Cytochrome C Nitrite Reductase (ccNIR) is a multiheme enzyme that converts nitrite to ammonia on each active site. The active site iron is bound to a protoporphyrin IX ring that is covalently linked to the enzyme's proteins.
The Fe-NO bond is linear and has six shared valence electrons. This is not a stable state for an Fe-NO bond. However, a bent seven electron configuration is too stable to undergo further reaction without considerable energy input. To compensate for this barrier, two rapid, consecutive, single electron reductions form an eight electron complex. The electron transfer occurs before a shift in geometry from a linear configuration to bent geometry.
Two protonations of the nitrogen lead to an increased N-O bond distance. The resulting intermediate is a hydroxylamine. further protonation of the hydroxylamine leads to the breakage of the N-O bond to form water. The oxidation of iron from Fe(II) to Fe(III), coupled with a further protonation of nitrogen leads to the release of ammonia.
The Cysteine ligated to the type 1 Cu center is located directly next to a Histidine in the primary structure of the amino acids. This Histidine is bound to the Type 2 Cu center responsible for binding and reducing nitrite. This Cys-His bridge plays an important role in facilitating rapid electron transfer from the type 1 center to the type 2.
As nitrite dispaces water, Cu is bound by both oxygens in a bidentate fashion. A nearby Asparagine residue hydrogen bonds to one of the newly formed oxygen ligands. An incomming electron reduces the Cu from an oxidation state (II) to state (I). This change in Cu facilitates a shift in nitrite binding so that the nitrogen is bound to Cu, and one oxygen has an extended bond length due to hydrogen bonding. A second hydrogen bond forms from Histidine and leads to the cleavage of the N-O bond. The Cu is now a five coordination compound bonded to nitric oxide and water. Nitric oxide is released as Cu is oxidized to state (II) and returns to the resting configuration.
The term assimilatory refers to the fact that the product of the enzymatic activity remains in the organism. In this case, the product is ammonia which has an inhibitive effect on assimilatory nitrate reductase, thus ensuring that the organism produces the ammonia according to its requirements.
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