Cytochrome component mitochondrial

Controversial results of oxygen radical detection in mitochondria have been described in the literature. Owing to experimental difficulties many authors were obliged to work with sub-mitochondrial particles instead of the whole mitochondria. However, it is quite possible that oxygen radical production by submitochondrial particles may be artificially enhanced due to exposure to oxygen. On the other hand, some analytical methods of superoxide detection such as cytochrome c reduction cannot be used due to the direct reduction of cytochrome by mitochondrial components. 

Cytochrome P-450 monooxygenase systems can be classified as of either two- or three-component type (43). Three-component-type systems are composed of cytochrome P-450 reductase, cytochrome P-450, and an iron-sulfur protein, whereas two-compo-nent-type systems are composed of cytochrome P-450 reductase and cytochrome P-450. Tlie results of many previous works have shown that almost all prokaryotes have three-component (mitochondrial) type cytochrome P-450 systems (29,45,46), and those of eukaryotes, such as yeast, are of the two-component (microsomal) type (47). Cytochrome... 

The studies show that if the cell unsaturated fatty acid content is low enough, the assembly of the mitochondrial membrane is profoundly affected. We have concentrated mainly on the mitochondrially synthesized components, but the mitochondrial level of cytochrome c, which is synthesized in the cytoplasm, was also reduced under lipid-depleted conditions. Since petite mutant cells, which lack a mitochondrial protein-synthesizing system, contain normal levels of cytochrome c when derepressed (Marzuki and Linnane, unpublished observation), this effect cannot be due to a simple feedback mechanism which coordinates the synthesis of cytoplasmically synthesized cytochrome components with mitochondrially synthesized components. More likely, it is a reflection of the altered physical structure of the membrane under these conditions, which may not allow the integration of cytochrome c into the membrane. This system offers great potential for resolving some of the problems of the mechanism of mitochondrial membrane assembly. 

Functionally and strucmrally, the components of the respiratory chain are present in the inner mitochondrial membrane as four protein-lipid respiratory chain complexes that span the membrane. Cytochrome c is the only soluble cytochrome and, together with Q, seems to be a more mobile component of the respiratory chain connecting the fixed complexes (Figures 12-7 and 12-8). 

To explain how H+ transfer occurred across the membrane Mitchell suggested the protons were translocated by redox loops with different reducing equivalents in their two arms. The first loop would be associated with flavoprotein/non-heme iron interaction and the second, more controversially, with CoQ. Redox loops required an ordered arrangement of the components of the electron transport system across the inner mitochondrial membrane, which was substantiated from immunochemical studies with submitochondrial particles. Cytochrome c, for example, was located at the intermembranal face of the inner membrane and cytochrome oxidase was transmembranal. The alternative to redox loops, proton pumping, is now known to be a property of cytochrome oxidase. 

Cytochrome bci is a multicomponent enzyme found in the inner mitochron-drial membrane of eukaryotes and in the plasma membrane of bacteria. The cytochrome bci complex functions as the middle component of the mitochondrial respiratory chain, coupling electron transfer between ubiquinone/ ubiquinol (see Figure 7.27) and cytochrome c. 

Cyanobacteria can synthesize ATP by oxidative phosphorylation or by photophosphorylation, although they have neither mitochondria nor chloroplasts. The enzymatic machinery for both processes is in a highly convoluted plasma membrane (see Fig. 1-6). Two protein components function in both processes (Fig. 19-55). The proton-pumping cytochrome b6f complex carries electrons from plastoquinone to cytochrome c6 in photosynthesis, and also carries electrons from ubiquinone to cytochrome c6 in oxidative phosphorylation—the role played by cytochrome bct in mitochondria. Cytochrome c6, homologous to mitochondrial cytochrome c, carries electrons from Complex III to Complex IV in cyanobacteria it can also carry electrons from the cytochrome b f complex to PSI—a role performed in plants by plastocyanin. We therefore see the functional homology between the cyanobacterial cytochrome b f complex and the mitochondrial cytochrome bc1 complex, and between cyanobacterial cytochrome c6 and plant plastocyanin. 

