The redox potential of cytochrome c-552 from Euglena gracillis: a thermodynamic study

Redox properties of cytochrome c

The redox properties of cytochromes (cyt) c, a ubiquitous class of heme-containing electron transport proteins, have been extensively investigated over the last two decades. The reduction potential (E degrees') is central to the chemistry of cyt c for two main reasons. First, E degrees' influences both the thermodynamic and kinetic aspects of the electron exchange reaction with redox partners. Second, this thermodynamic parameter is remarkably sensitive to changes in the properties of the heme and the protein matrix, and hence can be profitably used for the investigation of the solution chemistry of cyt c. This research area owes much to the exploitation of voltammetric techniques for the determination of E degrees' for metalloproteins, which dates back to the late 1970s. Since then, much effort has been devoted to the comprehension of the molecular factors that control E degrees' in cyt c, which include first coordination sphere effects on the heme iron, the interactions of the heme group with the surrounding polypeptide chain and the solvent, and also include medium effects related to the nature and ionic composition of the solvent, pH, the presence of potential protein ligands, and the temperature. This article provides an overview of the most significant advances made in this field recently.

Effects of Specific Anion-Protein Binding on the Alkaline Transition of Cytochrome c

The thermodynamic parameters of the alkaline transition of beef heart ferricytochrome c have been measured through direct electrochemistry experiments carried out at variable pH and temperature in the presence of different sulfate concentrations. Sulfate is known to bind specifically to cytochrome c in a sequential manner at two surface sites. The effects of such a specific binding reflect on the thermodynamics of the transition and can be satisfactorily interpreted within the frame of the Debye-Hückel theory with simple electrostatic considerations. In particular, the increase in the thermodynamic pKa values (extrapolated to I = 0) upon sulfate binding turns out to be a fully enthalpic effect which can be accounted for by considering the coulombic effects of the formation of ionic couple(s) on the protein surface. This study also shows that the apparent pKa values at finite ionic strength are only moderately affected by the nature of the anion in solution, and differences tend to vanish at high ionic strength.

Thermodynamics of the alkaline transition of cytochrome c

The apparent equilibrium constant (Kapp) of the alkaline transition (AT) of beef heart cytochrome c, obtained from pH titrations of the current intensities in cyclic voltammetry experiments, has been measured as a function of the temperature from 5 to 65 degrees C, at different ionic strength (I = 0.01-0.2 M). The temperature profile of the pKapp values is biphasic and yields two distinct sets of DeltaH degrees 'AT and DeltaS degrees 'AT values below and above approximately 40 degrees C. In the low-temperature range, the process is endothermic and is accompanied by a small positive entropy change, while at higher temperatures it becomes less endothermic and involves a pronounced entropy loss. The temperature dependence of the transition thermodynamics is most likely the result of the thermal transition of native ferricytochrome c from a low-T to an high-T conformer which occurs at alkaline pH values at a temperature comparable with above (Ikeshoji, T., Taniguchi, I., and Hawkridge, F. M. (1989) J. Electroanal. Chem. 270, 297-308; Battistuzzi, G., Borsari, M., Sola, M., and Francia, F. (1997) Biochemistry 36, 16247-16258). Thus, it is apparent that the transitions of the two native conformers to the corresponding alkaline form(s) are thermodynamically distinct processes. It is suggested that this difference arises from either peculiar transition-induced changes in the hydration sphere of the protein or to the preferential binding of different lysines to the heme iron in the two temperature ranges. Extrapolation of the Kapp values at null ionic strength allowed the determination of the thermodynamic equilibrium constants (Ka) at each temperature, hence of the "true" standard thermodynamic parameters of the transition. The pKa value at 25 degrees C was found to be 8.0. A pKapp value of 14.4 was calculated for the alkaline transition of ferrocytochrome c at 25 degrees C and I = 0.1 M. The much greater relative stabilization of the native state in the reduced as compared to the oxidized form turns out to be almost entirely enthalpic in origin, and is most likely due to the greater affinity of the methionine sulfur for the Fe(II) ion. Finally, it is found that the Debye-Hückel theory fits the ionic strength dependence of the pKapp values, at least qualitatively, as observed previously for the ionic strength dependence of the reduction potential of this protein class. It is apparent that the increase in the pKapp values with increasing ionic strength is for the most part an entropic effect.

