Bohr Effect - Mechanism

Mechanism

In deoxyhemoglobin, the N-terminal amino groups of the α-subunits and the C-terminal histidine of the β-subunits participate in ion pairs. The formation of ion pairs causes them to decrease in acidity. Thus, deoxyhemoglobin binds one proton (H+) for every two O₂ released. In oxyhemoglobin, these ion pairings are absent and these groups increase in acidity. Consequentially, a proton is released for every two O₂ bound. Specifically, this reciprocal coupling of protons and oxygen is the Bohr effect.

Additionally, carbon dioxide reacts with the N-terminal amino groups of α-subunits to form carbamates:

R−NH₂ + CO₂ R−NH−COO- + H+

Deoxyhemoglobin binds to CO₂ more readily to form a carbamate than oxyhemoglobin. When CO₂ concentration is high (as in the capillaries), the protons released by carbamate formation further promotes oxygen release. Although the difference in CO₂ binding between the oxy and deoxy states of hemoglobin accounts for only 5% of the total blood CO₂, it is responsible for half of the CO₂ transported by blood. This is because 10% of the total blood CO₂ is lost through the lungs in each circulatory cycle.