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Chapter 12

After studying this chapter, you should be able to:

• Understand the meaning of redox potential and explain how it can be used to predict the direction of flow of electrons in biologic systems.
• Identify the four classes of enzymes (oxidoreductases) involved in oxidation and reduction reactions.
• Describe the action of oxidases and provide examples of where they play an important role in metabolism.
• Indicate the two main functions of dehydrogenases and explain the importance of NAD- and riboflavin-linked dehydrogenases in metabolic pathways such as glycolysis, the citric acid cycle, and the respiratory chain.
• Identify the two types of enzymes classified as hydroperoxidases; indicate the reactions they catalyze and explain why they are important.
• Give the two steps of reactions catalyzed by oxygenases and identify the two subgroups of this class of enzymes.
• Appreciate the role of cytochrome P450 in drug detoxification and steroid synthesis.
• Describe the reaction catalyzed by superoxide dismutase and explain how it protects tissues from oxygen toxicity.

Chemically, oxidation is defined as the removal of electrons and reduction as the gain of electrons. Thus, oxidation is always accompanied by reduction of an electron acceptor. This principle of oxidation–reduction applies equally to biochemical systems and is an important concept underlying understanding of the nature of biologic oxidation. Note that many biologic oxidations can take place without the participation of molecular oxygen, eg, dehydrogenations. The life of higher animals is absolutely dependent upon a supply of oxygen for respiration, the process by which cells derive energy in the form of ATP from the controlled reaction of hydrogen with oxygen to form water. In addition, molecular oxygen is incorporated into a variety of substrates by enzymes designated as oxygenases; many drugs, pollutants, and chemical carcinogens (xenobiotics) are metabolized by enzymes of this class, known as the cytochrome P450 system. Administration of oxygen can be lifesaving in the treatment of patients with respiratory or circulatory failure.

In reactions involving oxidation and reduction, the free energy change is proportionate to the tendency of reactants to donate or accept electrons. Thus, in addition to expressing free energy change in terms of ΔG0′ (Chapter 11), it is possible, in an analogous manner, to express it numerically as an oxidation–reduction or redox potential (E0). The redox potential of a system (E0) is usually compared with the potential of the hydrogen electrode (0.0 V at pH 0.0). However, for biologic systems, the redox potential (E0) is normally expressed at pH 7.0, at which pH the electrode potential of the hydrogen electrode is −0.42 V. The redox potentials of some redox systems of special interest in mammalian biochemistry are shown in Table 12–1. The relative positions of redox systems in the table allow prediction of the direction of flow of electrons from one redox couple to another.

Table 12–1 Some Redox Potentials of ...

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