Electrostatic effects on the kinetics of oxidation-reduction reactions of c-type cytochromes

Photocatalytic Hedgehog Particles for High Ionic Strength Environments

High ionic strength environments can profoundly influence catalytic reactions involving charged species. However, control of selectivity and yield of heterogeneous catalytic reactions involving nano- and microscale colloids remains hypothetical because high ionic strength leads to aggregation of particle dispersions. Here we show that microscale hedgehog particles (HPs) with semiconductor nanoscale spikes display enhanced stability in solutions of monovalent/divalent salts in both aqueous and hydrophobic media. HPs enable tuning of photocatalytic reactions toward high-value products by adding concentrated inert salts to amplify local electrical fields in agreement with Derjaguin, Landau, Verwey, and Overbeek theory. After optimization of HP geometry for a model photocatalytic reaction, we show that high salt conditions increase the yield of HP-facilitated photooxidation of 2-phenoxy-1-phenylethanol to benzaldehyde and 2-phenoxyacetophenone by 6 and 35 times, respectively. Depending on salinity, electrical fields at the HP-media interface increase from 1.7 × 10⁴ V/m to 8.5 × 10⁷ V/m, with high fields favoring products generated via intermediate cation radicals rather than neutral species. Electron transfer rates were modulated by varying the ionic strength, which affords a convenient and hardly used reaction pathway for engineering a multitude of redox reactions including those involved in the environmental remediation of briny and salty water.

Ascorbate peroxidase, a key molecule regulating amphotericin B resistance in clinical isolates of Leishmania donovani

Amphotericin B (AmB), a polyene macrolide, is now a first-line treatment of visceral leishmaniasis cases refractory to antimonials in India. AmB relapse cases and the emergence of secondary resistance have now been reported. To understand the mechanism of AmB, differentially expressed genes in AmB resistance strains were identified by a DNA microarray and real-time reverse transcriptase PCR (RT-PCR) approach. Of the many genes functionally overexpressed in the presence of AmB, the ascorbate peroxidase gene from a resistant Leishmania donovani strain (LdAPx gene) was selected because the gene is present only in Leishmania, not in humans. Apoptosis-like cell death after exposure to AmB was investigated in a wild-type (WT) strain in which the LdAPx gene was overexpressed and in AmB-sensitive and -resistant strains. A higher percentage of apoptosis-like cell death after AmB treatment was noticed in the sensitive strain than in both the resistant isolate and the strain sensitive to LdAPx overexpression. This event is preceded by AmB-induced formation of reactive oxygen species and elevation of the cytosolic calcium level. Enhanced cytosolic calcium was found to be responsible for depolarization of the mitochondrial membrane potential and the release of cytochrome c (Cyt c) into the cytosol. The redox behavior of Cyt c showed that it has a role in the regulation of apoptosis-like cell death by activating metacaspase- and caspase-like proteins and causing concomitant nuclear alterations, as determined by terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) and DNA fragmentation in the resistant strain. The present study suggests that constitutive overexpression of LdAPx in the L. donovani AmB-resistant strain prevents cells from the deleterious effect of oxidative stress, i.e., mitochondrial dysfunction and cellular death induced by AmB.

The effect of ionic strength on the electron-transfer rate of surface immobilized cytochrome C

Horse heart cytochrome c was immobilized on four different self-assembled monolayer (SAM) films. The electron tunneling kinetics were studied in the different assemblies as a function of the ionic strength of the buffer solution using cyclic voltammetry. When cytochrome c is electrostatically immobilized, the standard electron exchange rate constant k0 decreases with the increase of the solution's ionic strength. In contrast, the protein covalently attached or ligated has a rate constant independent of the ionic strength. The inhomogeneity of electrostatically immobilized cytochrome c increases with the increase of the solution's ionic strength whereas that of the covalently attached protein is independent of the ionic strength. A comparison of these different electron-transfer behaviors suggests that the thermodynamically stable geometry of cytochrome c in the electrostatic assemblies is also an electron transfer favorable one. It suggests that the surface charges of cytochrome c are capable of guiding it into geometries in which its front surface faces the electron-transfer partner. The inhomogeneity observed in this study indicates that a distribution of cytochrome c orientations and thus a distribution of electron transfer rate constants exists.

