Use of specific lysine modifications to identify the site of reaction between cytochrome c and ferricyanide

Redox state-dependent aggregation of mitochondria induced by cytochrome c

Cytochrome c is known to play central role in apoptosis. Here, it is shown that ferricytochrome c, but not ferrocytochrome c is able to directly induce the aggregation of rat liver mitochondria, similar to the effect caused by magnesium ions at high concentrations. The aggregation was revealed by a decrease in light dispersion of mitochondrial suspension and it was confirmed by the optical microscopy. In the medium containing NADH and cytochrome c, mitochondrial aggregation was initiated only after exhaustion of NADH leading to oxidation of cytochrome c. The aggregation induced by 30 μM ferricytochrome c, but not by 5 mM MgCl(2), was completely inhibited by 30-100 μM ferricyanide, thus indicating that ferricyanide-cytochrome c specific interaction prevents mitochondrial aggregation. After completion of the aggregation caused by ferricytochrome c, this effect cannot be readily reversed by subsequent reduction of cytochrome c. The aggregation induced by ferricytochrome c and/or magnesium ions explains masking of the external NADH-oxidase activity of mitochondria in vitro reported in the literature. This new cytochrome c redox state-dependent phenomenon might also be involved in more complex mechanisms controlling aggregation (clustering) of mitochondria in vivo under the influence of pro-apoptotic factors and requires further study.

Tuning the Rate and pH Accessibility of a Conformational Electron Transfer Gate

Methods to fine-tune the rate of a fast conformational electron transfer (ET) gate involving a His-heme alkaline conformer of iso-1-cytochrome c (iso-1-Cytc) and to adjust the pH accessibility of a slow ET gate involving a Lys-heme alkaline conformer are described. Fine-tuning the fast ET gate employs a strategy of making surface mutations in a substructure unfolded in the alkaline conformer. To make the slow ET gate accessible at neutral pH, the strategy involves mutations at buried sequence positions which are expected to more strongly perturb the stability of native versus alkaline iso-1-Cytc. To fine-tune the rate of the fast His 73-heme ET gate, we mutate the surface-exposed Lys 79 to Ala (A79H73 variant). This mutation also simplifies ET gating by removing Lys 79, which can serve as a ligand in the alkaline conformer of iso-1-Cytc. To adjust the pH accessibility of the slow Lys 73-heme ET gate, we convert the buried side chain Asn 52 to Gly and also mutate Lys 79 to Ala to simplify ET gating (A79G52 variant). ET kinetics is studied as a function of pH using hexaammineruthenium(II) chloride (a6Ru2+) to reduce the variants. Both variants show fast direct ET reactions dependent on [a6Ru2+] and slower gated ET reactions that are independent of [a6Ru2+]. The observed gated ET rates correlate well with rates for the alkaline-to-native state conformational change measured independently. Together with the previously reported H73 variant (Baddam, S.; Bowler, B. E. J. Am. Chem. Soc. 2005, 127, 9702-9703), the A79H73 variant allows His 73-heme-mediated ET gating to be fine-tuned from 75 to 200 ms. The slower Lys 73-heme (15-20 s time scale) ET gate for the A79G52 variant is now accessible over the pH range 6-8.

Electron-transfer-mediated binding of optically active cobalt(III) complexes to horse heart cytochrome c

Optically active cobalt(II) complexes are used as reducing agents in the electron-transfer reaction involving horse heart cytochrome c. Analysis of the circular dichroism (CD) spectra of reaction products indicates that the corresponding cobalt(III) species of both enantiomers of [CoII(alamp)] (H2alamp=N,N'-[(pyridine-2,6-diyl)bis(methylene)]-bis[alanine]) are partly attached to the protein during electron transfer by coordination to an imidazole unit of one of the histidine residues. His-26 and His-33 are both solvent exposed, and the results suggest that one of these histidine residues acts as a bridge in the electron transfer to and from the haem iron of cytochrome c. The reaction is enantioselective: the ratio of the relative reactivity at 15 degrees C is 2.9 in favour of the R,R-enantiomer. A small induced CD activity in the haem chromophore reveals that some structural changes in the protein occur consecutively with the binding of the cobalt(III) complex.

