Cytochrome p450- like alpha subunits of E. coli sulfite reductase (SiR) multimerize with beta subunits to catalyze the NADPH dependent reduction of sulfite to sulfide. Beta subunits have an Fe4S4 cluster and a siroheme, while the alpha subunits (cysJ gene) are of the cytochrome p450 (CyPor) family having FAD and FMN as prosthetic groups and utilizing NADPH. Cypor (including cyt -450 reductase, nitric oxide synthase, and methionine synthase reductase) are ferredoxin reductase (FNR)-like proteins with an additional N-terminal FMN domain and a connecting sub-domain inserted within the flavin binding portion of the FNR-like domain. The connecting domain orients the N-terminal FMN domain with the C-terminal FNR domain.
Lu S et al.(2020). "CDD/SPARCLE: the conserved domain database in 2020.",