Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Electron Transport, Oxidative Phosphorylation, and the Regulation of ATP Synthesis
Electron transfer reactions are oxidation-reduction reactions

Above, we examined several enzymatic reactions in which hydrogen atoms or electrons are transferred from one molecule to another. We shall now return to such reactions and look at some of their quantitative aspects. Chemical Reactions Involving The transfer of electrons from one molecule to another are called oxidation-reduction, or redox, reactions. Compounds that donate electrons in such reactions are referred to as electron Donors or reducing agents, whereas compounds that accept electrons are called electron acceptors or oxidizing agents.

Oxidizing and reducing agents always function as conjugate redox pairs, much like acids and bases function as conjugate acid-Base Pairs (Sec. 4.8). Recall that acid-base reactions are described by the following general equation:

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A similar general equation can be written for oxidation-reduction reactions:

A typical example of a redox reaction is

in which ferrous iron (Fe²⁺) acts as an electron donor, and ferric iron (Fe³⁺) serves as an acceptor. Together, the Fe²⁺ and Fe³⁺ ions constitute a conjugate redox pair.

There are four principal ways in which electrons are transferred from one molecule to another.

1. Direct electron transfer. For example, the Fe²⁺-Fe³⁺ redox pair can transfer its electrons to the Сu⁺-Сu²⁺ pair.

Fe²⁺ + Сu²⁺ → Fe³⁺ + Сu⁺.

2. Transfer as hydrogen atoms. Recall that a hydrogen atom consists of a proton (Н⁺) and an electron (е^-). In this case, the general equation has the form

where АН₂ is a hydrogen (or electron) donor and А is a hydrogen acceptor; together they form a conjugate redox pair capable of reducing the electron acceptor В via the transfer of hydrogen atoms.

АН₂ + В → А + ВН₂.

3. Transfer of electrons from a donor to an acceptor in the form of a hydride ion (:Н^-) carrying two electrons, as occurs in the case of NAD-dependent dehydrogenases (Sec. 10.6).

4. Transfer via the direct interaction of an organic reducing agent with oxygen, leading to The formation of a product containing covalently bound oxygen. An example of such a reaction is The oxidation of a hydrocarbon to an alcohol.

In this reaction, the hydrocarbon serves as the electron donor, and the oxygen atom acts as the acceptor.

All four of these electron transfer mechanisms are utilized in living Cells. Therefore, to designate a single electron equivalent participating in a redox process, the neutral term reducing equivalent is frequently used. This term does not specify the precise form in which the electron transfer takes place—that is, whether the entity transferred is a bare electron, a hydrogen atom, a hydride ion, or oxygen incorporation resulting in an oxidized product. As we shall see later, electron transfer in Cell/35.html">Mitochondria occurs in various forms: as hydride ions, hydrogen atoms, and finally as simple electrons (during the terminal stages catalyzed by Cytochromes).

During the enzymatic oxidation of biological fuel molecules, pairs of reducing equivalents are typically removed, and each oxygen atom likewise accepts two reducing equivalents. Consequently, the transfer of a single pair of reducing equivalents from a substrate to oxygen is conventionally adopted as the "unit" of Biological Oxidation.