Structure of heme d₁-free cd₁ nitrite reductase NirS

A key step in anaerobic nitrate respiration is the reduction of nitrite to nitric oxide, which is catalysed by the cd₁ nitrite reductase NirS in, for example, the Gram-negative opportunistic pathogen Pseudomonas aeruginosa. Each subunit of this homodimeric enzyme consists of a cytochrome c domain and an eight-bladed β-propeller that binds the uncommon isobacteriochlorin heme d₁ as an essential part of its active site. Although NirS has been well studied mechanistically and structurally, the focus of previous studies has been on the active heme d₁-bound form. The heme d₁-free form of NirS reported here, which represents a premature state of the reductase, adopts an open conformation with the cytochrome c domains moved away from each other with respect to the active enzyme. Further, the movement of a loop around Trp498 seems to be related to a widening of the propeller, allowing easier access to the heme d₁-binding side. Finally, a possible link between the open conformation of NirS and flagella formation in P. aeruginosa is discussed.