An erythrocyte enzyme assay in a patient moving to high altitude reveals elevated levels of 2,3-bisphosphoglycerate. Which enzyme of the Rapoport-Luebering shunt directly synthesizes 2,3-BPG in red cells?

• A Enolase acting on 2-phosphoglycerate
• B Bisphosphoglycerate mutase acting on 1,3-bisphosphoglycerate ✓
• C Pyruvate kinase acting on phosphoenolpyruvate
• D Bisphosphoglycerate phosphatase acting on 2,3-BPG

Explanation

The Rapoport-Luebering shunt diverts 1,3-bisphosphoglycerate away from ATP generation: bisphosphoglycerate mutase converts it to 2,3-BPG, and bisphosphoglycerate phosphatase then hydrolyzes it to 3-phosphoglycerate, rejoining glycolysis. Elevated 2,3-BPG binds deoxyhemoglobin preferentially, shifting the oxygen dissociation curve rightward and improving tissue oxygen delivery at altitude. Option D describes the degradation step, not the synthetic one, making it the most tempting distractor.

Reference: Harper Illustrated Biochemistry, 32nd ed.

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