The insertion of the non-heme FeB cofactor into nitric oxide reductase from P. denitrificans depends on NorQ and NorD accessory proteins

Bacterial NO reductases (NOR) catalyze the reduction of NO into N₂O, either as a step in denitrification or as a detoxification mechanism. cNOR from Paracoccus (P.) denitrificans is expressed from the norCBQDEF operon, but only the NorB and NorC proteins are found in the purified NOR complex.Here, we established a new purification method for the P. denitrificans cNOR via a His-tag using heterologous expression in E. coli. The His-tagged enzyme is both structurally and functionally very similar to non-tagged cNOR. We were also able to express and purify cNOR from the structural genes norCB only, in absence of the accessory genes norQDEF. The produced protein is a stable NorCB complex containing all hemes and it can bind gaseous ligands (CO) to heme b₃, but it is catalytically inactive. We show that this deficient cNOR lacks the non-heme iron cofactor Fe_B. Mutational analysis of the nor gene cluster revealed that it is the norQ and norD genes that are essential to form functional cNOR. NorQ belongs to the family of MoxR P-loop AAA+ ATPases, which are in general considered to facilitate enzyme activation processes often involving metal insertion. Our data indicates that NorQ and NorD work together in order to facilitate non-heme Fe insertion. This is noteworthy since in many cases Fe cofactor binding occurs spontaneously. We further suggest a model for NorQ/D-facilitated metal insertion into cNOR.