Snapshots along an Enzymatic Reaction Coordinate:  Analysis of a Retaining β-Glycoside Hydrolase

The enzymatic hydrolysis of O-glycosidic linkages is one of the most diverse and widespread reactions in nature and involves a classic “textbook” enzyme mechanism. A multidisciplinary analysis of a β-glycoside hydrolase, the Cel5A from Bacillus agaradhaerens, is presented in which the structures of each of the native, substrate, covalent-intermediate, and product complexes have been determined and their interconversions analyzed kinetically, providing unprecedented insights into the mechanism of this enzyme class. Substrate is bound in a distorted 1 S 3 skew-boat conformation, thereby presenting the anomeric carbon appropriately for nucleophilic attack as well as satisfying the stereoelectronic requirements for an incipient oxocarbenium ion. Leaving group departure results in the trapping of a covalent α-glycosyl-enzyme intermediate in which the sugar adopts an undistorted 4 C 1 conformation. Finally, hydrolysis of this intermediate yields a product complex in which the sugar is bound in a partially disordered mode, consistent with unfavorable interactions and low product affinity.