Structure and Bonding
Myoglobin contains a porphyrin ring with an iron center. There is a proximal histidine group attached directly to the iron center, and a distal histidine group on the opposite face, not bonded to the iron.
Many functional models of myoglobin have been studied. One of the most important is that of picket fence porphyrin by James P. Collman. This model was used to show the importance of the distal prosthetic group. It serves three functions:
- To form hydrogen bonds with the dioxygen moiety, increasing the O2 binding constant
- To prevent the binding of carbon monoxide, whether from within or without the body. Carbon monoxide binds to iron in an end-on fashion, and is hindered by the presence of the distal histidine, which forces it into a bent conformation. CO binds to heme 23,000 times better than O2, but only 200 times better in hemoglobin and myoglobin. Oxygen binds in a bent fashion, which can fit with the distal histidine.
- To prevent irreversible dimerization of the oxymyoglobin with another deoxymyoglobin species
Read more about this topic: Myoglobin
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