In an enzymatic reaction, what is a substrate?

• A. The reactant (what participates in the reaction) in a enzymatic reaction
• B. The product created by the reaction
• C. A cofactor that binds to the enzyme but does not participate
• D. The enzyme that speeds up the reaction

Answer: (A)

Substrate is the molecule that binds to the enzyme’s active site and is transformed during the reaction—the reactant that the enzyme acts upon. After the chemical change, products are released. The product is what results from the reaction, not the molecule that participates. A cofactor is a helper that assists the enzyme but does not become the substrate in the main chemical step, and the enzyme itself is the catalyst that speeds up the reaction, not the substrate being transformed.