Catalysis fundamentals: most enzymes accelerate reactions primarily by what effect on the activation energy barrier (Ea)?

Introduction / Context:
Enzymes are biological catalysts that markedly increase reaction rates. Understanding how they achieve rate enhancement clarifies why pathways can proceed rapidly at physiological temperatures and pH without changing the net thermodynamics of the overall reaction.

Given Data / Assumptions:

• Activation energy (Ea) is the kinetic barrier between reactants and products.
• Rate depends exponentially on Ea according to Arrhenius-type relationships.
• Enzymes bind and stabilize the transition state relative to the ground state.

Concept / Approach:
By preferentially binding the transition state, enzymes lower the free-energy difference between reactants and the transition state (ΔG‡), thereby increasing the fraction of molecules crossing the barrier per unit time. The standard free-energy change ΔG°' of the overall reaction remains the same; enzymes do not alter equilibrium positions, only the path to reach them faster.

Step-by-Step Solution:

Verification / Alternative check:
Transition-state analog inhibitors bind enzymes tightly, supporting the idea of preferential transition-state stabilization as the central mechanism.

Why Other Options Are Wrong:

• Increasing Ea would slow reactions, not accelerate them.
• No change to Ea would not explain large rate enhancements.
• Enzymes do not make endergonic reactions spontaneous without coupling to an exergonic process (e.g., ATP hydrolysis).

Common Pitfalls:
Confusing kinetic acceleration with shifting equilibrium; enzymes speed both forward and reverse reactions equally when uncoupled.

Final Answer:
They decrease the activation energy