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

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Abstract
In this work we demonstrate how the reduction state of the Q‐pool determines the distribution of electron flow over the two quinol‐oxidising branches in Paracoccus denitrificans: one to quinol oxidase, the other via the cytochrome bc1 complex to the cytochrome c oxidases. The dependence of the electron‐flow rate to oxygen on the fraction of quinol in the Q‐pool was determined in membrane fractions and in intact cells of the wild‐type strain, a bc1‐negative mutant and a quinol oxidase‐negative mutant. Membrane fractions of the bc1‐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 the Q redox state was exceptionally strong corresponding to elasticity coefficients close to 2 or higher. In intact cells, the dependence was weaker. In uncoupled cells the dependence of the oxygen‐consumption rates on the fractions of quinol in the Q‐pool in the wild‐type strain and in the two mutants came closer to that found for the membrane fractions. We also determined the dependence for membrane fractions of the wild‐type in the absence and presence of antimycin A, an inhibitor of the bc1 complex. The dependence in the presence of antimycin A resembled that of the bc1‐negative mutant. These results indicate that electron‐flow distribution between the two quinol‐oxidising branches in P. denitrificans is not only determined by regulated gene expression but also, and to a larger extent, by the reduction state of the Q‐pool.