Adenosine triphosphate (ATP) is often described as the cell’s “energy currency.” The hydrolysis of ATP to adenosine diphosphate (ADP) and inorganic phosphate (Pi) in aqueous solution can be written schematically as
ATP(aq) + H2O(l) → ADP(aq) + Pi(aq)
Biochemists frequently study the energetics of ATP hydrolysis by coupling the process to a two-step laboratory reaction sequence.
Step 1 Glucose(aq) + Pi(aq) → Glucose-6-phosphate(aq) + H2O(l) ΔH° = +14.0 kJ mol⁻¹
Step 2 ATP(aq) + Glucose-6-phosphate(aq) → ADP(aq) + Glucose(aq) + Pi(aq) ΔH° = –46.5 kJ mol⁻¹
Answer the following.
(a) Using the two-step sequence, calculate the standard molar enthalpy change for ATP hydrolysis as written above.
(b) A separate calorimetric determination reports ΔH° = –30.5 kJ mol⁻¹ for ATP hydrolysis under identical conditions. Compare this literature value with your result from part (a) and determine the percent difference.
(c) Propose one biochemical or experimental factor that might explain the difference between the literature value and your calculated value. Provide a mechanistic rationale linked to the reaction environment.
(d) Discuss how the concept of energy coupling in biological systems relies on the property illustrated by your calculations in parts (a) and (b).