Mechanism of the allosteric change

X-ray crystallography revealed that oxyhemoglobin, the form which has four O₂ molecules bound, differs markedly in the quaternary structure of it from deoxyhe moglobin, the form with no O2 bound. In the absence of bound O₂, the Fe²⁺ lie to one side of the porphyrin ring, which itself is curved slightly As a molecule of O² binds to heme prosthetic group it pulls Fe²⁺ into the plane of the porphyrin ring as shown in figure given below, ?attening out the ring in the process Movement of the Fe²⁺ makes the proximal histidine to move. This shifts the position of helix F and regions of the polypeptide chain at either end of the helix. Therefore, movement in the center of the subunit is transmitted to surfaces, where it causes the ionic interactions which hold four subunit together in different position, thus changing the quaternary structure, leading to cooperative binding of O₂ to Hb.