What is the term for the molecule that the enzyme reacts with?

• A. Product
• B. Water
• C. Substrate
• D. Catalyst

Answer: (C)

The key idea is that in an enzyme-catalyzed reaction, the molecule that fits the enzyme and is transformed is called the substrate. The enzyme binds the substrate at the active site to form an enzyme–substrate complex, the chemical change occurs, and then the products are released. So the term for the molecule the enzyme reacts with is substrate. The other options don’t fit as well: the product is what’s produced after the reaction, not what’s acted on; water appears as a reactant in some reactions but isn’t the general term; and a catalyst refers to the enzyme itself (or another substance) that speeds up the reaction, not the molecule being transformed.