A student is asked to compare the reaction coordinate diagrams for an uncatalyzed and an enzyme-catalyzed hydrolysis of an ester in aqueous solution. The uncatalyzed reaction proceeds through a single transition state, whereas the enzyme-catalyzed reaction proceeds through two sequential elementary steps, each with its own transition state.
Energies (relative to reactants):
• Uncatalyzed transition state: 85 kJ mol⁻¹
• Catalyzed first transition state: 32 kJ mol⁻¹
• Catalyzed intermediate: 14 kJ mol⁻¹
• Catalyzed second transition state: 41 kJ mol⁻¹
• Products: –25 kJ mol⁻¹
(a) On a set of labeled axes (potential energy vs. reaction coordinate), sketch both the uncatalyzed and catalyzed pathways on the same diagram. Clearly indicate all transition states, the intermediate, reactants, and products.
(b) Using your diagram, identify the rate-determining step for the catalyzed pathway and justify your choice.
(c) Explain quantitatively how the catalyst affects the overall activation energy compared with the uncatalyzed reaction.
(d) Discuss whether the enzyme alters the equilibrium constant for the hydrolysis reaction. Support your answer with thermodynamic reasoning.