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Nitritereductase

-
This enzyme catalyses the deoxigenation of nitrite NO 2 to nitrogenmonoxide NO via a one-
electron reduction, one of the focal steps in dinitrification (see ch. 7). The enzyme is built up of
three identical subunits. Each of these subunits contains a catalytic type-II Cu centre (for the
activation of nitrite) and a type-I Cu centre for electron transfer (reduction of nitrite) [see inset
on p. 20 for the classification of copper enzymes). Reaction steps (cf. Fig. 17):

(1) Exchange of water for nitrite/nitrite activation (at {Cu-II})
-
+
+
(2) Formation of nitrosyl: NO 2 + 2H → NO + H 2O
-
+
(3) NO + e → NO (by {Cu-I})
(4) Reestablishment of the starting situation by exchange of NO for H 2O

N H O
2
(His)N OH 2 NO 2 - (His)N O O + O N
Cu Cu 2H (His)N
(His)N 2+ N(His) (His)N 2+ N(His) (His)N Cu N(His)
2+
H O
2
+
{Cu }
S(Met)
2+
{Cu } NO
{Cu} = (His)N Cu S(Cys)
N(His) (His)N OH 2
Cu
(His)N 2+ N(His)

Figure 17. The course of reaction catalysed by nitritereductase.

Oxigenases

Cytochrome P 450 is an oxygenase belonging to the haeme-type proteins. Axial ligands are a
cysteinate and – in the resting state – water. In the active state, the position occupied by water
is free. Cyt-P 450 detoxifies hydrophobic substrates (such as benzene) in the liver by conversion
to hydrophilic compounds (such as phenol) which are than secreted. The overall reaction can
be formulated as shown:

+
-
RH + O 2 + 2H + 2e → ROH + H 2O

The substrate RH is bonded by hydrophobic interaction into the protein pocket close to the
active centre of the active form of the enzyme. The course of reaction is illustrated in Fig. 18.
II
II
III
In the first step, Fe is reduced to Fe , followed by oxidative addition of O 2 (Fe + O 2 →
-
III
-
Fe -O 2 ), i.e. O 2 is reduced to superoxide, and further – by an external e source – to peroxide:
III
III
2-
III
-
-
Fe -O 2 + e → Fe -O . In the succeeding step, Fe transfers 2 electrons to the peroxo ligand.

One of the oxo groups is released and trapped by two protons to form water. The other oxo
V
group remains coordinated to iron. The intermediate thus formed can be formulated with Fe
IV •+
IV
IV
V
or Fe : {O=Fe } ↔ {O=Fe } , in the latter formulation (with Fe ) as a radical cation, with
the radical character being dislocated over the oxygen and part of the protein matrix. In the
final step, a hydrogen atom of the substrate is transferred to the ferryl oxygen, and the {OH}
transferred back to the substrate radical: