Electron transport chain components

Aminoglycosides must traverse the plasma membrane and, in the case of gramnegative bacteria, the outer membrane to gain access to the target ribosomes. Transport across the plasma membrane has been shown to require the proton motive force, and mutants deficient in electron transport chain components fail to transport aminoglycosides and are consequently resistant. ... 

Selected entries from Methods in Enzymology [vol, page(s)] Electron-transport chain [components, 69, 205, 206 sites of inhibition, 69, 676, 677] chloroplast [autoxidizable carriers, 69, 416, 417 DBMIB, 69, 422, 423 dichlorophenolindophenol and related carriers, 69, 418 ferricyanide, 69, 417, 418 isolated, 69,... 

Question How can the <> values of electron-transport-chain components be measured ... 

Measurements can be done using the technique of redox potentiometry. In experiments of this type, mitochondria are incubated anaerobically in the presence of a reference electrode [for example, a hydrogen electrode (Chap. 10)] and a platinum electrode and with secondary redox mediators. These mediators form redox pairs with Ea values intermediate between the reference electrode and the electron-transport-chain component of interest they permit rapid equilibration of electrons between the electrode and the electron-transport-chain component. The experimental system is allowed to reach equilibrium at a particular E value. This value can then be changed by addition of a reducing agent (such as reduced ascorbate or NADH), and the relationship between E and the levels of oxidized and reduced electron-transport-chain components is measured. The 0 values can then be calculated using the Nernst equation (Chap. 10) ... 

Zhang Y, Marcillat O, Giulivi C, Emster L, Davies KJ. 1990. The oxidative inactivation of mitochondrial electron transport chain components and ATPase. J Biol Chem 265 16330-16336. 

Cobalt electron-transport chain. Component of vitamin B12, which is essential for the metabolism... 

Explain why one cannot precisely predict the sites in the electron transport chain where the coupling of oxidation to phosphorylation occurs on the basis of the redox potentials of the electron transport chain components. 

The mitochondrial inner membrane (Fig. 7) contains proteins that act in concert to catalyze NADH and FADH2 oxidation by molecular oxygen. [See reactions (2) and (3) above.] These reactions are carried out in many small steps by proteins that are integral to the membrane and that undergo oxidation-reduction. These proteins make up what is called the mitochondrial electron transport chain. Components of the chain include iron proteins (cytochromes and iron-sulfur proteins), flavoproteins (proteins that contain flavin), copper, and quinone binding proteins. 

It is not inconceivable that such permeability effects are a result of an initial direct effect of herbicide binding to ATPase or an electron transport chain component and/or a protonophore action. However, most, if not all, the herbicides discussed in this section can be classed as multisite inhibitors as evidenced by their inclusion in previous chapters, particularly Chapter 1, and elsewhere in this chapter. Such a membrane partitioning would not be specific to the inner mitochondrial membrane but would occur in other... 

The ready reversibility of this reaction is essential to the role that qumones play in cellular respiration the process by which an organism uses molecular oxygen to convert Its food to carbon dioxide water and energy Electrons are not transferred directly from the substrate molecule to oxygen but instead are transferred by way of an electron trans port chain involving a succession of oxidation-reduction reactions A key component of this electron transport chain is the substance known as ubiquinone or coenzyme Q... 

FIGURE 21.3 % J and % values for the components of the mitochondrial electron transport chain. Values indicated are consensus values for animal mitochondria. Black bars represent %r red bars,. ... 

Although electrons move from more negative to more positive reduction potentials in the electron transport chain, it should be emphasized that the electron carriers do not operate in a simple linear sequence. This will become evident when the individual components of the electron transport chain are discussed in the following paragraphs. 

The electron transport chain system responsible for the respiratory burst (named NADPH oxidase) is composed of several components. One is cytochrome 6558, located in the plasma membrane it is a heterodimer, containing two polypeptides of 91 kDa and... 

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. 

