The Bohr Effect

This is an effect observed in hemoglobin. It was first observed by Christian Bohr (father of Niels Bohr). The Bohr Effect is the release of hydrogen ions from hemoglobin upon the binding of oxygen.

Hemoglobin exists in two states the R-state and the T-state. The R-state has a higher affinity for O2 than does the T-state. Each of these conformational states has a different set of stabilizing interactions.

The interactions responsible for the Bohr effect involve positively charged acidic groups. In the T-state these groups participate in ion pairs, and thus are stabilized in the protonated state (their pKas are raised). In the R-state these interactions are absent and consequently the acidic groups release protons. The ratio is ~0.6 protons released per O2 bound.

Importance of the Bohr Effect

The Bohr effect plays an important role in oxygen and carbon dioxide transport. The body uses a bicarbonate buffer system to maintain a steady blood pH, this is given by the equilibrium shown below.

CO2 + H2O ‹---›   H2CO3
H2CO3 ‹---›  H+ + HCO3-

The overall equilibrium is given by:

CO2 + H2O ‹---›   H+ + HCO3-

Another useful equilibrium is that of Hemoglobin (Hb) and Oxygen:

Hb + O2 ‹---›   HbO2 + H+

The conversion of CO2 to bicarbonate is mediated by the enzyme carbonic anhydrase.

The events of the transport system can be divided into two regions; the capillaries and the lungs.
The Lungs
In the lungs the partial pressure of O2 is high. This favors the binding of O2 by Hb, and thus releases protons. This proton release affects the bicarbonate equilibrium, causing it to shift towards CO2 formation. This serves to drive off the CO2 that we exhale.
The Capillaries
In the capillaries the cells produce CO2 through normal respiration. This shifts the bicarbonate equilibrium towards ionization. This increase in the hydrogen ion concentration shifts the Hb equilibrium back towards release of O2.

Thus we see that carbon dioxide is released in the lungs for exhalation, and oxygen is released in the capillaries where it is transported to muscles and other cells by Myoglobin.

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