Single answerDifficulty 3/5 · ChallengingOlympiadConcept explanationArgumentation
Making glutamine uphill
Topic: Energy coupling & ATP
Glutamine synthetase makes the amino acid glutamine. On its own, glutamate + NH₃ → glutamine + H₂O has ΔG°′ ≈ +14.2 kJ/mol, and ATP + H₂O → ADP + Pᵢ has ΔG°′ ≈ −30.5 kJ/mol. A biochemist sets up three tubes at 37 °C, each containing glutamate and NH₃, and measures glutamine after 30 minutes.
Data
| Tube | Also added | Glutamine formed |
|---|---|---|
| 1 | Glutamine synthetase, no ATP | None detectable |
| 2 | Glutamine synthetase + ATP | Large amount; ADP and Pᵢ appear one-for-one with glutamine |
| 3 | ATP first fully hydrolysed by a separate ATPase, then glutamine synthetase added | None detectable (the tube warmed very slightly) |
Which explanation accounts for all three tubes?
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