Mechanism of Action - H⁺ and CO2 function rapidly to facilitate the exchange of O2 and CO2 in the respiratory cycle. Like the effects of H⁺ and CO2, binding of 2,3-bisphosphoglycerate (BPG) (Figure 7.17) acts to lower the oxygen affinity of hemoglobin. BPG binds in the cavity between the chains (Figure 7.18), making electrostatic interactions with positively charged groups surrounding this opening. Comparison of the two hemoglobin conformations shown in Figure 7.12b reveals that this opening is much narrower in oxyhemoglobin than in deoxyhemoglobin. In fact, BPG cannot be accommodated in the oxy form. The higher the BPG content in red blood cells, the more stable the deoxy structure will be. A decrease in O2 affinity (oxygen release) is explained by stabilization of the deoxy structure. Increased BPG levels are also found in the blood of smokers who, because of the carbon monoxide in smoke, also suffer from a limitated oxygen supply.

Fetal Hemoglobin and BPG - A fetus must obtain oxygen from the mother's blood by exchange through the placenta. Fetal blood, therefore, must have a higher O2 affinity than the mother's blood. The human fetus has a hemoglobin different from the adult form. In the fetus the chains are replaced by similar, but distinctly different, polypeptides. These are called chains, so fetal hemoglobin (Hbf) has an 22 structure. The intrinsic oxygen affinity of Hbf is very similar to that of HbA, but Hbf has a much lower affinity for BPG than does HbA. This difference is largely due to the replacement of His 143 in the adult chain by a serine in the fetal chain. The positively charged His 143 in adult chains helps to bind the negatively charged BPG molecule, favoring the deoxy form (Figure 7.18). The concentration of BPG is about the same in the circulatory systems of mother and fetus. Under these conditions, Hbf will have less BPG bound than will HbA, and therefore Hbf will have a higher oxygen affinity at the same BPG concentration.

Other Effectors for Oxygen Release - The blood of birds contains inositol hexaphosphate (Figure 7.17), and fish use ATP for a similar purpose. All of these molecules have a strong negative charge and bind in the central cleft of deoxyhemoglobin. All of these allosteric effectors, including H⁺, CO2, and BPG, act in the same general manner-by biasing the conformational equilibrium in hemoglobin toward the deoxy form, favoring oxygen release. However, they interact at distinctly different sites, and therefore their effects can be additive, as illustrated for CO2 and BPG in Figure 7.19.