Question

Binding of oxygen to both myoglobin and hemoglobin in causes a conformational change in which a proximal histidine is moved toward the plane of the heme ring. In hemoglobin, this results in a sigmoidal O2 binding whereas myoglobin exhibits hyperbolic O2 binding. Explain this difference.

Answer 1

The hemoglobin's O₂ binding curve is sigmoidal curve due to co-operativity. We know, hemoglobin have 4 heme group. In deoxyhemoglobin, Fe²⁺ is in high spin state and paramagnetic. But in oxyhemoglobin, Fe²⁺ is in low spin state and diamagnetic. Fe²⁺ have 28% more size in high spin state compared to low spin state which is large enough not to fit in to the center of porphyrin ring. So the hemoglobin will distort the bonds around Fe in order to bind with oxygen. This will lead to co-operativity.

Co-operativity simply means, change in structure of one heme group will act as trigger which changes the structure of other three heme groups in the hemoglobin. ie, as said this is the conformational change in which a proximal histidine is moved toward the plane of the heme ring.This will help O₂ to bind, more easy, to the 2^nd 3^rd and 4^th heme groups.

(Oxyen molecule binding with one of the heme group of hemoglobin)

(binded Oa N- te histidine -2 2

And when hemoglobin reaches tissues, there is increased amount of CO₂. It will decrease the pH. So at acidic deoxygenation process favored(Bohr effect). Thus O₂ is released in tissues. So, increased amount of CO₂ and low pH(acidic) causes the decrease in affinity of O₂ in hemoglobin.

In the case of myoglobin, we know, it contains only one heme group. So co-operativity is not observed in myoglobin. This will give a hyperbolic curve. Myoglobin is found in the muscle tissue and there myoglobin strongly binds oxygen molecule. When required, myoglobin releases oxygen which is used by muscle cells.

O₂ binding affinity of hemoglobin is weaker than O₂ binding affinity of myoglobin. Because, hemoglobin travels from lungs to tissues and vice versa. In lungs O₂ pressure is high. So there hemoglobin binds with oxygen molecule. When it reaches tissues, there high CO₂ and low pH will lead to release of oxygen molecule. But in case of myogobin, it can be found in muscle tissues and it's function is to store oxygen which is brought by the hemoglobin and release oxygen only when it is needed by the muscle tissues. So myglobin bind oxygen more strongly than hemoglobin.

(O₂ Binding Curve for hemoglobin and myoglobin)

CMyoglobin uew Meiglobin) ptia presture turation