Environmental Engineering Reference
In-Depth Information
H
2
is an unavoidable byproduct of N
2
fixation. The evolution of H
2
reduces
the efficiency of N
2
fixation by consuming ATP. Many diazotrophs have an
uptake hydrogenase that catalyzes the recapture of the energy lost in H
2
pro-
duction. It is impossible to eliminate the hydrogenase reaction coincident with
N
2
fixation apparently because hydride ions must be bound to the active site
prior to binding N
2
[29].
2.2 Nitrogenase Structure and N
2
-fixation Genes
Nitrogenase is composed of two multi-subunit metalloproteins, termed Com-
ponent I (dinitrogenase, or the MoFe protein) and Component II (dinitrogenase
reductase, or the Fe protein). Both proteins have Fe-S reaction centers. The Fe
protein of Component II passes electrons to the MoFe protein, thereby reducing
the MoFe protein and allowing the reduction of N
2
to occur at the active site of
Component I [53]. Each cycle of N
2
fixation requires association of Component
I and II, hydrolysis of 2 MgATP, transfer of an electron from Component II to
Component I, and dissociation of the two proteins [17]. The rate-limiting step
for the reaction appears to be the dissociation of Component I and II [102]. The
cycle is repeated, with the Component I protein progressing to increasingly
reduced states until 8 electrons are transferred and N
2
is formed [17]. Nitro-
genase has a relatively slow turnover time of 5 electrons per second, thereby
catalyzing the reduction of 1 molecule of N
2
in 1.25 seconds [98, 102].
N
2
is not present the only substrate for nitrogenase. If N is not present, hy-
drogen is
2
formed from water. Other substrates that are reduced by nitrogenase
are acetylene (to ethylene and ethane), hydrogen cyanide,hydrogen azide, nitrou
oxide,
and carbon monoxide [98]. These other compounds are sometimes
competi
tive substrates or inhibitors, depending on the concentrations of substr
-
ates and
proteins [17]. Carbon monoxide is an inhibitor for all of these other
substrates
[29].
Nitrogenases consist of three gene families that contain molybdenum (Mo),
vanadium (V), or iron Fe in
(
)
the cofactor (FeMo cofactor in the case of the Mo
They are three
different, but evolutionarily related gene families.
enzyme).
conventional (Mo nitrogenase), alternative (V nitro-
genase,
vnfHDGK
encoded),or second alternative (Fe,
anfHDGK
encoded)
nitrogenases (Fig. 1).
These enzymes are termed
The nitrogenases are metalloproteins, and both components contain Fe. Com-
ponent I of the conventional Mo containing enzyme is composed of a tetramer
of α
2
β
2
subunits each containing two pairs of FeS clusters, P clusters and one
FeMo cofactor. The FeMo cofactor is composed of MoFe
3
S
3
linked to Fe
4
S
3
with 3 sulfurs [53] that lie within the α subunits of the protein. These sites
form the presumed active site for N
2
fixation [17, 53]. The P clusters lie at the
interfaces between the α and β subunits and are involved in shuttling electrons
between the Fe protein and the active site. The P clusters are composed of
