The table below lists two biochemical reactions that occur in nitrogen metabolism at 298K.
Label
Reaction (all species aqueous, pH ≈ 7)
ΔG∘ (kJmol−1)
(1)
Glutamate + NH₄⁺ ⇌ Glutamine + H₂O
+6.3
(2)
ATP + H₂O ⇌ ADP + Pᵢ + H⁺
–30.5
A biochemist proposes coupling reaction (1) to reaction (2) so that formation of glutamine becomes thermodynamically favorable.
(a) Identify every chemical species that appears in both reactions and therefore can act as a common intermediate when the two reactions are summed.
(b) Write the overall chemical equation obtained after adding the two reactions and canceling any intermediates. State clearly which species, if any, are canceled.
(c) Calculate the standard free-energy change, ΔGoverall∘, for the coupled process. Show all work, including units and the correct sign of the final value.
(d) Explain, using thermodynamic reasoning, why the coupled reaction is spontaneous under standard biochemical conditions even though reaction (1) alone is not. In your explanation, refer explicitly to both the calculated value of ΔGoverall∘ and the concept of reaction coupling.