title: Free energy coupling between H+-generating and H+-consuming pumps. Ratio between output and input forces.
abstract: The delta Gp/delta mu H ratio has been measured in mitochondria close to state 4 in the presence of various uncoupler or K+/valinomycin concentrations in media containing either 1 mM or 50 mM Pi. Care has been taken to control the factors affecting delta Gp and delta mu H which could lead to an artefactual increase of the delta Gp/delta mu H ratio above the highest accepted value for the H+/ATP stoichiometry (n = 4, synthesis + transport). In particular, to avoid overestimation of delta Gp due to inactivation of the ATPases at low delta mu H or to the presence of adenylate kinase, the static head state was approached from the side of net ATP synthesis and delta Gp was measured in a state close to static head but still maintaining a residual rate of aerobic phosphorylation. For each concentration of uncoupler or K+, the Pi concentration and/or the adenylate energy charge (EC) as a function of time have been measured as indicators of net ATP synthesis. Only the values of delta Gp measured during a decrease in Pi concentration and/or an increase in EC have been considered to be meaningful for calculations of delta Gp/delta mu H ratios. Both uncouplers and K+ transport cause a marked depression of delta mu H and a parallel depression of the rate of ATP synthesis. However the low rate of ATP synthesis taking place under conditions of low delta mu H eventually results, especially at high Pi concentrations, in a relatively large delta Gp. The delta Gp/delta mu H ratios obtained at the lower delta mu H values exceed 4 and approach 6. Although slightly higher delta Gp/delta mu H ratios are obtained with valinomycin-treated than with uncoupler-treated mitochondria, the pattern of the rise of the force ratio as delta mu H decreases is similar in both cases. An increase of the delta Gp/delta mu H ratio above 4, the maximal accepted H+/ATP stoichiometry is thermodynamically incompatible with the delocalized protonic coupling model.
mesh_terms: D000255:Adenosine Triphosphate / Q000096:biosynthesis; D000818:Animals; D004734:Energy Metabolism; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D008930:Mitochondria, Liver / Q000378:metabolism; D011188:Potassium / Q000378:metabolism; D011522:Protons; D051381:Rats; D013816:Thermodynamics; D014475:Uncoupling Agents / Q000494:pharmacology; D014634:Valinomycin / Q000494:pharmacology
chemical_list: D011522:Protons; D014475:Uncoupling Agents; D014634:Valinomycin; D000255:Adenosine Triphosphate; D011188:Potassium