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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. NADPH cytochrome p450 reductase (CYPOR) serves as an electron donor in several oxygenase systems and is a component of nitric oxide synthases and methionine synthase reductases. CYPOR transfers two electrons from NADPH to the heme of cytochrome p450 via FAD and FMN. Ferredoxin-NADP+ (oxido)reductase is an FAD-containing enzyme that catalyzes the reversible electron transfer between NADP(H) and electron carrier proteins such as ferredoxin and flavodoxin. Isoforms of these flavoproteins (i.e. having a non-covalently bound FAD as a prosthetic group) are present in chloroplasts, mitochondria, and bacteria in which they participate in a wide variety of redox metabolic pathways. The C-terminal domain contains most of the NADP(H) binding residues and the N-terminal domain interacts non-covalently with the isoalloxazine rings of the flavin molecule which lies largely in a large gap betweed the two domains. Ferredoxin-NADP+ reductase first accepts one electron from reduced ferredoxin to form a flavin semiquinone intermediate. The enzyme then accepts a second electron to form FADH2 which then transfers two electrons and a proton to NADP+ to form NADPH.
Jiyao W et al.(2023). "The conserved domain database in 2023.",