Ionic strength-dependent physicochemical factors in cytochrome c3 regulating the electron transfer rate

The effect of ionic strength on the macroscopic and microscopic redox potentials and the heme environment of cytochrome c3 from Desulfovibrio vulgaris Miyazaki F have been investigated by NMR and electrochemical methods. The redox potentials of this tetraheme protein are found to be ionic strength-dependent. Especially, the microscopic redox potentials of hemes 2 and 3 at the fourth reduction step increase significantly with increasing ionic strength, which is in contraction to the theoretical expectation. The coordinated imidazole proton signals are unaffected by ionic strength. However, the methyl and propionate proton signals of hemes 1 and 4 showed significant ionic strength dependencies that are distinct from those for hemes 2 and 3. This heme classification is the same as that found in the ionic strength dependencies of the microscopic redox potentials at the fourth reduction step. Furthermore, the effect of ionic strength on the electrostatic potentials at the heme irons has been examined on the theoretical basis. The electrostatic potential at heme 4 changes up to 1 M ionic strength, which was not expected from the observations reported on cytochromes so far. These results are discussed in connection with the reported anomalous ionic strength dependency of the reduction rate of cytochrome c3.

Anion binding to cytochrome c2: implications on protein-Ion interactions in class I cytochromes c

The binding of several inorganic and carboxylate anions to cytochrome c2 from Rhodopseudomonas palustris has been investigated by monitoring the salt-induced changes in the redox potential of the heme, using an interpretative model based on the extended Debye-Hückel equation. Most anions were found to interact specifically with the protein at one or multiple sites. Binding constants to the oxidized protein in the range 10(1)-10(2) m-1 were determined from the anion concentration dependence of the chemical shift of the isotropically shifted heme methyl resonances. For several anions the stoichiometry and strength of the binding to cytochrome c2 were found comparable with those determined for mitochondrial cytochromes c, in spite of the limited sequence similarity (less than 40%) and the lower positive charge of the bacterial protein. These analogies were interpreted as indicative of the existence of common binding sites which are proposed to be located in the conserved lysine-rich domain around the solvent-exposed heme edge, which is also the surface area likely involved in the interaction with redox partners. The changes in E degrees due to partial neutralization of the positive charge of cytochrome c2 due to specific anion binding were found comparable with those for the mitochondrial species.

Anion binding to mitochondrial cytochromes c studied through electrochemistry. Effects of the neutralization of surface charges on the redox potential

The redox potential of horse and bovine heart cytochromes c determined through cyclic voltammetry is exploited to probe for anion-protein interactions, using a Debye-Hückel-based model. In parallel, protein charge neutralization resulting from specific anion binding allows monitoring for surface-charge/E(o) relationships. This approach shows that a number of anions, most of which are of biological relevance, namely CI-, HPO(2-)4, HCO3-, NO3, SO(2-)4, CIO4-, citrate3- and oxalate2-, bind specifically to the protein surface, often in a sequential manner as a result of the presence of multiple sites with different affinities. The binding stoichiometries of the various anions toward a given cytochrome are in general different. Chloride and phosphate appear to bind to a greater extent to both proteins as compared to the other anions. Differences in binding specificity toward the two cytochromes, although highly sequence-related, are observed for a few anions. The data are discussed comparatively in terms of electrostatic and geometric properties of the anions and by reference to the proposed location and amino acid composition of the anion binding sites, when available. Specific binding of this large set of anions bearing different charges allows the electrostatic effect on Eo due to neutralization of net positive protein surface charge(s) to be monitored. (J)H NMR indeed indicates the absence of significant salt-induced structural perturbations, hence the above change in Eo is predominantly electrostatic in origin. A systematic study of protein surface-charge/Eo relationships using this approach is unprecedented. Values of 15-25 mV (extrapolated at zero ionic strength) are obtained for the decrease in Eo due to neutralization of one positive surface charge, which are of the same order of magnitude as previous estimates obtained with either mutation or chemical modification of surface lysines. The effects of the anion-induced decrease of net positive charge on Eo persist also at a relatively high ionic strength and add to the general effects related to the charge shielding of the protein as a whole due to the surrounding ionic atmosphere: hence the ionic strength dependence of the rate of electron transfer between cytochromes c and redox partners could also involve salt-induced changes in the driving force.

Cyclic voltammetry and 1H-NMR of Rhodopseudomonas palustris cytochrome c2. Probing surface charges through anion-binding studies

The effects of increasing concentrations of Cl-, ClO4-, and HCO3- on the redox potential of Rhodopseudomonas palustris cytochrome c2 indicate that the two polyatomic anions bind specifically to the protein at one site, while chloride simply exerts an ionic atmosphere effect. The change in E degree upon specific anion binding allows us to probe for the influence of surface charges on the redox potential of cytochromes c. The decrease in redox potential at null ionic strength (delta E degree I = 0) due to anion neutralization of one positive surface charge was found to be 23 mV with perchlorate and 33 mV with bicarbonate. These values compare reasonably well with previous theoretical predictions and estimates of the effect of charge alteration on the E degree values in cytochromes c chemically modified or mutated at surface lysines. These delta E degree values, determined on the unmodified protein, are unprecedented for c-type cytochromes. The anion-induced chemical shift changes of the hyperfine-shifted heme 1H-NMR resonances of the oxidized protein yield lower limit values of 53 M-1 and 18 M-1 for the affinity constant for specific HCO3- and ClO4- binding, respectively.

