Cytochrome P450, electron-transfer reactions

Dihaloelimination is a two-electron transfer reaction. Thompson et al. [377] reported reductive dichloroelimination of 1,1,2-TCA and TeCA by hepatic micro-somes from rat Ever, with VC and both tDCE and cDCE as metabolites. Reductive dichloroelimination from hexa- and pentachloroethane by microsomal cytochrome P450 was studied by Nastainczyk et al. [378]. The main products of the in vitro metabolism of hexa- and pentachloroethane were PCE (99.5%) and TCE (96%), respectively, with minor amounts of pentachloroethane (0.5%) and TeCA (4%), respectively, via reductive dechlorination. 

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... 

Natural proteides like hemoglobin, myoglobin, peroxidase, catalase, cytochrom a, b, P450, desoxigenase and chlorophyll contain the porphyrin system as a prostetic group. The porphyrin is bound coordinatively through the metal atom to the natural polymer or inserted in lipid-protein layers. Important properties are binding of small molecules, catalysis and electron transfer reactions. 

Electron-transfer reactions of amines are of significant importance in biochemical systems. Enzymes known to catalyze the oxidative dealkylation of amines include monoamine oxidase [16, 17], cytochrome-P450 [18, 184-186], horseradish peroxidase [187], hemoproteins [188, 189], and chloroperoxidase [187, 188]. N-dealkylation of amines by peroxidases are generally accepted to occur via one-electron transfer, whereas the role of electron transfer in reactions catalyzed by enzymes such as monoamine oxidase [16, 17] and cytochrome P-450 [18, 184, 185] is currently a topic of debate. 

Protein-mediated electron transfer is a device used in a diverse array of biological transformadons. Well-known electron transfer processes include the mitochondrial electron transport system, photosynthesis (Chapter 13), and nitrogen flxadon (Chapter 15). Less well known biochemical reactions in which electron transfer plays a crucial role include nitric oxide synthesis and the cytochrome P450 electron transport systems. Each of these mechanisms is briefly oudined. 

Mie, Y, Ikegami, M. and Komatsu, Y. (2010) Gold sputtered electrode surfaces enhance direct electron transfer reactions of human cytochrome P450s. Electochem. Commun., 12, 680. 

Fig. 5. Catalytic cycle of cytochrome P450. The substrate HR binds to the resting enzyme A to form intermediate B, which is reduced by one electron to form C and then reacts with dioxygen. The resulting ferric-peroxo intermediate D is reduced by one equivalent to form the transient oxyferrous intermediate E, which proceeds quickly to intermediate F with release of a molecule of water. F is designated Fe(V)=0 to indicate that it is oxidized by two equivalents greater than A and not to imply anything about the true oxidation state of the iron. Intermediate F then transfers an oxygen atom to the substrate to regenerate the resting enzyme. The peroxide shunt refers to the reaction of B with hydrogen peroxide to produce the intermediate F, which can then proceed to product formation.

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