An alternative cytochrome oxidase of Paracoccus denitrificans functions as a proton pump

Abstract

Deletion of the genes encoding subunit I of cytochrome aa 3 in P. denitrificans yields mutants that lack cytochrome aa 3 but which are still capable of oxidizing ubiquinol by O 2 via the cytochrome bc 1 complex, as revealed by myxothiazol-sensitive respiration on succinate. An alternative terminal oxidase must be responsible for the myxothiazol-sensitive activity. The H +/e − ratio of proton translocation by mutant cells approached 3.0 for oxidation of succinate by O 2, while it was near 2.0 either with ferricyanide as electron acceptor (engaging the bc 1 complex only), or with O 2 as acceptor in the presence of myxothiazol (engaging the ubiquinol oxidase). However, proton translocation coupled to oxidation of N, N, N′, N′-tetramethyl- p-phenylenediamine (TMPD) plus ascorbate amounted to much less than 1.0 H +/e −. This may be due to interference by another TMPD-oxidising activity that is not engaged during electron flux to oxygen via the cytochrome bc 1 complex. We conclude that Paracoccus can express an alternative cytochrome b-containing terminal oxidase, which translocates protons at least when operating in conjunction with the cytochrome bc 1 complex.