Nitrite reductase refers to any of several classes of enzymes that catalyze the reduction of nitrite. There are two classes of NIR's. A multi haem enzyme reduces NO₂⁻ to a variety of products. Copper containing enzymes carry out a single electron transfer to produce nitric oxide.

There are several types of iron based enzymes. Cytochrome cd₁, or Pseudomonas cytochrome oxidase contains two c and two d type hemes with two polypeptide chains. Different forms of this reductase catalyze the formation of nitric oxide or nitrous oxide. A version of this compound was originally called [Ferrocytochrome c-551:oxidoreductase]. It was initially considered an oxidase. It catalyzes the reduction of NO₂⁻ to NO. This tetraheme enzyme has two subunits, each containing a c-type and a d-type heme. The reduced d hemes bind nitrite and convert it to product.

(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 ccNIR protein uses six electrons and seven hydrogens to reduce nitrite to ammonia. The active site of the enzyme contains an iron in a +2 oxidation state. The oxidation level allows nitrite to bond more strongly than to the +3 state due to increased pi backbonding. This electronic effect transfers electron density into the nitrite antibonding orbital between nitrogen and oxygen. The occupation of the LUMO decreases the strength of the N-O bond. A second electronic effect is the hydrogen bonding of both oxygens to nearby amino acids. These acids are often arginine and Histidine. The interactions lengthen the N-O bonds and facilitate cleavage of an oxygen from nitrogen.

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