Recognition and modulation of cytochrome c's redox properties using an amphiphilic homopolymer

An amphiphilic homopolymer scaffold has been used to bind to the protein, cytochrome c. This interaction is analyzed using cyclic voltammetry, native gel electrophoresis, UV-visible absorption, and circular dichroism spectroscopy. The polymer binds to cytochrome c with micromolar affinity and the association of polymer with cytochrome c leads to a structural change of the protein. This conformational change exposes the heme unit of the protein, which affords an opportunity to reversibly modulate its electron-transfer properties. We have also shown that the electrostatic binding of polymer to cytochrome c can be used to disrupt its interaction with its natural partner, cytochrome c peroxidase.

Reductions by ferrocytochrome c peroxidase: 5. Kinetics of ferricyanide reduction

The kinetics of reduction of ferricyanide by yeast ferrocytochrome c peroxidase (CPPII) were investigated as a function of ionic strength in phosphate buffers at pH 7.0 and 25 ± 1 °C. The observed bimolecular rate constant (k12) is 8.4 × 104 M−1s−1in 0.1 M phosphate. The dependence of the reaction rate on ionic strength indicates a change of −9 on the protein at pH 7.0, which is in good agreement with the total charge of −11 estimated for CCPIIfrom its amino acid content. Substituting k12at infinite ionic strength [Formula: see text] into the Marcus cross relation yields an electron self-exchange rate constant [Formula: see text] for the FeIII/FeIIcouple of CCP of 7.2 × 10−5 M−1s−1. This value is over four orders of magnitude higher than that calculated for the FeIV/FeIIIcouple of CCP from literature data for cross-reactions with ferrocyanide at pH 7.0. Possible reasons for the large difference in the two CCP [Formula: see text] values are discussed. Literature data also allowed [Formula: see text] values for various other heme proteins to be determined from their cross-reactions with ferricyanide. The calculated rate constants vary by eight orders of magnitude, and the variation of [Formula: see text] with protein structure suggests that the redox reactivity of ferrous heme proteins towards ferricyanide is dependent on the spin state and coordination of iron, as well as on the accessibility of the heme. Keywords: cytochrome c peroxidase, ferricyanide, Marcus cross relation, electron self-exchange.

Molecular dynamics simulations of rubredoxin from Clostridium pasteurianum: changes in structure and electrostatic potential during redox reactions

Molecular dynamics simulations of Clostridium pasteurianum rubredoxin in the oxidized and reduced forms have been performed. Good agreement between both forms and crystal data has been obtained (rms deviation of backbone atoms of 1.06 and 1.42 A, respectively), which was due in part to the use of explicit solvent and counterions. The reduced form exhibits an unexpected structural change: the redox site becomes much more solvent-accessible, so that water enters a channel between the surface and the site, but with little actual structural rearrangement (the rms deviation of backbone atoms between the oxidized and reduced is 0.77 A). The increase in solvent accessibility is also seen, although to a much lesser extent, between the oxidized and reduced crystal structures of Pyrococcus furiosus rubredoxin, but no high resolution crystal or nuclear magnetic resonance solution data exist for reduced C. pasteurianum rubredoxin. The electrostatic potential at the iron site and fluctuations in the potential, which contribute to both the redox and electron transfer properties, have also been evaluated for both the oxidized and the reduced simulations. These results show that the backbone plays a significant role (62-70 kcal/mol/e) and the polar side chains contribute relatively little (0-4 kcal/mol/e) to the absolute electrostatic potential at the iron of rubredoxin for both forms. However, both groups contribute significantly to the change in redox state by becoming more polarized and more densely packed around the redox site upon reduction. Furthermore, these results show that the solvent becomes much more polarized in the reduced form than in the oxidized form, even excluding the penetrating water. Finally, the simulation indicates that the contribution of the charged side chains to the electrostatic potential is largely canceled by that of the counterions.

Pressure modulation of cytochrome-to-cytochrome electron-transfer. Models and enzyme reactions

The kinetics of electron-transfer involved in reactions of reduction of 2,6-dichlorophenol indophenol and Fe(CN)3-(6) by L-ascorbic acid and reduction of ferric cytochrome c by both L-ascorbic acid and reduced hydroxylamine oxidoreductase were studied as a function of three parameters: ionic strength, pressure (1-2000 bar) and temperature (4-20 degrees C) using the high-pressure stopped-flow method. From measurements, the thermodynamic parameters of activation volume (delta V++), and, when possible, activation enthalpy and entropy (delta H++ and delta S++) have been calculated. We found, for these four systems, that the pressure has revealed solvation effects involved in electron-transfer. For the reduction of ferric cytochrome c by reduced hydroxylamine oxidoreductase (a cytochrome-to-cytochrome electron-transfer), we have not obtained evidence for a conformational change.

