A simple calculation indicates that the passage of more than one proton is required to synthesize one molecule of ATP from ADP and Pi. Although the ΔG for this reaction under standard conditions is +7.3 kcal/mol, at the concentrations of reactants in the mitochondrion, ΔG is probably higher (+10 to +12 kcal/mol). We can calculate the amount of free energy released by the passage of 1 mol of protons down an electrochemical gradient of 220 mV (0.22 V) from the Nernst equation, setting n = 1 and measuring ΔE in volts:
556
ΔG(cal/mol) = −nFΔE = −(23,062 cal · V−1 · mol−1)ΔE
= (23,062 cal · V−1 · mol−1)(0.22 V)
= −5074 cal/mol, or −5.1 kcal/mol
Because the downhill movement of 1 mol of protons releases just over 5 kcal of free energy, the passage of at least two protons is required for synthesis of each molecule of ATP from ADP and Pi.