Electron transfer in cytochrome c: role of the polypeptide chain

Electronic connection between the quinone and cytochrome C redox pools and its role in regulation of mitochondrial electron transport and redox signaling

Mitochondrial respiration, an important bioenergetic process, relies on operation of four membranous enzymatic complexes linked functionally by mobile, freely diffusible elements: quinone molecules in the membrane and water-soluble cytochromes c in the intermembrane space. One of the mitochondrial complexes, complex III (cytochrome bc1 or ubiquinol:cytochrome c oxidoreductase), provides an electronic connection between these two diffusible redox pools linking in a fully reversible manner two-electron quinone oxidation/reduction with one-electron cytochrome c reduction/oxidation. Several features of this homodimeric enzyme implicate that in addition to its well-defined function of contributing to generation of proton-motive force, cytochrome bc1 may be a physiologically important point of regulation of electron flow acting as a sensor of the redox state of mitochondria that actively responds to changes in bioenergetic conditions. These features include the following: the opposing redox reactions at quinone catalytic sites located on the opposite sides of the membrane, the inter-monomer electronic connection that functionally links four quinone binding sites of a dimer into an H-shaped electron transfer system, as well as the potential to generate superoxide and release it to the intermembrane space where it can be engaged in redox signaling pathways. Here we highlight recent advances in understanding how cytochrome bc1 may accomplish this regulatory physiological function, what is known and remains unknown about catalytic and side reactions within the quinone binding sites and electron transfers through the cofactor chains connecting those sites with the substrate redox pools. We also discuss the developed molecular mechanisms in the context of physiology of mitochondria.

Electrocatalytically functional multilayer assembly of sulfite oxidase and cytochrome c

An electrocatalytically functional multilayer has been designed using two proteins, cytochrome c and sulfite oxidase, and a polyelectrolyte (polyaniline sulfonate). The two proteins were co-immobilized on the surface of a gold electrode in alternating layers by electrostatic interactions using the layer-by-layer technique. The formation of this fully electro-active multilayer is characterized by quartz crystal microbalance and electrochemical experiments. The electro-catalytic characterization of the device containing up to 12 layers is based on generation of an oxidation current after sulfite addition. Besides the electron-transfer mechanism, the role of the different components in the electron-transport chain is clarified. Kinetic data were extracted to characterize the multilayer function. This artificial multilayer assembly is expected to be useful in the biosensor and biofuel cell development.

Solution conformation of ferricytochrome c-551 from Pseudomonas stutzeri substrain ZoBell

The main chain protons and the majority of side chain protons have been assigned for the ferric form of Pseudomonas stutzeri substrain ZoBell (American Type Culture Collection 14405) cytochrome c-551. The chemical shifts were compared to those for the ferrous protein to determine the pseudocontact shift contribution. These observed values were compared to contributions calculated from the atomic coordinates of the ferrous cytochrome and an optimized effective room temperature g-tensor centered on the paramagnetic ferric iron. The agreement between observed and calculated values indicates that the conformations of the two forms are highly similar.

A periplasmic and extracellular c-type cytochrome of Geobacter sulfurreducens acts as a ferric iron reductase and as an electron carrier to other acceptors or to partner bacteria

An extracellular electron carrier excreted into the growth medium by cells of Geobacter sulfurreducens was identified as a c-type cytochrome. The cytochrome was found to be distributed in about equal amounts in the membrane fraction, the periplasmic space, and the surrounding medium during all phases of growth with acetate plus fumarate. It was isolated from periplasmic preparations and purified to homogeneity by cation-exchange chromatography, gel filtration, and hydrophobic interaction chromatography. The electrophoretically homogeneous cytochrome had a molecular mass of 9.57 +/- 0.02 kDa and exhibited in its reduced state absorption maxima at wavelengths of 552, 522, and 419 nm. The midpoint redox potential determined by redox titration was -0.167 V. With respect to molecular mass, redox properties, and molecular features, this cytochrome exhibited its highest similarity to the cytochromes c of Desulfovibrio salexigens and Desulfuromonas acetoxidans. The G. sulfurreducens cytochrome c reduced ferrihydrite (Fe(OH)3), Fe(III) nitrilotriacetic acid, Fe(III) citrate, and manganese dioxide at high rates. Elemental sulfur, anthraquinone disulfonate, and humic acids were reduced more slowly. G. sulfurreducens reduced the cytochrome with acetate as an electron donor and oxidized it with fumarate. Wolinella succinogenes was able to reduce externally provided cytochrome c of G. sulfurreducens with molecular hydrogen or formate as an electron donor and oxidized it with fumarate or nitrate as an electron acceptor. A coculture could be established in which G. sulfurreducens reduced the cytochrome with acetate, and the reduced cytochrome was reoxidized by W. succinogenes in the presence of nitrate. We conclude that this cytochrome can act as iron(III) reductase for electron transfer to insoluble iron hydroxides or to sulfur, manganese dioxide, or other oxidized compounds, and it can transfer electrons to partner bacteria.

