What is the name of the reactant that an enzyme acts upon?

The reactant that an enzyme acts upon is referred to as the substrate. An enzyme catalyzes the reaction involving the substrate by lowering the activation energy required for the reaction to proceed, thereby facilitating the conversion of the substrate into products.

In the context of enzyme activity, the substrate fits into a specific region of the enzyme known as the active site. This interaction is crucial for the enzyme's catalytic function, as it allows the enzyme to modify the substrate during the reaction. The term catalyst generally refers to substances that speed up chemical reactions, including enzymes themselves, but does not specifically denote the reactant. Product refers to the outcome of the reaction, generated after the substrate has been transformed by the enzyme.

Thus, when speaking about the substance that undergoes a chemical change as a result of the enzyme's action, the term substrate is the correct and precise terminology to use.