Ferredoxin—nitrite reductase
- UniProt: Q39161
The ferredoxin nitrite reductase ( NiR ) is an enzyme in plants, algae and cyanobacteria, which is localized in the plastids and reduces nitrite to ammonium. This is a reaction step in the metabolic pathway of the nitrate assimilation, in which the supplied nitrogen fertilization, for example, nitrates of the plant is made available as an ammonium ion.
The catalyzed reaction:
NiR is one of the Nitritreduktasen. Homologous enzymes in bacteria are capable of reducing sulfite.
Importance of nitrite reductase in the plant organism
Plants absorb nitrogen in the form of nitrate ( NO3- ) or ammonium (NH4 ) from the water or the ground. In the form of nitrate nitrogen absorbed must be converted to ammonium, to be ready for synthetic organic material. The first step of nitrate reduction is catalyzed by nitrate reductase. The nitrate reductase reduces nitrate to nitrite (NO2 - ). In a second step is then further reduced to nitrite ammonium, which catalyzes the nitrite.
Nitrite is a cell poison and must be implemented quickly. Since, inter alia, the affinity of nitrite to nitrite is high, the nitrite formed by nitrate reductase but is fully implemented.
Biochemistry
The plant nitrite reductase is nuclear encoded, but exclusively localized in the plastids. Therefore, the nitrite reduction is in contrast to the cytosol held nitrate reduction in the plastids instead.
The enzyme has three covalently bound cofactors: a 4Fe-4S iron-sulfur cluster and a FAD Sirohäm. The three cofactors form an electron transport chain and transfer electrons from the electron donor ferredoxin to nitrite. For a complete reduction of nitrite six electrons must be transferred:
The electrons come mainly from photosynthesis ( photosystem I), reduce the oxidized ferredoxin again. Alternatively, smaller quantities Ferrodoxins can also be reduced by NADPH. Then the plant in the root is dependent, since there can be no photosynthesis.