Electron-transfer reactions in cytochromes

Tosha T, Yoshioka S, Hori H, Takahashi S, Ishimori K, Morishima I (2002) Molecular mechanism of the electron transfer reaction in cytochrome P450cam-putidaredoxin roles of glutamine 360 at the heme proximal site. Biochemistry 41 13883-13893... 

Amaut LG, Formosinho SJ (1998) Modelling intramolecular electron transfer reactions in cytochromes and in photosynthetic bacteria rection centres. J Photochem Photobiol A Chem... 

The most conspicuous use of iron in biological systems is in our blood, where the erythrocytes are filled with the oxygen-binding protein hemoglobin. The red color of blood is due to the iron atom bound to the heme group in hemoglobin. Similar heme-bound iron atoms are present in a number of proteins involved in electron-transfer reactions, notably cytochromes. A chemically more sophisticated use of iron is found in an enzyme, ribo nucleotide reductase, that catalyzes the conversion of ribonucleotides to deoxyribonucleotides, an important step in the synthesis of the building blocks of DNA. 

The cytochromes are another group of haem proteins found in all aerobic forms of life. Cytochromes are electron carriers involving a Fe(ii)/Fe(m) redox system. They are a crucial part of the electron transfer reactions in mitochondria, in aspects of the nitrogen cycle, and in enzymic processes associated with photosynthesis. 

In order to obtain further information on the magnitude of the overall reaction volume and the location of the transition state along the reaction coordinate, a series of intermolecular electron-transfer reactions of cytochrome c with pentaammineruthenium complexes were studied, where the sixth ligand on the ruthenium complex was selected in such a way that the overall driving force was low enough so that the reaction kinetics could be studied in both directions (153, 154). The selected substituents were isonicotinamide (isn), 4-ethylpyr-idine (etpy), pyridine (py), and 3,5-lutidine (lut). The overall reaction can be formulated as... 

Willner I, Willner B (1991) Artificial Photosynthetic Model Systems Using Light-Induced Electron Transfer Reactions in Catalytic and Biocatalytic Assemblies. 159 153-218 Woggon W-D (1997) Cytochrome P450 Significance, Reaction Mechanisms and Active Site Analogues. 184 39 - 96... 

Since their first report of long-ranged electron-transfer reactions in ruthenium-modified cytochrome c [43], Gray and co-workers have studied re-... 

The simplest electron transfer reaction that cytochrome c can undergo, at least in principle, is the self-exchange reaction. The rate of this... 

The mechanism of electron transfer reactions in metal complexes has been elucidated by -> Taube who received the Nobel Prize in Chemistry for these studies in 1983 [xiv]. Charge transfer reactions play an important role in living organisms [xv-xvii]. For instance, the initial chemical step in -> photosynthesis, as carried out by the purple bacterium R. sphaeroides, is the transfer of electrons from the excited state of a pair of chlorophyll molecules to a pheophytin molecule located 1.7 mm away. This electron transfer occurs very rapidly (2.8 ps) and with essentially 100% efficiency. Redox systems such as ubiquinone/dihydroubiquinone, - cytochrome (Fe3+/Fe2+), ferredoxin (Fe3+/Fe2+), - nicotine-adenine-dinucleotide (NAD+/NADH2) etc. have been widely studied also by electrochemical techniques, and their redox potentials have been determined [xviii-xix]. 

Siddarth, P. and Marcus, R.A. (1993c) Correlation between theory and experiment in electron-transfer reaction in protein electronic couplings in modified cytochrome c and myoglobin derivatives. J. Phys. Chem. 97, 13078-13082. 

Hill, B. C., 1994, Modeling the sequence of electron transfer reactions in the single turnover of reduced, mammalian cytochrome c oxidase with oxygen, J. Biol. Chem. 269 2419n2425. 

Electron-transfer reactions between cytochrome c and cytochrome c peroxidase have been studied extensively because of the well-characterized structures and biophysical properties of the reactants [146-150]. It is well known that the resting ferric form of cytochrome c peroxidase is oxidized by hydrogen peroxide to compound I, which contains an oxyferryl heme moiety in which the iron atom has a formal oxidation state of 4-1- [146-150]. The other is a porphyrin n radical cation or organic radical (R +) localized on an amino acid residue of Trp-191 [151-154] this is formed by transfer of the oxidized equivalent to the amino acid side chain [150]. The site of electron transfer in the reduction of compound I has been controversial and two forms of compound II have been identified, (P)Fe =0 containing the oxyferryl heme Fe(IV) [155-158] and [(P)Fe ] + containing Fe(III) and the porphyrin % radical cation which oxidizes the amino acid side-chain to produce an organic radical [(P)Fe +, R" ] [159 165] as shown in Scheme 10. 

The mechanism of reduction of dioxygen has been partially clarified, mainly thanks to the low temperature trapping technique designed by Chance et al. [129], Although the precise mechanism is still not understood, it seems probable that dioxygen is reduced to water in two concerted two-electron steps (Fig. 3.3) in which certain intermediates have been identified (see Refs. 8, 92, 97-100, 129-133). One important feature of this particular mechanism is that it affords a switch from one-electron transfer reactions (of cytochromes c, a and Cu ) to effective two-electron steps in the reduction of Oj. The latter is necessary for thermodynamic and kinetic reasons and to effectively prevent release of toxic oxygen radicals from the active site (see Ref. 99). 

Electron carriers and electron-transfer proteins Electron-transfer reactions in photosynthesis involve electron carriers or electron-transfer proteins, including, among others, quinones, cytochromes, and iron-sulfur proteins. In the following, we present a summary of the carriers or associated proteins that are primarily involved in photosynthetic electron-transfer reactions, along with a listing in Fig. 20. Although ATP is not an electron carrier, it is included in the figure to remind us of the common components present in the structures of ATP and NAD(P) molecules [see Fig. 20 (A)]. 

S He, S Modi, DS Bendall and JC Gray (1991) The surface exposed residue tyrosine Tyr83 of pea plastocyanin is involved in both binding and electron transfer reactions with cytochrome f EMBO J 10 4011-4016... 

The experiments referred to in this chapter have also assisted the theoretical analysis of the role of temperature and pressure on biological electron transfer reactions. In the case of cytochrome c, an empirical approach could show that the heme iron is screened more efficiently from surface charges in the oxidized state (37). In general, solute-solvent interactions are directly influenced by temperature and pressure, and these interactions will affect the electrostatic interaction energies, which can be accounted for in terms of changes in the dielectric constant of both the solute and the solvent. Such interactions are of major importance in the understanding of electron transfer processes. 

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