solved by nature. Since nitrogen is a constituent of nearly all biomolecules,
nitrogen is essential for life. Although N 2 is abundant in the atmosphere,
molecular nitrogen is not reactive and most organisms are unable to directly
metabolize this source. Fortunately, certain prokaryote organisms have
acquired the ability to reduce dinitrogen to ammonia; these organisms play
a critical role in the nitrogen cycle (Figure 3.11). The cellular mechanism
that performs this reaction is the nitrogenase enzyme system. Nitrogenase
consists of two components, the iron protein and the molybdenum iron
protein, named according to the metal cofactors of each protein.
In nitrogenase, the substrate reduction is a multi-electron process:
N 2 +8H++8e−+16MgATP→2NH 3 +H 2 +16MgADP +16Pi (3.39)
This multi-step reaction involves three types of electron-transfer reaction
(Figure 3.13). The iron protein is reduced by cellular electron carriers such
as ferredoxin and flavodoxin. The reduced-iron protein forms a complex
with the molybdenum protein and transfers an electron in a MgATP-
dependent process. Electrons are transferred within the molybdenum
protein to the substrate at the active site. Nitrogenase is a relatively slow
enzyme, with a turnover time per electron of 5 s−^1 in comparison with a
rate of over 1000 s−^1 for many other enzymes, The slow rate reflects the
complexity of the reaction and the requirement of two different proteins.
As was found for the industrial application, the key to the enzymatic
process is the presence of metals. In nitrogenase, the substrate binds to a
large metal complex that consists of a molybdenum atom and several iron
atoms. The precise mechanism by which the enzyme is able to create
ammonia is still not fully understood and research is underway to under-
stand how this protein can perform the same reaction that requires
extremely high temperatures and pressures in the human-made protocol.
Part of these efforts have centered on the determination and characteriza-
tion of the metal cofactors using X-ray diffraction (see Chapter 15).
Figure 3.13A scheme showing the involvement of different molecular
complexes in the biological process of nitrogen fixation. Adapted from
Rees and Howard (1999).
68 PARTI THERMODYNAMICS AND KINETICS
FdOx
(Fld)
FdRed
(Fld)
ADP
Ox Red
Fe protein
2ATP
Red Native
exchange
ATP
Fe protein
2ADP
Fe protein
2ATP
MoFe protein
MoFe protein
SubstrateOx SubstrateRed
MoFe protein
