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Myoglobin is a simple oxygen
transport protein. Oxygen is carried to the myoglobin via hemoglobin. It
is then released to the muscle cells for respiration.
globin. This chain is
made up of seven alpha helical and six non-helical segments. The helical
segments are designated with letters A through H.
protoporphyrin IX shown
here in ball and stick representation. In
the centre of this group resides the iron ion. An iron-porphyrin group is
called a heme. In myoglobin, the heme
group is located in the ‘V’ shaped pocket between helices E and F (red). The oxygen
is coloured in red, nitrogen is coloured in blue, iron(II) is coloured in light
proximal histidine, is the only connection of
heme group with the protein. It is located on the helix F. This amino acid is
covalently bonded to the iron ion.
vs O2 discrimination.
In order to gain a better
insight on how myoglobin discriminates between O2 and CO, model
systems have been developed.
The main synthetic challenge represents the synthesis of a pentacoordinated iron(II) complex with a sixth
coordination site hindered to prevent Fe(III)-OO-Fe(III) bridging.
groups located on positions 5, 10, 15 and 20 of porphyrin ring.
2-methylimidazole occupies the fifth coordination place on the
iron(II) ion at the bottom of the ring and plays the role of the proximal
histidine. The "fence" effectively protects sixth coordination site
having the effect similar to that of the distal histidine and valine 68. In
this complex iron also lies approximately 0.4 angstroms out of the plane of the
tetradentate ring. It is displaced towards the imidazole ligand. This can be
seen better if you change the view to stick or wireframe.
Small molecules, like O2, can still reach the iron
center. Much like in oxy myoglobin, the O2 molecule binds end-on to the
Fe(II) ion. You'll note that there are actually three O atoms around the metal
centre. This is due to the disorder in the structure. The disorder occurs
because the dioxygen molecule adopts at least two orientations with respect to
a rotation around the Fe - O bond, even in the solid state. The X-ray single
crystal analysis "picks up" only the average. Hence the exact
position of the second oxygen atom cannot be precisely determined.
Other substituted porphyrin ligands have also been developed.
It contains a so-called “capped” porphyrin ligand . This “cap”, like dome, almost completely hinders one side of the ring causing severe distortions in the structure of carbonyl complex.
you want to learn more about myoglobin models see: