Question

Explain the Bohr effect. How does pH (and pKa) and CO2 affect the binding affinity of oxygen to hemoglobin in the lungs and in tissues? Which of the specific amino acids in the hemoglobin molecule are involved in facilitating the interactions that affect oxygen binding affinity. How is CO2 transported from the tissues to the lungs?

Answer 1

Bohr effect describes the affinity of hemoglobin for oxygen when there is any change in the pH or carbon dioxide level. When carbon dioxide level in the body increases, or when there is a decrease in the blood pH (acidic conditions), the affinity of hemoglobin for oxygen decreases, as a result of which, more oxygen is given up by hemoglobin in the tissues. This can be observed by a right-ward shift in the oxygen dissociation curve.

Decrease in pH or increase in the concentration of carbon dioxide in the blood lowers the affinity of hemoglobin for oxygen, both in the lungs and in the tissues. This is explained by the following graph.

Thus, at a particular partial pressure of oxygen in the lungs, the saturation of hemoglobin with oxygen is less (that is, the partial pressure of oxygen needs to be increased in order to achieve the level of saturation, that was obtained in case of physiological pH and normal carbon dioxide level). This also indicates that oxygen will be dissociated from hemoglobin more easily at the tissues, because of reduced affinity.

The amino acid Histidine is important in providing a proper oxygen binding affinity. There are two specific histidine molecules present in hemoglobin- proximal and distal. Oxygen binds to iron of the heme group and distal histidine. Binding with the distal histidine results in a stable interaction between hemoglobin and oxygen.

Carbon dioxide from tissues to the lungs is transported in three ways-

1. By binding to hemoglobin as carbaminohemoglobin.

2. By dissolving in the blood.

3. Mostly as bicarbonate ion. Carbon dioxide in the blood reacts with water and in presence of carbonic anhydrase, it dissociates into hydrogen ion and bicarbonate ion. This bicarbonate ion is transported to the lungs with the help of blood plasma.