca. 30 mV compared to that of wild type (485 mV). A similar increase in P0/P.+ redox midpoint potential for NH(L166) compared to that of wild type is also observed at pH 5, 6, and 9.5. The p0/P.+ midpoint potential of the NL(L166) mutant was comparable to that of wild type at all pH values. In contrast, for the ND(L166) mutant, the midpoint potential shows a markedly different pH dependency, being 25 mV higher than wild type at pH 5 but 20 mV lower than wild type at pH 9.5. The hydrogen bond interactions of the primary electron donor from Rhodospirillum (Rsp.) centenum were determined from the FT Raman vibrational spectrum which exhibits a 1616 cm(-1) band similar to what is seen in the NH(L166) and ND(L166) Rb. sphaeroides mutants. Comparison of the sequence of the L subunit determined for the Rsp. centenum RC with that of other species indicates that positions L166 and L168 are occupied by His residues. The stronger hydrogen bond between the conserved His L168 and the acetyl carbonyl group of P(L), observed in the primary donor of Rsp. centenum and of several bacterial species which are known to possess a histidine residue at the analogous L166 position, is proposed to be due to interactions between these two histidine residues.

Amino Acid Sequence↗
PubMed2005

Substituent effects on redox potentials and optical gap energies of molecule-like Au38(SPhX)24 nanoparticles.

A molecule-like substituent effect on redox formal potentials in the nanoparticle series Au(38)(SPhX)(24) has been discovered. Electron-withdrawing "X" substituents energetically favor reduction and disfavor oxidation, and give formal potentials that correlate with Hammett substituent constants. The ligand monolayer of the nanoparticles is shown, thereby, to play a strong role in determining electronic energies of the nanoparticle core and is more than simply a protecting or capping layer. The substituent effect does not, however, detectably change the HOMO-LUMO gap energy, being identical for the HOMO and LUMO levels and presumably inductive in nature.

Journal Article↗
PubMed1998

Redox potentials for yeast, Escherichia coli and human glutathione reductase relative to the NAD+/NADH redox couple: enzyme forms active in catalysis.

The flavoenzyme glutathione reductase catalyzes the NADPH-dependent reduction of glutathione disulfide, yielding two molecules of glutathione. The oxidation-reduction potentials, Eox/EH2 (two-electron reduced enzyme), for yeast, Escherichia coli, and human glutathione reductase have been determined between pH 6.0 and 9.8 relative to the nonphysiological substrate couple NAD+/NADH and were found to be -237, -243, and -227 mV (+/-5 mV) at pH 7.0 and 20 degreesC, respectively. The potential as a function of pH demonstrated slopes of -51, -45, and -42 mV/pH unit, respectively, at low pH and -37, -31, and -34 mV/pH unit, respectively, at high pH. The change in slope indicated pKa values of 7.4, 8.5, and 7.6, respectively. The slopes indicate that two protons are associated with the two-electron reduction of Eox at low pH and that only one proton is involved with the two-electron reduction of Eox at high pH, provided that the effects of nearby titratable residues are considered in the data analysis. The influence of four such groups, Cys50, Cys45, His456', and either Tyr107 or the flavin-(N3), has been included (residue numbering refers to the yeast sequence). The enzyme loses activity upon deprotonation of the acid-base catalyst at high pH. Since the pKa ascribed to the EH2-to-EH- ionization is lower than the pKa of the acid-base catalyst, both the EH2 and EH- forms of glutathione reductase must be catalytically active, in contrast to the closely related enzyme lipoamide dehydrogenase, for which only EH2 is active.

Catalysis↗
PubMed2006

The reaction of neuroglobin with potential redox protein partners cytochrome b5 and cytochrome c.

Previously identified, potentially neuroprotective reactions of neuroglobin require the existence of yet unknown redox partners. We show here that the reduction of ferric neuroglobin by cytochrome b(5) is relatively slow (k=6 x 10(2)M(-1)s(-1) at pH 7.0) and thus is unlikely to be of physiological significance. In contrast, the reaction between ferrous neuroglobin and ferric cytochrome c is very rapid (k=2 x 10(7)M(-1)s(-1)) with an apparent overall equilibrium constant of 1 microM. Based on this data we propose that ferrous neuroglobin may well play a role in preventing apoptosis.

