In erythrocytes, some 1,3-bisphosphoglycerate is diverted by bisphosphoglycerate mutase to 2,3-BPG instead of proceeding through phosphoglycerate kinase. What is the energetic consequence of this diversion for the cell?

• A No change in ATP yield because 2,3-BPG re-enters glycolysis upstream of the ATP-generating steps
• B Complete cessation of glycolytic flux distal to 1,3-BPG
• C Gain of one ATP per diverted molecule because 2,3-BPG carries a high-energy phosphate
• D Loss of one ATP per diverted molecule, since 2,3-BPG is dephosphorylated to 3-phosphoglycerate without ATP formation ✓

Explanation

The Rapoport-Luebering shunt bypasses the phosphoglycerate kinase step where 1,3-BPG would have yielded one ATP. Bisphosphoglycerate phosphatase then converts 2,3-BPG to 3-phosphoglycerate with no ATP recovery, so each diverted molecule costs the red cell one ATP. The payoff is 2,3-BPG itself, which binds deoxyhemoglobin and shifts the oxygen dissociation curve rightward. Flux continues past the shunt, so glycolysis is not halted.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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