Human methionine synthase reductase (MSR) restores methionine sythase which is responsible for the regeneration of methionine from homocysteine, as well as the coversion of methyltetrahydrofolate to tetrahydrofolate. In MSR, electrons are transferred from NADPH to FAD to FMN to cob(II)alamin. MSR resembles proteins of the cytochrome p450 family including nitric oxide synthase, the alpha subunit of sulfite reductase, but contains an extended hinge region. 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. CYPORs resemble ferredoxin reductase (FNR) but have a connecting subdomain inserted within the flavin binding region, which helps orient the FMN binding doamin with the FNR module. 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.
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