Ammonia monooxygenase

Hyman MR, CL Page, DJ Arp (1994) Oxidation of methyl fluoride and dimethyl ether by ammonia monooxygenase in Nitrosomonas europaea. Appl Environ Microbiol 60 3033-3035. 

Hyman MR, IB Murton, DJ Arp (1988) Interaction of ammonia monooxygenase from Nitrosomonas europaea with alkanes, alkenes, and alkynes. Appl Environ Microbiol 54 3187-3190. 

Juliette LY, MR Hayman, DJ Arp (1993) Inhibition of ammonia oxidation in Nitrosomonas europeae by sulfur compounds thioethers are oxidized to sulfoxides by ammonia monooxygenase. Appl Environ Microbiol 59 3718-3727. 

Rasche ME, MR Hyman, DJ Arp (1991) Factors limiting aliphatic chlorocarbon degradation by Nitrosomonas europaea cometabolic inactivation of ammonia monooxygenase and substrate specificity. Appl Environ Microbiol 57 2986-2994. 

The cometabolism of halogenated methanes has been examined in Nitrosomonas europaea and may putatively be mediated by ammonia monooxygenase. 

Ammonia monooxygenase in Nitrosomonas europaea is able to oxidize fluoromethane to formaldehyde (Hyman et al. 1994). 

Rasche et al. [410] reported that some terrestrial and marine nitrifiers had the capacity to oxidize methyl bromide to formaldehyde and bromide ion. They concluded that ammonia monooxygenase produced by the nitrifiers, which catalyzes the oxidation of ammonia to hydroxylamine, was responsible for the oxidation of methyl bromide to formaldehyde. 

Figure 18-19 The ammonia oxidation system of the bacterium Nitrosomonas. Oxidation of ammonium ion (as free NH3) according to Eq. 18-17 is catalyzed hy two enzymes. The location of ammonia monooxygenase (step a) is uncertain but hydroxylamine oxidoreductase (step b) is periplas-mic. The membrane components resemble complexes I, III, and IV of the mitochondrial respiratory chain (Fig. 18-5) and are assumed to have similar proton pumps. Solid green lines trace the flow of electrons in the energy-producing reactions. This includes flow of electrons to the ammonia monoxygenase. Complexes HI and IV pump protons out but complex I catalyzes reverse electron transport for a fraction of the electrons from hydroxylamine oxidoreductase to NAD+. Modified from Blaut and Gottschalk.315...

Cell-free extracts of N. europaea will only oxidize hydroxylamine, probably because the enzyme responsible for the initial oxidation of ammonia is associated with the membrane and so is lost on disruption of the cell. This enzyme, ammonia monooxygenase, also catalyzes the oxidation of methane.1540 While the methane has a higher Km value for oxidation by ammonia monooxygenase than for oxidation by a methane monooxygenase, it is still possible that both processes would be catalyzed by ammonia monoxygenase in the natural environment. 

Holmes, A.J., Costello, A., Lidstrom, M.E., and Murrell, J.C. (1995) Evidence that particulate methane monooxygenase and ammonia monooxygenase may be evolutionary related. FEMS Microbial. Lett. 132, 203-208. 

C554 is proposed to bind, and may thus be the electron exit heme. Cytochrome C554 also has two coplanar diheme pairs, which may indicate that it can also accept two electrons simultaneously. This cytochrome then transfers electrons to the membrane-bound tetraheme cytochrome Cm552 (see Section 4), which is a good candidate to reduce the membrane ubiquinone pool, from where electrons are partitioned between the ammonia monooxygenase reaction, the aerobic respiratory chain, and reverse electron transport. ... 

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... 

Francis, C. A., O MuUan, G. D., and Ward, B. B. (2003). Diversity of ammonia monooxygenase (amoA) genes across environmental gradients in Chesapeake Bay sediments. Geobiology 1, 129-140. 

Klotz, M. G., and Norton, J. M. (1995). Sequence of an ammonia monooxygenase subunit a- encoding gene from Nitrosospira sp NpAV. Gene 163, 159—160. 

Zahn, J. A., Arciero, D. M., Hooper, A. B., andDiSpirito, A. A. (1996). Evidence for an iron center in the ammonia monooxygenase from Nitrosomonas europaea. FEES Letters 397, 35—38. 

Rotthauwe, J. H., Witzel, K. P., and Liesack, W. (1997). The ammonia monooxygenase structural gene amoA as a functional marker molecular fine-scale analysis of namral ammonia-oxidizing populations. Appl. Environ. Microbiol. 63, 4704—4712. 

The ammonia-oxidizing bacteria oxidize ammonia to nitrous acid via hydroxyl-amine (NH2OH) (Lees, 1952 Hofman and Lees, 1953) ammonia is first oxidized to hydroxylamine by the catalysis of ammonia monooxygenase (AMO) (Dua et al., 1979 Hollocher et al., 1981). In this reaction, molecular oxygen is utilized. Then, hydroxylamine formed is oxidized to nitrous acid by the catalysis of hydroxylamine oxidoreductase (HAO). 

Ammonia monooxygenase is very unstable when the cells of N. europaea are destroyed the enzyme is mostly inactivated (Suzuki and Kwok, 1970 Suzuki et al., 1981). Therefore, little is known about the properties of the enzyme. The copper atom seems important for the enzyme to function, as the enzyme is inactivated by many cuprous chelating agents (Hooper and Terry, 1973 Wood, 1986), and the enzyme seems to be activated by cuprous ions (Ensign et al., 1993). Moreover, the enzyme purified from the heterotrophic nitrifier Paracoccus denitrificans has been found to be activated by the cuprous ion (Moir et al., 1996). However, the P. denitrificans enzyme may be a little different from the N. europaea enzyme, because it is not inhibited by acetylene (Moir et al., 1996), while the N. europaea enzyme is inhibited by the compound (Hooper and Terry, 1973). Electron paramagnetic resonance (EPR) studies show that iron is also important for the function of the enzyme (Zahn et al., 1996). The amino acid sequence of the N. europaea enzyme is deduced from DNA (Hyman and Wood, 1985 McTavish et al., 1993 Bergmann and Hooper, 1994), although firm evidence has not been obtained that the DNA really encodes the enzyme. 

Ammonia monooxygenase requires 2[H] (two hydrogen atoms or electrons) to oxygenate ammonia, as shown by equation (3.1). As the oxidation of ammonia to hydroxylamine by the cell-free extracts of N. europaea is activated by addition of cytochrome c-554 (Yamanaka and Shinra, 1974), the 2 [H] are thought to be supplied to the enzyme through the cytochrome (Suzuki and Kwok, 1981). Incidentally, the substrate for ammonia monooxygenase is ammonia, but not ammonium ion (Suzuki et al., 1974). 

Ammonia monooxygenase catalyzes also the oxidation of methane (Hyman and Wood, 1983) and carbon monoxide (Tsang and Suzuki, 1982) to methanol and carbon dioxide, respectively, in addition to the catalysis of the oxidation of ammonia. The oxidation of methane by the enzyme will be described again below in relation to the regulation of the methane formation by the ammonia-oxidizing bacteria. 

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