The reduction state of the Q-pool regulates the electron flux through the branched respiratory network of Paracoccus denitrificans

In this work we demonstrate how the reduction state of the Q-pool determine s the distribution of electron flow over the two quinol-oxidising branches in Paracoccus denitrificans: one to quinol oxidase, the other via the cytoc hrome bc(1) complex to the cytochrome c oxidases. The dependence of the ele ctron-flow rate to oxygen on the fraction of quinol in the Q-pool was deter mined in membrane fractions and in intact cells of the wild-type strain, a bc(1)-negative mutant and a quinol oxidase-negative mutant. Membrane fracti ons of the bc(1)-negative mutant consumed oxygen at significant rates only at much higher extents of Q reduction than did the wild-type strain or the quinol oxidase-negative mutant. In the membrane fractions, dependence on th e Q redox state was exceptionally strong corresponding to elasticity coeffi cients close to 2 or higher. In intact cells, the dependence was weaker. In uncoupled cells the dependence of the oxygen-consumption rates on the frac tions of quinol in the Q-pool in the wildtype strain and in the two mutants came closer to that found for the membrane fractions. We also determined t he dependence for membrane fractions of the wild-type in the absence and pr esence of antimycin A, an inhibitor of the bc(1) complex. The dependence in the presence of antimycin A resembled that of the bc(1)-negative mutant. T hese results indicate that electron-flow distribution between the two quino l-oxidising branches in P. denitrificans is not only determined by regulate d gene expression but also, and to a larger extent, by the reduction state of the Q-pool.