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Nitrogen cycle

N 2 N O
2
Biogenic Non- Denitri-
nitrogen biogenic NO fication
fixation nitrogen
fixation
Nitrification
NH 3 Nitrate Nitrite
Nitrite
Ammonification
Degra-
dation Assimilation

{C-N}

Figure 22: The nitrogen cycle. Processes involving the –III oxidation state of nitrogen are in
red. For nitrite and nitrate reductases see the previous chapter

In Fig. 23, the organisation of the metal centres of nitrogenase is depicted. The electrons
necessary for the reduction of dinitrogen are delivered by an iron protein containing a cubane-
-
like [4Fe,4S] ferredoxin. Primary e acceptor is the P cluster of the FeMoco, the iron-
2+
molybdenum-cofactor. Two ATP (activated by Mg ) are hydrolysed per electron transferred.
The FeMoco contains two P and two M clusters, arranged in such a way that the complete
cofactor attains C 2 symmetry. The P cluster is a double cubane containing the Fe 8S 7 core. The
reduction equivalents are then further transported to the M cluster, a Fe 7MoS 9 core, which is
responsible for the final activation and reduction of N 2. The cage formed by the metal centres
of the M cluster contains electron density which can be interpreted in terms of a nitrogen atom.
The M cluster is connected to the protein matrix by just one Cys and a His, the latter
coordinated to Mo. The coordination environment of Mo is supplemented by the vicinal
hydroxide and carboxylate of homocitrate. In which way activation and reduction of N 2 takes
place is unknown. In the case of insufficient molybdenum supply, or at low temperatures, a
vanadium-nitrogenase is activated (which is more efficient at lower temperatures than the Mo
version). An iron-only nitrogenases is also known.

-
M -Cluster CH2CO2 Gln
M
S Fe S Fe S O C -
S CH2CH2CO2
(Cys)S Fe S Fe Fe S Mo
N O-C
O
S Fe Fe S N(His)
S

Figure 23. Organisation of nitrogenase (top), and the structure of the M cluster (bottom).