Transient and time-resolved resonance Raman investigation of photoinitiated electron transfer in ruthenated cytochromes c

Ruthenation of exterior amino acid residues of heme proteins provides an effective means by which biological ET reactions can be studied within the context of highly complex protein environments. Resonance Raman spectroscopy can probe both ET kinetics and structural dynamics at the molecular level. Here we present the first comprehensive use of time-resolved and transient resonance Raman spectroscopies to examine photoinduced ET in cytochromes. Two ruthenated cytochromes c, Ru(lys72)-cyt.c and Ru(cyt102)cyt.c, were studied with TRRS using 10 ns laser pulses and with TRRRS on a 10 ns to 10 ms time scale. It was found that resonance Raman protocols can effectively trace ET kinetics and associated heme--protein structural dynamics. Care must be exercised, however, when drawing comparisons to measurements made by other methods (i.e., transient absorbance). The TRRS studies directly probe the heme and its local environment and reveal that the heme dynamics accompanying ET are very rapid relative to phenomenological ET kinetics. The heme and its local environment evolve to their equilibrium (ferrous) structure in less than 10 ns subsequent to ET, with no evidence for the existence of metastable heme pocket geometries analogous to those observed in the dynamic response of hemoglobins and oxidases. Finally, species-specific differences are observed in the photoinduced ET kinetics and heme structural dynamics. However, these differences are confined to nanosecond or faster time scales.

Possible Role of the Iron Coordination Sphere in Hemoprotein Electron Transfer Self-Exchange: (1)H NMR Study of the Cytochrome c-PMe(3) Complex

The rates of self-exchange electron transfer in the trimethylphosphine complex of cytochrome c have been measured by an NMR technique over a large range of ionic strengths. The rate constant is 1.56 x 10(4) M(-)(1) s(-)(1) at 23 degrees C (&mgr; = 0.34 M) at pH 6.9. Dependence on ionic strength of the rate constant is treated by van Leeuwen theory. Extrapolation of the rate constant to infinite ionic strength gives a rate constant of 3.9 x 10(5) M(-)(1) s(-)(1). This rate constant is compared with others reported for myoglobin and cytochrome b(5)(). The values for these systems range over 2 orders of magnitude with myoglobin-PMe(3) << cytochrome b(5)() < cytochrome c-PMe(3) < cytochrome c. Analysis of the data in terms of Marcus theory gives a reorganization energy, lambda, for self-exchange of 0.75 eV mol(-)(1) for cytochrome c-PMe(3). Substitution of Met-80 by PMe(3) appears to influence only weakly the rearrangement barrier to electron transfer.

Comparative Kinetic Analysis of Reversible Intermolecular Electron-Transfer Reactions between a Series of Pentaammineruthenium Complexes and Cytochrome c

In this kinetic and thermodynamic study, the reversible outer-sphere electron-transfer reactions between a series of Ru(NH(3))(5)L(3+/2+) complexes (L = etpy, py, lut) (etpy = 4-ethylpyridine; py = pyridine; lut = 3,5-lutidine) and cytochrome c were investigated as a function of ionic strength, buffer, pH, temperature, and pressure. Due to the low driving forces of these systems, it was possible to study all the reactions in both redox directions. The observed rate constants for various L are correlated on the basis of the ability of ligands on the ruthenium complex to penetrate the heme groove on cytochrome c. The measurements as a function of pressure enabled the construction of volume profiles for all investigated systems. The activation volumes for all of these processes are very similar: between -14.9 and -17.8 cm(3) mol(-)(1) for the reduction and between +14.7 and +17.8 cm(3) mol(-)(1) for the oxidation of the protein by Ru(NH(3))(5)L(2+/)(3+), respectively. The overall reaction volume varies between 27 and 35 cm(3) mol(-)(1), from which it follows that the transition state lies exactly halfway between reactant and product states on a volume basis in all cases. There is good agreement throughout between kinetic and thermodynamic data.