Weak interactions and molecular recognition in systems involving electron transfer proteins

Electrostatic interactions and other weak interactions between amino acid side chains on protein surfaces play important roles in molecular recognition, and the mechanism of their intermolecular interactions has gained much interest. We established that charged peptides are useful for investigating the molecular recognition character of proteins and their molecular interaction induced structural changes. Positively charged lysine peptides competitively inhibited electron transfer from reduced cytochrome f (cyt f or cytochrome c (cyt c) to oxidized plastocyanin (PC), due to neutralization of the negatively charged site of PC by formation of PC-lysine peptide complexes. Lysine peptides also inhibited electron transfer from cyt c to cytochrome c peroxidase. Likewise, negatively charged aspartic acid peptides interacted with the positively charged sites of cytfand cyt c, and competitively inhibited electron transfer from reduced cytfor cyt c to oxidized PC and from [Fe(CN)6]4- to oxidized cyt c. Changes in the geometry and a shift to a higher redox potential of the active site Cu of PC on oligolysine binding were detected by spectroscopic and electrochemical measurements, owing to the absence of absorption in the visible region for lysine peptides. Structural and redox potential changes were also observed for cyt f and cyt c by interaction with aspartic acid peptides.

Beta-thiopropionyl cytochromes c modified at lysyl residues: preparation and characterization of the monosubstituted horse cytochromes c

beta-Thiopropionyl derivatives of horse cytochrome c singly modified at each of 18 different lysine epsilon-amino groups have been prepared using sulfosuccinimidyl-2-(biotinamido)ethyl-1,3-dithiopropionate and purified to homogeneity by high-pressure liquid chromatography. These derivatives were characterized by determination of: (i) the location of the modification; (ii) reduction potentials; (iii) visible and NMR spectra: and by (iv) measurement of electron transfer activity with cytochrome-c oxidase. No significant changes in structure were indicated, except for the ferric forms of the derivatives modified at lysines 72, 73, and 79 which are discussed separately. The electron transfer activity of the beta-thiopropionyl cytochromes c with bovine heart cytochrome-c oxidase was decreased to extents dependent on the position of the modification. Aminoethylation, a secondary modification which reverses the charge change, restored the electron transfer rate to that observed with the unmodified cytochrome c, irrespective of the location of the primary modification. These results afford a direct experimental demonstration that alterations in kinetics with physiological electron transfer partners resulting from modifications which cause a change of the charge of surface side chains are solely due to the electrostatic effects. Of the many chemically modified cytochromes c prepared to date, the singly substituted beta-thiopropionyl cytochromes c are likely to be particularly useful as the thiol allows covalent linkage of any sulfhydryl-reactive reagent to a well-defined location on the protein surface by a simple procedure, even when the secondary modifier is relatively unstable, a crucial advantage not otherwise readily achieved.

Rapid kinetics of membrane potential generation by cytochrome c oxidase with the photoactive Ru(II)-tris-bipyridyl derivative of cytochrome c as electron donor

Yeast iso-1-cytochrome c covalently modified at cysteine-102 with (4-bromomethyl-4'-methylbipyridine)[bis(bipyridine)]Ru2+ (Ru-102-Cyt c) has been used as a photoactive electron donor to mitochondrial cytochrome c oxidase (COX) reconstituted into phospholipid vesicles. Rapid kinetics of membrane potential generation by the enzyme following flash-induced photoreduction of Ru-102-Cyt c heme has been measured and compared to photovoltaic responses observed with Ru(II)(bipyridyl)3 (RuBpy) as the photoreductant [D.L. Zaslavsky et al. (1993) FEBS Lett. 336, 389-393]. At low ionic strength, when Ru-102-Cyt c forms a tight electrostatic complex with COX, flash-activation results in a polyphasic electrogenic response corresponding to transfer of a negative charge to the interior of the vesicles. The initial rapid phase is virtually identical to the 50 microsecond transient observed in the presence of RuBpy as the photoactive electron donor which originates from electrogenic reduction of heme a by CuA. CuA reduction by Ru-102-Cyt c turns out to be not electrogenic in agreement with the peripheral location of visible copper in the enzyme. A millisecond phase (tau ca. 4 ms) following the 50 microsecond initial part of the response and associated with vectorial translocation of protons linked to oxygen intermediate interconversion in the binuclear centre, can be resolved both with RuBpy and Ru-102-Cyt c as electron donors; however, this phase is small in the absence of added H2O2. In addition to these two transients, the flash-induced electrogenic response in the presence of Ru-102-Cyt c reveals a large slow phase of delta psi generation not observed with RuBpy. This phase is completely quenched upon inclusion of 100 microM ferricyanide in the medium and originates from a second order reaction of COX with the excess Ru-102-Cyt c2+ generated by the flash in a solution.

