text: Carboxypeptidases are usually classified into one of several families based on their active site mechanism.
  * Enzymes that use a metal in the active site are called "metallo-carboxypeptidases" (EC number 3.4.17).  
  * Other carboxypeptidases that use active site serine residues are called "serine carboxypeptidases" (EC number 3.4.16).
  * Those that use an active site cysteine are called "cysteine carboxypeptidase" (or "thiol carboxypeptidases")(EC number 3.4.18).
  These names do not refer to the selectivity of the amino acid that is cleaved.