The Bohr Effect

Effect of pH Drop - A pH drop in the blood in the capillaries lowers the oxygen affinity of hemoglobin, allowing even more efficient release of the last traces of oxygen. The response of hemoglobin to changes in pH is called the Bohr effect. The overall reaction may be written

Hb-4O2 + nH+ <=> Hb-nH+ + 4O2

where n has a value somewhat greater than 2. Physiologically, this reaction has two consequences. First, in the capillaries, hydrogen ions promote the release of O2 by driving the reaction to the right. Then, when the venous blood recirculates to the lungs or gills, the oxygenation has the effect of releasing the H⁺ by shifting the equilibrium to the left. This, in turn, tends to release CO2 from the bicarbonate dissolved in the blood by the reversal of the bicarbonate reaction:

CO2 + H2O <=> HCO3^- + H+

The free CO2 can then be expired.

Mechanism of the Bohr Effect - Certain proton binding sites in hemoglobin have a higher affinity in the deoxy form than in the oxy form. A major contribution comes from histidine residue 146 at the C-terminus of each chain. In the deoxy form, His 146 can make a salt bridge with asp 94 in the same chain, if His 146 is protonated. His 146 has an abnormally high pKa because the salt bridge stabilizes the proton against dissociation. In the oxy form, however, this salt bridge simply cannot be formed, so the pKa falls to its normal value of about 6.5. Consequently, at blood pH (~7.4), His 146 is largely unprotonated in oxyhemoglobin. Therefore, a high concentration of protons, which favors protonation, also favors the deoxy form and thus promotes the release of oxygen.

Other residues are also involved in the Bohr effect, including the N-terminal amino groups of the chains. The basic mechanism is the same as for His 146. That is, protons associated with the residues are allosteric effectors favoring the deoxy conformation. The effect of lowering pH on the oxygen affinity of hemoglobin is illustrated in Figure 7.16. Note that a decrease in pH of only 0.8 units shifts the P50 from less than 20 mm Hg to over 40 mm Hg, more than doubling the oxygen unloaded to myoglobin.