Reaction of the Hansenula anomala flavocytochrome b2 and cytochrome b2 core with inorganic outer sphere redox compounds

The oxidation of reduced cytochrome b2 core and flavocytochrome b2 by three inorganic outer sphere compounds, Fe(CN)6(3-), Co(phen)3(3+) and Mn(CyDTA) (H2O)-, has been studied by stopped-flow. The reaction with Fe(CN)6(3-) is very rapid; the second order rate constants at 10 degrees C (pH 7) and I = 0.02 M are k = 1 x 10(8) M-1 s-1 and 1 x 10(7) M-1 s-1 for cytochrome b2 core and flavocytochrome b2, respectively. The reaction between cytochrome b2 core and Co(phen)3(3+), too fast at pH 7.0, has been characterized at 10 degrees C and pH 4.0; the second order rate constant is k = 2 x 10(7) M-1 s-1 and becomes 4 x 10(8) M-1 s-1 at pH 6.5. The reaction between flavocytochrome b2 and Co(phen)3(3+) has a second order rate constant k = 2 x 10(7) M-1 s-1 at pH 7.0, 10 degrees C. The oxidation of both proteins by Mn(CyDTA)(H2O)- is characterized by a second order rate constant k = 2.8 x 10(6) M-1 s-1 and 2.3 x 10(5) M-1 s-1 for cytochrome b2 core and flavocytochrome b2, respectively, at pH 7.0 and 10 degrees C. The reactivity of the b2 heme towards the outer sphere oxidants is higher than that reported for heme c in bacterial and eukaryotic cytochrome c. The larger delta E and the larger accessibility of the b2 heme can account for this result. The flavodehydrogenase domain seems to modulate the electron transfer also to these inorganic compounds, as found previously in the case of macromolecular electron acceptors.

Structural and functional features of Pseudomonas cytochrome c peroxidase

The secondary structure of Pseudomonas cytochrome c peroxidase (ferrocytochrome c: hydrogen-peroxide oxidoreductase, EC 1.11.1.5) has been predicted from the established amino acid sequence of the enzyme using a Chou-Fasman-type algorithm. The amount of alpha-helicity thus obtained is in agreement with previously obtained results based on circular dichroic measurements at far UV. The two heme c moieties of the enzyme have earlier been shown to have widely different characteristics, e.g., the redox potentials of the hemes differ with about 600 mV, and carry out different functions in the enzyme molecule. The structural comparisons made in this study enlighten the observed functional differences. The first heme in the polypeptide chain, heme 1, has in its environment a folding pattern generally encountered in cytochromes. In the region of the sixth ligand, however, profound differences are noted. The cytochromal methionine has been replaced by a lysine with a concomitant lowering of redox-potential thus making peroxidatic activity possible. Around heme 2, extra amino acid residues have been added to the peroxidase as compared with Rhodospirillum molischianum cytochrome c2 core structure in the 20's loop. After completion of the cytochromal fold around heme 2 an additional tail consisting of 25 residues is linked. This tail shows no stabilizing elements of secondary structure, but contains a strongly hydrophobic segment which suggests a possible membrane contact site of this extrinsic membrane protein. Heme 2 is concluded to have a cytochromal function in the molecule. To further elucidate the functional properties of the enzyme, a noncovalent two-fragment complex was produced by specific cleavage of the peroxidase by Pseudomonas elastase. The complex was studied with respect to its properties to the native enzyme. The two-fragment complex of Pseudomonas peroxidase retains the overall conformation of the native enzyme showing, however, no heme-heme interaction. Thus, a comparison of the properties of the native enzyme with those of the two-fragment complex permitted some conclusions to be drawn on the structure of the enzyme as well as the mechanism of heme-heme interaction. From the present results we conclude that the two distal heme surfaces in the peroxidase are oriented toward each other. This structural arrangement allows an inter-heme communication in the enzyme molecule and it also forms the structural basis for the enzyme mechanism. The structural comparisons also give insight into the evolution of an ancestral cytochrome c into an efficient peroxidase that has a versatile control mechanism in heme-heme interaction.

