| Summary: | Interaction of two redox enzymes of <i>Escherichia coli</i>, cytochrome <i>bo</i><sub>3</sub> and cytochrome <i>bd</i>-I, with ammonium sulfate/ammonia at pH 7.0 and 8.3 was studied using high-resolution respirometry and absorption spectroscopy. At pH 7.0, the oxygen reductase activity of none of the enzymes is affected by the ligand. At pH 8.3, cytochrome <i>bo</i><sub>3</sub> is inhibited by the ligand, with 40% maximum inhibition at 100 mM (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>. In contrast, the activity of cytochrome <i>bd</i>-I at pH 8.3 increases with increasing the ligand concentration, the largest increase (140%) is observed at 100 mM (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>. In both cases, the effector molecule is apparently not NH<sub>4</sub><sup>+</sup> but NH<sub>3</sub>. The ligand induces changes in absorption spectra of both oxidized cytochromes at pH 8.3. The magnitude of these changes increases as ammonia concentration is increased, yielding apparent dissociation constants <i>K</i><sub>d<i>app</i></sub> of 24.3 ± 2.7 mM (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> (4.9 ± 0.5 mM NH<sub>3</sub>) for the Soret region in cytochrome <i>bo</i><sub>3</sub>, and 35.9 ± 7.1 and 24.6 ± 12.4 mM (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> (7.2 ± 1.4 and 4.9 ± 2.5 mM NH<sub>3</sub>) for the Soret and visible regions, respectively, in cytochrome <i>bd</i>-I. Consistently, addition of (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> to cells of the <i>E. coli</i> mutant containing cytochrome <i>bd</i>-I as the only terminal oxidase at pH 8.3 accelerates the O<sub>2</sub> consumption rate, the highest one (140%) being at 27 mM (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>. We discuss possible molecular mechanisms and physiological significance of modulation of the enzymatic activities by ammonia present at high concentration in the intestines, a niche occupied by <i>E. coli</i>.
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