Intramolecular electron transfer cytochrome

The systems that we investigated in collaboration with others involved intermolecular and intramolecular electron-transfer reactions between ruthenium complexes and cytochrome c. We also studied a series of intermolecular reactions between chelated cobalt complexes and cytochrome c. A variety of high-pressure experimental techniques, including stopped-flow, flash-photolysis, pulse-radiolysis, and voltammetry, were employed in these investigations. As the following presentation shows, a remarkably good agreement was found between the volume data obtained with the aid of these different techniques, which clearly demonstrates the complementarity of these methods for the study of electron-transfer processes. 

In applying this principle to proteins, one would ideally like to modify a protein at one specific site with a number of related, substitution-inert, inorganic redox reagents, and then study the intramolecular electron transfer step as a function of a wide variety of variables (e.g., the redox potential and hydrophobicity of the redox reagent). Such a study is extremely difficult to carry out with large proteins, and none has been reported thus far. We have, however, found out that horseheart cytochrome c is amenable to modification at a single site by the... 

Other examples using pulse radiolysis include (a) studies on cytochrome c-di nitrate reductase from Thio-sphaera pantotropha to provide evidence for a fast intramolecular electron transfer from c-heme to rate constant for the reaction of... 

Cytochrome cd reduces nitrite by one electron to NO as the major product, providing a reducing system was used that does not reduce NO chemically (Kim and Hollocher, 1983, 1984 Mancinelli et al., 1986 ParretaL, 1976 Silvestrini et al., 1979 Timkovich et al., 1982 Yamanaka and Okunuki, 1963a). There is only one rapid kinetic study of cytochrome cd, with nitrite as the oxidant (Sil-vestrini et al., 1990). This work, carried out at pH 8.0, indicated that there was intramolecular electron transfer, c d, as in the O2 reaction, but that the... 

The kinetics of intramolecular electron transfer from Ru(II) to Fe(III) in ruthenium-modified cytochrome c has been studied [77-80]. In these studies electron transfer from electron-excited Ru(II) (bpy)3, which was added to the protein solution, to ruthenium-modified horse heart cytochrome c, (NH3)5Ru(III) (His-33)cyt(Fe(III)), was found to produce (NH3)5Ru(II) (His-33)cyt (Fe(III)) in fivefold excess to (NH3)5Ru(III) (His-33)cyt(Fe(II)). As in refs. 72 and 73, in the presence of EDTA the (NH3)5Ru(II)(His-33)cyt(Fe(III)) decays mainly by intramolecular electron transfer to (NH3)5Ru(III)(His-33)cyt(Fe(II)). The rate constant k — 30 3s 1 at 296 K and does not vary substantially over the temperature range 273-353 K. Above 353 K intramolecular Ru(II) - Fe(III) electron transfer was not observed owing to the displacement of methionine-80 from the iron coordination sphere. The distance of intramolecular electron transfer in this case is also equal to 11.8 A (see Fig. 19). 

Intramolecular electron transfer from Ru(II) to Fe(III) in (NH3)3Ru(II) (His-33)cyt(Fe(III)) induced by pulse-radiolysis reduction of Ru(III) in the (NH3)5Ru(III) (His-33)cyt(Fe(III)) complex were investigated [84]. The results obtained differ from those of refs. 77-80 where flash photolysis was used to study the similar electron transfer reaction. It was found [84] that, over the temperature range 276-317 K the rate of electron transfer from Ru(II) to Fe(III) is weakly temperature dependent with EA 3.3 kcal mol 1. At 298 K the value of kt = 53 2 s"1. The small differences in the temperature dependence of the electron tunneling rate in ruthenium-modified cytochrome c reported in refs. 77-80 and 84 was explained [84] by the different experimental conditions used in these two studies. 

Intramolecular electron-transfers through peptides have also been observed by Isied and coworkers using Ru(NH3)5 modified cytochrome c 55). Because of the kinetic inertness of both the ruthenium(II) and ruthenium(III), NMR and other physical techniques can be used to characterize the point of attachment of the ruthenium center. NMR and peptide mapping experiments showed that the ruthenium is bound to the His-33 site of cyt c (Fig. 2). The reduction potentials are +0.26 V for cyt c and +0.07 V for [(NH3)5Ru(His)]2 +. Upon reduction of the Ru(III)-cyt c(III) derivative with 1 equiv. of electrons, any Ru(II)-cyt c(III) produced should undergo... 