Oxidation is intimately linked to the activation of polycyclic aromatic hydrocarbons (PAH) to carcinogens (1-3). Oxidation of PAH in animals and man is enzyme-catalyzed and is a response to the introduction of foreign compounds into the cellular environment. The most intensively studied enzyme of PAH oxidation is cytochrome P-450, which is a mixed-function oxidase that receives its electrons from NADPH via a one or two component electron transport chain (10. Some forms of this enzyme play a major role in systemic metabolism of PAH (4 ). However, there are numerous examples of carcinogens that require metabolic activation, including PAH, that induce cancer in tissues with low mixed-function oxidase activity ( 5). In order to comprehensively evaluate the metabolic activation of PAH, one must consider all cellular pathways for their oxidative activation. 

It is important that mitochondrial oxygen radical production depends on the type of mitochondria. Recently, Michelakis et al. [78] demonstrated that hypoxia and the proximal inhibitors of electron transport chain (rotenone and antimycin) decreased mitochondrial oxygen radical production by pulmonary arteries and enhanced it in renal arteries. This difference is probably explained by a lower expression of the proximal components of electron transport chain and a greater expression of mitochondrial MnSOD in pulmonary arteries compared to renal arteries. 

The NADPH oxidase is in fact a multicomponent enzyme system that constitutes an electron transport chain from NADPH to O2. The components of this oxidase complex are now almost completely defined, and experiments performed primarily with CGD neutrophils have helped to identify these major constituents. 

Redox potentials were also used to arrange the electron carriers in their correct order. This procedure was applied to the cytochromes by Coolidge (1932). There were however serious difficulties. Electrochemical theory applies to substances in solution the values obtained are significantly affected by pH and the concentrations of the different components. Of the members of the electron transport chain only the substrates NAD+, NADP+, and cytochrome c are soluble. The other components were difficult to extract from tissue particles without altering their properties. Further, it was hard to determine their concentration and to decide on appropriate values for pH and oxygen concentration. Nevertheless, mainly from work by Ball (1938), at the time in Warburg s laboratory, an approximate order of redox potentials was drawn up ... 

Reviewing the criteria for inclusion of components into the electron transport chain, Slater (1958) highlighted considerations previously advanced by H.A. Krebs as necessary to establish a pathway, namely that the amounts of enzyme present must be commensurate with enzymic activity in the preparation, activity should be fully restored by the reintroduction of the postulated component into an inhibited or depleted preparation, and that the rates of oxidation and reduction of components must be at least as great as those in the system overall. Reduction of cytochrome b by the systems then in use was thought by Chance (1952) and Slater (1958) to be too slow for the inclusion of this cytochrome into the main chain. 

The first of these new, electron transferring components was coenzyme Q (CoQ). Festenstein in R.A. Morton s laboratory in Liverpool had isolated crude preparations from intestinal mucosa in 1955. Purer material was obtained the next year from rat liver by Morton. The material was lipid soluble, widely distributed, and had the properties of a quinone and so was initially called ubiquinone. Its function was unclear. At the same time Crane, Hatefi and Lester in Wisconsin were trying to identify the substances in the electron transport chain acting between NADH and cytochrome b. Using lipid extractants they isolated a new quininoid coenzyme which showed redox changes in respiration. They called it coenzyme Q (CoQ). CoQ was later shown to be identical to ubiquinone. 

The spatial separation between the components of the electron transport chain and the site of ATP synthesis was incompatible with simple interpretations of the chemical coupling hypothesis. In 1964, Paul Boyer suggested that conformational changes in components in the electron transport system consequent to electron transfer might be coupled to ATP formation, the conformational coupling hypothesis. No evidence for direct association has been forthcoming but conformational changes in the subunits of the FI particle are now included in the current mechanism for oxidative phosphorylation. 

The first cytochrome to be recognised as a component of the photosynthetic electron transport chain was cytochrome f [142]. The properties of cytochrome f have been reviewed [143,144], and amino-acid sequence information is available for pea, spinach, wheat and tobacco [145]. The axial ligand to the heme-Fe... 

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