Interaction of cytochrome c with cytochrome c oxidase: an understanding of the high- to low-affinity transition

The steady-state kinetics of high- and low-affinity electron transfer reactions between various cytochromes c and cytochrome c oxidase (ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1) preparations were studied spectrophotometrically and polarographically. The dissociation constants for the binding of the first and second molecules of horse cytochrome c (I = 15 mM) are 5.10(-8) M and 1.10(-5) M, respectively, close to the spectrophotometric Km values and consistent with the controlled binding model for the interaction between cytochrome c and cytochrome oxidase (Speck, S.H., Dye, D. and Margoliash, E. (1984) Proc. Natl. Acad. Sci. USA 81, 346-351) which postulates that the binding of a second molecule of cytochrome c weakens that of the first, resulting in low-affinity kinetics. While the Km of the polarographically assayed high-affinity reaction is comparable to that observed spectrophotometrically, the low-affinity Km is over an order of magnitude smaller and cannot be attributed to the binding of a second molecule of cytochrome c. Increasing the viscosity has no effect on the Vmax of the low-affinity reaction assayed polarographically, but increases the Km. Thus, the transition from high- to low-affinity kinetics is dependent on the frequency of productive collisions, as expected for a hysteresis model ascribing the transition to the trapping of the oxidase in a primed state for turnover. At ionic strengths above 150 mM, the rate of cytochrome c oxidation decreases without any correlation to the calculated net charge of the cytochrome c, indicating rate-limiting rearrangement of the two proteins in proximity to each other.

The effect of complex formation upon the reduction rates of cytochrome c and cytochrome c peroxidase compound II

The effect of complex formation between ferricytochrome c and cytochrome c peroxidase (Ferrocytochrome-c:hydrogen peroxide oxidoreductase, EC 1.11.1.5) on the reduction of cytochrome c by N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), reduced N-methylphenazonium methosulfate (PMSH), and ascorbate has been determined at low ionic strength (pH 7) and 25 degrees C. Complex formation with the peroxidase enhances the rate of ferricytochrome c reduction by the neutral reductants TMPD and PMSH. Under all experimental conditions investigated, complex formation with cytochrome c peroxidase inhibits the ascorbate reduction of ferricytochrome c. This inhibition is due to the unfavorable electrostatic interactions between the ascorbate dianion and the negatively charged cytochrome c-cytochrome c peroxidase complex. Corrections for the electrostatic term by extrapolating the data to infinite ionic strength suggest that ascorbate can reduce cytochrome c peroxidase-bound cytochrome c faster than free cytochrome c. Reduction of cytochrome c peroxidase Compound II by dicyanobis(1,10-phenanthroline)iron(II) (Fe(phen)2(CN)2) is essentially unaffected by complex formation between the enzyme and ferricytochrome c at low ionic strength (pH 6) and 25 degrees C. However, reduction of Compound II by the negatively changed tetracyano-(1,10-phenanthroline)iron(II) (Fe(phen)(CN)4) is enhanced in the presence of ferricytochrome c. This enhancement is due to the more favorable electrostatic interactions between the reductant and cytochrome c-cytochrome c peroxidase Compound II complex then for Compound II itself. These studies indicate that complex formation between cytochrome c and cytochrome c peroxidase does not sterically block the electron-transfer pathways from these small nonphysiological reductants to the hemes in these two proteins.

Charge interactions of cytochrome c with cytochrome c oxidase

The pyridoxal phosphate (PLP) modification of the lysine amino groups in cytochrome c causes decrease in the reaction rate with cytochrome c oxidase. The rate constants for (PLP)2-cyt. c, PLP(Lys 86)-cyt. c, PLP(Lys 79)-cyt. c and native cytochrome c (at pH 7.4, I = 0.02) are 3.6 X 10(-3) sec-1, 5.5 X 10(-3) sec-1, 5.2 X 10(-3) sec-1 and 9.8 X 10(-3) sec-1, respectively. In spite of the same positive charge of singly PLP-cytochromes c the reaction between PLP(Lys 86)-cyt. c and cyt. c oxidase exhibits the ionic strength dependence that differs from those of the PLP(Lys 79)-cyt. c. The rate constants at zero and infinite ionic strength for PLP(Lys 86)-cyt. c is 2-fold less than that for PLP(Lys 79)-cyt. c. The positively charged cytochrome c lysines 86 and 79 form two from four or five predicted complementary charge interactions with carboxyl groups on cytochrome c oxidase.

