title: Crystal structure of gamma-chymotrypsin in complex with 7-hydroxycoumarin.
abstract: The 1.8 A crystal structure of 7-hydroxycoumarin (7-HC) bound to chymotrypsin reveals that this inhibitor forms a planar cinnamate acyl-enzyme complex. The phenyl ring of the bound inhibitor forms numerous van der Waals contacts in the S1 pocket of the enzyme, with the p-hydroxyl group donating a hydrogen bond to the main-chain oxygen atom of Ser217, and the o-hydroxyl group forming a water-mediated hydrogen bond with the carbonyl oxygen of Val227. The structure of the acyl-enzyme complex suggests that the mechanism of inhibition of 7-HC involves nucleophilic attack by the Ser195 O(gamma) atom on the carbonyl carbon atom of the inhibitor, accompanied by the breaking of the 2-pyrone ring of the inhibitor, and leading to the formation of a cinnamate acyl-enzyme derivative via a tetrahedral transition state. Comparisons with structures of photoreversible cinnamates bound to chymotrypsin 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 conformation in contrast to the photoreversible cinnamates. In summary, the 7-HC-chymotrypsin complex provides basic insight into the inhibition of chymotrypsin by natural coumarins and provides a structural basis for the design of more potent mechanism-based inhibitors against a wide range of biologically important chymotrypsin-like enzymes.
authors.affiliation: International Centre for Chemical Sciences, H. E. J. Research Institute of Chemistry, University of Karachi, Karachi, 75270, Pakistan.
authors.forename: U
authors.initials: U
authors.lastname: Ghani
authors.forename: K K
authors.initials: KK
authors.lastname: Ng
authors.lastname: Atta-ur-Rahman
authors.forename: M I
authors.initials: MI
authors.lastname: Choudhary
authors.forename: N
authors.initials: N
authors.lastname: Ullah
authors.forename: M N
authors.initials: MN
authors.lastname: James
mesh_terms: D000818:Animals; D001665:Binding Sites; D002417:Cattle; D002918:Chymotrypsin / Q000037:antagonists & inhibitors* / Q000737:chemistry* / Q000378:metabolism; D002934:Cinnamates / Q000737:chemistry / Q000378:metabolism; D018360:Crystallography, X-Ray; D015195:Drug Design; D006860:Hydrogen Bonding; D008956:Models, Chemical; D008958:Models, Molecular; D011485:Protein Binding; D011487:Protein Conformation; D015842:Serine Proteinase Inhibitors / Q000737:chemistry* / Q000378:metabolism*; D013329:Structure-Activity Relationship; D014468:Umbelliferones / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D002934:Cinnamates; D015842:Serine Proteinase Inhibitors; D014468:Umbelliferones; C031477:7-hydroxycoumarin; D002918:Chymotrypsin; C117605:gamma-chymotrypsin