A biochemist is studying the biosynthesis of glutamine from glutamic acid and ammonia…

CHEMISTRY · Multi-part · A Level · Chemistry · Thermodynamics · Thermodynamics

Problem

A biochemist is studying the biosynthesis of glutamine from glutamic acid and ammonia under standard-state conditions (1 mol L−11\,\text{mol}\,\text{L}^{-1}, 298 K298\,\text{K}). The following standard free-energy changes are reported: Reaction 1: Glutamic acid + NH3_3 →\rightarrow Glutamine        ΔG∘=+14.2 kJ mol−1\Delta G^\circ = +14.2\,\text{kJ}\,\text{mol}^{-1} Reaction 2: ATP + H2_2O →\rightarrow ADP + Pi             ΔG∘=−30.5 kJ mol−1\Delta G^\circ = -30.5\,\text{kJ}\,\text{mol}^{-1} Assume that the phosphate ion produced in Reaction 2 instantly becomes part of an intermediate in Reaction 1. (a) Identify the chemical species that serves as a common intermediate between the two reactions. (b) Write the overall coupled reaction obtained by algebraically adding Reactions 1 and 2. Include all reactants and products, cancelling the intermediate. (c) Calculate the standard free-energy change, ΔG∘\Delta G^\circ (in kJ mol−1\text{kJ}\,\text{mol}^{-1}), for the overall coupled reaction. (d) Explain, in terms of thermodynamic favorability, why coupling Reaction 1 to the hydrolysis of ATP enables glutamine to be synthesized spontaneously under standard-state conditions.

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