Facilitated electron transfer, cytochrome

The reaction between HiPIP and cytochrome c or bacterial cytochromes784 involves specific sites on both proteins, although no kinetic evidence for association was found. Reaction between Chr. vinosum and Rhodopseudomonas gelatinosa HiPIPs with modified cytochrome c (trinitro-phenyllysine-13)785 was more rapid than for the unmodified cytochrome c since modification of Lys-13 destabilizes the heme crevice, and because the hydrophobic TNP group facilitates electron transfer by interacting with a hydrophobic region of the HiPIP. 

Adsorbed layers of cytochrome c3 have also been used to facilitate electron transfer of molecules that otherwise would not yield high electron-transfer rates. Facilitated electron transfer has been observed for flavodoxin and ferredoxin on cytochrome c3 modified basal plane pyrolytic graphite (26). 

In the electron transfer cytochromes the iron of heme is bound in a hexacoordinate low-spin state, with two protein ligands (typically histidine and/or methionine) above and below the heme plane. They serve as electron carrier proteins in mitochondria and endoplasmatic organelles as well as in bacterial redox chains. At least three classes of cytochromes, a, b and c, are known. They can alternate between an oxidized Fe(III) low-spin state with a single unpaired eleetron and a reduced Fe(II) low-spin form with no unpaired electrons. Since iron remains low spin, electron transfer is greatly facilitated. The best characterized family are the c cytochromes. 

COH, but also facilitated electron transfer from cytochrome c to H2O2, finally generating ROS. Therefore, GO/PP in the C. elegans body had the ability to accelerate and enhance electron transfer between the interface of juglone/02, H2O2/COH, or cytochrome C/H2O2, which impaired the inherent antioxidant defence system and eventually resulted in dramatic toxicity to the worms. 

In the hemoproteins, the central ion is iron. In protoporphyrins, central ion and ligand form the heme. The same heme is inserted into different protein molecules, yielding different molecular complexes with different catalytic properties. For example, cytochromes facilitate electron transfer, catalase converts H2O2 into H2O, and hemoglobin stores and transports oxygen. The same chelates (central ion and ligand) can perform... 

It has been shown [103] that cytochrome c exhibits different voltam-metric responses depending on the nature of the electrode surface. Various surface modifiers have been proposed to facilitate electron transfer to or from some conjugated proteins. The SERS spectroscopy has been used to elucidate the interfacial properties of cytochrome c in the presence of the promoters at both silver and gold electrodes [103, 106]. It has been demonstrated that the surface modifiers act either as the surface active agents with which cytochrome c is coadsorbed on the electrode, or they form a strongly adsorbed layer through which the electron exchange process between the native form of cytochrome c and the electrode takes place. 

Besides the permselective properties, the electrocatalytic properties of ECP films can be also used for the amperometric detection of some target molecules. Accordingly, electrodes modified with PPy, polythiophene (PTh), PAni, and their derivatives were found to catalyze the electrochemical oxidation of ascorbic acid [127-129], NADH [115, 116,130], dopamine [128], pyrrolo-quinoline quinone [131] as a coenzyme of some oxidoreductases, and quinone and derivatives [132, 133]. Selectivity exhibited by these materials could be enhanced by the introduction of an appropriate substituent onto the polymer backbone. So, a facilitated electron transfer between cytochrome c and carboxylic acid or carboxylate-substituted PPy [134] or polyindole [135] has been observed. As such an effect was not obtained with unsubstituted polymer films, the cytochrome c-polymer interaction was e lained on the basis of binding between the polymer substituents and the lysine residues on the redox protein. 

In the ci positional state, fast electron transfer from the Rieske protein to cytochrome Ci will he facilitated hy the close interaction and by the hydrogen bond between His 161 of the Rieske protein and a propionate group of heme Ci, but the Rieske cluster is far away from the quinone binding site. 

We see that electron transfer can be accompanied by loss of a proton and that E ° may become pH dependent. (See also Eq. 16-18.) Even with cytochrome c, although there is little structural change upon electron transfer, there is an increased structural mobility in the oxidized form.156 This may be important for coupling and could also facilitate associated proton-transfer reactions. For example, it is possible that in some cytochromes the imidazole ring in the fifth coordination position may become deprotonated upon oxidation. This possibility is of special interest because cytochromes are components of proton pumps in mitochondrial membranes (Chapter 18). 

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