Carboxypeptidase is a type of protease that cleaves peptide bonds at the C-terminus of a protein or peptide. Therefore, the substrate of carboxypeptidase is a protein or peptide with an amino acid residue at its C-terminus.

Here's a more detailed explanation:

* C-terminus: This refers to the end of a protein or peptide chain that has a free carboxyl group (-COOH).

* Peptide bond: This is the type of bond that links amino acids together in proteins.

* Substrate: This is the molecule that an enzyme acts upon.

In simpler terms, carboxypeptidase "nibbles" off amino acids from the end of a protein or peptide, one at a time.

Important Note: Different carboxypeptidases have different specificities for the amino acid at the C-terminus. For example, carboxypeptidase A prefers aromatic or aliphatic amino acids, while carboxypeptidase B prefers basic amino acids.