Electron carriers in oxidative phosphorylation

Cytochromes. Heme-containing proteins that function as electron carriers in oxidative phosphorylation and photosynthesis. 

Quinone A non-protein, lipid-soluble electron carrier in oxidative phosphorylation. Known also as coenzyme Q. 

Ubiquinones (UQ), often called coenzyme Qio, are electron carriers in oxidative phosphorylation and photosynthesis, respectively. Ubiquinones consist of quinoid nucleus (derived from the shikimate pathway), 4-hydroxybenzoate (derived from chorismate or tyrosine), and terpenoid moiety. Zeatin, a phytohormone, is a member of the cytokinin family involved in various processes of growth and development in plants. Most cytokinins are adenine-type, where the hydrogen of amino group at Ce position of adenine is replaced with an isoprenoid. 

Cytochrome c is an important biological intermediate in electron transfer. This metalloprotein, found in all cells, has a molecular weight of approximately 12,800 and contains 104 amino acids (in vertebrates). It is an electron carrier for oxidative phosphorylation, transferring electrons to 02. The energy released in... 

The transfer of phosphoryl groups is a central feature of metabolism. Equally important is another kind of transfer, electron transfer in oxidation-reduction reactions. These reactions involve the loss of electrons by one chemical species, which is thereby oxidized, and the gain of electrons by another, which is reduced. The flow of electrons in oxidation-reduction reactions is responsible, directly or indirectly, for all work done by living organisms. In nonphotosynthetic organisms, the sources of electrons are reduced compounds (foods) in photosynthetic organisms, the initial electron donor is a chemical species excited by the absorption of light. The path of electron flow in metabolism is complex. Electrons move from various metabolic intermediates to specialized electron carriers in enzyme-catalyzed reactions. 

We examine the function of flavoproteins as electron carriers in Chapter 19, when we consider their roles in oxidative phosphorylation (in mitochondria) and photophosphorylation (in chloroplasts), and we describe the photolyase reactions in Chapter 25. 

Site-specific inhibitors Site-specific inhibitors of electron transport have been identified and are illustrated in Figure 6.10. These compounds prevent the passage of electrons by binding to a component of the chain, blocking the oxidation/reduction reaction. Therefore, all electron carriers before the block are fully reduced, whereas those located after the block are oxidized. [Note Because electron transport and oxidative phosphorylation are tightly coupled, site-specific inhibition of the electron transport chain also inhibits ATP synthesis.]... 

Oxidative phosphorylation is susceptible to inhibition at all stages of the process. Specific inhibitors of electron transport were invaluable in revealing the sequence of electron carriers in the respiratory chain. For example, rotenone and amytal block electron transfer in NADH-Q oxidoreductase and thereby prevent the utilization of NADH as a substrate (Figure 18.43). In contrast, electron flow resulting from the oxidation of succinate is unimpaired, because these electrons enter through QH2, beyond the block. AntimycinA interferes with electron flow from cytochrome h Q-cytochrome c... 

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). 

The anisotropic organization of electron carriers across the membrane accounts for the vectorial transport of protons from the inside to the outside of the membrane, which occurs with the passage of electrons. The coupling of this proton gradient to a proton-translocating ATP synthase (also known as ATP synthetase) accounts for the chemiosmotic coupling in oxidative phosphorylation. 

In Chapter 10 the basic principles of oxidative phosphorylation, the complex mechanism by which modem aerobic cells manufacture ATP, are described. The discussion begins with a review of the electron transport system in which electrons are donated by reduced coenzymes to the electron transport chain (ETC). The ETC is a series of electron carriers in the inner membrane of the mitochondria of eukaryotes and the plasma membrane of aerobic prokaryotes. This is followed by a description of chemiosmosis, the means by which the energy extracted from electron flow is captured and used to synthesize ATP. Chapter 10 ends with a discussion of the formation of toxic oxygen products and the strategies that cells use to protect themselves. 

The iron-sulfur proteins play important roles as electron carriers in virtually all living organisms, and participate in plant photosynthesis, nitrogen fixation, steroid metabolism, and oxidative phosphorylation, as well as many other processes (Chapter 7). The optical spectra of all iron-sulfur proteins are very broad and almost featureless, due to numerous overlapping charge-transfer transitions that impart red-brown-black colors to these proteins. On the other hand, the EPR spectra of iron-sulfur clusters are quite distinctive, and they are of great value in the study of the redox chemistry of these proteins. 

The two pathways have in common the involvement of acetyl-CoA and thioesters, and each round of breakdown or synthesis involves two-carbon units. The differences are many malonyl-CoA is involved in biosynthesis, not in breakdown thioesters involve CoA in breakdown and involve acyl carrier proteins in biosynthesis biosynthesis occurs in the cytosol, but breakdown occurs in the mitochondrial matrix breakdown is an oxidative process that requires NAD and FAD and produces ATP by electron transport and oxidative phosphorylation, whereas biosynthesis is a reductive process that requires NADPH and ATP. 

The electron carriers in the electron transport chain are limited in number. The concentration of ADP in the cell is low relative to ATP (Sec. 10.3), so once ADP is phosphorylated, no more ATP can be synthesized. Similarly, when NAD+ is reduced to NADH, it is no longer available for further oxidation of fuel molecules. The individual components of the electron transport chain must also continually pass on then-electrons to the next carrier otherwise, they remain fully reduced and are imable to accept electrons from the preceding electron carrier. Even compounds that participate in the metabolic pathways that generate NADH are in limited supply (recall moiety conservation in Sec. 10.3), and they too can become limiting unless they are regenerated. 

A number of molecules known to be involved in electron transport and in oxidative phosphorylation reactions in biological systems are found in the lamellar material. One class of these compounds are the cytochromes. These substances are protein molecules which contain iron, chelated in tetrapyrrole pigment structures. These substances are electron carriers in biological systems. Electrons may be accepted by the forms in which the... 

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