ATP-regulation of cytochrome oxidase in yeast mitochondria: role of subunit VIa.

The role of the nuclear-encoded subunit VIa in the regulation of cytochrome oxidase by ATP was investigated in isolated yeast mitochondria. As the subunit VIa-null strain possesses a fully active and assembled cytochrome oxidase, multiple ATP-regulating sites were characterized with respect to their location and their kinetic effect: (a) intra-mitochondrial ATP inhibited the complex IV activity of the null strain, whereas the prevailing effect of ATP on the wild-type strain, at low ionic strength, was activation on the cytosolic side of complex IV, mediated by subunit VIa. However, at physiological ionic strength (i.e. approximately 200 mM), activation by ATP was absent but inhibition was not impaired; (b) in ethanol-respiring mitochondria, when the electron flux was modulated using a protonophoric uncoupler, the redox state of aa3 cytochromes varied with respect to activation (wild-type) or inhibition (null-mutant) of the cytochrome oxidase by ATP; (c) consequently, the control coefficient of cytochrome oxidase on respiratory flux, decreased (wild-type) or increased (null-mutant) in the presence of ATP; (d) considering electron transport from cytochrome c to oxygen, the response of cytochrome oxidase to its thermodynamic driving force was increased by ATP for the wild-type but not for the mutant subunit. Taken together, these findings indicate that at physiological concentration, ATP regulates yeast cytochrome oxidase via subunit-mediated interactions on both sides of the inner membrane, thus subtly tuning the thermodynamic and kinetic control of respiration. This study opens up new prospects for understanding the feedback regulation of the respiratory chain by ATP.