Animals↗
PubMed2003

Redox potential of human thioredoxin 1 and identification of a second dithiol/disulfide motif.

Thioredoxin (Trx1) is a redox-active protein containing two active site cysteines (Cys-32 and Cys-35) that cycle between the dithiol and disulfide forms as Trx1 reduces target proteins. Examination of the redox characteristics of this active site dithiol/disulfide couple is complicated by the presence of three additional non-active site cysteines. Using the redox Western blot technique and matrix assisted laser desorption ionization time-of-flight mass spectrometry mass spectrometry, we determined the midpoint potential (E0) of the Trx1 active site (-230 mV) and identified a second redox-active dithiol/disulfide (Cys-62 and Cys-69) in an alpha helix proximal to the active site, which formed under oxidizing conditions. This non-active site disulfide was not a substrate for reduction by thioredoxin reductase and delayed the reduction of the active site disulfide by thioredoxin reductase. Within actively growing THP1 cells, most of the active site of Trx1 was in the dithiol form, whereas the non-active site was totally in the dithiol form. The addition of increasing concentrations of diamide to these cells resulted in oxidation of the active site at fairly low concentrations and oxidation of the non-active site at higher concentrations. Taken together these results suggest that the Cys-62-Cys-69 disulfide could provide a means to transiently inhibit Trx1 activity under conditions of redox signaling or oxidative stress, allowing more time for the sensing and transmission of oxidative signals.

Amino Acid Motifs↗
PubMed2004

An evaluation of electrode insertion techniques for measurement of redox potential in estuarine sediments.

E(h) measurements by electrodes are commonly used to characterize redox status of sediments in freshwater, marine and estuarine studies, due to the relative ease and rapidity of data collection. In our studies of fine-grained estuarine seabeds, we observed that E(h) values measured in intact sediment cores were influenced by different electrode insertion techniques. Sediment E(h) measurements generated via lateral insertion of platinum electrodes through silicone-filled ports in acrylic cores were systematically more positive (on the order of 10-100 mV) than profiles generated via vertical insertion of platinum electrodes downward through the sediment-water interface of the same cores. A review of the literature indicated that while researchers routinely use both insertion techniques to measure E(h), no discrepancy in output has previously been reported. We discuss the results of three experiments conducted to determine if the discrepancy in output was caused by electrode poisoning by sulfides during the stepwise vertical insertion technique, or was caused by contact of the electrode with the silicone plug during the lateral insertion technique. We conclude that contact between the platinum surface of the electrode and the silicone plug biases the E(h) measurements, resulting in erroneously positive E(h) values. Insertion of electrodes into sediment through silicone plugs produced E(h) values that were an average of 105.6 mV (+/-10.4 SE) more positive than values generated upon electrode insertion directly into sediment. Thus, we recommend against using an insertion technique where the platinum electrode remains in contact with the silicone plug, as this method results in misclassification of sediment redox state and estimated depth of the redoxcline.

Electrochemistry↗
PubMed1988

Electrochemistry of flavonoids. Relationships between redox potentials, inhibition of mitochondrial respiration, and production of oxygen radicals by flavonoids.

We have investigated the redox behavior of a series of structurally related flavonoids employing cyclic voltammetry under physiological conditions. The flavonoids that auto-oxidized and produced oxygen radicals had oxidation potentials (E 1/2) significantly lower [-30 to +60 mV vs (SCE)] than those that did not undergo auto-oxidation (+130 to +340 mV vs SCE). The range of E 1/2 values for the auto-oxidizable flavonoids was comparable to the E 1/2 range reported for the optimum quinone induced production of superoxide (O2 pi) in mitochondrial NADH-CoQ reductase (complex I). The most potent flavonoid inhibitors of mitochondrial succinate-CoQ reductase (complex II) possessed hydroxyl configurations capable of supporting redox reactions. For a series of 3,5,7-trihydroxyflavones that differed by b-ring hydroxylation it was found that decreasing E 1/2 of the flavonoids was associated with decreasing I50 values towards succinoxidase. These findings suggest that the electrochemical properties of the flavonoids may contribute to their biological activity.

