Chemistry Reference
In-Depth Information
sensitive to oxygen,
3
the plant roots produce a haemoglobin with a high affinity for oxygen, leghaemoglobin (like
haemoglobins from insect larvae or lamprey, it has the classic 'globin fold' found in mammalian haemoglobins
and myoglobins
see Chapter 3), which maintains an anerobic environment around the enzyme (
Downie, 2005
).
All nitrogenases consist of two types of subunit, one of which contains a special Fe
e
S cluster, known as the
P-cluster, and a second, which contains an iron and sulfur-containing cofactor which includes a different metal.
This metal is usually Mo, hence, the cofactor is known as FeMoCo. However in some species, and under
conditions of particular metal bio-availability, Mo can be replaced by V or even by Fe. When Mo levels are low
and V is available, these “alternative” nitrogenases contain V. When both Mo and V levels are low, a third
nitrogenase is produced, which contains only Fe. However, by far the greatest advances in our understanding of the
structure and mechanism of nitrogenases have come from studies on the MoFe-nitrogenases from free living
nitrogen-fixing bacteria like Azotobacter, Clostridium, and Klebsiella.
The overall reaction catalysed by nitrogenase is
e
8H
þ
þ
8e
þ
16P
i
As we can see, the process of nitrogen fixation is extremely energy intensive, requiring both large amounts of
ATP and of reducing equivalents. The nitrogenase is made up of two proteins (
Figure 17.11
)
, termed the MoFe
protein and the Fe protein. The
N
2
þ
16ATP
þ
16H
2
O
2NH
3
þ
H
2
þ
16ADP
þ
/
a
2
b
2
heterotetrameric MoFe protein contains both the FeMo-cofactor and the
P-cluster, with the functional unit constituted by an
ab
dimer, containing one FeMo-cofactor and one P-cluster. In
contrast, the Fe protein is a homodimer, which binds a single [4Fe
e
4S] cluster at the interface between the two
FIGURE 17.11
Structures of the nitrogenase MoFe and Fe proteins. The MoFe protein is an
a
2
b
2
tetramer, with the alpha subunits shown in
magenta and the beta subunits shown in green. The Fe protein is a
g
2
dimer, with each subunit shown in blue. A MoFe protein binds two Fe
proteins, with each
ab
unit being a catalytic unit. One Fe protein is shown associating with one
ab
-unit of the MoFe protein. The relative positions
and structures of two bound MgADP molecules, the Fe protein [4Fe
e
4S] cluster, and MoFe protein P-cluster (8Fe-7S), and FeMo-cofactor
(7Fe-Mo-9S-homocitrate-X) are shown. Each is highlighted to the right. The flow of electrons is from the [4Fe
e
4S] cluster to the P-cluster to the
FeMo-cofactor. The element colour scheme is C gray, O red, N blue, Fe rust, S yellow, and Mo magenta. Structures from PDB files 1M1N for the
MoFe protein and 1FP6 for the Fe protein.
(From
Seefeldt, Hoffman, & Dean, 2009
.
Copyright 2009, with permission from Annual reviews, Inc.)
3. All known nitrogenases are irreversibly inhibited by molecular oxygen. So, if you cut into the root nodules of a common legume like pea or
bean, you will see that it has a blood-red colour due to the high levels of leghaemoglobin.
