The three reactions in the table occur in the cytosol of a cell at pH 7 and

CHEMISTRY · Multi-part · CBSE · Chemistry · Thermodynamics · Thermodynamics

Problem

The three reactions in the table occur in the cytosol of a cell at pH 7 and 298 K298\,\text{K}.
No.ReactionΔG∘′ (kJ mol−1)\Delta G^{\circ\prime}\,(\text{kJ}\,\text{mol}^{-1})
1Phosphoenolpyruvate + ADP → Pyruvate + ATP−31-31
2ATP + Mannose → Mannose-6-phosphate + ADP−25-25
3Mannose-6-phosphate → Fructose-6-phosphate+3+3
(The prime in ΔG∘′\Delta G^{\circ\prime} indicates biochemical standard state, where [H+]=10−7 M[H^+] = 10^{-7}\,\text{M}.) A metabolic engineer wants to convert phosphoenolpyruvate (PEP) and mannose directly into fructose-6-phosphate (F6P) and pyruvate without any net consumption or production of ATP or ADP. (a) Combine the provided reactions, using appropriate integer coefficients, to obtain the overall reaction that accomplishes the engineer’s objective. Show clearly how common intermediates cancel. (b) Calculate the overall ΔG∘′\Delta G^{\circ\prime} for the reaction you derived in part (a). (c) Discuss why the presence of the nucleotide intermediates ATP and ADP is essential for making the otherwise unfavorable isomerization of mannose-6-phosphate to fructose-6-phosphate proceed spontaneously. (d) Suppose reaction 1 were replaced by the simple hydrolysis of PEP: Phosphoenolpyruvate + H2OH_2O → Pyruvate + Pi  ΔG∘′=−62 kJ mol−1\Delta G^{\circ\prime} = -62\,\text{kJ}\,\text{mol}^{-1}. Re-calculate the overall ΔG∘′\Delta G^{\circ\prime} for the pathway under this substitution and comment on whether the pathway would be more or less thermodynamically favorable.

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