Unexpected dependence on pH of NO release from Paracoccus pantotrophus cytochrome cd1

A previous study of nitrite reduction by Paracoccus pantotrophus cytochrome cd1 at pH 7.0 identified early reaction intermediates. The c-heme rapidly oxidised and nitrite was reduced to NO at the d1-heme. A slower equilibration of electrons followed, forming a stable complex assigned as 55% cFe(III)d1Fe(II)-NO and 45% cFe(II)d1Fe(II)-NO+. No catalytically competent NO release was observed. Here we show that at pH 6.0, a significant proportion of the enzyme undergoes turnover and releases NO. An early intermediate, which was previously overlooked, is also identified; enzyme immediately following product release is a candidate. However, even at pH 6.0 a considerable fraction of the enzyme remains bound to NO so another component is required for full product release. The kinetically stable product formed at the end of the reaction differs significantly at pH 6.0 and 7.0, as does its rate of formation; thus the reaction is critically dependent on pH.