Figure 9.03. Increasing 2,3-diphosphoglyceric acid (DPG) concentration causes the hemoglobin-oxygen association-dissciation curve to shift to the right, ie. to decrease Hb's affinity for oxygen (P50 values are 15, 20, 27, 34, and 42 mm Hg from left to right).
DPG, a by-product of the normal glycolytic pathway, is formed in erythrocytes, and modulates (ie. changes) hemoglobin's affinity for oxygen. It decreases Hb's affinity for oxygen by stabilizing the deoxy form of hemoglobin so that it does not load oxygen so readily.
DPG's synthesis is increased by the alkalinity of the blood. DPG attaches to the alpha and beta chains of adult Hb, but not to Hb F of fetal hemoglobin so it does not modulate oxygen carriage by that species of Hb in the fetus. "Striped" Hb A, Hb having little DPG attached, like the curve to the far left, has an ODC much like Hb F.
Shortly before birth and afterward, Hb F is replaced by Hb A and DPG levels rise as well. Both of these changes act to shift the ODC to the right (ie. raise P50, lower affinity).