| Abstract
| - The copper-containing nitrite reductase from Alcaligenes faecalis S-6 was found to catalyzethe oxidation of nitric oxide to nitrite, the reverse of its physiological reaction. Thermodynamic and kineticconstants with the physiological electron donor pseudoazurin were determined for both directions of thecatalyzed reaction in the pH range of 6−8. For this, nitric oxide was monitored by a Clark-type electrode,and the redox state of pseudoazurin was measured by optical spectroscopy. The equilibrium constant(Keq) depends on the reduction potentials of pseudoazurin and nitrite/nitric oxide, both of which varywith pH. Above pH 6.2 the formation of NiR substrates (nitrite and reduced pseudoazurin) is favoredover the products (NO and oxidized pseudoazurin). At pH 8 the Keq amounts to 103. The results show thatdissimilatory nitrite reductases catalyze an unfavorable reaction at physiological pH (pH = 7−8).Consequently, nitrous oxide production by copper-containing nitrite reductases is unlikely to occur invivo with a native electron donor. With increasing pH, the rate and specificity constant of the forwardreaction decrease and become lower than the rate of the reverse reaction. The opposite occurs for the rateof the reverse reaction; thus the catalytic bias for nitrite reduction decreases. At pH 6.0 the kcat for nitritereduction was determined to be 1.5 × 103 s-1, and at pH 8 the rate of the reverse reaction is 125 s-1.
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