The structure for hemoglobin is very similar to myoglobin
except that it has a quaternary structure due to the presence
of four protein chain subunits. Actually, there are two pairs
of similar chains - two alpha types and two beta types. Each
protein chain subunit contains a heme group with the iron attached.
Each hemoglobin molecule can bind to a total of four oxygen molecules.
Individually, each of the chains is not that much different
than that of myoglobin, so the same principles apply that were
discussed previously. The diatomic oxygen molecules are shown
bonded to the heme group.
The specific intrachain and interchain bonding interactions,
are well known at this time. There are some changes in the structure
of the deoxyhemoglobin when oxygen reacts to form oxyhemoglobin
which is shown in the graphics on the left.
Hemoglobin - Chime
in new window
Oxygenation of the hemoglobin occurs in the blood as it passes
through the lungs. Oxygen in the air diffuses through the walls
of the lungs into surrounding blood capillaries. As oxygen binds
to the deoxyhemoglobin, a hydrogen ion is released. The hydrogen
ion in turn causes the release of carbon dioxide from the blood
into the lungs.
As the first oxygen is bond to one of the four heme groups,
the protein begins to undergo small shifts in the structure,
but sufficient to expel BPG (2,3-bisphosphoglycerate ion). This
molecule resides in the "cleft" between the protein
chains. See whether you can see this in the Chime model.
As soon as the BPG is expelled, a cascade of events causes
the other three heme groups to bind to three more oxygen molecules
in fast succession.
When the oxyhemoglobin reaches the vicinity of the tissue
cells, carbon dioxide, in excess from metabolism which has used
up the oxygen supplies, causes an increase in hydrogen ions.
The increase in hydrogen ions then initiates the process for
the hemoglobin to release the oxygen to the cells. Again BPG
begins to work its way into the hemoglobin and causes faster
loss of the oxygen molecules.
|Quiz: What is the definition
of quaternary structure?