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 sub unit VIa-null strain possesses a fully active and assembled cytochrome oxid ase. multiple ATP-regulating sites were characterized with respect to their location and their kinetic effect:: (a) intra-mitochondrial ATP inhibited the complex TV activity of the null strain, whereas the prevailing effect o f 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 physiol ogical ionic strength (i.e. approximate to 200 mM), activation by ATP was a bsent but inhibition was not impaired; (b) in ethanol-respiring mitochondri a, when the electron flux was modulated using a protonophoric uncoupler, th e odor state of aa(3) cytochromes varied with respect to activation (wild-t ype) or inhibition (null-mutant) of the cytochrome oxidase by ATP; (c) cons equently, 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 respons e of cytochrome oxidase to its thermodynamic driving force was increased by ATP for the wild-type but not for the mutant subunit. Taken together, thes e findings indicate that at physiological concentration, ATP regulates yeas t cytochrome oxidase via subunit-mediated interactions on both sides of the inner membrane, thus subtly tuning the thermodynamic and kinetic control o f respiration. This study opens up neu prospects for understanding the feed back regulation of the respiratory chain by ATP.