The specific reactants that an enzyme acts on are called

substrates

Explanation:

Substrates -

It is one of the biochemical species that is involved in a biochemical reaction, that reacts with the enzyme, in order to give the product, is known as a substrate.

In the very first step, the enzyme selectively binds with the substrate, to give an enzyme - substrate complex.

And, in the last step, complex further gives the final product of the biochemical reaction.