Ammonia Oxidation Mechanisms in Nitrifying Bacteria

Ammonia oxidation in nitrifying bacteria represents the first and rate-limiting step of the aerobic nitrogen cycle, converting ammonia to nitrite. This process is mediated by a two-enzyme pathway: ammonia monooxygenase catalyses the insertion of oxygen into ammonia to form hydroxylamine, and hydroxylamine oxidoreductase then converts hydroxylamine to nitrite. The reaction sequence is tightly coupled, ensuring efficient energy conservation through electron transfer and proton translocation across the cell membrane. Members of the genera Nitrosomonas and Nitrosospira dominate in soils and freshwater, while marine environments are largely inhabited by ammonia-oxidising gammaproteobacteria. Structural and biochemical studies have revealed that ammonia monooxygenase contains a copper-centered active site, whereas hydroxylamine oxidoreductase utilises multiple haeme cofactors to shuttle electrons. The interplay of these enzymes underpins global nitrogen turnover, influences fertiliser requirement, and affects greenhouse-gas emissions, particularly nitrous oxide. Understanding these mechanisms has led to the development of targeted inhibitors to manage nitrification in agricultural soils and to optimise nitrifying biofilms in wastewater treatment plants.

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Ammonia Oxidation Mechanisms in Nitrifying Bacteria publication trend

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Technical terms

Ammonia monooxygenase (AMO): A multi-subunit enzyme that catalyses the oxygen-dependent oxidation of ammonia to hydroxylamine.

Hydroxylamine oxidoreductase (HAO): A multi-haeme enzyme responsible for converting hydroxylamine to nitrite, coupling electron transfer to energy conservation.

Nitrification: The biological oxidation of ammonia to nitrate via nitrite, typically carried out in two sequential enzymatic steps by specialised bacteria or archaea.

Nitrite (NO2–): The intermediate product of ammonia oxidation, which serves as the substrate for subsequent oxidation to nitrate by nitrite-oxidising microorganisms.

References

1. Alcohols as inhibitors of ammonia oxidizing archaea and bacteria. FEMS Microbiology Letters (2023).
2. Oxidation of ammonia by Nitrosomonas europaea. Definite 18O-tracer evidence that hydroxylamine formation involves a monooxygenase.. Journal of Biological Chemistry (1981).
3. Effect of Salinity on Hydroxylamine Oxidation in a Marine Ammonia-Oxidizing Gammaproteobacterium, Nitrosococcus oceani strain NS58: Molecular and Catalytic Properties of Tetraheme Cytochrome c-554. Microbes and Environments (2010).