Chemistry Reference
In-Depth Information
N
imid
N
imid
N
imid
N
imid
+ O
2
N
N
Fe
Fe
N
N
N
~60 pm
Fe
NN
Fe
NN
NN
NN
N
- O
2
N
N
O
O
O
O
five-coordinate
square pyramidal
six-coordinate
octahedral
(side-on view)
(side-on view)
X
X
X
X
XX
X
X
linear
bent
chelated
dissociated
M
M
M
M
Figure 8.5
The geometry change at the iron centre of the heme unit in
myoglobin
or
hemoglobin
upon dioxygen
coordination. Potential modes of coordination of a diatomic molecule are also shown, with the bent
form favoured for dioxygen binding employed in the drawing of the complex.
This removes one of the common reactions of iron complexes 'in the beaker', and is an
example of the role and importance of the enveloping biopolymer in the natural system.
The Fe(II) centre, in the absence of oxygen, has a five-coordinate square pyramidal
geometry. When oxygen binds at the 'vacant' sixth site, opposite the histidine imidazole,
the molecule changes to six-coordinate octahedral geometry. This requires the Fe to move
from being displaced above the plane of the macrocycle ring (by
60 pm), as expected
for the square pyramidal shape, to lying in the plane, as expected for an octahedral shape
(Figure 8.5). In achieving this, the protein is required to adjust its conformation, to retain
the required Fe N(histidine) bond distance. Release of oxygen allows relaxation back to
the five-coordinate square-based pyramidal shape around the iron. No permanent change
in the protein is involved, and hence re-use of the protein can occur.
Another point to consider is the way dioxygen binds to the heme. In principle, there are
four ways a diatomic molecule can bind to a metal ion: linear, bent, chelated or dissociated
into two separate atoms (Figure 8.5). Examples of all four modes exist: linear is found in
carbonyl complexes M CO; bent is found in M NO
+
complexes; chelated is found for
CO in [IrCl(CO)(PR
3
)], and dissociated is found for H
2
following its addition to some four-
coordinate organometallic complexes. The very large number of atoms in a biomolecule
makes it very difficult to determine the structural detail accurately by X-ray crystal structure
analysis, which is not a problem encountered in the case of low molecular mass molecules.
The biological structure has been supported through examining low molecular weight model
compounds that bind dioxygen reversibly, and from spectroscopic methods. The 'bent'
mode (Figure 8.5) is now well defined for dioxygen binding in hemoglobin and myoglobin.
∼
8.2.1.2
Nonheme Oxygen-binding Iron Proteins
Whereas oxygen binding in humans and many other animals involves heme units, not
all life-forms bind and carry dioxygen in this way.
Hemerythrin
is a nonheme iron protein
used by
sipunculid
and
brachiopod
marine invertebrates for oxygen transfer and/or storage.
