Ferredoxin reductase (FNR), an FAD and NAD(P) binding protein, was intially identified as a chloroplast reductase activity, catalyzing the electron transfer from reduced iron-sulfur protein ferredoxin to NADP+ as the final step in the electron transport mechanism of photosystem I. FNR transfers electrons from reduced ferredoxin to FAD (forming FADH2 via a semiquinone intermediate) and then transfers a hydride ion to convert NADP+ to NADPH. FNR has since been shown to utilize a variety of electron acceptors and donors and has a variety of physiological functions including nitrogen assimilation, dinitrogen fixation, steroid hydroxylation, fatty acid metabolism, oxygenase activity, and methane assimilation in many organisms. FNR has an NAD(P)-binding sub-domain of the alpha/beta class and a discrete (usually N-terminal) flavin sub-domain which vary in orientation with respect to the NAD(P) binding domain. The N-terminal moeity may contain a flavin prosthetic group (as in flavoenzymes) or use flavin as a substrate. Because flavins such as FAD can exist in oxidized, semiquinone (one- electron reduced), or fully reduced hydroquinone forms, FNR can interact with one and 2 electron carriers. FNR has a strong preference for NADP(H) vs NAD(H).

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gi 2751151858   8 AAGQG---FRSWgaLRRIRGDDKPMLGARWIGWLFFGVKN 44
Cdd:cd06208   142 ATGTGiapFRSF--LRRLFREKHADYKFTGLAWLFFGVPN 179

These ferredoxin reductases are related to the NADPH cytochrome p450 reductases (CYPOR), but lack the FAD-binding region connecting sub-domain. Ferredoxin-NADP+ reductase (FNR) 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 between 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. CYPOR serves as an electron donor in several oxygenase systems and is a component of nitric oxide synthases, sulfite reducatase, and methionine synthase reductases. CYPOR transfers two electrons from NADPH to the heme of cytochrome p450 via FAD and FMN. CYPOR has a C-terminal FNR-like FAD and NAD binding module, an FMN-binding domain, and an additional connecting domain (inserted within the FAD binding region) that orients the FNR and FMN -binding domains. 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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                  ....*....|....*....|....*....|....*....|
gi 2751151858   8 AAGQG---FRSWgaLRRIRGDDKPMLGARWIGWLFFGVKN 44
Cdd:cd06208   142 ATGTGiapFRSF--LRRLFREKHADYKFTGLAWLFFGVPN 179

These ferredoxin reductases are related to the NADPH cytochrome p450 reductases (CYPOR), but lack the FAD-binding region connecting sub-domain. Ferredoxin-NADP+ reductase (FNR) 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 between 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. CYPOR serves as an electron donor in several oxygenase systems and is a component of nitric oxide synthases, sulfite reducatase, and methionine synthase reductases. CYPOR transfers two electrons from NADPH to the heme of cytochrome p450 via FAD and FMN. CYPOR has a C-terminal FNR-like FAD and NAD binding module, an FMN-binding domain, and an additional connecting domain (inserted within the FAD binding region) that orients the FNR and FMN -binding domains. 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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                  ....*....|....*....|....*....|....*....|
gi 2751151858   8 AAGQG---FRSWgaLRRIRGDDKPMLGARWIGWLFFGVKN 44
Cdd:cd06208   142 ATGTGiapFRSF--LRRLFREKHADYKFTGLAWLFFGVPN 179