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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. [ABSTRACT FROM AUTHOR]