Observation of fast release of NO from ferrous d1 haem allows formulation of a unified reaction mechanism for cytochrome cd1 nitrite reductases

Cytochrome cd1 nitrite reductase is a haem-containing enzyme responsible for the reduction of nitrite into nitric oxide (NO), a key step in the anaerobic respiratory process of denitrification. The active site of cytochrome cd1 contains the unique d1 haem cofactor, from which NO must be released. In general, reduced hemes bind NO tightly relative to oxidised hemes. Here we present experimental evidence that the reduced d1 heme of cytochrome cd1 from Paracoccus pantotrophus releases NO rapidly (k=65-200 s-1); this result suggests that NO release is the rate-limiting step of the catalytic cycle (turnover number = 72 s-1). We also demonstrate using a complex of the d1 haem and apomyoglobin, that the rapid dissociation of NO is largely controlled by the d1 haem cofactor itself. We present a reaction mechanism proposed to be applicable to all cytochromes cd1 and conclude that the d1 haem has evolved to have low affinity for NO, as compared to other ferrous haems