Which term refers to proteins that function as biological catalysts?

• A. Enzymes
• B. Catalysts
• C. Substrates
• D. Hormones

Answer: (A)

Enzymes are proteins that act as biological catalysts. They speed up chemical reactions inside living organisms by lowering the activation energy—the energy required to start a reaction—so processes occur more rapidly at body temperature. Each enzyme has a specific shape with an active site that binds its particular substrates, giving high specificity for the reactions it can catalyze.

While catalysts in general speed up reactions, not all catalysts are proteins that function in living systems. Substrates are the molecules that enzymes act on, not the agents that speed up the reaction. Hormones are signaling chemicals, not catalysts.