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ammonia oxidation I (aerobic)

Nitrification, the oxidation of ammonia to nitrate by microorganisms, is a key process in the global nitrogen cycle, resulting in nitrogen loss from ecosystems, eutrophication of surface and ground waters, and the production of atmospherically active trace gases. No known bacteria are capable of carrying out the whole process. Rather, ammonia oxidizers convert ammonia to nitrite, while nitrite oxidizers oxidize nitrite to nitrate. The "classical" ammonia-oxidizing bacteria are chemolithoautotrophs that utilize the process to generate sufficient energy for growth and maintenance reactions, including the reductive fixation of carbon dioxide. However, other organisms are able to oxidize ammonia to nitrite, including some marine archaea |CITS: [16177789][16894176][19794413]|, methane-oxidizing bacteria |CITS: [18650926][21682764]| and some heterotrophic bacteria |CITS:[6721486][2782877][2181927]|. The key difference between heterotrophic and autotrophic ammonia oxidation is that heterotrophic ammonia oxidation usually does not generate energy. It has been suggested that in these bacteria nitrification is used as a sink for excess reducing power generated by oxidative metabolism |CITS: [Robertson88][8253206]|. Autotrophic ammonia oxidizing bacteria include the genera |FRAME: TAX-35798|, |FRAME: TAX-914| and |FRAME: TAX-1227| (the best studied ammonia oxidizing organism is |FRAME: TAX-915|). All of these organisms possess the genes encoding the two essential enzymes, ammonia monooxygenase (AMO) and hydroxylamine oxidoreductase (HAO). The 4 electrons released in the oxidation of |FRAME: HYDROXYLAMINE| by HAO are channeled throgh an electron transfer chain to the quinone pool |CITS:[4372235][9808046]|. The nature of the electron transfer chain appears to differ among different types of organisms. The hypothesized chain in |FRAME: TAX-915| is described in |FRAME: PWY-7082| |CITS: [11807563][12209257][11004450]|. Many heterotrophic bacteria also possess genes encoding AMO and HAO activities. However, the enzymes found in these organisms can be very different from the ones found in the chemolithoautotrophs. While the second step of the pathway (the oxidation of hydroxylamine to nitrite) is catalyzed in autotrophs only by a complex hexameric |FRAME: CPLX-1321|, which is composed of three octaheme cytochrome c monomers (haoA) cross-linked between a tyrosine residue and the active site heme c group of the neighboring monomer, other types of enzymes have been reported from heterotrophs. HAOs consisting of a small (20 kDa) periplasmic monomer containing ferric iron that is non-heme and non iron-sulfur have been reported from several organisms, including |FRAME: TAX-266| |CITS: [8253206][8920986]| and a |FRAME:TAX-286| Sp. |CITS: [9082922]|. An HAO isolated from |FRAME: ORG-5975| was reported to be a homo-dimer of 68 kDa subunits that contains no detectable cofactors |CITS: [9049019]|. In addition, some heterotrophs lack the enzyme, but are still able to oxidize ammonia by other routes, as demonstrated in the pathway |FRAME: PWY-2242| |CITS: [8674977]|. Many organisms couple ammonia oxidation to |FRAME: DENITRIFICATION-PWY "denitrification"|. This has been shown for both heterotrophs |CITS: [2181927][8253206][9765887]| and autotrophs |CITS:[Bock95][15583163]|. In this process the nitrite produced by ammonia oxidation is reduced to nitric oxide, nitrous oxide, and possibly to nitrogen gas. For more information, see |FRAME: PWY-7084|.

from BIOCYC source record: META_AMMOXID-PWY
Type: pathway
Taxonomic scope
conserved biosystem

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