Its crystal structure (PDB file 1CPA)[21] showed the first parallel beta structure: a large, twisted, central sheet of 8 strands with the active-site Zn located at the C-terminal end of the middle strands and the sheet flanked on both sides with alpha helices. It is an exopeptidase that cleaves peptides or proteins from the carboxy-terminal end rather than internal to the sequence. Later a small protein inhibitor of carboxypeptidase was solved (PDB file 4CPA)[22] that mechanically stops the catalysis by presenting its C-terminal end just sticking out from between a ring of disulfide bonds with tight structure behind it, preventing the enzyme from sucking in the chain past the first residue.