Energy Metabolism in Erythrocytes
Because mature erythrocytes lack mitochondria, they cannot generate energy via oxidative phosphorylation. Anaerobic glycolysis serves as their sole source of ATP.
Erythrocytes lack the enzymes for the citric acid cycle (e.g., the conversion of oxaloacetate to malate) or gluconeogenesis (the conversion of pyruvate to phosphoenolpyruvate). A typical glycolytic reaction here is the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate by the corresponding dehydrogenase.
Rapoport-Luebering Shunt
This is a specific side pathway of glycolysis operating in erythrocytes. It bypasses the standard reaction catalyzed by phosphoglycerate kinase and consists of two main steps:
- Synthesis of 2,3-BPG: The enzyme bisphosphoglycerate mutase (or 1,3-bisphosphoglycerate mutase) converts 1,3-bisphosphoglycerate into 2,3-bisphosphoglycerate.
- Re-entry into Glycolysis: The enzyme 2,3-bisphosphoglycerate phosphatase cleaves inorganic phosphate ($P_i$) in the presence of water, converting 2,3-BPG into 3-phosphoglycerate, which continues down the classical glycolytic pathway to 2-phosphoglycerate.
Energy Balance of Glycolysis
The presence of the shunt significantly alters the net energy yield in the erythrocyte.
- Standard glycolysis (without shunt): The cell consumes 2 molecules of ATP (for hexokinase and phosphofructokinase-1 reactions) but produces 4 ATP (at the phosphoglycerate kinase and pyruvate kinase steps). The net gain is +2 ATP.
- Glycolysis involving the shunt: Because the phosphoglycerate kinase reaction is bypassed, substrate-level phosphorylation does not occur at this step. ATP consumption remains at 2 ATP, while synthesis occurs only at the pyruvate kinase step (+2 ATP). The net gain drops to 0 ATP.
Role of 2,3-Bisphosphoglycerate in Hypoxia
The 2,3-BPG synthesized during the shunt acts as a vital allosteric regulator of hemoglobin. It binds tightly to deoxyhemoglobin, stabilizing it in the tense (T-state).
This interaction decreases hemoglobin's affinity for oxygen and shifts the oxyhemoglobin dissociation curve to the right. Under hypoxic conditions, this mechanism is crucial: it facilitates oxygen release to peripheral tissues to compensate for oxygen deprivation, even though the erythrocyte itself sacrifices energetic efficiency (net ATP synthesis drops to zero).