Cytochrome system, electron transfer

Cytochromes are electron-transfer proteins having one or several haem groups. Cytochrome c binds to the protein by one or, more commonly two, thioether bonds involving sulphydryl groups of cysteine residues. The fifth haem iron ligand is always provided by a histidine residue. Cytochrome c has been proved to be a useful model system for studying the relationship between protein structure and thermostability due to the availability of its three-dimensional structure from a wide variety of organisms, both mesophiles and thermophiles. 

Adrenodoxin. Adrenodoxin is the only iron-sulfur protein which has been isolated from mammals. This protein from mitochondria of bovine adrenal cortex was purified almost simultaneously by Kimura and Suzuki (32) and Omura et al. (33). It has a molecular weight of 12,638 (34) and the oxidized form of the protein shows maximal absorbances at 415 and 453 nm. Adrenodoxin acts as an electron carrier protein in the enzyme system required for steroid hydroxylation in adrenal mitochondria. In this system, electron transfer is involved with three proteins cytochrome P. gQ, adrenodoxin and a flavoprotein. Reduced NADP gives an electron to Tne flavoprotein which passes the electron to adrenodoxin. Finally, reduced adrenodoxin transfers the electron to cytochrome Pas shown in Fig. 3. The mechanism of cytochrome P cq interaction with steroid, oxygen and adrenodoxin in mixed-function oxidase of adrenal cortex mitochondria has been reviewed by Estabrook et al. (35). 

A wide variety of different cytochrome-linked electron-transfer systems is encountered in bacteria respiratory chains with oxygen, nitrate or sulphate as electron acceptors, fumarate reductase systems and light-driven cyclic electron-transfer systems (Fig. 3). All these systems are composed of several electron-transfer carriers, the nature of which varies considerably in different organisms. Electron carriers which are most common in bacterial electron-transfer systems are flavoproteins (dehydrogenases), quinones, non-heme iron centres, cytochromes and terminal oxidases and reductases. One common feature of all electron-transfer systems is that they are tightly incorporated in the cytoplasmic membrane. Another important general property of these systems is that electron transfer results in the translocation of protons from the cytoplasm into the external medium. Electron transfer therefore... 

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. 

Scheme 10.3 Electron-transport systems associated with cytochrome P450 monooxygenases. Arrows indicate electron transfer.

Cyclic voltammetry and other electrochemical methods offer important and sometimes unique approaches to the electroactive species. Protein organization and kinetic approaches (Correia dos Santos et al. 1999, Schlereth 1999) can also be studied by electrochemical survey. The electron transfer reaction between cytochrome P450scc is an important system for... 

The electron transfer properties of the cytochromes involve cycling of the iron between the +2 and +3 oxidation states (Cytochrome)Fe + e" (Cytochrome)Fe ° = -0.3Vto+ 0.4V Different cytochromes have different side groups attached to the porphyrin ring. These side groups modify the electron density in the delocalized iz system of the porphyrin, which in turn changes the redox potential of the iron cation in the heme. 

P. Mitchell (Nobel Prize for Chemistry, 1978) explained these facts by his chemiosmotic theory. This theory is based on the ordering of successive oxidation processes into reaction sequences called loops. Each loop consists of two basic processes, one of which is oriented in the direction away from the matrix surface of the internal membrane into the intracristal space and connected with the transfer of electrons together with protons. The second process is oriented in the opposite direction and is connected with the transfer of electrons alone. Figure 6.27 depicts the first Mitchell loop, whose first step involves reduction of NAD+ (the oxidized form of nicotinamide adenosine dinucleotide) by the carbonaceous substrate, SH2. In this process, two electrons and two protons are transferred from the matrix space. The protons are accumulated in the intracristal space, while electrons are transferred in the opposite direction by the reduction of the oxidized form of the Fe-S protein. This reduces a further component of the electron transport chain on the matrix side of the membrane and the process is repeated. The final process is the reduction of molecular oxygen with the reduced form of cytochrome oxidase. It would appear that this reaction sequence includes not only loops but also a proton pump, i.e. an enzymatic system that can employ the energy of the redox step in the electron transfer chain for translocation of protons from the matrix space into the intracristal space. 

An important model system for electron transfer studies is the electron carrier cytochrome c (Cyt c). Its redox center is a heme, coordinated by a histidine and... 

C.H. Brubaker, Michigan State University In the case of the cytochromes, it has been proposed that electron transfer from the iron porphyrin may involve the pi system of the porphyrin and even nearby aromatic rings. Do you think that a similar thing may happen in the case of the reaction between these copper(I) p lastocyanins and the chromium(III) You seem to favor the idea that the important factor is that the Cr(III) be at a site that is reasonably close to the copper center. 

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. 

The large number of cytochromes identified contain a variety of porphyrin ring systems. The classification of the cytochromes is complicated because they differ from one organism to the next the redox potential of a given cytochrome is tailored to the specific needs of the electron transfer sequences of the particular system. The cytochromes are one-electron carriers and the electron flow passes from one cytochrome type to another. The terminal member of the chain, cytochrome c oxidase, has the property of reacting directly with oxygen such that, on electron capture, water is formed ... 

While cytochrome P-450 catalyzes the interaction with substrates, a final step of microsomal enzymatic system, flavoprotein NADPH-cytochrome P-450 reductase catalyzes the electron transfer from NADPH to cytochrome P-450. As is seen from Reaction (1), this enzyme contains one molecule of each of FMN and FAD. It has been suggested [4] that these flavins play different roles in catalysis FAD reacts with NADPH while FMN mediates electron... 

Recent advances in measuring the kinetics of the various electron-transfer steps in this system have been achieved by use of flash photolysis of ruthenated derivatives of cytochrome c (Ru-Cc) (17-19). In these studies [Ru(bpy)3]2+ is covalently bound to a surface residue at a site that does not interfere with the docking of cytochrome c to cytochrome c oxidase. Solutions are then prepared containing both Ru-Cc and cytochrome c oxidase, and the two proteins associate to form a 1 1 complex. Flash photolysis of the solution leads directly to the excitation of the RuII(bpy)3 site, which then reduces heme c very rapidly. This method thus provides a convenient means to observe the subsequent intracomplex electron transfer from heme c to cytochrome c oxidase and further stages in the process. 

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