Environmental Engineering Reference
In-Depth Information
3 Enzymes Involved in Ammonia Turnover
In addition to the interconversion of NH
3
/NH
4
and biomass, ammonia is either
produced or consumed by various microbial enzymes (Figure
1
; see Table
1
for a
comprehensive summary of some important enzymes and their properties). Accord-
ingly, ammonia is produced from dinitrogen (by nitrogenase; equation
5
) or from
nitrite, nitric oxide or hydroxylamine. The latter three reactions (equations
13
,
14
,
and
15
) are catalyzed by members of the respiratory multiheme cytochrome
c
(MCC) family (see Sections
4
and
5
for details; see Figure
2
in Section
4
for the
structure of heme
c
), of which the cytochrome
c
nitrite reductase (NrfA) is the best
characterized enzyme.
¼
þ
340 mV 3
½
ð
NO
2
þ
6e
þ
8H
þ
!
NH
4
þ
2H
2
O
0
0
NO
2
=
NH
4
13
Þ
5e
þ
5H
þ
!
NO
þ
NH
3
þ
H
2
O
ð
14
Þ
2e
þ
2H
þ
!
þ
NH
3
þ
ð
Þ
NH
2
OH
H
2
O
15
Conversely, ammonia is used as a substrate by the enzymes hydrazine synthase
(involved in the anammox process, equation
16
) and ammonia monooxygenase,
which catalyzes the first step in the nitrification process, i.e., oxidation of ammonia
to nitrate via nitrite (equation
17
).
3e
þ
3H
þ
!
NO
þ
NH
3
þ
N
2
H
4
þ
H
2
O
ð
16
Þ
2e
þ
2H
þ
!
NH
3
þ
O
2
þ
NH
2
OH
þ
H
2
O
ð
17
Þ
Hydrazine synthase has been recently purified from
Candidatus
Kuenenia
stuttgartiensis and characterized as an apparently anammox-specific heterotrimeric
complex (HzsABC) that contains two diheme cytochromes (gene products of
kuste2860 and kuste2861) [
14
,
38
]. In contrast, ammonia monooxygenase is a
membrane-integral copper-iron enzyme (see Chapter
6
in [
7
]).
4 Cytochrome
c
Nitrite Reductase as Paradigm
In principle, nitrite-reducing enzymes can be divided into two major classes.
The first class includes those enzymes reducing NO
2
to NO in the denitrification
pathway, the second class comprises those enzymes which reduce NO
2
directly to
NH
3
(Figure
1
). In bacterial denitrification, two groups of dissimilatory nitrite
reductases are known, either a cytochrome
cd
1
or a copper enzyme carrying a
type-1 and a type-2 Cu center. In nitrite-ammonifying bacteria there may exist two
different processes of nitrite reduction to ammonia, with different physiological
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