The promotion of self-association of horse-heart cytochrome c by hexametaphosphate anions

In the presence of the highly charged hexametaphosphate anion, horse heart cytochrome c aggregates to form stable protein complexes. The formation of protein aggregates has been detected by high-resolution 1H-NMR spectroscopy from an increase in the linewidth of resolved ferricytochrome c resonances with hexametaphosphate concentration. Alternatively, analytical ultracentrifugation reveals protein association from the increase in apparent sedimentation coefficients of cytochrome c in the presence of equimolar hexametaphosphate. Protein aggregation is dependent on the concentration of background electrolyte since in the range 10-150 mM sodium cacodylate alternative stabilisation of dimeric and trimeric complexes was observed by both NMR and analytical ultracentrifugation. A model is proposed for the mechanism of protein aggregation caused by polyphosphate binding to the surface of cytochrome c.

The location of the polyphosphate-binding sites on cytochrome c measured by NMR paramagnetic difference spectroscopy

Analyses of unimolecular electron self-exchange reactions provide a comparatively simple and direct approach to understanding biological electron transfer. Such studies are currently limited by a lack of well characterised aggregating systems. In the presence of sodium hexametaphosphate, cytochrome c forms stable protein aggregates as a result of binding hexametaphosphate at a single site on its surface (preceding paper in this issue of the journal). Here we report the location of the principal polyphosphate binding site on the surface of cytochrome c for both hexametaphosphate and a second polyphosphate, tripolyphosphate determined using 1H-NMR spectroscopy in conjunction with the relaxation probe potassium hexacyanochromium(III). Addition of either hexametaphosphate or tripolyphosphate to ferricytochrome c in the presence of the relaxation probe causes a decrease in intensity of several resonances in the paramagnetic difference spectrum, including Phe82 ortho/meta, Ile85 delta methyl and Ile9 gamma methyl. Together these effects put the site of polyphosphate binding close to lysines 13, 86, and 87. Additionally the effect of sodium tripolyphosphate and sodium trimetaphosphate on cytochrome c aggregation is described. The potential role of this site in anion-induced cytochrome c aggregation is discussed.

Electrostatic and steric control of electron self-exchange in cytochromes c, c551, and b5

The ionic strength dependence of the electron self-exchange rate constants of cytochromes c, c551, and b5 has been analyzed in terms of a monopole-dipole formalism (van Leeuwen, J.W. 1983. Biochim. Biophys. Acta. 743:408-421). The dipole moments of the reduced and oxidized forms of Ps. aeruginosa cytochrome c551 are 190 and 210 D, respectively (calculated from the crystal structure). The projections of these on the vector from the center of mass through the exposed heme edge are 120 and 150 D. For cytochrome b5, the dipole moments calculated from the crystal structure are 500 and 460 D for the reduced and oxidized protein; the projections of these dipole moments through the exposed heme edge are -330 and -280 D. A fit of the ionic strength dependence of the electron self-exchange rate constants gives -280 (reduced) and -250 (oxidized) D for the center of mass to heme edge vector. The self-exchange rate constants extrapolated to infinite ionic strength of cytochrome c, c551, and b5 are 5.1 x 10(5), 2 x 10(7), and 3.7 x 10(5) M-1 s-1, respectively. The extension of the monopole-dipole approach to other cytochrome-cytochrome electron transfer reactions is discussed. The control of electron transfer by the size and shape of the protein is investigated using a model which accounts for the distance of the heme from each of the surface atoms of the protein. These calculations indicate that the difference between the electrostatically corrected self-exchange rate constants of cytochromes c and c551 is due only in part to the different sizes and heme exposures of the two proteins.

Electron self-exchange in Pseudomonas cytochromes

Electron self-exchange has been measured by an NMR technique for cytochromes c551 from Pseudomonas aeruginosa and Pseudomonas stutzeri. The rate for P. aeruginosa cyt c551 is 1.2 x 10(7) M-1 s-1 at 40 degrees C in 50 mM phosphate at pH 7. For P. stutzeri, under the same conditions, the rate is 4 x 10(7) M-1 s-1. For both cytochromes, the rate was independent of ionic strength up to 0.5 M in added NaC1, the enthalpy of activation was 20 +/- 4 kcal mol-1, and the entropy of activation was 38 +/- 10 cal mol-1 deg-1.