Transient kinetics of the one-electron transfer reaction between reduced flavocytochrome b2 and oxidized cytochrome c. Evidence for the existence of a protein complex in the reaction

The one-electron transfer reaction from reduced flavocytochrome b2 (fully reduced by three electron equivalents) to ferricytochrome c, both purified from the yeast Hansenula anomala, has been studied using stopped-flow spectrophotometry in the course of a single turnover, for reactants initially mixed in a heme molar ratio equal to one. The cytochrome c reduction proceeded to completion through an apparently first-order process. Depending on the experimental conditions (concentrations and or ionic strength), the reduction is of second-order or first-order character. To interpret these kinetic results computer simulation studies have been performed based on a kinetic scheme involving, besides the formation of a complex before the electron transfer step, intramolecular electron transfer steps within flavocytochrome b2 to maintain the concentration of the specific electron donor center, the reduced cytochrome b2. As far as the cytochrome c reduction rate constant, ka, and its variations were concerned the simulated data showed that this complicated scheme could approximate a mechanism which is by far the simplest, involving only the two former steps. Such a scheme accounts firstly for the hyperbolic dependence of the rate of reduction of cytochrome c, ka, upon reductant concentrations which had provided clear evidence for the kinetic existence of a complex in the reaction pathway. At 5 degrees C the rate constant for the electron transfer is 380 s-1 with an activation energy of 13.8kJ mol-1 (3.3 kcal mol-1). Secondly it predicts the observed variations of ka with ionic strength and provides estimates of the rate constants of the binding step.

The oxidation-reduction kinetics of the reaction of cytochrome c1 with non-physiological redox agents

The kinetics of the oxidation-reduction reactions of cytochrome c1 with ascorbate, ferricyanide, triphenanthrolinecobalt(III) and N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) have been examined using the stopped-flow technique. The reduction of ferricytochrome c1 by ascorbic acid is investigated as a function of pH. It is shown that at neutral and alkaline pH the reduction of the protein is mainly performed by the doubly deprotonated form of ascorbate. From the ionic-strength-dependence studies of the reactions of cytochrome c1 with ascorbate, ferricyanide and triphenanthrolinecobalt(III), it is demonstrated that the reactions rate is governed by electrostatic interactions. The second-order rate constants for the reaction of cytochrome c1 with ascorbate, ferricyanide, TMPD and triphenanthrolinecobalt(III) are 1.4 . 10(4), 3.2 . 10(3), 3.8 . 10(4) and 1.3 . 10(8) M-1 . s-1 (pH 7.9, I = 0, 10 degrees C), respectively. Application of the Debye-Hückel theory to the data of the ionic-strength-dependence studies of these redox reactions of cytochrome c1 yielded for ferrocytochrome c1 and ferricytochrome c1 a net charge of --5 and --4, respectively. The latter value is close to that of --3 for the oxidized enzyme, calculated from the amino acid sequence of the protein. This implies that not a local charge on the surface of the protein, but the overall net charge of cytochrome c1 governs the reaction rate with small redox molecules.

Dielectric constant dependence of biological oxidation-reduction. 1. A model of polarity-dependent ferrocytochrome c oxidation

A theoretical model for the effect of the dielectric constant (c) of the solvent medium on ferrocytochrome c oxidation by ferricyanide is developed to account for the observed variations of the rate constant (k) of reactions in aqueous binary mixtures with alcohols (less than 5-10 mol% ethanol and propranolol). A correlation between k and c is found if ln k is expressed as a function of the Kirkwood parameter (c-1)(2c+1). The results of calculations indicate that the use of the 'overall dipole moment' of cytochrome c in oxidoreduction studies is likely to be unreliable. Instead, the decrease in k in alcohol/water mixtures is best explained--in conformity with Onsager's theory of the reaction field--by a polarity effect on the dipole moment of the cytochrome c heme upon diffusion of the polar solvent molecules into the low dielectric constant heme crevice.

