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FILL IN EVALUATION FORM |  |
CO-OPERATIVE BINDING OF OXYGEN
BY HAEMOGLOBIN
Binding of oxygen to haemoglobin is known as co-operative binding because the binding of successive O2 molecules facilitates binding of the next. Binding of the 1st O2 molecule increases the affinity of haemoglobin for oxygen and hence facilitates the binding of the 2nd O2 molelcule. Binding of the 2nd O2 molecule facilitates the binding of the 3rd O2 molecule and so on. The affinity of haemoglobin for the 4th O2 molecule is approximately 300 times that for the 1st. This co-operative binding explains the sigmoidal shape of the oxygen dissociation curve.
Haemoglobin can exist in two conformational states:
Relaxed (R) state - this state corresponds to the quaternary strucure of oxyhaemoglobin & favours oxygen binding
Tense (T) state - this state corresponds to the quaternary structure of deoxyhaemoglobin & has a lower binding affinity for oxygen
Binding of oxygen causes a change in the conformational state of the haemoglobin molecule bringing about a change in the position of the haem groups.
Transition from one state to another involves the breaking or formation of salt bridges between the polypeptide chains. When oxygen is taken up the 2 beta chains move closer together and when oxygen is released the chains move apart.
The reaction of oxygen with the iron molecule of the haem group is an oxygenation reaction, not oxidative, and the iron remains in the ferrous (2+) state.
Oxygen is transported as molecular O2 and is not ionically bound to the iron molecule. It is this very loose, reversible binding that enables oxygen to be taken up and released so readily.
2,3-Diphosphoglycerate:
Oxygen affinity of haemoglobin in red blood cells is lowered by a highly anionic organic phosphate known as 2,3-diphosphoglycerate (2,3-DPG or DPG, but sometimes written as 2,3-BPG). This molecule binds specifically to the deoxygenated form of haemoglobin and not the oxygenated form enabling the unloading of oxygen at the tissues.
2,3-DPG is present in red blood cells at similar concentrations to haemoglobin.
2,3-DPG binds to the central cavity of deoxyhaemoglobin by cross-linking the beta polypeptide chains. When oxygen binds to haemoglobin the two beta chains move closer together and the central cavity becomes smaller causing 2,3-DPG to be extruded from the haemoglobin molecule.