Intermolecular electron transfer, cytochrome

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... 

Reaction of Cytochrome cimu with Tris(oxalato)cobalt(III) The cytochrome c protein was also used as reductant in a study of the redox reaction with tris (oxalato)cobalt(III).284 Selection of the anionic cobalt(III) species, [Conl(ox)3]3 was prompted, in part, because it was surmised that it would form a sufficiently stable precursor complex with the positively charged cyt c so that the equilibrium constant for precursor complex formation (K) would be of a magnitude that would permit it to be separated in the kinetic analysis of an intermolecular electron transfer process from the actual electron transfer kinetic step (kET).2S5 The reaction scheme for oxidation of cyt c11 may be outlined ... 

Contzen J, Kostka S, Kraft R, Jung C. Intermolecular electron transfer in cytochrome P450cam covalently bound with tris(2,2 -bipyridyl)ruthenium(II) structural changes detected by FTIR spectroscopy. J Inorg Biochem 2002 91 607-17. 

For electron transfer from copper to heme, two dominant sets of pathways of comparable efficiency were predicted. In each set of pathways, the point of intermolecular electron transfer was from the backbone O of Glu i of amicyanin to the backbone N of Gly of cytochrome c-551i, and the entry of electrons to iron occurred either via the porphyrin ring or the His ligand. In one set of pathways the exit of electrons from copper occurred via the Cys copper ligand, and the phenolic side chain of Tyr was an intermediate between Cys and Glu. In the other set of pathways the exit of electrons from copper occurred via the Met copper ligand, and the backbone of Lys was an intermediate between Met and Glu i... 

Electron-Transfer Reactions. It is well known that thermal and photochemical electron-transfer reactions exhibit characteristic pressure dependences and associated volumes of activation (see Sections II, III, and VI). It is therefore realistic to expect that photoinduced thermal electron-transfer reactions will also exhibit a characteristic pressure dependence that should reveal mechanistic information on the nature of the reaction. Recent interest in the mechanistic understanding of long-distance electron-transfer reactions prompted an investigation of the effect of pressure on intramolecular electron transfer in ruthenium-modified cytochrome c [151] (a typical example of a closely related intermolecular electron-transfer reaction was... 

Lojou, E., F. Cutruzzola, M. Tegoni, and P. Bianco (2003). Electrochemical study of the intermolecular electron transfer to Pseudomonas aeruginosa cytochrome cd nitrite reductase. Electrochim. Acta 48, 1055-1064. 

Daff and co-workers have investigated the catalytic cycle which can be described in terms of five consecutive electron-transfer events. Lactate dehydrogenation results in the two-electron reduction of FMN. The two electrons are individually passed to the 2-heme (intramolecular electron transfer) and then onto cytochrome c (intermolecular electron transfer). Freeze quench EPR was used to demonstrate that during steady-state turnover of the enzyme approximately 75% of the flavin is in the semiquinone state, and hence that this is a catalytic intermediate. 

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. 

Finally, in this section we discuss the model compounds for the Type III binuclear Cua center. Electron transfer (eT) to and from Cua is remarkably fast, considering the distances to be traveled. Intermolecular eT from cytochrome c Cua, over a distance of -18-20 A, has a rate constant, k, equal to 6 X ICf s while intramolecular eT from Cua heme a, also over an -18- to 20-A distance, has a rate constant of 1 x lO s The Cua center exists in two redox states the reduced form with two cuprous ions (Cu -Cu ) and the fully delocalized mixed valent center (Cu -Cu Cu -Cu =Cu - -Cu ). In the... 

Cytochromes c, small blue-copper proteins, or an internal heme c group can function as natural electron acceptors for the dehydrogenases. Since these are soluble proteins and the genes have been cloned in most cases, they provide excellent possibilities to study electron transfer pathways in vitro and intermolecular as well as intramolecular pathways between a quinone and Cu or heme c in particular. 

Flavocytochrome b2 from Saccharomyces cerevisiae, a member of the FMN-dependent oxidoreductase superfamily, catalyzes the two-electron oxidation of lactate to pyruvate with subsequent electron-transfer to cytochrome c via the bound flavin [55], What distinguishes the enzyme from other family members is the N-terminal fusion of a heme-binding domain to the ySa-barrel structure, which hosts the primary active site. Rather than dumping the electrons from the reduced flavin hydroquinone onto molecular oxygen, they are transferred intramolecularly to the heme-binding domain and from there in a second intermolecular step to cytochrome c. 

Application of pulse-radiolysis techniques (see Sect. 1.4 for more details on the experimental set-up) revealed that the following intramolecular (Eqs 1.6 and 1.7) and intermolecular (Eq. 1.8) electron-transfer reactions, where cyt c represents cytochrome c, all exhibit a significant acceleration with increasing pressure. The reported volumes of activation are —17.7 0.9, -18.3 0.7, and —15.6 0.6 cm mol respectively, and clearly demonstrate a significant volume collapse upon reaching the transition state [63]. 

Kadkhodayan S, Coulter ED, Maryniak DM, Bryson TA, Dawson JH (1995) Uncoupling oxygen transfer and electron transfer in the oxygenation of camphor analogues by cytochrome P450cam. Direct observation of an intermolecular isotope effect for substrate C H activation. J Biol Chem 270 28042 28048... 

The complex between yeast cytochrome c peroxidase and cytochrome c, however, seams to present a different type of behavior with respect to the association process. As for most known complexes formed between electron transfer proteins, this is a week complex with a binding constant of the order of 10. In contrast to the other cases, these two proteins do not show a great extent of surface matching at the docking interface, nor do they show very specific amino acid interactions across the intermolecular surface. As a result, the predicted model of the structure of this complex is less well defined. 

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