Preferred sites for electron transfer between cytochrome c and iron and cobalt complexes

The K79G Mutation Reshapes the Heme Crevice and Alters Redox Properties of Cytochrome c

The two roles of cytochrome c (cyt c), in oxidative phosphorylation and apoptosis, critically depend on redox properties of its heme iron center. The K79G mutant has served as a parent protein for a series of mutants of yeast iso-1 cyt c. The mutation preserves the Met80 coordination to the heme iron, as found in WT* (K72A/C102S), and many spectroscopic properties of K79G and WT* are indistinguishable. The K79G mutation does not alter the global stability, fold, rate of Met80 dissociation, or thermodynamics of the alkaline transition (p K_a) of the protein. However, the reduction potential of the heme iron decreases; further, the p K_H of the trigger group and the rate of the Met-to-Lys ligand exchange associated with the alkaline transition decrease, suggesting changes in the environment of the heme. The rates of electron self-exchange and bimolecular electron transfer (ET) with positively charged inorganic complexes increase, as does the intrinsic peroxidase activity. Analysis of the reaction rates suggests that there is increased accessibility of the heme edge in K79G and supports the importance of the Lys79 site for bimolecular ET reactions of cyt c, including those with some of its native redox partners. Structural modeling rationalizes the observed effects to arise from changes in the volume of the heme pocket and solvent accessibility of the heme group. Kinetic and structural analyses of WT* characterize the properties of the heme crevice of this commonly employed reference variant. This study highlights the important role of Lys79 for defining functional redox properties of cyt c.

Electron-Transfer Reactions through the Associated Interaction between Cytochromec and Self-Assembled Monolayers of Optically Active Cobalt(III) Complexes: Molecular Recognition Ability Induced by the Chirality of the Cobalt(III) Units

Self-assembled monolayers (SAMs) of optically active Co(III) complexes ((S)-2/(R)-2) that contain (S)- or (R)-phenylalanine derivatives as a molecular recognition site were constructed on Au electrodes ((S)-2-Au/(R)-2-Au). Molecular recognition characteristics induced by the S and R configurations were investigated by measurements of electron-transfer reactions with horse heart cytochrome c (cyt c). The electrochemical studies indicate that the maximum current of cyt c reduction is obtained when the Au electrode is modified by 2 with a moderate coverage of approximately 4.0 x 10(-11) mol cm(-2). Since the Au electrode is not densely packed with the Co(III) units at this concentration, we conclude that the penetrative association process between cyt c and the Co(III) unit plays an important role in this electron-transfer system. The differences in the electron-transfer rates of (S)-2-Au and (R)-2-Au increase with increasing scan rates, a result indicating that the chiral ligand has an influence on the rate of association of the complexes with cyt c. 3-Au has a mixed monolayer composed of 2 and hexanethiol and exhibits electron-transfer behavior comparable to 2-Au. The difference in the association rates of (S)-3-Au and (R)-3-Au is larger than that between (S)-2-Au and (R)-2-Au, which indicates that the molecular recognition ability of 3-Au has been enhanced by filling the gap between molecules of 2 with hexanethiols. The differences in the oxidation rates of cyt c(II) between (S)-2-Au and (R)-2-Au and between (S)-3-Au and (R)-3-Au were larger than the differences in the rates of the reduction of cyt c(III); this suggests that the size of the heme crevice varies according to the oxidation state of cyt c.

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.

Electron transfer reactions in Zn-substituted cytochrome P450cam

We have investigated photoinduced electron transfer (ET) reactions between zinc-substituted cytochrome P450cam (ZnP450) and several inorganic reagents by using the laser flash photolysis method, to reveal roles of the electrostatic interactions in the regulation of the ET reactions. The laser pulse irradiation to ZnP450 yielded a strong reductant, the triplet excited state of ZnP450, (3)ZnP450, which was able to transfer one electron to anionic redox partners, OsCl(6)(2-) and Fe(CN)(6)(3-), with formation of the porphyrin pi-cation radical, ZnP450(+). In contrast, the ET reactions from (3)ZnP450 to cationic redox partners, such as Ru(NH(3))(6)(3+) and Co(phen)(3)(3+), were not observed even in the presence of 100-fold excess of the oxidant. One of the possible interpretations for the preferential ET to the anionic redox partner is that the cationic patch on the P450cam surface, a putative interaction site for the anionic reagents, is located near the heme (less than 10 A from the heme edge), while the anionic surface is far from the heme moiety (more than 16 A from the heme edge), which would yield 8000-fold faster ET rates through the cationic patch. The ET rate through the anionic patch to the cationic partner would be substantially slower than that of the phosphorescence process in (3)ZnP450, resulting in no ET reactions to the cationic reagents. These results demonstrate that the asymmetrical charge distribution on the protein surface is critical for the ET reaction in P450cam.

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.

Electrostatic interactions of 4-carboxy-2,6-dinitrophenyllysine-modified cytochromes c with physiological and non-physiological redox partners

An analysis of the effect of electrostatic properties of 4-carboxy-2,6-dinitrophenyllysine (CDNP-lysine) cytochromes c on their reactions with strongly and weakly binding redox partners is given. For strongly binding systems (cytochrome-c oxidase, cytochrome-c reductase, sulphite oxidase and yeast cytochrome-c peroxidase) the magnitude of the dipole moments of the CDNP cytochromes c determines their relative reactivities. For weakly binding redox agents, such as hexacyanoferrate(III), cobalt(III)tris(1,10-phenanthroline), azurin and plastocyanin, the electrostatic potential at the haem edge accounts for the greater part of the relative activities. Relative rate data were obtained from the literature. It is concluded that the dipole moment of native cytochromes c may account for an approx. 50-fold increase in the efficiency of its physiological activity towards membrane-bound enzymes. A correction on a formula to describe the contribution of a molecular dipole moment to the ionic strength dependence of a bimolecular rate constant (Koppenol, W. H. (1980) Biophys. J. 29, 493-508) leads to an equation nearly identical to that obtained by Van Leeuwen et al. (Van Leeuwen, J.W., Mofers, F.J.M. and Verrman, E.C.I. (1981) Biochim. Biophys. Acta 635, 434-439).

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.