Monomers of proteins result by the activity of the following enzyme
The action of a dipeptidase on a dipeptide would break the dipeptide into two amino acids (monomers).
A carboxypeptidase is a protease enzyme that hydrolyzes (cleaves) a peptide bond at the carboxy-terminal (C-terminal) end of a protein or peptide. This would need further breakdown to give monomers.
Pepsinogen is the inactive form of the enzyme pepsin.
Trypsin also acts on large peptides and need further breakdown to yield monomers.