Transmembrane pH and Electrical Gradients: Evaluation and Possible Role in Oxidative Phosphorylation

In suspensions of isolated mitochondria under phosphorylating conditions the respiratory chain from the NAD couple to Cytochrome c is near equilibrium with ATP synthesis. A transmembrane electrical potential of -0.11--0.17 V (negative inside) and a pH gradient of 0.4-0.8 pH units (alkaline inside) is observed. Approximately 67kJ/mol (16 kcal/mol) is required for ATP synthesis and 3-5 mol of H + would have to move across the membrane to provide enough energy to synthesize 1 mol of ATP if these reactions were coupled. The prokaryotic bacterium, Paracoccus denitrificans, has a respiratory chain and ATP synthesis efficiency similar to that of mitochondria. At an external pH of 7.6 the transmembrane electrical potential is -0.04 V, the pH gradient is approximately 0, and 50.4 kJ/mol (12 kcal/mol) is required for ATP synthesis. These data suggest that proton transport is not a primary intermediate in the mechanism of oxidative phosphorylation in either mitochondria or P. denitrificans.