Oxidative phosphorylation electron-transport system

The processes of electron transport and oxidative phosphorylation are membrane-associated. Bacteria are the simplest life form, and bacterial cells typically consist of a single cellular compartment surrounded by a plasma membrane and a more rigid cell wall. In such a system, the conversion of energy from NADH and [FADHg] to the energy of ATP via electron transport and oxidative phosphorylation is carried out at (and across) the plasma membrane. In eukaryotic cells, electron transport and oxidative phosphorylation are localized in mitochondria, which are also the sites of TCA cycle activity and (as we shall see in Chapter 24) fatty acid oxidation. Mammalian cells contain from 800 to 2500 mitochondria other types of cells may have as few as one or two or as many as half a million mitochondria. Human erythrocytes, whose purpose is simply to transport oxygen to tissues, contain no mitochondria at all. The typical mitochondrion is about 0.5 0.3 microns in diameter and from 0.5 micron to several microns long its overall shape is sensitive to metabolic conditions in the cell. 

Hatefi Y. The mitochondrial electron transport and oxidative phosphorylation system. Annu Rev Biochem 54 1015-1069, 1985. 

If the insecticide rotenone is added to a system capable of carrying out electron transport and oxidative phosphorylation, the step which is inhibited involves ... 

Although there is a 5-fold difference between the sizes of the mitochondrial genomes of yeast (84 kb) and mammals (16 kb), the number of proteins synthesized within mitochondria is similar. Proteins produced by mammalian mitochondria are those involved in electron-transport and oxidative-phosphorylation systems. These include cytochrome b, three subunits of cytochrome oxidase, one subunit of ATPase, and six subunits of NADH dehydrogenase. Apart from these differences, protein synthesis in mitochondria follows the same steps and mechanisms as those in the cytoplasm. 

Hatefi, Y. The Mitochondrial Electron Transport and Oxidative Phosphorylation System. Ann. Rev. Biochem. 54, 1015-1069 (1985). [A review that emphasizes the coupling between oxidation and phosphorylation.]... 

Ans. The rate of oxygen consumption depends on the rate at which electrons reach the final enzyme in the electron transport system, cytochrome oxidase. There is an absolute coupling of electron transport with oxidative phosphorylation. Therefore the rate of oxygen consumption depends on the availability of ADP. The more ADP available, the greater the rate of oxygen consumption. 

Different types of organisms such as daphnia, mussels, algae, and fish have been extensively incorporated in toxicity tests for water assessment systems [65], Most of these assays are developed as test systems with few as laboratory-based sensor systems. Membranes with their active enzyme system have also been implemented in the development of toxicity kits and sensors. An example is the MitoScan Kit (Harvard BioScience, Inc., Holliston, MA), which uses fragmented inner mitochondrial membrane vesicles isolated from beef heart (EPA, 2005 [9]). The submito-chondrial particles contain complexes of enzymes responsible for electron transport and oxidative phosphorylation. When specific toxins are in the sample, the enzyme reactions are slowed or inhibited, and these are monitored spectophotometrically at 340 mn. This is still in a bioassay test kit format but may be developed to optical sensor system. 

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. 

Figure 18-20 Electron transport system for oxidation of the nitrite ion to the nitrate ion by Nitrobacter. Only one site of proton pumping for oxidative phosphorylation is available. Generation of NADH for biosynthesis requires two stages of reverse electron transport.

Now that it is established that cestodes possess all the components of a electron transport system, is the latter functional Weinbach von Brand (952) failed to demonstrate either respiratory control or oxidative phosphorylation in T. taeniaeformis, although they regarded this as a technical rather than a physiological problem. However, there is good evidence that isolated mitochondria from M. expansa (124-127) and H. diminuta (663, 978) are capable of oxidative phosphorylation and respiratory control. The demonstration that a preparation of H. diminuta mitochondria will oxidise a range of substrates, exhibiting respiratory control, is shown in Table 5.14. Similarly, mitochondria from Diphyllo-bothrium latum can oxidise NADH (728) and succinate (729). It is likely that the classical mammalian-type part of the cytochrome chain in cestodes is capable of oxidative phosphorylation, but there is no evidence for ATP synthesis occurring on the alternative branch from the quinone or vitamin K/cytochrome b complex to cytochrome o. 

