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nitrate reductase
structure of nitrate reductase A from E. coli
Identifiers
EC no.	1.7.99.4
CAS no.	9013-03-0
Databases
IntEnz	IntEnz view
BRENDA	BRENDA entry
ExPASy	NiceZyme view
KEGG	KEGG entry
MetaCyc	metabolic pathway
PRIAM	profile
PDB structures	RCSB PDB PDBe PDBsum
Gene Ontology	AmiGO / QuickGO
Search PMC articles PubMed articles NCBI proteins

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.

Types

Eukaryotic

Eukaryotic nitrate reductases are part of the sulfite oxidase family of molybdoenzymes. They transfer electrons from NADH or NADPH to nitrate.

Prokaryotic

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.

Structure

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.

Mechanism

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 (IV) 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.

Regulation

Nitrate reductase (NR) is regulated at the transcriptional and translational levels induced by light, nitrate, and possibly a negative feedback mechanism. First, nitrate assimilation is initiated by the uptake of nitrate from the root system, reduced to nitrite by nitrate reductase, and then nitrite is reduced to ammonia by nitrite reductase. Ammonia then goes into the GS-GOGAT pathway to be incorporated into amino acids. When the plant is under stress, instead of reducing nitrate via NR to be incorporated into amino acids, the nitrate is reduced to nitric oxide which can have many damaging effects on the plant. Thus, the importance of regulating nitrate reductase activity is to limit the amount of nitric oxide being produced.

Inactivation of nitrate reductase

The inactivation of nitrate reductase has many steps and many different signals that aid in the inactivation of the enzyme. Specifically in spinach, the very first step of nitrate reductase inactivation is the phosphorylation of NR on the 543-serine residue. The very last step of nitrate reductase inactivation is the binding of the 14-3-3 adapter protein, which is initiated by the presence of Mg and Ca. Higher plants and some algae post-translationally regulate NR by phosphorylation of serine residues and subsequent binding of a 14-3-3 protein.

Anoxic conditions

Studies were done measuring the nitrate uptake and nitrate reductase activity in anoxic conditions to see if there was a difference in activity level and tolerance to anoxia. These studies found that nitrate reductase, in anoxic conditions improves the plants tolerance to being less aerated. This increased activity of nitrate reductase was also related to an increase in nitrite release in the roots. The results of this study showed that the dramatic increase in nitrate reductase in anoxic conditions can be directly attributed to the anoxic conditions inducing the dissociation of 14-3-3 protein from NR and the dephosphorylation of the nitrate reductase.

Applications

Nitrate reductase activity can be used as a biochemical tool for predicting grain yield and grain protein production.

Nitrate reductase can be used to test nitrate concentrations in biofluids.

Nitrate reductase promotes amino acid production in tea leaves. Under south Indian conditions, it is reported that tea plants sprayed with various micronutrients (like Zn, Mn and B) along with Mo enhanced the amino acid content of tea shoots and also the crop yield.

