We can measure the tendency to transfer electrons from any redox pair experimentally. All that is required is the formation of an electrical circuit linking a 1:1 (equimolar) mixture of the redox pair to a second redox pair that has been arbitrarily selected as a reference standard, so that we can measure the voltage difference between them (Panel 14–1). This voltage difference is defined as the redox potential; electrons move spontaneously from a redox pair like NADH/NAD+ with a lower redox potential (a lower affinity for electrons) to a redox pair like O2/H2O with a higher redox potential (a higher affinity for electrons). Thus, NADH is a good molecule for donating electrons to the respiratory chain, while O2 is well suited to act as the “sink” for electrons at the end of the chain. As explained in Panel 14–1, the difference in redox potential, ∆Eʹ0, is a direct measure of the standard free-energy change (∆G°) for the transfer of an electron from one molecule to another.
Electron transfers release Large amounts of Energy
As just discussed, those pairs of compounds that have the most negative redox potentials have the weakest affinity for electrons and therefore are useful as carriers with a strong tendency to donate electrons. Conversely, those pairs that have the most positive redox potentials have the greatest affinity for electrons and therefore are useful as carriers with a strong tendency to accept electrons. A 1:1 mixture of NADH and NAD+ has a redox potential of –320 mV, indicating that NADH has a strong tendency to donate electrons; a 1:1 mixture of H2O and ½O2 has a redox potential of +820 mV, indicating that O2 has a strong tendency to accept electrons. The difference in redox potential is 1140 mV, which means that the transfer of each electron from NADH to O2 under these standard conditions is enormously favorable, since ∆G° = –109 kJ/mole, and twice this amount of energy is gained for the two electrons transferred per NADH molecule (see Panel 14–1). If we compare this free-energy change with that for the formation of the phosphoanhydride bonds in ATP, where ∆G° = 30.6 kJ/mole (see Figure 2–50), we see that, under standard conditions, the oxidation of one NADH molecule releases more than enough energy to synthesize seven molecules of ATP from ADP and Pi. (In the cell, the number of ATP molecules generated will be lower because the standard conditions are far from the physiological ones; in addition, small amounts of energy are inevitably dissipated as heat along the way.)
The electron-transport properties of the membrane protein complexes in the respiratory chain depend upon electron-carrying cofactors, most of which are transition metals such as Fe, Cu, Ni, and Mn, bound to proteins in the complexes. These metals have special properties that allow them to promote both enzyme catalysis and electron-transfer reactions. Most relevant here is the fact that their ions exist in several different oxidation states with closely spaced redox potentials, which enables them to accept or give up electrons readily; this property is