Kinetics of the reactions of trioxodinitrate (cas 15435-67-3) and nitrite ions with cytochrome d in Escherichia coli

Add time:08/16/2019    Source:sciencedirect.com

The rate of reaction of trioxodinitrate (cas 15435-67-3) with reduced cytochrome oxidase d in membrane particles from Escherichia coli at pH 7 and 25°C depends linearly upon [HN2O3− over the concentration range studied (up to 0.05 mM) and is also first-order in cytochrome d. The known rate of decomposition of trioxodinitrate to give NO− and NO2− is about 4.5-times faster than the rate of reaction of reduced cytochrome d with trioxodinitrate, implying that cytochrome d reacts directly with NO−, with a trapping ratio of between 0.20 and 0.25, rather than with trioxodinitrate. The implications of the facile formation of the NO−-nitrosyl complex of cytochrome d for the mechanism of denitrification are discussed with particular reference to the mechanism of N-N bond formation. The reaction of reduced cytochrome d with nitrite (a decomposition product of trioxodinitrate) under these conditions is much slower than that with trioxodinitrate. The kinetics show a biphasic dependence of initial rate upon nitrite concentration. The rate data at low [NO2− are consistent with saturation of a high affinity site for nitrite, having Vmax = 4.29 · 10−9 M s−1 and Km = 0.034 mM. The existence of two binding sites for nitrite is consistent with the suggestion that the cytochrome bd complex contains two cytochrome d haems.

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