2. Bohr effect: The release of O2 from hemoglobin is enhanced when the pH is lowered (proton concentration [H+] is increased) or when the hemoglobin is in the presence of an increased pCO2. Both result in decreased oxygen affinity of hemoglobin and, therefore, a shift to the right in the oxygen-dissociation curve (Fig. 3.9). Both, then, stabilize the T (deoxy) form. This change in oxygen binding is called the Bohr effect. Conversely, raising the pH or lowering the concentration of CO2 results in a greater oxygen affinity, a shift to the left in the oxygen-dissociation curve, and stabilization of the R (oxy) form.
a. Source of the protons that lower pH: The concentration of both H+ and
CO2 in the capillaries of metabolically active tissues is higher than that observed in alveolar capillaries of the lungs, where CO2 is released into the expired air. In the tissues, CO2 is converted by zinc-containing carbonic anhydrase to carbonic acid: which spontaneously loses a H+, becoming bicarbonate (the major blood buffer):
The H+ produced by this pair of reactions contributes to the lowering of pH. This differential pH gradient (that is, lungs having a higher pH and tissues a lower pH) favors the unloading of O2 in the peripheral tissues and the loading of O2 in the lung. Thus, the oxygen affinity of the hemoglobin molecule responds to small shifts in pH between the lungs and oxygen-consuming tissues, making hemoglobin a more efficient transporter of O2.
b. Mechanism of the Bohr effect: The Bohr effect reflects the fact that the deoxy form of hemoglobin has a greater affinity for H+ than does oxyhemoglobin. This is caused by ionizable groups such as specific histidine side chains that have a higher pKa (see p. 6) in deoxyhemoglobin than in oxyhemoglobin. Therefore, an increase in the concentration of H+ (resulting in a decrease in pH) causes these groups to become protonated (charged) and able to form ionic bonds (salt bridges). These bonds preferentially stabilize the deoxy form of hemoglobin, producing a decrease in oxygen affinity. [Note: Hemoglobin, then, is an important blood buffer.]
The Bohr effect can be represented schematically as: where an increase in H+ (or a lower pO2) shifts the equilibrium to the right (favoring deoxyhemoglobin), whereas an increase in pO2 (or a decrease in H+) shifts the equilibrium to the left.
3. 2,3-BPG effect on oxygen affinity: 2,3-BPG is an important regulator of the binding of O2 to hemoglobin. It is the most abundant organic phosphate in the RBC, where its concentration is approximately that of hemoglobin. 2,3-BPG is synthesized from an intermediate of the glycolytic pathway (Fig. 3.10; see p. 101 for a discussion of 2,3-BPG synthesis in glycolysis).
a.
2,3-BPG binding to deoxyhemoglobin: 2,3-BPG decreases the oxygen affinity of hemoglobin by binding to deoxyhemoglobin but not to oxyhemoglobin. This preferential binding stabilizes the T conformation of deoxyhemoglobin. The effect of binding 2,3-BPG can be represented schematically as: b.