An enzyme is substrate-specific because of the shape of its

active site

Explanation:

In catalysis of a chemical reaction, enzymes interact with substrates, temporarily forming the so-called enzyme-substrate complex with them. In the formation of the secondary and tertiary structures of an enzyme (do not forget that the enzymes are proteins), certain locations in the molecule eventually appear that will serve to accommodate one or more substrates, just as a key lodges in the lock. These docking sites are called active sites and are on the enzyme surface. By tapping into the active sites, the substrates are close together and can react more easily.

Accordingly, we can state that an enzyme is substrate specific due to the shape of its active site.