As we saw in Chapter 2, the reduction potential (E) for a partial reduction reaction

Oxidized molecule + e⁻ ⇌ reduced molecule

is a measure of the equilibrium constant of that partial reaction. With the exception of the b cytochromes in complex III (CoQH₂–cytochrome c reductase), the standard reduction potential E°′ of the electron carriers in the electron-transport chain increases steadily from NADH to O₂. For instance, for the partial reaction

NAD⁺ + H⁺ + 2 e⁻ ⇌ NADH

the value of the standard reduction potential is −320 mV, which is equivalent to a ΔG°′ of +14.8 kcal/mol for transfer of two electrons. Thus this partial reaction tends to proceed toward the left; that is, toward the oxidation of NADH to NAD⁺.

In contrast, the standard reduction potential for the partial reaction

Cytochrome c_ox (Fe³⁺) + e⁻ ⇌ cytochrome c_red (Fe²⁺)

is +220 mV (ΔG°′ = −5.1 kcal/mol) for transfer of one electron. Thus this partial reaction tends to proceed toward the right; that is, toward the reduction of cytochrome c (Fe³⁺) to cytochrome c (Fe²⁺).

The final reaction in the electron-transport chain, the reduction of O₂ to H₂O

2 H⁺ + ½ O₂ + 2 e⁻ → H₂O

has a standard reduction potential of +816 mV (ΔG°′ = −37.8 kcal/mol for transfer of two electrons), the most positive in the whole series; thus this reaction also tends to proceed toward the right.

As illustrated in Figure 12-25, the steady increase in E°′ values, and the corresponding decrease in ΔG°′ values, of the carriers in the electron-transport chain favors the flow of electrons from NADH and FADH₂ (generated from succinate) to O₂. The energy released as electrons flow energetically “downhill” through the electron-transport chain complexes drives the pumping of protons against their concentration gradient across the inner mitochondrial membrane.