Sigmoidal relation between mitochondrial respiration and log ([ATP]/[ADP])out under conditions of extramitochondrial ATP utilization. Implications for the control and thermodynamics of oxidative phosphorylation.

Except for close to state 3, mitochondrial respiration has been observed to vary almost linearly with the extramitochondrial phosphorylation potential. For the understanding of the control, thermodynamics, and stoichiometries of oxidative phosphorylation, it is important if this linearity corresponds to an extension of a near-equilibrium flow-force relationship. Using three methods to determine the extramitochondrial ATP/ADP ratio, we observed that at high ATP/ADP ratios the relationship between respiratory rate and log (ATP/ADP) deviated in a sigmoidal fashion from linearity, if the amount of hexokinase present was modulated. In a titration with uncoupler, the sigmoidicity at high ATP/ADP ratios was absent. This difference between the flow-force relationships of these two experiments suggests that the sigmoidicity in the former case reflects a nonproportional flow-force relationship of the adenine nucleotide translocator. In the latter case, one measures the flow-force relationship of the redox-driven proton pumps alone, which turns out to be virtually linear. We determined the flow-force relation of the adenine nucleotide translocator for two ways of varying the force and confirmed the sigmoidicity in both cases. The implication is that the near-linearity of the flow-force relationships at intermediary respiratory rates does not correspond to an Onsager-type (near equilibrium) linearity. We discuss that this phenomenon requires the application of nonclassical forms of nonequilibrium thermodynamics and may be responsible for some of the control over oxidative phosphorylation that is exerted by the cytosolic ATP consuming processes.