THE 2.2-A RESOLUTION X-RAY CRYSTAL-STRUCTURE OF THE COMPLEX OF TRYPSIN INHIBITED BY 4-CHLORO-3-ETHOXY-7-GUANIDINOISOCOUMARIN - A PROPOSED MODEL OF THE THROMBIN INHIBITOR COMPLEX

The crystal structure of trypsin complexed to 4-chloro-3-ethoxy-7-guanidinoisocoumarin, a potent, irreversible inhibitor of both trypsin and thrombin, has been determined to 2.20-Å resolution and refined to an R factor of 0.15 by a geometric restraint least-squares procedure. The results provide positive structural evidence for a mechanism of irreversible inactivation of the enzyme via a quinone imine methide intermediate to form a covalent adduct with the active-site His-57 residue. The chloro acyl enzyme intermediate at Ser-195 is also present. H-bonding interactions formed in the active-site complex by both inhibitor models are summarized. The structures of the complexes of 4-chloro-3-ethoxy-7-guanidinoisocoumarin with porcine pancreatic elastase and bovine trypsin have been compared, and clearly their structures are quite different. Binding conformations of this isocoumarin inhibitor are modeled by active-site homology into the structure of human α-thrombin, which is an enzyme of pharmacological interest and whose structure has recently been solved. © 1990, American Chemical Society. All rights reserved.