Long range electron-transfer has also been demonstrated within the complex between zinc-substituted cytochrome c peroxidase and cyt c 59). The kinetics of intramolecular electron-transfer from Ru(II) to Fe(III) in ruthenium modified cyt c has also been investigated 58). 

A more sophisticated method involves the combination of metal-substituted proteins with electron acceptors covalently attached to the amino acid residue located at the protein surface. The [Ru(NH3)5]3+ complex attached to the histidine residue had been used in Zn-substituted myoglobin [71,72] or cytochrome b562 [73] as an electron acceptor. This design allows the distance between donor and acceptor to be fixed and is very useful for experimental analysis of intramolecular electron transfer in proteins. 

Scott JR, Willie A, MacLean M, et al. Intramolecular electron transfer in cytochrome b5 labeled with ruthenium(II) polypyridine complexes rate measurements in the Marcus inverted region. J Am Chem Soc 1993 115 6820-4. 

Scott JR, Fairris JL, McLean M, et al. Intramolecular electron-transfer reactions of cytochrome 5 covalently bonded to ruthenium(II) polypyridine complexes reorgani-zational energy and pressure effects. Inorg Chim Acta 1996 243 193-200. 

It would appear then that the redox properties of flavocytochrome 4>2 are well understood. While this is generally true, there are a number of aspects which remain controversial and it is these that will form the main focus of this article. There are three major questions which will be addressed (i) Does the transfer of redox equivalents from lactate to flavin really involve a carbanion intermediate (ii) What controls the intramolecular electron transfers from flavin to heme (iii) Where, on the surface of flavocytochrome 4>2> does cytochrome c bind prior to inter-molecular electron transfer ... 

Kobayashi, K., Koppenh fer, A., Ferguson, S. J., and Tagawa, S., 1997, Pulse radiolysis studies on cytochrome cd nitrite reductase from Thiosphaera pantotropha Evidence for a fast intramolecular electron transfer from c heme to d, heme, RiocAemtstry 36 1361 In 13616. 

In step 1, CcP compound I forms a complex with ferrocytochrome c followed by electron transfer (step 2) and dissociation (step 3) of oxidized cytochrome c. Step 4 represents the intramolecular electron transfer from Trp 191 to the iron giving Fe +Trp +, the intermediate required to oxidize the second molecule of ferrocytochrome c. In this mechanism, formation of the Trp 191 cation radical is essential for both electron transfer steps. 

Sulfite oxidase contains an oxo-molybdenum center and a 6-type cytochrome. The proposed catalytic sequence (254-256) for the enzyme is shown in Fig. 16. Oxidation of sulfite to sulfate, a two-electron process, occurs at the molybdenum center with concomitant reduction of the molybdenum from VI to IV. Electrons are removed from the enzyme by interactions of the heme of the 6-type cytochrome with exogenous cytochrome c, a one-electron process. Thus, the proposed mechanism of Fig. 16 involves two separate intramolecular electron transfers be-... 

Figure 3. Driving force-dependence of intramolecular electron transfer rates in Ru-ammine-His33 modified Zn-substituted cytochrome c ( ), and Ru-bpy-His33 modified Fe-cytochrome c ( ). Solid lines were generated using Eq. 1 and the following parameters Ru-ammine,, i=1.15 eV, Hab = 0.10 cm Ru-bpy, X = 0.74 eV, Hab = 0.095 cm". ...

Seven horse heart cytochrome C derivatives, each with a single sarcophaginate [Co(diAMsar)]3+ cation covalently attached to a specific surface carboxylate side chain, were synthesized. The experimental intramolecular electron transfer rate constants is nearly independent of the [Co(diAMsar)] + cation attachment site [320]. 

H. E.M., NazmudUnov, R.R., and Ulstrup, J. (2010) Approach to interfadal and intramolecular electron transfer of the diheme protein cytochrome c(4) assembled on Au(lll) surfaces. journal of Physical Chemistry B, 114, 5617-5624. 

Chi, Q.J., Zhang, J.D., Jensen, P.S., Nazmudtinov, R.R., and Ulstrup, J. (2008) Surface-induced intramolecular electron transfer in multi-centre redox metalloproteins the di-haem protein cytochrome q in homogeneous solution and at electrochemical surfaces. Journal of Physics Condensed Matter, 20, 374124. 

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