Does the reduction of c heme trigger the conformational change of crystalline nitrite reductase?

The structures of nitrite reductase from Paracoccus denitrificans GB17 (NiR -Pd) and Pseudomonas aeruginosa (NiR-Pa) have been described for the oxidiz ed and reduced state (Fulop, V., Moir, J. W. B., Ferguson, S. J., and Hajdu , J. (1995) Cell 81, 369-377; Nurizzo, D., Silvestrini, M. C., Mathieu, M., Cutruzzola, F., Bourgeois, D., Fulop, V., Hajdu, J., Brunori, M., Tegoni, M., and Cambillau, C. (1997) Structure 5, 1157-1171; Nurizzo, D., Cutruzzol a F., Arese, M., Bourgeois, D., Brunori, M., Cambillau, C., and Tegoni, M. (1998) Biochemistry 37, 13987-13996). Major conformational rearrangements a re observed in the extreme states although they are more substantial in NiR -Pd. The four structures differ significantly in the c heme domains. Upon r eduction, a His(17)/ Met(106) heme-ligand switch is observed in NiR-Pd toge ther with concerted movements of the Tyr in the distal site of the d(1) hem e (Tyr(10) in NiR-Pa, Tyr(25) in NiR-Pd) and of a loop of the c heme domain (56-62 in NiR-Pa, 99-116 in NiR-Pd). Whether the reduction of the c heme, which undergoes the major rearrangements, is the trigger of these movements is the question addressed by our study. This conformational reorganization is not observed in the partially reduced species, in which the c heme is p artially or largely (15-90%) reduced but the d(1) heme is still oxidized. T hese results suggest that the d(1) heme reduction is likely to be responsib le of the movements. We speculate about the mechanistic explanation as to w hy the opening of the d(1) heme distal pocket only occurs upon electron tra nsfer to the d(1) heme itself, to allow binding of the physiological substr ate NO2- exclusively to the reduced metal center.