Nitrite reductase (NiR) from Pseudomonas aeruginosa (EC 1.9.3.2) (NiR-Pa) is a soluble enzyme catalyzing the reduction of nitrite (NO2-) to nitric oxide (NO). The enzyme is a 120 kDa homodimer, in which each monomer carries one c and one d1 heme. The oxidized and reduced forms of NiR from Paracoccus denitrificans GB17 (previously called Thiosphaera pantotropha) (NiR-Pd) have been described [Fülop, V., et al. (1995) Cell 81, 369-377; Williams, P. A., et al. (1997) Nature 389, 406-412], and we recently reported on the structure of oxidized NiR-Pa at 2.15 A [Nurizzo, D., et al. (1997) Structure 5, 1157-1171]. Although the domains carrying the d1 heme are almost identical in both NiR-Pa and NiR-Pd oxidized and reduced structures, the c heme domains show a different pattern of c heme coordination, depending on the species and the redox state. The sixth d1 heme ligand in oxidized NiR-Pd was found to be Tyr25, whereas in NiR-Pa, the homologuous Tyr10 does not interact directly with Fe3+, but via a hydroxide ion. Furthermore, upon reduction, the axial ligand of the c heme of NiR-Pd changes from His17 to Met108. Finally, in the oxidized NiR-Pa structure, the N-terminal stretch of residues (1-29) of one monomer interacts with the other monomer (domain swapping), which does not occur in NiR-Pd. Here the structure of reduced NiR-Pa is described both in the unbound form and with the physiological product, NO, bound at the d1 heme active site. Although both structures are similar to that of reduced NiR-Pd, significant differences with respect to oxidized NiR-Pd were observed in two regions: (i) a loop in the c heme domain (residues 56-62) is shifted 6 A away and (ii) the hydroxide ion, which is the sixth coordination ligand of the heme, is removed upon reduction and NO binding and the Tyr10 side chain rotates away from the position adopted in the oxidized form. The conformational changes observed in NiR-Pa as the result of reduction are less extensive than those occurring in NiR-Pd. Starting with oxidized structures that differ in many respects, the two enzymes converge, yielding reduced conformations which are very similar to each other, which indicates that the conformational changes involved in catalysis are considerably diverse.