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This study investigates the relationships between the efficiency of oxidative phosphorylation (ATP/O) and respiratory flux in yeast mitochondria. To manipulate the electron flux through the respiratory chain, different substrates leading to NAD(P)H were used. By testing the effect of ADP either on respiratory rate in the presence or absence of oligomycin or on the level of NAD(P)H, on one hand, and the effects of uncouplers on respiration, on the other, we distinguished several categories of substrates: those for which the low respiration rate was mainly controlled by dehydrogenase activities and others for which the respiration was high and controlled downstream from the dehydrogenases. By using these different substrates, we observed that the ATP/O ratio decreased irrespective of the proton-motive force when the electron flux increased, unlike the situation when the respiratory rate was modulated by addition of the respiratory inhibitor. This result suggests that the oxidative phosphorylation efficiency depends on the value of the flux crossing the proton pumps. This relationship between efficiency (ATP/O) and electron flux was linked to a main control upstream from the respiratory chain. Such changes in the ATP/O ratio at least involved changes in the stoichiometry (H+/2e-) of the respiratory chain. Indeed, in non-phosphorylating mitochondria, the ratio of stoichiometries at site 2+3 over site 3 varied according to the proton-motive force. This cannot be explained by a variation in proton leak alone but involved both (i) a variable stoichiometry (H+/2e-) in relation to the electron flux value and (ii) different relationships between the variation in stoichiometry and the flux value at each coupling site.
Fitton et al. (Sun,) studied this question.