Enzymes nitrification

The rate of bacterial growth and hence the rate of nitrification is both temperature and pH dependent. Maximum bacterial activity occurs at about 28°C and a pH of about 8, Below a temperature of about 2°C, the reaction is very slow (Fig. 8.7). Below pH 5.5, the nitrifying bacteria decrease their activity, and below pH 4.5 the nitrification process is severely restricted lack of oxygen also inhibits nitrification. As noted above, oxidation of NH to NO is an enzyme-driven reaction and commonly the Km (see Chapter 7) under optimum conditions is observed to be somewhere around 2.5 mM. Some Km values below 2.5 mM are observed under high pH values when a large fraction of the ammonium is in the NH3 form. 

More simultaneous measurements of NH3 in the ocean and in the atmosphere are needed to reduce the considerable uncertainties of the ocean/atmosphere flux estimates. The ongoing acidification of the ocean will shift the NH3/NH4 equilibrium to NH. On the one hand this might have implication for the atmospheric distribution of NH3, since the uptake capacity of the ocean will be increased with unknown consequences for chemistry of the atmosphere (e.g. the aerosol formation) over the ocean. On the other hand this might have severe implications for the nitrification rates in seawater because they are influenced by the pH. When the pH drops from 8 to 7, nitrification rates can be reduced by 50% (Huesemann et al., 2002). (One explanation for this is that the ammonia monooxygenase enzyme uses rather NH3 than NH4 as substrate.) Most recently it was suggested that atmospheric NH3 serves as a foraging cue for seabirds such as the blue petrel (Nevitt ei a/., 2006) is an excretion product of... 

Schmidt, H. L., Werner, R. A., Yoshida, N., and Well, R. (2004). Is the isotopic composition of nitrous oxide an indicator for its origin from nitrification or denitrification A theoretical approach from referred data and microbiological and enzyme kinetic aspects. Rapid Communication Mass Spectrometry 18(18), 2036—2040. 

It has been argued that heterotrophic nitrification involves enzyme systems that are quite different from those of the autotrophs (Wehrfritz et al, 1993) and that heterotrophic nitrification cannot serve as an energy generating mechanism (Castignetti, 1990), as the autotrophic process does. In the aerobic denitrifiers, which are also capable of nitrification, the initial enzyme, AMO, appears to be quite similar to the enzyme in autotrophic nitrifiers. However, HAO differs significantly and in the heterotrophs, is a smaller, simpler enzyme that performs a two electron transfer (instead of the four electron transfer of the autotrophic HAO) and... 

Both NH3 oxidizers and N02 oxidizers, but especially the former, are susceptible to inhibition by a wide range of compounds, and several different modes of action have been documented (Bedard and Knowles, 1989). The two most common modes of inhibition are (1) interference with the active site of the primary enzyme (i.e., NH3 monooxygenase in AOB N02 oxidoreductase in NOB) by compounds that share structural homology with NH3 or N02 and (2) metal binding compounds, which interfere with the action or availability of copper in the NH3-oxidizing enzymes. In both NH3- and N02 -oxidizers, the susceptibility to inhibitors by key enzymes forms the basis of some methods used to measure the rate of nitrification in the environment (see above). 

Alternative pathways for denitrification have been proposed that do not involve the classical enzyme systems, but their existence has only recently been confirmed or investigated in detail. In the absence of O2, a number of elements and compounds have the potential to oxidize NH , including Mn(II), Mn02, NO, NO2 and NO2. Because anaerobic oxidation of NH directly to N2 bypasses the multiple oxidation steps required for coupled nitrification-denitrification... 

Oxidation of ammonia to nitrite, N02, and nitrate, N03, is called nitrification the reverse reaction is ammonification. Reduction from nitrite to nitrogen is called denitrification. All these reactions, and more, occur in enzyme systems, many of which include transition metals. A molybdenum enzyme, nitrate reductase, reduces nitrate to nitrite. Further reduction to ammonia seems to proceed by 2-electron steps, through an uncertain intermediate with a -fl oxidation state (possibly hyponitrite, N202 ) and hydroxylamine ... 

Nitrification—The process by which Nitro-somonasbacteria oxidize ammonium to nitrite, which is then oxidized by Nitrobacterto nitrate. Nitrogenase—The microbial enzyme that fixes dinitrogen, by cleaving its triple bond and forming ammonia. 

Reaction inhibitors slow reaction rates. Nitrogen mineralization and nitrification (conversion of organic nitrogen and ammonium to nitrate) rates in soils, for example, can be slowed temporarily by chemicals that specifically slow or stop the microorganisms involved. Toxic metals can also operate as enzyme inhibitors, by replacing the metal coenzyme portion of an enzyme and thereby inactivating it. 

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