The influence of pH on the kinetics of dissimilatory nitrite reduction in Paracoccus denitrificans

Abstract

An almost stoichiometric conversion of nitrite to nitrous oxide was observed during the nitrite reduction by Paracoccus denitrificans cells in a medium of pH 6.4. The N 2O accumulated in the reaction medium and was decomposed only after nitrite had been consumed; when the pH of the medium was higher than 7.3–7.4, nitrous oxide did not accumulate. The activity of N 2O reductase was, in the whole range of pH 6.4–9.2, higher than the activity of NO − 2 reductase, both activities showing the maximum at the pH higher than 8.0. Using an artificial donor, TMPD plus ascorbate, the maximum activity of NO − 2 reductase, but not N 2O reductase was shifted by about two pH units to acidic region. The activity of nitrite reductase declined in the presence of N 2O only at higher pH values. Cytochrome c, as a common electron donor for both N 2O and NO − 2 reductase, was more oxidized at pH < 7.3 in the presence of NO − 2 than in the presence of N 2O, the opposite being true at pH > 7.3. The increased flux of electrons to cytochrome c has for a constant pH value (6.4) no effect on their distribution over NO − 2 and N 2O. The results indicate that the distribution of electrons in the terminal part is determined by the different pH optima for NO − 2 reductase and N 2O reductase, and by a mutual dependence of activities of the two reductases due to the competition for redox equivalents from a substrate.