This depends on the type of enzyme. The enzyme pepsin breaks down proteins in the acidic conditions of the stomach. Pepsin has an optimum of pH 2.5 and a working range of between pH 1-4.

Exopeptidases remove the terminal amino acids from the protein chain and endopeptidases attack an internal peptide bond, as in trypsin, chymotrypsin, pepsin ... protein as substrate and ...