X‐ray crystallographic structure of a papain‐leupeptin complex

Abstract
The three‐dimensional structure of the papain‐leupeptin complex has been determined by X‐ray crystallography to a resolution of 2.1 Å (overall R‐factor = 19.8%). The structure indicates that: (i) leupeptin contacts the S subsites of the papain active site and not the S'subsites; (ii) the ‘carbonyl’ carbon atom of the inhibitor is covalently bound by the Cys‐25 sulphur atom of papain and is tetrahedrally coordinated; (iii) the ‘carbonyl’ oxygen atom of the inhibitor faces the oxyanion hole and makes hydrogen bond contacts with Gln‐19 and Cys‐25.