What triggers the change in the tertiary structure of hemoglobin? A. The ionic r

Question

What triggers the change in the tertiary structure of hemoglobin?

A. The ionic radius of the iron ion increases, allowing the iron ion to move out of the

plane of the heme, which pulls the proximal histidine residue.

B. The ionic radius of the iron ion decreases, allowing the iron ion to move into the plane

of the heme, which pulls the proximal histidine residue.

C. The ionic radius of the iron ion decreases, allowing the iron ion to move out of the

plane of the heme, which pulls the proximal histidine residue.

D. The ionic radius of the iron ion decreases, allowing the iron ion to move into the plane

of the heme, which pulls the distal histidine residue.

E. The ionic radius of the iron ion increases, allowing the iron ion to move out of the

plane of the heme, which pulls the distal histidine residue.

Explanation / Answer

The change in the tertiary structure of hemoglobin is triggered by

B. The ionic radius of the iron ion decreases, allowing the iron ion to move into the plane of the heme, which pulls the proximal histidine residue.

Explanation: In the deoxy form, the iron ion is not completely in the plane of the pyrrole rings, in fact it is about 0.4 angstroms below the plane of the ring. This downward shift is due to the proximal histidine ligand on the bottom of the coordination complex. However, when one of the monomers binds to an oxygen molecule, the iron ion gains a sixth coordination ligand, the oxygen molecule itself, and it pulled up 0.4 angstroms to the plane of the pyrrole rings. This shift upwards also pulls the proximal histidine group up as well.

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