For its activity, carboxypeptidase requires

A carboxypeptidase is a protease enzyme that hydrolyzes a peptide bond at the carboxy-terminal end of a protein or peptide. Humans, animals, and plants contain several types of carboxypeptidases that have diverse functions ranging from catabolism to protein maturation. It is a good illustration of the induced-fit theory, because the active site changes appreciably when the substrate binds. As the protein substrate binds to carboxypeptidase, the active site closes in around it. Hydrolysis of the peptide bond is most likely to occur if the terminal residue has an aromatic or bulky hydrocarbon side chain. A zinc ion ($Z{n}^{+2}$ ) is tightly bound near the active site and assists in catalysis. Three hydrogen bonding and electrostatic interactions are critical for the enzyme to recognize the terminal amino acid in the peptide chain. The intermediate is stabilized by interactions with $Z{n}^{+2}$ and the carboxypeptidase molecule. The last step is a proton transfer and cleavage of the peptide bond. This entire process requires considerable mobility of the carboxypeptidase A protein itself. So, for its activity, carboxypeptidase requires zinc and not iron, niacin or copper.