ATP synthesis by F-type ATP synthase is obligatorily dependent on the transmembrane voltage

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Abstract
ATP synthase is the universal enzyme that manufactures cellular ATP using the energy stored in a transmembrane ion gradient. This energy gradient has two components: the concentration difference (ΔpH or ΔpNa+) and the electrical potential difference ΔΨ, which are thermodynamically equivalent. However, they are not kinetically equivalent, as the mitochondrial and bacterial ATP synthases require a transmembrane potential, ΔΨ, but the chloroplast enzyme has appeared to operate on ΔpH alone. Here we show that, contrary to the accepted wisdom, the ‘acid bath’ procedure used to study the chloroplast enzyme develops not only a ΔpH but also a membrane potential, and that this potential is essential for ATP synthesis. Thus, for the chloroplast and other ATP synthases, the membrane potential is the fundamental driving force for their normal operation. We discuss the biochemical reasons for this phenomenon and a model that is consistent with these new experimental facts.