Animals↗
PubMed1998

The role of heme propionate in controlling the redox potential of heme: square wave voltammetry of protoporphyrinato IX iron (III) in aqueous surfactant micelles.

The proton equilibrium in aqueous surfactant solutions of hemin, (PPIX)Fe(H2O)+2, involving the propionic acid groups is reported (PPIX = protoporphyrinato IX). The surfactant used are sodium dodecyl sulphate (SDS), triton X-100 (TX-100) and hexadecyl trimethylammonium bromide (CTAB). The pKa values, determined spectroscopically from the pH variations of the Soret absorption, are found at ca. 3.5 in SDS and TX-100 micelles. In the cationic surfactant (CTAB) the absorbance of hemin is independent of the hydrogen ion concentration in solution and this surfactant form a salt-link with the heme propionate group. The dependence of mid-point potential on pH indicates that the heme propionate undergoes a redox-linked changed in pKa from 3.2-3.5 in the ferric form to 4.4-4.5 in the ferrous form (27 degrees C, mu = 0.2 M). The change in the mid-point potential per unit change in pH. delta E/delta pH, is ca. -59 mV. Replacement of the water molecules in hemin by tetrahydrofuran, (PPIX)Fe(thf)+2, induces a considerable shift of pKa of the heme propionate: 6.2-6.8 in the ferric form and 7.5-7.8 in the ferrous form. The lower value of pKa in diaquo hemin is attributed to stabilisation of the propionate by a hydrogen bond with coordinated water molecules. All these results may be interpreted in terms of the involvement of a heme propionate group in redox linked uptake of protons and the influence of hydrogen bonding and salt-link formation on the pKa of the proton equilibrium.

Cetrimonium↗
PubMed2001

Role of hydrogen bonding interactions to N(3)H of the flavin mononucleotide cofactor in the modulation of the redox potentials of the Clostridium beijerinckii flavodoxin.

The role of the hydrogen bonding interaction with the N(3)H of the flavin cofactor in the modulation of the redox properties of flavoproteins has not been extensively investigated. In the flavodoxin from Clostridium beijerinckii, the gamma-carboxylate group of glutamate-59 serves as a dual hydrogen bond acceptor with the N(3)H of flavin mononucleotide (FMN) cofactor and the amide hydrogen of the adjacent polypeptide backbone in all three oxidation states. This "bridging" interaction serves to anchor the FMN in the binding site, which, based on the E59Q mutant, indirectly affects the stability of the neutral flavin semiquinone by facilitating a strong and critical interaction at the FMN N(5)H [Bradley, L. H., and Swenson, R. P. (1999) Biochemistry 38, 12377-12386]. In this study, the specific role of the N(3)H interaction itself was investigated through the systematic replacement of Glu59 by aspartate, asparagine, and alanine in an effort to weaken, disrupt, and/or eliminate this interaction, respectively. Just as for the E59Q mutant, each replacement significantly weakened the binding of the cofactor, particularly for the semiquinone state, affecting the midpoint potentials of each one-electron couple in opposite directions. (1)H-(15)N HSQC nuclear magnetic resonance (NMR) spectroscopic studies revealed that not only was the N(3)H interaction weakened as anticipated, but so also was the hydrogen bonding interaction with the N(5)H. Using the temperature coefficients of the N(5)H to quantify and correct for changes in this interaction, the contribution of the N(3)H hydrogen bond to the binding of each redox state of the FMN was isolated and estimated. Based on this analysis, the N(3)H hydrogen bonding interaction appears to contribute primarily to the stability of the oxidized state (by as much as 2 kcal/mol) and to a lesser extent the reduced states. It is concluded that this interaction contributes only modestly (<45 mV) to the modulation of the midpoint potential for each redox couple in the flavodoxin. These conclusions are generally consistent with ab initio calculations and model studies on the non-protein-bound cofactor.

Amino Acid Substitution↗
PubMed2003

Determination of the redox potentials and electron transfer properties of the FAD- and FMN-binding domains of the human oxidoreductase NR1.

