Crystal structure of gamma-chymotrypsin in complex with 7-hydroxycoumarin

The 1.8 Angstrom crystal structure of 7-hydroxycoumarin (7-HC) bound to chy motrypsin reveals that this inhibitor forms a planar cinnamate acyl-enzyme complex. The phenyl ring of the bound inhibitor forms numerous van der Waal s contacts in the S1 pocket of the enzyme, with the p-hydroxyl group donati ng a hydrogen bond to the main-chain oxygen atom of Ser217, and the o-hydro xyl group forming a water-mediated hydrogen bond with the carbonyl oxygen o f Val227. The structure of the acyl-enzyme complex suggests that the mechan ism of inhibition of 7-HC involves nucleophilic attack by the Ser195 O-gamm a atom on the carbonyl carbon atom of the inhibitor, accompanied by the bre aking of the 2-pyrone ring of the inhibitor, and leading to the formation o f a cinnamate acyl-enzyme derivative via a tetrahedral transition state. Co mparisons with structures of photoreversible cinnamates bound to chymotryps in reveal that although 7-HC interacts with the enzyme in a similar fashion , the binding of 7-HC to chymotrypsin takes place in a productive conformat ion in contrast to the photoreversible cinnamates. In summary, the 7-HC-chy motrypsin complex provides basic insight into the inhibition of chymotrypsi n by natural coumarins and provides a structural basis for the design of mo re potent mechanism-based inhibitors against a wide range of biologically i mportant chymotypsin-like enzymes. (C) 2001 Academic Press.