What is the name for the reactant in an enzyme-catalyzed reaction?

In an enzyme-catalyzed reaction, the reactant is referred to as the substrate. This is the molecule that the enzyme acts upon to facilitate a biochemical reaction. Enzymes are highly specific proteins that bind to substrates at their active sites, lowering the activation energy required for the reaction to proceed. Once the enzyme catalyzes the conversion of the substrate, the resulting molecules are known as products.

The other terms provided don’t accurately describe the reactant in this context: a product is the result of the reaction; an inhibitor is a substance that decreases the activity of the enzyme, which would inactivate or reduce the reaction; an activator enhances or increases enzyme activity, but neither of these terms refers to the initial reactant involved in the reaction. Hence, the substrate is the correct term for the reactant in enzyme-catalyzed reactions.