Biological Electron Transport Chains

The various components of biological electron transport chains can be broadly divided into redox enzymes, redox proteins, and redox cofactors. This division is very convenient for our present discussion. In this section we briefly describe each of these component. 

Redox enzymes can further be subdivided into terms of the redox centers present in the enzyme. Several types of redox center are known, including those 

FIGURE 9.4. Ping-pong mechanism for the oxidation of glucose by oxygen catalyzed by glucose oxidase. 

Flavoenzymes are further subdivided into oxidases and dehydrogenases on the basis of the mechanism of the enzyme-catalyzed reaction. In oxidases the reduced flavin is reoxidized by molecular oxygen generating hydrogen peroxide. In the dehydrogenases the reduced flavin does not react with oxygen.  

Quinone containing redox enzymes are a much less widely studied group, and to date fewer examples have been characterized. The prosthetic group in this case is a quinone, pyrrolo-quinoline quinone (PQQ), and again it is a two-electron 

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Addition of the two half-reactions gives the expression for respiration on explicitly including the statement for the chemical energy obtained. The analytical simplicity of the halfreactions lays out the underlying essential result of the biological electron transport chain. Indeed, the electron transport chain of the mitochondrion achieves the separation into protons, ff, and electrons, e". (For the structure of the mitochondrion, refer to Figure 8.5, below.)... 

By virtue of its EPR properties, the electron transport chain of xanthine oxidase has proved particularly amenable to study. Indeed, it might be argued, despite glaring gaps in our knowledge, that this chain is better understood than is any other biological electron transport chain. We will therefore digress from our main topic of molybdenum, to summarize briefly this aspect of work on xanthine oxidase. 

From the data of literature it is known that water-soluble derivatives of fullerenes are able to be localized in mitochondria and influence their state as well as enzyme system (Foley et al., 2002). Such intracellular localization of fullerenes C60 could explain biologic effects under irradiation, because generation of free oxygen radicals in the cells occurs during emission of electrons from electron-transport chain of mitochondria. 

Van Hellemond, J.J., Klockiewicz, M., Caasenbeek, C.P., Roos, M.H. and Helens, A.C.M. (1995) Rhodoquinone and complex II of the electron transport chain in anaerobically functioning eukaryotes. Journal of Biological Chemistry 270, 31 065-31 070. 

Piericidins are the first compounds obtained by the screening search for insecticidal natural products among microbial metabolites.10 They were isolated from Streptomyces mobaraensis in 1963,11 and many piericidin derivatives have been found in microbial metabolites until now.12 Piericidins are not used as insecticides practically, but are important biological reagents because they have specific inhibitory activity toward the mitochondrial electron transport chain protein nicotinamide adenine dinucleotide (NADH)-ubiquinone reductase (complex I).13 Piericidin Ax (1 in Figure 1) is biosynthesized as a polyketide,14 but genes responsible for its biosynthesis are not yet identified. Total synthesis of piericidins A (1) was reported recently.15... 

Hannemann F, Bichet A, Ewen KM, Bernhardt R. Cytochrome P450 systems—biological variations of electron transport chains. Biochim. Biophys. Acta 2007 1770 330-344. 

Without biological electron transfer reactions (also called reduction/oxidation or redox reactions) life would not exist. Well-organized electron transfer reactions in a series of membrane-bound redox proteins form the basis for energy conservation in photosynthesis and respiration. The basic reaction is simply the transfer of electrons from the donor to the final electron acceptor. Perhaps the best example of these redox reactions, their importance for living organisms, and the nature of the different type of biocatalysts that are involved is the respiration chain present in the membranes of mitochondria. The membrane-bound nature of this electron transport chain, supporting electron transfer from NADH to O2 as... 

Cytochrome c occupies a prominent place in the mitochondrial electron-transport chain. Its water solubility, low molecular weight (12.4 kDa), stability, and ease of purification have allowed many experiments, which, when taken together, present a detailed picture of the structure and biological function of this... 

There are many examples of biological oxidation-reduction reactions. For example, the electron-transport chain of aerobic respiration involves the reversible oxidation and reduction of iron atoms in cytochrome c. 

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