Respiratory chain redox systems

In biochemical systems, acid-base and redox reactions are essential. Electron transfer plays an obvious, crucial role in photosynthesis, and redox reactions are central to the response to oxidative stress, and to the innate immune system and inflammatory response. Acid-base and proton transfer reactions are a part of most enzyme mechanisms, and are also closely linked to protein folding and stability. Proton and electron transfer are often coupled, as in almost all the steps of the mitochondrial respiratory chain. 

As in the respiratory chain (see p. 140), the light reactions cause electrons to pass from one redox system to the next in an electron transport chain. However, the direction of transport is opposite to that found in the respiratory chain. In the respiratory chain, electrons flow from NADH+H to O2, with the production of water and energy. 

Cytochrome c, a small heme protein (mol wt 12,400) is an important member of the mitochondrial respiratory chain. In this chain it assists in the transport of electrons from organic substrates to oxygen. In the course of this electron transport the iron atom of the cytochrome is alternately oxidized and reduced. Oxidation-reduction reactions are thus intimately related to the function of cytochrome c, and its electron transfer reactions have therefore been extensively studied. The reagents used to probe its redox activity range from hydrated electrons (I, 2, 3) and hydrogen atoms (4) to the complicated oxidase (5, 6, 7, 8) and reductase (9, 10, 11) systems. This chapter is concerned with the reactions of cytochrome c with transition metal complexes and metalloproteins and with the electron transfer mechanisms implicated by these studies. 

In both systems, membrane-bound ubiquinone plays crucial roles in the respiratory chain. Indeed, various quinones, including ubiquinone and menaquinone, are used to connect the redox reactions of various membrane proteins. In spite of the large amount of biochemical and biophysical data on quinone and quinone binding proteins, little structural... 

Figure 1 9F-1 Redox systems in the respiratory chain. P = phosphate ion. (From P. Karlson, Introduction to Modem Biochemistry. New York Academic Press, 1963, with permission.)...

The individual electron carriers of the four complexes of the respiratory chain, shown in Figure 14-4, are arranged in accordance with their redox potentials, with the transfer of electrons from NADH to oxygen associated with a potential drop of 1.12 V, and that of succinate to oxygen of 0.8 V. In the electron transport system, the electrons can be transferred as hydride ions (H ) or as electrons (e.g., in the cytochromes). 

The arrangement of components of the electron transport chain was deduced experimentally. Since electrons pass only from electronegative systems to electropositive systems, the carriers react according to their standard redox potential (Table 14-2). Specific inhibitors and spectroscopic analysis of respiratory chain components are used to identify the reduced and oxidized forms and also aid in the determination of the sequence of carriers. 

ATP is synthesized from ADP and phosphate during electron transport in the respiratory chain. This type of phosphorylation is distinguished from substrate-level phosphorylation, which occurs as an integral part of specific reactions in glycolysis and the TCA cycle. The free energy available for the synthesis of ATP during electron transfer from NADH to oxygen can be calculated from the difference in the value of the standard potential of the electron donor system and that of the electron acceptor system. The standard potential of the NADH/NAD+ redox component is —0.32 V and that of H2O/5O2 is -1-0.82 V therefore, the standard potential difference between them is... 

The oxidation by 02 of [H] (either as NADH or FADH, succinate or directly as active acetic acid) occurs via a sequence of electron transfer steps between redox components being incorporated (more or less deeply) into the inner mitochondrial membrane. As is shown in Fig. 11 in the above mentioned electron transport chain, referred to as respiratory chain, the electronic energy is successively decreased step by step. Oxygen is introduced only into the last step of the chemical events in the respiratory chain. Nature has developed a special enzyme system, the cytochrome oxidase, for the realization of oxygen reduction to water. It is assumed that this enzyme system provides the indispensible cooperation of four electrons and protons, respectively, which is required for oxygen reduction ... 

Cytochrome c oxidase is the terminal member of the respiratory chain in all animals and plants, aerobic yeasts, and some bacteria." " This enzyme is always found associated with a membrane the inner mitochondrial membrane in higher organisms or the cell membrane in bacteria. It is a large, complex, multisubunit enzyme whose characterization has been complicated by its size, by the fact that it is membrane-bound, and by the diversity of the four redox metal sites, i.e., two copper ions and two heme iron units, each of which is found in a different type of environment within the protein. Because of the complexity of this system and the absence of detailed structural information, spectroscopic studies of this enzyme and comparisons of spectral properties with 02-binding proteins (see Chapter 4) and with model iron-porphyrin and copper complexes have been invaluable in its characterization. 

In the biological reactions associated with metabolism systems of electron and hydrogen transport are associated with both respiration and photosynthesis. The electron transport or respiratory chains, involved with the oxidation of organic and inorganic substances by the micro-organisms include a series of components each of which can exist in two forms, i.e. oxidised or reduced forms. Each component of the system is characterised by a constant redox potential. 

The function of the enzymes of the mitochondrial respiratory chain is to transform the energy of redox reactions into an electrochemical proton gradient across the hydrophobic barrier of a coupling membrane. Isolated oligoenzyme complexes of the respiratory chain of mitochondria, cytochrome c oxidase, succinate ytochrome c reductase, and NADH CoQ reductase, are able to catalyze charge transfer in model systems, e.g., at a water/octane interface, which can be followed by a change in the interfacial potential at this interface [20-... 

Flavine mononucleotide (FMN) forms a part of the first complex of the respiratory chain of mitochondria, and its effect on the transmembrane potential value during redox reactions in the system NADH-Q6-O2 was shown by Ismailov et al. taking as an example bilayer lipid membranes.54 55 57,75... 

Iron and sulfur can be extracted from F. the resulting apoferredoxin is reactivated by iron(II) salts and sulfides. The synthesis of the iron-free protein has been achieved by the Merrifield technique. On account of their properties as redox systems (Fe +e" Fe ") the F. effect electron transport between enzyme systems but do not exhibit any enzymatic activity. They transport electrons in the respiratory chain, in photosynthesis, and in nitrogen fixation. The iron-sulfur protein P439 of the Fc4S4-type (Mr 11600) plays a role in photosynthesis. Conclusions can be drawn about the evolutionary histories of plants from the similarities and differences in the amino acid sequences. For the evolutionary history of F. in photosynthesis, see Lit.K F. with FejSj- and FejSg-clusters also occur in bacteria Lit. TrendsBiochem.Sci. 13,30-33(1988). FEMSMicrobiol. Rev. 54,155-176 (1988) Trends Biochem. Sci. 13,369 f. (1988). 

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