Redox reactions transfer electrons and energy

In preceding section described the addition of phosphate groups to ADP to make ATP as an endergonic reaction that can extract and store energy from exergonic reactions. Another way of transferring energy is to transfer electrons. A reaction in which one substance transfers one or more electrons to another substance is called an oxidation–reduction reaction or redox reaction.

Reductionis the gain of one or more electrons by an atom, ion or molecule.

Oxidationis the loss of one or more electrons.

Although oxidation and reduction are always defined in terms of traffic in electrons, we may also think in these terms when hydrogen atoms (not hydrogen ions) are gained or lost because transfers of hydrogen atoms involve transfers of electrons (H = H+ + e–). Thus, when a molecule loses hydrogen atoms, it becomes oxidized. Oxidation and reduction always occur together: As one material is oxidized, the electrons it loses are transferred to another material, reducing that material. In a redox reaction, we call the reactant that becomes reduced an oxidizing agent and the one that becomes oxidized a reducing agent. In both the combustion and the metabolism of glucose, glucose is the reducing agent and oxygen gas is the oxidizing agent. In a redox reaction, energy is transferred. Much of the energy originally present in the reducing agent becomes associated with the reduced product. (The rest remains in the reducing agent or is lost.) As we will see, some of the key reactions of glycolysis and cellular respiration are highly exergonic redox reactions.

The coenzyme NAD is a key electron carrier in redox reactions

In preceding Chapter, we described the role of coenzymes, small molecules that assist in enzyme-catalyzed reactions.

ADP acts as a coenzyme when it picks up energy released in an exergonic reaction and uses it to make ATP (an endergonic reaction). In a similar fashion, the coenzyme NAD(nicotinamide adenine dinucleotide) acts as an energy carrier, in this case in redox reactions.

NAD exists in two chemically distinct forms, one oxidized (NAD+) and the other reduced (NADH + H+). Both forms participate in biological redox reactions. The reduction reaction NAD+ + 2 H ®NADH + H+ is formally equivalent to the transfer of two hydrogen atoms (2 H+ + 2 e–). However, what is actually transferred is a hydride ion (H–, a proton and two electrons), leaving a free proton (H+). This notation emphasizes that reduction is accomplished by the addition of electrons. Oxygen is highly electronegative and readily accepts electrons from NADH. The oxidation of NADH + H+ by O₂, NADH + H+ + 1⁄2 O₂ ®NAD+ + H₂O is highly exergonic, with a ∆G = –52.4 kcal/mol (–219 kJ/mol). Note that the oxidizing agent appears here as “1⁄2 O₂” instead of “O.” This notation emphasizes that it is oxygen gas, O₂, that acts as the oxidizing agent. Just as ATP can be thought of as packaging free energy in bundles of about 12 kcal/mol (50 kJ/mol), NAD can be thought of as packaging free energy in bundles of approximately 50 kcal/mol (200 kJ/mol). NAD is the most common, but not the only, electron carrier in cells. As you will see, another carrier, FAD(flavin adenine dinucleotide), is also involved in transferring electrons during glucose metabolism.