The transport of cations in mitochondria

The present paper has reviewed several factors related to ion transport and examined the properties of cation transport in mitochondria. The analysis suggests that: (1) The concept that a metabolically dependent electrical potential across the mitochondrial membrane plays a role in determining ion fluxes and steady-state concentrations is not justified and the data indicate that such exchanges are generally electroneutral. (2) Generally, the influx and efflux of an ion proceed by the same mechanism with at least one exception. (3) There are indications that some of the steps in transport are common to several cations. (4) The idea that carrier or ionophoric molecules are involved in cation transport has been examined in some detail together with the possible involvement of some known mitochondrial components. In particular, a model has been introduced in which local charge imbalances produced by H+ fluxes serve as the driving force of transport. The molecules of the complex are arranged in series in a tripartite arrangement including a filter or gate, a nonselective channel and an H+-transferring portion linked to either electron transport or the ATPase. Parts of this model have been introduced by other investigators. Models in which different portions of channels have differing functions have been proposed previously for other transport systems.

Molecular interpretation of kinetic-ionic strength effects

A recent and important approach to investigating electron transfer mechanisms of redox proteins has been through kinetic-ionic strength studies. There is, however, significant controversy as to whether such studies (1) yield information regarding the charge (or location) of the electron transfer site or (2) more simply reflect the influence of net or overall protein charge on the electrostatic interactions. A critical analysis using different theoretical approaches is made of our recent work and of the bulk of the published non-physiological small molecule-protein and protein-protein kinetic ionic strength studies; it is concluded that (1) the approximated Bronsted-Debye-Huckel equation can not be used at all for protein redox reactions, (2) irrespective of the theoretical approaches discussed, such studies do not provide information regarding the charge of the electron transfer site, (3) it is the net charge of the reactants that control the electrostatic interactions, (4) both the equation derived by Wherland and Gray and the full Bronsted-Debye-Huckel equation provide reasonably good approximations of net protein charge, (5) pH changes quantitatively modulate net protein charge, and (6) thus, protein redox rates need to be electrostatically corrected if relevant interpretations of kinetic-ionic strength experiments are to be made.

The ionic strength dependence of the rate of a reaction between a small ion and a large ion with a dipole moment

The ionic strength dependence of the rate constant of a reaction between a small ion and a large ion with a dipole moment (e.g. a protein) is described. This description takes into account only the electrostatic interactions between the two ions. This approach agrees with the Marcus theory treatment of the electrostatic interactions and also with the Debye-Hückel theory which is based on changes in the activity coefficients of the reactants. The contribution of the dipole moment of the protein to the ionic strength dependence of the rate constant has been calculated. A method is described whereby one can calculated the charge of the protein without knowing the precise ionic strength dependence of the rate constant. Two applications are mentioned to illustrate the usefulness of the method.

The use of specific lysine modifications to locate the reaction site of cytochrome c with sulfite oxidase

The reduction of cytochrome c by beef liver sulfite oxidase was found to be strongly inhibited by high ionic strength, indicating the importance of electrostatic interactions to the reaction. The reaction rates of sulfite oxidase with singly trifluoroacetylated or trifluoromethylphenylcarbamylated cytochrome c derivatives were studied to determine the role of individual lysines in the reaction. The reaction rate was decreased by modification of the lysines immediately surrounding the heme crevice, the decreases following the order: Lys 13 greater than Lys 25 congruent to Lys 79 approximately equal to Lys 87 greater than Lys 8 approximately equal to Lys 27 approximately equal to Lys 72. Modification of lysines 22, 55, 88, 99, and 100 had no effect on the reaction rate. These results indicate that the interaction site on cytochrome c for sulfite oxidase is at the heme crevice region, and overlaps considerable with that for cytochrome oxidase.

The effect of pH and ionic strength on the steady-state activity of isolated cytochrome C oxidase

1. The turnover number and apparent Km of isolated beef-heart cytochrome c oxidase were found to increase continuously when the pH was lowered from 8.6 to 4.6 (turnover number 32-630 s-1). In this pH range neither irreversible denaturation of the enzyme nor an optimum for the turnover number was observed. 2. The turnover number of cytochrome c oxidase was found to be independent of ionic strength. It was concluded that the dependence of the activity of cytochrome c oxidase on ionic strength is caused by a change in the value of Km for cytochrome c. 3. The pH dependence of the turnover number of cytochrome c oxidase can be described by a simple model in which at least three sites on the complex of cytochrome c oxidase with cytochrome c (pKa 8.0, 6.5 and 4.8) can take up a proton.