| SYNTHESIS OF 2,3-BISPHOSPHOGLYCERATE
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| 2,3-Bisphosphoglycerate (2,3-BPG) (Fig. 11.8) is an important by-product of glycolysis in the RBC, sometimes reaching 5 mmol/L concentration, comparable with the molar concentration of Hb in the RBC. It is in fact the major phosphorylated intermediate in the erythrocyte, present at even higher concentrations than ATP (1-2 mmol/L) or inorganic phosphate (1 mmol/L). 2,3-BPG is a negative allosteric effector of the O2 affinity of Hb. It decreases the O2 affinity of deoxyhemoglobin, promoting the release of O2 in peripheral tissue. The presence of 2,3-BPG in the RBC explains the observation that the O2 affinity of purified HbA is greater than that of whole RBCs. 2,3-BPG concentration increases in the RBC during adaptation to high altitude and in anemia, promoting the release of O2 to tissues when the O2-tension and saturation of hemoglobin is decreased in the lung. Fetal Hb (HbF) is less sensitive than adult Hb (HbA) to the effects of 2,3-BPG, promoting efficient transfer of O2 across the placenta from HbA to HbF (see Chapter 4).
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Figure 11.8 Pathway for biosynthesis and degradation of 2,3-bisphosphoglycerate (2,3-BPG). BPG mutase catalyzes the conversion of 1,3-BPG into 2,3-BPG. The same enzyme has bisphosphoglycerate phosphatase activity, which hydrolyzes the 2-phosphate group, yielding 3-phosphoglycerate. Note that this pathway bypasses the phosphoglycerate kinase reaction, so that the overall yield of ATP per mol of glucose is decreased. |
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