1. An enzyme catalyzes a reaction by
(a) decreasing the energy of the substrate.
(b) deveasing the activation energy of the reaction.
(c) decreasing product stability.
(d) increasing the activation barrier of the reaction.

Enzymes act as biological catalysts that speed up chemical reactions essential for life without being consumed. The correct answer to the multiple-choice question is (b) decreasing the activation energy of the reaction. This mechanism allows reactions to occur rapidly under physiological conditions.

Correct Answer Explanation

Enzymes lower the activation energy (Ea), the energy barrier reactants must overcome to form products. By stabilizing the transition state through active site binding, enzymes provide an alternative pathway with reduced Ea. This increases reaction rates exponentially without altering equilibrium or free energy change (ΔG).

Option Analysis

• (a) Decreasing the energy of the substrate: Incorrect. Enzymes do not lower substrate energy levels; they modify the reaction pathway, leaving substrate and product energies unchanged.

• (b) Decreasing the activation energy of the reaction: Correct. Enzymes reduce Ea by transition state stabilization, bond strain, or optimal substrate orientation, enabling faster product formation.

• (c) Decreasing product stability: Incorrect. Product stability remains unaffected; enzymes influence kinetics, not thermodynamics.

• (d) Increasing the activation barrier of the reaction: Incorrect. Raising the barrier would slow reactions, opposite to catalysis.

Why Activation Energy Matters

Activation energy determines reaction feasibility at body temperature (~37°C). Without enzymes, many vital reactions like DNA replication would be too slow. Mechanisms include acid-base catalysis and covalent intermediates, all converging on Ea reduction.

Exam Preparation Tips

Focus on energy diagrams showing lowered Ea peaks for enzyme-catalyzed vs. uncatalyzed reactions. Practice similar MCQs from biochemistry textbooks or platforms like Khan Academy. Understanding distinguishes enzyme action from inhibitors, which may raise Ea.