When grown under aerobic conditions, the yeast produces two ATP molecules from one molecule of glucose by substrate-level phosphorylation in glycolysis. The two molecules of pyruvate produced can then be completely oxidized to CO2, and each yields a further 15 molecules of ATP. This leads to a slow decrease in the concentration of glucose, a steady production of CO2, and relatively little change in the amount of ATP. Also, the two molecules of NADH can be reoxidized to NAD+ by the electron-transport system. (This produces yet more ATP, as discussed in Chap. 14.)... 

NADH-ubiquinone reductase was isolated by Hatefi et al. in 1961 (27-B9). A procedure was developed for the resolution of the mitochondrial electron transport system into four enzyme complexes. Recently, a fifth fraction, which is capable of energy conservation and ATP-Pi exchange, was also isolated (30, 31). The overall scheme for the isolation of the five component enzyme complexes of the mitochondrial electron transport-oxidative phosphorylation system is given in Fig. 1. It is seen... 

Mitochondrial electron transport system Sulfuramid, chlorfenapyr Azocyclotin, cyhexatin, Fenbutatin-oxide, propargite, Tetradifon, diafenthiuron Uncouplers of oxidative phosphorylation Inhibitors of ATP synthase... 

ATP is produced from ADP (adenosine diphosphate) by coupling the release of electrons to the reaction of organic phosphates and ADP producing ATP. ATP has two modes of production substrate-level phosphorylation and oxidative phosphorylation. In the former, the electrons released by the energy source are absorbed by an intermediate product within the system. The electron absorption is accompanied by an energy release and ATP is formed. The electron-transport system is simple. 

Studies with beef-heart submitochondrial particles initiated in Green s laboratory in the mid-1950s resulted in the demonstration of ubiquinone and of non-heme iron proteins as components of the electron-transport system, and the separation, characterisation and reconstitution of the four oxidoreductase complexes of the respiratory chain. In 1960 Racker and his associates succeeded in isolating an ATPase from submitochondrial particles and demonstrated that this ATPase, called F, could serve as a coupling factor capable of restoring oxidative phosphorylation to F,-depleted particles. These preparations subsequently played an important role in elucidating the role of the membrane in energy transduction between electron transport and ATP synthesis. 

These are the energy producers within the cell. They generate energy in the form of Adenosine Tri-Phosphate (ATP). Generally, the more energy a cell needs, the more mitochondria it contains. Site for Kreb s Citric Acid Cycle Electron transport system and Oxidative Phosphorylation Fatty acid oxidation Amino acid catabolism Interconversion of carbon skeletons. 

Oxidative phosphorylation is the process in which ATP molecules are formed as a result of the transfer of electrons from the reducing equivalents, NADH or FADH2 (produced by glycolysis, the citric acid cycle and fatty acid oxidation) to oxygen by a series of electron carriers in the form of a chain located in the inner membrane of mitochondria. This is the final reaction sequence of respiration. Since the electrons are transferred by a series of electron carriers in the form of a chain, it is known as electron transport system (ETS). 

Both groups of reactions are found in bacteria (14), all higher animals (i5), and plants (16) however, oxidative phosphorylation is responsible for 90 % of the oxygen consumed (i 7). Oxidative phosphorylation is driven by the respiratory electron-transport system that is embedded in the lipoprotein inner membrane of eukaryotic mitochondria and in the cell membrane of prokaryotes. It consists of four complexes (Scheme I). The first is composed of nicotinamide adenine dinucleotide (NADH) oxidase, flavin mononucleotide (FMN), and nonheme iron-sulfur proteins 18,19), and it transfers electrons from NADH to ubiquinone. The second is composed of succinate dehydrogenase (SDH), flavin adenine dinucleotide (FAD), and nonheme iron-sulfur proteins (20), and it transfers electrons from succinate to ubiquinone 21, 22). The third is composed of cytochromes b and c, and nonheme iron-sulfur proteins (23), and it transfers electrons from ubiquinone (UQ) to cytochrome c 24). The fourth complex consists of cytochrome c oxidase [ferrocytochrome c 0 oxidoreductase EC 1.9.3.1 25)] which transfers electrons from cytochrome c to O2 26, 27). 

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