Correct answer = D. Thirteen of the approximately 100 polypeptides required for oxidative phosphorylation are coded for by mitochondrial DNA, including the electron transport components cytochrome c and coenzyme Q. Oxygen directly oxidizes cytochrome oxidase. Succinate dehydrogenase directly reduces FAD. Cyanide inhibits electron flow, proton pumping, and ATP synthesis. 

A second group of electron carriers in mitochondrial membranes are the iron-sulfur [Fe-S] clusters which are also bound to proteins. Iron-sulfur proteins release Fe3+ or Fe2+ plus H2S when acidified. The "inorganic clusters" bound into the proteins have characteristic compositions such as Fe2S2 and Fe4S4. The sulfur atoms of the clusters can be regarded as sulfide ions bound to the iron ions. The iron atoms are also attached to other sulfur atoms from cysteine side chains from the proteins. The Fe-S proteins are often tightly associated with other components of the electron transport chain. For example, the flavoproteins Flavin 1, Flavin 2, and Flavin 3 shown in Fig. 10-5 all contain Fe-S clusters as does the Q-cytochrome b complex. All of these Fe-S clusters seem to be one-electron carriers. 

The final group of mitochondrial redox components are one-electron carriers, small proteins (cytochromes) that contain iron in the form of the porphyrin complex known as heme. These carriers, which are discussed in Chapter 16, exist as several chemically distinct types a, b, and c. Two or more components of each type are present in mitochondria. The complex cytochrome aa3 deserves special comment. Although cytochromes are single-electron carriers, the cytochrome aa3 complex must deliver four electrons to a single 02 molecule. This may explain why the monomeric complex contains two hemes and two copper atoms which are also able to undergo redox reactions.1 2... 

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). 

The electron acceptor for complex III is cytochrome c, which, unlike the other cytochromes, is water soluble and easily released from mitochondrial membranes. Nevertheless, it is usually present in a roughly 1 1 ratio with the fixed cytochromes, and it seems unlikely that it is as free to diffuse as are ubiquinone and NAD+.121122 However, a small fraction of the cytochrome c may diffuse through the intermembrane space and accept electrons from cytochrome b-, which is located in the outer membrane.123 Cytochrome c forms a complex with cardiolipin (diphosphatidylglycerol), a characteristic component of the inner mitochondrial membrane.124... 

The tissue is minced or ground and suspended in sucrose buffer. The choice of buffer conditions is critical in order to avoid loss of protein and other components that may leak from the mitochondria. Cytochrome c is one of the easiest mitochondrial proteins to dislodge, and some is almost always lost. The buffer should be isotonic or hypotonic with a low ionic strength. Sucrose is an ideal buffer component because mitochondria are especially stable and remain intact under these conditions. 

The discussion to this point has focused on the isolation of intact mitochondria. By various chemical and physical treatments, mitochondria may be separated into their four components. This allows biochemists to study the biological functions of each component. For example, by measuring enzyme activities in each fraction, one can assign the presence of a particular enzyme to a specific region of the mitochondria. Studies of mitochondrial subfractions have resulted in a distribution analysis of enzyme activities in the four locations (Table E10.1). This type of study is often referred to as an enzyme profile or enzyme activity pattern and the enzyme may be considered a marker enzyme. For example, cytochrome oxidase, which is involved in electron transport, is a marker enzyme for the inner membrane. 

Cytochromes of this group are widely distributed. Examples are listed in Table 17. Cytochrome b is a component of the respiratory chain in many aerobic organisms, but only as a complex bcx in mitochondria (and be/ hi chloroplasts). The separation of be, may involve a modification of cytochrome b. Amino acid sequences of a number of mitochondrial cytochromes b are available.712... 

N.W. Downer, N.C. Robinson and R.A. Capaldi, Components of the mitochondrial inner membrane, 3, Characterization of a seventh different subunit of beef heart cytochrome c oxidase, Similarities between the beef heart enzyme and that from other species, Biochemistry 15 (1976) 2930-2936. 

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