Kinetics of the reduction of wild-type and mutant cytochrome c-550 by methylamine dehydrogenase and amicyanin from Thiobacillus versutus

To elucidate the kinetic properties of the methylamine dehydrogenase (MADH) redox chain of Thiobacillus versutus the reduction of cytochrome c-550 by MADH and amicyanin has been studied. Under steady state conditions, the rate constants of the reactions have been determined as a function of the ionic strength, both for wild type cytochrome c-550 and for mutants in which the conserved residue Lys14 has been replaced as follows: Lys14-->Gln (mutant [K14Q]cytochrome c-550) and Lys14-->Glu (mutant [K14E]cytochrome c-550). The second-order rate constant of the reduction of cytochrome c-550 by MADH shows a biphasic ionic-strength dependence. At low ionic strength the rate constant remains unchanged (wild type) or increases ([K14Q]cytochrome c-550) with increasing ionic strength, while at high salt concentrations the rate constant decreases monotonically as the ionic strength increases. It is suggested that conformational freedom exists in the association complex and that this is favourable for electron transfer. [K14Q]cytochrome c-550 and [K14E]cytochrome c-550 are reduced at rates 20-fold and 500-fold slower than wild-type cytochrome c-550 by MADH, due to a lower association constant. It is concluded that MADH possesses a negative patch with which cytochrome c-550 associates. Lys14 plays an important role in the formation of the reaction complex. The midpoint potentials of wild-type and mutant cytochrome c-550 have been determined by using cyclic voltammetry. [K14Q]cytochrome c-550 and [K14E]cytochrome c-550 show an increase in E0 of only 2 mV and 8 mV, respectively, compared to wild-type cytochrome c-550 (241 mV at pH 8.1). [K14Q]cytochrome c-550 and [K14E]cytochrome c-550 cytochrome c-550 are reduced by amicyanin at rates that are only slightly faster than for wild-type cytochrome c-550. The difference is partly attributable to the change in E0. High ionic strength results in a threefold increase in the rate in all three cases. These results indicate that charge interactions do not play a major role in the formation of the amicyanin/cytochrome c-550 reaction complex, suggesting an interaction at the hydrophobic patch of amicyanin. The reduction of cytochrome c-550 by MADH can be inhibited by Zn(2+)-substituted amicyanin. Ag(+)-amicyanin, however, has little effect on the reduction rate. These results suggest that MADH has a much higher affinity for Cu(2+)-amicyanin (substrate) than for Cu(+)-amicyanin (product). On the basis of these findings the roles of the components of the MADH redox chain are discussed.

Laser flash photolysis studies of electron transfer to the cytochrome b5-cytochrome c complex

Rate constants for electron transfer in the complex between recombinant rat mitochondrial outer membrane cytochrome b5 or the tryptic fragment of bovine liver cytochrome b5 and horse mitochondrial cytochrome c were measured by laser flash photolysis of 5-deazariboflavin-EDTA solutions. When an excess of cytochrome b5 was titrated with increasing amounts of cytochrome c at low ionic strength and electron transfer was initiated by a laser flash, both proteins were rapidly reduced by deazariboflavin semiquinone. The initial photoreduction was followed by a slower second-order reduction of b5 complexed oxidized cytochrome c by free reduced cytochrome b5. At an 8:1 ratio of cytochromes b5 to c, the pseudo-first-order rate constant for reduction of complexed cytochrome c increased 3-5-fold between ionic strengths of 5 and 40 mM, and then dropped precipitously at higher ionic strengths. The ionic strength dependent increase in rate constant is likely to be due to relief of steric hindrance via rearrangement of cytochrome c in the complex. The reaction rate showed no sign of saturation at any ionic strength, indicating a first-order rate constant greater than 10(4) s-1 within a transient ternary protein complex; i.e., interprotein electron transfer approaches the largest values previously reported for the stable binary protein complex (approximately 4 x 10(5) s-1). Our results emphasize the flexibility of electron-transfer protein complexes, which had previously been modeled in a single conformation with specific salt bridges. It appears that a variety of orientations can exist within such protein-protein complexes and that the population of conformations changes with ionic strength.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of surface charges on the redox potential of cytochrome c2 from the purple phototrophic bacterium Rhodobacter capsulatus

