Nitric oxide reductase (P450nor) from Fusarium oxysporum

In the present review we wanted to highlight the characteristic features of cytochtome P450 NADH-NO reductase (P450nor) from Fusarium oxysporum which belongs to the heme-thiolate protein family. This enzyme catalyzes the reduction of two NO molecules to N2O. The discovery, isolation, identification and crystallography are described in detail. Special emphasis was focused on the mechanism of NO reduction and possible electronic configurations of the 444 nm intermediate were discussed. Among heme-thiolate proteins nitric oxide reductase (P450nor) is unique since it catalyzes the conversion to dinitrogen oxide as a reductive process. However, it joins the typical physical characteristics of other P450 proteins including the ferric NO complex which can be considered as the enzyme-substrate complex of the enzyme. At a closer look some of its properties like a tilted structure and a shorter Fe-N distance indicate properties for a facilitated hydride transfer from NADH. The resulting intermediate forms the product in a subsequent reaction with the NO radical. For this rate-limiting step at physiological NO levels electron transfer is postulated as a common feature with other heme-thiolate mechanisms. P450nor seems to have an important role in protecting the fungus from NO inhibition of mitochondria especially when dioxygen becomes limiting.