Human novel reductase 1 (NR1) is an NADPH dependent diflavin oxidoreductase related to cytochrome P450 reductase (CPR). The FAD/NADPH- and FMN-binding domains of NR1 have been expressed and purified and their redox properties studied by stopped-flow and steady-state kinetic methods, and by potentiometry. The midpoint reduction potentials of the oxidized/semiquinone (-315 +/- 5 mV) and semiquinone/dihydroquinone (-365 +/- 15 mV) couples of the FAD/NADPH domain are similar to those for the FAD/NADPH domain of human CPR, but the rate of hydride transfer from NADPH to the FAD/NADPH domain of NR1 is approximately 200-fold slower. Hydride transfer is rate-limiting in steady-state reactions of the FAD/NADPH domain with artificial redox acceptors. Stopped-flow studies indicate that hydride transfer from the FAD/NADPH domain of NR1 to NADP+ is faster than hydride transfer in the physiological direction (NADPH to FAD), consistent with the measured reduction potentials of the FAD couples [midpoint potential for FAD redox couples is -340 mV, cf-320 mV for NAD(P)H]. The midpoint reduction potentials for the flavin couples in the FMN domain are -146 +/- 5 mV (oxidized/semiquinone) and -305 +/- 5 mV (semiquinone/dihydroquinone). The FMN oxidized/semiquinone couple indicates stabilization of the FMN semiquinone, consistent with (a) a need to transfer electrons from the FAD/NADPH domain to the FMN domain, and (b) the thermodynamic properties of the FMN domain in CPR and nitric oxide synthase. Despite overall structural resemblance of NR1 and CPR, our studies reveal thermodynamic similarities but major kinetic differences in the electron transfer reactions catalysed by the flavin-binding domains.

2,6-Dichloroindophenol↗
PubMed1977

Midpoint redox potentials of plant and algal ferredoxins.

Midpoint potentials of plant-type ferredoxins from a range of sources were measured by redox titrations combined with electron-paramagnetic-resonance spectroscopy. For ferredoxins from higher plants, green algae and most red algae, the midpoint potentials (at pH 8.0) were between --390 and --425 mV. Values for the major ferredoxin fractions from blue-green algae were less negative (between --325 and --390 mV). In addition, Spirulina maxima and Nostoc strain MAC contain second minor ferredoxin components with a different potential, --305 mV (the highest so far measured for a plant-algal ferrodoxin) for Spirulina ferrodoxin II, and --455 mV (the lowest so far measured for a plant-algal ferredoxin) for Nostoc strain MAC ferredoxin II. However, two ferredoxins extracted from a variety of the higher plant Pisum sativum (pea) had midpoint potentials that were only slightly different from each other. These values are discussed in terms of possible roles for the ferredoxins in addition to their involvement in photosynthetic electron transport.

Electron Spin Resonance Spectroscopy↗
PubMed2003

How cytochromes with different folds control heme redox potentials.

The electrochemical midpoint potentials (E(m)'s) of 13 cytochromes, in globin (c, c(2), c(551), c(553)), four-helix bundle (c', b(562)), alpha beta roll (b(5)), and beta sandwich (f) motifs, with E(m)'s spanning 450 mV were calculated with multiconformation continuum electrostatics (MCCE). MCCE calculates changes in oxidation free energy when a heme-axial ligand complex is moved from water into protein. Calculated and experimental E(m)'s are in good agreement for cytochromes with His-Met and bis-His ligated hemes, where microperoxidases provide reference E(m)'s. In all cytochromes, E(m)'s are raised by 130-260 mV relative to solvated hemes by the loss of reaction field (solvation) energy. However, there is no correlation between E(m) and heme surface exposure. Backbone amide dipoles in loops or helix termini near the axial ligands raise E(m)'s, but amides in helix bundles contribute little. Heme propionates lower E(m)'s. If the propionic acids are partially protonated in the reduced cytochrome, protons are released on heme oxidation, contributing to the pH dependence of the E(m). In all cytochromes studied except b(5)'s and low potential globins, buried side chains raise E(m)'s. MCCE samples ionizable group protonation states, heme redox states, and side chain rotamers simultaneously. Globins show the largest structural changes on heme oxidation and four-helix bundles the least. Given the calculated protein-induced E(m) shift and measured cytochrome E(m) the five-coordinate, His heme in c' is predicted to have a solution E(m) between that of isolated bis-His and His-Met hemes, while the reference E(m) for His-Ntr ligands in cytochrome f should be near that of His-Met hemes.

Bacteria↗