In cellular metabolism, the conversion of glutamic acid and ammonia into glutamine requires an input of free energy (ΔG=+14.2 kJ/mol) and cannot occur spontaneously. A living cell drives this endergonic synthesis by coupling it to the exergonic hydrolysis of ATP (ΔG=−30.5 kJ/mol). Which statement best explains the biochemical mechanism by which energy coupling allows this unfavorable reaction to proceed?
- A) The enzyme absorbs the heat released by ATP hydrolysis to raise the temperature in the active site, thereby overcoming the positive standard free energy barrier.
- B) ATP binds directly to the active site as a non-covalent allosteric activator, lowering the activation energy without participating in any phosphate transfer.
- C) ATP transfers a phosphate group to glutamic acid to form a high-energy phosphorylated intermediate, which then readily reacts with ammonia to yield glutamine.
- D) The inorganic phosphate produced by ATP hydrolysis binds covalently to ammonia, making it an unstable electrophile that spontaneously attacks glutamic acid.