References

1. PDB: 1Q16​; Bertero MG, Rothery RA, Palak M, Hou C, Lim D, Blasco F, Weiner JH, Strynadka NC (September 2003). "Insights into the respiratory electron transfer pathway from the structure of nitrate reductase A". Nature Structural Biology. 10 (9): 681–7. doi:10.1038/nsb969. PMID 12910261. S2CID 33272416.
2. Marschner, Petra, ed. (2012). Marschner's mineral nutrition of higher plants (3rd ed.). Amsterdam: Elsevier/Academic Press. p. 135. ISBN 9780123849052.
3. Moreno-Vivián, Conrado Cabello, Purificación Martínez-Luque, Manuel Blasco, Rafael Castillo, Francisco. Prokaryotic Nitrate Reduction: Molecular Properties and Functional Distinction among Bacterial Nitrate Reductases. American Society for Microbiology. OCLC 678511191.{{cite book}}: CS1 maint: multiple names: authors list (link)
4. Tavares P, Pereira AS, Moura JJ, Moura I (December 2006). "Metalloenzymes of the denitrification pathway". Journal of Inorganic Biochemistry. 100 (12): 2087–100. doi:10.1016/j.jinorgbio.2006.09.003. PMID 17070915.
5. Kuypers MM, Marchant HK, Kartal B (May 2018). "The microbial nitrogen-cycling network". Nature Reviews. Microbiology. 16 (5): 263–276. doi:10.1038/nrmicro.2018.9. hdl:21.11116/0000-0003-B828-1. PMID 29398704. S2CID 3948918.
6. "ENZYME entry: EC 1.7.99.4". ENZYME Enzyme nomenclature database. Retrieved 25 April 2019.
7. Blasco F, Iobbi C, Ratouchniak J, Bonnefoy V, Chippaux M (June 1990). "Nitrate reductases of Escherichia coli: sequence of the second nitrate reductase and comparison with that encoded by the narGHJI operon". Molecular & General Genetics. 222 (1): 104–11. doi:10.1007/BF00283030. PMID 2233673. S2CID 22797628.
8. Jones GJ, Morel FM (May 1988). "Plasmalemma redox activity in the diatom thalassiosira: a possible role for nitrate reductase". Plant Physiology. 87 (1): 143–7. doi:10.1104/pp.87.1.143. PMC 1054714. PMID 16666090.
9. Campbell WH (June 1999). "Nitrate Reductase Structure, Function and Regulation: Bridging the Gap between Biochemistry and Physiology". Annual Review of Plant Physiology and Plant Molecular Biology. 50 (1): 277–303. doi:10.1146/annurev.arplant.50.1.277. PMID 15012211. S2CID 22029078.
10. Tischner R (October 2000). "Nitrate uptake and reduction in higher and lower plants". Plant, Cell and Environment. 23 (10): 1005–1024. doi:10.1046/j.1365-3040.2000.00595.x.
11. Hille, Russ; Hall, James; Basu, Partha (2014). "The Mononuclear Molybdenum Enzymes". Chemical Reviews. 114 (7): 3963–4038. doi:10.1021/cr400443z. PMC 4080432. PMID 24467397.
12. Fischer K, Barbier GG, Hecht HJ, Mendel RR, Campbell WH, Schwarz G (April 2005). "Structural basis of eukaryotic nitrate reduction: crystal structures of the nitrate reductase active site". The Plant Cell. 17 (4): 1167–79. doi:10.1105/tpc.104.029694. PMC 1087994. PMID 15772287.
13. Taiz L, Zeiger E, Moller IM, Murphy A (2014). Plant Physiology and Development (6 ed.). Massachusetts: Sinauer Associates, Inc. p. 356. ISBN 978-1-60535-353-1.
14. ^ Allègre A, Silvestre J, Morard P, Kallerhoff J, Pinelli E (December 2004). "Nitrate reductase regulation in tomato roots by exogenous nitrate: a possible role in tolerance to long-term root anoxia" (PDF). Journal of Experimental Botany. 55 (408): 2625–34. doi:10.1093/jxb/erh258. PMID 15475378.
15. Wang Y, Bouchard JN, Coyne KJ (September 2018). "Expression of novel nitrate reductase genes in the harmful alga, Chattonella subsalsa". Scientific Reports. 8 (1): 13417. Bibcode:2018NatSR...813417W. doi:10.1038/s41598-018-31735-5. PMC 6128913. PMID 30194416.
16. Croy LI, Hageman RH (1970). "Relationship of nitrate reductase activity to grain protein production in wheat". Crop Science. 10 (3): 280–285. doi:10.2135/cropsci1970.0011183X001000030021x.
17. Dalling MJ, Loyn RH (1977). "Level of activity of nitrate reductase at the seedling stage as a predictor of grain nitrogen yield in wheat (Triticum aestivum L.)". Australian Journal of Agricultural Research. 28 (1): 1–4. doi:10.1071/AR9770001.
18. Mori, Hisakazu (2001). "Determination of Nitrate in Biological Fluids Using Nitrate Reductase in a Flow System". Journal of Health Science. 47 (1): 65–67. doi:10.1248/jhs.47.65. ISSN 1344-9702.
19. Ruan J, Wu X, Ye Y, Härdter R (1988). "Effect of potassium, magnesium and sulphur applied in different forms of fertilisers on free amino acid content in leaves of tea (Camellia sinensis L". J. Sci. Food Agric. 76 (3): 389–396. doi:10.1002/(SICI)1097-0010(199803)76:3<389::AID-JSFA963>3.0.CO;2-X.
20. Venkatesan S (November 2005). "Impact of genotype and micronutrient applications on nitrate reductase activity of tea leaves". J. Sci. Food Agric. 85 (3): 513–516. Bibcode:2005JSFA...85..513V. doi:10.1002/jsfa.1986.

External links

• UMich Orientation of Proteins in Membranes protein/pdbid-1q16
• Nitrate+reductases at the U.S. National Library of Medicine Medical Subject Headings (MeSH)

• Biology

• Enzymes
• Integral membrane proteins
• EC 1.7.99
• Protein families