Kinetics of Pseudomonas aeruginosa cytochrome c551 and cytochrome oxidase oxidation by Co(phen)3(3+) and Mn(CyDTA)(H2O)-

The reaction between reduced Pseudomonas cytochrome c551 and cytochrome oxidase with two inorganic metal complexes, Co(phen)3(3+) and Mn(CyDTA)(H2O)-, has been followed by stopped-flow spectrophotometry. The electron transfer to cytochrome c551 by both reactants is a simple process, characterized by the following second-order rate constant: k = 4.8 X 10(4) M-1 sec-1 in the case of Co(phen)3(3+) and k = 2.3 X 10(4) M-1 sec-1 in the case of Mn(CyDTA)(H2O)-. The reaction of the c-heme of the oxidase with both metal complexes is somewhat heterogeneous, the overall process being characterized by the following second-order rate constants: k = 1.7 X 10(3) M-1 sec-1 with Co(phen)3(3+) and k = 4.3 X 10(4) M-1 sec-1 with Mn(CyDTA)(H2O)- as oxidants; under CO (which binds to the d1-heme) the former constant increases by a factor of 2, while the latter does not change significantly. The oxidation of the d1-heme of the oxidase by Co(phen)3(3+) occurs via intramolecular electron transfer to the c-heme, a direct bimolecular transfer from the complex being operative only at high metal complex concentrations; when Mn(CyDTA)(H2O)- is the oxidant, the bimolecular oxidation of the d1-heme competes successfully with the intramolecular electron transfer.

Kinetics of electron transfer between two Hansenula anomala flavocytochrome b2 derivatives and two simple copper proteins (azurin and stellacyanin)

Two derivatives of Hansenula anomala flavocytochrome b2 have been prepared, one deprived of the flavin prosthetic group (deflavocytochrome b2), and the other consisting of the heme-b-carrying globule (b2 core). The redox potential of the heme in the two derivatives is -5 (+/- 5) mV and -10 (+/- 5) mV respectively, fairly similar to the value of -20 (+/- 5) mV reported for the holoenzyme, indicating a minor effect of the flavin and of the flavodehydrogenase domain on heme potential. The kinetics of azurin and stellacyanin reduction by both derivatives have been investigated. At pH 7.0, I = 0.2 M and 20 degrees C the second-order rate constants are: k = 8 X 10(5) M-1 S-1 for azurin reduction by deflavocytochrome b2; k = 1.6 X 10(6) M-1 S-1 for azurin reduction by b2 core; k = 1 X 10(7) M-1 S-1 for stellacyanin reduction by deflavocytochrome b2; k = 3 X 10(7) M-1 S-1 for stellacyanin reduction by b2 core. The change in pH markedly affects the kinetics in the case of azurin, but has no effect on stellacyanin reduction. The change in ionic strength has a significant effect when deflavocytochrome b2 is the reductant, indicating that the flavodehydrogenase domain plays a role in the stabilization of the transient kinetic complex by means of electrostatic interactions. The kinetic results are discussed in the framework of the Marcus theory.

Kinetics of electron transfer between mitochondrial cytochrome c and iron hexacyanides

The reduction of horse and Candida krusei cytochromes c by ferrocyanide has been studied by 1H NMR spectroscopy and the reaction found to involve a precursor complex of ferrocyanide bound to ferricytochrome c (pH* 7.4, 2H2O, I = 0.12, and 25 degrees C). The electron transfer rate constants for the reduction of the two ferricytochromes by associated ferrocyanide were found to be the same at 780 +/- 80 sec-1 but the association constants for binding of ferrocyanide to ferricytochrome c were significantly different: horse, 90 +/- 20 M-1 and Candida, 285 +/- 30 M-1. The different association constants partly accounts for the previously observed reactivity difference between horse and Candida cytochromes c. Comparison of the NMR data with data obtained by other kinetic methods has allowed the electron transfer rate constant for the oxidation of ferrocytochrome c by associated ferricyanide to be determined. This was found to be 4.6 +/- 1 X 10(4) sec-1.

