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Nitrite Reductase

Overview

Nitrite reductase is an enzyme that belongs to the Oxidoreductase family for catalyzing the six-electron reduction of nitrite into ammonia, using the reduced ferredoxin as the electron donor. It is a monomeric protein with molecular mass near 66 kDa characterized by a unique which contains a single [4Fe–4S] cluster and a single siroheme (which serves as the binding site for nitrite) via a bridging sulfur atom from a cysteine residue. The protein has a globular fold consisting of 3 alpha/beta domains with the siroheme-iron sulfur cofactor at the interface of the three domains. The siroheme is surrounded by several ionizable amino acid residues that facilitate the binding and subsequent reduction of nitrite.

Mechanism

The electron flow proceeds initially from reduced ferredoxin to the enzyme's [4Fe–4S] cluster and subsequently from the reduced cluster to the siroheme. Since ferredoxin is a one-electron donor, the enzyme must accumulate six electrons in one-electron steps before it can reduce nitrite.

Structure

Secondary Structures

Nitrite Reductase has only one chain consisting of 591 residues. There are 33 (33% of chain) and 33 (21% of chain).

Ligands

Siroheme is an iron-containing isobacteriochlorin, a modified tetrapyrrole similar in structure to both heme and chlorophyll. It is a heme-like prosthetic group used by nitrite reductase to carry out the six-electron reduction of nitrogen. Nitrite is reduced to ammonium while still bound to siroheme. The protein has a globular fold consisting of 3 alpha/beta domains with the siroheme-iron sulfur cofactor at the interface of the three domains. The siroheme is surrounded by several ionizable amino acid residues that facilitate the binding and subsequent reduction of nitrite. It has 3 iron atoms, one of which is in the siroheme. Has only one chain consisting of 591 residues. It consists of 33 helices (33% of chain) and 33 beta strands (21% of chain)

Ligand Binding to Active Sites

• binds Siroheme
• binds Iron-Sulfur Complex
• binds Potassium ion
• binds Chloride ion
• binds Chloride ion
• binds Nitrite