Relation of Stereochemical Behavior to Catalytic Mechanism

Over the past few decades the reactions of a number of retaining gly-cosylases have been reported to involve an oxocarbonium ion rather than a covalent intermediate. During the same period, however, a revised version of the traditional double-displacement mechanism has become widely accepted as the way retaining glycosidases function. Here, the required covalent intermediate is reached via an oxocarbonium ionlike transition state in which the anomeric C-l atom remains partly bonded to its original axial or equatorial substituent, with a second transition state intervening between the intermediate and cosubstrate. According to this model, the idea of an ion-pair intermediate is untenable—with the questionable exception of hen s egg-white lysozyme.13715 

a number of investigators report the catalysis of reactions by various retaining enzymes that they consider to be inconsistent with a mechanism requiring a covalent glycosyl-enzyme intermediate. For lysozyme, the crystallographic data show that OE2 of Asp52 and C-l of 0-MurNAc-GlcNAc- 

Relationship of Transition-State Structure to the Stereochemistry and Mechanism of Glycosylase Reactions 

Transition-state structures computed from multiple kinetic isotope-effect, data have provided important information on the catalytic mechanisms of glycosylases. This is exemplified by the refined transition-state structures derived by Schramm and his associates and used to probe the hydrolytic reactions catalyzed by nucleosidases and the ADPR (adenosine diphosphori-bosyl) transferase toxins of Vibrio cholerae and Corynebacterium diphthe- 

The modeled transition-state structures for hydrolysis by the a-glucosi-dase and the glucoamylase show significant oxocarbonium ion character with the D-glucosyl unit of each having a flattened 4C conformation consistent with a C-l-O-5 bond order of 1.92, even though opposite o-glucose anomers are formed from the substrate. The transition-state structures show modest differences but they do not predict the stereochemical outcome of the catalyzed reactions.157 

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