In enzymology, a ferredoxin-NADP reductase () abbreviated FNR, is an enzyme that catalyzes the chemical reaction 2 reduced ferredoxin + NADP + H 2 oxidized ferredoxin + NADPH The 3 substrates of this enzyme are reduced ferredoxin, NADP, and H, whereas its two products are oxidized ferredoxin and NADPH. It has a flavin cofactor, FAD. This enzyme belongs to the family of oxidoreductases, that use iron-sulfur proteins as electron donors and NAD or NADP as electron acceptors. This enzyme participates in photosynthesis. FNR provides a major source of NADPH for photosynthetic organisms. Nomenclature The systematic name of this enzyme class is ferredoxin:NADP oxidoreductase. Other names in common use include: adrenodoxin reductase, ferredoxin-NADP reductase, ferredoxin-NADP oxidoreductase, ferredoxin-nicotinamide adenine dinucleotide phosphate reductase, ferredoxin-nicotinamide-adenine dinucleotide phosphate (oxidized), reductase ferredoxin-TPN reductase, NADP:ferredoxin oxidoreductase, NADPH:ferredoxin oxidoreductase, reduced nicotinamide adenine dinucleotide phosphate-adrenodoxin, reductase, and TPNH-ferredoxin reductase Mechanism During photosynthesis, electrons are removed from water and transferred to the single electron carrier ferredoxin. Ferredoxin: NADP reductase then transfers an electron from each of two ferredoxin molecules to a single molecule of the two electron carrier NADPH. FNR utilizes FAD, which can exist in an oxidized state, single electron reduced semiquinone state, and fully reduced state to mediate this electron transfer. FNR has an induced-fit mechanism of catalysis. Binding of ferredoxin to the enzyme causes the formation of a hydrogen bond between a glutamate residue (E312) and a serine residue (S96) in the active site. The glutamate residue is highly conserved because it both stabilizes the semiquinone form of FAD and is a proton donor/acceptor in the reaction. The rate limiting step of the electron transfer reaction is the release of the first oxidized ferredoxin molecule after the reduction of FAD with one electron. This step is inhibited by the presence of oxidized ferredoxin and stimulated by the presence of NADP. The binding of NADP to the enzyme lowers the binding affinity of the enzyme for ferredoxin. This reaction can also operate in reverse to generate reduced ferredoxin, which can then be used in a variety of biosynthetic pathways. Some bacteria and algae use the molecule flavodoxin instead of ferredoxin as the single electron carrier molecule to be reduced or oxidized. Structure Plant-type ferredoxin: NADP reductase has two structural domains. The first domain is an antiparallel beta barrel at the amino terminus of the protein that contains the binding domain for the FAD cofactor. The second domain is