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Cu,Zn-Superoxidedismutase contains a catalytically active type-I copper centre and a
-
structural zinc centre, linked by bridging His ; Fig. 21. The enzyme catalyses the
disproportionation (dismutation) of superoxide to hydrogenperoxide and oxygen.

(Arg)
NH
+ 2

H H
O
(His)N II N(His)
Cu N(His)
(His)N N - N Zn N(His) Figure 21. Active centre of Cu,Zn-superoxidedismutase

O(Asp)
-
+
Overall reaction: 2O 2 + 2H → H 2O 2 + O 2

Reaction sequence:
II
-
II
-
(1) Cu (H 2O) + O 2 → Cu (O 2 ) + H 2O (exchange of water for hyperoxide)
-
-
II
I
-
-
(2) Cu (O 2 ) + O 2 → Cu (O 2 ) + O 2 (oxidation of external O 2 to O 2)
I
-
+
II
-
-
(3) Cu (O 2 ) + H → Cu (HO 2 ) (internal reduction of O 2 to peroxide at the Cu centre)
II
-
II
+
(4) Cu (HO 2 ) + H + H 2O→ Cu (H 2O) + H 2O 2 (separation of hydrogenperoxide)

7. Nitrogen fixation

Nitrogen fixation is the biogenic and non-biogenic transformation of elemental N 2 into
nitrogen compounds, affording to overcome the bonding energy between the two trebly bonded
nitrogen atoms (949 kJ/mol).The biogenic fixation, carried out by free living nitrogen-fixing
bacteria (Azotobacter) and cyanobacteria (“blue-green algae”, Anabaena), some archaea, and
by symbiotic bacteria associated with leguminous plants (Rhizobium), leads to ammonium
ions. Biogenic fixation accounts for about 60% of the overall nitrogen supply. Non-biogenic
non-anthropogenic fixation, which can occur by electric discharge (lightning) in the
troposphere, and by cosmic radiation in the stratosphere [N 2 → 2N; N + O 2 → NO + O →
NO x], accounts for 10%. For the overall conversions cf. Fig. 22. The remaining 30% of world-
wide N 2 fixation go back to the Haber-Bosch process and combustion of fossil fuels (natural
gas, coal, crude oil) and products produced from crude oil (petrol, gasoline, diesel).

Comparison between the Haber-Bosch process and the biogenic N 2 fixation:
Haber-Bosch biogenic
+
-
+
N 2 + 3H 2 ' 2NH 3 N 2 + 10H + 8e → 2NH 4 + H 2
(energy driven by: 16ATP + 16H 2O → 16ADP + 16P i)
Temperature: 500 °C Temperature: ca. 20 °C
Pressure: 200-450 bar Pressure: 1 bar
Catalyst: Fe (+ Al 2O 3 + K 2O + …) Catalyst: Nitrogenase (Fe/Mo- or Fe/V-S cluster)
Yield: 17% Yield: 75%
8
8
Annual production ca. 10 t Annual production ca. 10 t