4CPA:
reference: Refined crystal structure of the potato inhibitor complex of carboxypeptidase A at 2.5 A resolution., Rees DC, Lipscomb WN, J Mol Biol 1982 Sep 25;160(3):475-98. PMID: 7154070

This OCA Structure page
uses Jmol, developed by the Jmol Development Team (documentation).