Chemistry Reference
In-Depth Information
FIGURE 18.5
The major biological nitrogen transformation pathways are linked by their associated enzymes. Genes encoding enzymes that
conduct the important transformations include those for various nitrate reductases (nas, euk-nr, narG, napA), nitrite reductases (nir, nrf), nitric
oxide reductase (norB), nitrous oxide reductase (nosZ), nitrogenase (nif), ammonium monooxygenase (amo), hydroxylamine oxidoreductase
(hao), nitrite oxidoreductase (nxr), and hydrazine hydrolase (hh).
(From
Canfield et al., 2010
.
Copyright 2010 with permission from AAAS.)
When organisms die, their nitrogen is returned to the environment as NH
4
, the fate of which depends on
whether oxygen is available or not. In the presence of oxygen,NH
4
can be oxidised to nitrate (nitrification),
primarily by soil-living bacteria in a two-stage pathway. Initially NH
4
is oxidised to NO
2
by bacteria such as
Nitrosomonas species. In the first step, ammonium monooxygenase, a copper/iron enzyme oxidises NH
4
to
hydroxylamine, which is then oxidised to NO
2
by the haem enzyme, hydroxylamine oxidoreductase. NO
2
is
subsequently oxidised to NO
3
by a different group of nitrifying bacteria (Nitrosobacter), via the haem enzyme
nitrite reductase. All of the nitrifiers use the protons and electrons generated in the oxidation of NH
4
and NO
2
to
