Can an endergonic reaction with a ΔG of 12 kcal/mol be driven by ATP hydrolysis?

Can an endergonic reaction with a ΔG of 12 kcal/mol be driven by ATP hydrolysis?

• A. Yes, because ATP hydrolysis always makes ΔG negative
• B. No, the overall ΔG is still positive
• C. Yes, if an enzyme is present
• D. No, ATP hydrolysis cannot affect the reaction

Answer: (B)

Thermodynamics of coupled reactions: the overall ΔG must be negative for the process to proceed. The energy released by ATP hydrolysis is finite—roughly −7 kcal/mol per ATP under cellular conditions. If the endergonic step has ΔG of +12 kcal/mol, coupling it to the hydrolysis of one ATP gives +12 + (−7) ≈ +5 kcal/mol, still unfavorable. So a single ATP hydrolysis cannot drive this reaction. In principle, energy from two ATPs (about −14 kcal/mol) could make the total ≈ −2 kcal/mol, allowing the reaction to proceed, but that requires using more than one ATP. Enzymes can arrange the coupling, but they don’t change the total energetics; the sum of ΔG values determines feasibility.

Thermodynamics of coupled reactions: the overall ΔG must be negative for the process to proceed. The energy released by ATP hydrolysis is finite—roughly −7 kcal/mol per ATP under cellular conditions. If the endergonic step has ΔG of +12 kcal/mol, coupling it to the hydrolysis of one ATP gives +12 + (−7) ≈ +5 kcal/mol, still unfavorable. So a single ATP hydrolysis cannot drive this reaction. In principle, energy from two ATPs (about −14 kcal/mol) could make the total ≈ −2 kcal/mol, allowing the reaction to proceed, but that requires using more than one ATP. Enzymes can arrange the coupling, but they don’t change the total energetics; the sum of ΔG values determines feasibility.