Copper containing enzymes carry out a single electron transfer to produce nitric oxide.
The active site iron is bound to a protoporphyrin IX ring that is covalently linked to the enzyme's proteins.
The oxidation level allows nitrite to bond more strongly than to the +3 state due to increased pi backbonding.
A second electronic effect is the hydrogen bonding of both oxygens to nearby amino acids.
However, a bent seven electron configuration is too stable to undergo further reaction without considerable energy input.
[3] 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.
In the resting state, the Cu is also binding a water molecule that is displaced by nitrite.
[7] As nitrite displaces water, Cu is bound by both oxygens in a bidentate fashion.
A nearby Aspartic acid residue hydrogen bonds to one of the newly formed oxygen ligands.
The term assimilatory refers to the fact that the product of the enzymatic activity remains in the organism.