Direct measurement of the self-exchange rate of stellacyanin by a novel e.p.r. method

A method for reconstituting the blue copper protein stellacyanin with the stable copper isotopes 63Cu and 65Cu is reported. Small differences in the e.p.r. spectra of the two isotopic forms of stellacyanin have been used to monitor the electron self-exchange reaction of stellacyanin by rapid-freeze e.p.r. methods. The self-exchange rate constant (k11) for stellacyanin has been determined as 1.2 X 10(5) M-1 X S-1 at 20 degrees C. This value is in close agreement with values obtained from less-direct methods.

Steady-state kinetics of yeast cytochrome c peroxidase catalyzed oxidation of inorganic reductants by hydrogen peroxide

Rates of yeast cytochrome c peroxidase (ferrocytochrome c:hydrogen-peroxide oxidoreductase, EC 1.11.1.5) catalyzed oxidation of bis(tripyridine)cobalt(II) ion, penta(amine)pyridineruthenium(II) ion and ferrocyanide ion by hydrogen peroxide have been found to obey the empirical equation: (formula; see text) in the pH range 5 to 8, and at saturating H2O2 concentrations. [( S] and [CcP] are the concentrations of the reductant and the enzyme, respectively.) Values of k2 were found to be independent of the reductant. The term k0[S] is only significant with the cobalt and ruthenium complexes at high pH. The mechanism proposed to account for this rate equation differs significantly from previous mechanistic proposals. In particular, the rate data require the assignment of the rate-limiting step at high substrate concentrations to a slow electron-transfer within the enzyme, and not, as previously suggested, to saturation of substrate binding to the enzyme. Also, the term k0[S] implies that the reactive substrates, including the natural substrate (yeast cytochrome c), react with the hydrogen peroxide-heme complex and not with the radical species formed by reaction with hydrogen peroxide in the absence of reductants.

Tetracyano-2,2-bipyridineiron(iii), an improved electron acceptor for the spectrophotometric assay of beta-oxidation and of succinate dehydrogenase in intact mitochondria

A recently described direct reading assay for beta-oxidation and for succinate oxidation in intact mitochondria using [Fe(CN)6]3- as final electron acceptor [Osmundsen & Bremer (1977) Biochem. J. 164. 621--633] has been improved by using instead tetracyano-2,2-bipyridineiron(III) [Fe(CN)4(bpy)]-, which gives a 2.6 times greater absorbance change on reduction. Some physical and kinetic properties of [Fe(CN)4(bpy)]- are described. The use of exogenous cytochrome c(III) as electron acceptor was also tested; this gives the largest absorbance change, although the absolute rate of reaction is only approx. one-third of that using [Fe(CN)6]3- or [Fe(CN)4(bpy)]-.

Kinetic data for redox reactions of cytochrome c with Fe(CN)5X complexes and the question of association prior to electron transfer

Use of rigorous equilibration kinetics to evaluate rate constants for the Fe(CN)6 4- reduction of horse-heart cytochrome c in the oxidized form, cyt c (III), has shown that limiting kinetics do not apply with concentrations of Fe(CN)6 4- (the reactant in excess) in the range 2-10 x 10(-4) M, I = 0.10 M (NaCl). The reaction conforms to a first-order rate law in each reactant, and at 25 degrees C, pH 7.2 (Tris), it is concluded that K for association prior to electron transfer is less than 200 M-1. From previous studies at 25 degrees C, ph 7.0 (10(-1) M phosphate), I = 0.242 M (NaCl), a value K = 2.4 x 10(3) M-1 has been reported. Had such a value applied, some or all of the redox inactive complexes Mo(CN)8 4-, Co(CN)6 3-, Cr(CN)6 3-, Zr(C2O4)4 4- present in amounts 5-20 x 10(-4) M would have been expected to associate at the same site and partially block the redox process. No effect on rats was observed. With the reductants Fe(CN)5(4-NH2-py)3- and Fe(CN)5(imid)3-, reactions proceeded to greater than 90% completion and rate laws were again first order in each reactant. Rate constants (M-1 sec-1) at 25 degrees C, pH 7.2 (Tris), I = 0.10 M (NaCl), are Fe(CN)6 4- (3.5 x 10(4)), Fe(CN)5(4-NH2py)3- (6.7 x 10(5), and Fe(CN)5(imid)3- (4.2 x 10(5). Related reactions in which cyt c(II) is oxidized are also first order in each reactant, Fe(CN)6 3- (9.1 x 10(6)), Fe(CN)5(NCS)3- (1.3 x 10(6)), Fe(CN)5(4-NH2py)2- (3.8 x 10(6) at pH 9.4), and Fe(CN)5(NH3)2- (2.75 x 10(6) at ph 8). Redox inactive Co(CN)6 3- (1.0 x 10(-3) M) has no effect on the reaction of Fe(CN)6 3- which suggests that a recent interpretation for the Fe(CN)6 3- oxidation of cyt c(II), I = 0.07 M, may also require reappraisal.

