Question

Hemoglobin (Hb) can be viewed as having two quaternary states, a low oxygen affinity state (T), and a high oxygen affinity state (R). Which of the following statements about the binding of O₂ by Hb are true?

1) Upon binding a molecule of oxygen, Hb undergoes a conformational change that makes the binding of subsequent O₂ molecules easier.
2) The conformational change induced in Hb upon binding oxygen is the result of a small movement (0.2 Å) of the iron cation in the center of heme.
3) Site-directed mutagenesis studies have indicated that the cooperativity of O₂ binding in Hb is attributable to the movement of the F helix in Hb.
4) Site-directed mutagenesis studies in which the proximal His residues of the F helix have been replaced by glycines have indicated the mutant protein still shows cooperativity of O₂ binding.

	Only statements 1, 3, and 4 are true.
	Only statements 1, 2, and 4 are true.
	Only statements 1, 2, and 3 are true.
	All of the listed statements are true.

Answer 1

Only statements 1,2 and 3 are true.

When oxygen molecule binds to Fe atom of a heme group through its vacant sixth coordination site, the iron becomes low spin and therefore becomes more smaller in radius. As a result low spin Fe moves towards porphyrin plane and just fits in the hole generated by the four generated by four coordinating nitrogens of the porphyrin plane. This automatcally pulls the coordinated histidine. These changes in heme unit due to its combination with O2 forms the tigger of cooperativity phenomenon.

Statement 4, is wrong as His is important for the functioning Hb.