In Escherichia coli Ammonia Inhibits Cytochrome bo(3) But Activates Cytochrome bd-I

Interaction of two redox enzymes of Escherichia coli, cytochrome bo(3) and cytochrome bd-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 bo(3) is inhibited by the ligand, with 40% maximum inhibition at 100 mM (NH(4))(2)SO(4). In contrast, the activity of cytochrome bd-I at pH 8.3 increases with increasing the ligand concentration, the largest increase (140%) is observed at 100 mM (NH(4))(2)SO(4). In both cases, the effector molecule is apparently not NH(4)(+) but NH(3). 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 K(dapp) of 24.3 ± 2.7 mM (NH(4))(2)SO(4) (4.9 ± 0.5 mM NH(3)) for the Soret region in cytochrome bo(3), and 35.9 ± 7.1 and 24.6 ± 12.4 mM (NH(4))(2)SO(4) (7.2 ± 1.4 and 4.9 ± 2.5 mM NH(3)) for the Soret and visible regions, respectively, in cytochrome bd-I. Consistently, addition of (NH(4))(2)SO(4) to cells of the E. coli mutant containing cytochrome bd-I as the only terminal oxidase at pH 8.3 accelerates the O(2) consumption rate, the highest one (140%) being at 27 mM (NH(4))(2)SO(4). 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 E. coli.