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Nitrate reductase
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Nitrate reductase
Nitrate reductases are molybdoenzymes that reduce nitrate (NO−
3) to nitrite (NO−
2). This reaction is critical for the production of protein in most crop plants, as nitrate is the predominant source of nitrogen in fertilized soils.
Eukaryotic nitrate reductases are part of the sulfite oxidase family of molybdoenzymes. They transfer electrons from NADH or NADPH to nitrate.
Prokaryotic nitrate reductases belong to the DMSO reductase family of molybdoenzymes and have been classified into three groups, assimilatory nitrate reductases (Nas), respiratory nitrate reductase (Nar), and periplasmic nitrate reductases (Nap). The active site of these enzymes is a molybdenum ion that is bound to the four thiolate functional groups of two pterin molecules. The coordination sphere of the molybdenum ion is completed by one amino-acid side chain and oxygen and/or sulfur ligands. In Nap, the molybdenum is covalently attached to the protein by a cysteine side chain, and an aspartate side chain in Nar.
Prokaryotic nitrate reductases have two major types, transmembrane nitrate reductases (NAR) and periplasmic nitrate reductases (NAP). NAR allows for proton translocation across the cellular membrane and can contribute to the generation of ATP by the proton motive force. NAP cannot do so.
The transmembrane respiratory nitrate reductase is composed of three subunits; an 1 alpha, 1 beta and 2 gamma. It can substitute for the NRA enzyme in Escherichia coli, allowing it to use nitrate as an electron acceptor for anaerobic respiration. A transmembrane nitrate reductase that can function as a proton pump (similar to the case of anaerobic respiration) has been discovered in the diatom Thalassiosira weissflogii.
The nitrate reductase of higher plants, algae, and fungi is a homodimeric cytosolic protein with five conserved domains in each monomer: 1) an Mo-MPT domain that contains the single molybdopterin cofactor, 2) a dimer interface domain, 3) a cytochrome b domain, and 4) an NADH-binding domain that combines with 5) an FAD-binding domain to form the cytochrome b reductase fragment. There exists a Glycophosphatidylinositol-anchored variant that is found on the outer face of the plasma membrane. Its function is not clear.[needs update?]
In prokaryotic periplasmic nitrate reductase, the nitrate anion binds to Mo(IV). Oxygen transfer yields an Mo(VI) oxo intermediate with release of nitrite. Reduction of the Mo oxide and protonolysis removes the oxo group, regenerating Mo(IV).
Similar to the prokaryotic nitrate reduction mechanism, in eukaryotic nitrate reductase, an oxygen in nitrate binds to Mo in the +4 oxidation state, displacing a hydroxide ion. Then the Mo d-orbital electrons flip over, creating a multiple bond between Mo(VI) and that oxygen, ejecting nitrite. The Mo(VI) double bond to oxygen is reduced by NAD(P)H passed through the intramolecular transport chain.
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Nitrate reductase
Nitrate reductases are molybdoenzymes that reduce nitrate (NO−
3) to nitrite (NO−
2). This reaction is critical for the production of protein in most crop plants, as nitrate is the predominant source of nitrogen in fertilized soils.
Eukaryotic nitrate reductases are part of the sulfite oxidase family of molybdoenzymes. They transfer electrons from NADH or NADPH to nitrate.
Prokaryotic nitrate reductases belong to the DMSO reductase family of molybdoenzymes and have been classified into three groups, assimilatory nitrate reductases (Nas), respiratory nitrate reductase (Nar), and periplasmic nitrate reductases (Nap). The active site of these enzymes is a molybdenum ion that is bound to the four thiolate functional groups of two pterin molecules. The coordination sphere of the molybdenum ion is completed by one amino-acid side chain and oxygen and/or sulfur ligands. In Nap, the molybdenum is covalently attached to the protein by a cysteine side chain, and an aspartate side chain in Nar.
Prokaryotic nitrate reductases have two major types, transmembrane nitrate reductases (NAR) and periplasmic nitrate reductases (NAP). NAR allows for proton translocation across the cellular membrane and can contribute to the generation of ATP by the proton motive force. NAP cannot do so.
The transmembrane respiratory nitrate reductase is composed of three subunits; an 1 alpha, 1 beta and 2 gamma. It can substitute for the NRA enzyme in Escherichia coli, allowing it to use nitrate as an electron acceptor for anaerobic respiration. A transmembrane nitrate reductase that can function as a proton pump (similar to the case of anaerobic respiration) has been discovered in the diatom Thalassiosira weissflogii.
The nitrate reductase of higher plants, algae, and fungi is a homodimeric cytosolic protein with five conserved domains in each monomer: 1) an Mo-MPT domain that contains the single molybdopterin cofactor, 2) a dimer interface domain, 3) a cytochrome b domain, and 4) an NADH-binding domain that combines with 5) an FAD-binding domain to form the cytochrome b reductase fragment. There exists a Glycophosphatidylinositol-anchored variant that is found on the outer face of the plasma membrane. Its function is not clear.[needs update?]
In prokaryotic periplasmic nitrate reductase, the nitrate anion binds to Mo(IV). Oxygen transfer yields an Mo(VI) oxo intermediate with release of nitrite. Reduction of the Mo oxide and protonolysis removes the oxo group, regenerating Mo(IV).
Similar to the prokaryotic nitrate reduction mechanism, in eukaryotic nitrate reductase, an oxygen in nitrate binds to Mo in the +4 oxidation state, displacing a hydroxide ion. Then the Mo d-orbital electrons flip over, creating a multiple bond between Mo(VI) and that oxygen, ejecting nitrite. The Mo(VI) double bond to oxygen is reduced by NAD(P)H passed through the intramolecular transport chain.
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