Nitrite oxidoreductase

Nitrite oxidoreductase (NOR or NXR) is an enzyme involved in nitrification. It is the last step in the process of aerobic ammonia oxidation, which is carried out by two groups of nitrifying bacteria: ammonia oxidizers such as Nitrosospira, Nitrosomonas, and Nitrosococcus convert ammonia to nitrite, while nitrite oxidizers such as Nitrobacter and Nitrospira oxidize nitrite to nitrate. NXR is responsible for producing almost all nitrate found in nature.[1]

NXR belongs to the class of EC numbers 1.7.2- [2][3] where 1 describes an oxidoreductase, 1.7 describes nitrogen compounds as donors, and 1.7.2- describes cytochromes as acceptors.[3]

Structure

NXR is composed of 2 mainly known subunits; nitrite oxidoreductase α (NxrA), and nitrite oxidoreductase β (NxrB) (sometimes written as NorA and NorB).[3]  However, recent studies describe a third and fourth subunit, NxrC and NxrT[3][4] The enzyme’s known active site is on the NxrA subunit.[4] There are two types of NXR; one where the NxrA subunit is located in the periplasmic space of a cell, and the other where NxrA is located in the cytoplasm[5]

The enzyme is bound to the inner cytoplasmic surface of the bacterial membrane and contains iron-sulfur centers and a molybdenum cofactor.[6][7] The enzyme is relatively abundant, making up 10-30% of the total protein in these bacteria and forms densely packed structures on the membrane surface.[8] To date, little is known about the exact structure of NXR, but has been discovered to form tubule structures that are hundreds of nanometers long.[5]

Pathway
Reaction

NXR oxidizes nitrite into nitrate in aerobic nitrogen oxidizing bacteria as well as ammonia to nitrite in ammonia oxidizing bacteria or archaea.  When it oxidizes nitrite to nitrate, two electrons are shuttled into the respiratory chain. Electrons flow through the subunits of the enzyme through cytochrome c toward the terminal oxidase.[4] This reaction can be reversed to reduce nitrate to nitrite in anaerobic conditions, though the driving force of this reversal is poorly understood.[9]

Metabolism

In periplasmic NXR types, protons are derived from water and contribute to proton motive force, which then contributes to the cell’s energy budget. However, cytoplasmic NXR does not contribute to proton motive force.[4]  The two electrons that are generated from the nitrite oxidation are then donated to molecular oxygen, which yields energy.[5]  The NXR pathway for nitrite oxidation generally has a low energy yield (ΔG’ = -74 kj/mol NO2).[4]

See also

References
  1. Chicano, Tadeo Moreno; Dietrich, Lea; de Almeida, Naomi M.; Akram, Mohd.; Hartmann, Elisabeth; Leidreiter, Franziska; Leopoldus, Daniel; Mueller, Melanie; Sánchez, Ricardo; Nuijten, Guylaine H. L.; Reimann, Joachim; Seifert, Kerstin-Anikó; Schlichting, Ilme; van Niftrik, Laura; Jetten, Mike S. M. (2021-07-15). "Structural and functional characterization of the intracellular filament-forming nitrite oxidoreductase multiprotein complex". Nature Microbiology. 6 (9): 1129–1139. doi:10.1038/s41564-021-00934-8. ISSN 2058-5276. PMC 8387239. PMID 34267357.
  2. Caspi, Ron; Billington, Richard; Fulcher, Carol A; Keseler, Ingrid M; Kothari, Anamika; Krummenacker, Markus; Latendresse, Mario; Midford, Peter E; Ong, Quang; Ong, Wai Kit; Paley, Suzanne; Subhraveti, Pallavi; Karp, Peter D (2017-10-20). "The MetaCyc database of metabolic pathways and enzymes". Nucleic Acids Research. 46 (D1): D633–D639. doi:10.1093/nar/gkx935. ISSN 0305-1048.
  3. "ENZYME: 1.7.2.-". enzyme.expasy.org. Retrieved 2023-10-19.
  4. Daims, Holger; Lücker, Sebastian; Wagner, Michael (September 2016). "A New Perspective on Microbes Formerly Known as Nitrite-Oxidizing Bacteria". Trends in Microbiology. 24 (9): 699–712. doi:10.1016/j.tim.2016.05.004.
  5. Holmes, Dawn E.; Dang, Yan; Smith, Jessica A. (2019), "Nitrogen cycling during wastewater treatment", Advances in Applied Microbiology, Elsevier, vol. 106, pp. 113–192, doi:10.1016/bs.aambs.2018.10.003, ISBN 978-0-12-816975-9, retrieved 2023-10-19
  6. Spieck E, Ehrich S, Aamand J, Bock E (1998). "Isolation and immunocytochemical location of the nitrite-oxidizing system in nitrospira moscoviensis". Arch. Microbiol. 169 (3): 225–30. doi:10.1007/s002030050565. PMID 9477257. S2CID 21868756.
  7. Meincke M, Bock E, Kastrau D, Kroneck PMH (1992). "Nitrite oxidoreductase from Nitrobacter hamburgensis: redox centers and their catalytic role". Arch. Microbiol. 158 (2): 127–31. doi:10.1007/BF00245215. S2CID 6903429.
  8. Spieck E, Muller S, Engel A, Mandelkow E, Patel H (1996). "Two-dimensional structure of membrane-bound nitrite oxidoreductase from Nitrobacter hamburgensis" (PDF). J. Struct. Biol. 117 (2): 117–123. doi:10.1006/jsbi.1996.0076.
  9. Ward, B.B. (2011), "Measurement and Distribution of Nitrification Rates in the Oceans", Research on Nitrification and Related Processes, Part A, Elsevier, pp. 307–323, retrieved 2023-10-19


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