Coupled Reactions Oxidation Reduction

When an atom or a molecule gains electrons, it is said to become reduced; when it loses electrons, it is said to become oxidized. Reduction and oxidation are always coupled reactions: an atom or a molecule cannot become oxidized unless it donates electrons to another, which therefore becomes reduced. The atom or molecule that donates electrons to another is a reducing agent, and the one that accepts electrons from another is an oxidizing agent. It is important to understand that a particular atom (or molecule) can play both roles; it may function as an oxidizing agent in one reaction and as a reducing agent in another reaction. When atoms or molecules play both roles, they gain electrons in one reaction and pass them on in another reaction to produce a series of coupled oxidation-reduction reactions—like a bucket brigade, with electrons in the buckets.

Notice that the term oxidation does not imply that oxygen participates in the reaction. This term is derived from the fact that oxygen has a great tendency to accept electrons; that is, to act as a strong oxidizing agent. This property of oxygen is exploited by cells; oxygen acts as the final electron acceptor in a chain of oxidation-reduction reactions that provides energy for ATP production.

Oxidation-reduction reactions in cells often involve the transfer of hydrogen atoms rather than free electrons. Since a hydrogen atom contains one electron (and one proton in the nucleus), a molecule that loses hydrogen becomes oxidized, and one that gains hydrogen becomes reduced. In many oxidation-reduction reactions, pairs of electrons—either as free electrons or as a pair of hydrogen atoms—are transferred from the reducing agent to the oxidizing agent.

Two molecules that serve important roles in the transfer of hydrogens are nicotinamide adenine dinucleotide (NAD), which is derived from the vitamin niacin (vitamin B3), and flavin adenine dinucleotide (FAD), which is derived from the vitamin riboflavin (vitamin B2). These molecules (fig. 4.17) are coenzymes that function as hydrogen carriers because they accept hydrogens (becoming reduced) in one enzyme reaction and donate hydrogens (becoming oxidized) in a different enzyme reaction (fig. 4.18). The oxidized forms of these molecules are written simply as NAD (or NAD+) and FAD.

Each FAD can accept two electrons and can bind two protons. Therefore, the reduced form of FAD is combined with the equivalent of two hydrogen atoms and may be written as FADH2. Each NAD can also accept two electrons but can bind only one proton. The reduced form of NAD is therefore indicated by NADH + H+ (the H+ represents a free proton). When the reduced forms of these two coenzymes participate in an oxidation-reduction reaction, they transfer two hydrogen atoms to the oxidizing agent (fig. 4.18).

Production of the coenzymes NAD and FAD is the major reason that we need the vitamins niacin and riboflavin in our diet. As described in chapter 5, NAD and FAD are required to transfer hydrogen atoms in the chemical reactions that provide energy for the body. Niacin and riboflavin do not themselves provide the energy, although this is often claimed in misleading advertisements for health foods. Nor can eating extra amounts of niacin and riboflavin provide extra energy. Once the cells have obtained sufficient NAD and FAD, the excess amounts of these vitamins are simply eliminated in the urine.

Fox: Human Physiology, Eighth Edition

4. Enzymes and Energy


© The McGraw-Hill Companies, 2003

Reaction site




The rest of the molecule has the same structure as NAD+

■ Figure 4.17 Structural formulas for NAD+, NADH, FAD, and FADH2.

(a) When NAD+ reacts with two hydrogen atoms, it binds to one of them and accepts the electron from the other. This is shown by two dots above the nitrogen (N) in the formula for NADH.

(b) When FAD reacts with two hydrogen atoms to form FADH2, it binds each of them to a nitrogen atom at the reaction sites.



Oxidized state

Reaction site

Reaction site

+ 2 ( H

Reaction site nh2


Reduced state

The rest of the molecule has the same structure as FAD

Oxidized state


Reduced state

NAD is oxidizing agent (it becomes reduced)

NADH is reducing agent (it becomes oxidized)

■ Figure 4.18 The action of NAD. NAD is a coenzyme that transfers pairs of hydrogen atoms from one molecule to another. In the first reaction, NAD is reduced (acts as an oxidizing agent); in the second reaction, NADH is oxidized (acts as a reducing agent). Oxidation reactions are shown by red arrows, reduction reactions by blue arrows.

Test Yourself Before You Continue

1. Describe the first and second laws of thermodynamics. Use these laws to explain why the chemical bonds in glucose represent a source of potential energy and describe the process by which cells can obtain this energy.

2. Define the terms exergonic reaction and endergonic reaction. Use these terms to describe the function of ATP in cells.

3. Using the symbols X-H2 and Y, draw a coupled oxidation-reduction reaction. Designate the molecule that is reduced and the one that is oxidized and state which one is the reducing agent and which is the oxidizing agent.

4. Describe the functions of NAD, FAD, and oxygen (in terms of oxidation-reduction reactions) and explain the meaning of the symbols NAD, NADH + H+, FAD, and FADH2.

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