Organization of the Electron-Transport Chain

The electron-transport chain is composed of the four complexes listed in Table 14.1. The pattern of electron transfer within these complexes is shown in Fig. 14-2. 

Complex Enzymatic Function/Name Functional Components 

Question By what experimental means has this pattern of electron transfer been determined  

Two broad experimental approaches have been used examination of the effects of specific inhibitors, which block electron flow through a particular complex and use of synthetic redox couples, which are able to deliver electrons to specific complexes, depending on the relative 0 values of the complex and the redox couple. 

The sites of action of some of the commonly used inhibitors of the electron-transport chain are shown in Fig. 14-3. These sites have been established by application of the crossover theorem (Chap. 10). For example, the fungus-derived antibiotic antimycin A causes an increase in the level of reduced cytochrome b and a decrease in the level of reduced cytochrome C] (i.e., an increase in the level of oxidized cytochrome c() thus, it is inferred that antimycin A interacts with complex III. 

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Fig. 14-2 Schematic organization of the electron-transport chain in mitochondria.

The heliobacteria are not specifically related to any of the four other groups of photosynthetic prokaryotes. It is therefore of interest to study the electron transfer mechanism of Aese bacteria. Existing ambiguities in the organization of the electron transport chain were the incentive to further examine this chain by low-temperature EPR measurements under a variety of reducing conditions. 

Looking at this picture one is teiqpted to speculate that in the process of vertebrate evolution sulphur-transferases of the liver greatly increased in activity and underwent translocation from the soluble cytoplasm into the mitochondrial compartment. Perhaps this is related to organization of the electron transport chain and iBqportance of oxidative phosphozylatlon in homolo-thermic animals. However, these suppositions must be verified by more extensive and systematic studies. 

These energy-producing reactions are termed respiration processes. They require the presence of an external compound that can serve as the terminal electron acceptor of the electron transport chain. However, under anaerobic conditions, fermentation processes that do not require the participation of an external electron acceptor can also proceed. In this case, the organic substrate undergoes a balanced series of oxidative and reductive reactions, i.e., organic matter reduced in one step of the process is oxidized in another. 

All tissues except mature red blood cells are able to manufacture haem for use in the respiratory cytochrome proteins of the electron transport chain. However, the liver is an especially important site of haem synthesis because it (a) is a major organ of erythropoiesis in utero and (b) haem-containing cytochrome-P450 (CYP-450) enzymes play significant roles in hepatic detoxification of drugs, toxins and endogenous waste products (Section 6.4). 

Figure 18-5 A current concept of the electron transport chain of mitochondria. Complexes I, III, and IV pass electrons from NADH or NADPH to 02, one NADH or two electrons reducing one O to HzO. This electron transport is coupled to the transfer of about 12 H+ from the mitochondrial matrix to the intermembrane space. These protons flow back into the matrix through ATP synthase (V), four H+ driving the synthesis of one ATP. Succinate, fatty acyl-CoA molecules, and other substrates are oxidized via complex II and similar complexes that reduce ubiquinone Q, the reduced form QH2 carrying electrons to complex III. In some tissues of some organisms, glycerol phosphate is dehydrogenated by a complex that is accessible from the intermembrane space.

Aerobic metabolism is a highly efficient way for an organism to extract energy from nutrients. In eukaryotic cells, the aerobic processes (including conversion of pyruvate to acetyl-GoA, the citric acid cycle, and electron transport) all occur in the mitochondria, while the anaerobic process, glycolysis, takes place outside the mitochondria in the cytosol. We have not yet seen any reactions in which oxygen plays a part, but in this chapter we shall discuss the role of oxygen in metabolism as the final acceptor of electrons in the electron transport chain. The reactions of the electron transport chain take place in the inner mitochondrial membrane. 

These experiments pointed out that respiratory reduction of As(V) sorbed to solid phases can indeed occur in nature, but its extent and the degree of mobilization of the As(III) product is constrained by the type of minerals present in a given system. What remains unclear is whether micro-organisms can actually reduce As(V) while it is attached to the mineral surface, or if they attack a mono-layer of aqueous As(V) that is in equilibrium with the As(V) adsorbed onto the surface layer. If, as is the case for dissimilatory metal-reducing bacteria such as Geobacter sulfurreducens and Shewanella oneidensis (44,45), components of the electron transport chain are localized to the outer-membrane of some arsenate-respiring bacteria, direct reductive dissolution of insoluble arsenate minerals may be possible by attached bacteria. Too little is known at present about the topology... 

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... 

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