Four site-directed mutants of Rhodobacter capsulatus cytochrome c2, which substitute lysines at three positions with aspartate or glutamate, have been prepared. Mutations included the single charge substitutions K12D, K14E, and K32E and a double charge substitution K14E/K32E. Characterization of the ionic strength dependence of the wild-type and mutant redox potentials in the "nonbinding" buffer Tris-cacodylate suggests that (i) at zero ionic strength introduction of negatively charged groups stabilizes the oxidized state by 11-14 mV per charge and (ii) at high ionic strengths where the charged groups are masked, the effects of single charge substitutions are overcome; however, the redox potential of the double charge substitution is still affected. These results indicate that at physiological ionic strengths charge distribution only affects redox potential when the heme environment has been perturbed by a structural perturbation and that the determinants of redox potential in c-type cytochromes is primarily due to the local heme environment.

The electrode reaction of Euglena gracilis cytochrome c-552 at edge-oriented pyrolytic graphite

The direct electron transfer reaction of Euglena gracilis cytochrome c-552 at edge-oriented pyrolytic graphite electrodes was determined by cyclic voltammetry to be quasi-reversible, stable and reproducible. The presence of a persistent layer of irreversibly adsorbed cytochrome c-552 on the electrode surface was detected in these experiments. Heterogeneous electron transfer rate constants for both the diffusing and adsorbed forms of the protein are reported, and mechanistic aspects are addressed. The applicability of cytochrome c-552 as a complementarily charged analog of eucaryotic cytochrome c in interfacial bioelectrochemical studies is discussed.

Factors influencing redox potentials of electron transfer proteins

The redox potentials of electron transfer proteins vary over a wide range, even when the type of redox center is the same. Rees [Proc. Natl. Acad. Sci. USA (1985) 82, 3082-3085] proposed that this variation of redox potential partly reflects the different net charges of the proteins, and he presented a linear correlation between these two properties for 36 proteins. A review of the factors that influence protein redox potentials makes it clear that this linear correlation is fortuitous. The key factors influencing redox potentials are the contributions to the Gibbs energy difference between the two redox states, resulting from bonding interactions at the redox center, electrostatic interactions between the redox-center charge and polar groups within the protein and solvent, and redox-state conformational changes. The relative importance of these terms is likely to vary from protein to protein.

Amino acid sequence, haem-iron co-ordination geometry and functional properties of mitochondrial and bacterial c-type cytochromes

Cytochromes are found in all biological oxidation Systems which involve transport of reducing equivalents through organized chains of membrane bound intermediates, regardless of the ultimate oxidant (Keilin, 1966; Bartsch, 1978; Meyer & Kamen, 1982). Thus, cytochromes are present not only in the aerobic mitochondrial and bac-terial respiratory chain, but are also found in much more diversified procariotic Systems, including all varieties of facultative anaerobes (nitrate and nitrite reducers), obligate anaerobes (sulphate reducers and phototrophic sulphur bacteria), facultative photoheterotrophes (phototrophic non-sulphur purple bacteria), and the photoautotrophic cyanobacteria (blue-green algae). Among the different types of cytochromes occurring in the cell, the soluble c-type cytochromes (‘class I’, Meyer & Kamen, 1982) are the most abundant and best characterized group of proteins (Bartsch, 1978; Meyer & Kamen, 1982; Dickerson & Timkovitch, 1975; Lemberg & Barrett, 1973; Salemme, 1977; Ferguson-Miller, Brautigan & Margoliash, 1979). The amino acid sequences of more than 80 mitochrondrial and close to 40 bacterial cytochromes c are known (Meyer & Kamen, 1982; Dickerson & Timkovitch, 1975; Schwartz & Dayhoff, 1976; Dayhoff & Barker, 1978).

Effect of a molecular dipole on the ionic strength dependence of a biomolecular rate constant. Identification of the site of reaction

A theory is proposed for determining the location of a reaction site on a protein of known tertiary structure with an asymmetric charge distribution by an analysis of the effect of ionic strength on the rate of reaction of the protein with a small ion, using equations of Brønsted (J. N. Brønsted, 1922, Z. Phys, Chem. 102:169-207), Debye and Hückel (P. Debye and E. Hückel, 1923, Phys. Z. 24:185-206), and Kirkwood (J. G. Kirkwood, 1934, J. Chem. Phys. 2:351-361). The theory is based on the fact that the dipole moment of the transition complex differs from that of the protein, which will be reflected in the ionic strength dependence of the reaction. The location of the small ion with respect to the dipole axis of the protein can be calculated from this difference. For protein-protein reactions, an a priori assumption has to be made about the orientation of one of the reaction partners, since many different orientations of the reactants with respect to each other result in dipole moments of the same magnitude.

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.