Electron transfer between horse heart and Candida krusei cytochromes c in the free and bound states

Electron transfer between horse heart and Candida krusei cytochromes c in the free and phosvitin-bound states was examined by difference spectrum and stopped-flow methods. The difference spectra in the wavelength range of 540-560 nm demonstrated that electrons are exchangeable between the cytochromes c of the two species. The equilibrium constants of the electron transfer reaction for the free and phosvitin-bound forms, estimated from these difference spectra, were close to unity at 20 degrees C in 20 mM Tris-HCl buffer (pH 7.4). The electron transfer rate for free cytochrome c was (2-3).10(4) M-1.s-1 under the same conditions. The transfer rate for the bound form increased with increase in the binding ratio at ratios below half the maximum, and was almost constant at higher ratios up to the maximum. The maximum electron exchange rate was about 2.10(6) M-1.s-1, which is 60-70 times that for the free form at a given concentration of cytochrome c. The activation energy of the reaction for the bound cytochrome c was equal to that for the free form, being about 10 kcal/mol. The dependence of the exchange rate on temperature, cytochrome c concentration and solvent viscosity suggests that enhancement of the electron transfer rate between cytochromes c on binding to phosvitin is due to increase in the collision frequency between cytochromes c concentrated on the phosvitin molecule.

Ionic strength effects on cytochrome aa3 kinetics

1. The occurrence of an optimal ionic strength for the steady-state activity of isolated cytochrome aa3 can be attributed to two opposite effects: upon lowering of the ionic strength the affinity between cytochrome c and cytochrome aa3 increases, whereas in the lower ionic strength region the formation of a less active cytochrome c-aa3 complex limits the ferrocytochrome c association to the low affinity site. 2. At low ionic strength, the reduction of cytochrome c-aa3 complex by ferrocytochrome c1 proceeds via non-complex-bound cytochrome c. Under these conditions the positively charged cytochrome c provides the electron transfer between the negatively charged cytochromes c1 and aa3. 3. Polylysine is found to stimulate the release of tightly bound cytochrome c from the cytochrome c-aa3 complex. This property points to the existence of negative cooperativity between the two binding sites. We suggest that the stimulation is not restricted to polylysine, but also occurs with cytochrome c. 4. Dissociation rates of both high and low affinity sites on cytochrome aa3 were determined indirectly. The dissociation constants, calculated on the basis of pre-steady-state reaction rates at an ionic strength of 8.8 mM, were estimated to be 0.6 nM and 20 microM for the high and low affinity site, respectively.

The reduction of porphyrin cytochrome c by hydrated electrons and the subsequent electron transfer reaction from reduced porphyrin cytochrome c to ferricytochrome c

1. Hydrated electrons, produced by pulse radiolysis react with porphyrin cytochrome c with a bimolecular rate constant of 3-10(10) M-1 S-1 at 21 degrees C and pH 7.4. 2. After the reduction step an absorbance change with a half-life of 5 microns is observed with the spectral range of 430-470 nm. A relatively stable intermediate then decays with a half-life of 15 s. 3. The spectrum of the intermediate observed 50 microns after the generation of hydrated electrons shows a broad absorption band between 600 and 700 nm and a peak at 408 nm. The spectrum is attributed to the protonated form of an initially produced porphyrin anion radical. 4. Reduced porphyrin cytochrome c reacts with ferricytochrome c with a bimolecular constant of 2-10(5) M-1- S-1 in 2 mM phosphate pH 7.4, at 21 degrees C and of 2 - 10(6) M-1-S-1 under the same conditions but at 1 M ionic strength. It is proposed that electron transfer in an analogous exchange reaction between ferrocytochrome c and ferricytochrome c occurs via the exposed part of the haem.