Electron transfer between cytochrome c and metal hexacyanide complexes. Effect of thermodynamic driving force on the electron transfer rate

The electron transfer reactions between ferrocytochrome c and three isomorphic hexacyanide complexes, [Fe(CN)6]3-, [Os(CN)6]3- and [Ru(CN)6]3-, have been studied using the method of photoexcitation. The transfer rates for [Os(CN)6]3- and [Ru(CN)6]3- are, respectively, about 45- and 200-times higher than that of [Fe(CN)6]3-. A reorganization energy of approx. 0.8 eV was found for the cytochrome c-hexacyanide system when the data were analyzed according to the theory of Marcus.

Kinetics of electron transfer between cytochrome c and iron hexacyanides. Evidence for two electron-transfer sites

The electron-transfer reaction between ferrocytochrome c and ferricyanide has been studied by the method of photoexcitation. The observed transfer rate shows saturation behaviour at high ferricyanide concentration. Data analysis indicates that there are two binding sites of vastly different affinities at which electron transfer occurs. The binding constant for the strong binding site decreases from 1600 M-1 to 80 M-1 as the ionic strength increases from 15 mM to 140 mM. At 20 degrees C, the intramolecular electron-transfer rate for this site is 4.65 X 10(4) s-1, which gives an electron-transfer distance of approx. 9.7 A according to Hopfield's model.

Preferred sites on cytochrome c for electron transfer with two positively charged blue copper proteins, Anabaena variabilis plastocyanin and stellacyanin

Rate constants for the reactions of horse cytochrome c (E'0 of +260 mV) with the copper proteins Anabaena variabilis plastocyanin (E'0 of +360 mV) used as oxidant and stellacyanin (E'0 of +187 mV) used as reductant have been determined at 25 degrees C, pH 7.5 and 7.0, respectively, and an ionic strength of 0.10 M (NaCl). These rate constants were also measured with eight different singly substituted 4-carboxy-2,6-dinitrophenyl (CDNP) horse cytochrome c derivatives, modified at lysine-7, -13, -25, -27, -60, -72, -86, or -87 and with the trinitrophenyl (TNP) derivative modified at lysine-13. The influence of the modifications on the bimolecular rate constants for these reactions defines the region on the protein that is involved in the electron-exchange reactions and demonstrates that the preferred site is at or near the solvent-accessible edge of the heme prosthetic group on the "front" surface of the molecule. Both reactions are strongly influenced by the lysine-72 modification to the left of the exposed heme edge and, to this extent, behave similar to the earlier studied reaction with azurin. These effects span only an order of magnitude in rate constants and are thus many times smaller than those for the physiological protein redox partners of cytochrome c. While the preferred sites of reaction on the surface of cytochrome c for small inorganic complexes appear to be dependent only on the net charge of the reactants, with the copper proteins additional factors intervene. These influences are discussed in terms of hydrophobic patches and the distribution of charges on the surface of the four copper proteins so far examined.

A 1H-NMR longitudinal relaxation study of the interaction between cytochrome c and cytochrome c oxidase

The interaction between the oxidized forms of cytochrome c and cytochrome c oxidase (EC 1.9.3.1) has been investigated by 1H-NMR longitudinal relaxation measurements. It is found that relaxation of methyl groups on the heme ring of cytochrome c markedly deviates from a simple exponential behavior in the presence of small amounts of cytochrome oxidase. A comparison with the relaxation behavior of cytochrome c modified by 4-carboxy-3,5-dinitrophenyl at Lys-13 shows that the oxidase induces a conformation in native cytochrome c that is closely related to that of the derivative. It is suggested that this change in conformation consists of a rupture of the salt bridge between Lys-13 and Glu-90 and a concomitant perturbation of the methionine ligand.

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.

1H NMR studies of the electron exchange between cytochrome c and iron hexacyanides. Definition of the iron hexacyanide binding sites on cytochrome c

Binding of [Fe(CN)6]3-, [Cr(CN)6]3-, [Co(CN)6]3- and [Cr(C2O4)3]3- to horse, tuna and Candida krusei cytochromes c has been studied by high-resolution 1H NMR spectroscopy. All the reagents bind at the same sites. There are at least two binding sites, and probably three, on horse cytochrome c at pH 7. One of the sites is only a weak binding site and is far from the haem group, whereas the other site(s) is(are) at the haem crevice. Ka for binding of [Fe(CN)6]3- to trimethyllysine-72 of C. krusei ferricytochrome c is 140 +/- 15 M-1 at 27 degrees C and pH 7.

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.