In enzyme-catalyzed reactions, what term describes the energy barrier that enzymes lower to speed up reactions?

• A. Activation energy
• B. Free energy
• C. Gibbs energy
• D. Catalytic efficiency

Answer: (A)

Activation energy is the energy barrier that enzymes lower to speed up reactions. It represents the minimum energy reactants must have to reach the transition state and form products. Enzymes accelerate reactions by stabilizing that transition state and by properly orienting substrates within the active site, sometimes using catalytic residues to donate or accept protons or to shield charges. By making the transition state easier to achieve, they lower the activation energy and increase the rate, without changing the overall energy difference between reactants and products.

The overall free energy change of the reaction (Gibbs energy) stays the same with or without the enzyme, so enzymes don’t alter the equilibrium. They also don’t change the activation energy by themselves; rather, they provide a pathway with a lower barrier. Catalytic efficiency describes how effectively an enzyme speeds up a reaction, not the barrier itself.