Thermodynamics of proton transport coupled ATP synthesis

• The thermodynamic H+/ATP ratio in the chloroplast ATP synthase is 4.0 ± 0.1.
• ΔpH and Δφ are equivalent in shifting the ATP synthesis equilibrium.
• H+/ATP = 4.7 from structure points to energy losses within the enzyme.

The thermodynamic H+/ATP ratio of the H+–ATP synthase from chloroplasts was measured in proteoliposomes after energization of the membrane by an acid base transition (Turina et al. 2003 [13], 418–422). The method is discussed, and all published data obtained with this system are combined and analyzed as a single dataset. This meta-analysis led to the following results. 1) At equilibrium, the transmembrane ΔpH is energetically equivalent to the transmembrane electric potential difference. 2) The standard free energy for ATP synthesis (reference reaction) is ΔG°ref = 33.8 ± 1.3 kJ/mol. 3) The thermodynamic H+/ATP ratio, as obtained from the shift of the ATP synthesis equilibrium induced by changing the transmembrane ΔpH (varying either pHin or pHout) is 4.0 ± 0.1. The structural H+/ATP ratio, calculated from the ratio of proton binding sites on the c-subunit-ring in F0 to the catalytic nucleotide binding sites on the β-subunits in F1, is c/β = 14/3 = 4.7. We infer that the energy of 0.7 protons per ATP that flow through the enzyme, but do not contribute to shifting the ATP/(ADP·Pi) ratio, is used for additional processes within the enzyme, such as activation, and/or energy dissipation, due e.g. to internal uncoupling. The ratio between the thermodynamic and the structural H+/ATP values is 0.85, and we conclude that this value represents the efficiency of the chemiosmotic energy conversion within the chloroplast H+–ATP synthase.