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 (GLU312) and a serine residue (SER96) 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.
Read more about this topic: Ferredoxin-NADP+ Reductase
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