Iron, cytochrome heme protein

Electron transfer to 02 occurs stepwise, through a series of flavoproteins, cytochromes (heme-proteins), iron-sulfur proteins, and a quinone. Most of the electron carriers are collected in four large complexes, which communicate via two mobile carriers— ubiquinone (UQ) and cytochrome c. Complex I transfers electrons from NADH to UQ, and complex II transfers electrons from succinate to UQ. Both of these complexes contain flavins and numerous iron-sulfur centers. Complex III, which contains three cyto-... 

A recent summary25 of the activation of O2 by cytochrome P-450 (an iron(III)-heme protein with a axial cysteine thiolate ligand) concludes that the reactive form of this mono-oxygenase also contains an oxene-ferryl group (RS)(por)FeV=O. The mono-oxygenase chemistry of cytochrome P-450 has been modeled via the use of (TPP)Fei lCl(TPP=tetraphenylporphyrin) and (OEP)Fe JCl (OEP=octaethylporphyrin) with peracids,26,27 iodosobenzene,26,27... 

Zinc is also used in biological studies to gain information about non-zinc containing systems. It can be a convenient redox inactive replacement for the study of complex systems with multiple redox centers. For example, the mechanism of quenching the triplet state of zinc cytochrome c by iron(II) and iron(III) cytochrome c has been studied. Zinc insertion has been used to get around the difficulty of studying two heme proteins with the same absorption spectra and provides rate constants for iron and iron-free cytochrome c quenching.991... 

How does nature prevent the release of hydrogen peroxide during the cytochrome oxidase-mediated four-electron reduction of dioxygen It would appear that cytochrome oxidase behaves in the same manner as other heme proteins which utilize hydrogen peroxide, such as catalase and peroxidase (vide infra), in that once a ferric peroxide complex is formed the oxygen-oxygen bond is broken with the release of water and the formation of an oxo iron(IV) complex which is subsequently reduced to the ferrous aquo state (12). Indeed, this same sequence of events accounts for the means by which oxygen is activated by cytochromes P-450. 

In the field of enzyme catalysis, heme-proteins such as cytochrome P450, for example, exhibit both types of 0-0 bond cleavages in organic hydroperoxides and peroxy acids (178). Heterolytic cleavage of HOOH/ROOH yields H20 or the corresponding alcohol, ROH and a ferryl-oxo intermediate (Scheme 4). Homolytic 0-0 bond cleavage results in the formation of a hydroxyl (HO ) or an alkoxyl (RO ) radical and an iron-bound hydroxyl radical. 

This general approach has, however, serious limitations. The position of the site for attack (and therefore the electron transfer distance involved) is very conjectural. In addition, the vexing possibility, which we have encountered several times, of a dead-end mechanism (Sec. 1.6.4) is always present. One way to circumvent this difficulty, is to bind a metal complex to the protein at a specific site, with a known (usually crystallographic) relationship to the metal site. The strategy then is to create a metastable state, which can only be alleviated by a discernable electron transfer between the labelled and natural site. It is important to establish that the modification does not radically alter the structure of the protein. A favorite technique is to attach (NH3)5Ru to a histidine imidazole near the surface of a protein. Exposure of this modified protein to a deficiency of a powerful reducing agent, will give a eon-current (partial) reduction of the ruthenium(III) and the site metal ion e.g. iron(III) heme in cytochrome c... 

Iron occurs in every mammalian cell and is vital for life processes. It is bound to various proteins and found in blood and tissues. The iron-porphyrin or heme proteins include hemoglobin, myoglobin and various heme enzymes, such as cytochromes and peroxidases. Also, it occurs in non heme compounds, such as ferritin, siderophilin, and hemosiderin. Hemoglobin, found in the red blood cells, is responsible for transport of oxygen to the tissue cells and constitutes about two-thirds (mass) of all iron present in the human body. An adult human may contain about 4 to 6 grams of iron. 

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